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

Abstract

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