Potential limits on warming-enhanced nitrogen availability in mesic boreal forests

Abstract In mesic regions, where water availability is rarely limiting, temperature increases with climate change can prompt increases in primary production. These responses will be determined in part by nitrogen (N) and phosphorus (P) availability, yet the relative importance of N- versus P-cycling for supporting primary production and soil C is unclear. Prompted by previous studies across a mesic boreal forest transect, we hypothesized that enhanced N-cycling and availability supporting increased productivity in warmer forests results in greater relative P-limitation. By assessing soil N and P stocks and inputs across this transect we uncovered multiple lines of evidence supporting this hypothesis. First, soil N stocks and soil N:P are highest in the warmer forests while soil C:P and P stocks are maintained across latitude. Second, soil N:P increases with depth in the warmest forests where organic P is the dominant form of P, and where the lowest mineral soil P stocks are observed. Third, estimates of soil P residence time are not coupled with those of C, and soil C:P does not correlate with tree growth, indicating that P availability is not supported by warming-induced microbial cycling, as is the case for N. Consistent with this, green needle litterfall C:P was lowest in the warmest sites suggesting enhanced P-resorption in those forests. These results provide nuanced evidence for relative P limitation that may emerge with the enhanced N cycling and productivity driven by a warmer climate suggesting a constraint on an important, negative climate feedback in these forests.