Phosphorus Fractions of Red Soils in Guangdong Province of South China and Their Bioavailability for Five Crop Species

Abstract

Red soil, the main soil type in south China, has many limitations for crop production, especially low phosphorus (P) bioavailability. Currently, little is understood concerning P fractions and their bioavailability for crops. In the present study, P fractions were analyzed from 50 red soil samples collected from regions leading agricultural production in Guangdong Province of south China. The most abundant P forms were iron-bound P (Fe-P) and organic P (Org-P), which, combined, account for 64.4% ± 1.9% of total soil P. Both Org-P and Fe-P had less bioavailability for soybean in sand culture than the other sparingly soluble P forms tested, with growth reduced by 78.0% ± 4.2% or 35.4% ± 6.7% and P content reduced by 50.2% ± 8.4% or 68.7% ± 1.8% when P was supplied as Org-P or Fe-P, respectively, compared with plants supplied with KH2PO4. Further field experiment using five crop species showed that P bioavailability in red soils is very low, as reflected by an average of 53.9% ± 2.9% loss in shoot P content for all tested crops in soils without P fertilization compared with plots amended with P fertilizer. After evaluation of root-to-shoot ratio and specific root length, along with rhizosphere P depletion and acidification, it is reasonable to speculate that different crop species have evolved various strategies to overcome P deficiency in red soils. Among the tested crops, rapeseed displayed more rhizosphere acidification and lower carbon cost for maintaining root growth and subsequently possessed a superior ability to use and activate P from red soils than the other four crops.