# Decomposition of logging residues in immature rubber plantations and effect on soil fertility

Thibaut Perron, Arnaud Legout, Frédéric Gay, Mouman Soumahoro, Daouda Kouassi, Jean-Paul Laclau, Louis Mareschal

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Thibaut Perron, Arnaud Legout, Frédéric Gay, Mouman Soumahoro, Daouda Kouassi, et al. De-composition of logging residues in immature rubber plantations and effect on soil fertility. Journal of Environmental Management, 2025, 395, pp.127640. 10.1016/j.jenvman.2025.127640. hal-05394380

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Research article

# Decomposition of logging residues in immature rubber plantations and effect on soil fertility

Thibaut Perron<sup>a,b,c,*</sup>, Arnaud Legout<sup>d</sup>, Frédéric Gay<sup>a,b</sup>, Mouman Soumahoro<sup>c</sup>, Daouda Kouassi<sup>e</sup>, Jean-Paul Laclau<sup>f,g</sup>, Louis Mareschal<sup>f,g</sup>

## ARTICLE INFO

### Keywords:

Hevea brasiliensis

Biogeochemical cycle

Africa

Diachronic study

Nutrient release kinetic

## ABSTRACT

The decomposition of crop residues plays a crucial role in nutrient biogeochemical cycles, contributing to the maintenance of soil fertility over successive replanting cycles in tropical perennial crops. In rubber plantations, the main source of natural rubber worldwide, the role of crop residues (hereafter referred to as logging residues) in nutrient cycling has been little studied. This study aimed to gain insight into the dynamics of nutrient release during the decomposition of logging residues in intensively managed rubber plantations, until their complete decay. The impact of contrasting practices of logging residue management on soil nutrient stocks was determined over 3.5 years after clearcutting. The experiment was replicated at 2 sites with contrasting soil conditions in Côte d'Ivoire. Four treatments (replicated in four blocks at each site) were set up with increasing amounts of logging residues at planting. Initial stocks of nutrients, and then decomposition rates and nutrient releases were monitored over 3.5 years after clearcutting, for leaves, twigs, branches and trunks. Soil nutrient stocks were determined in the 0–30 cm layer from clearcutting (i.e., 6 months before planting) up to three years after planting. About 94 % of the total nutrients in logging residues were released over the first three years after replanting rubber trees. All nutrient stocks in the topsoil increased drastically in the treatments with large amounts of residues. Recovery rates were calculated to evaluate soil efficiency in retaining nutrients released from logging residues: for all nutrients (except N), recovery rates were below 100 % at both sites, indicating possible nutrient losses. This indicates that further studies are necessary for a thorough understanding of the fate of nutrients originating from logging residues, particularly regarding deep drainage losses and nutrient uptake by trees. This study highlights the importance of improving practices to maximize nutrient storage and uptake such as i) uniformly spreading logging residues across the plot, ii) sowing legume cover crops and iii) planting trees as early as possible after clearcutting. Leaving logging residues on-site, either partially or fully, could reduce dependence on mineral fertilizers and promote more sustainable natural rubber production.

## 1. Introduction

In tropical perennial crops, a better understanding of nutrient cycling is essential to optimise management practices, which will ultimately improve crop performance and maintain soil fertility. When crop residues are left on-site, the nutrients released during the decomposition process can maintain or improve the nutrient stocks in the soil. These

nutrients fluxes are especially important in the tropics where soils are generally nutrient-poor and become impoverished over the successive replanting cycles in the case of perennial crops (Panklang et al., 2022). The export of logging residue is likely to lead to a soil nutrient deficit and requires expensive compensations through fertilisers.

While crop residues management has been widely documented in terms of nutrient cycling (effects on soil fertility, tree growth,

<footnote><sup>a</sup> CIRAD, UMR ABSys, F-34398, Montpellier, France</footnote>

<footnote><sup>b</sup> ABSys, Univ. of Montpellier, CIRAD, INRAE, Institut Agro, Montpellier, France</footnote>

<footnote><sup>c</sup> Société Africaine des Plantations d'Hévéas (SAPH), Abidjan, Côte d'Ivoire</footnote>

<footnote><sup>d</sup> INRAE, BEF, F-54280, Nancy, France</footnote>

<footnote><sup>e</sup> Société des Caoutchoucs de Grand-Béréby (SOGB), San Pédro, Côte d'Ivoire</footnote>

<footnote><sup>f</sup> CIRAD, UMR Eco&Sols, 2 Place Viala, Cedex 2, 34060, Montpellier, France</footnote>

<footnote><sup>g</sup> Eco&Sols, INRAE, CIRAD, IRD, Univ. of Montpellier, Institut Agro, Montpellier, France</footnote>

<footnote>* Corresponding author. CIRAD, UMR ABSys, F-34398, Montpellier, France.
E-mail address: tperron.cirad@gmail.com (T. Perron).</footnote>
decomposition rates, etc.) in tropical planted forests, this practice has received little attention in rubber plantations although they are the main source of natural rubber in the world covering about 13 million hectares (IRSG, 2025). Most of these plantations are established on highly weathered soils with low chemical fertility. The early stage of a rubber plantation (0–6 years), called the immature phase because rubber harvesting generally begins 6 years after planting, is the most important in terms of nutrient management, since the nutrient requirements of the plantations peak at this period (Perron et al., 2021). A recent review pointed out that studies dealing with nutrient fluxes in the immature phase were scarce and that current knowledge was insufficient to design sustainable nutrient management practices through the establishment of nutrient budgets (Vrignon-Brenas et al., 2019).

Logging residues correspond to the trunks, branches, twigs, leaves and stumps of the old stand after clearcutting. In the past, logging residues were burned directly in the stands. Burning is now limited by commercial companies to improve the sustainability of the management. Logging residues are now commonly exported for economic (Simorangkir, 2006) and phytosanitary considerations, especially to limit the risk of white root diseases. The whole tree is crushed and used as biomass energy or only the trunk as timber. These practices result in considerable nutrient exports, as logging residues are rich in nutrients: 970 kg ha⁻¹ of N, 189 kg ha⁻¹ of P, 366 kg ha⁻¹ of K, 942 kg ha⁻¹ of Ca and 256 kg ha⁻¹ of Mg are stocked in the whole rubber tree stand ~40 years after planting, according to a study conducted in Côte d'Ivoire (Perron et al., 2021).

The amounts of nutrients contained in logging residues are at least twice as high as in rubber plantations than in *Eucalyptus* and *Acacia* plantations (Van Bich et al., 2018; Chen et al., 2023; Kulmann et al., 2023), as a result of longer rotations in rubber plantations. The effects of logging residues on nutrient cycling could thus be considerable in rubber plantations.

The decomposition of logging residues is an important process that contributes to the supply of nutrients to the soil. In rubber plantations, only one reference tackles this process, as far as we know. Logging residues from a 31-year-old stand in Malaysia were completely decomposed between 8 months after clearcutting for leaves and 33 months for branches (the decomposition of the trunk was not assessed) (Yew, 2001). However, this study does not provide the dynamics of decomposition, but only a time until complete decay.

Several studies conducted in *Eucalyptus* or *Acacia* plantations have reported that leaving part of the crop residues on site positively impacted soil fertility (Inail et al., 2022; Chen et al., 2023; Kulmann et al., 2023). A recent study conducted over 18 months after clearcutting in two young rubber plantations in Côte d'Ivoire showed that spreading a part (trunk exported) or all of the logging residues on-site could improve soil functions (carbon transformation and structure maintenance) in the 0–10 cm soil layer (Perron et al., 2022). However, the consequences of these practices on the stocks of bioavailable nutrients in the soil, in horizons deeper than 10 cm and over the complete period of logging residues decomposition, remain unknown.

In the tropics, the rapid decomposition of crop residues can lead to a substantial input of nutrients into the soil over a short period of time, especially in the case of rubber plantations where logging residues contain large amounts of nutrients. The time lag between the low nutrient requirements of the rubber trees the first two years after planting and the large amounts of nutrients likely to be released by the decomposition of logging residues the first months after clearcutting (given the inverse exponential shape of decay models) raises question on the fate of these nutrients. In sandy soils, under heavy rainfalls, significant losses of nutrients through deep leaching are expected, even though no data can corroborate this so far. In young (1-3-years old) rubber plantations, however, a recent study reported that losses through runoff and soil erosion were very low as long as a cover crop was sown, and even lower when combined with logging residues spread in the inter-rows (Perron et al., 2024).

We conducted a 3.5-year diachronic study in two immature rubber plantations established on contrasting soil types in Côte d'Ivoire. The objective was to gain insight into the dynamics of carbon and nutrient releases during the decomposition of logging residues in intensively managed rubber plantations. The specific objectives were: (1) to quantify the decomposition dynamics of logging residues and the associated release of nutrients; (2) to assess the effects of different logging residue management practices on soil nutrient stocks and (3) to estimate the soil's efficiency to store the nutrients released from decomposition.

## 2. Material and methods

### 2.1. Study sites

The study was carried out from December 2017 to June 2021 in two commercial rubber plantations in Côte d'Ivoire (Supplementary Fig. 1). The Bongo plantation belongs to the Société Africaine des Plantations d'Hévéas (SAPH) and is located in the southeast of the country (latitude 5°30'32.364"N, longitude 3°32'51.755"W, altitude 96 m). The Société de Caoutchouc de Grand-Béréby plantation (SOGB) is located in the southwest of the country (latitude 4°43'9.696"N, longitude 7°6'41.795"W, altitude 34 m). Both plantations lie within the rainforest ecological zone, where a sub-equatorial climate favourable to rubber tree cultivation prevails. Rainfall in southern Côte d'Ivoire is divided between two rainy seasons: a major one from May to July and a minor one from October to November, with two dry seasons from December to April and from August to September. From 2001 to 2021, average annual rainfall was 1758 mm at SAPH and 1701 mm at SOGB; mean annual temperature was 27.2 °C and 26.9 °C at SAPH and SOGB, respectively (Fig. 1).

The SAPH site is characterized by slight slopes (<5 %). Its soil is classified as an arenic Acrisol according to the World Reference Base for Soil Resources (IUSS Working Group WRB, 2015). The SOGB site features hilly areas (slopes of 10–25 %) and its soil is classified as a ferralic Acrisol. Soil texture is loamy sand at SAPH (11 % clay in the topsoil) and sandy loam at SOGB (23 % clay in the topsoil) (Table 1). The soils at both sites are acidic, with a pH<sub>H20</sub> of 4.3 and 4.7 in the 0–30 cm horizon at SAPH and SOGB, respectively. While total carbon and nitrogen are higher at SOGB, the concentration of available phosphorus in the 0–30 cm soil layer is higher at SAPH.

At both sites, the previous land use was tropical rainforest. Before clear-cutting, the stand at SAPH was 40 years old with 253 living trees ha⁻¹, and the one at SOGB was 38 years old with 233 living trees ha⁻¹. Weeds and undergrowth were chemically controlled by manual weeding.

### 2.2. Experimental design

Old rubber plantations were clear-cut in November 2017 at both sites. The trees were felled using a bulldozer, which pushed up the trunk, as is generally the case in rubber plantations. This felling method results in the stumps being left on the surface of the ground. The logging residues were spread in the interrow (method used in commercial rubber plantations). In the planting line, 1 m on the either side of the planting line, soil was kept bare.

Experimental plots were set up from December 2017 to January 2018, and the experimental design was the same at SAPH and SOGB. It consisted of four practices of logging residue management replicated four times in randomized blocks, giving 16 plots per site. Each plot had 80 trees and covered an area of 0.14 ha at SAPH and 0.16 ha at SOGB. The treatments were as follows.

- No Residue (NR): all logging residues (trunks, stumps, branches, twigs and leaves) were removed from the plot.
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Umbrothermal diagram at SAPH (a) and SOGB (b) over the period 2011–2021. Temperature data come from stations located in Adiaké and San Pedro, two cities close to SAPH and SOGB sites respectively (source: infoclimat.fr). The rainfall data come from recordings made at the 2 study sites. The histograms represent rainfall and the red lines the mean temperatures.</figcaption></figure>

- Partial Residues (PR): the trunks were removed from the plot. Stumps, branches, twigs and leaves were left on-site in the inter-rows.

- Residues (R): all logging residues were left on-site in the inter-rows.

- Double Residues (DR): a double input of logging residues was left on-site in the inter-rows (i.e., twice the amount of the R treatment).

A legume cover of *Pueraria phaseoloides* was sown broadcast in the inter-rows of all treatments (10 kg ha⁻¹ of moistened seeds) in February 2018. Sowing *Pueraria phaseoloides* is a common practice in industrial rubber plantations during the first months after clearcutting, especially to control soil erosion and weeds. Subsoiling to a depth of 80 cm, followed by an application of dolomite at 200 kg ha⁻¹ in the planting row, was carried out in early March 2018 in all plots. At SOGB, young rubber trees were planted in May 2018 with the IRCA230 clone spaced at 8 × 2.5 m (i.e., 500 trees ha⁻¹). At SAPH, planting took place in July 2018 with the IRCA41 clone spaced at 6 × 3 m (i.e., 555 trees ha⁻¹). A width of 1 m, of the soil surface on either side of the planting row is kept bare by frequent weeding (glyphosate applications). At planting, 40 g tree⁻¹ of N, P, K and Mg slow-release fertilizer (formulation: 15-8-9-3) was applied in the planting hole in all treatments.

## 2.3. Biomass and nutrient stocks in the old rubber stands

A full inventory of trees was carried out in the old rubber plantations in October 2017, just before the clearcut, at both sites. Ten trees were then selected in ten classes of basal area (from the smallest to the biggest tree in the stand) and destructively sampled. The aboveground biomass

was separated into four components: trunk, branches (4 cm < Ø < 20 cm), twigs (Ø ≤ 4 cm) and leaves. The stumps were uprooted down to a depth of approximately 1 m, and all lateral roots with a diameter greater than 1 cm were collected to reduce the risk of white root disease. The fresh weights of all the tree components were measured in the field. Subsamples of each component were collected and weighed for moisture content measurements. These samples were then oven-dried at 65 °C, from 2 days for leaves to 10 days for trunks, and crushed before chemical analyses (see Section 2.5).

## 2.4. Soil nutrient stocks

Soil nutrient stocks were assessed only in the most contrasting treatments, i.e., in No Residues, Residues and Double Residues (soil in the Partial Residues treatment was not sampled). Soil sampling was carried out in September 2017, just before the clearcut and then annually in April-May from 2018 to 2021, approximately at 0, 12, 24 and 36 months after planting. Soils were sampled in the middle of the inter-row down to a depth of 30 cm using a manual auger. In each plot, three soil cores (0–30 cm) were collected at different locations and mixed together in a composite soil sample, for a total of 4 composite soil samples per treatment (i.e., one per block). Soil samples were then air-dried, sieved at 2 mm and sent to the different laboratories for chemical analysis (see Section 2.5).

The bulk density of the 0–30 cm layer was measured annually at one location per plot (next to a sampling for chemical analysis), for a total of 4 samplings (i.e., one per block) using a metallic cylinder of ~250 cm³. Soil was then oven-dried at 105 °C for 24 h, sieved at 2 mm and weighed
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Biomass (t ha⁻¹) and nutrient (N, P, K, Ca and Mg in kg ha⁻¹) stocks in the different components of logging residues spread in the inter-rows of the three practices of logging residue management at SAPH and SOGB sites (n = 4). “PR” stands for Partial Residues, “R” for Residues and “DR” for Double Residues practices.</figcaption></figure>

<caption>**Table 1**</caption>

Soil properties at SAPH and SOGB in the 0–30 cm layer in September 2017. Data in brackets are standard deviations (n = 4). See section 2.6. for details on soil analysis methods.

<table><thead><tr><th></th><th>SAPH</th><th>SOGB</th></tr></thead><tbody><tr><td>pH<sub>H20</sub></td><td>4.3 (0.1)</td><td>4.7 (0.2)</td></tr><tr><td>Total C (%)</td><td>0.76 (0.12)</td><td>1.08 (0.27)</td></tr><tr><td>Total N (mg kg<sup>-1</sup>)</td><td>625 (75)</td><td>857 (147)</td></tr><tr><td>Av. P (mg kg<sup>-1</sup>)</td><td>7.3 (3.6)</td><td>4.0 (0.8)</td></tr><tr><td>Exch. K (cmol kg<sup>-1</sup>)</td><td>0.02 (0.00)</td><td>0.04 (0.02)</td></tr><tr><td>Ca (cmol kg<sup>-1</sup>)</td><td>0.24 (0.07)</td><td>0.30 (0.08)</td></tr><tr><td>Mg (cmol kg<sup>-1</sup>)</td><td>0.06 (0.01)</td><td>0.13 (0.05)</td></tr><tr><td>Clay (%)</td><td>11 (0)</td><td>23 (5)</td></tr><tr><td>Silt (%)</td><td>2 (0)</td><td>13 (4)</td></tr><tr><td>Sand (%)</td><td>87 (1)</td><td>64 (4)</td></tr></tbody></table>

without the coarse (>2 mm) components.

Soil nutrient stocks were calculated at equivalent soil mass to avoid any bias linked to changes in soil compaction over time and among treatments (Fowler et al., 2023), using the simpleESM R package (Ferchaud et al., 2023).

## 2.5. Dynamics of logging residue decomposition

Next to the experimental trial, 56 decomposition subplots of 1-m² composed of leaves, twigs (Ø < 4 cm) and branches (4 cm < Ø < 20 cm), protected by a 1 × 1 cm mesh net, were set up just after tree harvesting to quantify the decomposition kinetics of these components, at both sites. In addition, 32 sections of 1 m long trunks were laid out on the soil surface. The trunks, twigs, branches and leaves used were collected in the old stand clearcut in November 2017 at each site. The humidity level of the different compartments was measured at the beginning of the monitoring (samples were oven-dried 48 h at 65 °C and weighed). The

wet weights of leaves (1.1 kg), twigs (4.0 kg) and branches (14.0 kg) deposited within the 1 m² decomposition plots were proportional to those of the stand inventoried prior to clear-cutting. From December 2017 to December 2021, at regular time intervals until their complete decomposition (i.e., every 3 months during the first year, then every 6 months up to 48 months), 8 decomposition plots and 4 trunks were destructively sampled at random. For each component, dry weight, ash content and mineral concentrations were determined (see Section 2.5.). The decomposition of stumps was not assessed. We hypothesized that it was similar to that of trunks, given the similar carbon to nitrogen ratio between the two components according to our data (not shown here) and the deposition of the stump on the soil surface after excavation.

## 2.6. Chemical analysis of soil and plant

For nutrient concentrations in soil samples, total N was analysed with an elemental analyser (FlashSmart™, Thermo Fisher), available P with the Bray II method (Bray and Kurtz, 1945), and exchangeable K, Ca and Mg were extracted with the cobalthexamine reagent and then measured by atomic emission spectrometry (ICP-AES).

For nutrient concentrations in plant samples, N was analysed with an elemental analyser (FlashSmartTM, ThermoFisher). The P, K, Ca and Mg concentrations were determined using a spectrophotometer after digestion by hydrofluoric acid and double calcination.

## 2.7. Data analyses

All analyses and graphs were done using R software (version 3.6.2).

### 2.7.1. Allometric equations

Allometric equations were established to assess biomass and nutrient stocks contained in the logging residues deposited on the soil surface in
the experimental plots. The methodology described in Perron et al. (2021) was used. Briefly, log-transformed linear equations were determined to assess the relationship between the biomass or the nutrient stocks of a tree component and the diameter as the predictive variable:

$$ \ln(y) = \ln(a) + b\ln(x) \quad (1) $$

where y is the biomass or nutrient stock (kg tree⁻¹), x the predictive variable, a the intercept coefficient and b the scaling exponent.

The models were established at the site level given the contrasting conditions between sites (soil properties, clones, etc.). The Akaike Information Criterion (AIC) and adjusted coefficient of determination (adj. R²) were used to select the best individual models. The model with the lowest AIC and highest adj. R² was adopted. The adj. R² of all the allometric equations used was greater than 0.85. The selected models were applied to the stand inventory carried out in the former rubber plantation just prior to clearcutting to assess the biomass and nutrient stocks of each tree component.

In total, 50 allometric equations were fitted at the two study sites (Supplementary Table 1). They were used to predict the biomass and nutrient stocks in each logging residue component (leaves, twigs, branches, trunks and stumps) at both sites.

### 2.7.2. Dynamics of nutrient releases over time

We calculated the remaining nutrient rates over time in the different components of logging residues:

$$ RNR_{i,t} = \frac{C_{i,t} \times M_{i,t}}{C_{i,0} \times M_{i,0}} \times 100 \quad (2) $$

where RNRᵢₜ is the remaining nutrient rate of component i in % at time t; Cᵢₜ and C₀ are the nutrient concentrations of component i at time t and when the decomposition subplots were set up, respectively; Mᵢₜ and M₀ are the dry masses of the component i at time t and when the decomposition subplots were set up, respectively.

The decomposition of each component of logging residues was then fit to the single exponential decay model (Chen et al., 2023):

$$ \text{mod.RNR}_{i,t} = e^{-kt} \quad (3) $$

where mod.RNRᵢₜ is the modelled remaining rate of a nutrient (or biomass) of component i, expressed in %, at time t; k is the constant of decomposition and t the time, expressed in month, since the decomposition subplots were set up

To estimate the release of nutrients from all logging residues (leaves, twigs, branches, trunks and stumps together) in each treatment, we used Eq. (2) summing all the masses of the residue components and then applied Eq. (3) to obtain an estimated remaining rate of a nutrient of all the residue components, in % (mod.RNRᵢₜₐ). In total, 5 global models were fitted per treatment and site (one for each nutrient, i.e. N, P, K, Ca, Mg).

The amounts of nutrients released in the experimental plots from the decomposition of logging residues were then estimated:

$$ nR_{i,t} = \text{mod.RNR}_{i,t} \times M_0 \quad (4) $$

where nRᵢₜ is the amount (in kg ha⁻¹) of nutrient i released from logging residues at time t; mod.RNRᵢₜₐ is the modelled remaining rate of a given nutrient (in %) in all of the residue components together at time t and M₀ is the initial nutrient stock in the logging residues deposited at the soil surface in kg ha⁻¹, estimated through allometric equations when the decomposition subplots were set up.

### 2.7.3. Soil nutrient recovery rates

A comprehensive study of the fate of nutrients released during the decomposition of logging residues would have required the monitoring of labelled nutrients in the soil and in plants (e.g., Versini et al., 2014). In order to estimate the approximate soil efficiency to store the nutrients

released by the decomposition of logging residues we calculated a recovery rate based on the quantity of nutrient retained in the soil (in the 0–30 cm layer) compared with the amount of the same nutrient released by the decomposition of logging residues, over a given period of time, using the equation:

$$ RRI = \frac{\text{Soil.nS}_{t+3} - \text{Soil.nS}_{t-0.5}}{nR_{t+3} - nR_{t-0.5}} \quad (5) $$

where RRI is the recovery rate of nutrient i expressed in %; Soil.nSᵢₜ₃ and Soil.nSᵢₜ-0.5 are the soil nutrient stocks in the 0–30 cm layer of nutrient i (total N or available P or exchangeable Ca, Mg, K, expressed in kg ha⁻¹) 3 years after planting and 0.5 year before planting, respectively; nRᵢₜ₃ and nRᵢₜ-0.5 are the amounts of nutrient i released from the decomposition of logging residues (in kg ha⁻¹) at t+3 years after planting and 0.5 year before planting, respectively.

Although several input and output fluxes of nutrients can occur in the topsoil over the first years after replanting, a recovery rate of 100 % within a given period could suggest that all the nutrients from logging residues decomposition have been stored in the 0–30 cm soil layer. When the recovery rate was <100 %, we hypothesized that nutrients released from the decomposition of logging residues have not been totally stored in this soil layer while a recovery rate >100 % would suggest additional nutrient inputs into the soil such as atmospheric N fixation by the legume or atmospheric deposition.

### 2.7.4. Statistical analyses

Mixed linear models were used to test the effects of logging residue management practices on soil nutrient stocks, using lme4 R-package (Bates et al., 2020). Treatment and year of sampling were included as fixed effects with their interactions, and the block was included as random effect. Normality of residuals and homoscedasticity were checked. In cases of significant effects (P < 0.05), Tukey's HSD multiple comparisons of means (post-hoc test) were performed using the emmeans R package (Lenth et al., 2020).

## 3. Results

### 3.1. Nutrient stocks in the logging residues

The total biomass of logging residues left on-site in the R treatment was 188 t ha⁻¹ at SAPH and 261 t ha⁻¹ at SOGB (Fig. 2). In the DR treatment, biomass was approximately double, reaching 368 t ha⁻¹ at SAPH and 494 t ha⁻¹ at SOGB. For the PR treatment, total biomass was 95 t ha⁻¹ at SAPH—about 50 % less than in the R treatment due to the removal of trunks—and 96 t ha⁻¹ at SOGB, representing 73 % of the biomass in the R treatment following trunk removal.

In the R treatment, nutrient stocks in the logging residues spread in the inter-rows were as follows: N stocks were 978 and 1013 kg ha⁻¹, P stocks were 188 and 140 kg ha⁻¹, K stocks were 415 and 683 kg ha⁻¹, Ca stocks were 1339 and 2489 kg ha⁻¹, and Mg stocks were 283 and 445 kg ha⁻¹ at SAPH and SOGB, respectively. The trunk was the component containing the most nutrients in terms of stock. When excluding trunks, as in the PR treatment, the stocks were reduced to 665 and 411 kg N ha⁻¹, 106 and 55 kg P ha⁻¹, 249 and 270 kg K ha⁻¹, 582 and 696 kg Ca ha⁻¹, and 108 and 126 kg Mg ha⁻¹ at SAPH and SOGB, respectively.

### 3.2. Decomposition of logging residues

Leaves exhibited the fastest decomposition dynamics, with complete disappearance (0 % remaining mass) occurring within 18 months at both sites (Fig. 3). In contrast, trunks, branches, and twigs took up to 48 months to fully decompose after the start of the experiment. Based on the exponential decay models, the half-life of leaves was 4.5 months at SAPH and 4.0 months at SOGB (Table 2). The half-lives of trunks, branches, and twigs were similar, ranging from 13.9 to 16.3 months at
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Remaining mass (in %) of each logging residues component during the decomposition process at SAPH and SOGB (n = 8 for leaves, twigs and branches; n = 4 for trunks).</figcaption></figure>

**Table 2**

Constant of decomposition *k* of the exponential decay models (month⁻¹), coefficient of determination (R²) of the model, half-life time (t₀.₅ in month) and whole-life time (t₀.₉₅ in month) for each component of logging residues at SAPH and SOGB.

<table><thead><tr><th>Site</th><th>Components</th><th>k (month<sup>-1</sup>)</th><th>R<sup>2</sup></th><th>t<sub>0.5</sub> (month)<sup>a</sup></th><th>t<sub>0.95</sub> (month)<sup>b</sup></th></tr></thead><tbody><tr><td rowspan="4">SAPH</td><td>Trunks</td><td>0.0497</td><td>0.88</td><td>13.9</td><td>60.3</td></tr><tr><td>Branches</td><td>0.0437</td><td>0.91</td><td>15.9</td><td>68.6</td></tr><tr><td>Twigs</td><td>0.0461</td><td>0.85</td><td>15.0</td><td>65.0</td></tr><tr><td>Leaves</td><td>0.1522</td><td>0.88</td><td>4.5</td><td>19.7</td></tr><tr><td rowspan="4">SOGB</td><td>Trunks</td><td>0.0483</td><td>0.94</td><td>14.3</td><td>62.0</td></tr><tr><td>Branches</td><td>0.0262</td><td>0.71</td><td>26.5</td><td>114.3</td></tr><tr><td>Twigs</td><td>0.0426</td><td>0.74</td><td>16.3</td><td>70.3</td></tr><tr><td>Leaves</td><td>0.1712</td><td>0.90</td><td>4.0</td><td>17.5</td></tr></tbody></table>

$$a \ t_{0.5} = \frac{-\ln(0.50)}{k} ; b \ t_{0.95} = \frac{-\ln(0.95)}{k}$$

both sites, except for branches at SOGB, which had a longer half-life of 26.5 months.

### 3.3. *Nutrient releases from logging residues*

All nutrients contained in leaves were fully released within 18 months after clearcutting, whereas complete nutrient release from trunks, branches, and twigs took 48 months (Fig. 4). Overall, a net nutrient release was observed starting from the first month of decomposition, with the exception of nitrogen in branches at SAPH, calcium in branches and twigs at SAPH, and phosphorus in branches at SOGB.

According to the global decay models established for each logging residue management practice, the decomposition constant *k* ranged from 0.067 to 0.138 at SAPH and from 0.085 to 0.156 at SOGB (Table 3). Overall, the nutrient release kinetics were comparable across the three logging residue management practices.

Logging residues were spread in the inter-rows of the experimental plots six months before planting. At planting (year 0), between 33 % and 60 % of the nutrients had already been released (Table 4). For example, in the Residues treatment, 343 kg ha⁻¹ of N, 94 kg ha⁻¹ of P, and 212 kg ha⁻¹ of K were released at SAPH; while at SOGB, 494 kg ha⁻¹ of N, 55 kg ha⁻¹ of P, and 302 kg ha⁻¹ of K were released. By the end of the study,

when the rubber trees were three years old, between 94 % and 99 % of the nutrients in the logging residues had been released. This corresponds to 625 and 969 kg ha⁻¹ of N, 184 and 131 kg ha⁻¹ of P, 413 and 649 kg ha⁻¹ of K, 1253 and 2388 kg ha⁻¹ of Ca, and 261 and 429 kg ha⁻¹ of Mg released at SAPH and SOGB, respectively.

### 3.4. *Effect of logging residue management on soil nutrient stocks*

Spreading logging residues in the inter-rows significantly increased both the concentrations and stocks of all nutrients in the 0–30 cm soil layer compared to residue removal (NR treatment) at both sites (Fig. 5 and Supplementary Fig. 2). For most nutrients, differences between the No Residue treatment and the Residues and Double Residues treatments became evident from the first year after planting (approximately 1.5 years after spreading residues). The Residues and Double Residues treatments showed very similar soil nutrient stocks, with nearly all nutrient stocks continuing to increase until the end of the study, when the rubber trees were three years old. Over this period, soil N stocks in the Residues treatment rose from 3.0 to 5.0 t ha⁻¹ at SAPH and from 2.8 to 4.1 t ha⁻¹ at SOGB. In the Residue treatment, soil P stocks increased 8-fold and 11-fold, K stocks increased 4-fold and 17-fold, Ca stocks increased 5-fold and 9-fold, and Mg stocks increased 7-fold and 8-fold at SAPH and SOGB, respectively. In contrast, nutrient stocks in the No Residue treatment remained relatively stable throughout the study. For example, N stocks at SAPH were 3.0 t ha⁻¹ at the start and 3.4 t ha⁻¹ at the end of the study, while at SOGB, N stocks remained steady at 2.9 t ha⁻¹.

### 3.5. *Soil nutrient recovery rates*

For most nutrients—except nitrogen at SAPH and both N and Mg at SOGB in the Residues treatment—the recovery rate was less than 100 %, suggesting that nutrients released by the decomposition of logging residues were not all retained in the 0–30 cm soil layer (Fig. 6). The highest recovery rates were observed for N, while P had the lowest recovery at both sites. Recovery rates ranged from 6 % for P to 214 % for N at SAPH, and from approximately 0 % for P to 118 % for N at SOGB. At SAPH, recovery rates were similar between the Residues and Double Residues treatments for most nutrients, except for nitrogen, which showed a
<figure><img src="image_5.png" /><figcaption>**Fig. 4.** Remaining nitrogen, phosphorus, potassium, calcium and magnesium (in %) in the different components of logging residues during the decomposition process at SAPH and SOGB (n = 8 for leaves, twigs and branches; n = 4 for trunks).</figcaption></figure>

<caption>**Table 3**</caption>

Constant of decomposition $k_{all}$ (month$^{-1}$) from the global decay models taking into account all the components of logging residues together, depending on the nutrient and the practice of logging residue management at SAPH and SOGB.

<table><thead><tr><th rowspan="2">Site</th><th rowspan="2">Practice</th><th colspan="5">k<sub>all</sub> (month<sup>-1</sup>)</th></tr><tr><th>N</th><th>P</th><th>K</th><th>Ca</th><th>Mg</th></tr></thead><tbody><tr><td rowspan="4">SAPH</td><td>Partial residues</td><td>0.067</td><td>0.104</td><td>0.124</td><td>0.082</td><td>0.070</td></tr><tr><td>Residues</td><td>0.071</td><td>0.120</td><td>0.119</td><td>0.100</td><td>0.077</td></tr><tr><td>Double residues</td><td>0.075</td><td>0.138</td><td>0.115</td><td>0.119</td><td>0.083</td></tr><tr><td>Partial residues</td><td>0.098</td><td>0.085</td><td>0.105</td><td>0.094</td><td>0.112</td></tr><tr><td rowspan="2">SOGB</td><td>Residues</td><td>0.119</td><td>0.090</td><td>0.102</td><td>0.104</td><td>0.136</td></tr><tr><td>Double residues</td><td>0.144</td><td>0.094</td><td>0.101</td><td>0.112</td><td>0.156</td></tr></tbody></table>

higher recovery in the Residues treatment. Conversely, at SOGB, recovery rates were consistently higher in the Residues treatment compared to Double Residues for all nutrients.

# 4. Discussion

## 4.1. *Nutrient stocks in logging residues*

The amounts of nutrients in logging residues in our study are much higher than in other tropical perennial crops where a part of crop residues might be left on site after clearcutting the stand. In an *Eucalyptus* plantation in China (Chen et al., 2023), where stems are harvested 7 years after planting for paper production, N stock left on-site in the crop residues is ~250 kg ha$^{-1}$ (up to 4 times more in the Residues treatment of our trial), P stock is ~25 kg ha (up to 7 times more in our trial) and K stock ~225 kg ha$^{-1}$ (up to 3 times more in our trial) according to. In a 7-year-old *Acacia* plantation in Vietnam, 27 t ha$^{-1}$ of biomass are left on-site at replanting (including bark) (Van Bich et al., 2018). N, P and K stocks in crop residues (including bark) are respectively ~440 kg ha$^{-1}$

(up to 2.5 times more in our study), ~15 kg ha$^{-1}$ (up to 15 times more in our study) and ~61 kg ha$^{-1}$ (up to 13 more in our study).

The trunk stores a large amount of nutrients according to our results. Thus, whether trunks are left on-site (Residues treatment) or not (Partial Residues treatment) ends up to very different amount of nutrients returned, especially for Mg and Ca for which concentrations are high in this component (Perron et al., 2021). This could affect rubber tree growth and biomass production, especially in nutrient-poor soils where rubber trees are commonly cultivated. The positive effect of logging residues on tree growth or tree volume has been already reported in several studies conducted in tropical or temperate planted forests (Laclau et al., 2010; Wei et al., 2020; Inail et al., 2022). In their literature review on this topic in forest ecosystems, Achat et al. (2015) find that intensive harvest residues management practices (i.e., branches exported with foliage) result in a reduction of tree diameter by 4–7 % compared to practices including the return of part of the residues. In the case of rubber plantations, no such study has been carried out to our knowledge and further research in this field would be interesting.

## 4.2. Decomposition of logging residues and nutrient release

Our study presents for the first time a dynamic of logging residue decomposition until complete decay, and its impact on soil nutrient stocks in immature rubber plantations. The rapid decomposition of leaves, fully decomposed within 18 months, contrasts with the slower decomposition of trunks, branches, and twigs, which took 48 months. Such differences in decomposition rates among residue components have already been shown in several studies in *Eucalyptus* plantations (e. g., Hernández et al., 2009; Rocha et al., 2016; Chen et al., 2023) or rubber tree plantations (Yew, 2001). In this latter study, the only one dealing with residue decomposition in rubber plantations to our knowledge, the author reported a time until complete decomposition of 8 months for leaves and about 32 months for twigs and branches. The
<caption>**Table 4**</caption>

Nutrients released in soil (kg ha⁻¹) over time depending on the practices of logging residues management at SAPH and SOGB. The data in brackets represents the percentage of nutrient released at a given time compared to the initial mass deposited at the soil surface.

<table><caption>**Table 4 (continued)**</caption><thead><tr><th>Site</th><th>Practice<sup>a</sup></th><th>Year after planting</th><th>N (kg ha<sup>-1</sup>)</th><th>P (kg ha<sup>-1</sup>)</th><th>K (kg ha<sup>-1</sup>)</th><th>Ca (kg ha<sup>-1</sup>)</th><th>Mg (kg ha<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="12">SAPH</td><td rowspan="12">Partial R</td><td>-0.5</td><td>0 (0 %)</td><td>0 (0 %)</td><td>0 (0 %)</td><td>0 (0 %)</td><td>0 (0 %)</td></tr><tr><td rowspan="3">0</td><td>220 (33 %)</td><td>49 (46 %)</td><td>130 (52 %)</td><td>226 (38 %)</td><td>37 (34 %)</td></tr><tr><td>467 (70 %)</td><td>90 (84 %)</td><td>222 (89 %)</td><td>449 (77 %)</td><td>77 (71 %)</td></tr><tr><td>576 (86 %)</td><td>101 (95 %)</td><td>242 (97 %)</td><td>532 (91 %)</td><td>94 (87 %)</td></tr><tr><td rowspan="3">1</td><td>625 (94 %)</td><td>104 (98 %)</td><td>247 (99 %)</td><td>563 (96 %)</td><td>102 (94 %)</td></tr><tr><td rowspan="3">2</td><td>343 (34 %)</td><td>94 (51 %)</td><td>212 (51 %)</td><td>573 (45 %)</td><td>100 (37 %)</td></tr><tr><td>714 (72 %)</td><td>163 (88 %)</td><td>367 (88 %)</td><td>1061 (83 %)</td><td>204 (75 %)</td></tr><tr><td>872 (88 %)</td><td>179 (97 %)</td><td>404 (97 %)</td><td>1208 (95 %)</td><td>245 (90 %)</td></tr><tr><td rowspan="3">3</td><td>940 (94 %)</td><td>183 (99 %)</td><td>413 (99 %)</td><td>1253 (98 %)</td><td>261 (96 %)</td></tr><tr><td rowspan="3">-0.5</td><td>0 (0 %)</td><td>0 (0 %)</td><td>0 (0 %)</td><td>0 (0 %)</td><td>0 (0 %)</td></tr><tr><td>716 (36 %)</td><td>203 (56 %)</td><td>410 (49 %)</td><td>1248 (51 %)</td><td>207 (39 %)</td></tr><tr><td>1463 (74 %)</td><td>331 (91 %)</td><td>719 (87 %)</td><td>2158 (88 %)</td><td>409 (77 %)</td></tr><tr><td rowspan="12">Double R</td><td rowspan="12">Partial R</td><td>-0.5</td><td>0 (0 %)</td><td>0 (0 %)</td><td>0 (0 %)</td><td>0 (0 %)</td><td>0 (0 %)</td></tr><tr><td rowspan="3">0</td><td>182 (44 %)</td><td>21 (39 %)</td><td>125 (46 %)</td><td>300 (43 %)</td><td>61 (48 %)</td></tr><tr><td>340 (82 %)</td><td>42 (78 %)</td><td>228 (84 %)</td><td>568 (81 %)</td><td>108 (86 %)</td></tr><tr><td>389 (94 %)</td><td>50 (92 %)</td><td>258 (95 %)</td><td>654 (94 %)</td><td>121 (96 %)</td></tr><tr><td rowspan="3">1</td><td>404 (95 %)</td><td>52 (99 %)</td><td>266 (99 %)</td><td>682 (98 %)</td><td>124 (99 %)</td></tr><tr><td rowspan="3">2</td><td>859 (88 %)</td><td>107 (80 %)</td><td>554 (84 %)</td><td>2048 (84 %)</td><td>393 (91 %)</td></tr><tr><td>947 (97 %)</td><td>125 (93 %)</td><td>628 (95 %)</td><td>2312 (95 %)</td><td>423 (98 %)</td></tr><tr><td rowspan="3">-0.5</td><td>0 (0 %)</td><td>0 (0 %)</td><td>0 (0 %)</td><td>0 (0 %)</td><td>0 (0 %)</td></tr><tr><td rowspan="3">0</td><td>496 (50 %)</td><td>55 (41 %)</td><td>302 (45 %)</td><td>1124 (46 %)</td><td>239 (55 %)</td></tr><tr><td>859 (88 %)</td><td>107 (80 %)</td><td>554 (84 %)</td><td>2048 (84 %)</td><td>393 (91 %)</td></tr></tbody></table>

<table><thead><tr><th>Site</th><th>Practice<sup>a</sup></th><th>Year after planting</th><th>N (kg ha<sup>-1</sup>)</th><th>P (kg ha<sup>-1</sup>)</th><th>K (kg ha<sup>-1</sup>)</th><th>Ca (kg ha<sup>-1</sup>)</th><th>Mg (kg ha<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="12">Double R</td><td rowspan="12">Partial R</td><td>3</td><td>968 (99 %)</td><td>131 (97 %)</td><td>649 (98 %)</td><td>2388 (98 %)</td><td>429 (99 %)</td></tr><tr><td rowspan="3">-0.5</td><td>0 (0 %)</td><td>0 (0 %)</td><td>0 (0 %)</td><td>0 (0 %)</td><td>0 (0 %)</td></tr><tr><td>1109 (57 %)</td><td>114 (43 %)</td><td>586 (45 %)</td><td>2296 (48 %)</td><td>513 (60 %)</td></tr><tr><td>1774 (92 %)</td><td>216 (81 %)</td><td>1081 (83 %)</td><td>4074 (86 %)</td><td>792 (94 %)</td></tr><tr><td rowspan="3">1</td><td>1893 (98 %)</td><td>249 (94 %)</td><td>1229 (95 %)</td><td>4540 (96 %)</td><td>835 (99 %)</td></tr><tr><td>1914 (99 %)</td><td>259 (98 %)</td><td>1273 (98 %)</td><td>4663 (99 %)</td><td>842 (99 %)</td></tr></tbody></table>

<sup>a</sup> Partial R: Partial Residues; R: Residues; Double R: Double Residues.

differences in decomposition rates among residue components might be explained by several factors such as the climate, the surface area of the residues and their quality, especially characterized by nutrient concentration, the C:N ratio and the lignin content (Zhang et al., 2008). For a given quality of residue, temperature and rainfall are among the main drivers of decomposition (Giweta, 2020). The climate at SAPH and SOGB being quite the same, this may explain why we found a very similar dynamic of decomposition at both sites.

The decomposition of leaves in our study (t<sub>0.5</sub> = 4.0–4.5 months) is faster than that of leaves in *Eucalyptus* (t<sub>0.5</sub> = 9.8 months; Chen et al., 2023) or *Acacia* plantations (t<sub>0.5</sub> = 5.6; Van Bich et al., 2018) cultivated under tropical climate. The half-life time of twigs in our study (t<sub>0.5</sub> = 15.0–16.3 months) is also generally faster than that found in the two latter references (t<sub>0.5</sub> = 15.5–33.9 months). Beyond chemical composition of logging residues, the different decomposition dynamics in our study compared to *Eucalyptus*, *Acacia* plantations or rubber tree residues (in Yew, 2001) might be explained in part by the climate at our sites, featuring higher temperature and/or rainfall.

In our study, the dynamics of nutrient release shows different patterns. N is often used by microorganisms for their growth, and N accumulation might thus be observed over the decomposition process in some components with a high C:N ratio (i.e., when >30) (Liu and Sun, 2013; Zheng et al., 2017). This is the case in our study at both sites for branches, where N increases at the beginning (SAPH) or in the middle (SOGB) of the decomposition process. The same mechanism of microbial immobilization, resulting in net accumulation of nutrients in a component, occurs for P (at SOGB only) and Ca (at SAPH only) in branches. Such P and Ca immobilization in branches has already been reported during the decomposition of harvest residues of *Acacia* (Van Bich et al., 2018) and *Eucalyptus* (Rocha et al., 2016; Chen et al., 2023) in the tropics.

The global decay models taking into account all the components together show that K is rapidly released from logging residues, at a comparable speed between the two sites (constant of decomposition k<sub>all</sub> > 0.102), which is consistent with the presence of this nutrient in ionic form (K⁺) in plant tissues. For the other nutrients, the constants of decomposition between SAPH and SOGB are more contrasting. Dynamics of release of N and Mg are much higher at SOGB than SAPH; while it is the contrary for P. The soils at SAPH are characterized by much greater availability of P than SOGB; while concentrations of N and Mg are higher at SOGB compared to SAPH. The reason for such differences in nutrient release might be their contrasting availability in soils between sites: the more limiting the nutrient in soil, the slower the rate of release from logging residues. Nutrient availability in soil drives the degree of microbial immobilization and therefore the speed of litter or
<figure><img src="image_6.png" /><figcaption>**Fig. 5.** Evolution of the stocks (t ha⁻¹) of total N, available P, exchangeable K, Ca and K in the 0–30 cm soil layer, from 0.5 year before planting to 3 years after planting, depending on the practices of logging residue management at SAPH and SOGB (n = 4). The asterisks indicate a significant difference of soil nutrient stocks between treatments (t), year (y) or for the treatment and year interaction (ty). \* for *P* < 0.05; \*\* for *P* < 0.01; \*\*\* for *P* < 0.001 and NS for a non-significant difference.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Recovery rate of nutrients from logging residues in the 0–30 cm soil layer (%). This is the ratio between the delta nutrient stock in soil and the nutrient release from logging residues over the same period, depending on the practices of logging residues management (Residues and Double Residues) at SAPH and SOGB (n = 4).</figcaption></figure>

residues decomposition (Hobbie and Vitousek, 2000).

### 4.3. The management of logging residues affects soil nutrients

Our diachronic study shows, over 3.5 years, that the spreading of logging residues significantly increases soil nutrient stocks in the 0–30 cm soil layer compared to plots without residues. This increase is visible from the planting (year 0) for some nutrients and continues throughout the study period. Logging residues are therefore an appropriate practice that can substantially contribute to maintaining soil fertility, which is critical for the sustainability of rubber plantations. However, our results to date indicate that the complete removal of logging residues does not lead to soil nutrient depletion: soil nutrient stocks remain stable up to three years after planting under this management practice at both study sites. This outcome may be attributed to the presence of the leguminous cover crop *Pueraria phaseoloides*, which is known to enhance soil fertility (Clermont-Dauphin et al., 2016; Xu et al., 2023). However, on the mid-to long term, a decrease in soil fertility especially due to uptake by trees might be expected in plots where logging residues are removed. This situation could happen very soon as the stand's nutrient requirement peaks between 2 and 5 years old, reaching up to 80 kg ha⁻¹ y⁻¹ of N, 14 kg ha⁻¹ y⁻¹ of P and 34 kg ha⁻¹ y⁻¹ of K during this period (Perron et al., 2021).

The positive impact of logging residues on soil chemical properties has been widely documented in planted forests in tropical or temperate contexts However, the increase in available nutrient stocks over time when logging residues are left on-site is particularly pronounced in our study. For instance, potassium stocks increased by a factor of 15 at SOGB and 6 at SAPH over a 1.5-year period in the 0–30 cm soil layer. These values are considerably higher than those reported in other studies conducted in *Eucalyptus* plantations, where K content increased by only 1.2 in the 0–20 cm layer over one year (Chen et al., 2023) r by 1.1 in the 0–5 cm layer over one year (Rocha et al., 2016). The reason is that logging residues in rubber plantations store a great amount of nutrients compared to other tropical perennial crops where harvest residues might be economically valorised (as timber or for paper production for example) and rotation cycles are shorter (only ~7 years in *Eucalyptus* plantation against ~35 years in rubber plantation). Recent diachronic studies conducted in young rubber plantations, in the same experimental design as this trial, have also reported significant improvement in soil health and soil nematode abundance when logging residues are

left on-site against residue removal (Perron et al., 2022; Kouakou et al., 2023).

### 4.4. Can the soil store the nutrients from the decomposition flux?

This nutrient flux, corresponding to an input into soil, is the largest compared to the other fluxes (input or output) occurring in the soil of an immature rubber plantation. For example, N outputs in the early stage of a plantation (from about 0 to 2 years old) are as follows: rubber stand's uptake is ~20 kg ha⁻¹ y⁻¹ (Perron et al., 2021), N uptake by *Pueraria phaseoloides* cover crop is 30 kg ha⁻¹ y⁻¹ (Vrignon-Brenas et al., 2019), leaching is between 2 and 30 kg ha⁻¹ y⁻¹ (Vrignon-Brenas et al., 2019) and losses through runoff and soil losses are almost null when proper management is implemented, i.e., with cover crop and logging residues in the inter-rows (Perron et al., 2022). Inputs of N are as follows: fertilizer application is very variable but the recommendation in industrial plantations is about 35 kg ha⁻¹ y⁻¹ in Côte d'Ivoire (Allé et al., 2015), atmospheric deposition range between 8 and 20 kg ha⁻¹ y⁻¹ (Vrignon-Brenas et al., 2019) and finally input through rubber litterfalls is very low since rubber trees start defoliating from 1 to 2 years old.

As logging residues are spread 6 months before the planting of the new rubber cycle, large amount of nutrient (from 33 % to 60 %) is already released at the time of planting. During this 6-month period, the risk of nutrient losses through leaching and erosion is high because the soil is bare as cover crop *Pueraria phaseoloides* is not yet developed. Deep leaching is not assessed in this study but we do determine the efficiency of soil to store the nutrient released from logging residue decomposition through a recovery rate in the 0–30 cm layer. Recovery rates of K, Ca and Mg are <100 % at both sites, which indicates that these nutrients are not totally stored in the 0–30 cm soil layer. As nutrient uptake by trees is rather low until 2 years old, this result suggests that a great part of K, Ca and Mg is leached in layers deeper than 30 cm or lost in runoff water. The recovery rates of N are greater than 100 % at both sites, suggesting that inputs into the soil are higher than outputs, over the study period. The most likely additional N inputs is the atmospheric fixation of N by the legume cover crop, which can provide up to ~200 kg N ha⁻¹ y⁻¹ in the first 2 years after its sowing (Clermont-Dauphin et al., 2016). N losses are very likely, especially through leaching of nitrate (Perron et al., 2022) which is little retained on soil particles, but also through soil losses and runoff until the spreading of the legume on the soil surface (Perron et al., 2024). The very low recovery rates of P at both sites
highlight the adsorption of this nutrient on oxy-hydroxides of Al and Fe in these highly weathered tropical soils. To better understand the fate of nutrients in soil, further studies on nutrient fluxes and biogeochemical cycles are needed in immature rubber plantations.

The nutrient recovery rates in the Residues treatment are higher than in the Double Residues treatment at SOGB only, for all nutrients. This means that nutrients from residue decomposition are less stored in the 0–30 cm soil layer in the Double Residues treatment than in the Residues treatment. The main reason for this is that excessive nutrient inputs from residue decomposition in the Double Residues treatment may have led to greater losses. Even though soils at SOGB are more clayey (and therefore have a greater ability to retain ions), these greater losses at SOGB than at SAPH might be due to the much higher amount of nutrients contained in the logging residues compared to SAPH (1.6, 1.9 and 1.6 times higher at SOGB than SAPH for K, Ca and Mg respectively). We may hypothesize that the maximum storage capacity of the soil was reached at SOGB. The same situation is observed for N only at SAPH.

### 4.5. Consequences for management of immature rubber plantations

The nutrient input through logging residue decomposition could provide a continuous supply of nutrients, reducing the need for mineral fertilizers on the full rotation. In tropical *Eucalyptus* plantations, harvest residues combined with low rates of fertilizers result in similar nutrient uptake by trees than plots where high doses of fertiliser are applied (Bizerra Barros et al., 2021). Logging residues could therefore substitute, to a greater or lesser extent depending on the soil nutrient limitations, to mineral fertilisation. So far, the ability of logging residues to supply nutrients to rubber trees has never been assessed. The use of inverse exponential decay models, determined for each residue component, could be useful to estimate the amount of nutrients released over time (Chen et al., 2023) in order to design a decision-making tool for nutrient management (Vrignon-Brenas et al., 2019). Studies including labelled residues could be appropriate to gain insight into the role of logging residues in plant nutrition (Versini et al., 2014) and more generally to study the fate of nutrients in the soil-plant system.

In addition, the recovery rates <100% for some nutrients suggest the need to adopt appropriate management practices to limit nutrient losses during the first years after planting. For example, practices that minimise nutrient leaching, such as soil cover and residue placement in the plot, could improve nutrient use efficiency. In addition, the use of legume cover crops not only helps to reduce soil erosion (Perron et al., 2024) but also contributes to the cycling of nutrients, thereby improving overall soil fertility. The very low recovery rates of P suggest that additional management practices, such as the application of phosphate fertilizers, fractioning of inputs, or the use of P-efficient cover crops (Sauvadet et al., 2021), may be necessary to optimise P availability.

The rapid release of nutrients during the initial months of residue decomposition may suggest the potential for early nutrient availability to newly planted rubber trees. However, the logging residues are placed on the inter-rows, an area that the fine roots of young rubber trees would not reach before the age of 2–3 years. Further studies on fine root projection are needed to better understand the capacity of nutrient uptake by young rubber trees. The spreading of logging residues more uniformly in the plot (i.e., not stacked in the inter-rows, as mulched residues for example) could make it possible for the rubber roots to gain access more rapidly to the nutrients released during decomposition. Further long-term studies are needed to assess the fate of nutrients stored in the soil and their uptake by rubber trees throughout the plantation cycle.

Finally, removal of logging residues (or burning) at replanting is a common and old practice, especially to limit the development of the white root disease caused by *Rigidoporus lignosus* (Nandris et al., 1987). In our study, the plots including logging residues spread in the inter-rows did not show higher tree mortality due to white root disease than plots with residue removal. Tree density remained high 3 years

after planting (>95% of living trees, data not shown), in all treatments of the experiment at both sites.

### 5. Conclusion

This study highlights the significant role of logging residues in nutrient cycling in immature rubber plantations. Logging residues decompose rapidly, with leaves fully decayed within 18 months and woody components taking up to 48 months for complete decomposition. The substantial amount of nutrients stored in these residues contributes to a drastic increase in soil nutrient stocks during the first three years after planting. In contrast, removal of logging residues results in stable soil nutrient stocks over the study period, indicating no immediate soil impoverishment. However, over multiple replanting cycles, we may hypothesize that such nutrient export is unsustainable for tropical soil management, and it could eventually lead to significant soil degradation. Our results show that leaving logging residues on-site, either partially or fully, may limit soil impoverishment, reduce dependence on mineral fertilisers and promote more sustainable natural rubber production.

However, our results also suggest considerable nutrient losses, most likely due to deep drainage. This finding underscores the importance of additional practices to minimise these losses, such as uniformly spreading logging residues across the plot avoiding a width of 2 m of bare soil in the planting rows and sowing legume cover crops as early as possible after clearcutting the old stand.

Overall, our results provide valuable insights into the biogeochemical cycling of nutrients in young rubber plantations. Further research on nutrient fluxes —such as leaching, nutrient uptake by the rubber stand and legume cover crop, litterfall, and root development— is needed to develop more appropriate nutrient management practices in immature rubber plantations.

#### CRediT authorship contribution statement

**Thibaut Perron:** Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Arnaud Legout:** Writing – review & editing, Validation, Formal analysis. **Frédéric Gay:** Project administration, Funding acquisition, Conceptualization.

**Mouman Soumahoro:** Investigation, Data curation. **Daouda Kouassi:** Investigation, Data curation. **Jean-Paul Laclau:** Writing – review & editing, Validation, Supervision, Methodology, Conceptualization.

**Louis Mareschal:** Writing – review & editing, Validation, Supervision, Methodology, Conceptualization.

#### Funding information

This study was carried out as part of the FERTIM project funded by the *Institut Français du Caoutchouc* (IFC) and by the Michelin, SIPH and SOCFIN companies.

#### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

#### Acknowledgments

We would like to thank the managers of the SAPH plantation (Eric Cavalc and Richmond Konan) and the SOGB plantation (Jean-Luc Hofs and his predecessor Aurélien Masson) for providing facilities and labour during the sampling operations. We also appreciate Antoine Manizan at SOGB and Jean-Marc Toto at SAPH for their significant assistance in supervising the fieldwork. Finally, we thank the laboratories that analysed the water and soil samples, including the UMR Eco&Sols
laboratory at the CeMEB-certified ECOTROP Platform (Montpellier, France), the Cirad Laboratory of Water, Soil and Plant Analysis (US Analyses, Cirad, Montpellier, France), the SOGB laboratory (at SOGB plantation, Grand-Béréby, Côte d'Ivoire) and the LAMA laboratory (UAR IMAGO, Dakar, Senegal).

# Appendix A. Supplementary data

Supplementary data related to this article can be found online at
https://doi.org/10.1016/j.jenvman.2025.127640.

# Data availability

The data that has been used is confidential.

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