N availability after long-term organic farming in irrigated and rain-fed Mediterranean semi-arid grassy crops

Summary

The use of manures and rotation with legumes in organic farming systems does not always guarantee the adequate nutrient supply to crops. We studied post-harvest N availability in a series of Mediterranean semi-arid rain-fed and irrigated organic fields after 18 years of conversion and compared these with conventional fields nearby. In organic irrigated soils the use of legumes and the application of moderate amounts of manures resulted in higher amounts of soil organic C and potentially mineralisable N. In contrast, in organic rain-fed soils that did not incorporate legumes in their rotation and used low amounts of manures, soil organic C and potential mineralisable N were lower. In organic irrigated soils changes in organic matter quality resulted in a lower potential net N mineralisation per unit of SOC than in conventional irrigated soils.