Explore the vast range of titles available.

Precision agriculture has mostly emphasized variable-rate nutrients, seeding, and pesticide application, but at several research sites, variable-rate irrigation equipment has been developed to explore the potential for managing irrigation spatially. The modifications to commercial machines are relatively straightforward, but costly; thus economic analyses have not been positive at current grain price: water cost ratios. However, with increased attention to conservation of water during drought, with increased contention for environmental, recreational, municipal, and industry use, or with regulatory constraints, conclusions regarding profitability or desirability of variable-rate irrigation may change. The objectives of this paper are to: 1) define and describe site-specific irrigation, 2) discuss the opportunities for conservation using site-specific irrigation, 3) present case studies from production and research fields that illustrate these opportunities, and 4) discuss critical research needs to fully implement precision irrigation and thus realize these opportunities for conservation. The opportunities for conservation discussed include situations where non-cropped areas exist in a field for which irrigation can be turned completely off, situations where a reduced irrigation amount provides specific benefits, and finally, situations where optimizing irrigation amount to adapt to spatial productivity provides quantitative benefits. Results from the case studies provide estimates of the potential for water conservation using precision irrigation that range from marginal to nearly 50 percent in single years, and average from eight to 20 percent, depending on the previous irrigation management strategy employed. Critical research needs include improved decision support systems and real-time monitoring and feedback to irrigation control.

Core Ideas Estimates of NDVI in upland cotton were influenced by field heterogeneity. Row–column design and spatial analysis improved the precision of NDVI estimates. Extensive rank changes in genotype mean estimates were observed across models. Controlling for experimental error attributable to field heterogeneity is important in high‐throughput phenotyping studies that enable large numbers of genotypes to be evaluated across time and space. In the current study, we compared the efficacy of different experimental designs and spatial models in the analysis of canopy spectral reflectance data collected on upland cotton (Gossypium hirsutum L.). Canopy spectral reflectance, as measured by normalized difference vegetation index (NDVI), was measured at first bloom on three upland cotton performance trials conducted in Florence, SC, during 2014 and 2015. The relative efficiency and estimates of genotype effects were compared among randomized complete block, an α‐lattice incomplete block, row–column incomplete block, nearest neighbor adjusted, and spatially correlated error models. The row–column model provided the greatest improvement in the precision of genotype effect estimates compared with the randomized complete block model. Genotype rankings based on NDVI varied substantially between the randomized complete block and alternative models, particularly at 5 and 10% selection intensities. These results suggest that the use of more complex experimental designs and spatial analyses should be routinely considered to minimize experimental error due to field heterogeneity and improve the precision and reliability of traits measured using high‐throughput phenotyping systems. These findings also indicate that further research into the effects of field heterogeneity on the relationship between NDVI and lint yield in upland cotton is warranted.

Nonpoint source N from riverine origin is a major water quality problem throughout the world. Nitrogen removal from a contaminated (6.6 mg L-1, NO3(-)N) stream was evaluated in this study using an in-stream wetland (ISW). The ISW was established at the exit of a 425-ha USDA Water Quality Demonstration watershed in the Coastal Plain of North Carolina. It ranged in depth from about 0.2 to 2 m, and it was <1% (3.3 ha) the size of the watershed. The ISW dramatically lowered mean stream NO3-N from 6.6 to 2.0 mg L-1. Nitrate-N mass removal was highly correlated to inflow NO3-N (r = 0.93) in the warmer months when biological processes were more active. Ammonia-N mass removal was opposite that of NO3-N. It was highly correlated to inflow NH3-N (r = 0.81) during the cooler months. Removal of both NO3-N and total-N (NO3-N + TKN) were positively correlated to temperature with r values of 0.77 and 0.62, respectively. Total annual N removal for the ISW was approximately 3 kg ha-1 d-1, which was about 37% of the inflow N. The ISWs appear to be very good landscape features for mitigating excess nonpoint source N in the southeastern Coastal Plain of the USA. As such, they are a good complement to other best management practices for improved water quality.

Riparian zones are an important conservation practice because they can decrease the entry of sediments and nutrients into sensitive aquatic ecosystems. We evaluated the effectiveness of a Coastal Plain riparian zone in decreasing the movement of phosphorus (P) into a black water stream from an overloaded swine manure spray field. Soil P concentrations (Mehlich 3 P, M3P; and total P, TP) were measured in a spray field, grass strip, mid-riparian, and stream edge continuum. Dissolved P (DP) was measured in ground water wells located in the spray field, grass strip, and stream edge and in in-stream grab samples. The spray field and grass strip areas had high soil M3P concentrations. Low M3P concentrations were detected in soils in the mid-riparian and stream edge areas, indicating effective retention of P by the grass strip area. Elevated DP concentrations were detected in the spray field and grass strip wells, while stream edge wells were low. The riparian zone contributed to decreased DP concentrations between the grass strip and stream edge wells. Furthermore, stream grab samples were consistently low in DP concentrations. We conclude that a riparian zone can effectively limit the movement of P-enriched sediments and prevent DP-enriched ground water from entering a local stream, even in a heavily loaded situation.

Agricultural nonpoint source pollution (NPS) is a major water quality concern throughout the USA and the world. Concerns over agricultural NPS are heightened where intensive agricultural operations exist near environmentally sensitive waters. To address these environmental concerns, a Water Quality Demonstration Project involving federal, state, and local agencies; private industry; and local landowners was initiated in 1990 on the Herrings Marsh Run (HMR) watershed in the Cape Fear River Basin in Duplin County, NC. Best Management Practices (BMPs) to reduce nutrient losses to the environment included nutrient and animal waste management plans, soil conservation practices, and an in-stream wetland (ISW). Stream nitrate-N and ammonia-N were measured at the watershed outlet and at three subwatersheds outlets from 1990-1998 to evaluate the effectiveness of the BMPs. The project was divided into pre-ISW (September 1990-May 1993) and post-ISW (June 1993-December 1998) time periods because the majority of the BMPs were implemented at the time of the ISW establishment. Post-ISW stream nitrate-N concentrations were significantly reduced on the HMR watershed (56%) and on each of the three subwatersheds (4% to 56%). The HMR watershed nitrate-N concentrations were reduced from 2.01 to 0.88 mg/L. One subwatershed had stream nitrate-N concentrations reduced from 5.63 to 2.74 mg/L. Nitrate-N mass export from the HMR watershed was significantly reduced on an annual basis from 7.14 to 3.88 kg/ha. Ammonia-N concentrations and mass export from the HMR watershed were unchanged from the pre- to post-ISW periods. The results of this study indicate that the implemented BMPs were effective in reducing nitrogen loss from the HMR watershed.

Waste handling systems for confined swine production in the upper South (approximately 32-37° N and 79-93° W) depend mainly on anaerobic lagoons and application of the waste effluent to cropland. The main objective of this study was to evaluate the quality of 'Coastal' bermudagrass [Cynodon dactylon (L.) Pers.] hay receiving effluent generated from a raw swine waste treatment system designed to reduce P and K concentrations and delivered by subsurface drip irrigation (SDI) compared with hay produced from commercial N fertilizer. Eight treatments, consisting of commercial N fertilizer or effluent, each irrigated at two irrigation rates (75 and 100% of estimated evapotranspiration) and two lateral spacings (0.6 and 1.2 m), were compared with a control treatment of commercial N fertilizer without irrigation. Three harvests were taken in each of 2 yr and five of the six evaluated using wether sheep (30-45 kg). Greatest dry matter intake (DMI) per unit body weight occurred for the control vs. all irrigated treatments (1.94 vs. 1.77 kg 100-1 kg; P = 0.02; SEM = 0.11). Among irrigated treatments, DMI was greatest from commercial N vs. effluent (1.81 vs. 1.71 kg 100-1 kg; P = 0.05; SEM = 0.11). Dry matter intake was similar for the 75% rate treatments and the non-irrigated treatment (mean, 1.87 kg 100-1 kg) but was reduced for the 100% rate (1.94 vs. 1.72 kg 100-1 kg; P = 0.03; SEM = 0.11). Hay from the 75% rate was more digestible than hay from the 100% rate (527 vs. 508 g kg-1; P = 0.03; SEM = 21). The SDI system functioned well, and lateral spacing did not alter hay quality. Treated waste from a raw waste treatment system was readily delivered by SDI at the recommended rate to produce bermudagrass hay of adequate quality for ruminant production systems.

Denitrification in constructed wetlands can be very important in the treatment of swine lagoon effluent when land application areas are limited. The objectives of this investigation were to determine (i) the denitrification enzyme activity (DEA) in the marsh sediments of marsh-pond-marsh (MPM) constructed wetlands, (ii) changes in DEA with additions of carbon and nitrate, and (iii) the response of DEA to different wastewater N loading rates. Swine wastewater was applied to six MPM wetlands located at North Carolina A&T State University, Greensboro, NC, at rates of 4 to 35 kg N ha(-1) d(-1). Soil samples were obtained from the top 25 mm of the marsh sections on four dates for determination of DEA via the acetylene blockage method (blocked at N2O). Headspace N2O was measured via gas chromatography. In the control treatment, they ranged from 0.06 to 1.13 and 0.16 to 0.79 mg N2O-N kg(-1) soil hr(-1) in the first and second marshes, respectively. In both marshes, the DEA rate was significantly increased with the addition of nitrate but not by glucose, indicating that nitrate was a clear limiting factor for denitrification. The DEA in both the control and the amended treatments increased dramatically with increased wastewater N loading, and the increases were generally more pronounced in the first marsh. The DEA values produced in the absence of acetylene blockage did not increase with wastewater N loading rate. Denitrification enzyme activity levels in the marsh sections of the MPM were generally consistent with a highly denitrifying environment.

Riparian zones are recognized as landscape features that buffer streams from pollutants, particularly nitrogen. The objectives of this experiment were to (i) assess denitrification activity within a riparian zone and (ii) determine the influence of physical, chemical, and landscape features on denitrification. This experiment was conducted from 1994 to 1997 in North Carolina on a riparian zone contiguous to a spray field that was heavily loaded with swine lagoon wastewater. Denitrification enzyme activity (DEA) was measured on soils collected from (i) the soil surface, (ii) midway between the soil surface and water table, and (iii) above the water table. The DEA ranged from 3 to 1660 microgram N2O-N kg(-1) soil h(-1). The DEA was highest next to the stream and lowest next to the spray field. Nitrate was found to be the limiting factor for denitrification. The DEA generally decreased with soil depth; means for the surface, middle, and bottom depths were 147, 83, and 67 microgram N2O-N kg(-1) soil h(-1), respectively. These DEA values are higher than those reported for riparian zones adjoining cropland of the southeastern United States, but are lower than those reported for a constructed wetland used for treatment of swine wastewater. Regression analysis indicated that soil total nitrogen was the highest single factor correlated to DEA (r2 = 0.65). The inclusion of water table depth, soil depth, and distance from the spray field improved the R2 to 0.86. This riparian zone possessed sufficient soil area with high denitrifying conditions to be a significant factor in the removal of excess nitrogen in the ground water.

Ammonia (NH3) volatilization is an undesirable mechanism for the removal of nitrogen (N) from wastewater treatment wetlands. To minimize the potential for NH3 volatilization, it is important to determine how wetland design affects NH3 volatilization. The objective of this research was to determine how the presence of a pond section affects NH3 volatilization from constructed wetlands treating wastewater from a confined swine operation. Wastewater was added at different N loads to six constructed wetlands of the marsh-pond-marsh design that were located in Greensboro, North Carolina, USA. A large enclosure was used to measure NH3 volatilization from the marsh and pond sections of each wetland in July and August of 2001. Ammonia volatilized from marsh and pond sections at rates ranging from 5 to 102 mg NH3-N m(-2) h(-1). Pond sections exhibited a significantly greater increase in the rate of NH3 volatilization (p < 0.0001) than did either marsh section as N load increased. At N loads greater than 15 kg ha(-1) d(-1), NH3 volatilization accounted for 23 to 36% of the N load. Furthermore, NH3 volatilization was the dominant (54-79%) N removal mechanism at N loads greater than 15 kg ha(-1) d(-1). Without the pond sections, NH3 volatilization would have been a minor contributor (less than 12%) to the N balance of these wetlands. To minimize NH3 volatilization, continuous marsh systems should be preferred over marsh-pond-marsh systems for the treatment of wastewater from confined animal operations.

Dissolved phosphorus (DP) can be released from wetlands as a result of flooding or shifts in water column concentrations. Our objectives were to determine the long-term (1460 d) DP retention and release characteristics of an in-stream wetland, and to evaluate how these characteristics respond to flooding, draining, and changes in DP concentrations. The studied in-stream wetland drains an agriculturally intensive subwatershed in the North Carolina Coastal Plain region. The wetland's DP retention and release characteristics were evaluated by measuring inflow and outflow DP concentrations, DP mass balance, and DP movement across the sediment-water column interface. Phosphorus sorption isotherms were measured to determine the sediment's equilibria P concentration (EPCo), and passive samplers were used to measure sediment pore water DP concentrations. Initially, the in-stream wetland was undersized (0.31 ha) and released 1.5 kg of DP. Increasing the in-stream wetland area to 0.67 ha by flooding resulted in more DP retention (28 kg) and low outflow DP concentrations. Draining the in-stream wetland from 0.67 to 0.33 ha caused the release of stored DP (12.1 kg). Shifts both in sediment pore water DP concentrations and sediment EPCo values corroborate the release of stored DP. Reflooding the wetland from 0.33 to 0.85 ha caused additional release of stored DP into the outflowing stream (10.9 kg). We conclude that for a time period, this in-stream wetland did provide DP retention. During other time periods, DP was released due to changes in wetland area, rainfall, and DP concentrations.