Gross nitrogen process rates in temperature 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

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DOI:

http://dx.doi.org/10.1016/j.foreco.2004.03.016

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