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

L are used commonly in agricultural systems Legume mulches are important sources of N for cereal crop producas a source of N for subsequent crops and for maintion, particularly for organic and resource-poor producers. A field taining soil N levels. This use is particularly important study was conducted using a direct method to determine if the amount in the humid tropics where N fertilizers often are not of N in cereal crops derived from either the shoots or roots of precedeconomically feasible due to poor market and infraing tropical legume cover crops was affected by their chemical compostructure development (Palm and Sanchez, 1991). To sition and mineralization potential. Desmodium ovalifolium Guill. & date, studies attempting to quantify the legume N contriPerr. [ D. adscendens (Sw.) DC. and Pueraria phaseoloides (Roxb.) bution to subsequent crops have been conducted mainly Benth.], were grown in 6.0-m2 microplots and foliar-labeled with 99 in temperate agroecosystems and have dealt primarily atom % 15N urea. A cereal sequence of maize (Zea mays L.)–rice (Oryza sativa L.)–maize followed the legumes. Cereal accumulation with aboveground legume N, ignoring root N because of of legume N from either the shoot (shoot leaf litter) or the rootthe difficulty of harvesting roots and nodules. Moreover, soil sources was evaluated by spatially separating the legume N these assessments of N cycling in cover crop based prosources. This was achieved by interchanging surface applications of duction systems have often relied on indirect methods nonlabeled and 15N-labeled legume shoots with in situ 15N-labeled and that evaluate plant and soil N pools (Ditsch et al., 1993; nonlabeled legume roots. Initially the Desmodium shoot N source Luna-Orea and Wagger, 1996), N release from cover contained 316 kg N ha 1 and roots contained 12.5 kg N ha 1. Pueraria crop residue (Ranells and Wagger, 1991; Luna-Orea et shoots and root N sources initially contained 262 and 14.8 kg N ha 1, al., 1996), and N uptake by a summer crop (Hargrove, respectively. About 90 g kg 1 of the initial N of each legume shoot 1986; Clark et al., 1994). 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 Nitrogen-15 methodology is useful for resolving N root-soil N sources were recovered. Of the 181 kg N ha 1 accumulated dynamics, whereby 15N-labeled legume cover crops are aboveground by the cereal sequence, the contribution of shoot plus harvested and applied as N sources for subsequent grain root-soil N sources was 200 g kg 1 from Desmodium and 150 g kg 1 crops (Varco et al., 1989; Jordan et al., 1993; Harris et from Pueraria. Cereal N was derived primarily from mineralization al., 1994). Varco et al. (1993) found that 600 g kg 1 of of soil organic matter present before the legumes and possibly from the N was mineralized and subsequently lost from 15NN deposition (precipitation and dry) occurring during the cereal crop labeled hairy vetch (Vicia villosa Roth) residue 30 d sequence. After harvest of the last cereal crop, 13 and 180 g kg 1 of after surface application, yet an average of only 60 g the initial legume N was present as inorganic and organic N fractions, kg 1 was recovered as soil inorganic N for two growing respectively, in the top 75 cm of soil. Even though Pueraria shoots had a lower C:N ratio and concentration of polyphenols than Desmodium seasons. In Australia, Ladd and associates (Ladd et al., shoots, the relative contributions of the shoot N source were similar 1981, 1983; Ladd and Amato, 1986) reported fieldfor both legumes. Decomposition of legume residues, particularly grown wheat (Triticum aestivum L.) recovered between legume shoots, make a meaningful contribution to the N economy of 11 and 280 g kg 1 N from 15N-labeled medic (Medicago cereal crops grown in the tropics. The legume cover crops (root littoralis L.) and an additional 40 g kg 1 recovery by a shoot) contributed nearly 280 g kg 1 of the aboveground N in the second wheat crop. When 15N-labeled red clover (Trifofirst cereal crop and as much as 110 g kg 1 of the N in the third crop lium pratense L.) residue was applied at maize planting, during the 15-mo sequence of cereals. 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 K.M. Glasener, American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, 900 Second Street, these studies, that of Varco et al. (1989) made an indirect NE, Suite 205, Washington, DC 20002; M.G. Wagger and R.J. Volk, estimate of the contribution of legume root N to subseDep. of Soil Science, North Carolina State Univ., Box 7619, Raleigh, quent crops. Only a few direct estimates of N contribuNC 27695-7619; C.T. MacKown, USDA-ARS, Grazinglands Research tion from legume shoot and root residues are available Lab., 7202 W. Cheyenne Street, El Reno, OK 73036. Research supported by the Trop Soils Program and funded in part by Grant no. (Harris and Hesterman, 1990; Russell and Fillery, 1996). DAN-1311-G-00-1049-00 from the USAID. Received 21 June 2001. The objectives of this study were to: (i) quantify the Nov. 2000. *Corresponding author (karlglasener@cs.com). N contribution from two tropical legume cover crops of differing chemical characteristics (i.e., potentially difPublished in Soil Sci. Soc. Am. J. 66:523–530 (2002). 524 SOIL SCI. SOC. AM. J., VOL. 66, MARCH–APRIL 2002 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. blocks. Desmodium and Pueraria seed were coated with a ferent rates of decomposition and nutrient release) to sugar-water sticker, inoculated with CIAT 4099 and CIAT a subsequent maize–rice–maize sequence using 15N 3287 Rhizobia, respectively, pelleted with 100 g of CaCO3 methodology, (ii) determine the effect of plant part (powder) per kg of seed, and planted in 50-cm wide rows at (shoot vs. root) on recovery of legume-derived 15N by a seeding rate of 27 kg ha . During an 8-mo growth period, grain crops, and (iii) quantify the legume-derived 15N legumes were maintained weed free by hand and pest free remaining in various soil N fractions at the end of the with applications of malathion (O,O-dimethyl phosphorodithcereal crop sequence. ioate of diethyl mecaptosuccinate). Weeds were allowed to grow in the fallow plots during the 8-mo legume growth period. MATERIALS AND METHODS At termination of legume cover crop growth, weeds in the fallow plot were cut and left on the soil surface. 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 Cover Crop Nitrogen-15 Labeling (0–2%) fine-loamy, mixed, isohyperthermic, Typic DystroIn mid-December (early in legume development to minipepts. Selected soil physical and chemical characteristics of mize root damage), two 6-m microplots containing five lethe surface 20 cm are as follows: 41% sand, 41% silt, 18% gume rows were sectioned off within each plot using galvaclay, bulk density 1.02 Mg m , 13.8 g C kg , 1.8 g N kg , nized steel collars (20 cm high) driven into the soil ≈15 cm to effective CEC 7.0 cmolc kg 1 (1 M NH4OAc, pH 7; 1 M KClreduce lateral flow of water and N. Sixteen microplots were extractable Al), 880 g kg 1 Al saturation, and pH 4.6 (1:1, installed, eight for each species representing two N sources soil:water). Rainfall and temperature data for the site, covand four replications. Within each legume plot, one of the ering the 21-mo study period, are presented in Fig. 1. microplots was randomly selected for foliar treatment with N. Shoots from the N-labeled microplot were exchanged Cover Crop Establishment later with shoots from the adjacent unlabeled microplot to spatially separate N-labeled root from N-labeled shoot From June through October 1993, a 1 ha, 6to 19-yr-old sources of legume N for the following maize–rice–maize sesecondary forest was cut and all aboveground vegetation was quence. carried off the experimental site and burned. Negligible litter Nitrogen-15 was applied to growing legumes beginning in remained on the soil surface, which was essentially undismid-May 1994 when Desmodium shoot dry matter accumulaturbed. The top 75 cm of soil contained 98 kg N ha 1 as tion was 9.7 Mg ha 1 (184 kg N ha 1 ) and Pueraria was 4.2 inorganic N just before the legume cover crops were planted. Mg ha 1 (144 kg N ha 1 ). The soil surface beneath the legume In mid-November, 10by 15-m plots of two tropical legumes canopies was not visible. Desmodium and Pueraria began flow[D. ovalifolium and P. phaseoloides (tropical kudzu)] and fallow check were established and randomized in each of four ering just prior to initiation of N foliar labeling. To prevent GLASENER ET AL.: NITROGEN-15 LABELED LEGUME NITROGEN SOURCES 525 seed set and to minimize N translocation from roots to shoots, containing 60 to 100 g total N was transferred into a 104mL specimen cup, and 0.4 g MgO, 0.2 g of Devarda’s alloy flowers were clipped during the N labeling period, dried, and later combined with the shoot N source. A total of 2.58 g (ground to a powder with a ball-mill), and 7.0 g K2SO4 (to increase solution osmotic potential and reduce H2O vapor N (4.3 kg N ha 1 ) as urea labeled with 99 atom % N was applied to each microplot in four equal foliar applications of pressure) were added. A 6-mm diameter acid-washed filter paper disk, acidified with 10 L of 2.5 M KHSO4, was sus0.645 g N each (18 and 30 May and 6 and 13 June). A 12-d interval occurred between the first and second N applications pended above the extract-reagent mixture to trap volatilized NH3. The cup was capped, gently swirled, and incubated at because of rainy condit