Labile phosphorus forms in irrigated and rainfed semiarid Mediterranean grassy crops with long-term organic or conventional farming practices

The availability of organic fertilisers plays a major role in organic farming systems. Such systems exclude the use of synthetic fertilisers, whilst aiming to optimise internal nutrient cycling. The low availability of manures, particularly in dry areas, can lead to negative nutrient balances in many organic farming systems. Such negative nutrient balances are mainly found for P and K. In this paper, we aim to study the availability of P in irrigated and rainfed semiarid Mediterranean grassy crops with long-term organic and conventional farming practices. NaHCO3 extracts were prepared from an array of soils from 16 plots, covering organic and conventional management in rainfed and irrigated conditions. Inorganic (NaHCO3-Pi) and organic P (NaHCO3-Po) were analysed in the extracts and related to soil properties (carbonate content, pH, organic C and N content). Rainfed, organically managed soils showed low P availability compared to conventionally managed soils. However, organically managed irrigated soils showed slightly higher NaHCO3-Pi than conventional soils. This is due to the fertilisation regime applied to the irrigated, organically managed systems. Such systems received four-fold the amount of manure applied to organically managed rainfed soils. In soils with low P availability, NaHCO3-Pi was largely depleted while NaHCO3-Po remained nearly unchanged. In soils with good or moderate P availability, NaHCO3-Pi appeared mainly to be regulated by soil organic matter (organic C and N). In conditions of low P availability, NaHCO3-Pi was mainly regulated by chemical processes related to soil pH and carbonate content. The regulation of NaHCO3-Po was less clear. Under low P availability, NaHCO3-Pi depletion occurred mainly in soils with high organic C and N and low pH. In low P soils with a high pH and carbonate content, NaHCO3-Pi appeared to be geochemically protected. In calcareous soils, management practices need to increase or maintain the level of soil organic C to facilitate mobilisation of the P reserve.