To minimize groundwater contamination by NO−3 and to maximize efficient crop N use, it is necessary to better characterize NO−3 leaching in various cropping systems. Our objectives were to compare the contributions of hairy vetch (Vicia villosa Roth) and NH+4 N sources to NO−3 leaching in corn (Zea mays L.) production, and to compare NO−3 leaching losses in a corn production system utilizing hairy vetch as a N source and winter cover crop with leaching losses in one utilizing an NH+4 N source and a rye (Secale cereale L.) cover crop. Nitrogen, sometimes enriched with 15N, was applied to corn grown on a Maury silt loam (fine, mixed, mesic Typic Paleudalf) in lysimeters as vetch‐ or NH+4‐N at rates between 10.5 and 14.0 g N m−2 yr−1. Treatments (N source/cover crop) were: NH+4‐N/fallow, vetch‐N/fallow, NH+4‐N/rye, and vetch‐N/vetch; mean 3‐yr cumulative NO−3 leaching losses were 6.11, 4.85, 0.35, and 2.51 g N m−2, respectively. Total recovery of added 15N in leachate was <5% for all treatments, and the percentage of leached NO−3 derived from labeled N sources was generally <15%. In the winter‐fallow lysimeters, effects of N source on the fraction of water input discharged, concentration of NO−3, and quantity of NO−3 leached varied with year and time of year; differences in N‐source availability and mulch effects were sometimes indicated. Generally, cover crop effects were larger than N‐source effects. The NH+4 N source/rye cover system leached consistently less NO−3 than the vetch N source/vetch cover system, even though the fraction of water discharged was not consistently different.

Three summer squash (Cucurbita pepo L.) cultivars were grown using conventional tillage and no-till soil management practices during 1991 and 1992 in the mountain regions of Georgia. Soil bulk density and N content as well as crop dry weight, leaf area, and yield were monitored to assess the potential for using conservation tillage in squash production. Soil bulk density of the surface (0 to 10 cm) layer under no-till exceeded. that under conventional tillage at planting by 0.25 Mg·m-3, and 1 month after planting by as much as 0.16 Mg·m-3. However, growth-limiting bulk densities (>1.45 Mg·m-3) did not occur. Total soil N to a 30-cm depth was similar for the two tillage regimes. There were no significant cultivar × tillage interaction effects on plant dry weight, leaf area, or crop yield. Total yields were similar for the two tillage regimes; however, early yield during 1991 was 27% less using no-till. There is potential for the use of conservation tillage in summer squash production in the southeastern United States. However, the current lack of registered herbicides for weed control and possible early market price incentives are likely disadvantages to widespread acceptance of such cultural practices.

Long‐term management practices affect the reserve of mineralizable soil N, and so can influence the amount of supplemental N fertilizer required in crop production. This study was conducted to (i) evaluate the residual effects of long‐term N fertilization and winter cover cropping on corn (Zea mays L.) N nutrition, and (ii) examine the ability of selected soil indices to detect management‐induced differences in soil N availability. In 1986, N fertilizer and winter cover crops were eliminated from plots which, from 1976 through 1985, had received varying tillage treatments, N fertilizer additions, and either hairy vetch (Vicia villosa Roth), rye (Secale cereale L.), or no winter cover crop. A history of N fertilization increased corn yield and N uptake (by an average of 20.4 kg N/ha). A history of winter cover cropping with hairy vetch increased corn yield and N uptake (by an average of 28.0 kg N/ha). Rye cover cropping generally had small or inconsistent effects relative to no cover crop. Tillage generally had insignificant effects on corn yield and N uptake. Soil N availability indices were determined on surface samples (0–15 cm) taken 2 wk after corn planting. The anaerobic incubation provided a poor index of N availability. Total soil C and Kjeldahl N were affected by tillage, though not by cover crop or fertilization history, and were marginally correlated with crop response. The autoclave index was only slightly superior to total soil C and Kjeldahl N as a N‐availability index. The soil NO3‐N concentration was highly correlated with corn yield, and N uptake. Though this study was conducted for 1 yr at one site, results indicate that measurement of surface soil NO3‐N made shortly after corn planting can provide a valid index of the effects of past crop and soil management practices on soil N availability to corn.

No abstract

Jabro et al., 1993; Jemison et al., 1994; Lengnick and Fox, 1994). Most of the evaluation studies have cali-Nitrate leached from soils can contaminate drinking water and brated a model with an individual data set and then pose a health risk at concentrations Ͼ 10 mg N L Ϫ1 . Computer models conducted a validation with additional independent data may be useful management tools for estimating NO 3 leaching, but they need to be calibrated and validated before use. The objective sets. Calibration of the model has been typically accomof this work was to calibrate and validate LEACHN to simulate soil plished by adjusting input parameters and rate constants NO 3 , soil NH 4 , water drainage, and NO 3 leaching in a Cecil sandy so that output values are as close as possible to measured loam (fine, kaolinitic, thermic Typic Kanhapludults). The calibration values in the calibration data set. This approach to testwas done by determining rate constants and parameters under laboraing a model has the disadvantage of requiring a calibratory conditions. The validation data was obtained from a two-year tion data set. In addition, the approach can be very timestudy with conventionally tilled corn (Zea mays L.) during summer consuming, especially if the range of values explored for and either a rye (Secale cereale L.) cover crop or fallow conditions each input parameter and rate constant is relatively during winter. Water drainage collected by tiles was automatically large. Furthermore, since this process adjusts several measured, subsampled, and analyzed for inorganic N concentrations.input parameters and rate constants at the same time, During the cold season, LEACHN underestimated soil NH 4 and NO 3 in at least half of the cases. During the warm season, the model

Further improvement of residue decomposition models may help to optimize the use of N released from cover crop residues. CERES models are widely used to simulate crop‐soil systems and have a common subroutine that describes N dynamics (CERES‐N). In this work, we tested CERES‐N with results from a litterbag study over 120 d in which crimson clover (Trifolium incarnatum (L.), rye (Secale cereale( L.), wheat (Triticum aestivum( L.), and oat (Avena sativa L.) residues were allowed to decompose on the soil surface. The field data were used to (i) evaluate the original CERES‐N and a modification of CERES‐N that allows users to enter actual sizes for the carbohydrates, cellulose, and lignin pools and (ii) determine the best‐fitting rate constants for decomposition of the carbohydrates and cellulose pools in the original CERES‐N and in CERES‐N with user‐supplied pool sizes. CERES‐N with the original rate constants or with rate constants and parameters proposed by other workers either underestimated (root mean square error RMSE = 1.23 g N m−2) or overestimated (RMSE = 2.18 g N m−2) N remaining in the crop residues after 120 d of decomposition. CERES‐N with user‐supplied pool sizes improved the simulation of N remaining after 120 d (RMSE = 0.69 g N m−2), but the simulated values generally tended to overestimate the field data. The best‐fitting rate constants for carbohydrates and cellulose pools were 0.67 and 0.021 d−1, respectively, for the original CERES‐N (RMSE = 0.77 g N m−2), and 0.17 and 0.011 d−1 for CERES‐N with user‐supplied pool sizes (RMSE = 0.55 g N m−2). Future work should further evaluate these fitted rate constants under field conditions.

Modification of tillage and herbicide use patterns to reduce adverse environmental effects on soybean [Glycine max (L.) Merr.] production may impact arthropod populations and damage in soybean. The objectives of this field study were to examine the effect of reduced tillage and herbicide use on populations of foliage‐inhabiting phytophagous and beneficial arthropods and their damage in soybean. Tillage treatments of no‐tillage, chisel‐plow with disking, and moldboard plow with disking and herbicide regimes of high (preplant and postemergence control), reduced (postemergence control), and no herbicide use were established in a 2‐yr rotation of corn (Zea mays L.), winter wheat (Triticum aestivum L.), and soybean. Phytophagous and predatory arthropods were sampled from soybean foliage with the shake‐cloth technique throughout the season in 1988, 1990, and 1992. Tillage treatments did not consistently affect any arthropod groups except stink bugs (Pentatomidae), which were more abundant in no‐tillage than plow‐tillage treatments, and bigeyed bugs (Geocoris spp.), which were more abundant in both plow‐tillage treatments than in no‐tillage. Herbicide regime also had no consistent effect on abundance of any taxa. Additionally, tillage and herbicide regimes had little consistent effect on soybean lepidopteran defoliation and pod damage caused by the bean leaf beetle [Cerotoma trifurcata (Forster)] over the 3 yr. There were extensive stand losses due to rodent damage (cotton rat; Sigmodon hispidis Say & Ord) in the no‐tillage, no‐herbicide system in 1992, where crabgrass [Digitaria sanguinalis (L.) Scop.] was prevalent. These results suggest that tillage and herbicide use practices can be modified without greatly affecting arthropod populations and their damage in soybean. However, these results imply that the potential for managing arthropod populations by modified tillage and herbicide use is limited in soybean.

No abstract