SUITABILITY OF PEANUT RESIDUE AS A NITROGEN SOURCE FOR A RYE COVER CROP

Leguminous winter cover crops have been utilized in conservation systems to partially meet nitrogen (N) requirements of succeeding summer cash crops, but the potential of summer legumes to reduce N requirements of a winter annual grass, used as a cover crop, has not been extensively examined. This study assessed the N contribution of peanut (Arachis hypogaea L.) residues to a subsequent rye (Secale cereale L.) cover crop grown in a conservation system on a Dothan sandy loam (fine-loamy, kaolinitic, thermic Plinthic Kandiudults) at Headland, AL USA during the 2003-2005 growing seasons. Treatments were arranged in a split plot design, with main plots of peanut residue retained or removed from the soil surface, and subplots as N application rates (0, 34, 67 and 101 kg ha) applied in the fall. Peanut residue had minimal to no effect on rye biomass yields, N content, carbon (C) /N ratio, or N, P, K, Ca and Zn uptake. Additional N increased rye biomass yield, and N, P, K, Ca, and Zn uptakes. Peanut residue does not contribute significant amounts of N to a rye cover crop grown as part of a conservation system, but retaining peanut residue on the soil surface could protect the soil from erosion early in the fall and winter before a rye cover crop grows sufficiently to protect the typically degraded southeastern USA soils.


INTRODUCTION
characteristics (Mitchell & Teel, 1977;Touchton et al., 1984;Oyer & Touchton, 1990;Reeves et al., 1993; In the southeastern USA, legume crop residues Torbert & Reeves, 1996).Typically, legumes are have been evaluated in conservation tillage systems to planted after harvest in the fall, terminated in the improve crop production and enhance soil physical spring, and a summer crop is planted into that residue.
A major benefit usually associated with legumes is the potential reduction in nitrogen (N) fertilizer expenses for subsequent cash crops.
Legume N in symbiosis with Rhizobium bac teria contributes to succeeding non-legume crops upon decomposition of legume top and root material (Bruulsema & Christie, 1987;Touchton et al., 1984).Winter annual legumes, such as crimson clover (Tri folium incarnatum L.) and hairy vetch (Vicia villosa Roth.), are utilized as N sources for summer crops (Touchton et al., 1984;Brown et al., 1985;Reeves, 1994).Sunn hemp, a summer legume, has also been shown to decrease corn N requirements in the south ern USA (Balkcom & Reeves, 2005).In addition, sum mer cash legumes have also been examined as an N source for subsequent crops.Researchers in the U.S. Corn Belt have found that alfalfa (Medicago sativa L.) and soybean [Glycine max ( L.) Merr.], can decrease the fertilizer N requirements of a succeeding corn (Zea mays L.) crop (Bruulsema & Christie, 1987;Bundy et al., 1993;Morris et al., 1993).Although peanut is a legume that is widely grown in the southeastern USA, no previous research has examined the N contribution of peanut residues to a rye cover crop utilized in a con servation system.Therefore, our objective was to com pare the N response and subsequent uptake of selected nutrients for rye grown in a conservation tillage sys tem following removal or retention of peanut residue across four N rates.

MATERIAL AND METHODS
In October 2002, an experiment was estab lished in Headland, AL, USA (85°19'15" W, 31°21'38" N) on a Dothan sandy loam.The experimental area was rotated to a different location each year to utilize pea nut residue from the previous peanut crop, but the ex periment remained on a Dothan sandy loam.Treat ments were arranged with a split-plot structure in a ran domized complete block design (n = 4).Main plots consisted of either retention or removal of peanut resi dues from the soil surface following mechanical har vest of peanut pods.Peanut residue was removed by mechanically raking into windrows and baling the pea nut residue.The average peanut biomass was estimated by weighing the baled residue.A subsample of the residue was dried at 55°C for 72 h and ground to pass a 2-mm screen with a Wiley mill (Thomas Scientific, Swedesboro, NJ)1 then further ground to pass a 1-mm screen with a Cyclone grinder (Thomas Scientific, Swedesboro, NJ) 1 .The peanut residue was analyzed for total C and N by dry combustion in a LECO CN-2000 analyzer (Leco Corp., St. Joseph, MI) 1 .An additional 0.5 g subsample was digested in a 70:30 mixture of nitric and perchloric acid overnight (Hue & Evans, 1986) and analyzed for total P, K, Ca, and Zn using an inductively coupled argon plasma spectrophotom eter (Jarrel-Ash Division/Fisher Scientific Co., Waltham, MA) 1 .A rye cover crop was drilled at 101 kg ha -1 across the experimental area on 20 November 2002, 30 October 2003, and 15 November 2004.Sub plot treatments were N rates (0, 34, 67, and 101 kg N ha -1 ) broadcast-applied in the fall, as NH 4 NO 3 , to the cover crop.Nitrogen was applied to the rye cover crop on 21 November 2002, 14 November 2003, and 3 December 2004.Plot dimensions were 7.3 m wide and 12.2 m long.
Rye biomass production was measured the fol lowing spring, prior to termination, on 23 April 2003, 8 April 2004, and 11 April 2005 by cutting all the aboveground biomass at the soil surface randomly within each plot on a 0.25 m 2 area.Samples were dried at 55 o C for 72 h and weighed to determine total biom ass production.A subsample of the dried rye biomass from each plot was ground, and analyzed for total C, N, P, K, Ca, and Zn using the procedures described above.Total biomass of the rye multiplied by the con centration of selected nutrients was used to determine the uptake of individual nutrients.All response vari ables were analyzed using the MIXED procedure (Littell et al., 1996) and the LSMEANS PDIFF option to distinguish between treatment means (release 9.1; SAS Institute Inc.; Cary, NC).Data were analyzed in relation to year, peanut residue, N rate, and their in teractions as fixed effects in the model, while replica tion, replication • peanut residue, replication • nitro gen, and replication • year were considered random.Single degree-of-freedom contrasts were used to evalu ate linear and quadratic effects of N rates for each re sponse variable.If a single degree-of-freedom contrast indicated a significant linear or quadratic response, the specified regression model was fit with the PROC REG procedure (SAS Institute, 2004).Treatment differences were considered significant if P £ 0.10 a priori.

RESULTS AND DISCUSSION
Peanut residue biomass and selected nutrient concentrations are shown in Table 1.Variability in nu trient concentrations existed among years, however in 2005 the K concentration was 72% lower than the con centrations observed during 2003 and 2004.The N concentration was 14 g kg -1 across all three years of the experiment.This N concentration was comparable to that reported by Balkcom et al. (2004) for post-har vest peanut residue.Based on the average residue pro duction and N concentration, the peanut residue had a total N accumulation of nearly 46 kg ha -1 .This amount represents approximately 50% of the recommended N rate for small grain production in Alabama (Mask et al., 1987).However, the amount required for rye uti lized as a cover crop would be less than the amount for rye to maximize grain production.This measured amount of N could increase rye biomass production and enhance benefits associated with winter cover crops, such as controlling erosion, improving infiltra tion, and increasing organic C inputs (Reeves, 1994).Since much of this peanut residue N is present in the organic form, not all the N would be immedi ately available for plant uptake by the following rye cover crop.Decomposition of the residue by soil mi crobes is required and what portion of the N the mi crobes do not use during the decomposition process will be potentially available for plant uptake and/or N loss pathways (e.g.leaching).Despite the peanut resi due containing significant amounts of N, P, K, and Ca, peanut residue only influenced rye biomass yields, Ca uptake and to a much lesser degree the N concentra tion of the rye cover crop (Table 2).These effects were dependent on the year and N level as indicated by the observed three way interactions.
Biomass levels were different among years within a given N rate, regardless of whether or not they followed peanut residue (Figure 1).Biomass levels also differed across different N rates within years when pea nut residue was retained or removed.Although the three-way interactions were significant, Figure 1 illus trates that peanut residue had little effect on rye bio mass yield compared to the particular growing season and N level applied.This finding was similar for N concentration and Ca uptake.
Interactions were also observed among cer tain variables between years and peanut residue (Table 2).The interaction observed for N concentra tion resulted from an inconsistent N concentration in rye observed across years.During the first two years, the N concentration following peanut residue was lower as compared to removed peanut residue, but was higher the last year of the study (data not shown).The lower N concentration observed following re tained peanut residue indicates that the peanut resi due could have immobilized N, which is supported by the incubation study conducted by Balkcom et al. (2004).However, during the 2005 growing season, N Table 2 -Analysis of variance probabilities following the removal and retention of peanut residues on the soil surface, subsequent N rates, and the interaction between these effects on rye biomass yield, N concentration, N uptake, C/N ratio, P uptake, K uptake and Ca uptake at the Wiregrass Research and Extension Center in Headland, AL USA from 2003-2005.
Source df Rye biomass yield N concentration Nuptake C/Nratio P uptake K uptake Ca uptake Zn uptake concentration was higher following the retention of peanut residue.Since the N concentration and C/N ratio are related due to the relatively constant C con centration of plant tissues, the interaction for C/N ra tio between year and peanut residue was similar to that of N concentration.The yield potential of the rye appeared to in crease each year of the experiment, although the ex periment did not remain in the same location each year (Figure 1).Additional N generally increased rye bio mass levels, and although the response was not con sistent across years or peanut residue levels, additional N above 101 kg ha -1 may have increased rye biomass in some cases.However, it is unrealistic to expect growers to apply high rates of an expensive input, like N, to a cover crop, which will not be harvested for grain.On the other hand, as previously mentioned, po tential benefits associated with cover crops are en hanced as the management of the cover crops in creases.Reiter et al. (2003) reported that cover crop biomass production should be > 4500 kg ha -1 for a high residue cereal crop conservation tillage system in Ala bama.Based on our results, a minimum of 34 kg N ha -1 is required to attain this level of high residue pro duction for conservation systems (Figure 1).Additional N will increase biomass production, but the cost of this N must be weighed against the anticipated benefits of the high residue.Presently, the benefits, associated with an incremental increase in N rate above a speci fied minimum biomass level, required for high residue are difficult to quantify.
Other nutrients also responded to additional N applied in the fall (Table 2).The response of additional N was also linear for other nutrients, except K uptake during the 2004 growing season (Table 3).The reason for increased response of rye biomass and N uptake to additional N would be expected since most crops respond positively to increased N availability.Increases in the uptake of P, K, and Zn are also related.As ad ditional N is applied to rye, growth increased and sub sequent uptake of selected nutrients also increased.As a result, P, K, and Zn uptakes increased as N rate in creased.The minimal effect of peanut residue on rye biomass and nutrient uptake may be attributed to the C/N ratio of the residue (Table 1), which has been shown to indicate the likelihood of N mineralization.Low ratios (i.e.< 20 to 1) result in net N mineraliza- Nitrogen uptake, P uptake, and K uptake, were measured in kg ha -1 , while Zn uptake was measured in g ha -1 .tion, while high ratios (i.e.> 30 to 1) result in net im mobilization of N (Tisdale et al., 1993).The limited response to other nutrients present in the peanut resi due indicates that these nutrients were also not avail able to rye in greater quantities compared to rye grow ing where peanut residue was removed.Also, where peanut residue was removed, peanut roots remained.However, the nutrient contribution of peanut roots to the rye cover crop also appears to be minimal.
Although peanut is a legume, the residue re maining in the field after peanut harvest did not con tribute significant amounts of N to a rye cover crop based on biomass yield over a 3-yr period.As a re sult, N rates applied to cereal cover crops, such as rye, should not be reduced following peanut.As expected, rye did respond positively to additional N applications, but 34 kg N ha -1 was adequate to enhance biomass pro duction to the level required to qualify as a high resi due system on this sandy Coastal Plain soil.However, southeastern peanut producers should retain peanut residue in the field to protect the highly weathered soil surface of Ultisols from erosion and potentially in crease soil organic matter contents, which will improve soil physical and chemical properties.

Figure 1 -
Figure 1-Rye biomass yields measured following the application of four fertilizer N rates to plots with and without peanut residue retained on the soil surface during the 2003-2005 growing seasons at the Wiregrass Research and Extension Center in Headland, AL USA.

Table 1 -
Dry matter peanut residue yield, C/N ratio, concentration, and mass basis of selected nutrients (C, N, P, K, Ca, and Zn) measured after peanut harvest at the Wiregrass Research and Extension Center in Headland, AL USA from2002-2004.
1 Numbers in parentheses represent standard deviations n=4. 2 Concentrations are reported on an ash-free basis.

Table 3 -
Regression equations for N, P, K, and Zn uptake as a function of fertilizer N rate at the Wiregrass Research and Extension Center in Headland, AL USA from2003-2005.