C. C. Truman

Assessing Indices for Predicting Potential Nitrogen Mineralization in Soils under Different Management Systems

A reliable laboratory index ofN availability would be useful for making N recommendations, but no single approach has received broad acceptance across a wide range of soils. We compared several indices over a range of soil conditions to test the possibility of combining indices for predicting potentially mineralizable N (N 0 ). Soils (0-5 and 5-15 cm) from nine tillage studies across the southern USA were used in the evaluations. Long-term incubation data were fit to a first-order exponential equation to determine N 0 , k (mineralization rate), and N 0 * (N 0 estimated with a fixed k equal to 0.054 wk -1 ). Out of 13 indices, five [total C (TC), total N (TN), N mineralized by hot KCI (Hot_N), anaerobic N (Ana_N), and N mineralized in 24 d (Nmin_24)] were strongly correlated to N 0 (r > 0.85) and had linear regressions with r 2 > 0.60. None of the indices were good predictors ofk. Correlations between indices and N 0 * improved compared with N 0 , ranging from r = 0.90 to 0.95. Total N and Hush of CO 2 determined after 3 d (Fl_CO2) produced the best multiple regression for predicting N 0 (R 2 = 0.85) while the best combination for predicting N 0 * (R 2 = 0.94) included TN, Fl_CO2 Cold_N, and NaOH_N. Combining indices appears promising for predicting potentially mineralizable N, and because TN and Fl_CO2 are rapid and simple, this approach could be easily adopted by soil testing laboratories.

Simulated rainfall study for transport of veterinary antibiotics-mass balance analysis.

Occurrence of human and veterinary antibiotics has been reported in various environmental compartments. Yet, there is a lack of information verifying the transport mechanisms from source to environment, particularly the transport of veterinary antibiotics as a non-point source pollutant. A rainfall simulation study was conducted to address surface runoff as a possible transport mechanism of veterinary antibiotics introduced in the field and mass balance was calculated with supplementary surface and depth soil measurement. Seven veterinary antibiotics that are the most abundantly used in agriculture for therapeutic and non-therapeutic (growth-promotion) purposes were examined in this study, including tetracycline (TC), chlortetracycline (CTC), sulfathiazole (STZ), sulfamethazine (SMZ), erythromycin (ETM), tylosin (TYL), and monensin (MNS). Runoff in aqueous and sediment phases was collected every 5 min for 1h with varied rainfall intensity and additional surface (0-2 cm) and depth (2-30 cm) soil samples were collected after rainfall simulation for mass balance analysis. Quantification of antibiotic concentration in all collected samples was based on solid phase extraction (SPE) followed by measurement with high-performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS). MNS showed the highest concentration in runoff aqueous samples (0.22 mg plot(-1)), while ETM showed the highest concentration in runoff sediment samples (0.08 mg plot(-1)). The highest concentration of each applied antibiotic in surface soil samples occurred at different locations. This result might indicate the mobility of these compounds in surface soil varies due to different physicochemical properties among the antibiotics. Further, the analysis results showed that all of the subject antibiotics had penetrated into the subsurface; yet, no residuals were found for STZ, suggesting this compound might have penetrated even deeper into the soil. These results indicate that aqueous or sediment erosion control might reduce the transport of veterinary antibiotics in the environment.

Effect of antecedent soil moisture on splash detachment under simulated rainfall.

Antecedent soil water content is an important variable affecting soil erosion processes and may be responsible for much of the variation in splash and wash erosion rates. The purpose of this study was to determine the effect of antecedent water content on splash detachment and on soil and hydraulic variables that control splash. Five soils ranging in texture from sandy loam to clay were exposed to simulated rainfall with an intensity of approximately 64 mm h−1 for 1 h. Wash, splash, runoff, and infiltration were measured for near-saturated (-0.5 kilopascal) and air-dried soils in 0.14 m2 Al erosion pans. Changes in the soil surface were monitored with a fall-cone device. Prewetting significantly reduced runoff occurring over a 60-minute rainfall period for the Heiden clay only. Prewetting reduced splash detachment rates for all soils except the Miami silt loam, with the greatest reduction for the Heiden clay. Total splash for the air-dried and prewetted Heiden clay was 770.1 and 169.4 g in 60 minutes, respectively. Prewetting increased soil shear strength following 60 minutes of rainfall for all soils. Development of a surface seal controlled the amount of soil detached from the Miami silt loam. Miami silt loam had the highest final shear strength values for air-dried and prewetted soils and the highest difference between initial and final shear strength (Δτ) among prewetted soils. Prewetted wash and splash sediment size was larger than that for air-dried sediments. Antecedent soil moisture conditions prior to rainfall influenced the amount of splash detachment and the physical processes that control amount of splash.

RELATIONSHIPS BETWEEN RAINFALL INTENSITY AND THE INTERRILL SOIL LOSS-SLOPE STEEPNESS RATIO AS AFFECTED BY ANTECEDENT WATER CONTENT

Rainfall intensity (I), antecedent water content, and slope steepness affect soil loss from interrill areas. Objectives of this field study were to (i) test the validity of the assumption that interrill soil loss is a function of I2, (ii) determine effect of antecedent water content on interrill erodibilities (Ki) for two slope steepnesses, (iii) evaluate the slope factor used in the water erosion prediction project (WEPP), and (iv) evaluate the product of I, flow discharge, and slope steepness as a possible interrill erosion model. Four soils, ranging in texture from sandy loam to clay, were exposed to sequences of simulated rainfall at three intensities (50, 70, and 100 mm h−1). Soil loss and runoff were measured from air-dried and prewetted flat plots and from air-dried and prewetted ridged plots. For each plot type (slope steepness), b values for E = alb varied between air-dried and prewetted conditions with no apparent trend. Values of b ranged between 0.91 and 2.05, although most b values did not approach 2.0. Prewetting had no effect on Heiden clay and Cecil sandy loam Ki values for the model E = KiI2, but it decreased Ki values for Miami silt loam and Broughton clay by at least 21 and 31%, respectively. Increased slope steepness associated with ridged plots reduced the degree of surface sealing as indicated by decreased soil strength. Values of Ki for E/Sf = KiI2, where Sf = WEPP slope factor, were different for different slopes. Sf was a function of soil type, antecedent water content, and individual interrill erosion processes controlling sediment delivery.

Rainfall Simulator and Plot Design for Mesoplot Runoff Studies

There is need to evaluate the effects of agricultural production activities on sedimentation, pesticide, and nutrient losses under controlled, simulated rainfall on plots large enough (generally exceeding 50 m2) to incorporate realistic slope lengths and dominant processes that control runoff and sediment yield from “field-size” areas. Therefore, a rainfall simulator system was developed for 600-m2 plots (mesoplots) to evaluate runoff (water and sediment) and agrichemical movement from fields for different tillage practices and chemical applications. The rainfall simulator applies water with irrigation sprinklers spaced 3 m apart on two irrigation laterals arranged 14.6 m apart along the plot length. Runoff and sediment were collected in a V-shaped trough and directed to a flume for measuring and sampling. Simulated rainfall of 25 mm/h with a median drop diameter of 1.52 mm had a coefficient of uniformity of 91 over the plot area. Water was applied and sediment and runoff collected from two mesoplots for six events over a corn-growing season.

Comparison of three measures of resistance of soil surface seals to raindrop splash

Abstract The relationship between rainsplash and three measures of soil strength was investigated for surface soil seals using a laboratory rainfall simulator and erosion pans. A significantly high correlation was found between pocket penetrometer and torvane strengths ( R 2 = 0.49) , and a lower correlation between both pocket penetrometer ( R 2 = 0.24) and torvane ( R 2 = 0.24) strengths and the fall-cone strength was found. Rainsplash was highly correlated ( R 2 = 0.49) with rainfall kinetic energy (KE) divided by the fall-cone shear strength (FC) but was not related to penetrometer and torvane strengths. Addition of a silt content term to the KE/FC linear equation slightly decreased R2. Improvements in the prediction of rainsplash can be achieved by including terms defining the macro- and microtopography, such as slope steepness, surface roughness, and depressional storage.

EFFECTS OF PESTICIDE, SOIL, AND RAINFALL CHARACTERISTICS ON POTENTIAL PESTICIDE LOSS BY PERCOLATION-A GLEAMS SIMULATION

ABSTRACT Potential pesticide loss in soil percolate is influenced by pesticide persistence and sorption by soil constituents (organic matter). Pesticide persistence, expressed as half-life (ti/2), changes with soil depth as microbial activity and soil properties change. Little is known, however, how these changes influence potential pesticide transport out of the root zone. Objectives of this study were to investigate relative differences in potential pesticide losses from the root zone by percolation due to 1) different soil surface and subsurface textures and pesticide ti/2» and 2) interactions between pesticide ti/2 and timing of rainfall after pesticide application. The GLEAMS (Groundwater Loading Effects of Agricultural Management Systems) model and a 50-year historical rainfall record at Tifton, Georgia, were used to simulate pesticide losses by percolation from three soils ranging in surface texture from sand to sandy clay loam. Hypothetical pesticides had surface ti/2 of 5, 15, 30, and 60 d and a range of subsurface ti/2 (2.5-360 d), and were applied to continuous com {Zea maize, L.) at 2 kg ha~ ^ as surface spray at planting each year on 1 April. Simulated pesticide losses by percolation increased with increased surface and subsuiface t|/2» and decreased with increased KQC (adsorption constant based on soil organic matter) values. Potential pesticide leaching was greatest for Lakeland sand and least for Greenville sandy clay loam. Rainfall timing affected simulated pesticide loss by percolation, especially for nonpersistent pesticides. For short pesticide ti/2 (0-5 d), excessive rainfall events within 1 ti/2 were largely responsible for simulated pesticide loss by percolation. Results indicate that changes in pesticide t|/2 in surface and subsurface horizons of different soils influence potential pesticide leaching from the root zone, and models (i.e., GLEAMS) can be used to provide comparative analysis of soil-pesticide-climate interactions. For example, depending on soil type and pesticide K^^ and surface Xyp^ values, potential leaching losses increased two to seven times as subsurface tiy2 increased six times.

Gleams-TC: a two-compartment model for simulating temperature and soil water content effects on pesticide losses

Many pesticide fate and transport models, including the GLEAMS model, overestimate pesticide degradation during its later stages of residence in soil. Except for runoff events shortly after pesticide application, models using equilibrium sorption kinetics often underestimate sediment-transported pesticides. To address concerns about transport of low levels of pesticides by runoff to sensitive ecosystems over annual or longer time cycles, GLEAMS was modified using a two-compartment pesticide-state model and algorithms for adjustment of degradation rates for temperature and soil water contents. Two pesticide pools, labile and nonlabile, were linked using first-order kinetics, with the forward and reverse rate constants between these pools as the only two additional inputs required for the model, GLEAMS-TC. GLEAMS-TC was calibrated successfully using 3 years of field data. Long-term pesticide persistence in soil was represented. Comparing model predictions with observed data, GLEAMS-TC simulated observed pesticide sediment transport, whereas GLEAMS underestimated observed data by a factor of 4. Sensitivity to rates of pesticide exchange between the two pools and impact on runofflosses were demonstrated with 50-year simulations. GLEAMS-TC is a research tool for investigating changes in pesticide state in soils as a function of exposure time, environmental variables, and interrelationships between pesticide degradation, mobility, and fate and transport in the environment.

Conservation tillage to effectively reduce interrill erodibility of highly-weathered Ultisols

Highly weathered Southeastern soils traditionally cropped under conventional tillage systems are drought-prone and susceptible to runoff and soil loss. We quantified differences in infiltration, runoff, soil loss, and interrill erodibilities (Ki) for three soils: Compass loamy sand, Decatur silt loam, and Tifton loamy sand managed under conventional- (CT), strip- (ST), and/or no-till (NT) systems with and without a residue cover (rye [Secale cerale L.]) (+C/-C) and with and without paratilling (+P/-P). Duplicate plots (1 m2 [~10 ft2]) on each tillage treatment received simulated rainfall (50 mm h-1 [2 in hr-1] for two hours). Runoff and sediment yields were continuously measured, and Ki values were calculated from measured data. The Water Erosion Prediction Project (WEPP) model was used to extend experimental data to long-term annual trends. For the Compass soil, NT-C plots increased runoff by as much as 43% and sediment yields by as much as 10-fold compared to NT+C plots. The NT+P+C plots decreased runoff by as much as 70% and sediment yields by 24-fold compared to CT-P-C. For the Decatur soil, NT+P plots decreased runoff by as much as 71% and sediment yields by as much as 2.7-fold compared to NT-P plots. The NT+P+C plots decreased runoff by as much as 73% and sediment yields by as much as 11.8-fold compared to CT-P-C. For the Tifton soil, ST+P+C plots decreased runoff by as much as 44% and sediment yields by as much as 2.7-fold compared to CT-P-C plots. Calculated Ki values for the Compass, Decatur, and Tifton soils were 0.37, 0.40, and 0.24, respectively. Residue cover decreased effective interrill erodibilities (Kieff) values by 11%, 2-fold, and 2.6-fold for the Decatur, Tifton, and Compass soils, respectively; Paratilling decreased Kieff values by 3-fold for the Compass and Decatur soils. The NT and/or ST systems had lower Kieff values than Ki values from corresponding CT-P-C treatments (Compass = 4- to 37-fold; Decatur = 4- to 13-fold; Tifton = 2-fold). Converting from a CT to a NT or ST system reduced predicted runoff (Compass = 1.7-fold; Decatur = 10% to 17%; Tifton = 1.6- to 2.3-fold) and sediment yields (Compass = 10- to 12-fold; Decatur = 6- to 33-fold; Tifton = 7.3- to 12.1-fold). The most benefit of NT or ST, as quantified by the maximum difference in 100-year predicted runoff and sediment yields, was for the Compass (78%) and Tifton (75%) soils for runoff and for the Compass (10.3-fold) and Decatur soils (9.7-fold) for sediment. Conservation tillage systems (NT, ST) coupled with surface residue cover and/or paratilling are effective in reducing runoff and sediment yields from highly-weathered soils by lowering effective Ki values.

Fenamiphos transport, transformation, and degradation in a highly weathered soil

Fenamiphos, a nematicide used on corn and sorghum, quickly oxidizes into two metabolites which have similar activities and toxicities, yet are more mobile and persistent than the parent compound. Given the soil and climatic conditions of the southeastern U.S., fenamiphos and its metabolites could be transported from the application site and contaminate off-site water bodies. A three-year study was conducted to evaluate (1) degradation and transport of the fenamiphos parent (Fp) and its metabolites (sulfoxide, Fx, and sulfone, Fo) from a 0.34 ha field site, and (2) the utility of the GLEAMS (Groundwater Loading Effects of Agricultural Management Systems) model in describing system response and simulating pesticide transport. Each year, fenamiphos was applied at 6.7 kg ha–1 a.i., broadcast and incorporated into the upper 100 mm soil layer before planting each crop. Concentrations of fenamiphos and its metabolites were determined from soil samples taken within the root zone at 50 mm intervals to a depth of 300 mm and from subsurface tile outflow at selected times throughout each sweet corn (Zea mays L.) and hybrid pearl millet (Pennisetum glaucum (L.) R. Br.) growing season. The GLEAMS model was used to simulate runoff, lateral subsurface flow (LSF), and Fp, Fx, and Fo losses from the Cowarts loamy sand. An average of 6 and 21% of the total rainfall + irrigation was measured as runoff and LSF, respectively. GLEAMS model simulations were correlated with measured runoff (R2 = 0.81) and LSF (R2 = 0.89). Field half-lives (t1/2) were determined by comparing observed concentrations in soil by depth and time to those simulated with the GLEAMS model. Average t1/2 values from measured field data were 5, 28, and 14 days for Fp, Fx, and Fo, respectively. For the three-year study, about 6.2% of the total amount of applied fenamiphos (Ftot = Fp + Fx + Fo) was measured in LSF, while less than 0.1% of the applied fenamiphos was measured in surface runoff. Fx was the dominant compound measured and simulated in the root zone and LSF, with 70 to 99% of measured Ftot being Fx. Calibration of the GLEAMS model provided fit of the field data that indicated (1) Fp dissipated rapidly while the two metabolites (Fx and Fo) formed (average Fp t1/2 = 5.5 d); (2) t1/2 values for all compounds remained relatively constant during 1987 and 1988, then numerically decreased in 1989; (3) coefficient of transformation (CT) values for Fx and Fo decreased from 1987 to 1989; and (4) CT values describing transformational changes from Fp to Fx were greater than those describing transformational changes from Fx to Fo. Decreases in t1/2 and CT values for Fp, Fx, and Fo with continued use over the three-year study is characteristic of enhanced microbial degradation.