Soil Aggregate Types and Water-Stable Aggregation of an Oxisol As Affected By Crop Sequence Under No-till.

Nowadays, it is estimated that the current area under NT in Brazil is 26 million ha, approximately.  No-tillage influences on soil organic matter dynamics (food source for microorganisms). The food source influences the type of organisms that live in the soil. The type of organisms existing in the soil, consequently, can influence soil aggregation. The aimed of this study was to evaluate cropping sequences under no-tillage on soil aggregation. The experimental design consisted of four treatments (cropping sequences) with three replications. The cropping sequences included: continuous corn (Zea mays L.) as summer crop followed by corn as winter crop (CC/C); continuous soybean (Glycine max (L.) Merrill) as summer crop followed by sunn hemp (Sunn hemp juncea L.) as winter crop (CS/SH); soybean - corn rotation as summer crops followed by sunn hemp as winter crop (SCR/SH); and soybean - corn rotation as summer crops followed by corn as winter crop (SCR/C). Treatments were established in 2002 and soil samples from 0 to 10 cm depth were obtained in 2012. Soil aggregates were identified as physiogenic, biogenic and intermediate. In each aggregate type were measured the relative amounts and the water-stable aggregation. Higher amount of physiogenic aggregates was observed on all crop sequences, which suggests the effect of intrinsic chemical and physical soil attributes on soil aggregation. However, higher water-stable aggregation was observed on biogenic type aggregates on CC/C, SCR/C and SCR/SH crop sequence, which are those including Poaceae species (grass) on them.  This fact suggests that the higher cellulose content of these plants mediate crop species effects on soil structure indirectly through their influence on soil microbes (food source for microorganisms). The results showed that crop sequence with frequent use of Poaceae represents suitable options to increase soil water-stable aggregation and consequently decrease the vulnerability to soil structural degradation.