Litter inputs drive patterns of soil nitrogen heterogeneity in a diverse tropical forest: Results from a litter manipulation experiment

Plant functional traits affect soil nutrient cycling in many ecosystems, but high levels of diversity make it challenging to identify their influence in tropical rainforests. Here, we used a litter manipulation experiment to demonstrate that differences in litter chemistry alone are sufficient to drive changes in soil nitrogen (N) cycling within a two-year period in a lowland tropical rainforest. Previously, we used airborne imaging spectroscopy to identify ten 0.25 ha plots on the Osa Peninsula of southwestern Costa Rica, each containing 10-20 emergent trees, with relatively high or low mean canopy N content (+/- 0.4 standard deviations of the landscape mean; n = 5 each). Plots with high canopy N content had higher soil inorganic N concentrations, faster net N cycling rates, and greater nitrous oxide (N2O) emissions. Because abiotic conditions were similar among the plot types, we hypothesized that litter inputs drove the formation of these high and low soil N patches. We tested this hypothesis with a reciprocal litter transplant experiment. After two years of monthly treatments, litter inputs from the high canopy N plots increased soil N availability in the low canopy N plots by 30% but had no measurable effects on net N cycling rates or N2O emissions. Surprisingly, litter inputs from the low canopy N plots increased soil N availability and net nitrification in the high canopy N plots. While somewhat counterintuitive from a canopy N perspective, these results may reflect a more nuanced effect of litter chemistry. Although low canopy N plot litter had lower N content than high canopy N plot litter, it leached relatively more and higher quality soluble carbon (C), which we speculate may have had a positive priming effect, releasing N from more recalcitrant compounds. Additionally, low canopy N plot litter supported higher rates of free-living N fixation, which could have directly increased soil N availability. Overall, our results show that tropical tree assemblages with differing foliar chemistry can directly influence soil properties through litter inputs, and that this effect may be mediated not only by litter N, but also relative C solubility and chemistry.