Legumes
Many legume species respond strongly to different photoperiod and temperature regimes. Soybean accumulated only half as much biomass in the DS than in the WS at AVRDC, while N accumulation in the DS was only 10 to 20% less than in the WS. Soybean biomass and N yields at MMSU and BRCI compared favorably with yields obtained in Taiwan (Thönnissen Michel, 1996) and in Texas (Munoz et al., 1983) where soybean was grown at high seeding densities for hay production. Indigofera yields at MMSU were about one half of the lowest yields obtained at AVRDC, and of the reported yields by Thönnissen Michel (1996) and Batilan et al. (1989). Indigofera has small seeds and seedlings emerge slowly making it more vulnerable to variable soil conditions such as soil crusting and compaction. Heterogeneous seed quality of the indigenous indigofera seed used at MMSU and a strong rainfall at 1 wk after sowing followed by soil crusting may have been responsible for the poor performance of indigofera, which is the main green manure crop used in this area of the Philippines (Garrity and Flinn, 1988). These factors make it difficult to propagate indigofera as a short term (60–74 d) GM. Mungbean yields were inferior to those reported by Meelu and Morris (1988), who obtained mungbean yields comparable to our soybean yields. They stressed the importance of the effect of the environment on N accumulation by GM species which implies that for optimal N accumulation the GM species must be adapted to the physical environment they will experience during growth.
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 ( 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 ( 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.
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