A model of nitrate leaching from agricultural systems in Virginia's Northern Neck

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Virginia Water Resources Research Center, Virginia Polytechnic Institute and State University

A model (VT-CROPS) was developed to simulate the long-term effects of nitrogen (N) leaching to groundwater in the Northern Neck region of Virginia and, ultimately, to the Chesapeake Bay. VT-CROPS simulates N fate and transport in a soil-plant-atmosphere continuum in a vertical slice between two crop rows, enabling consideration of nonuniform fertilizer placement and root growth patterns. VT-CROPS models atmospheric, soil and crop subsystems. Atmospheric conditions (rainfall, temperature, solar radiation) may be entered directly by the user or generated by using a stochastic climatic generator. The soil subsystem simulates runoff, infiltration, drainage and soil-water redistribution, N immobilization, nitrification, mineralization, denitrification, and advective N transport. The crop subsystem simulates plant N uptake, and vegetative and reproductive growth in response to soil and climatic factors, explicitly for maize or wheat. VT-CROPS simulates soybean in a crop rotation, empirically accounting for leaf area and root growth. The model is capable of simulating long-term cropping sequences under minimum and conventional tillage . practices for continuous maize or for rotations involving maize, wheat, soybean, and fallow.

Critical internal model parameters were calibrated through comparison of output to field data. The sensitivity of output to input variables was determined. Model output is most sensitive to the climatic variables. Model-predicted crop performance variables-grain and total dry matter yields and N content-and soil N content were compared with available field data from two sites over a three-year period for maize. Data from six sites over a one-year period were tested for wheat. Predictions for maize and total N content were fairly accurate, with a tendency to greater error in dry years. Predictions for wheat were somewhat less accurate, but incomplete field data precluded determining the source of discrepancies.

Long-term model predictions, for two-year crop rotations with minimum and conventional tillage, were evaluated by comparing performance variables with literature values. Appropriate responses were obtained for N transformation processes. Mass conservation for soil water and N were good. Maize performance variables were within the range of literature values, and were higher under minimum till. Wheat yields and N contents were somewhat higher than values reported in the literature. Nitrogen load is correlated to drainage and water use over the short run, and to rainfall and drainage over longer periods. Minimum tillage did not increase N load to groundwater. Over a year, nitrogen load was periodic, with most leaching taking place from January through April. More than 50% of the N load over a rotation was lost during an extended fallow period that followed soybeans. Nitrogen load increased with fertilizer rates; however, the N leaching fraction was optimal at rates of 150-200 kg/ha.

The model was applied to the Virginia counties of Richmond, Westmoreland, Lancaster, King George, and Northumberland to assess the potential for long-term N leaching to groundwater. Soil surveys indicated that 34 soil map units occurred within 123,000 hectares of cultivated land. To reduce the number of simulations, principal component analysis and cluster analysis were used to subdivide the cultivated area into 10 land units based on different soil properties. Historical climatic data from the area were used to calibrate the stochastic climatic generator.

Analyses were performed to determine long-term crop performance and N loads to groundwater and surface waters in the study area over a 26-year period (13 rotations). Two management systems were applied to the land units. The first management system consisted of a rotation of minimum-tilled maize, conventionally tilled wheat, minimum-tilled soybeans, and a fallow period. The second management system had a similar cropping sequence, but all crops were conventionally tilled. In both cases, fertilizer was applied at a rate of 150 kg-N/ha/crop. With the exception of two land units, mean yield, water use, and N uptake over the simulation were fairly uniform among the land units. Runoff and drainage were highly variable between land units and over time within units. Mineralization, denitrification, and N load were highly variable both between land units and over time. Nitrogen load ranged from 66 to 131 kg/ha/rotation between land units.

Long-term average N loads and N concentrations from the cultivated area and from the total area of the study region were estimated. For this analysis, it was assumed that 80% of the cultivated area was under minimum till and 20% under conventional tillage. An area-weighted average of 5.4 million kg-N/ha/year, or 29% of total N applied, is discharged to groundwater, with an average drainage concentration of 9.9 mg/I. The average N concentration from the study area (including uncultivated areas) to groundwater is estimated at 5.1 mg/I. Average N concentration to the Chesapeake Bay from all sources, after dilution with runoff, is 4.5 mg/I, which is lower than the drinking water standard for nitrate N of 10 mg/I.

The possibility of using sewage sludge as a replacement for, or in consort with, N fertilizer was investigated for a typical land unit, under a conventionally tilled maize-wheat-soybean-fallow rotation. Simulations were conducted with 100%, 50%, and 0% sludge (CN ratio of 12). With fertilizer N augmenting the sludge, the total N input (250 kg/ha) was the same for each treatment. Mean yields were similar for 50% and 0% sludge, but lowered by 10% and 16%, respectively, for maize and wheat with 100% sludge. Discrepancies in yields were attributed to the fact that mineralization rates of sludge are not high enough to supply the crop N requirement during periods of peak uptake. Nitrate leaching was reduced by 41 % and 25% with 100% and 50% sludge applications, respectively.