Measurement and modeling of phosphorous transport in shallow groundwater environments

摘要

Leaching of phosphorus (P) from agricultural soils, especially those that are sandy, is adversely impacting P-limited ecosystems like Florida's Everglades. A more developed understanding of P and water management strategies and their effects on P leaching is needed to achieve reductions in subsurface P losses, especially from intensively managed dual cropping systems under plastic mulch in shallow water regions. We compared the effects of conservation P and water management strategies with traditional practices on P transport to groundwater. A 3-year experiment was conducted on hydrologically isolated plots with plastic-mulched successive cropping systems to compare high (HEI) and soil test based recommended (REI) external input (water and fertilizer P) systems with traditional sub-irrigation (seepage), and REI with a potential water conservation subsurface drip irrigation system (REI-SD) with regard to groundwater P concentrations above and below the low conductivity spodic horizon (Bh). The REI treatments had higher available storage for rainfall and P than HEI. Use of both REI systems (REI = 2098 μg/L and REI-SD = 2048 μg/L) reduced groundwater P concentrations above the Bh horizon by 33% compared to HEI (3090 μg/L), and results were significant at the 0.05 level. Although the subsurface drip system saved water, it did not offer any groundwater quality (P) benefit Mixing and dilution of influent P below the low conductivity Bh horizon between treatments and with the regional groundwater system resulted in no significant differences in groundwater P concentration below the Bh horizon. Groundwater P concentrations from this study were higher than reported elsewhere due to low soil P storage capacity (SPSC), high hydraulic conductivity of sandy soils, and a high water table beneath crop beds. The HEI system leached more P due to ferilizer P in excess of SPSC and used higher irrigation volumes compared with REI systems. Despite a 40% difference in the average amount of added fertilizer P between HEI (187 kg P_2O_5/ha) and REI (124 kg P_2O_5/ha), soil Mehlich 1 P (M1P) values were similar for both systems while they received P_(input). Soil M1P for REI and REI-SD increased to a maximum of 55 mg/kg while they received P_(input), and then gradually decreased after P_(input) ceased. However, M1P for HEI increased steadily to a maximum of 145 mg/kg by the end of the study with continued P_(input). Mehlich-1P measured six years after the study still showed relatively high levels of P, a legacy effect of P_(input). The main factors influencing groundwater P concentration varied by seasons. During fall with frequent rainfall, the concentrations were influenced mainly by M1P and P_(input), and highlight a need for greater focus on P_(input) management (vs. water management) during this season. However, during the dry period of spring, a greater focus on irrigation management is required since depth to water table and rainfall also become contributing factors. Three multivariate models (r~2 = 0.67 to 0.93), for spring, fall, and annual periods, were developed for predicting groundwater P concentrations for a wide range of water and P inputs (0 to 191 kg P_2O_5/ha of P_(input))- The uniqueness of these models is that they use readily available hydrologic (rainfall and water table depth), management (P_(input)). and soil (M1P) data commonly monitored by growers when managing water and nutrient inputs on agricultural landscapes. The development of similar models may not be necessary for other agro-ecosystems in similar regions since long-term data collected in these regions may be applied, with verification, to the models presented here.