Assessment of erosion rates during rehabilitation of hardened volcanic soils (tepetates) in Tlaxcala
The emergence of tepetates (hardened volcanic ash layers) on the surface after erosion of the overlying horizon is widespread all over the Trans-Mexican Volcanic Belt, and in particular in the State of Tlaxcala where it covers 15% of the state surface. The rehabilitation of tepetates can be a solution to the lack of arable land and to environmental problems related to surface runoff and soil erosion. The purpose of this investigation was to evaluate the effect of organic farming on erosion rates and to determine the relationship between runoff, soil loss, rain erosivity (EI30), soil organic carbon (SOC), vegetation cover (V) and aggregate stability (AS), by multiple regression analysis performed for individual erosive events (36) and for cumulative annual values.
Erosion and runoff rates were measured from 2003 to 2005 in five farmer’s plots (580 to 2,020 m2) of 3% slope inclination. Two plots were reclaimed and cultivated since 2002 and three since 1989. Three different management techniques were applied in plots reclaimed in 1989: “conventional”, “improved” and “organic”, corresponding mainly to increasing incorporation of organic matter. Soil loss in plots reclaimed in 2002 ranged from 8.6 to 19.1 ton×ha-1×year-1 under conventional management and from 5.5 to 14.1 ton×ha-1×year-1 under organic farming. In plots reclaimed in 1989 the erosion rates are up to three times smaller than in recently reclaimed tepetates, with soil losses ranging from 1.1 to 5.6 ton×ha-1×year-1 and no significant (P<0.05) differences between management techniques. For individual events, runoff and soil loss are significantly dependant on EI30, SOC and V (r2=0.73 and 0.54 respectively). For annual values, the model accounted for an even larger proportion of the variance (r2=0.94 and 0.87, respectively). Even moderate incorporation of fresh organic matter significantly increased aggregate stability in both recently and older reclaimed tepetates. However, the percolation stability index was not correlated to SOC nor to erosion rates as we expected. In recently reclaimed tepetates, organic management enhanced carbon accumulation and vegetation cover, reducing runoff and soil loss in relation to conventional management.