Over 60% of tropical forests are recovering from disturbance, and soil nitrogen availability may limit biomass recovery. Here we use several indicators to ask how fast the nitrogen cycle recovers following agricultural abandonment in the Atlantic Forest of southern Bahia, Brazil. We hypothesized that as forests age, total and available soil pools of N will increase as will soil emissions of nitrous oxide (N20). Our study system in the state of Bahia, Brasil, contains some of the last remnants of relatively intact Atlantic Forest - a biodiversity hotspot that once covered the entire east coast of Brazil and has been ~85% deforested.. We tested our hypotheses across a chronosequence of secondary forest sites adjacent to a large block of mature forest. We selected 5, 0.1ha plots of age 16, 29, 45 and >100 (mature) years old, and measured total soil N (0-10cm), inorganic N pools, and rates of N mineralization, nitrification, and soil (N20) soil emissions. We sampled in April, August, and November of 2016 to determine potential seasonality in N cycling metrics.
Results/Conclusions
We found total soil nitrogen, nitrate, and ammonium pools did not significantly increase with forest age (p=0.22, 0.60 and 0.48, respectively). While mature forests had the highest rates of N mineralization on average (1.3 +/- 0.14 ug N day-1 in mature forests compared to 0.75 +/- 0.12 ug N day-1 in 16-year-old forests), this variation was not statistically correlated with forest age (p=0.19). N20 fluxes in November were significantly higher in the oldest compared to the youngest forest sites (4.43 +/-0.7 ug N-N20 m-2 hr-1 in the 16-year-old forest and 13.084 +/- 2.8 ug N-N20 m-2 hr-1 in the mature forests; p = 0.03). However, these differences were not statistically significant in August (p=0.19) when fluxes exhibited no trend with forest age. In neither month were N20 soil emissions significantly influenced by soil moisture (R2=0.01 & R2=0.001; p=0.46, respectively). Together, these N cycling indicators suggest that the N cycle in these forests recovers quickly, but N remains to be a limiting nutrient in Atlantic Forests of Bahia, Brasil following >100 years of agricultural abandonment.