Evaluation of the root zone water quality model for predicting water and NO3–N movement in an Iowa soil

Evaluation of computer models with field data is required before they can be effectively used for predicting agricultural management systems. A study was conducted to evaluate tillage effects on the movement of water and nitrate–nitrogen (NO3–N) in the root zone under continuous corn (Zea mays L.) production. Four tillage treatments considered were: chisel plow (CP), moldboard plow (MP), no-tillage (NT), and ridge-tillage (RT). The root zone water quality model (RZWQM: V.3.25) was used to conduct these simulations. Three years (1990–1992) of field observed data on soil water contents and NO3–N concentrations in the soil profile were used to evaluate the performance of the model. The RZWQM usually predicted higher soil water contents compared with the observed soil water contents. The model predicted higher NO3–N concentrations in the soil profile for MP and NT treatments in comparison with CP and RT treatments, but the magnitude of simulated NO3–N peak concentrations in the soil profile were substantially different from those of the observed peaks. The average NO3–N concentrations for the entire soil profile predicted by the model were close to the observed concentrations except for ridge tillage (percent difference for CP=+5.1%, MP=+12.8%, NT=+18.4%, RT=−44.8%). Discrepancies between the simulated and observed water contents and NO3–N concentrations in the soil profile indicated a need for the calibration of plant growth component of the model further for different soil and climatic conditions to improve the N-uptake predictions of the RZWQM

Alachlor Dissipation in Shallow Cropland Soil

A soil-column laboratory experiment and a 2-yr field-sampling study evaluated the overall dissipation of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl) acetamide]. Theses studies also measured the effect of no-till and chisel-plow tillages on alachlor leaching and dissipation. In the top 30 cm layer of soil, the overall half-life was 3 d or less, and the time to 90% dissipation ranged from 17 to 30 d. In no-fill soil, alachlor dissipated slightly faster, and more was transported into the 10 to 30 cm soil layer. Weather conditions promoting the movement of alachlor into the soil, however, weakened the effect of tillage on the dissipation rate. Most of the alachlor present in the soil, regardless of tillage, was found in the top 10 cm at all times. Of the alachlor applied to 30-cm tall soil columns, only 0.4% was removed by water flowing from chisel-plow columns and 1.6% from no-till columns. The results show that tillage was not a key factor in alachlor dissipation and leaching. Alachlor leaching was also a minor component in overall dissipation

Tillage and Nitrogen Management Effects on Nitrate-Nitrogen in the Soil Profile

A field study was conducted on Nicollet loam soils to determine the effects of tillage and nitrogen (N) management on the residual nitrate-nitrogen (NO3-N) in the soil profile. Two replications of two tillage systems (no-till and moldboard plow) and two N management practices (single application of N at the rate of 175 kg/ha and three applications totaling 125 kg/ha) were used for continuous-corn production. Soil samples were taken periodically each year to a depth of 150 cm at three locations within each of the six experimental plots (9 m apart, one along and two on either side of a tile drain running down the center of the plot). No-till and three N applications with a lower total application rate reduced the residual NO3-N in the soil profile. Regression analysis of the data showed less residual NO3-N under no-till system having a definite decreasing trend with depth. Eight years (1984-1991) of data indicate that corn yields were not affected by tillage or N management practices

Movement of NO3-N and atrazine through soil columns as affected by lime application

Lime (CaCO3) applied to the soil, to minimize or neutralize the soil pH, can influence the fate and transport of other chemicals in soil. This study was conducted to investigate the effect of lime application on the movement of NO3-N and atrazine through soil columns under saturated and unsaturated conditions

Agronomic and environmental soil testing for phosphorus and threshold levels in soils

Greater knowledge of soil phosphorus (P) is needed to develop application recommendations for Iowa farmers. This project provides more data on the topic by addressing both agronomic and water quality issues

Spatio-temporal analysis of yield variability for a corn-soybean field in Iowa

Spatio-temporal analyses of yield variability are required to delineate areas of stable yield patterns for application of precision farming techniques. Spatial structure and temporal stability patterns were studied using 1995- 1997 yield data for a 25-ha field located near Story City, Iowa. Corn was grown during 1995-1996, and soybean in 1997. The yield data were collected on nine east-west transects, consisting of 25 yield blocks per transect. The two components of yield variability, i.e., large-scale variation (trend) and small-scale variation, were studied using median polishing technique and variography, respectively. The trend surface, obtained from median polishing, accounted for the large-scale deterministic structure induced by treatments and landscape effects. After removal of trend from yield data, the resulting yield residuals were used to analyze the small-scale stochastic variability using variography. The variogram analysis showed strong spatial structure for the yield residuals. The spatial correlation lengths were found to vary from about 40 m for corn to about 90 m for soybean. The range parameter of the variograms showed a significant correlation coefficient of –0.95 with the cumulative growing season rainfall. The total variance of 1995 corn yield was partitioned as 56% trend, 37% small-scale stochastic structure, and 7% as an interaction of both. Yield variance of 1996 corn was about 80% trend and 20% small-scale stochastic structure. Contrary to corn years, the total yield variance for soybean in 1997 was partitioned as about 25% trend and about 75% small-scale stochastic structure. The significant negative correlation of range with rainfall shows that small-scale variability may be controlled by factors induced directly or indirectly by rainfall. More years of data are required to substantiate these relationships. The lack of temporal stability in large-scale and small-scale variation suggest that longer duration yield data analyses are required to understand and quantify the impact of various climatic, and management factors and their interaction with soil properties on delineation of areas under consistent yield patterns before applying variable rate technology

Effect of Soil Surface Submergence and a Water Table on Vegetative Growth and Nutrient Uptake of Corn

Effects of excessive-water stress on corn vegetative growth and nutrient uptake were investigated in environmentally controlled growth chambers. Two excessive-water treatments (soil surface submergence and water table at 15-cm depth) and four excessive-water stress levels (equivalent to 90, 180, 270, and 360 cm-day of stress as defined by SEW30 concept) were imposed at 21 days after emergence. Data on plant growth parameters (i.e., height, leaf area, dry matter, and shoot uptake of N, P, and K) were compared for both water-table positions. Corn plants were significantly larger when a water table was imposed at the 15-cm depth than when the surface was submerged at all excessive-water stress levels. Plant nutrient uptake also was greater when a water table was maintained at 15 cm below the surface than when the surface was submerged. Nutrient uptake decreased significantly with increasing stress level for the submerged-surface treatment, but the trend was not consistent for the water-table treatment

Role of Directly Connected Macropores on Pathogen Transport to Subsurface Drainage Water

Pathogen contamination of water supplies is now considered one of the top water quality issues in the United States and worldwide. Continual application of livestock manure may contribute to nonpoint source pollution by releasing microbial pathogens including bacteria, virus, and protozoa, through runoff and subsurface drainage water to surface and ground water. Many studies have been conducted in the laboratories and fields to understand the preferential flow through macropores. But no experiments in the field have been conducted to examine the breakthrough curve of pathogen and/or Escherichia coliform (E.coli) with directly connected macropores. The objective of this research is to address the transport of pathogens (specifically the indicator organism E. coli) through soils, and more specifically the role of macropores in the transport of E. coli to subsurface drains. A greater understanding and more theoretical modeling approach is needed to understand the role of directly connected macropores on pathogen transport to subsurface drainage