Effects of nitrogen additions on annual nitrous oxide fluxes from temperate forest soils in the northeastern United States

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Abstract
Nitrous oxide (N2O) fluxes between aerobic soils and the atmosphere were measured each month for 2 years (except during the period of snow cover) in control and N‐fertilized plots in a red pine plantation and a mixed hardwood forest in the northeastern United States. Nitrogen was added as NH4NO3; application rates were 0 (control), 37 (low‐N), and 120 (high‐N) kg N/ha/yr in year 1. In year 2, application rates were 0 (control), 50 (low‐N), and 150 (high‐N) kg N/ha/yr. A total of 2520 individual flux measurements were made over the 2 years. Mean emission rates (μg N/m2/h) from the control plots were low (pine: year 1 = 0.17, year 2 = 1.10; hardwood: year 1 = 0.27, year 2 = 0.29). Fertilization resulted in only small increases in N2O effluxes even in the second year, with low rates in the low‐N plots (pine: year 1 = 2.19, year 2 = 3.87; hardwood: year 1 = 1.16, year 2 = 0.77) and also in the high‐N plots (pine: year 1 = 0.71, year 2 = 5.24; hardwood: year 1 = 1.12, year 2 = 0.90). The maximum N2O loss was 0.350 kg N/ha/yr from the red pine high‐N plot. Weak seasonal flux trends were noted in both stands, with highest efflux rates in spring and mid to late summer. No statistically significant relationships between fluxes and soil temperatures, soil moisture, or soil NO3− or NH4+ were detected. Low rates of net nitrification in both the control plots and the fertilized plots are thought to be responsible for the low N2O emissions. Small increases in net nitrification were measured in the forest floor of the the pine stand, with rates in the control, low‐N, and high‐N plots of 1.5, 4.5, and 8.5 kg N/ha/yr, respectively. Net nitrification rates in the hardwood plots were 0.1, 0.9, and 0.6 kg N/ha/yr in the control, low‐N, and high‐N plots, respectively. Low NH4+ availability in the forest floor and low NH4+ concentrations in soil solution below the rooting zone (0.5–0.7 m) suggest that NH4+ availability for nitrification is limited by competition with plant uptake and microbial immobilization, and that this competition is not alleviated by 2 years (50–150 kg N/ha/yr) of N addition.