A review of leguminous fertility-building crops, with particular ...

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A review of leguminous fertility-building crops, Defra project OF0316 3. How much N is captured? Italian ryegrass was found to be a more suitable control plant than either winter rape or winter rye (Mueller & Thorup-Kristensen, 2001). As in the example above, estimates may also be influenced by the method used to measure fixation. McNeill & Wood (1990) reported that values obtained using the N-difference method were approximately 25% lower than those estimated using the 15 N isotope dilution method. Similarly, under a high N regime, the N-difference method was found to underestimate the amount of fixed N in harvested clover material by up to 33% (Høgh-Jensen & Kristensen, 1995). Boller & Nösberger (1987), however, found that, on average, the difference method gave a 10% higher estimate of fixation than the isotope dilution method for mixtures of grass with either red or white clover. A study by Loges et al. (2000a) compared estimates of fixation by the Ndifference, isotope dilution and natural abundance methods for pure legume and grass/legume mixtures. Estimates using the isotope dilution method were between 89 and 104% of those obtained with the N-difference method, whereas estimates using the natural abundance method were only 52-61% those with the difference method. These values were based on sampling of the above-ground herbage. Differences between the methods were less marked (67-97%) when the calculations included below-ground plant components and changes in soil mineral-N. Nevertheless, it is clear that there are large uncertainties in any measurments of N-fixation. An additional limitation of these methods is that they are most suited to determining the accumulation of N in the above-ground parts of the plant. Few measurements include the N in roots and in stubble below sampling height. These components may be particularly important in providing N to following crops. In the study by Loges et al. (2000a), estimates using the N difference, isotope dilution and natural abundance methods were on average 33, 46 and 75% greater when based on total plant material + soil mineral N than when based on just the aboveground, harvested material. McNeill et al. (1997) estimated that the total below-ground N for subterranean clover plants was equivalent to 73% of that in the shoots. Thus, much of the below-ground N can be present as fine roots and rhizosphere soil. It was concluded that standard root recovery procedures are likely to underestimate the total N accretion and turnover. Nitrogen-fixation has also been determined from the rate at which acetylene is reduced by the nodules when the gas is injected into the soil around the legume roots. The acetylene is reduced by nitrogenase in the nodules, the same enzyme responsible for the N-fixation reaction. However, this method is generally considered to be unsuitable for estimating the amount of N fixed during the season because of uncertainties about the exact equivalence between the Nfixation and acetylene reduction processes and the need to interpolate from a limited number of short-term measurements. It is of greater value for comparative, short-term measurements of relative rates of fixation: in these circumstances, results are more satisfactorily reported in terms of rates of nitrogenase activity. Methodological difficulties make it almost impossible to get an accurate measure of fixed N. All methods have advantages and disadvantages. The literature has used a number of methods, so that estimates are not always directly comparable. Written by S Cuttle, M Shepherd & G Goodlass Page 46
A review of leguminous fertility-building crops, Defra project OF0316 3. How much N is captured? The majority of N-fixation values in the following sections were obtained using the N-difference and isotope dilution methods. Where the less-reliable acetylene reduction method was used, this is stated in the text. Most estimates refer to fixation in the above-ground herbage. Estimates of the quantities of N fixed by legumes that can be grown under UK conditions are considered below. Results are included from non-UK studies where they provide an indication of the fixation that can be achieved under conditions that are more (or less) favourable than those in this country. In particular, much of the information about fixation by white clover originates from studies in New Zealand where the temperate climate and longer growing season particularly favours clover. Legumes grown in studies in southern Australia and in the dryland cereal areas of North America are more likely to be affected by summer drought unless irrigated. Data from other countries in north Western Europe are more directly comparable with UK conditions but, further east, are likely to have a more continental climate with warmer summers and colder winters. For the purposes of modelling the potential for legumes in European dairy farming, Topp et al. (2002) proposed eight agro-climatic zones for Northern Europe. The north of the UK was grouped with Ireland and the southern UK grouped with France, the Netherlands and Belgium. Highest yields of fertilised grass and of monocultures of red clover and lucerne were allocated to this latter zone. Mixtures of grass with white clover, red clover or lucerne also performed best in this zone. White clover is by far the most important legume for use in grazed pastures. Both white and red clover are important as silage crops. Red clover is the most common leguminous green manure grown on organic farms in the UK (Phillips et al., 2002; Watson, 2002). Other forage legumes and green manures include lucerne, vetches, trefoil and lupins. The range of grain legumes includes winter and spring varieties of field beans, peas, lupins and specialised crops such as lentils and soybeans. Peas and beans are the most commonly grown, mainly for use as animal feed (Taylor & Cormack, 2002). The potential of each of these crops to fix atmospheric N is reviewed in the following sections. 3.1.1. White clover (Trifolium repens) There have been many studies of the amount of N fixed by white clover, reflecting its importance in pastures in this country and more especially in New Zealand. White clover is generally grown as a mixture with grass and differences in the proportions of grass and clover undoubtedly contribute to the wide range of fixation values observed. Factors affecting the growth of white clover/grass mixtures have been reviewed by Kessler & Nösberger (1994). Typical estimates of annual fixation are listed in Table 3.1. White clover monocultures are relatively drought resistant but less so when grown in mixtures with grass (Thomas & Bowling 1981). Estimates of N fixation by white clover span a wide range of values and in spite of the abundant information, there is still uncertainty about the quantities fixed. Estimates of the amounts of biological N-fixation by grass/clover pastures in the UK have been reviewed by Wood (1996). It was concluded that white clover had the potential to fix 100-200 kg Written by S Cuttle, M Shepherd & G Goodlass Page 47
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