Measuring the Effects of Long-term Nitrogen Fertilization on Soil Carbon Cycling in Two Tropical Forests

Human alteration of the global nitrogen (N) cycle is widespread and increasing, with the potential to alter ecosystem functioning and biogeochemical cycles. The effects of N enrichment differ by ecosystem, and likely depend on background N availability. Previous studies have shown effects of N enrichment on soil organic matter (SOM) dynamics, although the size and direction of change are not consistent. Our study uses a long-term (15 year) N fertilization experiment in two Puerto Rican forests to test the effect of N enrichment on SOM cycling. The two forests are from different elevations in the Luquillo Long-Term Ecological Research Site and differ in background soil N content. Past work showed an increase in bulk soil carbon (C) driven by an increase in the mineral-associated pool (Cusack et al. 2011). We show data from soils collected following an additional 11 years of fertilization to measure changes through time in both bulk soil and three SOM pools separated using density fractionation. Fertilized plots showed different trends in the two forests. Fertilized plots had greater bulk soil C in the relatively low-N, upper elevation forest, but showed no difference in the lower elevation forest. There has not been a significant change in soil C content since the last sampling in either forest. To better understand potential mechanisms leading to changes in soil C with fertilization, we are analyzing radiocarbon of soil samples collected 11 years apart to model the mean system age of the mineral-associated OM fraction separated by density fractionation. Changes in mean system age of mineral-associated OM will allow us to elucidate if the increase in soil C storage is due to increasing inputs of OM to the mineral-associated fraction or decreased decomposition of mineral-associated OM. This research will contribute to our understanding of how long-term N enrichment affects soil C dynamics across a gradient of tropical forest soils with different nutrient availability and allow for better prediction of the ecosystem consequences of present and future increases in N availability.