In the South American savanna, no-tillage (NT) is implemented to improve the
sustainability of cropping systems. At the beginning of the rainy season,
however, more plant nutrients may be leached under NT than under conventional
tillage (CT) because of more pronounced organic matter mineralisation and
increased pore continuity. To test this hypothesis, we analysed the chemical
composition of the soil solution under conventional (CT) and no-tillage (NT)
soybean (Glycine max (L.) Merr.) fields in the Brazilian
savanna (Cerrados).
We collected the soil solution at 0.15, 0.3, 0.8, 1.2, and 2 m depth and the
precipitation in 1- to 3-day intervals from 28 October to 23 December 1998 on
plots under CT and NT, replicated 3 times. We determined pH, electrical
conductivity (EC), Ca, K, Mg, Na,
NO3– ,
NH4+,
Cl–, and total organic carbon (TOC) concentrations
in soil solution and precipitation.
Soil solution pH under NT was 0.3–0.8 units lower than under CT and was
inversely related to TOC concentrations (average under NT, 1.02; CT, 0.70 mmol
C/L) at all depths. Average Cl– , Ca, and Mg
concentrations at 0.15–0.3 m depth were significantly higher under CT
(1.09, 1.1, and 0.25 mmol/L) than under NT (0.50, 0.83, and 0.17
mmol/L), respectively. No difference was observed in average Na (0.09
mmol/L) and NO3–
concentrations (2.2 mmol/L) between CT and NT. At 0.8–2 m, average
NO3– (0.30 mmol/L),
Cl– (0.18 mmol/L), Ca (0.19 mmol/L), Mg
(0.05 mmol/L), and Na (0.04 mmol/L) concentrations under CT were
significantly lower than under NT
(NO3– , 0.38;
Cl– , 0.40; Ca, 0.23; Mg, 0.09; Na, 0.06
mmol/L). In the monitored period, the Cl–
which had accumulated during the dry season and which was applied with KCl
fertiliser on 29 October reached a depth of 0.3 m under CT and of 1.2m under
NT.
The results suggest higher mineralisation rates and faster leaching in the NT
than in the CT systems, being the combined result of different plowing
practices and different cropping sequences.