Predicting Water, Sediment and NO3-N Loads under Scenarios of Land-use and Management Practices in a Flat Watershed

Changes in land-use or management practices may affect water outflow, sediment, nutrients and pesticides loads. Thus, there is an increasing demand for quantitative information at the watershed scale that would help decision makers or planners to take appropriate decisions. This paper evaluates by a modeling approach the impact of farming practices and land-use changes on water discharge, sediment and NO₃-N loads at the outlet of a 51.29 km² watershed of central Iowa (Walnut Creek watershed). This intensively farmed (corn-soybean rotation) watershed is characterized by a flat topography with tiles and potholes. Nine scenarios of management practices (nitrogen application rates: increase of current rate by 20, 40%, decrease of current rate by 20, 40 and 60%; no tillage) and land-use changes (from corn-soybean rotation to winter wheat and pasture) were tested over a 30 yr simulated period. The selected model (Soil and Water Assessment Tool, SWAT) was first validated using observed flow, sediment and nutrient loads from 1991 to 1998. Scenarios of N application rates did not affect water and sediment annual budgets but did so for NO₃-N loads. Lessening the N rate by 20, 40 and 60% in corn-soybean fields decreased mean NO₃-N annual loads by 22, 50 and 95%, respectively, with greatest differences during late spring. On the other hand, increasing input N by 20 and 40% enhanced NO₃-N loads by 25 and 49%, respectively. When replacing corn-soybean rotation by winter wheat, NO₃-N loads increased in early fall, immediately after harvest. Pasture installation with or without fertilization lessened flow discharge, NO₃-N and sediment delivery by 58, 97 and 50%, respectively. No-tillage practices did not significantly affect the water resource and sediment loads. Finally, such realistic predictions of the impact of farming systems scenarios over a long period are discussed regarding environmental processes involved.     

Chestnut Blight: Biological Control by Transmissible Hypovirulence in Endothia parasitica

Hypovirulence in Endothia parasitica is caused by a cytoplasmic determinant that is transferred by hyphal anastomosis in host tissue and in culture. Transmission of this determinant affects the virulence of the fungus to the extent that host invasion by previously virulent isolates is limited.     

BTEX Sorption by Montmorillonite Organo-Clays: TMPA,Adam, HDTMA

Organo-clay can be utilized for the containment of environmentalpollutants originating from waste sites or accidental spills. Abatch study was conducted using organo-clays produced from aWyoming montmorillonite (SWy) and three organic cations(trimethylphenylammonium (TMPA), trimethylammonium adamantane(Adam), and hexadecyltrimethylammonium (HDTMA)) to characterizeBTEX (benzene, toluene, ethylbenzene, o-, m-, p-xylene) sorption. Sorption data were fitted to two models,with Freundlich resulting in greater correlations of the datathan the Langmuir model (R ² at P ≤0.001-0.05). The Freundlich conditional index (n _f),which describes the experimental sorption characteristics,decreased curvilinearly with organic-cation molecular weights,thus suggesting organo-clays with smaller cations had greaterhydrocarbon retention. Sorption of BTEX followed the order ofTMPA > Adam > HDTMA organo-clays. A similar sequence in themagnitudes of log K _d and log K _omsupportedthis finding. Positive log K _om/K _ow valuesfor TMPA and Adam derivatives indicated there was a greaterretention of BTEX by these organo-clays than octanol. The orderof log K _om for SWy-HDTMA, although concentration-dependent, was analogous to the log K _ow order,indicating partitioning was the dominant sorption mechanism forthe HDTMA-clay. Isotherms for SWy-TMPA and SWy-Adam followed aconvex up pattern. In contract, a concave up curvature, notedfor SWy-HDTMA isotherms, was probably caused by a cosorptiveenhancement process resulting from an effective increase in organic matter content of the organo-clay due to furtherhydrocarbon sorption,in concurrence with a decrease inadsorbate activity coefficients. Values of binding affinityconstant, K _f, for SWy-TMPA were consistently higherthan SWy-Adam. The K _f values determined for totalBTEX sorption by TMPA and Adam derivatives were higher thanthose for the individual hydrocarbons. With SWy-HDTMA, the same order was observed for benzene and toluene; however, ethylbenzene and xylenes had greater K _f values thanthat for the BTEX mixture, possibly due to higher partitioningaffinity of the larger alkylbenzenes. With SWy-HDTM, thesequence of K _f values was: ethylbenzene > m-xylene > p-xylene > o-xylene > toluene >benzene. Trends for SWy-TMPA and SWy-Adam were in contrast tothat of the partitioning order, suggesting that adsorption, ratherthan partitioning, was the primary sorption mechanism for thesetwo organo-clays. With respect to the equilibriumconcentrations, the sorbed amounts for total BTEX mixture weregenerally higher than those for the individual compounds. Ascompared to benzene and toluene, the large-size alkylbenzenesshowed greater partitioning affinity due to their high hydrophobicity.     

Relationship Between Six Years of Corn Yields and Terrain Attributes

Crop yield, soil properties, and erosion are strongly related to terrain attributes. The objectives of our study were to examine the relationship between six years of corn (Zea mays L.) yield data and relative elevation, slope, and curvature, and to develop a linear regression model to describe the spatial patterns of corn yield for a 16 ha field in central Iowa, USA. Corn grain yield was measured in six crop years, and relative elevation was measured using a kinematic global positioning system. Slope and curvature were then determined using digital terrain analysis. Our data showed that in the four years with less than normal growing season precipitation, corn yield was negatively correlated with relative elevation, slope, and curvature. In the two years with greater than normal precipitation, yield was positively correlated with relative elevation and slope. A multiple linear regression model based on relative elevation, slope, and curvature was developed that predicted 78% of the spatial variability of the average yield of the transect plots for the four dry years. This model also adequately identified the spatial patterns within the entire field for yield monitor data from 1997, which was one of the dry years. The relationship between terrain attributes and corn yield spatial patterns may provide opportunities for implementing site-specific management.     

Simulated long-term nitrogen losses for a midwestern agricultural watershed in the United States

Adequate knowledge on the movement of nutrients under various agricultural practices is essential for developing remedial measures to reduce nonpoint source pollution. Mathematical models, after extensive calibration and validation, are useful to derive such knowledge and to identify site-specific alternative agricultural management practices. A spatial-process model that uses GIS and ADAPT, a field scale daily time-step continuous water table management model, was calibrated and validated for flow and nitrate-N discharges from a 365 ha agricultural watershed in central Iowa, in the Midwestern United States. This watershed was monitored for nitrate-N losses from 1991 to 1997. Spatial patterns in crops, topography, fertilizer applications and climate were used as input to drive the model. The first half of the monitored data was used for calibration and the other half was used in validation of the model. For the calibration period, the observed and predicted flow and nitrate-N discharges were in excellent agreement with r² values of 0.88 and 0.74, respectively. During the validation period, the observed and predicted flow and nitrate-N discharges were in good agreement with r² values of 0.71 and 0.50, respectively. For all 6 years of data, the observed annual nitrate-N losses of 26 kg ha⁻¹ for the entire simulation were in excellent agreement with predicted nitrate-N losses of 24.2 kg ha⁻¹. The calibrated model was used to investigate the long-term impacts of nitrate-N losses to changes in the rate and timing of fertilizer application. Results indicate that nitrate-N losses were sensitive to rate and timing of fertilizer application. Modeled annual nitrate-N losses showed a 17% reduction in nitrate-N losses by reducing the fertilizer application rate by 20% and switching the application timing from fall to spring. Further reductions in nitrate-N losses require conversion of row cropland to pasture and/or replacement of continuous corn or corn–soybean rotation systems with alternative crops.     

Irrigation management for groundwater quality protection

Deep percolation flow below agricultural and can transport nitrate and pesticide residues to underlying groundwater. Irrigated agriculture in dry climates can also contaminate groundwater with salt from irrigation water and with trace elements such as selenium leached from the vadose zone. Groundwater contamination by agricultural chemicals can be minimized by using best management practices (BMPs) for crop production (including low-input sustainable agriculture or other source control) and for irrigation. Irrigation systems should be designed and managed for zero or minimum deep percolation during the growing seasons to keep fertilizer and pesticides in the root zone as long as possible. At other times, irrigation efficiencies can be lower to produce enough deep percolation water for leaching salts out of the root zone. Because of spatial variability and preferential flow, however, some deep percolation and movement of chemicals may still occur, even if the irrigation efficiency is 100%. BMPs should be developed to minimize such deep percolation flow.     

Transgenic ‘Mailing 26’ apple expressing the attacin E gene has increased resistance toErwinia amylovora

Summary Apple (Malus domestica) transgenic T1 was obtained byAgrobacterium tumefaciens-mediated transformation of Malling 26 rootstock using the plasmid binary vector pLDB 15. pLDB 15 contains within its T -DNA a gene encoding the lytic protein attacin E. The integration of the attacin E gene into the apple genome was confirmed by Southern analysis. Northern analysis indicated the presence of an attacin E mRNA in plants inoculated withErwinia amylovora. After inoculation ofin vitro grown plants of T1, Malling 26, and Malling 7 (resistant control) withE. amylovora, the loglo of the inoculum concentration lethal to 50% of the plants was 5.4, 4.4, and 5.6, respectively. In greenhouse trials for resistance to fire blight, T1 was significantly more resistant than ‘Mailing 26’.     

Evaluation of the Accuracy of a Central Iowa Soil Survey and Implications for Precision Soil Management

The movement towards precision agriculture has led to calls for soil maps that are more detailed and accurate than those offered in standard NCSS soil surveys. Studies have shown that soil variability can be greater than depicted in soil surveys; in fact, delineations that contain at least 50% of the soil mapped are considered satisfactory for soil survey purposes. Lacustrine plains are relatively flat and often have parent materials with uniform properties. Because soils are usually mapped using soil–landform relationships one might expect soil maps in these areas to be less accurate than average; it is difficult to delineate between map units using soil–landform relationships in such subtle landscapes. We grid-mapped a field containing lacustrine-derived soils in central Iowa and used the grid to evaluate the soil survey for accuracy. Two major and two minor soils, as determined by the area they occupy in the field, were present. For the field as a whole, the two major soils were correctly identified by the soil survey at least 63% of the time. The two minor soils were correctly identified 33% of the time or less by the soil survey. Large-scale soil mapping is expensive because of the time involved to create them in the field and in the office. Therefore, it is only economically beneficial to produce a detailed map if the map leads to significant alterations in the way a field is managed. In fields that may have uniform soil properties, it may be more cost-effective to conduct a reconnaissance survey first and then decide if more detailed mapping is required.     

RZWQM simulated effects of crop rotation, tillage, and controlled drainage on crop yield and nitrate-N loss in drain flow

Accurate simulation of agricultural management effects on N loss in tile drainage is vitally important for understanding hypoxia in the Gulf of Mexico. An experimental study was initiated in 1978 at Nashua, Iowa of the USA to study long-term effects of tillage, crop rotation, and N management practices on subsurface drainage flow and associated N losses. The Root Zone Water Quality Model (RZWQM) was applied to evaluate various management effects in several previous studies. In this study, the simulation results were further analyzed for management effects (tillage, crop rotation, and controlled drainage) on crop production and N loss in drain flow. RZWQM simulated the observed increase in N concentration in drain flow with increasing tillage intensity from NT (no-till) to RT (ridge till) to CP (chisel plow) and to MP (moldboard plow). It also adequately simulated tillage effects on yearly drain flow and yearly N loss in drain flow. However, the model failed to simulate lower corn and soybean yields under NT than under MP, CP, and RT. On the other hand, RZWQM adequately simulated lower yearly drain flow and lower flow-weighted N concentration in drain flow under CS (corn-soybean) and SC (soybean-corn) than under CC (continuous corn). The model adequately simulated higher corn yield under CS and SC than under CC. Applying the newly suggested N management practice for the Midwest of controlled drainage, the model simulated a 30% reduction in drain flow and a 29% decrease in N losses in drain flow under controlled drainage (CD) compared to free drainage (FD). With most of the simulations in reasonably close agreement with observations, we concluded that RZWQM is a promising tool for quantifying the relative effects of tillage, crop rotation, and controlled drainage on N loss in drainage flow. Further improvements on simulated management effects on crop yield and N mineralization are needed, however.