A Holistic Assessment of the Soil Health and Agronomic Effects of Dryland Crop Rotations.

Dryland agroecosystems face unique challenges for improving soil health and agroecosystem function. Severe water limitation in semi-arid climates leads to high erosion rates and low carbon (C) inputs, which constrains soil organic C (SOC) accrual, soil aggregation, and healthy microbial communities. Many dryland farmers use fallow periods, where no crops are grown and weeds are controlled, to increase water availability and yield of the following crop. However, fallow periods further exacerbate C limitation in dryland soils, and require high herbicide use, particularly for no-till systems. Reducing the duration and frequency of fallow periods to effectively intensify cropping systems has the potential to increase C input to the soil by boosting overall productivity, and reduce herbicide requirements through greater plant competition. However, few studies have examined the effects of cropping system intensification on soil health and management outcomes on working farms. We quantified SOC, water-stable aggregation, fungal and microbial biomass, arbuscular mycorrhizal colonization of winter wheat roots, 6-year yield histories, and fertilizer and herbicide use on 96 dryland, no-till fields in the semi-arid High Plains. Three levels of cropping system intensity from wheat-fallow to continuous (no summer fallow) rotations were represented along a potential evapotranspiration gradient that increases from northwestern Nebraska to southeastern Colorado. After accounting for soil texture and other covariates to a depth of 10 cm, continuous rotations had 17% higher SOC concentrations, higher aggregate stability, and three times greater fungal biomass and mycorrhizal fungal colonization of wheat roots than low-intensity wheat-fallow systems. Continuous rotations achieved the same or greater overall grain production as wheat-fallow and mid-intensity rotations while using less herbicide and the same amount of fertilizer. Overall, we found that cropping system intensification can provide numerous soil health and yield benefits with potentially fewer chemical inputs, and that these effects are robust amidst significant variability in soil types, climates, and management styles.