Reduced tillage and compost effects on soil aggregate stability of a silt-loam Luvisol using different aggregate stability tests

Aggregate stability of soils informs about their relative strengths against erosive forces and mechanical disruption; however, the many methods of assessment are not of equal potential in discriminating management effects. Moreover, the index to mimic the actual behaviour of field soils is not readily discernable from such assessment. In this study involving a silt-loam Luvisol on long-term tillage trial in Central Germany, we assessed topsoil aggregate stability 2.5 years after initiating the experiment using selected laboratory methods from which 12 indices were derived. Our aim was to assess treatment effects on aggregate stability as discriminated by the indices, explore the relationships among them, and relate them to simulated erosion of the soil. Treatments were reduced tillage and conventional tillage each with compost at 0 and 5 t ha(-1). The indices included mean-weight diameter after dry (MWDd) and wet (MWDw) sieving, sand-corrected water-stable aggregates (%WSA(cfs)), aggregate stability index (ASI), percolation stability (PS), sealing index (SI); and tensile strength of 24.16, 16-8 and 8-4 mm air-dry (TSad) and re-moistened (TSrm) aggregates. The SI method used rainfall simulation which enabled measurements of runoff and mass of eroded sediment (MASED(eroded)). Tillage- x -compost interactions on aggregate stability were consistently non-significant. Four of the indices discriminated between the tillage systems; three (MWDw, WSA(cfs) and TSrm(16-8)) showed higher stability under reduced tillage while ASI showed otherwise. Two (ASI and SI) favoured compost non-application. MASED(eroded) correlated with PS, %WSA(cfs) and MWDd (r = -0.53*, -0.51* and 0.49*, respectively), with best fit as logarithmic, exponential and polynomial, respectively. Also, MASED(eroded) was explained by PS and two of the aggregate size fractions from dry sieving (p <= 0.002; R-2 = 0.700). To enhance aggregate stability and reduce water erosion in this soil, reduced tillage is a good candidate while compost may be unnecessary in the short run. The indices PS, %WSA(cfs) and MWDd can mimic water erodibility of the soil. The inclusion of MWDd usually associated with wind erosion among these indices found relevant for water erosion is remarkable.