A reliable laboratory index of N availability would be useful for making N recommendations, but no single approach has received broad acceptance across a wide range of soils. We compared several indices over a range of soil conditions to test the possibility of combining indices for predicting potentially mineralizable N (N 0 ). Soils (0-5 and 5-15 cm) from nine tillage studies across the southern USA were used in the evaluations. Long-term incubation data were fit to a fi rst-order exponential

No abstract

No abstract

Abstract:Seasonal cracking of the soil matrix results in poor estimates of runoff and infiltration by simulation models due to the changing soil storage conditions. In this study, soil surface elevation changes were measured every two weeks and soil crack volume was calculated for a two-year period at the USDA-Agricultural Research Service, Riesel Y-2 watershed in central Texas. Soil anchors were placed in triplicate at depths of 0Ð15, 0Ð45, 0Ð90, 1Ð50 and 2Ð5 m and relative movement from a monument at 4Ð5 m was measured. Soil movement was translated into crack volume assuming isotrophic shrinkage. A crack flow model was developed for this study that computes crack volume from crack potential, soil depth and soil moisture. Simulated crack volume followed the seasonal trends found in the measured crack volume and was in general agreement with a regression R 2 D 0Ð84. The crack model was incorporated into SWAT (Soil and Water Assessment Tool), a comprehensive hydrologic model. Regression analysis was performed on measured and simulated daily surface runoff with an R 2 D 0Ð87 indicating good agreement. The model was able to simulate surface runoff accurately in winter months when cracks were swelled closed and in the fall recharge events of 1998 when crack volume went from 70 to 10 mm. The relationships between measured crack volume and hydrologic variables simulated by the model were also examined and discussed.

No abstract