Green manure as a tool for improving C and N balance of Mediterranean cropping systems.

It is well known that high input cropping systems may cause the decay of soil fertility, thus limiting crop yield and soil resilience. Furthermore the typical environmental conditions of Mediterranean area (high temperature coupled with high moisture in autumn and spring periods) usually enhance this trend because of an increased SOM degradation. In this context, Green Manures, especially with legume crops, may represent a useful tool for improving soil fertility also reducing external energy inputs because they allow to increase soil N and P availability for the following crop and at the same time, contribute to the conservation of SOM and soil biological, physical and chemical properties (Mc Vay et al., 1989). On the other side, winter non leguminous crops, thanks to their high C up N ratio could improve C storage in soil, thus resulting also effective in capturing post-harvest soil N, so reducing autumn N leaching (Upendra, 2007). A mixture of legume and non legume cover crops could be more suitable to both increase organic matter and reduce the potential N leaching. Nevertheless, adequate quantities of C and N are not easy to predict, because biotical (weeds) and a-biotical factors (rainfall and temperature) could limit growth and N-fixing of the crops used for green manuring. This paper shows the results of a two year trial on the effects of different green manure treatments (leguminous, non leguminous and mixtures) on C and N balance and yield of maize. This two year experiment showed that legume crop green manure can give a maize yield not different from mineral fertilization. The application of high quality organic input such as Vetch (with low lignin/N and C/N ratio) could provide a more efficient use of nutrients by releasing N quickly for food crops. Green manuring made with mixed crops and pure oat cause a significant reduction in yield of the following crops, because of lack in available N due to high C/N ratio of biomass. Nevertheless application of lower quality plant residues (high in lignin and low in N), such as oat, may maintain or increase SOC, and build-up soil organic N in the slowly turning pool (Sanchez, 1989), but only additional N fertilization can make this tool sustainable for farm economics. High N surplus at harvest was assessed for all treatments including C, showing that nitrate leaching and ground water pollution could involve organic farming too. Growing winter N catch crops between the summer crops may be one way to reduce this problem. Those crops can uptake the residual inorganic N present in soils at the end of the summer growing season as well as that, which will be mineralized during the following winter period, till the beginning of the next summer season (Rodrigues, 2001). Furthermore extreme variability in biomass and N supplies with green manure suggest to integrate this tool with other techniques of soil fertility management.