Net primary production, nitrogen cycling, biomass allocation, and resource use efficiency along a topographical soil water and nitrogen gradient in a semi-arid forest near an arid boundary

The objective of this study was to investigate how plants maintain productivity under a limited supply of water and N along the topographical soil water and N gradients in semi-arid forests. We investigated forest structure and productivity, N cycling, and water and N use by plants at three different slope positions in a forested area near an arid boundary on a loess plateau in China. Net primary production (NPP) and aboveground N uptake decreased as soil water and/or N availability decreased on upper slopes; however, NPP and aboveground N uptake were only slightly lower than those of more humid forest ecosystems. Water use efficiency (WUE), N use efficiency (NUE), and fine root biomass increased as soil water and/or N supply decreased with altitude. High NUE was linked to higher N mean residence time, caused by higher N resorption efficiency rather than increasing N productivity. Our results suggest that NPP and N uptake can be maintained by increasing WUE and NUE and increasing fine root biomass in water and N co-limited semi-arid forest ecosystems near arid boundaries. Such changes in resource use and acquisition strategy can affect production and N cycling via plant-soil feedback systems.