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

Aims Natural N-15 abundance provides integrated information about nitrogen (N) input, transformation and output, indirectly reflecting N cycling traits within terrestrial ecosystems. However, relationships between natural N-15 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 delta N-15 of soils and plants in a semi-arid grassland; (ii) detect the relationships between delta N-15 of soils and four major N cycling processes (i.e. mineralization, nitrification, denitrification and ammonia volatilization); and (iii) determine whether delta N-15 of soils can be used as an indicator of N cycling in this semi-arid grassland. Methods The 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. delta N-15 values of topsoils (0-10 cm), surface soils (0-2 cm) and plants were measured in 2006. Differences in delta N-15 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 delta N-15 values of topsoils were linked to the four N cycling processes to investigate their relationships. Important Findings The delta N-15 values of topsoils (5.20-5.96 parts per thousand) were substantially higher than the delta N-15 values of plants (2.51-2.93 parts per thousand) and surface soils (1.44-2.92 parts per thousand) regardless of grazing intensities. The N-15-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 delta N-15 values in topsoils than in surface soils. In addition, the delta N-15 values of topsoils were positively correlated with the delta N-15 values of both plants and surface soils. Grazing, especially the high-intensity grazing (5.33 sheep ha(-1)), resulted in a significant decrease in delta N-15 of surface soils. However, no statistically significant variations in delta N-15 of topsoils and plants were found in response to grazing. The delta N-15 values of topsoils exhibited significant dependence on the cumulative rates of NH3 volatilization, net nitrification and denitrification in 2005 but not in 2006.