Intensifying rotations increases soil carbon, fungi, and aggregation in semi-arid agroecosystems

Increasing soil organic carbon (SOC) is a critical but daunting challenge in semi-arid agroecosystems. For dryland farmers, low levels of SOC and aggregation exacerbate the risks of farming in a water-limited environment − risks that will compound with climate change. Many dryland farmers in semi-arid climates use year long periods called summer fallow, where no crops are grown and weeds are controlled, to store rainwater and increase the yield of the following crop. In semi-arid climates around the world, dryland farmers are increasingly replacing summer fallow with a crop, a form of cropping system intensification. Cropping system intensification has the potential to increase SOC, but the drivers of this effect are unclear, and may change based on environmental conditions and management strategy. We quantified SOC, water-stable aggregates, and fungal and microbial biomass on 96 dryland, no-till fields in the semi-arid Great Plains, USA, representing three levels of cropping system intensity from wheat-fallow to continuous (no summer fallow) rotations along a potential evapotranspiration gradient. Cropping system intensity was positively associated with SOC, aggregation, and fungal biomass, and these effects were robust amidst variability in environmental and management factors. Continuous rotations averaged 1.28% SOC at 0-10 cm and had 17% and 12% higher SOC concentrations than wheat-fallow in 0-10 cm and 0-20 cm depths, respectively. Aggregate stability in continuous rotations was about twice that in wheat-fallow rotations. Fungal biomass was three times greater in continuous rotations than wheat-fallow, but was not significantly different from mid-intensity rotations. Using structural equation modeling, we observed that continuous cropping, potential evapotranspiration, % clay content, and fungal biomass together explained 50% of the variability in SOC, and that SOC appears to enhance aggregation directly and as mediated through increases in fungal biomass. Overall, the model suggests that cropping system intensity increases SOC both directly, through greater C inputs to soil, and indirectly, by increasing fungal biomass and aggregation. Our findings suggest that continuous cropping has the potential to provide gains in SOC and soil structure that will help offset C emissions and enhance the resilience of dryland agroecosystems.