SOIL ORGANIC-CARBON, MICROBIAL BIOMASS AND CO2-C PRODUCTION FROM 3 TILLAGE SYSTEMS

Organic matter is a major sail component which is influenced by tillag e. This paper quantifies the effect of no-till, chisel tillage and plo w tillage on the content and depth distribution of organic carbon and microbial biomass after 12 years of each tillage system. The soil was typical of the Argentine Rolling Pampa. The resistance of organic matt er to mineralization was evaluated by means of an incubation test. In the no-till and chisel tillage systems, crop debris accumulated within the top 5 cm of soil, especially in the no-till system, Consequently, organic carbon was 42-50% higher (P=0.01) in the no-till soil than in the soil from the plow and chisel tillage systems. Biomass carbon and soil basal respiration (0-10 day period) were noticeably stratified u nder no-till and chisel tillage, while they were uniform from 0 to 15 cm in the plowed soil. The metabolic quotient of the biomass (basal re spiration/biomass) was regulated in all cases by the coarse plant debr is content of the soil (r(2) = 0.79, P = 0.01). A doubled exponential model was fitted to CO2-C values produced during 160 days of incubatio n (r(2) greater than or equal to 0.95). This shows that soil carbon dy namics can be described as being composed of two pools: one labile, an d one resistant to microbial attack, The proportion of total carbon mi neralized and the decomposition of soil-resistant carbon in 160 days i n the no-till and chisel tillage soils were high at the soil surface, but decreased with depth. In plowed soil, these parameters were consta nt from 0 to 20 cm. The organic matter at the soil surface under the n o-till and chisel tillage systems was more readily degradable than und er plow tillage in the laboratory experiment. Carbon inputs from crops were estimated to be similar between tillage systems. Consequently, i n situ accumulation of labile forms of organic matter under a no-till system may be ascribed to a decrease in the mineralization intensity o f the soil organic matter. Soil temperature determinations suggested t hat plowed plots were warmer than no-tilled plots, and this phenomenon could lead to a decrease of microbial respiration in straw-covered so il.