Belowground insights into nutrient limitation in northern hardwood forests

Soil nutrient environments are changing in forests of the northeastern United States due to decades of anthropogenic nitrogen (N) emissions and acidic deposition, causing N enrichment and cation depletion, and possibly alleviating N limitation to forest growth. We asked whether biotic demand for phosphorus (P) or calcium (Ca) exceeded that for N and used an ingrowth core approach to test belowground responses to different nutrients. We tested fine root foraging for nutrients (N, P, or Ca) in three mid-age (26-30 years) and mature (a parts per thousand yen100 years) northern hardwood forest stands in the Bartlett Experimental Forest (BEF), NH, and in one mature forest stand in the Hubbard Brook Experimental Forest (HBEF), NH, USA. Fine root colonization of cores responded clearly to Ca in mature forest at HBEF, responded to P in mid-age forests at BEF, and responded primarily to N in mature forests at BEF. Net N mineralization potential was higher in soils of mid-age than mature forests at BEF, with roots responding to N where N availability was low and to P or Ca where N availability was high. Nutrients elicited no responses from either fungi or phosphatase activity in mid-age forests, but in mature forests at BEF, N enhanced phosphatase activity. While no straightforward pattern emerged among the different mechanisms of nutrient acquisition that we tested, our results do suggest that P and Ca can be important limiting nutrients in these northern hardwood forests when N availability is relatively high. We hypothesize that the interacting effects of disturbance by forest harvest and N deposition can cause a transient P limitation to forest growth, and that other nutrients become more limiting as forests age.