Plant litter quality affects the accumulation rate, composition, and stability of mineral-associated soil organic matter

Authors: CORDOVA, SILVIA - Iowa State University; Olk, Daniel; DIETZEL, RANAE - Iowa State University; MUELLER, KEVIN - Cleveland State University; ARCHONTOUILIS, SOTIRIOS - Iowa State University; CASTELLANO, MICHAEL - Iowa State University

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/9/2018
Publication Date: 7/13/2018
Publication URL: https://handle.nal.usda.gov/10113/6097431
Citation: Cordova, S.C., Olk, D.C., Dietzel, R., Mueller, K., Archontouilis, S., Castellano, M. 2018. Plant litter quality affects the accumulation rate, composition, and stability of mineral-associated soil organic matter. Soil Biology and Biochemistry. 125:115-124. https://doi.org/10.1016/j.soilbio.2018.07.010.
DOI: https://doi.org/10.1016/j.soilbio.2018.07.010

Interpretive Summary: The organic part of soil, the soil organic matter, plays several important roles in soil performance, including holding large amounts of essential plant nutrients, maintaining available water in soil, and feeding soil microorganisms. Despite its central importance, we do not fully understand its formation nor even its primary source materials. We challenged recent claims that soil organic matter is derived mostly from residues of soil microorganisms by incubating plant materials in soil. We found that the plant materials became part of soil organic matter regardless of the type of plant materials. Therefore plant materials are generally a significant source material of soil organic matter. These results demonstrate the importance of managing plant materials to maintain favorable levels of soil organic matter. The results are useful to researchers of soil organic matter and of soil performance, and to farm producers who strive to maintain soil organic matter levels in their fields.

Technical Abstract: Mineral-associated organic matter (MAOM) is a relatively large and stable fraction of total soil organic matter (SOM). High-quality plant litters characterized by high rates of mineralization have been hypothesized to promote the accumulation of MAOM with greater efficiency than low-quality litters because most MAOM is microbial-derived, and high-quality litters maximize the synthesis of microbial products. However, the effect of litter quality on MAOM is inconsistent. We conducted 46-d incubations of four plant litters (alfalfa, oats, maize and soybean) in two low-C subsoils (sandy loam and silt loam) with and without nutrient addition. Our short-term incubations allowed us to focus on the effect of litter quality when differences in litter-C mineralization were maximum. We hypothesized that: i) high-quality plant litters promote more efficient accumulation of MAOM-C than low-quality plant litters; ii) MAOM in soils incubated with high-quality litters has a greater proportion of microbial-derived carbohydrates than MAOM in soils incubated with low quality litters, iii) nutrient addition increases the proportion of litter-C that is transferred to MAOM-C; and iv) neither plant litter quality nor nutrient addition affects the potential mineralization of accumulated MAOM-N. Plant litter quality had a much larger effect on litter-C mineralization rate and MAOM-C accumulation than soil type or nutrient addition. Soils amended with high-quality oat and alfalfa litters had greater MAOM-C accumulation than soils amended with low-quality maize and soybean litters. However, soils amended with high-quality litters also had greater litter-C mineralization than soils amended with low-quality litters. As a result, the accumulation of MAOM-C per unit of litter-C mineralization was lower in soils amended with high- vs. low-quality litters (0.65 vs. 1.39 g MAOM-C accumulated g-1 C mineralized). Although cellulose and hemicelluose indices of accumulated MAOM were greater for maize and soybean than oats and alfalfa, most carbohydrates in MAOM were plant-derived regardless of litter quality. At the end of the incubations, more of the accumulated MAOM-N was potentially mineralizable in soils amended with high quality litters. Nevertheless, most (75%) of the litter-C remained as residual litter; 12% was mineralized to CO2 and 13% was transferred to MAOM. Given the importance of residual litter, our results point to recent work that indicates stabilization of slowly decomposing plant-derived litters is an important mode of long-term SOM stabilization in MAOM.