Soil Nitrogen Availability Affects Belowground Carbon Allocation and Soil Respiration in Northern Hardwood Forests of New Hampshire

Plant nutrient acquisition in forests requires respiration by roots and mycorrhizae. Belowground carbon allocation and soil respiration should thus reflect plant effort allocated to nutrient uptake, for example in conditions of different nutrient availabilities controlled by site quality or stand history. Soil respiration, belowground C allocation, and fine root biomass were measured in three sites differing in nutrient availability in the northern hardwood forests of the White Mountains of New Hampshire. Annual soil respiration and belowground C allocation measured in two stands at each site were lowest at Jeffers Brook, the site with highest nutrient availability, and higher at Hubbard Brook and Bartlett Experimental Forests. Neither soil respiration nor belowground C allocation differed significantly between mid-aged (31-41 year old) and older stands (> 80 year old) within the sites, despite higher fine root (< 1 mm) biomass in old stands than mid-aged stands. During the growing season, soil respiration was low where net nitrogen mineralization and net nitrification were high across an extensive sample of thirteen stands and annual belowground C allocation decreased with increasing nitrification across the six intensively studied stands. Available P was not related to soil respiration. The relationships among N availability, belowground C allocation, and soil respiration support the claim that forests allocate more C belowground in ecosystems with low availability of a limiting nutrient.