Plant Phosphorus Uptake in a Soybean-Citrus Intercropping System in the Red Soil Hilly Region of South China*1

A field microplot experiment was conducted in the red soil hilly region of South China to evaluate plant phosphorus (P) uptake under soybean and citrus monoculture and the soybean-citrus intercropping system using the 32P tracer technique. P fertilizer was applied at three depths (15, 35, and 55 cm). The experimental results showed that the planting pattern and 32P application depth significantly affected the characteristics of P uptake by soybean and citrus. Under the soybean-citrus intercropping system, considerable competition was observed when the 32P fertilizer was applied to the topsoil (15 cm); therefore, the 32P recovery rate declined by 41.5% and 14.7% for soybean and citrus, and 32P supplying amount of topsoil to soybean and citrus decreased by 346.8 and 148.1 mg plot−1, respectively, compared to those under the monoculture. However, 32P recovery of soybean was promoted when 32P fertilizer was applied to the deeper soil layers (35 and 55 cm) under soybean-citrus intercropping. Under the soybean monoculture, 32P fertilizer could hardly be used by soybean when 32P fertilizer was applied at the 55 cm depth or below, with the recovery rate being less than 0.1%; it was up to 0.253% by soybean under intercropping. The higher P recovery of soybean under soybean-citrus intercropping when P was applied in the deeper soil layers was because part of the P nutrient that the citrus absorbed from the deeper soil layers could be released into the topsoil and then it could be used by the soybean.