Changes in Nitrogen Cycling During Tropical Forest Secondary Succession on Abandoned Pastures

Advisor: Erika Marin-Spiotta; Includes Figures, Appendix and Bibliography.Nitrogen (N) plays two important roles in Earth's climate. As a plant nutrient, the availability of N affects plant growth and the uptake of carbon (C) from the atmosphere into plant biomass. The accumulation of C in long-lived biomass and in soils contributes to reducing the amount of CO2, a greenhouse gas, in the atmosphere. Humans have altered the N cycle, affecting the potential for terrestrial ecosystems to sequester C and mitigate climate change. Land-use change, specifically deforestation and reforestation, can affect N availability for plant growth and N2O production. Highly-weathered tropical soils are not expected to be N-poor, but long-term or intensive agricultural use can deplete soil nutrients, with implications for forest recovery. Despite the importance of N, there are still large gaps in our understanding of N dynamics during succession. Nitrogen limitation in pasture soils and early successional forests increases the demand for N-fixing tree species, but studies show a greater abundance of N-fixing species and in turn, fixation potential, in older wet tropical forests. Nitrogen mineralization rates can be high in agricultural soils, yet lower in younger forests compared to mature ones. Here we used a chronosequence approach to study how N availability changes with reforestation on former pastures and identify potential factors explaining observed patterns. Our results showed that net N mineralization and nitrification rates peaked in later successional forests, indicating changes in soil N availability during reforestation of pastures. The reference forest rates were lower than those in the late successional forests, suggesting potential effects of differences in species composition with post-agricultural succession. Nitrogen mineralization and nitrification rates were related to the concentration of N in the free light fraction and to the presence of N-fixing trees. The successional trends in N cycling rates varied by sampling period, suggesting a complex relationship in the recovery of N availability during forest succession