LAND-USE EFFECTS ON ORGANIC-CARBON, NITROGEN, AND SULFUR CONCENTRATIONS IN MACROAGGREGATES OF DIFFERENTLY TEXTURED BRAZILIAN OXISOLS

The objective of this study was to assess the influence of land use on soil aggregate size distribution and the consequences for organic C, N, and S concentrations in bulk soil and macroaggregates. The properti es of a loamy and a clayey Oxisol used for continuous cropping, pastur e and reforestation were compared with those of the native savannah (' 'Cerrado''). We measured aggregate size fractionation, C, N, and S con centrations in bulk soil, small (0.25-2 mm, SMA), and large macroaggre gates (2-8 mm, LMA), and carried out a mineralization experiment with intact and crushed LMA. The aggregate size distribution pastures was n ot different from native Cerrado. Reforestation and plowing caused hig her percentages of smaller aggregates which was more pronounced in the loamy than in the clayey soil. Total concentrations of C, N, and S we re higher in the clayey (CL 21.5-23.3 g kg(-1); N: 1.2-1.4 g kg(-1); S : 178-213 mg kg(-1)) than in the loamy soil (C: 7.8-10.3 g kg(-1); N 0 .5-0.7 g kg(-1); S: 87-132 mg kg(-1)) LMA of the loamy soil had higher C,N, and S concentrations than the bulk soil. SMA and both macroaggre gate fractions of the clayey soil did not differ from the bulk soil. 7 1% of potentially mineralizable N in LMA of the loamy soil were only m ineralized after aggregate disruption. In contrast, there were only sm all differences between crushed and intact IMA of the clayey soil. The refore, we considered conventional tillage suitable For clayey soils. The loamy soil would require a more soil conserving system like no-til l or crop-pasture rotation to improve sustainability.