Associations between organic matter fractions and the active soil microbial biomass

The active pool of organic matter plays an essential role in the short-term of nutrients turnover in soil. An approach to characterizing this fraction is through densimetric techniques which isolate soil light fractions. Cropping and tillage systems are determinants of the amount and distribution of soil organic matter, especially in the upper layers of the soil profile. Our objectives were to evaluate the distribution and dynamics of carbon in different density fractions in order to provide a better understanding of soil fertility changes induced by contrasting types of soil management: plow tillage, no-tillage and pasture. The total and active microbial biomass pools and microbial activity were also determined. The experiment was performed on a Typic Argiudoll from the Argentinean Pampa. Organic carbon was highest under pasture, but there were no differences between the others two treatments for the 0-15 cm layer. Under the pasture and no-tillage treatments, organic carbon decreased with depth. The light fraction (density less than or equal to 1.6 g ml-1) was higher under no-tillage than in the plowed soil, indicating that this fraction was more sensitive to management than was total carbon. The carbon mineralized in 160 d of incubation from different density fractions followed the order: light fraction > medium fraction (1.6-2 g ml-1) > heavy fraction (greater than or equal to 2 g ml-1), presumably because of an increase in chemical and physical protection of organic matter in the heavier fractions. Total soil microbial biomass was stratified under the pasture and no-tillage treatments. Basal respiration was significantly associated with the availability of carbon in the light fraction (r2 = 0.98. P < 0.001) and carbon in the soil microbial biomass (r2 = 0.88, P < 0.001). The active microbial biomass differed (P < 0.05) between pasture (29 microgram C g-1), no-tillage (19 microgram C g-1) and plow tillage (9 microgram C g-1). The active microbial biomass, as a fraction of the total soil microbial biomass, increased with depth in all treatments, but especially in plow tillage soils. There was a positive and strong association between the availability of carbon in the light fraction per unit of active soil microbial biomass and the ratio between the respiration in 10 d and the active microbial biomass (r2 = 0.93, P < 0.001). Our results suggest that no-tillage produced the accumulation of carbon in the soil light fraction and increased the potential carbon mineralization. Consequently this tillage treatment can conserve the potential fertility of soil under cultivation.