Tillage affects soil aggregation parameters linked with wind erosion

Geometric mean diameter (GMD), erodible fraction (EF), and dry aggregate stability (DASt) are soil parameters deduced by dry sieving that are used to identify soil susceptibility to wind erosion. Values of GMD, EF, and DASt have been calculated for different soil types but limited information is available on the effects of tillage on these parameters. In order to asses this influence we analyzed their variation in an Entic Haplustoll of Argentina during a 2 year sampling period. This soil was submitted to three tillage systems during the 7 years prior to sampling: no-till (NT), vertical tillage (VT) and conventional tillage (CT). We also analyzed the dry stability of each aggregate size fraction coarser than 0.84 mm (DASi). Results showed that tillage produced significant differences in all analyzed parameters. Average parameter values were, by treatment: GMD (2.37 mm) and DASt (88%) the highest and EF (20%) the lowest in NT, and GMD (0.88 mm) and DASt (49%) the lowest and EF the highest (49%) in CT. VT showed intermediate values of these parameters. As a consequence of soil disturbance by tillage, variations with time in CT were higher for GMD (SD=0.42), EF (SD=8.26) and DASt (SD=16.31) than in VT (SD=0.31, 5.71 and 8.00, respectively) and NT (SD=0.31, 2.75, and 1.99, respectively). GMD calculated with a regression equation based on soil textural fractions, OM and CaCO3 contents (Hagen, pers. comm.), was similar to the measured GMD in soils with low tillage disturbance (NT and VT) but it was much higher than the measured GMD in highly disturbed soils (CT). WEQ overestimated the wind erosion of the studied soil by 25 t ha(-1) year(-1) in NT and 9 t ha(-1) year(-1) in VT when the potential wind erosion (I factor) was calculated with an EF value obtained from the regression equation given by Fryrear et al. [Fryrear, D.W., Krammes C.A., Williamson D.L., Zobeck T. M. 1994. Computing the wind erosion fraction of soils. Soil Water Conserv. 49:183-188.]. NT exhibited greater quantities of aggregates coarser 19.2 and 6.4 mm than VT and CT, and lesser quantities of fine aggregates (0.84 and 2 mm) than CT on most sampling dates. The variability of the 19.2 mm sized aggregates between sampling dates was greater in NT (SD=18) than in VT (SD=6) and CT (SD=4), while the variability of the 6.4 mm sized aggregates (SD=5.5) was similar in all tillage systems. A time-dependent trend toward an increase of the 0.84 mm sized aggregates and a decrease of the 19.2 mm sized was observed in CT, indicating that tillage was degrading 19.2 mm aggregates into 0.84 nun aggregates. DASi of all sized aggregates was lower in NT (8.1%) than in VT (14.1%) and CT (23. 1%), and was also less variable between sampling dates in NT (SD=1.1), than in VT (SD=4) and CT (SD=7.6). A negative relationship between aggregate size and DASi was found (y=-1.755x+86.46, R-2=0.56, P<0.001). Aggregates formed in NT were 5 to 7% more stable than VT aggregates and 13 to 16% more stable than CT aggregates. We concluded that tillage practices affect the parameters deduced from dry aggregate size distribution at different rates in the studied soil. Therefore, further studies should be developed to obtain reliable correction factors for these parameters on the basis of previous management conditions. (C) 2007 Elsevier B.V. All rights reserved.