Influences of organic fertilization and solarization in a greenhouse on particle-size fractions of a Mediterranean sandy soil

The effects of a composted organic amendment and solarization on the organi c matter (OM) of a sandy soil were determined by means of particle-size fra ctionation and analysis of carbon and nitrogen contents. After 2 years, tot al soil carbon increased under organic fertilization but did not significan tly change with solarization. As a consequence of the climatic conditions i n the greenhouse, the carbon concentrations (g kg-l fraction) of the partic le-size fractions were lower than those found for temperate soils and close r to those for tropical soils. The carbon amounts (g kg(-1) soil) and carbo n:nitrogen ratios, which were highest in fractions >200 mum? reflected the short-term influence of the industrially processed organic amendment, rich in composted coarse plant debris. In contrast, the characteristics of the O M associated with each fraction were not significantly affected by solariza tion. In comparison with other coarse-textured temperate or tropical soils, carbon concentrations in fine silt (2-20 mum) and clap (0-2 mum) fractions were very low. This suggests a "greenhouse effect", together with a high r ate of carbon mineralization affecting fine silt and clay fractions.