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 distributed across three sites in the White Mountain National Forest of New Hampshire. At Bartlett Experimental Forest (BEF), which is underlain by granite, we have three young, three mid-aged, and three mature stands. At both Hubbard Brook Experimental Forest (HB), on granodiorite, and Jeffers Brook (JB) on amphibolite (metamorphosed basalt), we have one mature and one mid-aged forest stand. 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. These relatively modest rates are designed to alter site fertility while minimizing artifacts associated with high doses of fertilizer. These treatments allow us to test for NP co-limitation and to challenge balanced forest nutrition and thereby induce mechanisms that maintain co-limitation, such as those represented in the Multiple Element Limitation Model (MEL).
Foliar N:P in the untreated mid-age and mature stands indicate that most of our sites are P-limited rather than N-limited. Consistent with P limitation, plots receiving P moved into the co-limited range, and those receiving N were even more P limited after 5 years of treatment, 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 surprising because temperate forests on glaciated soils have been presumed to be N limited, but they are consistent with predictions of the 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.