The effects of increased N input on soil C and N dynamics in seasonally dry tropical forests: An experimental approach

• CO2 and NO2 fluxes, and net N-transformations increase with precipitation.
• Soil organic C concentrations decrease with elevated N input.
• Elevated N input increases CO2 and N2O fluxes in the driest region.
• N enrichment increases only CO2 fluxes in the wettest region.

Although tropical forests play a disproportionate role in regulating global C and N cycles, the impact of elevated N-deposition on these ecosystems is not well studied. Our goal was to determine whether elevated N input had an effect on soil C and N cycling in seasonally dry tropical forests along a precipitation gradient (with mean annual precipitation ranging from 540 mm to 1040 mm). We used a microcosm N-fertilization experiment to explore how soil C and N dynamics respond to N addition. NO2 fluxes and net N-mineralization and nitrification rates in soils increased considerably with mean annual precipitation. In contrast, CO2 fluxes decreased with increased mean annual precipitation. Organic C concentrations in soil from both the driest and the wettest forest sites decreased significantly with elevated N input. However, microbial biomass responded to N enrichment differently across sites. In soils from the driest site, microbial biomass C and N concentrations declined significantly with elevated N input. In contrast, in soils from the wettest site, microbial biomass C and N immobilization increased. Net N-mineralization and nitrification rates did not respond to elevated N input. In soils from the driest site, elevated N input resulted in an increase in C and N losses due to CO2 and N2O fluxes. However, elevated N input to soils from the wettest site did not consistently affect CO2 and N2O fluxes. In soils from the wettest site, the main response to N enrichment was an increase only in CO2 fluxes. We conclude that the reactions of seasonally dry tropical forest soils to increases in N deposition will vary, depending on the details of the site's water limitations, which appear as a regulatory mechanism of the N cycle in these soils.