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Most biogeochemical models simulate water dynamics using the tipping bucket approach, which has been often found to be too simplistic to represent vadose zone dynamics adequately under shallow groundwater conditions. Recently, a solution to the Richards equation using the Mualem–van Genuchten model (Rich‐vGM) has been added into the EPIC (Environmental Policy Integrated Climate) model to address this shortfall. Its performance was tested using lysimeters operating under free drainage (FD) and at a shallow water table (60‐ [WT60] and 120‐cm depth [WT120]). Model accuracy was also compared with the upgraded tipping bucket‐based method implemented into EPIC (the variable saturation hydraulic conductivity method [VSHC]). Soil water content (SWC) data were split into calibration and validation subsets. Model evaluation also included annual evapotranspiration (ET), percolation (PRK), and upward water movements to assess underlying soil water balance factors. The submodels provided accurate and similar results upon comparison with SWC measures under FD (Nash–Sutcliffe coefficient [NSE] = 0.26 and 0.61 using VSHC and Rich‐vGM, respectively). The Rich‐vGM model accurately reproduced observed SWC and ET (e.g., NSE = 0.70 and percentage bias [PBIAS] = −3.7% for WT120, respectively) although it slightly overestimated PRK (PBIAS = 47.8%, on average). Instead, VSHC proved unable to correctly simulate shallow groundwater conditions (e.g., NSE = −1.85 for WT60 SWC). Under shallow groundwater conditions, the Rich‐vGM method is recommended, despite the additional data required and the need to define the bottom boundary conditions according to water table fluctuations. In conclusion, the Richards solver introduced and tested in EPIC improved the model's ability to represent complex biophysical and biogeochemical processes in terrestrial ecosystems associated with the hydrological balance. Core Ideas: EPIC Richards solver was tested in a 9‐yr experiment under different groundwater conditions.The new Richards algorithms were compared with the tipping‐bucket based method.Both methods predicted similar soil water content under free drainage.Water content and ET were better predicted by Richards under shallow water table.Under specific conditions, percolation was overestimated for both approaches. [ABSTRACT FROM AUTHOR]

In a pine forest, urea hydrolysis rates can vary widely. The rate of urea hydrolysis is controlled by the amount of urease enzyme, temperature, pH, urea concentration, but at the soil surface, soil water potential is an especially critical factor because it may vary widely in short periods of time due to precipitation, dews, and rapid evaporation. The objective of this study was to describe the quantitative relationship between soil water and urea hydrolysis rates in a loblolly pine (Pinus taeda L.) forest floor. Hydrolysis rates were measured in a pine forest with time after urea was applied in seven studies performed at different times that varied widely in soil water content and temperature. Separately in a laboratory study, the urea hydrolysis rates were measured in pine needles and in the humified layer at different water potentials. Results showed that although well-defined relationships were found between urea hydrolysis rates and forest floor water potentials, the urea hydrolysis rates measured in the field could not be predicted based on these relationships and the water potential of the forest floor pine needles at the time of urea application. However, the percent of urea hydrolyzed at 3 and 29 d could be predicted accurately from the initial water potential of new needles and an interaction between initial water potential and relative humidity days. [ABSTRACT FROM AUTHOR]

Soil water availability affects NH3 loss from surface applied urea, partly due to its influence on urea hydrolysis rates. However, the quantitative effects of soil water on hydrolysis rates are not well understood. Our objective was to quantify how soil matric potential affects urea hydrolysis rates across a wide range of soil matric potentials. Urea hydrolysis rates were measured based on disappearance of urea at various soil water potentials in soils limed to a pH of 8.2 to eliminate confounding by soil pH. Urea-N was applied on a soil mass basis (1200 µg urea-N g-1 soil, 0.2-2.4 mol urea L-1) in Exp. I and at a constant urea-N soil solution concentration of 40,000 µg urea-N g-1 soil solution (1.43 mol urea L-1) in Exp. II. Differences in relative rates of urea hydrolysis between both experiments suggested that in Exp. I there was a confounding effect of urea-N solution concentration on the response of urea hydrolysis to soil matric potential. In Exp. I, relative rates of hydrolysis changed little at intermediate soil matric potentials between -0.1 and -1.0 MPa because urea-N concentration in solution increased as matric potential decreased. In Exp. II, relative rates were greatest at the highest matric potentials (>-0.24 MPa) and decreased more rapidly as matric potential decreased, approaching zero at a matric potential of approximately -10 MPa. These results also suggest that research to describe urea hydrolysis rates as affected by soil water content should be based on urea-N concentrations in soil solution and not urea-N concentrations on a soil mass basis. [ABSTRACT FROM AUTHOR]

The relationship between soil test P (STP) and dissolved reactive P (DRP) in runoff has been shown to vary with soil type due to differences in soil properties. This study was conducted to determine the effect of mineralogy on P sorption behavior and DRP losses in runoff using simulated rainfall. Nine different soft types were sampled from four different fields to provide a range in STP. Unamended soils were packed into runoff boxes for use in a rainfall simulation study (7.5 cm h-1 for 30 min). A mineralogical analysis and adsorption-desorption isotherm was conducted on one representative sample from each soil type. Results indicated that P retention for adsorption and desorption in separated day fractions and whole soils was well correlated to Al bearing minerals such as hydroxy-interlayered-vermiculite (HIV), gibbsite, and amorphous Al. However, P retention was negatively related to kaolinite content, which was also confirmed by isotherms conducted on pure day minerals. Based on the isotherm results, all soils were split into two groups based on the ratio of HIV/ksolinite. Soils with a HIV/kaolinite ratio >0.5 had a significantly lower concentration of DRP in runoff for a given soil water soluble P level compared with soils with a ratio <0.5. [ABSTRACT FROM AUTHOR]