Modeling Solute Transport in Soil Under Conservation Agriculture Production Systems in the Philippines.

This study aimed to model and to compare the solute-transport behavior of soil under conservation agriculture production and plow-based systems in the Philippines. Undisturbed soil core samples were taken from both production systems from experimental sites of SANREM-CRSP in Claveria, Misamis Oriental, Philippines.  Miscible displacement and adsorption experiments were performed at the laboratory facilities of the College of Engineering and Agro-industrial Technology, University of the Philippines Los Baños.  A stochastic method, following continuous-input soil column tests, determined the dispersivity of the soil samples while a laboratory flow-through method, using pulse-input soil column tests, calculated the retardation factor.  Both the soil dispersivity and the retardation factor were optimized using the CXTFIT model to fit the observed values to the Convection-Dispersion Equation. Upper-layer plow-based soil, with a combined lowest retardation factor of 1.26 and dispersivity of 17.5 cm, had the highest peak concentration of 0.67C0 and the shortest time-to-peak of 44 s.  Soils under conservation agriculture production systems, having a high retardation factor of 6.54 in spite of the highest value of dispersivity of 27.3, exhibited the lowest peak concentration of 0.44C0 after nearly 7 min of peaking time.  These results may be indicative of higher soil organic carbon content in soils under conservation agriculture.  Model efficiencies ranging from 77% to 98% signify that the Convection-Dispersion Equation is able to adequately predict solute transport in these soils. Simulations of solute transport in response to changing soil organic carbon content for a ten-year period were performed.  Increasing the organic carbon by 30% in soils under conservation agriculture reflects a 9.6% decrease in peak concentration and a 3.1% increase in time-to-peak.  On the other hand, there are minimal changes in terms of both peak concentration and time-to-peak in soils under plow-based systems assuming 3% decrease in organic carbon. Model simulation results have practical implications on both the economics of crop production in terms of agro-chemical application and on groundwater quality protection.