Soil nitrogen and phosphorus pools and fluxes along a boreal forest climate transect suggest potential limits on warming-enhanced nutrient availability

Abstract:

High latitude ecosystems are vulnerable to the climate warming projected to occur within this century directly affecting plant and soil processes and impacting soil carbon (C) storage. The net effect of these responses will be determined in part by nitrogen (N) and phosphorus (P) availability. In line with previous studies across a mesic boreal forest transect, we hypothesize that N-cycling and availability will be greatest in the warmest climate forests, but that this enhanced N-cycling results in greater relative P-limitation. We have three lines of evidence suggesting that enhanced N availability supporting increased productivity with warming may be limited by available P with further future climate warming in these forests. First, larger soil N stocks and inputs to the soil profile and a tight correlation between organic soil C:N and tree growth was observed with climate warming across this transect. In contrast no differences in the organic horizon soil P stocks were evident along this transect despite the 3-fold increase in litter P inputs. Second, organic soil C:P exhibited no correlation with tree growth, suggesting that P is less coupled to soil carbon processing in these forests; correspondingly, needle litter P concentrations did not vary across regions despite significant decreases in N with latitude. The availability of P in these forest soils may be reduced relative to N as a consequence of enhanced productivity, supported by enhanced N availability, as well as the reduced geological source of P with decreased latitude. However, needle litter N:P did not differ by region, and an increase in N:P from green to brown needle litter suggests a relative increase in P-resorption relative to N in the coldest region forests. Third, the unexpected lack of evidence for increased retention of P relative to N in tree needle litterfall with climate warming in these forests suggests that enhanced productivity and thus soil C stock maintenance supported by increased N availability in the warmest climate forests may be limited by the availability of P with further climate warming. Future research should include the relative importance of internal recycling, parent material sources of P and their controls related to forest ecosystem responses to climate change.