@article{hong_white_weisz_gumpertz_duffera_cassel_2007, title={Groundwater nitrate spatial and temporal patterns and correlations: Influence of natural controls and nitrogen management}, volume={6}, ISSN={["1539-1663"]}, DOI={10.2136/vzj2006.0065}, abstractNote={To use shallow groundwater NO3–N concentration as an indicator of groundwater quality requires understanding its patterns, correlations, and controls across space and time. Within a study comparing variable‐rate and uniform N management, our objectives were to determine groundwater NO3–N patterns and correlations at various spatial and temporal scales and their association with natural controls and N management. Experiments in a random, complete block design were conducted in a 2‐yr crop rotation in North Carolina that included one variable‐rate and two uniform N management treatments to wheat (Triticum aestivumL.) and corn (Zea maysL.). We measured groundwater NO3–N and depth every 2 wk at 60 well nests, sampling the 0.9‐ to 3.7‐m depth. Field‐mean NO3–N varied with time from 5.5 to 15.3 mg NO3–N L−1These variations were correlated primarily with concurrent changes in water table elevation and depth. Mean NO3–N exhibited two preferred states: high when the water table was shallow and low when the water table was deep. Temporal NO3–N fluctuations greatly exceeded treatment effects. Treatments appeared to affect NO3–N temporal covariance structure. Groundwater NO3–N spatial patterns and correlations were associated mostly with saturated hydraulic conductivity and water table fluctuations and appeared influenced by subsurface lateral flow. When treatment effects became consistently significant later in the study, they overrode natural controls, and NO3–N was spatially uncorrelated or exhibited shorter spatial correlation ranges and patterns associated predominantly with treatments.}, number={1}, journal={VADOSE ZONE JOURNAL}, publisher={Soil Science Society of America}, author={Hong, Nan and White, Jeffrey G. and Weisz, Randy and Gumpertz, Marcia L. and Duffera, Miressa G. and Cassel, D. Keith}, year={2007}, month={Feb}, pages={53–66} }
 @article{duffera_white_weisz_2007, title={Spatial variability of Southeastern US Coastal Plain soil physical properties: Implications for site-specific management}, volume={137}, ISSN={["1872-6259"]}, DOI={10.1016/j.geoderma.2006.08.018}, abstractNote={Our objectives were to describe the field-scale horizontal and vertical spatial variability of soil physical properties and their relations to soil map units in typical southeastern USA coastal plain soils, and to identify the soil properties, or clusters of properties, that defined most of the variability within the field. The study was conducted on a 12-ha field in Kinston, NC. A 1:2400 scale soil survey had delineated three soil map units in the field: Norfolk loamy sand, Goldsboro loamy sand, and Lynchburg sandy loam. These are representative of millions of hectares of farmland in the Coastal Plain of the southeastern USA. Sixty soil cores were taken to ∼ 1-m depth, sectioned into five depth increments, and analyzed for: soil texture as percentage sand, silt, and clay; soil water content (SWC) at − 33 and − 1500 kPa; plant available water (PAW); saturated hydraulic conductivity (Ksat); bulk density (BD); and total porosity. A penetrometer was used to measure cone index (CI) at each sample location. Variography, two mixed-model analyses, and principal components analysis were conducted. Results indicated that soil physical properties could be divided into two categories. The first category described the majority of the within-field variability and included particle size distribution (soil texture), SWC, PAW, and CI. These characteristics showed horizontal spatial structure that was captured by soil map units and especially by the division between sandy loams and finer loam soils. The second class of variables included BD, total porosity, and Ksat. These properties were not spatially correlated in the field and were unrelated to soil map unit. These findings support the hypothesis that coastal plain soil map units that delineate boundaries between sandy loams versus finer loam soils may be useful for developing management zones for site-specific crop management.}, number={3-4}, journal={GEODERMA}, publisher={Elsevier BV}, author={Duffera, Miressa and White, Jeffrey G. and Weisz, Randy}, year={2007}, month={Jan}, pages={327–339} }
 @article{duffera_robarge_mikkelsen_1999, title={Estimating the availability of nutrients from processed swine lagoon solids through incubation studies}, volume={70}, ISSN={["0960-8524"]}, DOI={10.1016/S0960-8524(99)00039-5}, abstractNote={Potential environmental hazards from the excess accumulation of swine (Sus scrofa domesticus) manure in eastern North Carolina and new state guidelines on treatment alternatives have necessitated the reevaluation of best management practices for disposal of swine waste (manure and effluent) as a fertilizer source on local crop land. Creation of a value-added product is one viable means of utilizing and economically redistributing the nutrients in swine manure. Incubation studies using four agricultural soils from eastern North Carolina were conducted with pelletized processed swine lagoon solids (PSLS) (1.7% N, 2.5% P, 0.12% Cu and 0.18% Zn) composed of dewatered swine lagoon sludge plus rock flour. The PSLS was added at three application rates (0, 200, 400 mg N kg−1 soil) and incubated for 16 weeks at 25°C. The soil was sampled at 0, 1, 2, 4, 8, 12, and 16 weeks and analyzed for NO3–N, NH4–N and Mehlich III-extractable P, Zn, and Cu. High concentrations of NH4–N and low concentrations of NO3–N were present in the soils shortly after addition of PSLS. After the second week, extractable NH4–N dropped to <4 mg kg−1, while the soil concentration of NO3–N increased rapidly. The amount of NO3–N generated reached 90% of its final value after the fourth week. Across the four soils, 24–35% of the added N, 15–50% of the added P, 20–50% of the added Zn and 15–20% of the added Cu was extractable after 8 weeks. The PSLS is an excellent source of P, but may require additional N if used as a fertilizer source for most row crops.}, number={3}, journal={BIORESOURCE TECHNOLOGY}, author={Duffera, M and Robarge, WP and Mikkelsen, RL}, year={1999}, month={Dec}, pages={261–268} }
 @article{duffera_robarge_mikkelsen_1999, title={Greenhouse evaluation of processed swine lagoon solids as a fertilizer source}, volume={22}, ISSN={["0190-4167"]}, DOI={10.1080/01904169909365748}, abstractNote={Abstract The highly concentrated nature of the swine (Sus scrofa domesticus) production industry in the Coastal Plain region of North Carolina, along with economic constraints that discourage transport of the swine effluent more than a few kilometers from the site of production, result in frequent and repeated application of swine effluent to the same agricultural fields or pastures. Creation of value‐added products and marketing as a fertilizer is an alternative means of redistributing nutrients that are concentrated regionally. Fertilizer equivalency greenhouse studies were conducted to evaluate the potential for processed swine lagoon solids (PSLS) formulated from dewatered swine lagoon solids and rock flour to serve as a fertilizer source for agricultural crops. Sweet corn (Zea mays L.), sorghum (Sorghum bicolor L.), and field bean (Phaseolus vulgaris L.) were grown for six weeks, and common bermudagrass (Cynodon dactylon L. Pers.) was grown for 18 weeks on a Norfolk sandy loam (fine‐loamy, Kaolinitic...}, number={11}, journal={JOURNAL OF PLANT NUTRITION}, author={Duffera, M and Robarge, WP and Mikkelsen, RL}, year={1999}, pages={1701–1715} }
 @article{duffera_robarge_1999, title={Soil characteristics and management effects on phosphorus sorption by highland plateau soils of Ethiopia}, volume={63}, ISSN={["1435-0661"]}, DOI={10.2136/sssaj1999.6351455x}, abstractNote={Differences in crop and fertilizer management are known to influence P retention by soils. An experiment was conducted to study the effect of soil characteristics and management practices on P sorptoin behavior of the highland plateau soils of Ethiopia. Surface samples from two Vertisols, an Andisol, and an Alfisol were collected from farmers' fields, research station farms, and from non‐cultivated‐non‐fertilized areas. Phosphorus sorption data were obtained by equilibrating 3‐g soil samples in 30 mL of 0.01 M CaCl2 containing various amounts of KH2PO4 Inorganic P fractions were determined by the Hedley P fractionation scheme. There was little variation in P sorption among Vertisol samples of alluvial origin as a result of cultivation‐fertilization practices. For soils of volcanic origin (Vertisol2 and Andisols), and the Alfisol, samples collected from farmers' fields sorbed more P than the non‐cultivated and research station samples. Least amounts of applied P sorbed by the non‐cultivated Andisol samples reflect the relatively large amounts of resin extractable P initially present in these soils and demonstrate that labile P initially present in the soil can influence subsequent P sorption. Stepwise regression analysis of the P sorption data showed that resin P accounts for 81% of the variation in P sorption at 0.2 mg P L−1 in solution. The highest amount of P was sorbed by samples collected from farmers' fields and was mainly due to the practice of continuous cropping with minimal P fertilization, which depletes labile P, and therefore requires higher levels of P fertilization for optimum crop yield.}, number={5}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Duffera, M and Robarge, WP}, year={1999}, pages={1455–1462} }
 @article{duffera_robarge_1996, title={Characterization of organic and inorganic phosphorus in the highland plateau soils of Ethiopia}, volume={27}, ISSN={["0010-3624"]}, DOI={10.1080/00103629609369742}, abstractNote={Abstract Surface horizon samples from two Vertisols, an Andisol, and an Alfisol were collected from farmers’ fields, research station farms, and from uncultivated/ nonfertilized areas to characterize the organic and inorganic forms of phosphorus (P) in the highland plateau soils of Ethiopia using the Hedley soil P fractionation scheme. The total P values ranged from 226 mg‐kg‐1 in the Akaki Vertisol samples developed on alluvial deposits to 1570 mgkg‐1 in the Andisol samples, where the HCl fraction dominates the inorganic soil P pool. The Alfisol samples contained 400 mg‐kg‐1 of total P, with the NaOHand residual P being the dominant P fractions. The resin inorganic phosphorus (Pi) and bicarbonate Pi fractions generally accounted for less than 15% of the total P in all soils, and were positively correlated with organic C. The NaOH P fraction, which was most prominent in the surface horizon samples of the Alfisols, accounted for 4–15% of total P. The HCl P fraction, ranged from 1% in the Alfisols to 46% in...}, number={15-17}, journal={COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS}, author={Duffera, M and Robarge, WP}, year={1996}, pages={2799–2814} }