High nitrate retention during winter in soils of the hubbard brook experimental forest

Stream export of nitrogen (N) as nitrate (NO3-; the most mobile form of N) from forest ecosystems is thought to be controlled largely by plant uptake of inorganic N, such that reduced demand for plant N during the non-growing season and following disturbances results in increased stream NO3- export. The roles of microbes and soils in ecosystem N retention are less clear, but are the dominant controls on N export when plant uptake is low. We used a mass balance approach to investigate soil N retention during winter (December through March) at the Hubbard Brook Experimental Forest by comparing NO3- inputs (atmospheric deposition), internal production (soil microbial nitrification), and stream output. We focused on months when plant N uptake is nearly zero and the potential for N export is high. Although winter months accounted for only 10 - 15% of annual net Enitrification, soil NO3- production (0.8-1.0 g N m(-2) winter(-1)) was much greater than stream export (0.03 - 0.19 N m(-2) winter(-1)). Soil NO3- retention in two consecutive winters was high (96% of combined NO3- deposition and soil production; year 1) even following severe plant disturbance caused by an ice-storm (84%; year 2) We show that soil NO3- retention is surprisingly high even when N demand by plants is low. Our study highlights the need to better understand mechanisms of N retention during the non-growing season to predict how ecosystems will respond to high inputs of atmospheric N, disturbance, and climate change.