Nitrogen Versus Phosphorus Limitation: A Factorial Fertilization Experiment in Temperate Hardwood Forests.

Experimental tests of N and P limitation in temperate forest systems are few, and those few have been short-term with very high rates of fertilization. In 2011 we began long-term low-level additions of N, P, and N+P in 13 forest stands of different ages distributed across three sites in the White Mountain National Forest of New Hampshire: Bartlett Experimental Forest (BEF), which is underlain by granite, Hubbard Brook Experimental Forest (HB), on granodiorite, and Jeffers Brook (JB) on amphibolite (metamorphosed basalt). Each stand has four plots treated annually with N (30 kg N/ha/yr as NH4NO3), P (10 kg P/ha/yr as NaH2PO4), both N and P, and control. Foliar N:P in the untreated stands indicate that most of our sites are P-limited rather than N-limited, and foliar P resorption was more complete than N resorption. Consistent with P limitation, after 5 years of treatment plots receiving P moved into the co-limited range, and those receiving N were even more P limited, according to their foliar N:P ratios. Tree growth responded more to P addition than to N addition in mid-aged and mature stands (P=0.02). These results are consistent with predictions of the Multiple Element Limitation (MEL) model, parameterized for our experimental conditions. Given the evidence for P limitation to aboveground growth, we expected to find reduced root biomass under P addition, but instead we found greater root biomass. Perhaps root turnover was reduced by P addition, consistent with reduced microsite depletion; greater root biomass need not reflect greater investment belowground. Soil respiration was reduced by N addition where initial N availability was low, which suggests that root or microbial activity was limited by N. A better understanding of the capacity of ecosystems to balance the acquisition of limiting resources is needed to manage ecosystems in the face of continuing environmental change.