Soil Structure Affects Seasonal Microbial Activity in Restored Grassland Agroecosystems.

Seasonal changes in soil microbial habitat and organic inputs may affect soil ecosystem services such as carbon (C), nitrogen (N), and phosphorous (P) storage and cycling in ways not captured by single sampling events.  We investigated seasonal turnover of soil aggregate ecology to integrate physical, chemical and biological drivers of soil ecosystem services.  Restoration of perennial agroecosystems was expected to increase microbial biomass, activity, and soil aggregation, balancing soil organic matter storage and plant production-based landowner income.  Soil aggregate turnover and intra-aggregate extracellular enzyme activity was measured within three bioenergy agroecosystems:  continuous corn, planted prairie, and fertilized planted prairie.  Physical protection of soil organic matter, measured by mean weighted diameter of soil aggregates, was greater in fertilized prairie compared to prairie and corn systems (P=0.002).  Greater aggregation corresponded with greater microbial biomass and enzyme activity in fertilized prairie; however, unfertilized prairie systems had greater root inputs (P<0.001).  Thus, N fertilizer addition increased microbial driven soil aggregation, which may lead to greater processing and protection of soil organic matter. Within aggregates, phosphatase and N-acetyl glucosaminidase activity was lower in microaggregates (<250 µm) than large macroaggregates (>2000 µm; P<0.02), but cellobiohydrolase showed the opposite response (P=0.06), indicating C, N, and P cycling may be differentially affected by soil aggregation.  Seasonal responses of potential extracellular enzyme activity differed by cropping system, increasing across the growing season in both corn and prairie, but peaking in mid-growing season in fertilized prairie (P<0.0001).  These data show agroecosystem management affects physical and biological constraints on soil C, N, and P cycling and storage.