Chemical differences in cover crop residue quality are maintained through litter decay

Authors: THAPA, RESHAM - North Carolina A&t State University; CABRERA, MIGUEL - University Of Georgia; Schomberg, Harry; REBERG-HORTON, CHRIS - North Carolina State University; POFFENBARGER, HANNA - University Of Kentucky; Mirsky, Steven

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2023
Publication Date: 7/23/2023
Citation: Thapa, R., Cabrera, M., Schomberg, H.H., Reberg-Horton, C., Poffenbarger, H., Mirsky, S.B. 2023. Chemical differences in cover crop residue quality are maintained through litter decay. Ecological Applications. https://doi.org/10.1371/Journal.pone.0289352.
DOI: https://doi.org/10.1371/Journal.pone.0289352

Interpretive Summary: Cover crop mixtures provide multiple ecosystem services depending on their overall decomposition and nitrogen (N) release rates. In order to provide best recommendations to growers, we must understand how variables regulating litter decay such as C:N ratio and litter chemistry (carbohydrates, holo-cellulose, and lignin) are influenced by legume-grass biomass proportions and manure management. Therefore, we conducted field experiments to assess how legume-grass biomass proportions and broadcasting poultry litter affect litter quality before and during decomposition. Overall results demonstrated that: i) increasing legume proportions increased labile carbohydrates but decreased C:N ratios as well as holo-cellulose and lignin fractions, ii) broadcasting poultry litter improved initial quality of litters dominated by grasses but not legumes, iii) either including some legumes and/or broadcasting poultry litter is recommended to avoid early-season N stress to subsequent cash crops, and iv) chemical changes in decomposing litters are related to proportional mass loss during litter decay. Our work will be used to improve process-based models and decision support tools for estimating residue persistence and N release in conservation tillage systems across diverse environments. Furthermore, farmers and ag professionals can use our results to help shape their cover crop and manure management decisions for long-term soil carbon accrual and stabilization from cover crops adoption.

Technical Abstract: As plant litter decomposes, its mass exponentially decreases until it reaches a non-zero asymptote. However, decomposition rates vary considerably among litter types as a function of their overall quality (i.e., carbon:nitrogen (C:N) ratio and litter chemistry). We investigated the effects of hairy vetch (HV: Vicia villosa Roth):cereal rye (RYE: Secale cereale L.) biomass proportions with or without broadcasted poultry manure on overall litter quality before and during decomposition. As HV biomass proportions increased from 0 to 100%, the relative susceptibility of HV:RYE mixtures to microbial decomposition increased due to: (i) decrease in the initial C:N ratio (87:1 to 10:1 in 2012 and 67:1 to 9:1 in 2013), (ii) increase in the non-structural labile carbohydrates (33 to 61% across years), and (iii) decrease in the structural holo-cellulose (59 to 33% across years) and lignin (8 to 6% across years) fractions. Broadcasted poultry manure decreased the overall initial quality of HV-dominated litters and increased the overall initial quality of RYE-dominated litters. Across all HV:RYE biomass proportions with or without poultry manure, chemical changes during litter decay were related to proportional mass loss. Therefore, the relative decrease in carbohydrates and the concomitant increase in holo-cellulose and lignin fractions were more pronounced for fast decomposing litter types, i.e., litters dominated by HV rather than RYE. While our results suggest possible convergence of litter C:N ratios at ~80% of the cumulative mass loss, initial differences in litter chemistry neither converged nor diverged. Therefore, we conclude that initial chemistry of litter before decomposition exerts strong control on its chemical composition throughout the decay continuum.