| A model (VTCROPS) 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. VTCROPS 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. 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. 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.;Model predicted crop performance variables and soil N content were compared with available field data for maize and for wheat.;Long-term model predictions, for two year crop rotations with minimum and conventional tillage, were evaluated by comparing performance variables with literature values. 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 strongly periodic, with most leaching taking place from January through April. Nitrogen load increased with fertilizer rates, however, N leaching fraction was optimal around 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.;Analyses were performed to determine long-term crop performance and N loads to ground and surface waters in the study area over a 26 year period (13 rotations). With the exception of two land units, mean yield, water use and N uptake over the simulation was fairly uniform among the land units. Runoff, drainage, mineralization, denitrification and N load were highly variable both between land units and over time.;Long-term average N loads and N concentrations from the cultivated area and from the total area of the study region were estimated.;The possibility of using sewage sludge as a replacement for, or in consort with N fertilizer was investigated. (Abstract shortened by UMI.). |
