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a Univ. of Florida, Inst. of Food and Agric. Sciences, Dep. of Agronomy, Gainesville, FL 32611 (C. M. Cherr current address: Univ. of California, Graduate Group in Ecology and Dep. of Plant Sciences, Davis, CA 95616)
b Univ. of Florida, Inst. of Food and Agric. Sciences, Dep. of Entomology and Nematology, Gainesville, FL 32611
* Corresponding author (jmscholberg{at}ifas.ufl.edu)
Received for publication January 28, 2005.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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12.2 Mg ha–1 and 172 kg N ha–1 in 14 wk). However, rapid N loss (45–58% within 4 wk after death) occurred after sunn hemp death. Winter GM growth (2.0–4.0 Mg ha–1 and 51–104 kg N ha–1 in 18–20 wk) appeared limited by low LAI response to growing degree days (GDD) and was not affected by previous sunn hemp GM. Sunn hemp residues and living winter legume together contained 120 to 125 kg N ha–1 at time of final sampling before sweet corn planting. Corn rotated with sunn hemp plus winter GM and supplemented with 133 kg synthetic N ha–1 produced marketable ear yields similar to monoculture corn fertilized with 200 kg synthetic N ha–1, but the practical value of this benefit is low. In northern Florida, winter and/or summer GM use may substitute only for a relatively small portion of synthetic N rates.
Abbreviations: C, sweet corn • GDD, growing degree days • GM, green manure • L, winter legume • LAI, leaf area index • PAR, photosynthetically active radiation • SH, sunn hemp • WAD, weeks after death • WAE, weeks after emergence
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Much GM research has been conducted in temperate or tropical regions on relatively fine-textured soils (Cherr et al., 2006b). Results of such studies may not extend to intermediate regions such as northern Florida, where freezing winters kill tropical GM legumes and temperate GM legumes often appear poorly adapted to warm, variable temperatures and sandy soils. Additionally, information about dry weight and N content of GM as they change over a growing season is often lacking. For example, Mansoer et al. (1997) and Karpenstein-Machan and Stuelpnagel (2000) provide two of the few reported GM growth analyses. Few studies, if any, quantify how GM growth is affected by temperature and light levels. Because growing time and weather in on-farm settings often differ from that in research trials, it is difficult for producers to predict potential GM biomass and N accumulation based only on the end-of-season values reported in the literature.
Decomposition and N release generally occur faster for residues with lower C:N and lignin:N ratios and lower polyphenol concentrations (Andren et al., 1992; Vigil and Kissel, 1995; Lomander et al., 1998). Carbon:N and lignin:N ratios are usually lower for legumes compared with nonlegumes and for leaves and flowers compared with stems (Somda et al., 1991). Rapid decay and N loss of leafy legumes in warm, humid weather limits their usefulness as GM for subsequent spring crops in Florida. Cobo et al. (2002) and Collins et al. (1990) showed recalcitrant stem materials capable of slowing decomposition and N release of leaves in homogenized mixtures. Because it is a legume with high accumulation of stem material, sunn hemp may be a leguminous GM capable of retaining much N during decomposition.
Soil incorporation of plant residues may speed decomposition and N release by buffering temperature and water regimes relative to the surface. Mansoer et al. (1997), Schomberg et al. (1994), and Thonnissen et al. (2000) showed more rapid decomposition and N release of soil-incorporated residues compared with residues left on the soil surface. Use of reduced tillage or zero tillage may therefore aid N retention in residues such as sunn hemp. Additionally, legumes may respond to "starter" N available early during their growth (Starling et al., 1998). Therefore, in northern Florida, it is possible that performance of temperate GM legumes growing over the winter may be enhanced when following a summer GM such as sunn hemp. Compared with the use of summer or winter GM alone, such a combined GM sequence may provide greater N benefit to spring-planted economic crops if more N is contained in GM materials at time of economic crop planting.
We investigated use of sunn hemp (grown during summer) and/or a winter legume of blue lupin (winter 2001–2002) and Cahaba white vetch (winter 2002–2003) as N sources for sweet corn in a reduced-tillage system on a sandy soil in northern Florida (i.e., Candler or Lake fine sands). We hypothesized that use of sunn hemp, winter legume, or sunn hemp plus winter legume as GM would result in ear yield benefit for a subsequent crop of sweet corn. We also hypothesized that more N would be contained in GM materials at time of corn planting, and greater ear yield benefit would be provided, with combined use of sunn hemp plus winter legume GM compared with sunn hemp or winter legume alone. Our primary objectives were to determine the amount of ear yield benefit from GM use and to evaluate effects on ear yield when GM was substituted for part of the optimal synthetic N rate. To improve understanding of GM potential in our environment, our secondary objectives were to document GM biomass and N accumulation by tissue fraction during growth, to relate GM dry weight and N accumulation to light interception, and to quantify decomposition and N release by the summer GM over the winter. Evaluation of the effects of GM use on corn root dynamics, soil C and N pools, plant-parasitic root-knot nematodes (Meloidogyne spp.), and weeds are presented elsewhere (Cherr, 2004; Cherr et al., 2006a).
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Treatments consisted of sweet corn planted after: sunn hemp followed by lupin (Tifblue 78, winter 2001–2002) or vetch (Cahaba white, winter 2002–2003, treatment denoted as SH–L–C), sunn hemp followed by winter fallow (denoted as SH–C), summer fallow followed by lupin or vetch (denoted as L–C), or summer and winter fallow (denoted as C). Sunn hemp was planted on 7 Aug. 2001 and 19 July 2002 (with 76 cm between-row spacing) and was killed with paraquat (N,N'-dimethyl-4,4'-bipyridinium dichloride, 1.1 kg a.i. ha–1, 31 Oct. 2001) or glyphosate [N-(phosphonomethyl) glycine, 2.1 kg a.i. ha–1, 30 Oct. 2002]. Lupin was planted on 19 Nov. 2001 with identical spacing as sunn hemp, and was killed with mowing and an application of glyphosate (1.1 kg a.i. ha–1, 12 Apr. 2002). Due to increase of root-knot nematode (Meloidogyne spp.), lupin was replaced with vetch in the second year (see Results and Discussion). Vetch was planted on 15 Nov. 2002 using a grain drill with between-row spacing of 19 cm, and was not chemically killed before sweet corn planting. Sweet corn (variety GS 0966, Syngenta) was planted 26 Apr. 2002 and 7 Apr. 2003 with between-row spacing of 76 cm. Corn population was 5.5 plants m–2 in 2002 and 8.2 plants m–2 in 2003. Because final corn plant population in 2002 appeared somewhat low, and because the same corn seed was used in 2002 and 2003, seeding rate recommended by University of Florida IFAS personnel was revised upward in 2003. All plantings except those of sunn hemp in 2001 and vetch were conducted with a rip-strip planter. All treatments were mowed before corn planting and after corn ear harvest. Weeds were controlled during corn growth with a single postemergence application of glyphosate (2.1 kg a.i. ha–1) in 2002 and single postemergence applications of glyphosate (1.0 kg a.i. ha–1), pendimethalin [N-(1-ethylpropyl)-3-4-dimethyl-2,6 dinitrobenzamine, 65 g a.i. ha–1], and metolachlor [2-ethyl-6-methyl-N-(1'-methyl-2'-methoxyethyl)-chloroacetylanilide, 2.1 kg a.i. ha–1] in 2003.
Sweet corn in each rotational level (SH–L–C, SH–C, L–C, and C) was fertilized with 0, 67, or 133 kg NH4NO3–N ha–1, which represent zero, one-third, and two-thirds the recommended synthetic N rate for sweet corn on sandy soil in Florida (i.e., Candler or Lake fine sands). Two additional C treatments without GM were fertilized with 200 and 267 kg NH4NO3–N ha–1. Synthetic N rates are hereafter denoted as 0N, 67N, 133N, 200N, and 267N, in respective order of their description above. For sweet corn in each treatment, synthetic N was applied as NH4NO3 in three equal applications: at emergence, 3, and 5 WAE. Phosphorus and K were applied to all plots as recommended from soil testing (30 kg P ha–1 and 60 kg K ha–1 on 14 Aug. 2001, and 35 kg P ha–1 and 85 kg K ha–1 on 19 Aug. 2002). All treatments were arranged in four randomized complete blocks with individual plots 7.6 by 8.8 m. Throughout the study, sunn hemp residues were left intact on the soil surface until mowing (immediately before planting sweet corn and after sweet corn harvest only) and strip tillage (during planting of sunn hemp and sweet corn). During planting of winter GM crops, tractor tires pushed roughly half of sunn hemp plants onto the ground. No visible residue loss resulting from wind occurred during the course of the study.
Sampling Procedure and Sample Analysis
During both years, GM plants were sampled from at least six plots in the experiment at periodic times to measure GM dry weight and N accumulation. Each GM crop was sampled at five or six equally spaced time intervals with sunn hemp, lupin, and vetch sampled every 2, 4, and 3 WAE, respectively. Just before termination of GM growth, GM plants were sampled from all plots in the experiment to determine maximum dry weight and N concentration. Dead sunn hemp residue also was sampled from one treatment in each block at 4, 6, 10, 12, and 16 WAD in 2002 and 2, 4, 6, 8, 11, 14, and 18 WAD in 2003. Decomposition was therefore quantified for material in situ, without alterations in size, homogeneity, or moisture content. At each sampling event, plants were sampled from a 61-cm length of row with uniform plant stands. Entire plants were removed after excavation around as much of the root system as possible, generally to a depth of 50 to 60 cm. Very little root mass appeared to exist below this depth for all GM species. All samplings, including samplings of sunn hemp conducted after its death, were conducted with this procedure. Sample plants were separated into leaves, stems, roots, and reproductive tissues (flowers and pods, where existing). Roots were washed clean of soil and debris. Total sample leaf area, and leaf, stem, root, and reproductive (flowers/pods) fresh weights were taken for each sample. Leaf area was determined with an LI-3000 (Li-Cor, Lincoln, NE). Dry weights were recorded for sample tissues after oven drying at 65°C for 72 h. All sample tissues were ground in a Wiley mill (Thomas Scientific, Swedesboro, NJ) to pass through a 2-mm screen, and a thoroughly mixed 5-g portion of each grinding was subsequently stored. Grindings were digested using a modification of the aluminum block digestion procedure of Gallaher et al. (1975), diluted, filtered, and analyzed for total Kjeldahl N at the Analytical Research Lab (Univ. of Florida, Gainesville; USEPA Method 351.2; Jones and Case, 1991).
For each sample LAI was determined by sample leaf area divided by the respective ground area. Daily values for GDD for all GM were calculated as (Taverage – Tbase)(1 d), where Taverage equals average daily temperature (°C). Base temperature (Tbase) estimates from early season leaf number data (following procedure of Sinclair et al., 2004) from sunn hemp (10°C) and lupin and vetch (5°C, Cherr et al., 2005, data not shown) corresponded to values reported by other investigators for similar crops (Qi et al., 1999; Jenni et al., 2000; Shield et al., 2002). Daily values for intercepted PAR were calculated as I0(1 – e–k*LAI), where I0 equals average daily incident PAR and k equals leaf angle coefficient (assumed to be 0.7 for all GM species in the study). Daily values for LAI were estimated graphically from sampled values.
Sweet corn ear harvest was conducted in an inner area of the plot kept free from destructive biomass and soil sampling (roughly 4.6 by 4.6 m), allowing harvest of the central 4.6 m of row length from each of the six inner rows of corn (out of a total of 10 rows in each plot). Representative subsamples of ears from harvest were graded using USDA standards (USDA, 1997) and the same procedure as described above was used to determine total Kjeldahl N concentration of ears.
Statistical Analysis
Analysis of variance (ANOVA) was conducted to evaluate effects of rotation and sweet corn fertilization on GM tissue dry weights and N contents. No ANOVA is reported for the initial crop of sunn hemp because the field was uniformly managed before this crop. For the initial winter GM (lupin), the ANOVA model did not include sweet corn fertilization rate, as no sweet corn had yet been planted. Otherwise, ANOVA model consisted of previous GM crop, previous sweet corn synthetic N fertilization rate, interactions of these effects, and block. Analysis of variance was conducted for dry weight and N content from final GM samplings (immediately before GM termination). Best-fit linear or quadratic models were determined by regression procedure provided by Statistical Analysis Systems (SAS Institute, 2002) to relate LAI of all GM crops to accumulated GDD. Similar procedure was used to relate dry weight and N accumulation of all GM crops to estimates of cumulative intercepted PAR.
To assess the effect of GM use and N application on corn ear yield and ear N content, ANOVA was conducted using marketable ear yield and total ear N content data from all 14 treatments. The ANOVA model included year, treatment, year x treatment interaction, and block. Because year x treatment interaction was significant at 0.05 level for both corn ear yield and ear N content, additional ANOVA was conducted for corn ear yield and ear N content within each year. For both corn ear yield and ear N content, Duncan's Multiple Range Test (
= 0.05) is shown for treatments within each year.
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Decomposition
Sunn hemp was killed after 12 and 14 wk of growth in 2001 and 2002, respectively (Fig. 2). Residue N losses from sunn hemp totaled 58 and 45% of initial N in the first 4 WAD in 2001–2002 and 2002–2003, respectively (Fig. 2B, 2D). Final residue N losses were 115 and 113 kg N ha–1 in 16 wk of 2001–2002, and in 18 wk of 2002–2003, respectively (79 and 65% of initial N content in 2001–2002 and 2002–2003, respectively). Dry weight decomposition losses (44–48% in 16–18 WAD) were much less than those of N, but also demonstrated most losses during the first 2 to 4 WAD (Fig. 2A, 2C). While leaves and flowers decomposed rapidly (complete loss within 6–12 WAD), stem dry weight at all dates after 2 to 4 WAD changed relatively little. Results were similar to those of Mansoer et al. (1997) studying sunn hemp surface decomposition in Alabama, although N losses in our study were somewhat greater. Root dry matter loss was almost as slow as that of stems. Nitrogen content of stems and roots remaining at the final sampling date were roughly 30 and 60 kg N ha–1 in 2002 and 2003, respectively (Fig. 2B, 2D).
Lupin and Vetch
Freezing temperatures occurred on only 12 d during lupin growth (winter 2001–2002; –5.7°C lowest recorded temperature) and on 24 d during vetch growth (winter 2002–2003, –6.6°C lowest recorded temperature, Fig. 1A, 2B). Rainfall during lupin and vetch growth was 22 and 51 cm, respectively (University of Florida, 2004). Lupin and vetch accumulated 4.0 ± 0.2 and 2.0 ± 0.1 Mg ha–1 dry weight (Fig. 4A
, 4C) and 104 ± 6 and 51 ± 6 kg N ha–1 (Fig. 4B, 4D) in 20 and 18 wk of growth, respectively. Similar results have been obtained in Florida (Gallaher, 1991) and New Mexico (Guldan et al., 1996). However, results were much lower than findings in temperate regions with finer soils and longer growing times (Forbes et al., 1970; Abdul-Baki et al., 1996; Ranells and Wagger, 1996; Sainju and Singh, 2001).
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Maximum growth phase of lupin and vetch did not begin until about 12 WAE (Fig. 4A, 4D). Growth time of winter legumes could not be extended as a result of the need to plant sweet corn and for daily maximum temperatures well over 30°C (at which these temperate legumes generally do not survive).
Leaf Area Indices of Green Manures as Functions of Growing Degree Days
Sunn hemp produced maximum LAI values of 3.6 and 6.1 m2 m–2 in 2001 and 2002, respectively. However, neither lupin nor vetch generated LAI greater than 1.5 m2 m–2 (Fig. 3). Growing degree days accumulated for all GM at time of peak LAI were 980 (sunn hemp, 2001), 1190 (sunn hemp, 2002), 1630 (lupin), and 1070 (vetch)°C d. Leaf area index for all GM crops demonstrated quadratic responses to accumulated GDD (Fig. 3, omitted data points represent sample dates at which sunn hemp LAI response to GDD was confounded by leaf senescence at the onset of cold weather.) However, the coefficient of the quadratic response term of sunn hemp LAI was greater than that of the winter legumes by a factor of roughly eight. This resulted in a much more rapid increase of sunn hemp LAI in response to GDD when compared with lupin and vetch. Response of lupin and vetch LAI to GDD were not significantly different from each other. This indicates that LAI of lupin and vetch differed only because of year-to-year variation in temperature (expressed as GDD). Because colder temperatures reduced GDD during the winter of 2003 as compared with the winter of 2002, vetch achieved lower LAI than lupin. Response of sunn hemp LAI to GDD did not differ between years according to regression.
Growth of Green Manures as Function of Intercepted Radiation
Cumulative incident PAR during growth of GM crops was 640 (sunn hemp, 2001), 785 (sunn hemp, 2002), 650 (lupin, before onset of reproduction), and 720 (vetch) MJ m–2 (Fig. 1C, 1D). However, cumulative light interception by lupin (before onset of reproduction) and vetch was only about 130 and 120 MJ m–2, respectively, while that for sunn hemp was about 350 and 510 MJ m–2 (2001 and 2002, respectively; Fig. 5C
, 5D). Although large differences in total plant dry weight and N accumulation existed between GM species and between years when viewed as a function of time after emergence (Fig. 5A, 5B), intercepted PAR explained most variability observed in total plant dry weight and N accumulation between different GM species and growing seasons (Fig. 5C, 5D). Response of dry weight and N accumulation to intercepted PAR did not differ significantly between any GM species or between sunn hemp in different years of growth according to regression.
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Total Nitrogen Contained in Green Manure Material before Sweet Corn Planting
In both years, GM material (including both sunn hemp residues and living winter legume) in SH–L–C contained 120 to 125 kg N ha–1 before sweet corn planting (Fig. 6
). After N release from dead sunn hemp over the winter, GM materials in SH–C treatments contained about 60 to 90 kg N ha–1 less than GM material in SH–L–C. As a result of the differences between lupin growth in 2001–2002 and vetch growth in 2002–2003 (discussed above), winter legumes alone (L–C treatments) provided 115 kg N ha–1 before sweet corn planting in 2002, but only 44 kg N ha–1 in 2003.
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Analysis of variance indicated that dry weight and N accumulation of winter GM legumes were not affected by previous sunn hemp GM, nor was GM affected by differences in sweet corn N fertilization rates in the second year of study (data not shown). As soil N levels were not quantified, we do not know if this occurred because growth and N accumulation of GM legumes did not respond to increased soil N, or if soil N was not retained by the time such legumes would take it up. As a result of very low N retention on sandy soils, soil N testing is generally not recommended in Florida (Sartain, 2001). Given the rapid N release from sunn hemp during decomposition (see below) and the relatively slow growth of lupin and vetch in our environment, it does not appear practical to expect lupin or vetch to take up N released from a previous GM in this environment.
Despite accumulation of a large fraction of stem material, N release from sunn hemp residue was relatively rapid. These N losses amounted to 45 to 58% of initial N in the first 4 WAD and 65 to 79% loss after 16 to 18 WAD. In our study, sunn hemp residues were left largely intact until immediately before corn planting, with about half of the material pushed down onto the ground during planting of the winter GM. In Alabama Mansoer et al. (1997) homogenized sunn hemp residue by mowing and experienced somewhat less N loss (57–67% after 16 wk). However, the lower N loss may have been attributable to colder weather and use of litterbags (for which GM residues were first oven dried). Given the findings of numerous other investigators (for example, Schomberg et al., 1994; Thonnissen et al., 2000), it appears that soil incorporation would unacceptably intensify long-term N loss from overwintering residue in our environment. In Alabama Mansoer et al. (1997) found greater N loss from sunn hemp residue over the winter when it was soil incorporated instead of left on the soil surface.
One to two wk before planting sweet corn, GM materials in SH–L–C treatments contained 120 to 125 kg N ha–1 (Fig. 6). Nitrogen contained in GM materials in SH–C and L–C treatments was lower than that contained in SH–L–C treatments in one or both years. Generally, use of GM had no effect on marketable ear yield in the absence of synthetic fertilizer, and only SH–L–C resulted in consistent increases in marketable ear yields compared with monoculture corn when supplemented with 67 or 133 kg synthetic N ha–1 (Table 1). Although marketable ear yields were not consistently greater with SH–L–C compared with SH–C or L–C treatments, use of SH–C or L–C did not result in consistent ear N content or yield benefit compared with monoculture corn at identical synthetic N rates. Therefore, use of combined summer plus winter GM appears to have advantage over use of summer GM alone or winter GM alone as N source for spring-planted sweet corn in this environment.
Nonetheless, the practical value of GM use was low when compared with C treatments receiving the recommended N rate (200N) or more (267N, Table 1). Compared with C 200N and C 267N, data showed conclusively that sweet corn rotated with any GM and fertilized with no N (0N) or one-third the recommended N rate (67N) produced far lower marketable ears. Although sweet corn in SH–L–C 133N and (in 1 of 2 yr) L–C 133N produced ear yields statistically similar to C 200N, these GM treatments represent only a one-third reduction in synthetic N rate. When marketable corn ear yields were significantly greater for C 267N compared with C 200N (in 2003), no GM treatment produced marketable ear yields similar to C 267N. Ear yields from corn in SH–C treatments and fertilized with 133N were always lower than ear yields in C 200N and C 267N treatments—probably resulting from low N content of sunn hemp residues after winter decomposition (Table 1, Fig. 6).
These findings are consistent with those of Balckom and Reeves (2005), who showed an N replacement value of under 60 kg ha–1 for sunn hemp in field corn production on a Compass loamy sand in Alabama. Nitrogen benefits from GM use in our study were much lower than those found for sweet corn on Lamoine and Elk silt loams in Kentucky and Maine, respectively (Griffin et al., 2000; Cline and Silvernail, 2002).
High N losses from overwintering sunn hemp residue were contrary to our management goals. However, our findings suggest greater N benefit may be provided to economic crops planted immediately after sunn hemp termination or planted into suppressed or dying sunn hemp. If moderate N supply with less residue is desired, killing sunn hemp after interception of roughly 200 MJ ha–1 (achievable in 8–9 wk in our experiment) should yield about 85 kg N ha–1 and 5 Mg dry matter ha–1 (Fig. 4C, 4D) of approximately half stem and one-third leaf by dry weight (Fig. 2A, 2B).
Generally, GM studies make little attempt to evaluate how GM dry weight and N accumulation, as well as relative sizes of tissue fractions, change over time. This information is critical. Even in environments with similar climate and soils, differences in weather or time of GM growth create great variability in GM dry weight and N accumulation and subsequent decomposition. Results comparable with ours could be quite valuable for GM selection and management. Knowing LAI response to GDD for possible GM species in a specific environment, researchers may be able to estimate potential growth and N accumulation for these GM species using predicted values for average daily temperatures and PAR. An understanding of tissue partitioning patterns in GM over time may also be valuable to provide rough estimates for potential rate of N release for GM terminated at different points in a growing season, although this was not investigated in our study.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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In our environment, use of GM could not substitute for all or two-thirds of the recommended rate of synthetic N without significant loss of sweet corn ear yield. Lupin and vetch accumulated relatively low amounts of N, and sunn hemp exhibited rapid N loss during winter decomposition. Use of economic crops planted immediately after sunn hemp death or planted into suppressed/dying sunn hemp may be more appropriate for use of this GM in warm-humid environments on sandy soils.
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ACKNOWLEDGMENTS |
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NOTES |
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The Plant Genome | |||
= roots, 
= stems, 
= leaves, 
= flowers. Error bars reflect standard error values. Black arrows indicate sunn hemp death.
) and 2002 (
), lupin during 2001–2002 (
), and vetch (
) during 2002–2003. Error bars reflect standard error values. Regression models shown are for both years of sunn hemp together (ySH) and lupin and vetch together (yL). Circled values were not included in regression model for sunn hemp as they represent sample dates at which leaf senescence occurred.
