Organic and inorganic P reserves in rain-fed and irrigated calcareous soils under long-term organic and conventional agriculture

In low input farming systems the reserves of organic and inorganic P may be a significant source of P to crops. To clarify this we investigated organic and inorganic P reserves in an array of irrigated and rain-fed calcareous soils of grassy crops, with long term organic or conventional farming practices. Organic P (Po) reserves appeared to be more sensitive than inorganic P reserves to changes in soil fertilisation regime. Organic P reserves were sensitive to the quantity of P added, rather than to the quality of the input. In soils receiving manures, there was no correlation between organic P reserves and soil organic C and N or soil carbonate content. In contrast, inorganic P reserves in soils receiving manure were positively linked to soil total N and to organic C (in irrigated organic soils) and negatively to carbonate content. This suggests that the accumulation of inorganic P pools in calcareous soils receiving manure may be enhanced in soils with high organic matter content. As indicated by changes in the N/Po ratio, soil carbonates protected total organic P pools from mineralisation in soils receiving adequate amounts of P fertilisers. In contrast, in soils receiving low amounts of manures we observed low amounts of organic P even in soils with a high carbonate content. Therefore, in conditions of P starvation, organic P reserves protected by carbonates may be slowly mobilised. Despite this mobilisation, in soils receiving low amounts of P, no relationships were observed between labile Pi pools and organic P reserves. In contrast in soils in soils receiving sufficient amounts of P we observed a relationship between organic P reserves and labile Pi pools. In irrigated soils inorganic P reserves were positively correlated with labile P pools, even when fertilisation was moderately low. The application of manure to these latter soils favoured the links between organic P reserves and labile P forms.