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 thi s 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 laye rs of the soil profile. Our objectives were to evaluate the distributi on and dynamics of carbon in different density fractions in order to p rovide a better understanding of soil fertility changes induced by con trasting types of soil management: plow tillage, no-tillage and pastur e. The total and active microbial biomass pools and microbial activity were also determined. The experiment was performed on a Typic Argiudo ll From the Argentinean Pampa. Organic carbon was highest under pastur e, but there were no differences between the others two treatments for the 0-15 cm laver. Under the pasture and no-tillage treatments, organ ic carbon decreased with depth. The light fraction (density less than or equal to 1.6 g ml(-1)) was higher under nu-tillage than in the plow ed soil, indicating that this fraction was more sensitive to managemen t than was total carbon. The carbon mineralized in 160 d of incubation from different density fractions followed the order: light fraction>m edium 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 phys ical protection of organic matter in the heavier fractions. Total soil microbial biomass was stratified under the pasture and no-tillage tre atments. Basal respiration was significantly associated with the avail ability: of carbon in the light fraction (r(2)=0.98, P < 0.001) and ca rbon in the soil microbial biomass (r(2)=0.88, P < 0.001). The active microbial biomass differed (P < 0.05) between pasture (29 mu g C g(-1) ), no-tillage (19 mu g C g(-1)) and plow tillage (9 mu g C g(-1)). The active microbial biomass, as a Fraction of the total soil microbial b iomass. increased with depth in all treatments, but especially in plow tillage soils. There was a positive and strong association between th e availability of carbon in the light fraction per unit of active soil microbial biomass and the ratio between the respiration in 10 d and t he active microbial biomass (r(2)=0.93, P < 0.001). Our results sugges t that no-tillage produced the accumulation of carbon in the soil ligh t fraction and increased the potential carbon mineralization. Conseque ntly this tillage treatment can conserve the potential fertility of so il under cultivation. (C) 1998 Elsevier Science Ltd. All rights reserv ed.