Plant Litter Quality Affects the Accumulation Rate, Composition, and Stability of Mineral-Associated Soil Organic Matter.

Plant litter quality can affect the stabilization and mineralization of soil organic matter (SOM). Recent concepts suggest that high-quality plant litters characterized by low C/N ratios should lead to faster and more efficient accumulation of mineral-associated organic matter (MAOM). However, the effect of litter quality on SOM is variable. Our objective was to isolate the effect of plant litter quality on the rate and efficiency of MAOM accumulation in the fine mineral soil fraction (i.e., soil <53 µm). We conducted a long-term incubation of four consecutive 46-day cycles. Two low-carbon (C) B horizon soils (sandy loam and silt loam) were amended with four plant litters (maize, soybean, oats, and alfalfa; 8.7 g C kg^-1 dry soil added per cycle), and mixed with and without the addition of essential nutrients. We used the four consecutive incubation cycles to maximize the possibility of measuring a change in MAOM. We measured the efficiency of litter-C transfer to MAOM as g MAOM-C per g CO₂-C accumulation. Results showed that soil type and plant litter quality had strong effects on C mineralization and accumulation of MAOM, however, the effect of nutrient addition was relatively small. Soils amended with oats and alfalfa litter (i.e., high-quality) accumulated more MAOM-C than soils amended with maize and soybean litter (i.e., low-quality). However, due to greater CO₂-C production, the accumulation of MAOM-C in soils amended with oats and alfalfa litters was two-fold less efficient compared to soils amended with soybean and maize litters. Moreover, assays for potentially mineralizable nitrogen at the end of the incubation cycles indicated that accumulated MAOM-N was less stable in soils amended with oats and alfalfa compared to soils amended with corn and soybeans.