Oscar J. Santanatoglia

Soil organic carbon, microbial biomass and CO2-C production from three tillage systems

Organic matter is a major soil component which is influenced by tillage. This paper quantifies the effect of no-till, chisel tillage and plow 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 matter 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 under 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 respiration/biomass) was regulated in all cases by the coarse plant debris content of the soil (r2=0.79, P=0.01). A doubled exponential model was fitted to CO2-C values produced during 160 days of incubation (r2≥0.95). Thisshows that soil carbon dynamics can be described as being composed of two pools: one labile, and one resistant to microbial attack. The proportion of total carbon mineralized and the decomposition of soil-resistant carbon in 160 days in the no-till and chisel tillage soils were high at the soil surface, but decreased with depth. In plowed soil, these parameters were constant from 0 to 20 cm. The organic matter at the soil surface under the no-till and chisel tillage systems was more readily degradable than under plow tillage in the laboratory experiment. Carbon inputs from crops were estimated to be similar between tillage systems. Consequently, in situ accumulation of labile forms of organic matter under a no-till system may be ascribed to a decrease in the mineralization intensity of the soil organic matter. Soil temperature determinations suggested that plowed plots were warmer than no-tilled plots, and this phenomenon could lead to a decrease of microbial respiration in straw-covered soil.

Effect of temperature on soil microbial biomass and its metabolic quotient in situ under different tillage systems

Variations in the microbial biomass and the in situ metabolic quotient (qCO2) due to climatic conditions were determined in a typical soil from the Argentine Rolling Pampa. Microbial C was evaluated by fumigation-incubation and qCO2 was calculated using soil respiration in the field. An inverse relationship between microbial C and soil temperature was fitted to a model (r2=0.90, P=0.01). No significant association with the soil water content was detected because the soil was generally near field capacity and thus water availability did not limited microbial growth and activity. Values of qCO2 increased (r2=0.89, P=0.01) as the result of metabolic activatìon, likely induced by a higher maintenance energy requirement at high temperatures. The highest values of qCO2 were obtained when microbial C was the lowest, which was attributed to self consumption of microbial C in the presence of high temperatures. Consequently, microbial C was generally higher (P=0.05) in winter than in summer. Therefore, when microbial C is used as an index of soil biological activity, the influence of temperature should be taken into account.