Disturbance by soil mixing decreases microbial richness and supports homogenizing community assembly processes.

The spatial heterogeneity of soil's microhabitats warrants the study of ecological patterns and community assembly processes in the context of physical disturbance that disrupts the inherent spatial isolation of soil microhabitats and microbial communities. By mixing soil at various frequencies in a 16-week lab incubation, we explored the effects of physical disturbance on soil bacterial richness, community composition, and community assembly processes. We hypothesized that well-mixed soil would harbor a less rich microbial community, with community assembly marked by homogenizing dispersal and homogeneous selection. Using 16S rRNA gene sequencing, we inferred community assembly processes, estimated richness and differential abundance, and calculated compositional dissimilarity. Findings supported our hypotheses, with > 20% decrease in soil bacterial richness in well-mixed soil. Soil mixing caused communities to diverge from unmixed controls (Bray-Curtis dissimilarity; 0.75 vs. 0.25), while reducing within-group heterogeneity. Our results imply that the vast diversity observed in soil may be supported by spatial heterogeneity and isolation of microbial communities, and also provide insight into the effects of physical disturbance and community coalescence events. By isolating and better understanding the effects of spatial heterogeneity and disconnectivity on soil microbial communities, we can better extrapolate how anthropogenic disturbances may affect broad soil functions.

Improved soil biological health increases corn grain yield in N fertilized systems across the Corn Belt.

Nitrogenous fertilizers have nearly doubled global grain yields, but have also increased losses of reactive N to the environment. Current public investments to improve soil health seek to balance productivity and environmental considerations. However, data integrating soil biological health and crop N response to date is insufficient to reliably drive conservation policy and inform management. Here we used multilevel structural equation modeling and N fertilizer rate trials to show that biologically healthier soils produce greater corn yields per unit of fertilizer. We found the effect of soil biological health on corn yield was 18% the magnitude of N fertilization, Moreover, we found this effect was consistent for edaphic and climatic conditions representative of 52% of the rainfed acreage in the Corn Belt (as determined using technological extrapolation domains). While N fertilization also plays a role in building or maintaining soil biological health, soil biological health metrics offer limited a priori information on a site's responsiveness to N fertilizer applications. Thus, increases in soil biological health can increase corn yields for a given unit of N fertilizer, but cannot completely replace mineral N fertilization in these systems. Our results illustrate the potential for gains in productivity through investment in soil biological health, independent of increases in mineral N fertilizer use.