Soil Organic Phosphorus Changes After Brachiaria Cultivation.

Organic phosphorus, accounting for 50 - 80% of total P in soil, is an important source of P available to plant. Cultivation of cover crop (Brachiara ruziziensis) wasobserved to increase soil P availability but the mechanisms are not fully understood. The objective of this study was to determine the changes in organic P transformations in soil as a result of cover crop rotations using a chemical fractionation approach.

The field experiment was established in 1998 in Botucatu, São Paulo State, Brazil. The soil is a Haplortox, sandy loam, with 670 g kg^-1 sand and 210 g kg^-1 clay.  In 2006 the rotation system was changed to Brachiaria ruziziensis (in half of the plots) instead of triticale, black oat, pearl millet, and 0 and 35 kg P ha^-1 were applied as triple superphosphate (soluble) and natural Arad rock phosphate (reactive) in 1998; 2001; 2006 and 2008. The experimental design was a 3 x 2 factorial with three P treatments with and without Brachiaria, and four replications for each treatment. Six soil samples were randomly taken at 0-5 cm from each plot to make one composited sample. Organic P in soils was fractionated into labile (NaHCO₃ extractable), moderately labile (HCl extractable), fulvic and humic bound fractions. Brachiaria cultivation significantly increased 0.5 M NaCO₃ extractable organic P at the 0-5 cm soil layer in the control and the plots receiving water soluble P fertilizers, but the increase was not significant for the plots treated with reactive rock phosphate. Cover crop rotation also increased P fraction bound to humic acids in the soils receiving P fertilizers. However, fulvic acids bound P was not affected by Brachiaria. These results indicate that Brachiaria rotation increases soil P availability by contributing to labile and humic acid bound organic P.