Effects of different techniques of green manure on N balance of the rotation tomato-melon.

Alternative strategies for satisfy N requirements of crops are crucial for organic farming. This aim could be successfully reached by green manure tough it presents several uncertainties due to the need of coincidence of the biomass N mineralization and crop N uptake (Korsaeth, 2002). Besides, this microbially driven process is influenced by chemical composition of manure and pedoclimatic factors (Breland, 1994) that shows high variability among sites and years, thus increasing the uncertainties about N balance (Korsaeth, 2002). Changing times of green manure burying may regulate their chemical composition (Kankanen et al., 1998) and mineralization rate too (Yadvinder-Singh et al., 1992). Legume biomasses are quickly mineralized in Mediterranean area if buried at early stages, while burying at late stages or burying mixed crops (legume crops+cereals) may reduce mineralization rates, because of higher C/N ratio and lignin content. In this trial, different techniques of green manuring were compared with the aim to evaluate the effects on yield and N balance of an extensive horticultural cropping system. Green manure with legume crops buried at early stages is able to gain yields not different from mineral fertilization. Late burying of manures leads to an increase in N supplies but reduces its availability for the following crop. This results in higher content of soil mineral N in post-harvest periods. In the environments with rainy autumn, not only mineral fertilization but also green manure can cause nitrate pollution, because large amounts of N from buried biomasses may be mineralized in the post-harvest period of the following crop, thus increasing the risk of nitrate leaching.