CHEMICAL COMPOSITION OF CROP BIOMASS IMPACTS ITS DECOMPOSITION

Authors: Johnson, Jane; Barbour, Nancy; Weyers, Sharon

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/7/2006
Publication Date: 1/1/2007
Citation: Johnson, J.M., Barbour, N.W., Lachnicht Weyers, S.L. 2007. Chemical Composition of Crop Biomass Impacts Its Decomposition. Soil Science Society of America Journal. 71(1):155-162.

Interpretive Summary: We sought to understand how the chemical make-up of plant material relates to how plants decompose. We chose five crops: alfalfa, corn, cuphea, soybean and switchgrass. We chose these crops because they have different amount of sugars, starch, and nitrogen-containing materials which break down quickly. These five crops also contain different amounts of lignin. Lignin is a very tough substance found in plants that breaks down slowly. We calculated the rate of plant break down in two parts. The first part breaks down very quickly within weeks and a second part the breaks down slowly taking months to years. We used a statistical method called stepwise multivariate analysis. This method shows us which plant chemical best described how a plant breaks down. Since we knew what was in the starting material, the method did a better job of describing the quick part of plant break down, but not the slow part. The information reported will help predict how quickly plant material breaks down, which is important for understanding carbon storage. The impact of this information is to improve carbon storage models used by scientists and improve scientists' understanding of the carbon cycle. Such knowledge may lead to improved recommendations on ways to increase carbon stored in the soil; thus, providing policy-makers insight for improving agriculture and environmental policy.

Technical Abstract: Understanding the interaction between plant components and their subsequent decomposition provides insights on how plant quality differences may influence carbon (C) sequestration within a given management system. Our hypothesis was that decomposition is a function of biochemical composition when all other variables are constants (e.g., particle size, temperature and moisture). Recognizing the challenge of reconciling laboratory and field studies, this study examined the decomposition dynamics of five selected crops with varying composition under controlled temperature and moisture regimes. Residue materials were partitioned into leaf, stem and root organs to give a clearer indication of compositional control on decomposition. Plant quality varied among species (alfalfa (Medicago sativa L.), corn (Zea mays L.), cuphea (Cuphea viscosissima X Cuphea lanceolata), soybean (Glycine Max L. merr.) and switchgrass (Panicum virgatum L.)) and organs (leaves, stems and roots). A two-component decomposition model was used to describe decomposition observed over 498 days. Stepwise multivariate regression indicated initial nitrogen (N) concentration, starch, total lignin and acid-insoluble ash (AI ash) were the four best predictors (r2=0.83) for the rate of active component decomposition (Ka). However, the rate of passive decomposition (Kp) was not readily predicted by initial composition. The best four-component model (r2=0.43) identified by stepwise multivariate regression for Kp included AI ash, hemicellulose, N concentration and C:N ratio. Rate constants are a function of the duration of incubation period, thus making direct comparison among separate experiments difficult. Chemical recalcitrance especially of corn roots appears to slow root decomposition; thus, chemical recalcitrance may contribute to the key role that roots play in soil organic C sequestration.