Net N mineralization and nitrification rates in a forested ecosystem in northeastern Taiwan

Abstract

We used the buried-bag in situ incubation technique to quantify net rates of N mineralization and nitrification in a forest ecosystem in northeastern Taiwan. At the near-ridge top site, NH 4 + concentrations in the organic horizon (O horizon; 2–0 cm) ranged from 14.5 to 58.5 mg N kg −1 and NO 3 − varied from 2.5 to 7.8 mg N kg −1. In the lower part of the watershed, NH 4 + concentrations in the O horizon were between 10.3 and 28.8 mg N kg −1 and NO 3 − varied from 2.6 to 20.1 mg N kg −1. There were slightly lower soil NH 4 + concentrations during the summer than other seasons. Using the buried-bag technique, we estimated net mineralization rates to vary between 0.02 and 0.28 mg N kg −1 soil per day and net nitrification rates between 0.02 and 0.26 mg N kg −1 soil per day at the upper site. Rates of both processes were lower during the summer and were positively related to soil moisture content. We speculate that differences in soil texture are likely related to the differing pattern in concentrations of inorganic soil N between the two sites. N mineralization was positively correlated with nitrification rate ( r=0.820, P<0.001) and relative nitrification (slope of this relationship) was 0.709, indicating the dominance of nitrate cycling relative to ammonium cycling. Our results show that improved knowledge about rates of fundamental ecosystem nutrient cycling processes in watersheds in Taiwan can contribute to our understanding of regional variation in rates of ecosystem function.