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 dairy manure (CDM) versus interseeding a legume into grass pasture on aggregate stability and soil C and nitrogen (N) contents. CDM was added to a mixture 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 and 5–15 cm) were sampled approximately 1.5 years after study implementation and wet 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 ( 250 μm) and mean weight diameter (MWD) than CDM added at 22.4 or 33.6 Mg ha− 1. Addition of CDM at 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.