The path less taken: Long-term N additions slow leaf litter decomposition and favor the physical transfer pathway of soil organic matter formation

Understanding soil organic matter (SOM) formation as a balance between soil microbial access to organic plant inputs and protection by chemical recalcitrance and mineral associations can greatly improve our projections of this important terrestrial carbon pool. However, gaps remain in our understanding of the processes controlling the formation and destabilization of SOM and how these processes are affected by persistent global changes, such as nitrogen (N) deposition. To assess how elevated N deposition influences decomposition dynamics and the fate of plant inputs in a temperate deciduous forest, we coupled a reciprocal transplant leaf litter decomposition study with an analysis of the distribution of SOM in mineral associated and particulate organic matter fractions at a long-term, whole-watershed, N fertilization experiment. Nearly 30 years of N additions slowed leaf litter decomposition rates by about 11% in the fertilized watershed, regardless of the watershed from which the initial litter was collected. An apparent consequence of the altered rates of decomposition was that the proportion of SOM in light particulate organic matter in soil from the fertilized watershed was about 40% greater than that of the reference watershed, and was positively correlated with the bulk soil carbon to nitrogen ratio. Collectively, our results suggest that N saturation in a temperate forest alters SOM formation by slowing decomposition and favoring the accumulation of particulate organic matter as opposed to microbially processed mineral associated organic matter. • 30 years of experimental N additions slowed leaf litter decay in a temperate forest. • N additions increased the share of SOM in the light particulate fraction. • Bulk soil C:N positively correlated to fraction SOM in light particulate fraction. • N additions favor physical transfer of plant inputs as pathway for SOM formation. • N fertilization caused high rates of nitrification in soils with high C:N.