Legume cover crops can supply all or most of the N required by a subsequent crop if legume biomass is of sufficient quantity and N mineralization is approximately synchronous with crop demand. Three 2-yr crop rotation cycles were conducted on a Lamoine silt loam (fine, illitic, nonacid, frigid Aeric Epiaquept) soil in Maine to (i) evaluate biomass and N accumulation of alfalfa (Medicago sativa L.), winter rye (Secale cereale L.), and hairy vetch (Vicia villosa Roth subsp. villosa) plus winter rye cover crops; (ii) determine sweet corn (Zea mays L.) response to legume and fertilizer N sources in a barley (Hordeum vulgare L.)–sweet corn rotation; and (iii) assess the accuracy of the presidedress soil nitrate test (PSNT) and leaf chlorophyll N test (LCNT) for distinguishing N-responsive and nonresponsive sweet corn. Both legumes accumulated more N than rye grown alone, although total biomass was similar. Sweet corn following rye always exhibited a linear response to N fertilizer (up to 156 kg N ha-1), but generally exhibited no response to added N following either alfalfa or hairy vetch plus winter rye (VR). Both PSNT and LCNT were 75% accurate in identifying plots responsive to additional fertilizer N. The legume cover crops grown were able to replace all or nearly all of the N fertilizer required by a subsequent sweet corn crop, with fertilizer replacement values (FRVs) of 58 to 156 kg N ha-1 in a short-season environment. These cover crops are a viable alternative source of N, greatly reducing or eliminating the need for N fertilizer.
Abbreviations: FRV, fertilizer replacement value • LCNT, leaf chlorophyll N test • PSNT, presidedress soil nitrate test • VR, hairy vetch plus winter rye • DM, dry matter
INTRODUCTION
PRODUCTION systems that utilize N-fixing legumes as a primary N source for subsequent nonlegume crops include full-season green manure crops, interseeded legumes, and cover crops. Each of these options present significant management challenges to producers interested in reducing fertilizer N inputs or producing crops without fertilizer N. For example, full-season green manure crops, while potentially having the greatest impact on soil quality and pest/weed cycles (Altieri, 1995; Biederbeck et al., 1998), may not be economically viable because of the loss of income from that field for an entire growing season. Interseeding legumes with or into a standing crop is common in small grains like wheat (Triticum aestivum L.) and oat (Avena sativa L.). Bruulsema and Christie (1987), Hesterman et al. (1992), and Stute and Posner (1995) demonstrated that this system can result in significant contributions of N to a subsequent corn crop. However, interseeding into a widely spaced row crop like corn or soybean [Glycine max (L.) Merr] may require specialized equipment and additional field operations, and competition between the interseeded and main crops also can be problematic (Kumwenda et al., 1993).
Cover crops, generally grown over the winter between harvest of one crop and planting of a subsequent crop, can overcome at least some of the obstacles associated with green manure and interseeded crops. It is well-documented that cover crops can supply sufficient N for production of a subsequent grain crop with little or no supplemental N fertilizer. McVay et al. (1989) found minimal corn yield response to N fertilizer following hairy vetch or crimson clover (Trifoilium incarnatum L.) cover crops, compared with corn following wheat. Burket et al. (1997) compared sweet corn yield following clover (Trifolium pratense L.), rye, and rye plus pea (Pisum sativum L.) cover crops, finding that both legume cover crops replaced approximately 150 kg fertilizer N ha-1. Cover crop N contributions to vegetable crops also can be substantial. Stivers and Sheehan (1991) evaluated the use of cover crops for tomato (Lycopersicon esculentum var. esculentum), demonstrating that yields were similar using cover crops and fertilizer N sources. There were minimal responses to fertilizer N following legume cover crops. Skarphol et al. (1987) used legume cover crops to supply N for snap bean (Phaseolus vulgaris L.). Bean yields following legume cover crops were similar to yields obtained with 90 kg fertilizer N ha-1 without a legume cover crop.
The use of cover crops in cool, northern climates can present additional production challenges, including: (i) limited opportunity for cover crop seeding and establishment; (ii) potentially small accumulation of cover crop biomass and N, especially if the cover crop is seeded after the main crop harvest; and (iii) the rate of cover crop decomposition and N mineralization, a function of both cover crop composition and temperature, may not keep pace with subsequent crop demand. In fact, these criteria of successful establishment, sufficient biomass and N accumulation, and synchrony between N mineralization and crop demand must be met for a cover crop to be viable in any crop system. The availability of reliable in-season N tests (either tissue- or soil-based) would better allow timely decisions on supplemental N applications following cover crops. Our research addresses these potential constraints to cover crop use in short-season conditions, evaluating cover crop options within a 2-yr barley–sweet corn rotation. The use of a cool-season small grain as the first-year crop allows additional flexibility for cover crop seeding and establishment; cover crops can either be interseeded with the barley or sown after barley harvest. In both cases, there is usually sufficient time for cover crop establishment. Sweet corn, as the test crop, responds to legume and fertilizer N application. It can be grown in a short (90–100 d) season environment, although economic return is usually less for later planted crops. The PSNT (Magdoff et al., 1984; Fox et al., 1989) developed for field corn has been successfully used in sweet corn production (Heckman et al., 1995), while the accuracy of the LCNT (Blackmer and Schepers, 1995) has not been assessed for sweet corn. The specific objectives of our research were to evaluate: (i) alfalfa, winter rye, and VR cover crop biomass and N accumulation; (ii) response of a subsequent sweet corn crop to cover crop and fertilizer N application; and (iii) accuracy of PSNT and LCNT in predicting sweet corn response to cover crop and fertilizer N sources.
Materials and methods
Research was conducted from 1992 to 1995 at the University of Maine Sustainable Agriculture Research Farm in Stillwater, ME (44°56' N, 68°42' W). Soil type was a Lamoine silt loam (USDA-SCS Soil Survey Staff, 1992), on a 0 to 2% slope. Soil nutrient levels in 1992, from analysis by the University of Maine Analytical Laboratory, were: pH 6.3 (1:1, soil:water), cation exchange capacity (CEC) 8.8 cmol kg-1, 9.1 kg P ha-1, 215 kg K ha-1, 323 kg Mg ha-1, and 2276 kg Ca ha-1, determined using a pH 3.0 1 M NH4Ac extractant and inductively coupled plasma emission spectroscopy. Soil organic matter was 45 g kg-1, measured by loss on ignition. Phosphorus was applied as triple superphosphate (0–46–0 N–P–K) and K was applied as KCl (0–0–60 N–P–K), before barley and sweet corn planting in each rotation cycle, according to soil test recommendations from the University of Maine Analytical Laboratory. In 1994 and 1995, Zn (2.25 kg ha-1) was applied prior to planting sweet corn, as soil Zn levels were less than 1.0 mg kg-1 soil. Three rotation cycles (2-yr each) were conducted, beginning in 1992 (Cycle I), 1993 (Cycle II), and 1994 (Cycle III). Barley and cover crop establishment occurred during Year 1 and cover crop incorporation and sweet corn production during Year 2 of each rotation cycle. Calendar dates for field operations and crop and soil sampling are provided in Table 1 and climate information is shown in Table 2 . Barley was seeded at 125 kg ha-1, using a grain drill without packer wheels, at 20 cm row spacing. Alfalfa (cv. Saranac) was broadcast (13.5 kg ha-1) immediately after barley planting on appropriate plots (3.25 by 10.7 m) and the entire experimental area was cultipacked to enhance seed-to-soil contact. Barley was harvested but yields were not measured. Following barley harvest, straw was incorporated using a tractor-mounted rototiller, except in plots interseeded with alfalfa. Winter rye (cv. Aroostook; 112 kg ha-1) and hairy vetch (cv. Madison; 56 kg ha-1) plus winter rye (56 kg ha-1) were then planted using a grain drill, with 20 cm row spacing. Newly planted plots were then cultipacked.
The experimental design was a randomized complete block with four replications. Treatments were arranged factorially, with the two factors being cover crop and N rate applied to sweet corn. Preliminary analysis of data was accomplished via analysis of variance (ANOVA) with main effects (rotation cycle, cover crop, N rate applied to sweet corn) and interactions. Significant interactions between rotation cycle and treatment (cover crop, N rate) led us to present data for each rotation cycle separately. Cover crop biomass and N accumulation were compared using least significant difference (LSD) only if ANOVA indicated significant treatment differences at 
= 0.05. To further partition cover crop and N rate effects on sweet corn, single degree of freedom contrasts were utilized to evaluate response to N fertilizer rate (N linear, N quadratic), compare cover crops (legume vs. rye, alfalfa vs. VR), and evaluate cover crop by N rate interactions. Regression equations for N rate response were developed only if linear or quadratic contrasts were significant within that cover crop system. Regression equations were calculated using treatment means, as they are intended to describe the general relationship between N rate and crop yield. Following Hesterman et al. (1992), FRVs were calculated for legume cover crops only if sweet corn yield following the legume cover crop was significantly higher than yield following winter rye without N fertilizer.
Results and discussion
Preliminary ANOVA using a combined dataset (across rotation cycles) indicated significant cycle by cover crop interactions for most cover crop parameters. For this reason, rotation cycles were analyzed separately. Cover crop biomass in the fall was generally quite small [<0.5>-1; data not shown]. Total cover crop biomass just prior to incorporation in late May ranged from approximately 3.7 to 6.9 Mg DM ha-1 (Table 3) , demonstrating that these cover crops are well adapted to a moderately cool growing environment. Total biomass for rye and VR was similar to that found in more southern locations, including Maryland (Clark et al., 1994; Shipley et al., 1992), North Carolina (Rannells and Wagger, 1996), and Georgia (McVay et al., 1989). In Cycles I and III, VR produced significantly more above-ground biomass than either alfalfa or rye grown alone. In Cycle II, the proportion of hairy vetch in the mixture was very low (<10%> date and harsh winter conditions without prolonged snow cover. As expected, root biomass accumulation by alfalfa was higher than either annual cover crop option (rye, VR).
Cover crop N content (the product of biomass accumulation and tissue N concentration) varied widely, from 52 to 209 kg N ha-1 (Table 4) . In Cycles I and III, both legume cover crops had higher N content than rye grown alone, generally accumulating two to three times as much N. In these two rotation cycles, the N content of alfalfa and VR were similar, as were above- and below-ground tissue N concentration. The N content we observed for alfalfa (105–174 kg N ha-1) is higher than the 20 to 75 kg N ha-1 found by Hesterman et al. (1992), although they incorporated the alfalfa 3 to 4 wk earlier. Stute and Posner (1995) found that dormant alfalfa accumulated 40 to 50 kg N ha-1 during the seeding year when underseeded with oat, but did not provide estimates for spring regrowth immediately prior to incorporation. Hairy vetch herbage, when grown alone, generally contains 35 to 45 g N kg-1 DM (McVay et al., 1989; Skarphol et al., 1987) and mineralizes very rapidly when incorporated into the soil. Growing this legume in mixture with a small grain generally dilutes tissue N concentration for the herbage as a whole. Even with this dilution effect, VR accumulated as much N as alfalfa and had similar tissue N concentration (Table 4). As mentioned above, hairy vetch growth during Cycle II was very limited, thus tissue N concentration and N content were similar to rye grown alone. Rye is a standard cover crop in many areas because of its winterhardiness and tolerance of late planting. As we found in our research, it can produce a substantial amount of biomass as a winter cover crop, generally from 2.5 to 5.5 Mg DM ha-1 in northern climates (Tollenaar et al., 1993, in Ontario; Kuo et al., 1996, in Washington). However, tissue N concentration is commonly 10 to 15 g kg-1 or less, especially after seedhead emergence, so total N accumulation may be low.
Conclusions
Sweet corn following alfalfa or VR generally did not respond to additional N fertilizer, as it did following rye grown alone. Alfalfa and VR cover crops supplied all or nearly all of the N required by sweet corn, with FRV ranging from 58 to 156 kg N ha-1. In one rotation cycle (Cycle II), however, severe winter conditions limited survival of both legume cover crops, resulting in no significant N contribution from the legumes. Both the PSNT and LCNT, using previously established critical values of 25 mg kg-1 (Heckman et al., 1995) and 43.5 SPAD units (Jemison and Lytle, 1996), respectively, could identify responsive sites with an accuracy of about 75%.
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