δ15N of soil available N along a 3200 km long transect in Northern China

Background/Question/Methods

Nitrogen (N) isotope is widely considered as an integrator of N cycling in terrestrial ecosystems, and has the potential to reveal spatial and temporal patterns of N cycling and predict the response of N cycles to natural and human disturbances. At the global scale, a pattern of decreased δ¹⁵N of soil and plant leaf with increasing mean annual precipitation (MAP) has been demonstrated for more two decades. However, the mechanisms causing this pattern are not clear. One possibility is that N losses relative to pool sizes are greater at drier sites and, because N lost (volatilization, denitrification and leaching) is typically depleted in ¹⁵N, leading to ¹⁵N-enrichment of soil and plants. Accordingly, we expect that δ¹⁵N of both ammonium and nitrate will decline with increasing precipitation. To test this hypothesis, we explored a regional pattern of ¹⁵N natural abundance for plant leaves, bulk soil as well as soil nitrate and ammonium along a 3200-km grassland transect in northern China. Soil samples were obtained from 36 sites, extending from Xinjiang Province at West to Inner Mongolian at East. The entire transect covered a MAP gradient from 30 to 500 mm.

Results/Conclusions

Soil ammonium concentration decreased and then increased along the MAP gradient, with a breakpoint at MAP around 200 mm. However, the δ¹⁵N of ammonium decreased monotonically with increasing precipitation. High and positive δ¹⁵N values in the arid region suggested a strong ammonia volatilization, associated with high pH values (on average 8.2). Nitrate concentration displayed no clear trend when MAP > 100 mm. Extremely high concentration (> 100 mg N kg^-1) was found when MAP < 100 mm, which may be caused by long-term atmosphere deposition and the lack of biological uptake. The δ¹⁵N of soil nitrate increased with increasing MAP when MAP < 200 mm, and then decreased when MAP > 200 mm. The MAP accounted for 50% and 19% of the variation of the δ¹⁵N-NO₃^- values in these two distinguishable precipitation regions, respectively. Our results suggest that along this 3200-km grassland transect, when MAP > 200 mm, δ¹⁵N values of both ammonium and nitrate decrease with increasing precipitation and thus support the global δ¹⁵N pattern. However, under extreme aridity, biological and abiotic controls on soil N dynamics may yield different ¹⁵N pattern and such nonlinear response to precipitation should be further studied and incorporated into biogeochemical models.