Arbuscular mycorrhizal fungi (AMF) have the potential to increase plant nutrient acquisition and enhance soil aggregation, which are key processes influencing the sustainability of dryland agroecosystems in semi-arid climates. In conjunction with no-till, cropping system intensification (reducing the frequency of bare fallow in crop rotations) may enhance populations of AMF and other soil fungi through greater belowground inputs of carbon. A better understanding of the links between crop rotation, soil fungi, nutrient acquisition, and soil aggregation is required to assess the potential for cropping system intensification to increase soil organic carbon (SOC) and availability of plant-essential nutrients like phosphorus (P). We assessed SOC, water-stable aggregates, and microbial phosholipid fatty acids on 72 dryland no-till fields. Three levels of cropping system intensity from wheat-fallow to continuous rotations were represented along a potential evapotranspiration (PET) gradient that increases from northwestern Nebraska to southeastern Colorado. On a subset of 53 fields, we assessed AMF colonization of winter wheat roots and whole-plant P concentrations at wheat heading. We used multiple linear regressions to quantify the effects of cropping system intensity on SOC, aggregation, fungal biomass, and AMF colonization. We also assessed the relationships between AMF and wheat P concentrations, and total fungi and aggregation across a wide range of soil types, climates, and management histories.
Intensified cropping systems had greater SOC, AMF colonization of wheat roots, fungal biomass, and aggregate mean weight diameter to 10 cm depth compared to wheat-fallow systems when clay content, PET, and number of years in no-till were included as covariates. Continuous rotations had 17% higher SOC concentrations and 2 times larger aggregate mean weight diameter compared to wheat-fallow, and mid-intensity rotations were intermediate of the two. Plant P concentrations increased with % AMF colonization of wheat roots when PET and applied P fertilizer were included as covariates. Additionally, aggregate mean weight diameter increased with fungal biomass, independent of climate, soil type, or management. These results suggest that robust and positive effects of crop rotation on SOC and P availability are mediated by enhanced AMF symbioses and fungal biomass. In semi-arid environments, reducing the duration of fallow periods and increasing the belowground supply of C through cropping system intensification can foster growth of fungi that enhance ecosystem services.