Manure from livestock is an important source of N for crop production in many areas, but efficient management of manure is critical to improve the economics of manure use and to minimize the impact on water quality. A field study was conducted on an Enosburg fine sandy loam (sandy over loamy, mixed, nonacid, mesic Mollic Haplaquent) in northwestern Vermont to evaluate the effect of dairy‐manure and N‐fertilizer application on corn (Zea mays L.) yields and soil profile NO3 in a silage production system. Treatments consisted of a factorial arrangement of manure (0 and 9 Mg ha‐1, dry‐matter basis), N rate (56 and 112 kg ha‐1 as NH4NO3), and time of N application (planting or six‐leaf stage), as well as 0 and 168 kg N ha‐1 rate at planting (with and without manure). Yields and N uptake were increased by N fertilizer and by manure. Without manure, grain and silage yields were increased by fertilizer N to the 112 kg ha‐1 rate in all years; with manure, N fertilizer did not increase yields significantly. Time of application had little or no effect on yield. Plant uptake of N followed a similar pattern but with somewhat more pronounced effects. A presidedress NO3 soil test reflected N availability, as indicated by relative yields. Manure application rates were equivalent, in terms of yield response, to 73 to 122 kg fertilizer N ha‐1 in individual years, which represented 27 to 44% of the total manure N in the year of application. Sampling of the 1.5‐m soil profile before planting and after harvest showed increases in soil NO3 that were related to the amounts of manure and fertilizer N applied. Some decreases in NO3 were measured from fall to spring sampling times, but net losses were minimal where <60 kg ha‐1 NO3‐N was present in the fall. Application of manure resulted in similar or slightly lower soil profile NO3 than agronomincally equivalent rates of fertilizer N.

Measurement of greenhouse gas (GHG) fluxes between the soil and the atmosphere, in both managed and unmanaged ecosystems, is critical to understanding the biogeochemical drivers of climate change and to the development and evaluation of GHG mitigation strategies based on modulation of landscape management practices. The static chamber-based method described here is based on trapping gases emitted from the soil surface within a chamber and collecting samples from the chamber headspace at regular intervals for analysis by gas chromatography. Change in gas concentration over time is used to calculate flux. This method can be utilized to measure landscape-based flux of carbon dioxide, nitrous oxide, and methane, and to estimate differences between treatments or explore system dynamics over seasons or years. Infrastructure requirements are modest, but a comprehensive experimental design is essential. This method is easily deployed in the field, conforms to established guidelines, and produces data suitable to large-scale GHG emissions studies.

Efficient use of N fertilizer for corn (Zea mays L.) production is important for increasing economic return to the grower and for minimizing the potential impact on water quality. Time and rate of application are important management tools for improving N efficiency. This experiment was conducted for 3 yr on two nonirrigated southern Minnesota soils-a Mt. Carroll silt loam (fine-silty, mixed, mesic Mollie Hapludalf) and a Webster clay loam (fine-loamy, mixed, mesic Typic Haplaquoll)-to evaluate the effect of time and rate of N application on corn yield, N uptake, and residual soil N03-N. Nitrogen as (NH 4 hS0 4 was applied in a factorial arrangement of N rate (low and high) and time of application (at planting (PL), eight-leaf stage (SL), or split evenly between the two times (SP)). A zero N control and a very high N rate at PL were also included. Nitrogen rates were 75, 150, and 225 kg ha-• on the Mt. Carroll, and 100, 200, and 300 kg ha-• on the Webster. Grain and total dry matter (DM) yield, and plant uptake of N were increased by N application in five of six site years, in most cases up to the high N rate. Delayed N application (SL or SP vs. PL) resulted in either no effect or a slight decrease in DM and in variable effects on N uptake, depending on the year and location. Residual NO:;-N in the 1.5 m profile ranged from 150 to 400 kg ha-• for most treatments in the fall but was 50 to 70o/o lower the following spring. Residual NO:; in the fall was consistently increased by delayed application of the high N rate from the PL to SL stage, with most of the increase occurring in the upper 0.6 m of the profile. The decrease in residual NO:; from fall to spring, attributed in part to leaching beyond the sampled zone, minimized the potential carryover effect for the next year's production and indicated a potential for greater environmental impact where N application was delayed. Dry matter production, N uptake, and residual N03-N were affected by unusually dry periods in midsummer of all3 yr, especially at the Webster site.

A gronomy J our n al • Volume 101, I s sue 4 • 2 0 0 9 727 ABSTRACT Due to a lack of surface residue and organic matter inputs, continuous corn (Zea mays L.) silage production is one of the most demanding cropping systems imposed on our soil resources. In this study, our objective was to determine if using cover/companion crops and/or applying low-solids liquid dairy manure could improve physical, chemical, and biological soil properties and overall soil quality. Corn was grown for 4 yr on a Bertrand silt loam in rotation with a living mulch of kura clover (KC, Trifolium ambiguum L.) or June-interseeded red clover (Trifolium pratese L.), and continuously with June-interseeded Italian ryegrass (IR, Lolium multifl orum L.), September-seeded winter rye (Secale cereale L.), or no cover crop. Extractable P and K, pH, soil organic matter (SOM), active C, water-stable aggregates, bulk density, penetrometer resistance, and microbial biomass/diversity were measured, and the Soil Management Assessment Framework (SMAF) soil quality index (SQI) was determined. Cover/ companion crop treatments generally had more large macroaggregates, greater aggregate mean-weight diameter, and larger quantities of total microbial biomass and most lipid/microbial groups than no-cover treatments. Manure and starter fertilizer additions resulted in signifi cant cover/companion crop treatment eff ects on extractable P and K. Liquid dairy manure alone did not improve any soil quality indicators. Although soil quality benefi ts of cover crops and manure are typically attributed to additions of organic C, we found no signifi cant treatment eff ects on SOM content. However, the active, or labile, C fraction, was signifi cantly increased by cover crops and showed good relationships with aggregate stability and microbial biomass. Overall, use of cover/companion crops appears benefi cial for corn silage systems, but it may take more than 4 yr for some soil quality indicators to fully respond.

A soil test for nitrogen availabilty to corn (Zea mays, L) has gained wide acceptance in the northeast region of the United States. The test involves sampling the surface 30 cm of soil during the early part of the growing season. The N0 3 -N present at that time is correlated with the probability of obtaining a yield increase by using sidedress nitrogen fertilizer. The test has been evaluated in 272 yr-site N response experiments in Vermont, Pennsylvania, Connecticut, New York, and New Hampshire. The various states in the region that are now offering a N soil test (Vermont, Pennsylvania, Connecticut, and Maine) have different methods of making recommendations based on soil-test levels. However, 20 to 30 mg NO 3 -N/kg soil is about the critical range above which there is a low 1103

Management of fertilizer N on corn (Zea mays L.) can greatly affect the efficiency of N use and the potential for adverse environmental effects. Field studies were conducted on two nonirrigated southern Minnesota soils — a Webster clay loam (fine‐loamy, mixed, mesic Typic Haplaquoll) and a Mt. Carroll silt loam (fine‐silty, mixed, mesic Mollic Hapludalf) — to determine the effect of time and rate of N application on recovery of fertilizer‐derived N (FDN) in corn grain and stover and in the soil. Nitrogen rates of 75 and 150 kg ha‐1 on the Mt. Carroll soil and 100 and 200 kg ha‐1 on the Webster were applied at planting or at the eight‐leaf stage as (NH4)2SO4 to the same plots from 1982 to 1984. Enriched 15N was applied to separate microplots each of the first 2 yr to allow measurement of FDN. Grain yield responded to applied N in 5 of 6 site‐yr, but not to time of application. Uptake of FDN in grain was increased by higher N rate in all cases, but by delayed application in only one site‐year. Total plant FDN recovery ranged from 31 to 60% at the low N rate and from 24 to 45% at the high rate. Both yields and FDN recovery were affected by unusually dry midseason conditions. Fertilizer‐derived N recovery from the soil after harvest ranged from 25 to 56%, with a large proportion at the high N rate in inorganic forms, especially with the late application, which increased the potential for leaching losses. Residual uptake of FDN by grain ranged from 1 to 10% of the initial N rate. The difference method for estimating FDN recovery gave different results from the 15N method, emphasizing the importance of examining both labeled and nonlabeled N pools for complete inter‐pretation of 15N studies.

Ammonia gas is the only significant basic gas that neutralizes atmospheric acid gases produced from combustion of fossil fuels. This reaction produces an aerosol that is a component of atmospheric haze, is implicated in nitrogen (N) deposition, and may be a potential human health hazard. Because of the potential impact of NH3 emissions, environmentally and economically, the objective of this study was to obtain representative and accurate NH3 emissions data from large dairy farms (>800 cows) in Wisconsin. Ammonia concentrations and climatic measurements were made on 3 dairy farms during winter, summer, and autumn to calculate emissions using an inverse-dispersion analysis technique. These study farms were confinement systems utilizing freestall housing with nearby sand separators and lagoons for waste management. Emissions were calculated from the whole farm including the barns and any waste management components (lagoons and sand separators), and from these components alone when possible. During winter, the lagoons' NH3 emissions were very low and not measurable. During autumn and summer, whole-farm emissions were significantly larger than during winter, with about two-thirds of the total emissions originating from the waste management systems. The mean whole-farm NH3 emissions in winter, autumn, and summer were 1.5, 7.5, and 13.7% of feed N inputs emitted as NH3-N, respectively. Average annual emission comparisons on a unit basis between the 3 farms were similar at 7.0, 7.5, and 8.4% of input feed N emitted as NH3-N, with an annual average for all 3 farms of 7.6 +/- 1.5%. These winter, summer, autumn, and average annual NH3 emissions are considerably smaller than currently used estimates for dairy farms, and smaller than emissions from other types of animal-feeding operations.

A number of soil tests have been proposed to predict crop response to added P or to assess potential for soil P loss to runoff waters. A series of four separate experiments were conducted over a 10‐yr period to evaluate soil test methods on a total of 163 Vermont and New York field soils. The experiments included the following: (i) a pot study with alfalfa grown in the greenhouse with 31 soils either unfertilized or fertilized with 18 mg P kg−1; (ii) routine chemical analysis on 54 soils; (iii) a 360‐d incubation study with 24 soils receiving either 0, 20, or 40 mg P kg−1 as CaH2PO4, in which soils were analyzed for desorption and adsorption and the equilibrium P concentration (EPC0); and (iv) another set of 54 agricultural soils incubated with 0 or 40 mg P kg−1 and analyzed for CaCl2, distilled water, and ammonium acetate (Vermont 1)–extractable P (VT1P) and EPC0 Although P extracted by VT1 was significantly correlated with P removed by F extractants, it was better correlated with the ratio of F‐extractable P/Al extracted by either acetate or F. Phosphorus additions increased VT1P, as well as P extracted by acetate + F (Vermont 2 [VT2]), and they decreased reactive soil Al (VT1Al) and P adsorption. The amount of P needed to increase VT1P by a certain amount was directly related to the amount of Al in the VT1 extract. Phosphorus availability to plants, CaCl2‐extractable P, and the EPC0 were all more closely related to VT1P than P extracted by solutions containing F, such as Mehlich 3 (M3), Bray and Kurtz 1 (BK1), and VT2. In a number of instances the ratio VT2P/VT1Al had a better relationship with CaCl2P and EPC0 than did VT1P. Thus, the fraction of reactive Al that has reacted with P (as estimated by VT1P or the ratio of VT2P/VT1Al) appears to be a better indicator of P availability and potential P desorption to runoff water than is P extracted with F.