Gross nitrogen process rates in temperate forest soils exhibiting symptoms of nitrogen saturation

In order to examine how fundamental soil N cycling processes are affected by elevated N inputs to temperate forest ecosystems, we made concurrent laboratory measurements of gross rates of nitrogen (N) mineralization, nitrification, nitrate (NO3-) consumption, ammonium (NH4+) immobilization, nitrous oxide (N2O) and nitric oxide (NO) production, and NO consumption in soils from the Harvard Forest Chronic N Amendment Study. Gross nitrification rates varied with N addition rate in a manner that was entirely consistent with patterns of NO3- leaching and NO emissions that have been previously observed in field studies. Gross nitrification was elevated above controls in soils from a pine stand receiving 5 and 15 g N m^-2 per year, and in soils from a hardwood stand receiving 15 g N m^-2 per year. Gross nitrification tended to increase with decreasing soil pH, suggesting that acid-tolerant nitrifying bacteria predominate in these soils. Different patterns of inorganic N consumption in the two stands may provide some clues to understanding the more rapid onset of N saturation that has been historically observed in the pine stand. Absolute rates of NH4+ immobilization, and rates of NO3- consumption per unit of available NO3-, each tended to decrease with increasing N addition in the hardwood stand, but did not vary significantly in the pine stand. Gross NO production rates increased in a manner that was consistent with nitrification rate increases, and represented up to 19% of gross nitrification. Production of N2O was generally =<15% of NO production and <1% of gross nitrification. Consumption of NO represented >=96% of gross NO production and may have contributed up to 25% of total NO3production.