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INTEGRATED AGRICULTURAL SYSTEMS

Tomato Crop Response to Short-Duration Legume Green Manures in Tropical Vegetable Systems

^a The Asian Vegetable Res. & Dev. Ctr., P.O. Box 42, Shanhua Tainan, Taiwan People's Republic of China
^b IRRI, P.O. Box 933, Manila 1099, Philippines
^c Bukidnon Resources Co., Inc. (BRCI), Diklum, Manola Fortich, Bukidnon 8703, Philippines
^d Dep. of Plant Nutrition, Technische Universität München, Freising-Weihenstephan, D-85350 Germany

schmidhalter{at}weihenstephan.de

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
The potential of legume green manure (GM) as an alternative to mineral N fertilizer in tropical horticulture has received scant attention. The feasibility of meeting N needs of tomato (Lycopersicon esculentum Mill.) with GM was studied in six field experiments at three locations in major vegetable growing areas of Taiwan and the Philippines between 1993 and 1995. Legume biomass, N₂ fixation and N accumulation, and tomato yield and N uptake were quantified within a 6-mo experiment cropping pattern. Yields of GM-amended tomato crops were compared with those amended with fertilizer N (0–150 kg N ha^-1). The residual effect of the fertilizing method of a second crop (maize; Zea mays L.) was estimated at AVRDC by measures of biomass and N uptake 30 d after sowing. Legume N recovery in tomato crops was traced with ¹⁵N at Mariano Marcos State University (MMSU). Soybean [Glycine max (L.) Merr.] harvested at 60 to 74 d accumulated a minimum of 2.8 Mg ha^-1 biomass and 100 kg ha^-1 N in all locations and seasons. A maximum of 6 Mg biomass ha^-1 and 140 kg N ha^-1 was reached in the wet season (WS) at AVRDC. Indigofera (Indigofera tinctoria L.) and mungbean [Vigna radiata (L.) Wilcz.] biomass yields were more variable and always inferior than soybean yields. Tomato yields across locations ranged from 3 to 70 Mg fruit ha^-1. Tomato yields responded to GM N in the WS in Taiwan and in the northern Philippines, comparing favorably with fertilizer at 38 to 120 kg N ha^-1. No response to GM N was found in the dry season (DS) at AVRDC or at Bukidnon Resources Company, Inc. (BRCI). The ¹⁵N experiments showed that only a small fraction of legume N (9–15%) was recovered by the tomato crop at MMSU. Maize biomass and N uptake, following the tomato crop, was increased with soybean GM compared with the control in the AVRDC WS and DS. Tomato yield response to GM N is high on infertile soils and tomato N requirement can be substituted fully or partially by GM, depending on soil N mineralization.

Abbreviations: AVRDC, Asian Vegetable Research and Development Center • BRCI, Bukidnon Resources Company, Inc. • DS, dry season • GM, [legume] green manure • IRRI, International Rice Research Institute • MMSU, Mariano Marcos State University • WS, wet season

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
VEGETABLE PRODUCTION SYSTEMS in the tropics and elsewhere are mostly intensive, because vegetables are high-value cash crops. High fertilizer rates are commonly applied to maximize yields. There is an urgent need for the implementation of alternative methods to reduce excessive use of mineral fertilizers and to improve soil fertility and vegetable quality.

The age-old practices of green manuring, application of compost, crop rotation, and inter- and relay-cropping, which were used in various soil fertility programs for developing countries until the early 1960s, have declined with the increased use of mineral fertilizer (Singh, 1975). A major benefit of legume green manures (GM) is the contribution of N to the soil via N₂ fixation. Because the role of N from organic sources such as GM is tied to complex microbial cycling of C and N, the availability and effects of legume N are more difficult to predict than those of chemical fertilizer N (Groffman et al., 1987). Most recent research on GM has focused on staple crops, especially on rice (Oryza spp.) (Ladha et al., 1989). Few published results exist for tropical vegetable production systems, although organic manuring is still a common practice in some vegetable farms in India and Nepal (Babha Tripathi, Nadia, India, personal communication). Stivers and Shennan (1991) and Abdul-Baki and Teasdale (1993) reported tomato yield following legume GM and mulch comparable to those obtained with synthetic fertilizers in the USA, but Lennartsson (1990) showed that vegetable yields following GM did not outfield those grown after fallow in the UK. Investigations are needed to evaluate the potential role of legume GM in tropical horticulture and to estimate the risks to production before promoting it as a widespread practice for farmers.

The objective of this study was to assess the feasibility of meeting N needs of tomato with legume GM at one location in southern Taiwan and two locations in the Philippines through integration of the legumes into established vegetable cropping patterns of these areas. This cropping strategy was tested for its location specificity by quantifying legume biomass, N₂ fixation, N accumulation, and tomato yield and N uptake. Tomato N nutrition was monitored by NO₃–sap samplings in the southern Philippines. To quantify the contribution of various N sources to tomato plants, legumes were labeled with ¹⁵N in an additional experiment in the northern Philippines and ¹⁵N was traced in tomato plants.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
Field Trials
Six field experiments in major vegetable growing areas of Taiwan and the Philippines were conducted between 1993 and 1995: four experiments on the experiment farm of the Asian Vegetable Research and Development Center (AVRDC) in southern Taiwan and one each in the north and south of the Philippines. At AVRDC, two experiments were conducted during the wet season of 1993, with average air temperatures of 27.6°C and a total rainfall (6-mo experiment period) of 1348 mm. Two experiments were conducted in the dry season of 1993–1994, with average air temperatures of 21.6°C and a total rainfall (6 mo) of 52.9 mm. The cropping history of the field used for the 20 yr prior to our experiments was a rotation of vegetables grown in the DS and flooded rice grown in the WS. The soil is of the Take series (loamy, mixed, hyperthermic Fluvaquentic Entochrepts), with pH (H₂O) of 8.2, total Kjeldahl N of 0.7 g kg^-1 (Bremner, 1965), and total C 6.4 g kg^-1 (Walkley–Black method). Prior to the first run of our experiments, the field area was cropped with maize for about 1 mo, to obtain a homogeneous soil mineral N distribution. Maize stubble was removed before the trial started.

In the Philippines, the first experiment was conducted on the experiment farm of the Mariano Marcos State University (MMSU) in Batac, Ilocos Norte (IRRI rainfed lowland consortium site). The rainfed lowlands of the province Ilocos Norte are characterized by intensive cropping systems, although soil fertility and rainfall distribution appear unfavorable (Tripathi, 1995). Rice is grown during the wet season and upland crops (legumes, maize, and vegetables) are grown in the dry season. Average air temperature during the DS experiment period of 6 mo was 27.3°C (max. 33°C, min. 20°C). After strong rainfall events in October (175 mm), no more rainfall occurred within the 6-mo experiment period. The soil is a Fluvaquentic Ustropept (fine-silty, mixed isohyperthermic), with pH (H₂O) of 8.1, total Kjeldahl N of 0.7 g kg^-1 (Bremner, 1965), and total C of 5.9 g kg^-1 (Walkley–Black method). To obtain a homogeneous soil mineral N distribution, this soil has been previously cropped to one rice crop without N fertilizer application. Rice straw was removed from the field before the trail started.

The second experiment in the Philippines was conducted in collaboration with the tomato processing company Bukidnon Resources Company, Inc. (BRCI), at their experiment farm in San Juan, Bukidnon, Mindanao. An extensive subsistence farming is practiced in this area. Soils in Bukidnon are rich in organic matter and of volcanic origin. Average air temperatures during the 6 mo were 24.1°C (max. 28.3°C, min. 19.8°C). Total rainfall during the experiment period was 1471 mm, with an average daily rainfall of 25 mm. The soil is a clayey, kaolinitic, hyperthermic Ultisol, with pH (H₂O) of 5.7 (after liming with 5 Mg ha^-1 CaCO₃), total Kjeldahl N of 2.1 g kg^-1 (Bremner, 1965), and total C of 19.5 g kg^-1 (Walkley–Black method).

Experiment Design
Asian Vegetable Research and Development Center Experiments
Experiments were run simultaneously on two fields, each with different bed systems; raised or low beds. The raised beds were 45 cm high and 2 m wide, with 2-m furrows between the beds. The furrows were sown with rice (O. sativa L.) and permanently flooded. The low beds were 20 cm high and 2 m wide, with 50-cm-wide irrigation furrows between beds. Both experiments (raised and low beds) were adjacent, such that the soil type, the cropping history, and meteorological conditions were the same. The experiment design for each experiment was a randomized complete block. Treatment plots measured 2 by 6 m, with four replicates. The eight treatments were (Table 1) two legume species, two green manure management (mulch and incorporation), and four controls having weed-free fallow (instead of legumes) and receiving 0, 30, 60, or 120 kg N ha^-1 applied to the tomato crop. In the DS treatment, plots were split into three equal-sized subplots prior to planting the vegetable crop to study N release in planted (tomato; cabbage, Braissica oleracea var. capitata L.) vs. unplanted plots (Thönnissen Michel, 1996).

Bukidnon Resources Company, Inc., Experiment
Experimental design was a randomized complete block with split plots and three replicates. Main treatment plots were 4.8 by 4.6 m; subtreatment plots grown to tomato were 4.8 by 3 m and subtreatments kept unplanted were 4.8 by 1.5 m. The eight treatments were comparable to those at AVRDC and MMSU, with the difference that mungbean, which is locally grown, replaced indigofera at BRCI (Table 1).

Green Manure and Tomato Crop
Legumes and tomato crops were grown in a 6-mo experimental cropping pattern (Table 2) . Legumes were inoculated with a Rhizobium strain mixture, specific for each legume. Bacterial strains were provided by the Soil Science Department of the Chung Hsing University in Taichung, Taiwan for AVRDC trials, the Soil Microbiology Unit at IRRI for the MMSU trial, and the Department of Agriculture for the BRCI trial. Legumes were hand-sown at 80 seeds m^-2 for soybean and 1.32 g m^-2 for indigofera at AVRDC and MMSU. Soybean and mungbean were hand-sown at 55 seeds m^-2 at BRCI. Phosphorus at 35 kg P ha^-1 as superphosphate and K at 83 kg K ha^-1 as KCl were broadcast in all beds before legume sowing at all three locations. At BRCI, soil was limed at a rate of 5 Mg ha^-1 CaCO₃. From 60 to 74 d after sowing, legumes were cut at ground level, chopped into 5- to 10-cm pieces, and either incorporated by rototilling to the 15-cm depth or left as mulch on the soil surface, as required for each treatment (Table 2). At BRCI, legumes for the incorporation treatment were left on the soil surface for 1 wk before incorporation.

Plant Analysis
Legumes were sampled at 60 to 74 d (Table 2). Plants from 0.5 m² at AVRDC and BRCI and from a microplot of 0.64 m² at MMSU (see ¹⁵N experiment) of each treatment replicate, which was afterwards excluded from further sampling, were carefully dug out to a depth of 15 to 20 cm and the soil was separated from the roots. Shoots, roots, and nodules were dried at 60°C for 72 h and weighed for biomass determination. Plant samples were ground in a Wiley Laboratory Mill Model 4 (Thomas Scientific, Philadelphia) to pass through a 1-mm sieve, subsampled, and ground again in a vibrating sample mill (Heiko T1-100, Heiko Seisakusho Ltd., Tokyo, Japan). Nitrogen content in shoots and roots including nodules were determined by the Kjeldahl distillation method (Bremner, 1965).

At tomato harvest, marketable fruit fresh weight, and fresh and dry weights and N content of tomato fruits and plants were determined. Maize plants (30 d; including roots) were pulled out from the soil and biomass and total N were determined as a relative indicator of the inorganic N available in the soil after tomato harvest.

Plant Petiole Sap Nitrate Analysis
Tomato was sampled weekly for plant petiole sap nitrate content (sap N) at BRCI between 0600 and 1000 h, 2 d after the second weekly irrigation. Sampling took place from 42 d after GM application (9 Aug. 1995) to 91 d after GM application (26 Sept. 1995). The fifth leaf (counted from the top) of five randomly selected plants per treatment was collected, in order to sample the most recently matured leaf (Drews and Fischer, 1989). Petioles were chopped into 1-cm pieces and squeezed with a garlic press. Petiole sap was diluted by 50 times with distilled water, and mixed thoroughly for 1 min. One drop of this solution was poured onto two reaction zones of Reflectoquant nitrate test strips, and sap N was determined by refractometric reading on the RQ-flex instrument (RQflex, Merck, Darmstadt, Germany).

Nitrogen Fixation and Nitrogen-15 Experiment
Biological Nitrogen Fixation
The amount of N acquired through biological N₂ fixation by legumes was estimated using the N difference method (Talbott et al., 1985). Legumes and reference plants were grown in a small experiment conducted in parallel in a field adjacent to the main field experiment. Seeding rates and harvest dates were the same as those for legumes in the main field experiment. A nonnodulating soybean line (provided by the NifTAL Project, Hawaii) was used as a non-N₂–fixing reference plant for soybean and an upland rice variety (IF 600 80-46A) as a reference for indigofera. Plants were grown on 8-m² plots, with two replicates.

Enriched Nitrogen-15 Balance in Tomato
Production of Nitrogen-15 Labeled Legume Plant Material
Legumes were enriched with ¹⁵N by foliar application (Zebarth et al., 1991) of 1 0.5% urea (30 atom % ¹⁵N) solution at a total rate of 10 kg N ha^-1. The ¹⁵N fertilizer was split for progressive foliar applications at 21, 28, 35, 42, and 48 d.

Application of Nitrogen-15 Labeled Legume Material as Green Manure
One day before legume harvest, metal frames measuring 0.8 by 0.8 by 0.3 m (length by width by height) (microplots) were pushed into the soil to a depth of 25 cm in soybean and indigorera incorporation and mulch treatments of the main field experiment. Legumes within the metal frames were removed, including roots. On the same day, ¹⁵N-labeled legumes were carefully dug out to a depth of 20 cm, the soil was separated from the roots, and the legumes were chopped into 5- to 10-cm pieces and applied (incorporated or as mulch) to microplots of the corresponding treatments in the main field experiment.

Legume Nitrogen-15 Recovery in Vegetables
Two tomato seedlings were transplanted into each microplot. Tomato fruit yield and plant biomass, N and ¹⁵N content of tomato fruits and plant were determined to calculate ¹⁵N recovery in the tomato plant. Percent N in tomato derived from soybean (S¹⁵N) was calculated using

(1)

where a is the ¹⁵N atom % excess of tomato and b is the ¹⁵N atom % excess of soybean. Recovery of soybean ¹⁵N by tomato was calculated using

(2)

where TN is the total N uptake by tomato and AN is the amount of soybean N applied (Harris and Hesterman, 1990).

Statistical Analysis and Presentation of Yield Data
Data were analyzed by analysis of variance (ANOVA) procedure using JMP Version 2 (SAS Inst., 1989) and SAS version 6.03 (SAS Inst., 1991). Yields and N accumulations of legumes, tomato crops, and maize of the raised beds are presented on the basis of planted area, without allowance for space occupied by rice.

	Results

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
Legumes
Soybean grown for 60 to 74 d accumulated a minimum of 2.8 Mg total biomass ha^-1 and 106 kg N ha^-1 in all locations and seasons in Taiwan and the Philippines (Table 3) . A maximum of 5.9 Mg biomass ha^-1 and 140 kg N ha^-1 was achieved in the wet season at AVRDC. Indigofera yields were more variable and always inferior to those of soybean. Mungbean biomass and N accumulation at BRCI were comparable to average indigofera yields of 0.9 Mg biomass ha^-1 and 25 kg N ha^-1. Soybean biomass in the wet season at AVRDC was nearly double that produced in the dry season, while indigofera biomass production was less affected by the season. At AVRDC, indigofera produced greater biomass yields on raised beds, while soybean biomass production was not influenced by the bed system. More than 90% of the legume N in 60- to 74-d soybean and indigofera was found in the shoot.

Tomato Yield, Nitrogen Uptake
Tomato yield in response to GM management and N fertilizer rates differed depending on season and location (Table 4) . Greatest tomato yields of 60 to 70 Mg ha^-1 were achieved with 150 kg N ha^-1 at MMSU and 120 kg N ha^-1 at AVRDC in the DS. High tomato yields (40 Mg ha^-1) were obtained in the control in the DS at AVRDC and at BRCI although no N was applied. The addition of 38 and 30 kg N ha^-1 doubled tomato yields compared with the control at MMSU and in the WS at AVRDC, respectively, while 30 kg N ha^-1 increased yields only by 13% in the DS at AVRDC, and had no effect at BRCI. A strong seasonal effect occurred at AVRDC, as DS yields were 10 times greater than those of the WS. Tomato yields responded lineraly to fertilizer N applications of 38 to 150 kg N ha^-1 at MMSU (r² = 0.81**), and 30 to 120 kg N ha^-1 in the DS at AVRDC (low beds: r² = 0.69**; raised beds: r² = 0.54*). Simple regression coefficients between tomato yields and applied fertilizer N were significant neither in the WS at AVRDC nor at BRCI. With the exception of MMSU (r² = 0.66**) tomato yields did not correlate with GM N amendments.

Tomato N was doubled with soybean GM compared with the control in raised beds in the wet season at AVRDC and at MMSU, comparing favorably with that with 120 kg N ha^-1 at AVRDC and to that with 38 kg N ha^-1 at MMSU (Table 5) . In the DS at AVRDC and BRCI, tomato N was not increased by green manuring compared with the control. Tomato N was correlated with fertilizer N applied at MMSU (r² = 0.90**), in the DS (low beds: r² = 0.71**; raised beds: r² = 0.55**) and in the WS (low beds: r² = 0.56**) at AVRDC. Tomato N accumulation in controls differed greatly between experiments, with 20 kg N ha^-1 in the WS at AVRDC and at MMSU, compared with 70 to 90 kg N ha^-1 in the DS at AVRDC and at BRCI (Table 5).

At BRCI, tomato yields and N uptake did not respond to any of the treatments (Tables 4 and 5). However greatest concentration of nitrate in tomato petiole sap (1000–1472 mL NO₃–N L^-1 plant sap) was found in 30, 60, and 120 kg N ha^-1 fertilizer treatments in early stages (7 wk after transplanting), while an average of 600 mL NO₃–N L^-1 plant sap was measured in control and GM treatments (data not shown). Thereafter, nitrate sap contents decreased gradually in all treatments and reached an average of 200 mL NO₃–N L^-1 at 9 wk after transplanting. From 10 wk after transplanting until final tomato harvest, nitrate sap dropped further in all treatments ranging between 9 and 100 mL NO₃–N L^-1 plant sap.

Residual Effect on Maize
All four GM treatments in raised beds and soybean GM in low beds increased maize biomass and N compared with control in the WS at AVRCD (Table 6) . In the DS, maize biomass and N were markedly enhanced by soybean GM in raised beds and by soybean mulch in low beds. The residual effect of soybean GM applied to tomato on the following maize was similar to that of 120 kg N ha^-1. In both seasons, greater maize biomass was found on raised beds than on low beds.

Recovery of GM ¹⁵N in tomato at MMSU was comparable among soybean and indigofera (Table 7) , indicating that 8.5 to 15% of legume N was taken up by the tomato crop. Slightly higher GM ¹⁵N was recovered in early-transplanted tomato. Of the ¹⁵N taken up by tomato, 59 to 70% accumulated in the fruits.

	Discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

Tomato Yield
Strong seasonal differences between tomato yields at AVRDC can primarily be explained by different night temperatures in the WS and DS. Sugiyama et al. (1966) reported that high night temperatures (<20°C) inhibit pollination of the tomato crop and consequently reduce fruit set. Tropical storms which temporarily flood the field, and bacterial, fungal and viral diseases are further responsible for the variable and low tomato yields in the WS (Hossain, 1992). In the WS at AVRDC, low regression coefficients between N fertilizer applied and tomato yields may have been caused by high N losses via leaching. Leaching losses inhibited the soil N accumulation before tomato transplanting in fallow plots. The more gradual release of GM N during decomposition compared with the timely application of fertilizer N may explain the relatively high tomato yields when amended with GM in the WS at AVRDC. In contrast to the DS, tomato yields in the WS did not respond to N fertilizer rats above 60 kg N ha^-1. Limited tomato yield response to high fertilizer rates has also been reported by Garrison et al. (1967) and Stivers and Shennan (1991).

Although tomato yields responded highly to dertilizer N at AVRDC in the DS, they were not closely related to application levels of GM N. Green manures undergo decomposition in order to release N. This process is so closely tied to complex microbial cycling of C and N, that the availability and effects of GM N are more difficult to predict than those of chemical fertilizers (Groffman et al., 1987). In the DS, nitrate accumulation due to soil N mineralized during the 2-mo fallow period in the control may have favored tomato crop development relative to those in legume treatments, where soil nitrate contents were low (Thönnissen Michel, 1996), as our legumes may have acted as nitrate catch crops (George et al., 1994). The depletion of soil nutrients, particularly P, the immobilization of soil N, the alteration of soil structure and exacerbated phytotoxicity from upland crops may have contributed to the short term advantages of fallow compared with legume treatments (Hamid et al., 1984). Since control and fertilizer treatments starting with the same initial NO₃ level as legume treatments were lacking, the N-supplying capacity of GM for tomato production in the DS may have been underestimated at AVRDC and at BRCI.

Liming and the application of poultry manure may have enhanced soil N mineralization at BRCI so that tomato crop N needs were met to such an extent that yields did not respond to GM or fertilizer N. High soil N mineralization in AVRDC in the DS and BRCI soil (Thönnissen Michel, 1996) resulted in tomato yields of 40 Mg ha^-1 after 2 mo of fallow (control and N fertilizer treatments), while low soil N mineralization at MMSU resulted in control tomato yield of 12.6 Mg ha^-1. Similar results were found by Stivers and Shennan (1991), where tomato yields after winter fallow were as high as those amended with legume GM or fertilizer N. Wien and Minotti (1987) reinforced the concept of fallow–GM as important for soil mineralization and N nutrition by reporting that tomato forages efficiently for soil N, obtaining only 30 to 40% from fertilizer sources.

The N-supplying capacity of the GM amendment declined after 8 wk, about the time of early fruit development, in all six field experiments (Thönnissen Michel, 1996), which we assume was detrimental to maximum tomato plant growth and yield. To achieve an optimal tomato plant nutrition using GM, an integrated approach combining organic with mineral N fertilizer could be most promising in the DS. Mineral N fertilizer (30–60 kg N ha^-1) could be applied to tomato plants starting 8 wk after GM application.

The congruence of N-release kinetics from GM with the N-uptake dynamics of the subsequent crop is a key consideration for GM management. At MMSU, a greater proportion of N mineralized from decomposing GM appears to coincide with tomato N demand of early-transplanted tomato plants, as higher yields and N uptake were achieved. Results also confirm that the efficiency of GM or fertilizer N use largely depends on crop demand (Appel, 1994), the ability of soils to supply N by mineralization of organic N (Campbell et al., 1981), and the growth and climatic conditions for the subsequent crop.

Nitrogen-15 Recovery in Plant
Low indigofera shoot biomass led to low recoveries of applied ¹⁵N. Nitrogen-15 recoveries in both legume species in our study were lower than in studies of Zebarth et al. (1991) and Vasilas et al. (1980), where 30% and 57% were recovered by alfalfa (Medicago sativa L.) and red clover (Trifolium pratense L.), and 44 to 67% by soybean, respectively. Higher labeled urea and greater quantities of labeled N fertilizer applied in their studies may have lead to higher ¹⁵N recoveries than in the present study. The distribution of ¹⁵N enrichment in soybean was comparable to results described by Vasilas et al. (1980), where highest enrichment was found in the seed, with hardly any in the roots.

Higher ¹⁵N enrichment (50–71; Table 7) in tomato fruit than in the plant compares with results of Ladd et al. (1981), who found higher enrichment in reproductive plant parts of wheat. The ¹⁵N recovery obtained in tomato plants is within the reported range (7–25%) by various crops grown subsequent to the application of ¹⁵N labeled legume residues (Vallis, 1983; Yaacob and Blair, 1980; Norman et al., 1990; Müller and Sundman, 1988; Harris and Hesterman, 1990). Recoveries of applied ¹⁵N in the subsequent crop plus soil were high (Ladd et al., 1981; Müller and Sundman, 1988), giving evidence that the ability of the soil to retain plant-derived N is strong compared with the ability of the subsequent crops and different loss mechanisms to remove it (Müller and Sundman, 1988). Harris et al. (1994) recovered 19% of the applied lugume N in microbial biomass, 38% of legume N applied in nonbiomass organic fractions; and only small amounts (<5%) of legume N were recovered in the inorganic fraction. Seligman et al. (1986) suggested that some of the added organic ¹⁵N was incorporated into stable soil organic N pools to be mineralized at a rate approaching that of the stable soil N fraction.

Residual Effect
Maize was for more efficient in N use than was tomato. The lower the tomato N uptake in the WS at AVRDC, the higher the residual effect on maize grown after tomato. Smaller N uptake by maize in the DS than the WS was due to less legume N applied, greater tomato N uptake and stronger N immobilization. The residual accumulation was comparable to that of fertilizer N application in both seasons. The strong response of maize to GM can be due to a remineralization of N partly immobilized after GM application. The N-supplying potential of GM for succeeding nonlegume crops estimated from the accumulation of inorganic N in bare fallow soil (Bowen et al., 1988) may differ strongly, depending on the succeeding crop. Tendencies towards a higher N recovery by crops with incorporated rather than mulched GM (Varco et al., 1989) were mostly confirmed with tomato, while differences in maize were not consistent.

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
Tomato yield response to GM N was high on poor soils and N could be substituted fully or in part, depending on soil N mineralization. The effect of GM N on tomato yields was marginal on fertile soils. Crops succeeding tomato (e.g., maize) may benefit from residual GM N. Further research on decomposition and N-release of legume GM, and on specific N-uptake patterns by different crops in contrasting environments, is required to further understand gaps in meeting N needs of tomato crops with GM. This should lead to the development of practicable recommendations for farmers as to optimal application method and opportunity for use of GM in specific cropping patterns.Boddey Chalk Victoria Matsui 1984; Bremmer 1965; Eaglesham Ayawabaa Rangarao Eskew 1981; George Ladha Buresh Garrity 1992; George Singleton Bohlool 1988; SAS Institute 1989; SAS Institute 1991

Received for publication August 12, 1998.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 

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