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Soil Science Society of America Journal 66:523-530 (2002)
© 2002 Soil Science Society of America

DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION

Contributions of Shoot and Root Nitrogen-15 Labeled Legume Nitrogen Sources to a Sequence of Three Cereal Crops

Karl M. Glasener*,a, Michael G. Waggerb, Charles T. MacKownc and Richard J. Volkb

a American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, 900 Second Street, NE, Suite 205, Washington, DC 20002
b Dep. of Soil Science, North Carolina State Univ., Box 7619, Raleigh, NC 27695-7619
c USDA-ARS, Grazinglands Research Lab., 7202 W. Cheyenne Street, El Reno, OK 73036

* Corresponding author (karlglasener{at}cs.com)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Legume mulches are important sources of N for cereal crop production, particularly for organic and resource-poor producers. A field study was conducted using a direct method to determine if the amount of N in cereal crops derived from either the shoots or roots of preceding tropical legume cover crops was affected by their chemical composition and mineralization potential. Desmodium ovalifolium Guill. & Perr. [= D. adscendens (Sw.) DC. and Pueraria phaseoloides (Roxb.) Benth.], were grown in 6.0-m2 microplots and foliar-labeled with 99 atom % 15N urea. A cereal sequence of maize (Zea mays L.)–rice (Oryza sativa L.)–maize followed the legumes. Cereal accumulation of legume N from either the shoot (shoot + leaf litter) or the root-soil sources was evaluated by spatially separating the legume N sources. This was achieved by interchanging surface applications of nonlabeled and 15N-labeled legume shoots with in situ 15N-labeled and nonlabeled legume roots. Initially the Desmodium shoot N source contained 316 kg N ha-1 and roots contained 12.5 kg N ha-1. Pueraria shoots and root N sources initially contained 262 and 14.8 kg N ha-1, respectively. About 90 g kg-1 of the initial N of each legume shoot was recovered in the total aboveground tissues from the three cereal crops, while 490 g kg-1 of Desmodium and 280 g kg-1 of Pueraria root-soil N sources were recovered. Of the 181 kg N ha-1 accumulated aboveground by the cereal sequence, the contribution of shoot plus root-soil N sources was 200 g kg-1 from Desmodium and 150 g kg-1 from Pueraria. Cereal N was derived primarily from mineralization of soil organic matter present before the legumes and possibly from N deposition (precipitation and dry) occurring during the cereal crop sequence. After harvest of the last cereal crop, 13 and 180 g kg-1 of the initial legume N was present as inorganic and organic N fractions, respectively, in the top 75 cm of soil. Even though Pueraria shoots had a lower C:N ratio and concentration of polyphenols than Desmodium shoots, the relative contributions of the shoot N source were similar for both legumes. Decomposition of legume residues, particularly legume shoots, make a meaningful contribution to the N economy of cereal crops grown in the tropics. The legume cover crops (root + shoot) contributed nearly 280 g kg-1 of the aboveground N in the first cereal crop and as much as 110 g kg-1 of the N in the third crop during the 15-mo sequence of cereals.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
LEGUMES ARE USED commonly in agricultural systems as a source of N for subsequent crops and for maintaining soil N levels. This use is particularly important in the humid tropics where N fertilizers often are not economically feasible due to poor market and infrastructure development (Palm and Sanchez, 1991). To date, studies attempting to quantify the legume N contribution to subsequent crops have been conducted mainly in temperate agroecosystems and have dealt primarily with aboveground legume N, ignoring root N because of the difficulty of harvesting roots and nodules. Moreover, these assessments of N cycling in cover crop based production systems have often relied on indirect methods that evaluate plant and soil N pools (Ditsch et al., 1993; Luna-Orea and Wagger, 1996), N release from cover crop residue (Ranells and Wagger, 1991; Luna-Orea et al., 1996), and N uptake by a summer crop (Hargrove, 1986; Clark et al., 1994).

Nitrogen-15 methodology is useful for resolving N dynamics, whereby 15N-labeled legume cover crops are harvested and applied as N sources for subsequent grain crops (Varco et al., 1989; Jordan et al., 1993; Harris et al., 1994). Varco et al. (1993) found that 600 g kg-1 of the N was mineralized and subsequently lost from 15N-labeled hairy vetch (Vicia villosa Roth) residue 30 d after surface application, yet an average of only 60 g kg-1 was recovered as soil inorganic N for two growing seasons. In Australia, Ladd and associates (Ladd et al., 1981, 1983; Ladd and Amato, 1986) reported field-grown wheat (Triticum aestivum L.) recovered between 11 and 280 g kg-1 N from 15N-labeled medic (Medicago littoralis L.) and an additional 40 g kg-1 recovery by a second wheat crop. When 15N-labeled red clover (Trifolium pratense L.) residue was applied at maize planting, 150 g kg-1 was recovered in the harvested crop and 570 g kg-1 was retained by the soil (Harris et al., 1994). Of these studies, that of Varco et al. (1989) made an indirect estimate of the contribution of legume root N to subsequent crops. Only a few direct estimates of N contribution from legume shoot and root residues are available (Harris and Hesterman, 1990; Russell and Fillery, 1996).

The objectives of this study were to: (i) quantify the N contribution from two tropical legume cover crops of differing chemical characteristics (i.e., potentially different rates of decomposition and nutrient release) to a subsequent maize–rice–maize sequence using 15N methodology, (ii) determine the effect of plant part (shoot vs. root) on recovery of legume-derived 15N by grain crops, and (iii) quantify the legume-derived 15N remaining in various soil N fractions at the end of the cereal crop sequence.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
A field study was conducted at La Jota Research Station (16°01' S and 65°25' W; 400 m above sea level), situated in the sub-Andean foothills of eastern Bolivia, on a gently sloping (0–2%) fine-loamy, mixed, isohyperthermic, Typic Dystropepts. Selected soil physical and chemical characteristics of the surface 20 cm are as follows: 41% sand, 41% silt, 18% clay, bulk density 1.02 Mg m-3, 13.8 g C kg-1, 1.8 g N kg-1, effective CEC 7.0 cmolc kg-1 (1 M NH4OAc, pH 7; 1 M KCl-extractable Al), 880 g kg-1 Al saturation, and pH 4.6 (1:1, soil:water). Rainfall and temperature data for the site, covering the 21-mo study period, are presented in Fig. 1 .



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  		Fig. 1. Monthly and 11-yr mean (1982–1993) precipitation and temperature at La Jota Experiment Station, Bolivia, 1993 to 1995. Temperature not recorded during March and April 1994. Horizontal bars indicate crop duration of legumes and cereal crops.

 
Cover Crop Establishment
From June through October 1993, a 1 ha, 6- to 19-yr-old secondary forest was cut and all aboveground vegetation was carried off the experimental site and burned. Negligible litter remained on the soil surface, which was essentially undisturbed. The top 75 cm of soil contained 98 kg N ha-1 as inorganic N just before the legume cover crops were planted. In mid-November, 10- by 15-m plots of two tropical legumes [D. ovalifolium and P. phaseoloides (tropical kudzu)] and fallow check were established and randomized in each of four blocks. Desmodium and Pueraria seed were coated with a sugar-water sticker, inoculated with CIAT 4099 and CIAT 3287 Rhizobia, respectively, pelleted with 100 g of CaCO3 (powder) per kg of seed, and planted in 50-cm wide rows at a seeding rate of 27 kg ha-1. During an 8-mo growth period, legumes were maintained weed free by hand and pest free with applications of malathion (O,O-dimethyl phosphorodithioate of diethyl mecaptosuccinate). Weeds were allowed to grow in the fallow plots during the 8-mo legume growth period. At termination of legume cover crop growth, weeds in the fallow plot were cut and left on the soil surface.

Cover Crop Nitrogen-15 Labeling
In mid-December (early in legume development to minimize root damage), two 6-m2 microplots containing five legume rows were sectioned off within each plot using galvanized steel collars (20 cm high) driven into the soil {approx}15 cm to reduce lateral flow of water and 15N. Sixteen microplots were installed, eight for each species representing two 15N sources and four replications. Within each legume plot, one of the microplots was randomly selected for foliar treatment with 15N. Shoots from the 15N-labeled microplot were exchanged later with shoots from the adjacent unlabeled microplot to spatially separate 15N-labeled root from 15N-labeled shoot sources of legume N for the following maize–rice–maize sequence.

Nitrogen-15 was applied to growing legumes beginning in mid-May 1994 when Desmodium shoot dry matter accumulation was 9.7 Mg ha-1 (184 kg N ha-1) and Pueraria was 4.2 Mg ha-1 (144 kg N ha-1). The soil surface beneath the legume canopies was not visible. Desmodium and Pueraria began flowering just prior to initiation of 15N foliar labeling. To prevent seed set and to minimize N translocation from roots to shoots, flowers were clipped during the 15N labeling period, dried, and later combined with the shoot 15N source. A total of 2.58 g 15N (4.3 kg N ha-1) as urea labeled with 99 atom % 15N was applied to each microplot in four equal foliar applications of 0.645 g 15N each (18 and 30 May and 6 and 13 June). A 12-d interval occurred between the first and second 15N applications because of rainy conditions. Each microplot was divided into quadrants to assure uniform distribution of the 15N solution sprayed on the upper plant canopy using a hand-held mister. The solution contained 300 mL H2O (volume necessary to maximize foliar coverage while minimizing drip, potential 15N loss, and soil contamination), a wetting agent to maximize absorption, and the 15N-labeled urea. Immediately following each foliar application, microplots were covered with a transparent plastic shelter for 2 d to prevent rainfall from washing 15N off the leaves.

Sampling and Analysis of Nitrogen- 15-Labeled Microplots
Ten days after the final 15N foliar split-application, legume shoots were cut at the soil surface and removed and then leaf litter was collected. A subsample of shoots was weighed, air-dried, ground to 1 mm, and analyzed for polyphenolic and lignin concentrations using the procedure described by Palm and Sanchez (1991) and Van Soest (1963), respectively. The remaining shoots and leaf litter were weighed, subsampled for moisture determination, oven dried at 65 °C, reweighed, and ground to 1 mm. Obtaining representative subsamples is frequently a problem with 15N field-tracer studies; thus the oven-dried and ground plant subsamples were thoroughly mixed in a twin-shell blender, and 5-g subsamples were ground to a fine powder in a dental amalgam ball mill. Finally, plant subsamples ({approx}5 mg) containing at least 100 µg N were weighed into Sn capsules for total N and 15N determinations with an automated flash-combustion analyzer coupled to an isotope ratio mass spectrometer (RoboPrep and TracerMass, Europa Scientific, Crewe, Cheshire, UK).1

The present study was designed to leave the labeled roots undisturbed and to follow the recovery of 15N from the combined root-soil system by subsequent crops. Immediately following removal of legume shoots, four soil cores were taken at five depths (0–10, 10–25, 25–40, 40–60, and 60–75 cm), air-dried, and sieved (2mm). Obtaining representative soil subsamples is often a problem with 15N field-tracer studies, more so than with plants due to the inherent variability of soil N. Thus the air-dried, sieved root-soil samples were thoroughly mixed in a twin-shell blender and 5-g subsamples were ground to a fine powder in a dental amalgam ball mill. Finally, subsamples of root-soil ({approx}40 mg) containing {approx}25 to 80 µg N were weighed into Sn capsules for total N and 15N determinations with an automated flash-combustion analyzer coupled to an isotope ratio mass spectrometer.

Inorganic Soil Nitrogen and Nitrogen-15 Analysis
Air-dried and sieved soil samples were shaken with 2 M KCl (20 g soil 100 mL-1) for 1 h, filtered, and analyzed for total inorganic N (NH4 + NO3) with an automated flow injection ion analyzer (QuickChem IV, Lachat Instruments, Milwaukee, WI). Inorganic N was isolated for 15N analysis by the method of Brooks et al. (1989) as follows. An extract volume containing 60 to 100 µg total N was transferred into a 104-mL specimen cup, and 0.4 g MgO, 0.2 g of Devarda's alloy (ground to a powder with a ball-mill), and 7.0 g K2SO4 (to increase solution osmotic potential and reduce H2O vapor pressure) were added. A 6-mm diameter acid-washed filter paper disk, acidified with 10 µL of 2.5 M KHSO4, was suspended above the extract-reagent mixture to trap volatilized NH3. The cup was capped, gently swirled, and incubated at 40 °C for 6 d. Following incubation, the paper disk was removed, dried in a vacuum microcentrifuge, wrapped in a Sn capsule and analyzed for 15N. The soil organic 15N fraction was determined by subtracting the inorganic 15N from total 15N.

Cereal Cropping Sequence
On 24 June 1994, the harvested unlabeled and 15N-labeled legume shoots and litter were used to establish microplots for the cereal cropping sequence. Within each replicate, 15N-labeled shoots and litter from the original foliar 15N-labeled microplot were removed and surface-applied to an adjacent microplot containing only unlabeled roots, that is, legumes in this new microplot had not received the foliar 15N application. Then, an equal weight of shoots plus litter (means for Desmodium and Pueraria were 19.2 and 8.6 Mg ha-1, respectively) from this unlabeled legume microplot were surface-applied to the microplot containing only the 15N-labeled root-soil N source. After redistributing surface residue, the microplots were left undisturbed for 7 d before annual crops were planted. Legumes outside the microplots were cut and left on the soil surface as mulch.

Maize (‘Across 8136’), the first cereal crop of the sequence, was sowed 30 June 1994 ({approx}30700 plants ha-1) using a jab planter and a 65- by 50-cm grid pattern within the microplots, the border area between microplots, and the fallow check plots. Post-emergence weed control and Desmodium regrowth proved challenging throughout the first maize crop, necessitating bi-weekly applications of 1 kg a.i. ha-1 of paraquat [1, 1'-dimethyl-4 4'-bipyridinium ion (dichloride salt)] in combination with hand weeding. Pests, particularly a severe Spodoptera frugiperda (J.E. Smith) infestation triggered by a 10-yr record drought, were controlled with weekly sprayings of methamidophos [O,S-dimethyl phosphoramidothiate] and malathion. Approximately 6 wk after planting the first maize crop, all microplots were fertilized by hand with 29, 111, 21, and 16 kg ha-1 of P, K, Ca, and Mg, respectively. Fertilizer trials had not been conducted in this region, so fertilizer tests for macronutrients were made and applications were conservatively calculated as twice the nutrient removal from an average local maize and rice yield of 2.3 and 2.9 Mg ha-1, respectively. Aboveground parts of mature maize plants were harvested on 7 Dec. 1994 from each microplot. Grain and stover were weighed, and subsampled for moisture, total N, and 15N determinations. Grain yield was adjusted to a 155 g kg-1 moisture basis. After removal of the maize plants an equal weight of unlabeled maize stover was surface-applied to the microplots.

Upland rice (‘Bluebell’) followed maize in the cereal sequence and was planted 15 Dec. 1994 ({approx}100000 plants ha-1) on a 40- by 25-cm planting grid with a jab planter. Before rice emergence, the area was sprayed with paraquat (1 kg a.i. ha-1) to kill existing vegetation. As with the maize crop, post-emergent weed control and Desmodium regrowth proved challenging, requiring five hand cultivations. Additionally, weekly spray applications of malathion were necessary to control various insect stresses. Fertilization rates for all nutrients were doubled for the rice crop as visual P deficiencies were observed in {approx}10% of the maize plants. Aboveground parts of mature rice plants were harvested 7 Apr. 1995 from each microplot, rice grain and straw were weighed, and subsampled for moisture, total N, and 15N determinations. The labeled rice straw was then returned to the respective microplots, while all grain was removed from the field and oven-dried at 65 °C for 24 h. Rice yields were not adjusted to a specific moisture basis due to logistical constraints, though local experience indicates oven drying at 65 °C for 24 h yields an approximate moisture basis of 80 g kg-1.

Maize followed rice as the last crop in the cereal sequence and was planted 3 May 1995. Cultural practices were the same as previously described for the first maize crop, with the exception of doubling the original fertilization rate of the first maize crop as detailed above. At maturity the aboveground parts of the last maize crop were harvested on 15 Sept. 1995 and processed as previously described for the first crop of maize in the cereal sequence.

Soil sampling was conducted at the end of each cereal crop season as previously described. Additionally, tissue, grain, and soil samples were analyzed for total N and 15N enrichment as previously described. Total plant C was determined with a CHN elemental analyzer (Model 2400, Perkin Elmer, Norwalk, CT).

Recovery Calculations and Statistical Analyses
Recoveries of foliar-applied 15N-urea were calculated using the sum of atom % excess 15N in the shoot plus soil (0 to 75 cm). Recoveries of legume sources of 15N in the following cereal crops were determined after each cereal harvest by measuring the atom % excess 15N in aboveground tissues and the top 75 cm of the soil profile. For the legume root-soil 15N source, it was assumed that the 15N label was contained entirely in the root, even though a portion of the root-soil 15N source was present as inorganic N before planting the first cereal crop. The amount of N contained in the legume roots of each 15N-labeled microplot was assumed to be proportionally equal to that measured following excavation of legume roots in an identical 15N labeling experiment (Glasener et al., 1998) that was conducted simultaneously. Consequently, the recoveries from this source would represent an upper limit for N derived entirely from legume roots.

The experiment was a split plot design with four replications. Each legume main plot had two randomized subplots consisting of the legume root and shoot 15N sources contained in microplots. Nitrogen-15 recovery data was analyzed by legume species, 15N source, and the interaction of these factors using PROC GLM (SAS, 1991). Pre-planned contrasts were implemented to distinguish significant differences among treatment effects.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Legume Cover Crop Traits
In mid-June 1994, when 15N-labeled cover crops were harvested, Desmodium had produced {approx}2.9-fold more shoot dry matter (17.0 Mg ha-1) than Pueraria (5.9 Mg ha-1), and had a N concentration that was only 16 g kg-1 compared with 33 g kg-1 for Pueraria (Table 1). Consequently, the C:N ratio of Desmodium was {approx}2-fold greater than that of Pueraria. Lignin concentrations were similar for the two legumes, while the polyphenol concentration of Desmodium shoots exceeded that of Pueraria by {approx}1.5-fold (Table 1).


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  		Table 1. Aboveground dry matter and selected chemical traits of Desmodium and Pueraria legume cover crops labeled with 15N.

 
Recovery of Foliar-Applied Nitrogen-15
About 750 g kg-1 of the 15N-urea applied to the canopy of the legume cover crops was recovered in the aboveground tissues (shoots, flowers, and leaf litter) and the top 75 cm of soil in the 15N-labeled microplots. Less than 40 g kg-1 of the foliar applied 15N-urea was found in the root-soil N pool, with the organic fraction of the root-soil N pool containing more of the foliar-applied 15N than the inorganic fraction (Table 2). Recoveries of 15N as inorganic, organic, and total N fractions in the soil were greater from Pueraria than Desmodium microplots. Previous study of 15N labeling of legume canopies produced similar results; the effectiveness of 15N foliar labeling of Desmodium and Pueraria measured an average recovery of 780 g kg-1 in shoots and only 41 g kg-1 in the excavated roots and soil (Glasener et al., 1998). Vasilas et al. (1980) also found the leaves and stems of soybean [Glycine max (L.) Merr.] to be the dominant sinks for foliar-applied 15N, and no more than 16 g kg-1 of the applied N was translocated to the roots. Morris and Weaver (1983) applied 15N-labeled urea to foliage of soybean and recovered an average of 680 g kg-1 in the shoots and 17 g kg-1 in the roots after three sampling dates. We observed some spray drift beyond the perimeter of the 15N-labeled microplots that would account partially for the unrecovered foliar applied 15N in our study. Additional losses may have occurred as urea was absorbed into the leaf (Wittwer et al., 1963), hydrolyzed, and then volatilized as NH3.


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  		Table 2. Recoveries of foliar-applied 15N-urea in the shoot and root-soil components of microplots grown with Desmodium and Pueraria legume cover crops.

 
Within 15N-labeled microplots, total N in the top 75 cm of the soil profile was equivalent (averaging 8250 kg ha-1) for the two legume cover crops (Table 3). Rather than disturb the soil to excavate legume roots to measure their N content, the contribution of legume root N to the soil total N pool of the microplots was calculated using data from our previous research (Glasener et al., 1998) with identical experimental conditions (i.e., same time period, identical growing conditions, same legumes) in which shoot and root dry matter and N content were measured. Estimated root N was 12.5 kg ha-1 for Desmodium and 14.8 kg ha-1 for Pueraria (Table 3), much less than the 268 kg N ha-1 of Desmodium shoots (leaf + stem + flowers) that was 1.4-fold greater (P <= 0.05) than the 190 kg N ha-1 of Pueraria shoots. Total N supplied by the Desmodium shoot source (shoot + leaf litter) was 1.2-fold greater than that supplied by Pueraria (Table 3), even though Pueraria had more leaf litter N (72 kg N ha-1) than Desmodium (48 kg N ha-1). Because the legume root N was a minor component of the total root-soil N pool, the 15N enrichment of the root-soil pool was <0.003 atom % excess 15N, a value much less than the 0.990 (Desmodium) and 1.235 (Pueraria) atom % excess 15N of the legume shoot N sources (Table 3).


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  		Table 3. Total N and 15N enrichment of microplot N sources supplied by labeled legume shoot and root-soil (0- to 75-cm depth) components and the calculated N content of roots in the 15N-labeled microplots.

 
Soil Inorganic Nitrogen
Total inorganic N in the top 75 cm of the soil profile declined from 98 kg ha-1 after removal of aboveground native vegetation and before planting the legumes, to {approx}18 kg ha-1 during growth of legumes and weeds in the fallow plots (Table 4). The soil inorganic N differential following native vegetation clearing and establishment of the legumes was likely the consequence of mineralization of belowground native vegetation and subsequent N uptake by the legume and weeds present in the fallow plots during cover crop establishment. Following legume establishment, soil inorganic N then appeared to increase soon after cutting aboveground vegetation (legumes or fallow weeds) from the plots and planting the first cereal crop of maize in the 15N-labeled legume and fallow plots. At this time, before the onset of rapid growth of the first cereal crop of maize, inorganic N in plots with Pueraria cover crop had 15 kg N ha-1 more soil inorganic N than plots with Desmodium, and 23 kg N ha-1 more than the initially fallow check plots. At the end of each crop in the cereal sequence, no significant differences (P <= 0.05) in the amount of soil inorganic N were observed among the legume cover crop plots and plots that were fallow when the cover crops were grown (Table 4).


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  		Table 4. Total inorganic N (NO3 + NH4) from 1993 to 1995 in the top 75 cm of soil in microplots planted with a legume cover crop followed by a sequence of three cereal crops.

 
The initially greater soil inorganic N of Pueraria plots occurred even though the amount of N supplied by the shoot N source was 54 kg ha-1 less than that of Desmodium. Differences in the chemical composition of the legume shoots (Table 1) may have contributed initially to greater net mineralization of Pueraria shoot N. Residue decomposition and subsequent N mineralization are often enhanced when tissues have high N concentrations (low C:N ratio) and low concentrations of lignin and polyphenols. Others have found when residue N concentration exceeded 17.0 g kg-1, net N mineralization occurred readily (Aber and Melillo, 1980; Constantinides and Fownes, 1994). The high N concentration (low C:N ratio) of Pueraria compared with that of Desmodium would likely have a greater potential for net N mineralization. In addition, N mineralization of legume leaf tissue has correlated well with soluble polyphenol concentration and polyphenol:N ratios <0.5 (Vallis and Jones, 1973; Palm and Sanchez, 1991; Oglesby and Fownes, 1992). Less polyphenols and a polyphenol:N ratio of 0.3 in Pueraria shoots would be consistent with initially greater N mineralization and accumulation of soil inorganic N than that of Desmodium shoots (polyphenol:N ratio of 0.8).

Contribution of Legume Nitrogen Sources to Soil Nitrogen Fractions
Amounts of inorganic and total N in the top 75 cm of the soil profile were measured after harvest of each crop grown in the cereal sequence. The relative contributions and amounts of soil organic N derived from the 15N-labeled legume root-soil and shoot N sources were consistently greater than those recovered in the soil inorganic N fraction (Table 5). The percentages of the N derived from labeled legume shoot and root-soil sources and recovered in the soil N fractions was similar for both legume species. After harvest of the first cereal crop of maize, recoveries of legume-derived N in the soil inorganic N fraction appeared to decline from 59 to {approx}14 g kg-1 after harvest of each of the last two crops in the cereal sequence. In contrast, during the sequence of three cereal crops, recoveries of legume-derived N in the soil organic N fraction appeared to remain constant at {approx}160 g kg-1. At the end of the three-cereal crop sequence, percentage recoveries of soil total N derived from the legume shoot N sources (Pueraria, 90 g kg-1; Desmodium, 140 g kg-1) were not significantly different from the root-soil N source (Pueraria, 160 g kg-1; Desmodium, 380 g kg-1). The chemical composition traits of Desmodium (Table 1) that would make it more resistant than Pueraria to decomposition and N mineralization had no apparent effect on the percentage contribution of the N sources into the soil N pool. However, amounts of legume shoot N recovered in the soil N fractions of Desmodium microplots appeared to be greater than those of Pueraria microplots, because the amount of the shoot N source supplied by Desmodium (316 kg ha-1) exceeded that of Pueraria (262 kg ha-1).


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  		Table 5. Recoveries of legume N sources as inorganic (NO3 + NH4) and organic N in the top 75 cm of soil after harvests of three sequential cereal crops preceded by a legume cover crop.{dagger}

 
Contribution of Legume Nitrogen Sources to the Following Cereal Crops
Because fertilizer N was not used, N available to the cereal crops would be derived from mineralization of soil organic matter, mineralization of legume mulches, and any N associated with precipitation and dry deposition. High elevation tropical rain forests can receive as much as 18 kg N ha-1 annually in bulk precipitation (Veneklaas, 1990). Aboveground dry matter and total N accumulation (Table 6) by the first cereal crop of maize after a cover crop of Desmodium (3.2 Mg ha-1, 57 kg N ha-1) was not significantly different than that of Pueraria (4.8 Mg ha-1, 57 kg N ha-1), even though plots with Desmodium had residues containing nearly 52 kg N ha-1 more than that of Pueraria. Significant differences in these traits due to a 15N-labeled N source were unexpected and have no reasonable cause. The overall mean grain yield of 1.7 Mg ha-1 (data not shown) was apparently less than the local average yield of 2.3 Mg ha-1 and was probably affected by low rainfall during the growing season (July to November 1994 rainfall equaled only 75 cm; 49% less than the 11-yr average of 146 cm). For the second crop of rice and the third crop of maize in the cereal sequence, aboveground dry matter and N accumulation were unaffected by species of the legume cover crop and N source (Table 6). In contrast to the first crop of maize, the third cereal crop of maize in the sequence had 1.9-fold more dry matter and 1.5-fold more N accumulation. This greater response for dry matter and N accumulation was most likely the consequence of increased amounts of K, P, Ca, and Mg fertilizer used and a more favorable climate (Fig. 1) during the cropping cycle of the third cereal crop.


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  		Table 6. Total aboveground dry mater (grain + vegetative), N accumulation, and N derived from legume cover crop N sources following harvest of three cereal crops grown in sequence.

 
Cereal crop accumulation of legume N derived from the root-soil N source was unaffected by legume species and appeared to decrease after the first crop of maize in the cereal sequence (Table 6). Averaged across species, the legume root-soil N source contributed 50 g kg-1 of the total N in the first crop of maize to no >20 g kg-1 of the total N accumulated by each of the following two cereal crops. Because the legume shoot N source contained substantially more N (Table 3), the contribution of legume shoot N to total N accumulated by each cereal crop was considerably greater (P <= 0.01) than that of the legume root-soil N source (Table 6). Legume shoot N contributed {approx}230 g kg-1 of the total N in the first maize crop. Subsequent contributions from legume shoot N declined to {approx}170 g kg-1 for the second cereal crop of rice, and finally to {approx}90 g kg-1 for the last crop of maize.

These results demonstrate that degradation of legume residues, particularly legume shoots, make a meaningful contribution to the N economy of cereal crops grown in the tropics. A substantial contribution (280 g kg-1, root + shoot sources) to the N needs of cereals is realized for the crop immediately following the legume cover crop and extends even to the third cereal crop (110 g kg-1) grown in a sequence across a 15-mo period.

Recoveries of Legume Nitrogen Sources
Although the chemical composition of the legume shoots differed (Table 1), an expected greater mineralization of Pueraria shoot residue N was not obvious as a greater N recovery by the cereal crops. Between 19 and 43 g kg-1 of the N initially contained in the legume shoot N sources of Desmodium and Pueraria was accumulated by each of the grain crops in the cereal sequence (Table 7). Differences in recoveries of N derived from the shoot N sources of the two legumes were not significant and totaled <100 g kg-1 of the N initially supplied. The actual N fertilizer value of mulches can be reduced by losses of N as the mulch decomposes. Up to 400 g kg-1 of the N in low C:N mulches can be lost as NH3 (Larsson, et al., 1998). In contrast, the percentages of N recovered from the legume root-soil N source were substantially greater (P <= 0.01) than those from the residues of legume shoots (Table 7). Total recovery of the legume root-soil N source by the three cereal crops was 280 g kg-1 for Pueraria and 490 g kg-1 for Desmodium. Even though a greater percentage of the N initially present in the legume root-soil N source was recovered in the cereal crops, the actual contribution of the root-soil N source to the N nutrition of the cereal crops was much less than the shoot N source (Table 6). At most, the legume root-soil N source contributed 6.1 kg N ha-1, while as much as 30.4 kg N ha-1 was contributed by the shoot N source.


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  		Table 7. Summary of 15N recoveries by a sequence of three cereal crops following a legume cover crop with 15N-labeled shoot and root-soil components.

 
Mean total N recoveries for the 15N-labeled legume N sources in the three cereal crops plus top 75 cm of the soil profile was 210 g kg-1 for the shoot N source and 660 g kg-1 for the root-soil N source (Table 7). These results demonstrate the largest source of legume N for the cereal crops was derived from legume shoots, primarily because of the abundance of N contained by the legume shoots placed on the soil surface. The legumes (shoot + root) provided an important source of N to the cereal crop sequence, contributing between 152 (Pueraria) and 198 g kg-1 (Desmodium) of the total aboveground N accumulated by the three cereal crops. The contribution of legume N continued to be detected even with the last crop of maize in the three-cereal crop sequence, but was minor compared with the contribution of N derived from the total native soil N pool of 8250 kg N ha-1.

Regardless of cover crop species, legume shoots were substantially greater contributors of N to the cereal cropping sequence than were the root-soil N sources. Composition differences between the two legumes, most notably the C:N ratio, were expected to result in more N provided by Pueraria than Desmodium. In a decomposition study using mesh bags, Luna-Orea et al. (1996) estimated that {approx}800 g kg-1 of the initial N in Pueraria residue was released during the first 12 wk compared with 550 g kg-1 for Desmodium. In the present study, however, the greater N content of Desmodium offset the potentially faster N mineralization rate of Pueraria. Nevertheless, cereal crop 15N recovery values suggest considerable overestimation of N contributions from cover crop residues based on a mesh bag technique.


   NOTES
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 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Research supported by the Trop Soils Program and funded in part by Grant no. DAN-1311-G-00-1049-00 from the USAID.

1 The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service or USDA-ARS of the product named, nor criticism of similar ones not mentioned. Back

Received for publication June 21, 2001.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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