Different agricultural practices accomplish distinct controls on microbial C an N dynamics. The dynamics of microbial C and N in soils under no-tillage system (NT) were evaluated in the crop sequences corn/wheat/soybean and soybean/wheat/soybean in soils with different texture, areas and periods under NT at the "Santa Branca" farm, Tibagi, Paraná State, Brazil (50 ° 23 ' W, 24 ° 36 ' N). The experiment was installed at four sites, three of which had been NT cultivated in the previous 12 years, with the following textures: clay (PD12-TmuA); sandy-clay (PD12-TmeA), and sandy-clay-loam (PD12-TA); and one site 22 years under NT with texture class sandy-clay-loam (PD22-TA). During an 18 months period soils were sampled nine times in four layers (0-2.5; 2.5-5; 5-10 and 10-20 cm). Microbial C was different in PD12-TA and PD22-TA in the deepest soil layers, due to the time of NT implantation. The microbial N level in the PD22-TA averaged 30.8 kg ha-1 higher than in PD12-TA, while the variations in microbial N in the 0-20 cm layer were smaller. No significant differences were observed in the amounts of microbial C and N among the crop sequences. Regarding soil texture, the averages of microbial C and N in the 0-20 cm layer were higher for PD12-TmuA, except 20 days after wheat sowing (Jun/01). Contrasting to the 5-10 and 10-20 cm layers, in PD12-TA and PD12-TmeA the differences between microbial C levels were not significant in the 0-5 cm layer. Our results indicate that the amount of microbial C and N is favored by long-term NT and the clayey soil texture.
soil texture; crop sequence; nitrogen immobilization; organic matter
