Soil organic matter in the granulometric fractions of an oxissol under different uses and management systems

Abstract

Soil organic matter (SOM) is an important component of agriculture sustainability. A clayey oxisol was studied by SOM particle size fractions in a citrus orchard managed (for a 9 year period) with a) Arachis prostrata ground cover; b) spontaneous vegetation controlled by mowing and tillage; c) bare soil. Also the same soil was studied under a 20 year period of annual crops (conventional tillage) and native forest. The orchard ground covers increased the soil C and N stocks when compared to orchard bare soil and annual crops. For all situations, the largest proportion of the SOM was found in the clay fraction (0-2 mm) (35 to 48 % of the total) followed by the silt fraction (2-20mm) (13 to 30 %). Soil C stock was reduced by 33 % with annual crops and by 19 % with bare soil orchard, and was slightly higher with orchard ground covers, when compared to native forest. The carbon losses in the layer 0-10 cm were 23 to 45% comparing the native forest to the crops, being larger with annual crops for 20 years and smaller at orchard with A. prostrata ground cover. The greatest SOM losses were in the 50-2,000 mm (vegetal debris + coarse sand), and fine sand (50-200mm) fractions. The greater recuperation of SOM in the orchard with leguminous cover was in the 20-50mm fraction when compared to annual crops.