iRelying more on biological N~ fixation has been suggested as a way to meet one of the major challenges of agricultural sustainability. A study was conducted to compare the fate of applied legume and fertilizer N in a long-term cropping systems experiment. Nitrogen-15-1abeled red clover (Trifolium pratense L.) and (NH4)2SO4 were applied microplots within the low-input and conventional cropping systems of the Farming Systems Trial at the Rodale Institute Research Center in Pennsylvania. The ~SN was applied to soil and traced into corn (Zea mays L.) in 1987 and 1988. Residual ~SN was also traced into second-year spring barley (Hordeum vulgare L.). Legume and fertilizer ~SN remaining in soil was measured and loss of N was calculated by difference. More fertilizer than legume N was recovered by crops (40 vs. 17% of input), more legume than fertilizer N was retained in soil (47 vs. 17% of input), and similar amounts of N from both sources were lost from the cropping systems (39% of input) over the 2-yr period. More fertilizer than legume N was lost during the year of application (38 vs. 18% of input), but more legume than fertilizer N was lost the year after application (17 vs. 4% of input). Residual fertilizer and legume ~SN was distributed similarly among soil fractions. Soil microbial biomass was larger in the legume-based system. A larger, but not necessarily more active, soil microbial biomass was probably responsible for the greater soil N supplying capacity in the legume-based compared with fertilizerbased system. M ANAGING NITROGEN INPUTS in crop production systems to achieve economic and environmental sustainability is a major challenge facing agriculture. Relying: less on commercial fertilizer N and more on biological N2 fixation by legumes has been suggested as a way to meet this challenge (Keeney, 1982; National Academy of Sciences, 1989). Nitrogen-15 methodology is recognized as a valuable tool for determining the fate and behavior of N applied in the environment (Hauck, 1971(Hauck, , 1982 L'Annunziata and Legg, 1984). Field experiments using SN have studied the recovery of fertilizer N by crops and have documented that use efficiency varies due to a number of factors, including timing and method of N application, tillage method, and climate. A well-managed, firstyear, single-harvested crop recovers between 50 and 70% of applied fertilizer N (Allison, 1966; Stanford, 1973). In addition, 10 to 40% of applied fertilizer N may remain in soil, 5 to 10% may be lost by leaching, and 10 to 30% may be lost to the atmosphere in gaseous forms (Kundler, 1970; Westerman et al., 1972).Studies evaluating the fate of ~SN from legume residues decomposing under field conditions concluded that: (i) <30% of legume N was recovered by a subsequent nonlegume crop; (ii) large amounts of legume N were retained in soil, mostly in organic forms; (iii) total recovery of le- 910gume N in crops and soils after 1 yr averaged 70 to 90%; and (iv) <5% of legume N from the original application was recovered by a second nonle...

Frost‐seeding a legume into an established stand of winter wheat (Triticum aestivum L.) or interseeding a legume into a small grain at planting has potential to provide the benefits of a legume green manure while still allowing for the harvest of a revenue‐producing crop. Field studies were conducted at three Michigan locations to quantify N accumulation by alfalfa (Medicago sativa L.) and red clover (Trifolium pratense L.) frost‐seeded into winter wheat or interseeded with oat (A vena sativa L.) and to evaluate the response of a subsequent corn (Zea mays L.) crop to legume and fertilizer N. Cropping sequences included corn following either wheat, wheat frost‐seeded with a legume, oat, or oat interseeded with a legume. Corn was planted either no‐till following wheat or conventionally (moldboard plow) following oat. Frost‐seeding and interseeding alfalfa and red clover had no effect on small grain yield, and stands of alfalfa and red clover were adequate (>13 plants/sq ft) even though N fertilizer had been applied to the small grains. Nitrogen accumulation did not differ among alfalfa or red clover cultivars, and averaged 80, 50, and 116 lb N/acre for alfalfa; and 96, 99, and 176 lb N/acre for red clover frost‐seeded into winter wheat at the three locations. When interseeded with oat, alfalfa contained an average of 44 lb N/acre and red clover contained an average of 33 lb N/acre prior to fall plowing. Corn response to the frost‐seeded legumes differed among locations due primarily to differences in precipitation during the weeks just prior to and following corn planting. When soil water was adequate, corn grain yields following the small grain seeded with a legume were 4 to 62% greater than following the small grain without the legume. With below‐normal precipitation following corn planting, corn grain yields in the legume systems were reduced by 3 to 27%, primarily due to delayed and reduced emergence. Fertilizer replacement values based on grain yield ranged from 0 to 49 lb N/acre for alfalfa and from 0 to 113 lb N/acre for red clover. Response of corn to the preceding legume differed by year, location, and seeding method.

With the current interest in sustainable agricultural systems, the use of legumes in crop rotations to provide N to subsequent crops is increasing. The objective of this study was to quantify the N contribution from different alfalfa (Medicago sativa L.) plant parts to a subsequent corn (Zea mays L.) crop, various soil fractions, and a 2nd yr spring barley (Hordeum vulgare L.) crop. The study was conducted at two field locations in Michigan, on a Capac loam (fineloamy, mixed, mesic, Aerie Ochraqualf) in East Lansing (EL) and on an Oshtemo sandy loam (coarse‐loamy, mixed, mesic, Typic Hapludalf) at the Kellogg Biological Station (KBS) in Hickory Corners. Alfalfa shoots and roots/crowns labeled with 15N were applied separately to microplots in Fall 1985 and Spring 1986 at a rate equivalent to 112 kg N ha−1. Corn was harvested and soil was sampled from all microplots in Fall 1986 and analyzed for 15N. Corn recovered 17 and 25% of the alfalfa‐15N applied to the loam and sandy loam soils at ELand KBS, respectively. Alfalfa‐15N remaining in soil averaged 46% of the initial input for both locations. Most (96%) of the alfalfa‐15N remaining in soil was recovered in the organic fraction, with microbial biomass accounting for 18% of this recovery. More 15N was recovered by corn and in soil from alfalfa shoots than roots/crowns at both locations, and from spring‐incorporated than fall‐incorporated plant material on the loam soil. Only 1% of the alfalfa‐15N from the original application was recovered by a 2nd yr spring barley crop at both locations

The efficacy of enhanced-efficiency (EE) nitrogen (N) fertilizer formulations in reducing N loss and improving the efficiency of urea-based fertilizer products in forage production is unclear. This study compared ammonium nitrate (AN), urea, four EE urea N formulations [A/-(n-butyl) thiophosphoric triamide (NBPT)-treated urea, NBPT and dicyanamide-treated urea, a polymer-coated urea (PGU), and a maleic-itaconic copolymer-treated urea (MICPU)], urea-ammonium nitrate (UAN), and two EE UAN formulations (NBPT-treated UAN, NBPT and dicyanamide-treated UAN) in forage bermudagrass [Cynodon dactylon (L.) Pers.] production. The experimental design was a randomized complete block design with four replications in each of two sites and 2 yr (2008)(2009). Treatment applications were made at the rate of 168 kg N ha ' spring dormancy-break (ca. 30 April) and after the second harvest (ca. 25 July; total of 336 kg N ha ^ season). Response variables included trapped ammonia (NH3) and forage yield, production efficiency, N concentration, N uptake, recovery of applied N, and nitrate concentration. Urea treated with NBPT reduced NH3 volatilization and, in some situations, increased agronomic response relative to urea. Addition of NBPT produced results similar to AN and UAN, and it was never detrimental relative to untreated urea. The MIGPU treatment was ineffectual relative to urea alone. The PGU reduced NH3 volatilization and improved N concentration in the forage but did not improve other agronomic characteristics. Use of UAN solutions produced results that were generally intermediate in response between urea and ammonium nitrate and were not improved by NBPT addition. Adding NBPT can reduce NH3 volatilization and increase the efficiency of urea, but further research is necessary to understand the limits of this additive.

Broiler production is increasing rapidly in the Southern Coastal Plain, and little research has been conducted to evaluate broiler litter applications on the sandy soils of the region. We conducted a 4‐yr field study to determine optimum rates of broiler litter, its economic value, changes in soil tests to a depth of 90 cm, and effects on pathogens and nematodes. Summer crops were cotton (Gossypium hirsutum L.), pearl millet [Pennisetum glaucum (L.) R. Br.] for grain, and peanut (Arachis hypogaea L.). Winter crops were wheat (Triticum aesitivum L.) and oilseed canola (Brassica napus L.). Litter rates were 0, 4.5, 9.0, and 13.5 Mg ha−1 for each crop. Litter application increased yields of cotton, pearl millet, wheat, and canola and decreased yield of peanut. Average crop value increase from application of a megagram of broiler litter was estimated to be $42 ha−1 yr−1 when the application was made to all crops and $68 ha−1 yr−1 when none was applied to peanut. The mean cost of applied litter was approximately $12 Mg−1. Surface soil P, K, Cu, Zn, and Mn were increased in direct relation to litter rate. Data indicate that it would be prudent to limit applications to about 4.5 Mg ha−1. Litter applications had little effect on soil nematodes, but Rhizoctonia limb rot (Rhizoctonia solani AG‐4) of peanut increased. Lodging of canola, due to Sclerotinia spp., was doubled by any application of broiler litter.