Natural 15N abundance in soils and plants in relation to N cycling in a rangeland in Inner Mongolia

Abstract

AimsNatural 15N abundance provides integrated information about nitrogen (N) input, transformation and output, indirectly reflecting N cycling traits within terrestrial ecosystems. However, relationships between natural 15N abundance and N cycling processes are poorly understood in China. Here, our primary objectives were to (i) examine the effects of grazing at varying levels of intensity on δ15N of soils and plants in a semi-arid grassland; (ii) detect the relationships between δ15N of soils and four major N cycling processes (i.e. mineralization, nitrification, denitrification and ammonia volatilization); and (iii) determine whether δ15N of soils can be used as an indicator of N cycling in this semi-arid grassland.MethodsThe field experiment was conducted within the long-term (17-year) grazing enclosures in a semi-arid grassland in Inner Mongolia. Five grazing intensities (0.00, 1.33, 2.67, 4.00 and 5.33 sheep ha-1) were designed. δ15N values of topsoils (0-10 cm), surface soils (0-2 cm) and plants were measured in 2006. Differences in δ15N of soils and plants between the five grazing intensities were examined. Rates of four soil N cycling processes were measured periodically during the 2005 and 2006 growing seasons. The δ15N values of topsoils were linked to the four N cycling processes to investigate their relationships.Important FindingsThe δ15N values of topsoils (5.20-5.96‰) were substantially higher than the δ15N values of plants (2.51-2.93‰) and surface soils (1.44-2.92‰) regardless of grazing intensities. The 15N-depleted N losses during microbial decomposition of organic matter in concert with the downward movement of residual substrate over time are the possible causes of higher δ15N values in topsoils than in surface soils. In addition, the δ15N values of topsoils were positively correlated with the δ15N values of both plants and surface soils. Grazing, especially the high-intensity grazing (5.33 sheep ha-1), resulted in a significant decrease in δ15N of surface soils. However, no statistically significant variations in δ15N of topsoils and plants were found in response to grazing. The δ15N values of topsoils exhibited significant dependence on the cumulative rates of NH3 volatilization, net nitrification and denitrification in 2005 but not in 2006. © The Author 2010.