BASAL AND SPECIFIC MICROBIAL RESPIRATION IN SEMIARID AGRICULTURAL SOILS - ORGANIC AMENDMENT AND IRRIGATION MANAGEMENT EFFECTS

Evolution of microbial activity in semiarid agricultural soils was stu died measuring the basal (CO2 evolved) and specific microbial respirat ion (qCO(2)). The effects of straw residue management (buried or remov ed), irrigation (dry or irrigation farming), and chemical composition of the soils (calcareous, gypsiferous, or saline) were analyzed. Seaso nal samplings were carried out over a 2-year period. Both irrigation m anagement and harvest residue management, as well as soil chemical com position, influence basal respiration significantly. Straw burying and water addition increased soil respiration in every sampling. On annua l average, calcareous soils, with low levels of gypsum and salts and h igher percentage of organic carbon, evolved the highest rates of carbo n dioxide. Basal and specific microbial respiration were positively an d significantly correlated. The release of carbon dioxide per unit of microbial biomass (qCO,) was considerable in the experimental soils, e specially with buried straw, although basal respiration was low. High levels of qCO(2) were related to severe climatic conditions and manage ment disturbances (ploughing action, fluctuation of organic carbon con tent), stressing factors that rejuvenate microbial population.