Agriculture Reference
In-Depth Information
fertilisers (Stockdale
et al.
2001) providing they are effectively nodulated. The sustainability of
legume use to supply crops with N in either organic or conventional farming systems (Ridley
et al.
2004) depends on the:
1 fixation of sufficient N in the legume biomass;
2 ability of the soil community to increase mineralisation of organic N; and
3 capacity of farming practices to maximise the beneficial soil fertility and environmental
effects of legumes and minimise their negative effects (e.g. increased acidity and N
leaching).
Nitrogen fixation by legume biomass may be efficient but the availability of the residues to
subsequent crops could be improved. Watson
et al.
(2002b) reviewed farm-scale nutrient
budgets for 88 organic farms in temperate climates and found that although all of the N budgets
showed a surplus, N use efficiency was low (average 0.3). Similarly, in another review, Berry
et
al.
(2002) concluded that although legumes had the potential to supply sufficient N to crops,
there was usually a shortfall. Where legume pastures are incorporated only once every few
years, organically produced crop residues and manures tend to have low N contents and slow
mineralisation rates. Improving legume-N availability to subsequent crops could be achieved
by (i) careful timing of organic residue incorporation, (ii) management practices that improve
the quality of legumes as microbial substrates and (iii) matching crop type with N mineralisa-
tion dynamics (Berry
et al.
2002).
For legumes to maintain adequate N availability to plants, reductions in soluble N inputs
must be compensated for by increased mineralisation of organic N (Bloem
et al.
1994). Poten-
tially mineralisable N is frequently higher in organic farming systems (Reganold
et al.
1993,
Drinkwater
et al.
1995, Gunapala and Scow 1998, Liebig and Doran 1999, Hauggaard-Nielsen
et al.
2003). Bloem
et al.
(1994) attributed the higher average rates of N mineralisation (30%)
for integrated compared with conventional farming to higher soil organic matter contents, but
there was no difference in gross N transformations between a pair of organic and conventional
arable farms (Cookson
et al.
2005a). Mite diversity in a conventional soil under wheat was cor-
related with gross N immobilisation (Osler
et al.
2004), so this may also be relevant in organic
farming systems because mites can increase the diversity of soil fauna (Mäder
et al.
2002, Hole
et al.
2005).
The sustainability of using legumes to supply crop requirements for N also depends on the
capacity of organic management practices to maximise beneficial effects of legumes while
minimising the potential for N leaching. Intercropping with legumes can increase the effi-
ciency with which soil nutrients are used (Hauggaard-Nielsen
et al.
2003). Steen Jensen and
Hauggaard-Nielsen (2003) advocated increased use of legumes in farming systems because of
beneficial environmental effects including improved soil structure, erosion protection,
increased biological diversity, stimulation of rhizosphere organisms, acidification of alkaline
soils and reduced energy use and carbon dioxide (CO
2
) production on and off the farm.
However, asynchrony in plant demand for N and its release from organic matter can cause
nitrate leaching, especially post harvest, and the acidifying effect of legumes is detrimental in
acid soils (Fillery 2001, Steen Jensen and Hauggaard-Nielsen 2003, Ridley
et al.
2004). Kirch-
mann and Bergström (2001) reviewed nitrate leaching from organic and conventional farms.
They concluded that although the average nitrate leaching over a rotation was lower in the
organic systems investigated, when the lower intensity of N input in organic farming was taken
into account, there was no difference (Kirchmann and Bergström 2001). In this evaluation,
there were insufficient data to compare nitrate leaching based on yield. Management options
for controlling N leaching were discussed by Kristensen
et al.
(1995), Steen Jensen and Haug-
gaard-Nielsen (2003) and Ridley
et al.
(2004) and well-managed organic systems have the
