Recent adoption of national rules for organic crop production have stimulated greater interest in meeting crop N needs using manures, composts, and other organic materials. This study was designed to provide data to support Extension recommendations for organic amendments. Specifically, our objectives were to (i) measure decomposition and N released from fresh and composted amendments and (ii) evaluate the performance of the model DECOMPOSITION, a relatively simple N mineralization/immobilization model, as a predictor of N availability. Amendment samples were aerobically incubated in moist soil in the laboratory at 22 degrees C for 70 d to determine decomposition and plant-available nitrogen (PAN) (n = 44), and they were applied preplant to a sweet corn crop to determine PAN via fertilizer N equivalency (n = 37). Well-composted materials (n = 14) had a single decomposition rate, averaging 0.003 d(-1). For uncomposted materials, decomposition was rapid (>0.01 d(-1)) for the first 10 to 30 d. The laboratory incubation and the full-season PAN determination in the field gave similar estimates of PAN across amendments. The linear regression equation for lab PAN vs. field PAN had a slope not different from one and a y-intercept not different than zero. Much of the PAN released from amendments was recovered in the first 30 d. Field and laboratory measurements of PAN were strongly related to PAN estimated by DECOMPOSITION (r(2) > 0.7). Modeled PAN values were typically higher than observed PAN, particularly for amendments exhibiting high initial NH(4)-N concentrations or rapid decomposition. Based on our findings, we recommend that guidance publications for manure and compost utilization include short-term (28-d) decomposition and PAN estimates that can be useful to both modelers and growers.

For many plants growth in elevated CO, leads to reduced rates of photosynthesis. To examine the role that leaf ontogeny plays in the acclimation response, we monitored photosynthesis and some related parameters at short intervals throughout the ontogenetic development of tobacco (Nicofiana fabacum 1.) leaves under ambient (350 p L L-')-and high (950 WL L-')-CO, conditions. l h e pattern of photosynthetic rate over time was similar between the two treatments and consistent with the expected pattern for a typical dicot leaf. However, the photosynthesis pattern in high-C0,-grown tobacco was shifted temporally to an earlier maximum and subsequent senescent decline. Ribulose-l,5-biphosphate carboxylase/oxygenase activity appeared to be the main factor regulating photosynthetic rates in both treatments. Therefore, we propose a new model for interpreting the acclimation response. Lowered photosynthetic rates observed during acclimation appear to be the result of a shift in the timing of the normal photosynthetic stages of leaf ontogeny to an earlier onset of the natural decline in photosynthetic rates associated with senescence.

Winter cover crops hold potential to capture excess NO3− and reduce leaching by recycling nutrients. The objective of this study was to compare winter NO3‐N leaching losses under winter‐fallow and a winter cereal rye (Secale cereale L.) cover crop following the harvest of sweet corn (Zea mays L.) or broccoli (Brassica oleracea var. italica Plenck). Leachate was sampled with passive capillary wick samplers that apply a suction of 0 to 5 kPa to the soil‐pore water and intercept leachate in a pan of known area. Without disturbing the over‐laying soil profile, 32 samplers (0.26 m2) were installed at a depth of 1.2 m in a Willamette loam (fine‐silty mixed mesic Pachic Ultic Argixeroll). The randomized complete‐block split plot design of this cover crop‐crop rotation study (initiated in 1989) has cropping system (winter fallow vs. winter cereal rye) as main plots and three N application rates, ranging from 0 to 280 kg N ha−1 yr−1, as subplots. At the recommended N rate for the summer crops, NO3 leaching losses were 48 kg N ha−1 under sweet corn‐winter‐fallow for winter 1992–1993, 55 kg N ha−1 under broccoli‐winter‐fallow for winter 1993–1994, and 103 kg N ha−1 under sweet corn‐winter‐fallow for winter 1994–1995, which were reduced to 32, 21, and 69 kg N ha−1, respectively, under winter cereal rye. For the first two winters, most of the variation (61%) in NO3− leaching was explained by N rate (29%), cereal rye N uptake (17%), and volume of leachate (15%). Seasonal, flow‐weighted concentrations at the recommended N rate were 13.4 mg N L−1 under sweet corn‐winter‐fallow (1992–1993), 21.9 mg N L−1 under broccoli‐winter‐fallow, and 17.8 mg N L−1 under sweet corn‐winter‐fallow (1994–1995), which were reduced by 39, 58, and 22%, respectively, under winter cereal rye.

Sublethal amounts of 2,4-D [(2,4-dichlorophenoxy) acetic acid] were applied to several vegetable crops in order to determine effect on yield and quality in a simulated drift experiment. Tomato (Lycopersicon esculentum Mill.) and root crops were most sensitive to 2,4-D; as little as 2.1 g/ha of acid distorted tomato fruit shape and elongated radish (Raphanus sativus L.) roots. All root crops were rendered unmarketable by 10.4 g/ha and gross yields were reduced by exposure to 104 g/ha. Yield of peppers (Capsicum frutescens L. var. grossum) was increased by exposure to 2.1 g/ha and severely depressed by 104 g/ha. Cucumber (Cucumis sativus L.) fruit shape was distorted at 11 g/ha and yield was reduced slightly. Lettuce (Lactuca sativa L.), onion (Allium cepa L.), and cabbage (Brassica oleracea L. var. capitata) were least sensitive to 2,4-D. Exposure to 20.8 g/ha did not reduce yields of lettuce or cabbage. Onion yields were reduced by application of 104 g/ha. Bush bean (Phaseolus vulgaris L.) yield was decreased by exposure to 22 g/ha, but potato (Solarium tuberosum L.) yield increased at 16 g/ha. Herbicide residues in the crop foliage were proportional to the degree of exposure. Residue analysis combined with foliar symptoms may be useful in predicting crop damage following 2,4-D drift.