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The use of no-tillage has notably increased in the Pampas region of Argentina during the last 10 yr. Two tillage experiments with contrasting previous agricultural use, degraded and non-degraded soils, were evaluated in the southeast of Buenos Aires province, Argentina. The objectives were to: (i) quantify the effects of tillage and N fertilization on quantity and vertical distribution of C and N in the soil organic matter (SOM) and particulate organic matter (POM) fractions as well as potentially mineralizable N (PMN), and (ii) evaluate these fractions as indicators of soil quality. Tillage systems were conventional tillage (CT), minimum tillage (MT), and no-tillage (NT) (main plots), and N fertilization rates were 0, 120, and 150 kg [ha.sup.-1] (subplots). Total organic C (TOC), total N (TN), POM-C, POM-N, and PMN were measured at 0- to 7.5- and 7.5- to 15-cm soil depth. in Exp. I (degraded soil) TOC was greater under NT (27 g [kg.sup.-1]) than under CT (24 g [kg.sup.-1]) in the 0-N treatments. No differences in TOC and TN were found in Exp. II at 0 to 7.5 cm (non-degraded soil). Carbon in POM and POM-N were greater under NT in the fractions of 212 to 2000 and 53 to 212 [micro]m at 0 to 7.5 cm, but they were similar or greater under CT at 7.5- to 15-cm depth in Exp. I. Stratification of TOC, TN, and POM were observed under NT in Exp. I. Potentially mineralizable N was greater under NT (62 mg [kg.sup.-1]) in Exp. I, however, no differences in PMN were observed in Exp. II. Carbon in POM 212 to 2000 [micro]m and PMN were the more sensitive indicators of tillage effects, mainly in Exp. I.

An evaluation of the effect of N fertilization mowing and burning on the microbial biomass (MBM) of tallgrass prairie proves that MBM is important for the conservation of N in tallgrass prairie. Mowing decreases the amount of microbial C (MC) and microbial N (MN) in vegetation, while the addition of nitrogen increases MN and reduces the amount of MC. However, the presence of MC and MN in tallgrass prairies emphasizes the need for the cycling of these nutrients.