Variations in nitrogen-15 natural abundance of plant and soil systems in four remote tropical rainforests, southern China

The foliar stable N isotope ratio (δ15N) can provide integrated information on ecosystem N cycling. Here we present the δ15N of plant and soil in four remote typical tropical rainforests (one primary and three secondary) of southern China. We aimed to examine if (1) foliar δ15N in the study forests is negative, as observed in other tropical and subtropical sites in eastern Asia; (2) variation in δ15N among different species is smaller compared to that in many N-limited temperate and boreal ecosystems; and (3) the primary forest is more N rich than the younger secondary forests and therefore is more 15N enriched. Our results show that foliar δ15N ranged from −5.1 to 1.3 ‰ for 39 collected plant species with different growth strategies and mycorrhizal types, and that for 35 species it was negative. Soil NO3− had low δ15N (−11.4 to −3.2 ‰) and plant NO3− uptake could not explain the negative foliar δ15N values (NH4+ was dominant in the soil inorganic-N fraction). We suggest that negative values might be caused by isotope fractionation during soil NH4+ uptake and mycorrhizal N transfer, and by direct uptake of atmospheric NH3/NH4+. The variation in foliar δ15N among species (by about 6 ‰) was smaller than in many N-limited ecosystems, which is typically about or over 10 ‰. The primary forest had a larger N capital in plants than the secondary forests. Foliar δ15N and the enrichment factor (foliar δ15N minus soil δ15N) were higher in the primary forest than in the secondary forests, albeit differences were small, while there was no consistent pattern in soil δ15N between primary and secondary forests.