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  Examining the contributions of maize shoots, roots, and manure to stable soil organic carbon pools in tropical smallholder farming soils

  Continuous inputs of organic matter are vital for sustaining soil organic carbon (SOC) and productivity of soils in smallholder crop-livestock systems. However, the dynamics of the different inputs i.e. maize shoots, roots and manure used are poorly understood. Along with organic inputs, use of mineral fertilizers can alter the nutrient stoichiometry of organic matter inputs and have implications for SOC turnover. This study sought to understand how maize-based inputs and alterations to nutrient stoichiometry contribute to stable SOC pools. We hypothesized that higher quality litter (i.e., manure) contributes more than maize residues to stable SOC pools and that N, P and S additions, designed to balance the stoichiometry of inputs to reflect the stable fine fraction of soil organic matter (C:N:P:S-10,000:833:200:143) results in greater SOC stabilization. We used a 13C natural abundance approach, where the C4 maize residues were incubated for 11 months to trace C stabilization into different SOC pools within a C3 soil. Contrary to our expectations, we observed greater recovery and stabilization of shoot-derived C (2 X more than manure and 1.63 X more than roots) in the mineral-associated organic matter (MAOM) fraction. Mineral N, P and S additions reduced new C recovery in MAOM by 40 % compared to no mineral nutrient’s additions. Our study highlights the importance of residue retention as a strategy to maintain SOC and soil health in smallholder systems, and our results challenge the idea that nutrient additions increase C stabilization of added residues.
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  Priming mechanisms providing plants and microbes access to mineral-associated organic matter

  Mineral-associated organic matter (MAOM) is considered a stable reservoir for soil nutrients that influences long-term soil carbon (C) and nitrogen (N) dynamics. However, recent experimental and theoretical evidence shows that root exudates may mobilize MAOM, thereby providing plants and microbes access to a large and N-rich pool. Given the mechanisms underlying MAOM C and N mobilization remain largely untested, we examined direct and indirect pathways by which root exudates destabilize this nutrient pool in laboratory mesocosms. We simulated root exudation with 13C-labeled oxalic acid to test whether root exudates are directly capable of mobilizing MAOM from mineral surfaces; and with 13C-labeled glucose to test whether indirect stimulation of microbial and extracellular enzyme activity leads to MAOM decomposition. We also tested the potential for oxalic acid and glucose to mobilize MAOM in an additional subset of sterilized soils to clarify the potential for non-microbial pathways of MAOM destabilization. Over the course of the 12-day MAOM incubation with and without simulated exudates, we measured C cycling (CO2 respiration rates, 13C–CO2 efflux), N cycling (inorganic N pools, gross N mineralization) and related microbial processes (enzyme activities and microbial community composition via phospholipid fatty acid analysis). Both of the simulated root exudates enhanced MAOM-C mineralization, with cumulative respiration increasing 35–89% relative to the water-only control. Likewise, glucose additions enhanced the production of an exo-cellulase and a chitinase by up to 130% and 39%, respectively, while oxalic acid enhanced oxidative enzyme activities up to 91% greater than control rates. We observed a positive association between glucose-induced shifts in enzyme activities, MAOM-C mineralization, and gross ammonification. Oxalic acid additions were associated with initial increases in fungal relative abundance and in sterile soils appeared to stimulate the release of metals and dissolved organic nitrogen into exchangeable pools. Our results indicate that common root exudates, like glucose and oxalic acid, can significantly increase the turnover and potential release of C and N from MAOM through indirect (e.g., enzyme induction) and direct (e.g., mobilization of metal oxides) mechanisms.