Soil resource heterogeneity in terms of litter aggregation promotes nitrous oxide fluxes and slows decomposition

To better understand the role of resource heterogeneity in decomposition and nitrous oxide (N2O) flux we systematically altered the degree of plant litter aggregation in soil, from uniformly distributed to highly aggregated. In laboratory incubations, we distributed 4.5 g of dried clover shoots (Trifolium pratense L.) in two particle sizes (1 or >5 mm) into 1, 3, or 9 patches versus uniformly distributed. Soil moisture content was also varied to manipulate soil oxygen (O2) concentrations. In moist soil (50% water-filled pore space, WFPS), litter aggregation delayed the peak litter decomposition rate by 3–5 days compared to uniformly distributed litter regardless of the litter particle size. In contrast, under near-saturated soil conditions (80% WFPS) litter aggregation suppressed decomposition throughout the 26-day incubation period. This significant interaction between litter aggregation and soil moisture treatments suggests that diffusion of soil resources (likely O2) plays an important role in the influence of litter aggregation on decomposition. Specifically, O2 diffusion may more adequately meet O2 consumption rates when litter is distributed than when aggregated. In contrast to the temporary influence of aggregation on litter decomposition, N2O fluxes under 50% WFPS conditions were consistently greater and on average 7.9, 7.2, and 4.7-fold greater than fine aggregated litter (1, 3, and 9 patches, respectively) than when uniformly distributed. Coarse litter aggregation also stimulated N2O emissions, but not as much as fine litter. Under field conditions with growing maize (Zea mays L.), litter aggregation also stimulated N2O emissions. The results suggest that litter aggregation plays a role in N2O flux from agricultural soils and it might be manipulated to provide an additional N2O mitigation strategy.