Green manure and compost effects on n-p dynamics in mediterranean organic stockless systems

Stockless systems, where plant and animal production are uncoupled, represent a common condition in Mediterranean areas. In organic systems, soil fertility is mainly managed by green manures, whereas composts of plant origin represent suitable soil conditioners. The low P content of these materials, together with the shortage of animal waste availability, determine potential nutrient depletion overtime and, in calcareous soils, P mining conditions. In a two-year field experiment in Central Italy, the effect of green manure vetch (Vicia villosa) (GM+) was compared with a control left fallow (GM-), combined with P-based fertilizers (F) on organic maize (Zea mays). Yields and N-P dynamics were evaluated by direct measurements, nutrient balance and efficiency index. N balance was higher than in GM+ GM- (+71.8 and -23.6 kg N ha-1, respectively). P enriched compost with RP (rock-phosphate) (EP), showed significant improvement in P use efficiency (meanly +34%) than other treatments (unfertilized control (RP), not P enriched compost (NEP)) and P equilibrium in the short run. On the other hand, P-fertilizers and maize residues soil incorporation determined P surplus in a long perspective (up to +10 kg P ha-1). Results emphasize the possibility of P reintegration in stockless organic systems without animal wastes.