Xiankai Lu

TL;DR: The data is synthesized to assess the status of the anthropogenic N(r) emissions and N deposition as well as their impacts on different ecosystems, including empirical critical loads for different ecosystems.

TL;DR: The results suggest that long-term N addition can contribute measurably to soil acidification, and that shortage of Ca and Mg should receive more attention than soil exchangeable Al in tropical forests with elevated N deposition in the future.

TL;DR: It is reported that P limitation is more widespread and much stronger than previously estimated and often occurs in other regions, suggesting that previous studies have underestimated the importance of altered P supply on aboveground plant production in natural terrestrial ecosystems.

TL;DR: The results suggest that high-N additions can decrease plant diversity in tropical forests, but that this response may vary with rate of N addition.

Abstract: There is increasing concern over the impact of atmospheric nitrogen (N) deposition on forest ecosystems in the tropical and subtropical areas. In this study, we quantified atmospheric N deposition and revealed current plant and soil N status in 14 forests along a 150 km urban to rural transect in southern China, with an emphasis on examining whether foliar d 15 N can be used as an indicator of N saturation. Bulk deposition ranged from 16.2 to 38.2 kg N ha � 1 yr � 1 , while the throughfall covered a larger range of 11.7–65.1 kg N ha � 1 yr � 1 . Foliar N concentration, NO3 leaching to stream, and soil NO3 concentration were low and NO3 production was negligible in some rural forests, indicating that primary production in these forests may be limited by N supply. But all these N variables were enhanced in suburban and urban forests. Across the study transect, throughfall N input was correlated positively with soil nitrification and NO3 leaching to stream, and negatively with pH values in soil and stream water. Foliar d 15 N was between � 6.6% and 0.7%, and was negatively correlated with soil NO3 concentration and NO3 leaching to stream across the entire transect, demonstrating that an increased N supply does not necessarily increase forest d 15 N values. We proposed several potential mechanism that could contribute to the d 15 N pattern, including (1) increased plant uptake of 15 N-depleted soil NO3 , (2) foliage uptake of 15 N-depleted NH4 1 , (3) increased utilization of soil inorganic N relative to dissolved organic N, and (4) increased fractionation during plant N uptake under higher soil N availability.