Winter Crops Residues Decomposition IN NO-Tillage SYSTEM IN SAVANNAH CLIMATE.

In no-tillage system, plant residues are expected to maintain the soil covered for long periods of time in order to control soil erosion. Therefore, the dynamic of plant residues decomposition can be different according to the species. The study of residue decomposition under field conditions is a requirement for optimizing the crop options rotation in no-tillage systems. The purpose of this study was to evaluate the plant residue decomposition over time of winter crops in no-tillage system. A field experiment was conducted during the 2008/2009 growing season at Jaboticabal, SP, Brazil (21^o14′S, 48^o17′W and altitude of 550 m), with tropical/megathermal climate zone or Koppen's Aw. The mean annual rainfall is 1417 mm, with an annual distribution peaking in October to March and a relatively dry season in April to September. The winter crops, sowed in February-March, 2009 were corn, sunflower, oilseed radish, pearl millet, pigeon pea, grain sorghum and sunn hemp.  Corn, sunflower and sorghum residues were collected after the grain harvest. The other winter crops residues were obtained after cut by a shredder, at flowering. The period of time to measure the plant decomposition was (15, 30, 60, 120 and 180 days). The data were adjusted to exponential model (Y = 100.e^‑kt); where Y is the plant residue dry mass (%) over time (days), and k is the decomposition constant rate. Oilseed radish residues showed the fastest decomposition rate, losing 50% of their dry mass within 75 days. High resistance for decomposition was observed in corn, sunn hemp and sorghum residues, which reduced 50% of their dry mass within 156, 135 and 126 days, respectively. However, winter crops residues with low decomposition rates, which persist on soil surface for long periods, are required in order to control the soil erosion until next summer crop sowing.