method gives an indication of the benefits of including a N2-fixing plant in the rotation, it does not confirm whether the advantages are due to sparing of soil N or an actual contribution from N2-fixation.
A development of this approach is to include a range of fertilizer N applications as experimental treatments when a cereal crop is grown in the subsequent season. The residual benefit of the previous legume crop is then expressed in equivalent units of fertilizer N required to match the amount of N provided by the legume (Fig. 5.1).
This method has a number of potential pitfalls. N fertilizers should be managed as normally recommended to gain a true comparison with the benefit of the legume, and this often means that they should be applied in split doses rather than all at sowing, otherwise losses may be high and the residual N benefits of the legume will be overestimated. For example, in India, comparisons of the N benefits from four green manure legumes showed that all gave rice yields greater than those achieved with the addition of 120 kg N ha-1as urea (Beriet al., 1989b). This does not mean that the green manures actually supplied 120 kg N ha-1, as in most cases the amount of N in the shoots turned into the soil was less than this. Instead the results suggest that the efficiency of use of urea-N was poor in the alkaline soil in which these experiments were conducted.
15N measurements of residual effects
There are a few direct measurements of the amounts of legume N recovered by the following non-legume crops using15N-labelled legume residues (Ladd, 1981). The major advantage of this method is that it is a direct measurement, as the proportion of the residue N recovered by the test crop is estimated in the same way as fertilizer recovery or fertilizer utilization efficiency, i.e.:
%N from legume stover = (Etest crop/Elegume stover)´100%
where E = atom %15N excess.
It is essential that the legume residues are uniformly labelled with15N, otherwise the above equation is invalid. There is also a danger that15N-labelled residues of plants grown under controlled conditions may have a very different composition than the residues of plants grown in the field, which will influence the speed with which N is released. Leaves of glasshouse-grown15N-labelled soybean plants contained 3.2% N (a C : N ratio of 13) compared with 1.5% N (C : N ratio of 28) in the leaf litter of field-grown plants (Bergersenet al., 1992). Stover of field-grown soybeans is lignified with a C : N ratio of 45 (Toomsanet al., 1995).
There is evidence (Foxet al., 1990) that the use of15N-labelled residues may underestimate the true amounts of N made available to subsequent crops if the inorganic15N mineralized during the decomposition of the labelled residues can sub- stitute for unlabelled inorganic N that would otherwise have been immobilized – the process of pool substitution (Jenkinsonet al., 1985). It is also possible that addition of legume N may increase the availability of native soil N – that is, cause a real and positive priming effect or ‘added nitrogen interaction’ (Jenkinsonet al., 1985). A
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Chapter 5
Fig. 5.1. A hypothetical example of calculation of fertilizer equivalence values by use of an N-fertilizer response curve. The dotted line shows how the residual benefit is calculated, in this case to be equivalent to 60 kg N ha-1of N as urea. The response curves give examples of the types of crop response often found after growing a legume: (a) shows a case where there is no residual N benefit from growing the legume; (b) shows an example where the residual benefit can be ascribed solely to an N effect, as the legume effect is substituted by increased amounts of fertilizer N; (c) shows a case where the residual benefit cannot be explained solely on the basis of N effects, as the benefit is maintained when larger amounts of N are added.
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comparison of results from several studies suggested that use of15N-labelling tends to give smaller estimates of N recovery from legume residues compared with N balance estimates, presumably due to pool substitution effects (Giller and Cadisch, 1995).
A further variation of this approach is to use an isotope dilution method, where unlabelled organic residues are added to plots previously labelled with15N and the contribution of N is calculated as described earlier (Chapter 4) by comparison of the15N-enrichments of plants grown in plots with or without addition of residues (Suwanaritet al., 1986; Kumar Raoet al., 1987). If this isotope dilution method and
15N-labelled residues are both used separately (in what have been termed ‘mirror image’ plots), this should allow the relative importance of pool substitution effects to be determined (provided that all of the soil N that mineralizes is uniformly labelled).
In a series of experiments using this approach to measure residual benefits from groundnut to maize or rice in northeast Thailand, variability was disappointingly large and obviated conclusions being drawn with any certainty (McDonaghet al., 1993; Toomsanet al., 1995). The results tended to indicate that priming effects were more important than pool substitution, although others have found little evidence for priming effects (Cadischet al., 1998). McDonaghet al. (1993) concluded that this isotope dilution approach was unlikely to yield useful results.
N contributed below ground in roots and nodules
Even where the above-ground biomass of the N2-fixing plant is removed, there may be significant amounts of N added to the soil in the form of dead roots, nodules and other contributions from rhizodeposition. Recovery of roots of grain legumes has indicated that only 10% or less of the plant N is present in roots at harvest (e.g.
Kipe-Nolt and Giller, 1993). Considerably more N may be present in the root systems of growing plants. Although nodules are rich in N, they often constitute such a small proportion of the below-ground biomass that the amounts of N in nodules are small, though some N2-fixing trees such asErythrinaare exceptions (Nygren and Ramirez, 1995) (Chapter 11). The N in roots and nodules of annual legumes may be translocated to the shoots during senescence, but less than 30% of the N in roots is usually remobilized (Peoples and Gifford, 1997). Standing crop measurements of roots tend to underestimate the amounts of N that may be contributed to the soil as fine roots are formed and senesce continuously (e.g. Schroth and Zech, 1995). As discussed above, grazing or cutting of legume shoots causes senescence of roots and nodules and part of the N released below ground will be recovered by a companion crop through N transfer, or can later be reabsorbed by the legume when it regrows.
The methods used to determine residual benefits often include below-ground N contributions from N2-fixation. Modifications of these methods can be applied to separate out what benefit is derived solely from roots and nodules of the legumes. For example, N yield of a subsequent crop can be assessed both in paired plots where the biomass is removed or returned, and where a non-nodulating genotype of the legume was grown (e.g. McDonaghet al., 1993; Toomsanet al., 1995). The residual benefit from the above-ground legume biomass can be distinguished by including additional
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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.