Spatial distribution of soil nitrogen in a jujube-crop intercropping ecosystem Horizontal variation of nutrient uptake and apparent loss in jujube/crop intercropping ecosystem Characteristics of phosphorus uptake by plants in soybean (Glycine MaxL. merr) and citrus (Citrus poonensis Hort ex Tanaka) intercropping system Impact of stalk mulching on the microclimate of intercropping ecosystems in upland red soil