N 2 -fixing black locust intercropping improves ecosystem nutrition at the vulnerable semi-arid Loess Plateau region, China.

The Loess Plateau in northwestern China constitutes one of the most vulnerable semi-arid regions in the world due to long-term decline in forest cover, soil nutrient depletion by agricultural use, and attendant soil erosion. Here, we characterize the significance of N ₂ -fixing Robinia pseudoacacia L. and non-N ₂ -fixing Juglans regia L. for improving nutrient availability and water retention in soil by comparing a range of biological and physicochemical features in monoculture and mixed plantations of both species. We found that N ₂ -fixing Robinia facilitates the nitrogen and phosphorus composition of non-N ₂ -fixing Juglans in the mixed stand as a consequence of improved soil nutrient availability, evident as higher levels of nitrogen and labile carbon compared to mono-specific stands. This demonstrates that intercropping N ₂ -fixing Robinia with non-N ₂ -fixing woody plants can greatly improve soil carbon and nitrogen bioavailability as well as whole-plant nutrition and can potentially mediate water retention with additional sequestration of soil organic carbon in the range of 1 t C ha ^-1  year ^-1 . Thus, intercropping N ₂ -fixing woody species (e.g. Robinia pseudoacacia or Hippophae rhamnoides L.) with locally important non-N ₂ -fixing tree and shrub species should be considered in afforestation strategies for landscape restoration.
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