Organic matter of a soil amended with composted sludges and affected by simulated processes of soil degradation

Under the dry Mediterranean climate, continuous degradation of soil structure is caused by water-erosion and wildfire because of the removal or destruction of the organic matter of the soil surface. These effects, normally, are avoided with the organic amendment of the soil surface. To model this process, indices of organic matter content of a forest soil classified as Rendzic Leptosol have been investigated in representative soil samples of Zarra (Valencia, Spain). Under laboratory conditions, soil samples amended with three rates of composted sludges (0, 5 and 10% w/w) were subjected to three energy levels of simulated water-erosion (EO, El and E2, corresponding to 0, 100 and 600 J.cm(-3), respectively) and two levels of heat (F0 and F1, corresponding to 25 degreesC and 500 degreesC of highest soil temperature, respectively) in a randomised complete block experimental design. For all treatments, the relative index of organic matter (rate of variation referenced to the content in the control) and the soil macroaggregates/microaggregates ratio were calculated. After the highest level of simulated degradative agents (the combination of E2 and F1 treatments), the relative organic matter index ranged from a 33% decrease in the unamended soil to a 15% increase in the soil which received the highest rate of sludge. The amendment of this coarse-textured soil with composted sludges at the rates of 5 and 10% (w/w) diminished the breakdown of macroaggregates; into microaggregates but their organic matter decreased more slowly than in the unamended soil. As a consequence of the simulated soil water erosion process, the organic matter in the unamended soil was concentrated around the particles of fine sand and that would provide their combustion by the heating induced treatment than for the amended soil samples.