Agronomic and Environmental Implications of Soil Resource Heterogeneity at Small Scales.

Soil resource spatial heterogeneity is a common property of most natural and managed soils; however, its functional influence over soil and plant processes is poorly known.  To understand how plant litter aggregation affects nitrogen cycle processes including N mineralization, plant N acquisition, nitrate leaching, and N₂O and total N loss we distributed ¹⁵N-labeled Trifolium pratensis litter in soil across an aggregation gradient (from uniformly distributed to highly aggregated) into which we planted maize.  Under N-limited conditions maize was 14% more productive where litter was aggregated.  Litter aggregation did not affect root to shoot ratio; however, total belowground C allocation appeared to be greater in response to uniformly distributed than to aggregated litter.  Plant N acquisition was greater in response to aggregated than uniformly distributed litter.  Litter aggregation also increased litter-derived N mineralization by 20%, shoot N by 18% and root N by 33% relative to uniformly distributed litter.  Of the litter-N added to the soil, 15% on average was unaccounted for at the end of growing season regardless of litter aggregation.  Nitrate leaching was also similar across the litter aggregation gradient (1-2% of added litter-N); however, N₂O emissions were significantly greater when the litter was aggregated than when uniformly distributed.  Results suggest that resource spatial heterogeneity alone can alter N cycling processes, and thus the management of sub-plant scale resource heterogeneity to optimize agronomic and environmental services in agricultural systems should be investigated further.