Cultivation of non-irrigated spring wheat in temperate free-grazing steppe improved both ecosystem and canopy water use efficiency.

The temperate steppe in northern China is important for sandstorm control and food/livestock production. Understanding the influence and regulatory control of cultivation on the water balance and water use efficiency (WUE) of this water-limited region would promote the sustainability of local ecosystem and food supply. This study combined eddy covariance system observational data and the Shuttleworth-Wallace model to investigate evapotranspiration (ET) and its composition in paired sites, including a free-grazing steppe site and an adjacent site reclaimed for spring wheat cultivation in Xilinhot, Inner Mongolia. Further, analysis of the WUE of both the ecosystem (WUE) and the canopy (WUE) under the two sites showed that the mean daily gross primary productivity (GPP) of the cultivation site was 3.84 gC·m·d, i.e., 15.7% higher than that of the free-grazing site (3.32 gC·m·d). Compared with the free-grazing site (1.76 kgHO·m·d), the mean daily ET of the cultivation site (1.40 kgHO·m·d) was reduced by 20.7%. The difference in ET was due mainly to suppression of evaporation at the cultivation site from increased shading associated with a higher leaf area index (LAI). The largely increased GPP of the cultivation site fundamentally contributed to the 54.7% higher WUE (4.75 gC·kgHO) in comparison with the free-grazing site (3.08 gC·kgHO). The WUE of the cultivation site was 57.9% higher than that of the free-grazing site. The variation of transpiration of the free-grazing site explained 64% of the change of WUE. These results indicate that land use differences in the temperate steppe area changed vegetation productivity substantially. Moreover, ecosystem ET and its composition, as well as large-scale land use change, might influence the regional water use pattern and mass balance. Our findings help clarify the impact of typical land use change on regional WUE, and could promote development of visionary and effective strategies for the use of the limited resources in arid-semiarid regions.