| 摘 要： |
Purpose Drylands are generally drought-stressed and have low soil fertility. While negative effects of drought on vegetation productivity are undisputed, it is unclear whether water limits productivity alone or in combination with low nitrogen (N) availability (N supply relative to plant N demand). Methods One aridity transect was established in each - the Loess Plateau (LP), Inner Mongolia Plateau (MP), and Tibetan Plateau (TP). Concentrations and delta N-15 of soil and leaf at the species level were measured, and ecosystem N processes were simulated using DeNitrification-DeComposition model. Results Model simulations of total N supply and its components (N deposition, biological N-2 fixation, and gross N mineralization) and modelled gross primary productivity (plant N demand) decreased with increasing aridity. delta N-15(soil) and community-weighted delta N-15(leaf) of non-N-2-fixing species increased with aridity, however, difference in delta N-15 between soil and leaf (delta N-15(Soil-leaf)) exhibited an increasing trend with aridity in MP and TP. delta N-15(Soil-leaf) at LP was variable but did not change significantly with aridity. These results implied that N availability increased with aridity in MP and TP but remained low due to high plant N demand in LP. We found that delta N-15(Soil-leaf) is a better indicator of N availability than delta N-15(soil) and delta N-15(leaf). Vegetation productivity was negatively correlated with water availability, but not with N availability along aridity gradients, indicating that vegetation productivity in these ecosystems was constrained by water rather than N availability. Conclusion Whether vegetation productivity is limited by N availability in drylands depends on how much water is available for plant growth. |
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