Elizabeth B. Haney

Nutrient delivery from the Mississippi River to the Gulf of Mexico and effects of cropland conservation

Excessive nutrients transported from the Mississippi River Basin (MRB) have created a hypoxic zone within the Gulf of Mexico, with numerous negative ecological effects. Furthermore, federal expenditures on agricultural conservation practices have received intense scrutiny in recent years. Partly driven by these factors, the USDA Conservation Effects Assessment Project (CEAP) recently completed a comprehensive evaluation of nutrient sources and delivery to the Gulf. The modeling framework used in the CEAP Cropland National Assessment, or Cropland CEAP, consists of the Agricultural Policy/Environmental eXtender (APEX) and Soil and Water Assessment Tool (SWAT) models. This CEAP modeling framework was successfully calibrated for flow, sediment, and nutrients at 38 sites and validated at an additional 17. Simulation results indicated that cultivated cropland was the dominant source of nitrogen (N) and phosphorus (P) to both local waters and the Gulf, but this was not true for each water resource region within the MRB. In addition, the results showed that point sources remain significant contributors of P loads, especially in the Tennessee and Arkansas/Red River basins where point source P loads exceeded those from cultivated cropland. Similarly, urban nonpoint sources were significant nutrient sources. The Upper Mississippi, Lower Mississippi, and Ohio basins contributed the largest amounts of nutrients delivered to the Gulf. The high delivery areas near the Mississippi River main stem, from which 87% of N and 90% of P was predicted to reach the Gulf, also coincided with elevated nutrient yields to local waters. Conservation practices established on agricultural lands within the MRB were predicted to have reduced nutrient loads to the Gulf by 20% as compared with a no conservation condition. The results indicate the importance of targeted implementation of conservation practices and consideration of local water and/or Gulf impacts depending on program goal(s). The present application illustrates the value of the Cropland CEAP modeling framework as a useful, science-based tool to evaluate pollutant sources and delivery and effects of agricultural conservation practices.

Soil Organic C:N vs. Water-Extractable Organic C:N

Traditionally, soil-testing laboratories have used a variety of methods to determine soil organic matter, yet they lack a practical method to predict potential N mineralization/immobilization from soil organic matter. Soils with high microbial activity may experience N immobilization (or reduced net N mineralization), and this issue remains unresolved in how to predict these conditions of net mineralization or net immobilization. Prediction may become possible with the use of a more sensitive method to determine soil C:N ratios stemming from the water-extractable C and N pools that can be readily adapted by both commercial and university soil testing labs. Soil microbial activity is highly related to soil organic C and N, as well as to water-extractable organic C (WEOC) and water-extractable organic N (WEON). The relationship between soil respiration and WEOC and WEON is stronger than between respiration and soil organic C (SOC) and total organic N (TON). We explored the relationship between soil organic C:N and water-extractable organic C:N, as well as their relationship to soil microbial activity as measured by the flush of CO2 following rewetting of dried soil. In 50 different soils, the relationship between soil microbial activity and water-extractable organic C:N was much stronger than for soil organic C:N. We concluded that the water-extractable organic C:N was a more sensitive measurement of the soil substrate which drives soil microbial activity. We also suggest that a water-extractable organic C:N level >20 be used as a practical threshold to separate those soils that may have immobilized N with high microbial activity.

Comparison of Wheat Yield Simulated Using Three N Cycling Options in the SWAT Model

The Soil and Water Assessment Tool (SWAT) model has been successfully used to predict alterations in streamflow, evapotranspiration and soil water; however, it is not clear how effective or accurate SWAT is at predicting crop growth. Previous research suggests that while the hydrologic balance in each watershed is accurately simulated with SWAT, the SWAT model over or under predicts crop yield relative to fertilizer inputs. The SWAT model now has three alternative N simulation options: 1) SWAT model with an added flush of N (SWAT-flush); 2) N routines derived from the CENTURY model (SWAT-C); and 3) a one-pool C and N model (SWAT-One). The objective of this study was to evaluate the performance of SWAT-flush, SWAT-C, and SWAT-One as they affect wheat yield prediction. Simulated yields were compared to wheat yields in a 28-year fertilizer/wheat yield study in Lahoma, OK. Simulated yields were correlated with actual 28-year mean yield; however, none of the available N cycling models predicted yearly yields. SWAT-C simulated average yields were closer than other N sub-models to average actual yield. Annually there was a stronger correlation between SWAT-flush and actual yields than the other submodels. However, none of the N-cycling routines were able to accurately predict annual variability in yield at any fertilizer rate. We found that SWAT-C or SWAT-flush are the most viable choices for accurately simulating long-term average wheat yields although annual variations in yield prediction should be taken into consideration. Further research is needed to determine the effectiveness of SWAT-C and SWAT-flush in determining average and annual yield in various farming regions and with numerous agronomic crops.

Re-defining and quantifying inorganic phosphate pools in the soil and water assessment tool

The soil and water assessment tool (SWAT), a large-scale hydrologic model, is used to estimate phosphate (P) loading in streams and water bodies. The labile, active, and stable P pools are currently used to represent P cycling in SWAT; however, these pools are conceptual without any chemical basis. The current structure allows SWAT to reasonably represent P cycling; however, restructuring and incorporation of recent research results should produce more accurate P simulations. This paper presents a redefined SWAT structure using four inorganic soil P pools (water soluble, labile, active, and stable) combined with soil extraction methods to determine initial values for each pool. The redefined structure was selected by examining the chemical characteristics of laboratory tests relative to soil P pools and analyzing the relationships between test results. Water soluble P, labile P and active P are determined using water, H3A, Mehlich 3 extraction, respectively. Stable P is determined with an acid digestion. Redefining SWAT’s inorganic soil P pools based on soil and extractant chemistry will improve the defensibility, credibility, and the accuracy of SWAT P routines in water resource planning, management, and decision making.