Mechanisms of aggregation in a silty loam under different simulated management regimes

Silty loams of the rolling Pampa of Argentina are affected by different soil degradation levels. It is still poorly understood how soil degradation affects the regeneration of a desirable topsoil structure in pampean silty loams. A greenhouse pot experiment was carried out to evaluate changes in soil aggregation after 4 and 12 months exposure to varying: (i) soil management regimes: pasture, conventional tillage (CT) and eroded conventional tillage (CTer); (ii) water regimes: constant field capacity (FC) and wetting-drying cycles (W/D); and (iii) vegetation: sterilized and ryegrass. After 4 months (October-January), the size of aggregates measured by their mean weight diameter (MWD) after dry sieving, varied from 1.78 to 3.54 mm. This was significantly increased by soil degradation level (pasture < CT < CTer), FC, and ryegrass. The stability of these aggregates, measured by their MWD after wet sieving, varied from 0.75 to 5.09 mm. This was mainly due to highly significant (P < 0.001) "vegetation x water regime" interactions. Soil W/D cycles increased aggregate stability in ryegrass pots, but decreased stability in sterilized pots. In FC pots, wet MWD was similar in sterilized and ryegrass pots. This shows the prevalence of abiotic mechanisms of cohesion development under a constant water regime. Results after 12 months of experiments differed little from those found after 4 months. The exception was soil wet MWD in sterilized pots under W/D cycles, which increased significantly from 4 to 12 months. In the studied silty loam, soil aggregation was maximized after 4 months in ryegrass pots under W/D cycles. Soil degradation level caused only minor effects on aggregate stability. (C) 2004 Elsevier B.V. All rights reserved.