Monitoring and modeling of subsurface drainage and nitrate leaching under various land covers

This dissertation includes the study of hydrologic cycling and nitrogen dynamics under various land covers for the subsurface drained agriculture in Iowa through field investigation and modeling approaches. Land covers included conventional corn-soybean rotation, winter rye cover crop in corn-soybean rotation, kura clover as a living mulch for corn, and perennial forage. Field experiments consisted of two parts: one was conducted in a crop field at a plot-scale including all the land covers near Gilmore City, Iowa from 2006 to 2008 and the other was conducted in non-weighing lysimeters with winter rye cover crop and bare soil during 2006-2008. The RZWQM-DSSAT model was tested against the measured data from the plot-scale study and the evaluated model was subsequently used to simulate the long-term impacts of winter rye cover crop on hydrologic cycling and nitrogen dynamics. Overall, the results suggest that subsurface drainage water quality in terms of NO3-N contamination can be effectively improved by converting conventional corn-soybean rotation into perennial forage, but at present there would be little economic return for the grasses and it may also alter the local hydrologic cycle. Planting corn in established kura clover living mulch also reduced the annual flow-weighted NO3-N concentration and NO3-N loss in the subsurface drainage flow, but the corn yield in kura clover treatment was significantly reduced. Although not significantly impacting total NO3-N loss in the plot-scale study, rye significantly reduced the NO3-N concentration in soil water within the soil profile, and showed a potential in reducing subsurface drainage and NO3-N loss in the non-weighing lysimeter and the long-term simulation studies. Therefore, rye cover crop has the potential to be an excellent cropping option under an integrated concern for the environment and economy

Effects of Cover Crops in Reducing Nitrate-Nitrogen Leaching in Iowa

Nitrate-nitrogen (N03-N) has been deemed a main source of pollutant for both shallow groundwater and surface water bodies. The main source of N03-N in the Mississippi River Basin (MRB) is linked to tile drainage (Lowrance, 1992; David et al. , 1997). Approximately 25% of agricultural land is artificially drained in Iowa (Baker et al., 2004) and subsurface drainage is the main source of N03-N loss. Schilling and Zhang (2004) reported that while Iowa accounts for 5% of the area of the MRB it contributes approximately 25% of N03-N load (23 lb-N acre-1) over a 28-year period from 1972 to 2000. Plot scale experiments measured N03-N loss of 23 to 49 lb-N acre-1 year1 in northeast Iowa (Weed and Kanwar, 1996), 24 to 27lb-N acre-1 in central Iowa (Baker et al., 1975; Baker and Johonson, 1981 ; Kanwar et al., 1983) and 6 to 56 lb-N acre-1 year1 in northwest Iowa (Lawlor, et al. , 2008)

Effect of different land covers on nitrate-nitrogen leaching and nitrogen uptake in Iowa

Nitrate-nitrogen (NO 3 -N) loading from subsurface drainage is an environmental concern in the Midwest. The majority of NO 3 -N loading occurs in April, May and June when the crops are not planted or just establishing. In this study, NO 3 -N leaching was monitored under alternative land covers and in a corn-soybean rotation. Land cover treatments in a 2-year field experiment included: 1) corn-soybean rotation initiated with corn in 2006 and fallow in late fall and early spring (fallow-Corn-fallow-Soybean, fCfS); 2) corn-soybean rotation initiated with soybean in 2006 and fallow in late fall and early spring (fallow-Soybean-fallow-Corn, fSfC); 3) corn-soybean rotation initiated with corn in 2006 with rye cover crop (rye-Corn-rye-Soybean, rCrS); 4) corn-soybean rotation initiated with soybean in 2006 with rye cover crop ( rye-Soybean-rye-Corn, rSrC); 5) Corn with established kura clover as a living mulch (kura-Kura-kura-Corn, kKkC); and 6) Pasture as a perennial grass treatment (PP). Subsurface drainage volume and NO 3 -N concentration were monitored. Suction lysimeters were installed to extract the soil water solution for NO 3 -N analysis. Biomass of spring cover crops was sampled to analyze nitrogen (N) content. The objectives of this study were: 1) to determine NO 3 -N loss through subsurface drainage as affected by different land covers; 2) to investigate the NO 3 -N concentrations in the soil water under different land covers and 3) to quantify the nitrogen uptake by different cover crops in the spring. The results from the two-year study indicated that the annual average NO 3 -N loss for fCfS and fSfC treatments was 37.5 kg N ha -1 and that the rCrS and rSrC treatments reduced NO 3 -N leaching by 3.8 kg N ha -1 during April, May and June. kKkC and PP treatments resulted in 39.7% and 59.9% annual NO 3 -N leaching reduction, respectively, when compared to the average NO 3 -N loss of fCfS and fSfC treatments. Rye followed by soybean reduced the NO 3 -N concentration in the soil solution significantly (56.4%) at the 30- and 60-cm depths, and PP treatment showed the lowest NO 3 -N concentration at those two depths. The average nitrogen uptake by rye was 33.3 kg N ha -1 at growth termination, and the average N uptake was 59.9 kg N ha -1 for kura clover and 33.2 kg N ha -1 for pasture in early June. This study suggested that winter rye cover crop, kura clover as a living mulch and perennial pasture land covers have positive effects on NO 3 -N loss reduction under the weather condition encountered during this study period in Iowa

Simulating Long-Term Impacts of Winter Rye Cover Crop on Hydrologic Cycling and Nitrogen Dynamics for a Corn-Soybean Crop System

Planting winter cover crops into corn-soybean rotations is a potential approach for reducing subsurface drainage and nitrate-nitrogen (NO3-N) loss. However, the long-term impact of this practice needs investigation. We evaluated the RZWQM2 model against comprehensive field data (2005-2009) in Iowa and used this model to study the long-term (1970-2009) hydrologic and nitrogen cycling effects of a winter cover crop within a corn-soybean rotation. The calibrated RZWQM2 model satisfactorily simulated crop yield, biomass, and N uptake with percent error (PE) within ±15% and relative root mean square error (RRMSE) \u3c30% except for soybean biomass and rye N uptake. Daily and annual drainage and annual NO3-N loss were simulated satisfactorily, with Nash-Sutcliffe efficiency (NSE) \u3e0.50, ratio of RMSE to standard error (RSR) \u3c0.70, and percent bias (PBIAS) within ±25% except for the overestimation of annual drainage and NO3-N in CTRL2. The simulation in soil water storage was unsatisfactory but comparable to other studies. Long-term simulations showed that adding rye as a winter cover crop reduced annual subsurface drainage and NO3-N loss by 11% (2.9 cm) and 22% (11.8 kg N ha-1), respectively, and increased annual ET by 5% (2.9 cm). Results suggest that introducing winter rye cover crops to corn-soybean rotations is a promising approach to reduce N loss from subsurface drained agricultural systems. However, simulated N immobilization under the winter cover crop was not increased, which is inconsistent with a lysimeter study previously reported in the literature. Therefore, further research is needed to refine the simulation of immobilization in cover crop systems using RZWQM2 under a wider range of weather conditions