Studies showing the prevalence of nitrogen (N) and phosphorus (P) co-limitation across ecosystems have contributed to the interest in understanding interactions of the biogeochemical cycling of multiple elements. Soil microbial production of extracellular enzymes provides a framework for examining the interactions among Carbon (C), N, and P demands and immobilization by decomposer organisms. Resource allocation theory suggests that microbes preferably allocate toward the acquisition of the element which most limits their growth. Consequently the availability of the limiting element should influence the relative allocation to production of different enzymes. We tested N and P co-limitation of decomposer microorganisms in northern hardwood forests by quantifying the activities of soil extracellular enzymes in a long-term factorial N×P fertilization experiment in the White Mountain National Forest, NH. We measured the activity of N-acquiring enzymes alanine aminopeptidase and β-N-acetylglucosaminidase, the P-acquiring enzyme acid phosphatase, and the C-acquiring enzyme β-glucosidase using a fluorometric method, after 4 years of treatment with N (30 kg N ha-1 yr-1 as NH4NO3), P (10 kg P ha-1 yr-1 as NaH2PO4), N + P, or nothing (control).
Results/Conclusions
The relative allocation on P- vs N-acquiring enzymes declined in response to P addition, but did not respond to N or N+P addition. The relative allocation on C- vs N- and P-acquiring enzymes increased in response to N addition. We found no interactive or additive effects of N+P additions that would suggest co-limitation. We conclude that the activities of decomposer microorganisms in this forest region experience P limitation rather than N limitation or N and P co-limitation, and that elevated N increases microbial demand for C.