The relationship between soil microbial activity and microbial biomass, soil structure and grassland management

The question of how soil structure interacts with microbial biomass is poorly understood. Most research on soil structure and soil microbial activity has been based on laboratory measurements of soil properties that are indirectly indicative of soil structure, and very few have used direct field data. This study assessed soil structural quality in situ by visual evaluation of soil structure method (VESS) and measured microbial activity related to soil carbon (C) and nitrogen (N) cycles under various grassland management to assess whether soil structure is correlated with microbial activity, both of which are regulated by agricultural management. Soil structure (indicated by Sq score) was strongly negatively correlated with both soil respiration and enzyme activity, indicating a decline in microbial activity with poorer soil structural quality. Both frequent reseeding and N fertilizer application were positively correlated with enzyme activity indicating that these management activities, planned to improve yield can have positive impacts on C input to the soil as well. The increase in enzyme activity under higher stocking rate was perhaps driven by soil C re-location to depth by animal trampling. The strong correlation between soil structure and C and microbial activity and C indicated a two-way ‘bridge’ function of soil C regulated by management. Good structure is supportive of soil organic matter decomposition by supplying optimal physical conditions, which supply food source for soil microbes and the soil processes of soil C dominated by microbial activity are beneficial for soil structure formation. However, further research is required to better understand this two-way C ‘bridge’ function.