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

The foliar stable N isotope ratio (delta N-15) can provide integrated information on ecosystem N cycling. Here we present the delta N-15 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 delta N-15 in the study forests is negative, as observed in other tropical and subtropical sites in eastern Asia; (2) variation in delta N-15 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 N-15 enriched. Our results show that foliar delta N-15 ranged from -5.1 to 1.3 aEuro degrees for 39 collected plant species with different growth strategies and mycorrhizal types, and that for 35 species it was negative. Soil NO3 (-) had low delta N-15 (-11.4 to -3.2 aEuro degrees) and plant NO3 (-) uptake could not explain the negative foliar delta N-15 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 delta N-15 among species (by about 6 aEuro degrees) was smaller than in many N-limited ecosystems, which is typically about or over 10 aEuro degrees. The primary forest had a larger N capital in plants than the secondary forests. Foliar delta N-15 and the enrichment factor (foliar delta N-15 minus soil delta N-15) were higher in the primary forest than in the secondary forests, albeit differences were small, while there was no consistent pattern in soil delta N-15 between primary and secondary forests.