Modeling nitrate transport in soils under multiple cropping systems in Aurora, Isabela, Philippines

This study was conducted to model and to compare the nitrate transport in conventionally-tilled and no-till soil under multiple cropping systems. The study also aimed to simulate the effect of varying fractions of soil organic carbon on nitrate transport. Undisturbed soil columns were taken from both production systems in Aurora, Isabela [Philippines]. Miscible displacement continuous-input determined the dispersivity while pulse-input column test calculated the retardation factor. CXTFIT/Excel was used to optimize the values of dispersivity and retardation factors in the Convection-Dispersion Equation. A dispersivity of 14.55 cm and retardation factor of 1.19 for the upper layer of conventionally-tilled soil showed the highest peak concentration of 0.69 C sub 0 and the shortest time-to-peak of 134 s. The upper layer of no-till soil with a dispersivity of 3.05 and retardation factor of 2.92 exhibited the lowest peak concentration of 0.39 C sub 0 and a longer peaking time of 12 minutes. Results were attributed to the formation of macropores in tilled soils and higher organic carbon for no-till soils. For longer soil column, a higher dispersivity value of 24.29 cm and retardation factor of 1.83 together with a peak concentration value of 0.24 C0and pore volumes to peak of 0.52 were observed for no-till soils while a lower dispersivity of 17.21 cm and retardation factor of 1.42 plus peak concentration of 0.23 C sub 0 and pore volumes to peak of 0.51 resulted for conventionally-tilled soil. These values showed the presence of interconnected macropores and a greater amount of organic carbon content in no-till soils. In contrast, low values of dispersivity and retardation factor may be due to the discontinuity of pore channels and possible slight compaction of soil for conventionally-tilled soil. Model performance based on NSE ranging from 81% to 93% indicates that the Convection-Dispersion equation satisfactorily modelled the nitrate transport in these soils. Simulations on the effects of varying fractions of organic matter on nitrate transport for a ten-year period showed a 19.2% decrease in peak concentration and 15.4% increase in peaking time for 30% increase in organic carbon in no-till soil. For conventionally-tilled soil, a 3% decrease in organic carbon showed 6.8% increase in peak concentration while peaking time remained unchanged. No-till production systems provide low values of dispersivity and high retardation factors of soils. These transport properties makes fertilizer and pesticides stay in the soil for a longer period of time, thus, performing its intended function while minimizing nutrient leaching into the groundwater system.