Rapid changes in microbial biomass and aggregate size distribution in response to changes in organic matter management in grass pasture

Adding high quantities of organic matter can increase carbon (C) inputs to soil and help maintain soil structure. This study investigated short-term effects of application of different levels of composted dairymanure (CDM) versus interseeding a legume into grass pasture on aggregate stability and soil C and nitrogen (N) contents. CDM was added to amixture of perennial grasses at 22.4, 33.6 or 44.8 Mg ha−1. A grass–legume treatment was established by interseeding alfalfa (Medicago sativa) into the grass mixture. A no-input control was sampled as a reference. Soils (0–5 and5–15 cm)were sampled approximately 1.5 years after study implementation andwet sieved to obtain four aggregate size classes: large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm), microaggregates (53–250 μm) and silt and clay fraction (b53 μm). Significant CDM influences were found in the 5–15 cmdepth. The addition of 44.8 Mg CDM ha−1 and alfalfa resulted in higher proportions ofmacroaggregates (>250 μm) andmeanweight diameter (MWD) than CDM added at 22.4 or 33.6 Mg ha−1. Addition of CDMat low dose rate and alfalfa did not affect total soil or aggregate-associated organic C or N. However, addition of CDM at 44.8 Mg ha−1 and alfalfa resulted in higher total soil microbial biomass C and N compared to CDM added at 22.4 and 33.6 Mg ha−1. Large macroaggregates were found to be positively correlated with total soil microbial biomass C (R=0.81, p=0.002). In conclusion, compared to a low application rate of CDM, addition of a high application rate of CDM or alfalfa interseeding resulted in higher total soil microbial biomass C and N and macroaggregates, and these changes in microbial biomass and aggregation occurred very rapidly.

Tunsisa T. Hurisso, Jessica G. Davis, Joe E. Brummer, Mary E. Stromberger, Maysoon M. Mikha, Michelle L. Haddix, Matthew R. Booher, Eldor A. Paul (2013). Geoderma 193-194: 68-75

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