Effect of no-tillage and conventional tillage systems on the chemical composition of soil solid phase and soil solution of Brazilian savanna oxisols

No-tillage (NT) cropping systems are becoming increasingly important in the Brazilian savanna. To evaluate their sustainability we compared soil chemical properties in 1- to 3-year-old NT systems following 9 to 11 years of conventional tillage (CT) with systems where CT was continuously in place for 12 years. In the rainy season 1997/98, NT was cropped with soybean and CT with corn while in the rainy season 1998/99 both systems were cropped with soybean. Soil solid phase samples were taken from the 0-0.15, 0.15-0.3, 0.3-0.8, 0.8-1.2, and 1.2-2 m layers on three spatially Separated plots under each of NT and CT. Soil solution samples were collected weekly at 0.15, 0.3, 0.8, 1.2, and 2 m soil depth during two rainy seasons (14 October to 28 April 1997/98 and 1998/99). We determined soil moisture contents, pH, the concentrations of exchangeable cations, the electrical conductivity (EC) of the soil solution, and the concentrations of Al, C, Ca, Cl-, K, Mg, Mn, Na, NH4+, NO3-, P, S, and Zn in solid soil and soil solution samples. Differences in soil solid phase properties and moisture content between NT and CT were small, few were significant. Under NT, the average solution pH was significantly lower (5.5), Al (26 mu g l(-1)), Mn (17 mu g l(-1)) and total organic C concentrations (TOC, 65 mg l(-1)) were higher than under CT (pH: 6.0, Al: 14 mu g l(-1), Mn: 14 mu g l(-1), TOC: 5.5 mg l(-1)). Irrespective of the different crops in the first rainy season, under NT, the EC (205 mu S cm(-1)), Ca (17 mg l(-1)), and Mg (2.9 mg l(-1)) concentrations at 0-0.3 m depth were lower than under CT (EC: 224 mu S cm(-1), Ca: 25 mg l(-1), Mg: 5.6 mg l(-1)). At 1.2-2 m depth, the reverse order was observed (EC: 124 mu S cm(-1) under NT and 84 mu S cm(-1) under CT, Ca: 11 mg l(-1) under NT and 7.5 mg l(-1) under CT, Mg: 3.1 mg l(-1) under NT and 1.8 mg l(-1) under CT). Our results indicate that enhanced soil acidification because of higher rates of organic matter mineralization and a more pronounced nutrient leaching because of increased pore continuity may limit the sustainability of NT.