Crop residue decomposition and stabilization in soil organic matter

Cropland soils may be sources or sinks for atmospheric CO2. In general, it is assumed that C input into the soil and soil organic matter (SOM) levels are linearly related. This gives rise to environmental concerns regarding the removal of crop residue. In recent years, it has been shown that residue incorporation increases SOM levels to only a small extent, and high C input is not directly beneficial for SOM stabilization. Similar observations have been reported from a well-documented long-term field experiment at Puch, Germany, which contradicted the predicted (linear) relationship between C inputs (1-5 Mg C ha-1 y-1) and SOM changes. Several factors have been suggested to explain the relationship between high C inputs and small observed increases of SOM: (i) alteration of soil physical properties, affecting residue mineralization and protection; (ii) differences in residue input quality, recalcitrant belowground versus labile aboveground inputs; (iii) decomposition of native SOM through priming effects of incorporated residues; (iv) partitioning of residue C between protected and less protected SOM fraction; and (v) translocation of part of the unprotected C to the subsoil. The aim of this thesis was to ascertain whether these factors can really explain the limited increases of SOM often observed in the context of increasing crop residue inputs. In order to quantify the effect of crop residue quality and quantity on soil physical structure and SOM stabilization, 13C-labeled wheat residues with variable quality (leaves, stems, roots) and quantity were added to the soil and incubated for 2 months. Soil aggregation generally increased with higher residue additions, but the proportion of residue C protected within aggregates decreased. The protection of aboveground biomass residues (leaves and stems) was more reduced than belowground (root) residues at high additions. However, regardless of residue type, SOM decomposition increased with higher crop residue addition. The decrease of residue protection within aggregates and the increase of SOM mineralization led to a decrease in the rate of C stabilization within SOM by higher residue additions. To explore the mechanisms how crop residue quality (leaves, stems, roots) and quantity effect residue and SOM mineralization, with a special focus on the priming effect, an incubation study was conducted over a period of 4 months. The added C was traced in CO2 and in microbial biomass, and enzyme activities were measured. Roots were least decomposed and the mineralization of aboveground biomass residue disproportionally increased with higher residue additions. However, roots caused much higher SOM priming than leaves and stems. The C source partitioning and enzyme activities revealed that SOM priming was mainly controlled by residue-feeding microorganisms. To quantify the relationship between residue decomposition (i.e. quality effect), input levels, and priming, a new unifying model (logistic & power functions) was proposed. The model enabled the estimation of threshold values for mineralization of low and high residue additions above which incremental priming was maximal: i.e. ca. 20% for roots, 29-44% for stems and 39-51% for leaves. SOM priming depended on residue quality and decreased with increasing C additions. Nonetheless, priming was a power function of residue mineralization, whereby the threshold for strong increases in priming was lower for root decomposition than for aboveground residues. In order to determine the effect of long-term C inputs (straw- or root-dominated) on changes in SOM contents and partitioning of added C between SOM fractions, the soil was sampled (top- and subsoil) from a field experiment started in 1983. Where, five organic amendments (either with straw or root dominated C inputs) were combined with different N fertilization rates. C input driven by straw incorporation was highest and increased with N fertilization. The density fractionation approach was used to separate topsoil SOM fractions. Total SOM content showed an increase with C inputs, which was mainly explained by the free light fraction of SOM. Despite high inputs, straw contributed little to the free light fraction, but prevented C losses from the mineral-associated SOM fraction (ρ >1.6 g cm-3), which were observed in the absence of straw addition. In contrast to topsoil, subsoil SOM contents decreased with N fertilization, thus also with C input. Above- (straw) and belowground (root) residues showed opposite effects on SOM fractions. Root C remained longer in the light fractions and was responsible for topsoil SOM increase with N fertilization. Straw decomposed rapidly (from light fractions), and sustained the most stable mineral-associated SOM fraction. Overall, results from incubation studies and the field experiment reveal that increasing amounts of aboveground residue addition improve soil aggregation. However, low physical protection and disproportionally increased residue mineralization decreases residue stabilization in SOM. Roots are recalcitrant to decomposition, but cause stronger and higher priming effects than aboveground residues. Nevertheless, high aboveground residue mineralization protects C in the most stable mineral-associated SOM fraction. Low root mineralization indicates that root litter can mainly stay in the unprotected free light SOM fraction, but roots can increase SOM losses through priming effects. The often described minor increase of SOM after organic matter input reflects the opposing behaviors of root and aboveground residues in SOM stabilization.