plots where no legume was grown previously and an equivalent amount of legume biomass to that produced is transferred.
There are only a few studies where15N-labelled roots have been added to soil and direct recovery of the N has been measured. Bergersenet al. (1992) found that negligible amounts of N were recovered from soybean roots using this method. The N content of roots is often so limited that it actuallyincreasesduring decomposition as the wide C : N ratios of the roots result in immobilization of N (Lehmann and Zech, 1998; Urquiagaet al., 1998).
The use of15N-labelling has been proposed for estimating amounts and turn- over of N in root systems of growing plants (Janzen and Bruinsma, 1989). Labelling of the root system can be achieved by exposure of plants to15N-labelled ammonia (Janzen and Bruinsma, 1989), or to leaf application of ammonium or urea (Ledgard et al., 1985a; Russell and Fillery, 1996b; McNeill et al., 1997) as N is rapidly distributed throughout the plant. The total amount of root N, including the N in fine roots and that which has been lost to the soil through rhizodeposition, can be calculated from the15N-enrichment of a ‘clean’ root fraction (Russell and Fillery, 1996a). The main assumption is that the root system is uniformly labelled with15N, so that if the15N-enrichment measured from a sample of the rhizosphere soil is less than that of the clean root, this is due to isotope dilution by soil N. The equation that can be used for calculating root N in the soil surrounding the roots is:
Nfine roots and rhizodeposits= (Eclean root´Erhizosphere soil)/Nclean root
where E = atom %15N excess.
The accuracy of estimates of total root N made using this method have not yet been fully verified; other potential problems are contamination of the soil during leaf labelling, and differences in15N-enrichment between nodules and roots. Results sug- gest that below-ground N contributions may have been underestimated substantially in the past, and may amount to 50% of the total amount of N in pasture legumes and 40% in crops at peak biomass (McNeillet al., 1997, 1998; Rochesteret al., 1998). As indicated, some of this N may be remobilized to grain during maturation of the plants.
It is clear that all estimates of N2-fixation, or of the residual benefits of legumes, that ignore contributions of N below ground will be underestimates, but by what magnitude remains a subject for future research.
will be limited and any mineralized N will tend to be used immediately by the microorganisms for growth or be immobilized. Release of N into the soil (net mineralization) for use by plants is thus a balance of the processes of mineralization and immobilization. The C : N ratio (usually expressed as g C to g N) is a useful guide as to whether net release of N from organic material is likely to occur during the early stages of decomposition. The C : N ratio also provides an indication of how rapidly a plant material is likely to be decomposed (e.g. Cornforth and Davis, 1968;
Frankenberger and Abdelmagid, 1985). Plant residues with a high C : N ratio (say
> 20 : 1) are likely to decompose slowly, with initial net immobilization of N, whereas residues with a smaller C : N ratio are likely to decompose more rapidly, with net mineralization of N occurring right from the beginning. Legume residues commonly have C : N ratios of less than 20 : 1 and therefore tend to release N and decompose rapidly (Palmet al., 2001).
This is, however, a gross simplification. Decomposition is dependent on the enzymatic cleavage of chemical bonds within the plant material. Soluble, low molecular weight substances such as glucose or amino acids are rapidly attacked by microorganisms, whereas insoluble polymeric materials tend to be cleaved primarily by slow-growing microorganisms (i.e. those with a slow basal rate of metabolism), so that breakdown of more complex substrates takes longer. Both the physical structure and the chemical composition of a plant residue thus determine whether or not it is resistant to decomposition. In particular, legume residues that contain a lot of lignins and polyphenolic compounds tend to be resistant to decay (Vallis and Jones, 1973; Palm and Sanchez, 1991; Constantinides and Fownes, 1993). The (lignin + polyphenol) : N ratio can give an excellent prediction of the N mineralization rate in legume residues, and decomposition of residues that contained less than 2% N resulted in a net immobilization of N over the first 6 weeks (Foxet al., 1990). Many legumes have tissues rich in reactive polyphenols, which bind strongly to proteins and render the N resistant to microbial attack (Handayantoet al., 1995). As leaves age, the N content decreases and the lignin content increases, so that older tissues decompose more slowly (Joachim and Kandiah, 1936). Older plant residues also tend to be physically harder and therefore less readily attacked by the soil fauna, which play an important role in decomposition by ‘comminuting’ or breaking up the residues into small fragments with a greater surface area for microbial attack.
Further discussion of the decomposition process is beyond the scope of this book and for more information on this subject readers are referred to Swiftet al.
(1979), Jenkinson (1981) and Cadisch and Giller (1997).
Managing organic residues for N release and maintenance of soil fertility Green plant material tends to contain little lignin, which is laid down in plants as a structural component in secondary thickening of cell walls, and thus generally decomposes more rapidly than grain legume stover or woody tissues. Decomposition of shoots of forage legumes or prunings of legume trees can be rapid – several reports suggest that 40% or more of the N in legume shoot material can be released in less
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than 2 weeks after addition to the soil (e.g. Cornforth and Davis, 1968; McDonagh et al., 1995a). This means that the N may be released before the crop is sufficiently large to take full advantage of it, and that much of the N may be lost from the system.
The management of nutrient release from organic residues to meet crop demands is a major challenge for research and a principal objective of the Tropical Soil Biology and Fertility (TSBF) programme (Woomer and Swift, 1994). Soil fertility, and N supply from soils in the longer term, is closely related to the maintenance of soil organic matter. Unfortunately there appears to be a direct trade-off: residues that release N readily for crops contribute little to build-up of soil organic matter, and recalcitrant residues that take a long time to decompose, or form stable complexes, have a poor capacity to supply nutrients (Palmet al., 2000).
Legumes as feed and fodder
The quality characteristics of plant materials that determine their rates of decomposition, principally the N, lignin and polyphenol contents, are essentially the same as those that determine the utility of the plant materials as animal feed (e.g.
Topps, 1992; Kumar and D’Mello, 1995; Chesson, 1997). Animals are surprisingly inefficient at utilizing N, often assimilating less than 10% of the N in the feed.
Excretion of N by animals in urine is a major source of N loss from cropping systems, as hydrolysis of urea to ammonia causes development of high localized concentrations of ammonia and the high pH that favours volatilization of ammonia gas (Chapter 10). The quality of the diet fed to animals can influence the capacity of resulting manure to release N (Mafongoyaet al., 2000; Delveet al., 2001), but the storage and handling of manure have a much stronger influence on the ability of animal manure to supply nutrients. Cattle manure collected from kraals in Africa can be as much as 90% sand (Gilleret al., 1997).
Conclusions
Any fixed N left in the field in the form of dead legume residues can be a source of N for other crops. The availability of the N for uptake will depend on the rate of mineralization of N in relation to the demand of growing crops. N in green manures is generally not only greater in quantity but also more available for rapid mineralization, as green legume residues have a smaller C : N ratio and lignin content than residues of mature grain legume crops. The large polyphenol contents found in some legumes have a strong influence on the capacity of their residues to release N in soil and in animal feeds.
The benefits of N2-fixation by legumes to cereals growing in intercrops or to grasses growing in mixed swards are less clear. In many cases no benefit to the N status of cereals has been seen when they are intercropped with legumes. In cases where a benefit is found, it is mainly due to sparing of soil N rather than direct transfer from the legume. As the contact time between grasses and legumes growing
in mixed swards is generally so much longer than that of most intercrops, it would be expected that the transfer of fixed N from the legume would be a more significant process, and this generally seems to be the case. One last plea whilst on this subject is that the definition of nitrogen transfer of Henzell and Vallis (1977) should generally be adopted to include all of the possible mechanisms listed in Table 5.1 and not only that of ‘excretion’, which is perhaps least likely to be of agronomic significance.
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