Nitrogen cycling in forest soils across climate gradients in Eastern China

A N-15 tracing study was carried out to investigate the potential gross nitrogen (N) dynamics in thirteen forest soils in Eastern China ranging from temperate to tropical zones (five coniferous forests, six deciduous broad-leaf forests, one temperate mixed forest, one evergreen broad-leaf forests ecosystems), and to identify the major controlling factors on N cycling in these forest ecosystems. The soil pH ranged from 4.3 to 7.9 and soil organic carbon (SOC) ranged from 6.6 g kg(-1) to 83.0 g kg(-1). The potential gross N transformation rates were quantified by N-15 tracing studies where either the ammonium or nitrate pools were N-15 labeled in parallel treatments. Gross mineralization rates ranged from 0.915 mu g N g(-1) soil day(-1) to 2.718 mu g N g(-1) soil day(-1) in the studied forest soils. The average contribution of labile organic-N (M (Nlab) ) to total gross mineralization (M (Nrec) +M (Nlab) ) was 86% (58% to 99%), indicating that turnover of labile organic N plays a dominant role in the studied forest ecosystems. The gross mineralization rates in coniferous forest soils were significantly lower (ranging between 0.915 and 1.228 mu g N g(-1) soil day(-1)) compared to broad-leaf forest soils (ranging from 1.621 to 2.718 mu g N g(-1) soil day(-1)) (p < 0.01). Thus, the dominant vegetation may play an important role in regulating soil N mineralization. Nitrate production (nitrification) occurred via two pathways, oxidation of NH (4) (+) and organic N the forest soils. Correlations with soil pH indicated that this is a key factor controlling the oxidation of NH (4) (+) and organic N in theses forest ecosystems. NH (4) (+) oxidation decreased with a decline in pH while organic N oxidation increased. The climatic conditions (e.g. moisture status) at the various sites governed the NO (3) (-) -N consumption processes (dissimilatory NO (3) (-) reduction to NH (4) (+) (DNRA) or immobilization of NO (3) (-) ). Total NO (3) (-) consumption and the proportion of total NO (3) (-) consumption to total NO (3) (-) production decreased with an increase in the drought index of ecosystems, showing that strong interactions appear to exist between climatic condition (e.g. the drought index), N mineralization and the rate of DNRA. Interactions between vegetation, climatic conditions govern internal N cycling in these forests soils.