Size and Content of Organic Particles in the Casts of Aporrectodea caliginosa and Lumbricus rubellus (Model Experiment)
Authors:   V.V. Dokuchaev Soil Science Institute, Frolov Oleg A. ,   Lomonosov Moscow State University, Milanovskiy Evgeniy Yu. ,   Institute of Physicochemical and Biological Problems in Soil Science, RAS
Published: December 2022

Being part of a wide variety of soil invertebrates, earthworms play an important role in soil organic matter (SOM) accumulation, mixing and transformation. The goal of this study is to detect organic and mineral particles in the particle-size distributions (PSD) of the casts of Aporrectodea caliginosa and Lumbricus rubellus. The two hypotheses of this study are as follows: (a) earthworms change PSD by grinding organic matter (OM), and (b) PSD without OM does not vary in all the variants. For the first time ever, the authors studied PSD before and after OM oxidation in casts. For the first time ever, they also described the increase in the content of mineral particles in the casts of A. caliginosa and L. rubellus that was not observed in the control samples. The soil did not contain particles of >100 gm (based on the performed particle-size distribution analysis). The experimental site was located 15 km to the north of V. V. Alekhin Central Black Earth State Biosphere Reserve. In 1947, a black earth plot having an area of 0.6 hectares was ploughed under regularly mowed virgin motley grass-meadow vegetation within the Reserve territory (51°34'12.5"N 36°05'22.5" E). In this study, we used a model experiment based on microcosms with earthworms. We took soil from the arable black earth horizon of Kursk Region (51°37'17.1" N; 36°15'42.0" E). This type of soil was Protocalcic Chernozem (Loamic, Pachic). The microcosms belonged to four variants: soil, soil&litter, soil&litter and worms (A. caliginosa), soil&litter and worms (L. rubellus). All the variants had four replications. We took a total of 24 samples (an average sample from 10 different parts of the microcosm) from each variant based on replications and sampling timing (Figure 2). We measured the total content of C after dry combustion in an oxygen stream at 1,000 °C with the AN-7529 carbon analyzer (Gomel Plant of Measuring Devices, Republic of Belarus) using the method of automatic coulometric titration. For our PSD analysis, we used the laser diffractometer Malvern Mastersizer 3000E with a helium-neon red light at a wavelength of 632.8 nm, and the 600ml Hydro LV dispersing device. The measurement ranges of particle sizes were from 0.01 to 2,000 gm (Malvern Panalytical Inc., GB). We determined PSD in soil samples and casts before and after OM oxidation. The laboratory model experiment variants had four replications. We performed a carbon content analysis in three dimensions for each sample. We obtained PSD results in six replications, each of which being an average value of three sample suspension scans. The figures show arithmetic mean values for the replications and the confidence intervals of a standard deviation at the significance level (a = 0.05) calculated using Excel (2010). We made an analysis of variance (ANOVA) and a principal component analysis (PCA) using additive logarithmic ratio transformation for data normalization. The contribution of the earthworm A. caliginosa to SOM accumulation is insignificant. The TOC in the casts of A. caliginosa is 0.32± 0.06% higher vs. the reference variant “soil.” The TOC in the soil with the epigeic soil-litter earthworm L. rubellus (4.99± 0.4%) and its casts (5.03±0.24%) is significantly higher vs. other experiment variants (Figure 3). Earthworms changed the soil PSD, which led to a redistribution of particles (Table 1). Owing to the intake of organic particles, earthworms increased the share of coarse sand in the sand fraction (vs. the particle fraction (PF) of the control sample - soil without litter and earthworms) for A. caliginosa (very fine sand +1.05%, fine sand +1.07%, medium sand +0.4%, coarse sand +0.22%) and L. rubellus (very fine sand +3.36%, fine sand +4.7%, medium sand +2.24%, coarse sand +1.03%) (Figure 4). The earthworms A. caliginosa concentrate mineral particles of fine sand (+0.46%), medium sand (+0.37%), and coarse sand (+0.07%) in their casts, while L. rubellus concentrate silt particles (+3.8%) and fine sand (+0.36%) (Figure 5). The loss of vol.(%) after oxidation in all fractions in all the variants is caused by soil organic matter (Table 2). We used PCA to assess the effect of earthworm species and litter on the size and content of organic particles in casts and soil (Figure 6). The PCA results show important fractions for detection of organic (>100 pm) and mineral (250-500 pm, 500-1,000 pm) particles in the PSD. We assessed the effect of the size and content of organic particles in casts using ANOVA (Table 3). The most important factors are earthworm species and litter (based on the partial n-square). We assume that the source of mineral particles in the casts of A. caliginosa are phytoliths from the litter of Acer platanoides (L). The earthworms L. rubellus have a stronger effect on soil vs. A. caliginosa. The study does not confirm some of our hypotheses. Earthworms change PSD through OM grinding, but the PSD without OM is different in all the variants. We hypothesize in our paper that the reason is the destruction of phytoliths from litter and their accumulation in casts. One may distinguish between organic and mineral components in samples through determination of PSD before and after organic matter removal. We recommend determining a particle-size distribution both before and after organic matter removal from initial samples. The paper contains 5 Figures, 3 Tables, 54 References. The Authors declare no conflict of interest.