Coupled C and N dynamics along a natural mean annual temperature gradient
The terrestrial carbon (C) and nitrogen (N) cycles are coupled to one another during many primary ecosystem processes like photosynthesis, primary productivity, and decomposition. Therefore, an understanding of their coupled behavior will be necessary to understand ecosystem function as changing climate increases mean annual temperature (MAT). The assessment of ecosystem function over gradients in MAT is usually challenged by covariation in vegetation, pedology, precipitation and soil chemistry. In the work presented here, measurements were made over a unique natural temperature gradient in a tropical wet montane forest in Hawai’i where, because of its volcanic history, confounding variables are relatively constant. Along this gradient, prior research has focused on soil respiration, litterfall C:N, gross primary production (GPP) and total belowground carbon flux, but have not been linked to N availability. To begin to make these linkages, we compared the bioavailability of N to gross primary production (GPP) and the ratio of belowground to aboveground C allocation across the gradient. Ammonium (NH₄⁺) and nitrate (NO₃⁻) availabilities were measured using ion exchange resins (PRS™ probes, Western Ag) over one month. We also measured the abundance of the gene amoAthat controls nitrification in soils.
NH₄⁺ and NO₃⁻ availability to plant roots was highly correlated to MAT (r²=0.71) and mirrored the pattern of GPP with increasing MAT (r²=0.82). MAT was positively related to the abundance of amoA. Results also suggested that the ratio of belowground to aboveground biomass C decreased with increasing MAT (r²= 0.64) and MAT was tightly linked to this ratio (r²=0.67). These results are consistent with a plant physiological response in biomass C partitioning to increasing MAT. Further, N availability was only weakly related to this observed change in carbon partitioning (r²= 0.19) or GPP (r²= 0.35). The observation that N availability and nitrification increase while belowground-aboveground C allocation decreases across the gradient may indicate that this allocation strategy is only used by plants when N is in short supply or that some other factor, such as P availability, is exerting strong control over plant productivity.