Net nitrogen mineralization and nitrification of different landscape positions in a lowland subtropical rainforest in Taiwan

Abstract

Measurements of soil nitrogen (N) transformation rate are needed to be able to assess the plant availability and forest ecosystem losses of N, but little is known about lowland subtropical rainforests. The objectives of the present study were to determine the rates of N mineralization and nitrification in different seasons (January and August 2006) and in different landscape positions (footslope and summit) in the Nanjenshan forest of southern Taiwan, where vegetation types and soil properties vary among different landscape positions. Net N mineralization and nitrification were measured using 28-day in-situ open core and capped core incubations. The results from January 2006 showed that the concentrations of ammonium-N (NH4 +-N), mineral N (NH4 +-N plus NO3 −-N) and nitrate-N (NO3 −-N) were not significantly different between open and capped cores, or between summit and footslope. However, the results of August 2006 showed that the NH4 +-N concentrations of the summit soils were significantly higher than those of the footslope soils (P < 0.05), and that NO3 −-N concentrations in the open cores were significantly lower than those in the capped cores because of higher rainfall in the summer. In general, concentrations of mineral N and NO3 −-N in August were higher than those in January (both P = 0.0003), and NH4 +-N concentrations were significantly different between the different landscape positions (P < 0.05). There were larger soil C and N and microbial N pools at the summit position; however, footslope soils showed higher net N mineralization and nitrification rate as expressed on the basis of per unit C or N. Our results suggested that the substrate properties of the footslope position contributed to the higher net N mineralization and nitrification rate, and that the differences in N transformation rates between the landscape positions appeared to be related to vegetation type.