Contribution of phosphorus issued from crop residues to plant nutrition

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Abstract
The contribution of crop residue phosphorus to ryegrass nutrition was analyzed by isotopic dilution methods, i.e. direct labelling of crop residue phosphorus and labelling of phosphate ions in the soil solution. Three types of crop residues were investigated: ryegrass aerial parts and roots and wheat straw. Our experiments showed a high dependence of phosphorus uptake on the residue and soil characteristics. Compared with the absorption of phosphorus derived from diammonium phosphate, the use of root residue phosphorus was similar and the absorption was higher for the aerial part and straw residues. We suggest it to be due to the protecting effect of the vegetal structure, the phosphate ions being slowly released into the soil resulting in a lower fixation on the soil adsorbing sites. The competition between the soil microorganisms and the plant for nitrogen assimilation was demonstrated. The addition of residues with high C/N (C/N ratios of ryegrass roots and wheat straw were 32 and 90, respectively) decreased the absorption of soil and residue nitrogen by the crop which was less effective in the case of residues with a lower C/N (C/N ratio of ryegrass aerial parts was 13). The presence of residues decreased the uptake of phosphorus derived from a soil poorly supplied with phosphorus. In a soil rich in phosphorus, higher amounts of phosphorus were used and the effect of residues on the absorption of phosphorus derived from the soil was less conspicuous.