Is nitrogen a primary driver of decomposition rates in nitrogen-rich tropical forests? Exploration of the spatially and chemically explicit role of nitrogen

Background/Question/Methods

High carbon:nitrogen (C:N) ratios of plant litter compared with C:N requirements of microbial biomass suggest a fundamental N limitation to litter decomposition, as demonstrated by a number of experiments. However, other studies have found that multiple nutrients and C chemistry can exert concerted control over litter decomposition, particularly in tropical rainforests on highly weathered and commonly N-rich soils. These apparently conflicting results may point toward a more nuanced N control over decomposition than simple stoichiometry suggests. I explore the scale, spatial variability, and chemical forms of N that may be particularly important for driving decomposition patterns in N-rich tropical forests. I use three studies which investigated: (1) litter decomposition across five diverse Neotropical forests using seven common litter types from the Long-term Intersite Decomposition Experiment Team (LIDET); (2) N-fertilization effects on litter and soil organic matter (SOM) decomposition in lower and upper elevation tropical rainforests in Puerto Rico; (3) spatial patterns in decomposer enzyme activities across an urban-remote tropical forest gradient in Puerto Rico where soil N levels vary strongly.

Results/Conclusions

Results from these studies provide evidence for spatially and chemically explicit N control over decomposition in tropical forests. Initial litter N content was strongly positively associated with decomposition rates across the LIDET tropical forests and litter types. Results from the Puerto Rican N fertilization experiment also demonstrated strong effects of mineral N availability on decomposition patterns. Decomposer enzyme activities increased in litter and soil in N fertilization plots in both forests, and decomposition rates for a common substrate increased in the lower elevation forest. Low density SOM in surface soils, composed primarily of litter tissue, strongly declined with N fertilization in both of the forests. Thus, changes in decomposition rates with added mineral N differed spatially between the litter layer and mineral soil, with stronger responses in SOM. Finally, decomposer enzyme activities across the urban-remote gradient were best predicted by soil mineral N levels, with no relationship to total soil N. Together, these results demonstrate that N can provide a dominant control on litter decomposition in tropical rainforests. Specifically, initial litter N content appears to be a primary driver of decomposition in the litter layer, whereas mineral N availability may be a dominant control over decomposition of minimally processed SOM in surface soils. Spatially and chemically explicit aspects of N control over decomposition merit further investigation in tropical forests.