Relations between nitrogen leaching and food productivity in organic and conventional cropping systems in a long-term field study
Introduction
The environmental impact of agricultural practices has received much focus over recent decades. Nitrogen (N) leaching has been one of the issues of concern, due to its negative influence on ground water quality and the eutrophication of coastal waters. Gaseous N losses have also been focused upon, mainly due to their contribution to the greenhouse effect. As a result, a wide range of new agronomic approaches has been developed to reduce the risk of N losses from agricultural soils. Organic farming (without mineral fertilizer or pesticide use) has been suggested as an alternative to conventional farming systems in order to enhance environmental quality (Poudel et al., 2002). Kirchmann and Bergstrom (2001) reviewed a number of studies comparing nitrate leaching from organic and conventional farming systems, and found that the nitrate losses from organic farming systems were indeed somewhat lower than those from conventional ones, a fact which could be explained by lower nutrient inputs in the organic systems. In most cases, however, the lower nutrient inputs in organic cropping systems also result in lower yield levels, relative to conventional systems (Kirchmann and Bergstrom, 2001, Kratochvil et al., 2004). Reduced yields per area imply that more land is needed to produce the same amount of food. Dalgaard et al. (1998) estimated that a change from conventional to organic dairy farming at a national scale in Denmark would demand a 47% increase in area for dairy farming in order to maintain production. There is an increasing global demand for food, due both to the increase in the population and to an increase in per capita demand for meat (Bradford, 1999, Nonhebel, 2005). Additionally, there is an increasing use of land for purposes other than food production (Nonhebel, 2005). Consequently, the ideal cropping system should maximize the production of human nutrients per unit area, and minimize the undesirable effects on the environment, resulting in a low ratio between emitted pollutants and food produced (loss-to-production ratio, LFP-ratio).
Literature reporting the relation between N losses and food production is scarce. Kirchmann and Bergstrom (2001) attempted to relate N leaching to yields through a literature review, but came to no clear-cut answer, due to the very limited number of articles that provided the necessary data. In some more recent studies, yields have been related to N leaching (Syväsalo et al., 2006) and to greenhouse gas emissions (Flessa et al., 2002). Flachowsky (2002) related losses of N and P and methane emission to the amount of animal proteins produced, and Weiske et al. (2006) calculated greenhouse gas emissions per kg milk. In a more indirect approach, Dalgaard et al. (1998) used N surplus as an indicator for risk of N loss, and calculated the N surplus per liter of milk and N surplus per kg pig meat. For comparison of farms with mixed production (e.g. cash-crops and livestock), however, the produce needs to be converted into a common unit.
In the current study, data from the ongoing Apelsvoll cropping system experiment in southeast Norway (Eltun, 1994) are used. The crop rotations were selected to match typical farms in the region, with arable crops only (arable systems) or with a mixture of arable and forage crops for milk production (mixed dairy systems), cropped either conventionally or organically. If the systems were to be compared with respect to the extent to which they meet all human nutritional requirements, the amounts of produced energy, protein and important vitamins and micronutrients should be considered (e.g. McIntyre et al., 2001). Since the experiment was not designed to meet such requirements, but rather to study yields and environmental impacts (Eltun, 1994), it was decided to convert all produce into metabolizable energy consumable by humans. For the mixed dairy systems, this involved a conversion from energy in fodder to estimated energy in meat and milk.
When studying the environmental impact of farming practice, it has been emphasized that the whole production chain should be considered, particularly for greenhouse gas emissions (Flessa et al., 2002, Weiske et al., 2006), where the majority of impacts usually occur off-farm (Wood et al., 2006). Although gaseous losses were estimated, more focus has been placed on measured N leaching and runoff in the present study, and the boundary of the analysis was defined as the farm. Since most of the environmental impacts resulting from the production of input factors also occur off-farm (Wood et al., 2006), energy budgets were not considered.
Comparing conventional and organic systems is a challenge, since the crop rotations used both in practice (Knudsen et al., 2006) and thus in experiments (e.g. Kirchmann and Bergstrom, 2001, Nguyen and Haynes, 1995, Torstensson et al., 2006) usually differ. The choice of crops and rotations has a large influence on production and nutrient flows. In a cropping system experiment with a holistic approach, as presented here, the measured output factors may be used to evaluate the effects of management and input factors, but not the internal processes of the systems related to individual crops or the order of crops in the rotation. To evaluate such effects, the complex structure of mass flows, nutrient transformations and their interactions with plant growth, soil and climatic conditions would need to be considered, requiring the use of dynamic modelling tools (e.g. Cabrera et al., 2005). This was, however, beyond the scope of this paper.
The present study is aimed at comparing the cropping systems in terms of (i) yields and production of metabolizable energy for human consumption; (ii) major N flows and mass balances of N; and (iii) N loss-to-production ratios, as an expression of the environmental efficiency of food production.
Section snippets
Experimental site and treatments
A detailed description of experimental design and cropping system management is presented elsewhere (Eltun, 1994, Korsaeth and Eltun, 2000, Riley and Eltun, 1994). Briefly, a 3.2 ha experiment with tile-drained plots was established in 1988/1989 on loam soil, with imperfect to poor natural drainage, on Apelsvoll farm of the Norwegian Institute for Agricultural and Environmental Research, Arable Crops Division, 60°42′N, 10°51′E, altitude 250 m. The major soil groups are classified as Aquic
Yields
Among the arable systems, the yields of barley, wheat and oats were 4.1 ± 0.2, 4.0 ± 0.3 and 5.0 ± 0.2 Mg ha−1, respectively. The yields of barley and wheat in the mixed dairy systems were 4.2 ± 0.2 and 3.8 ± 0.3, respectively. There were significant differences within each group. The largest overall cereal yields were measured in CA1. Yields of barley and oats were similar in CA1 and CA2, but wheat yields were lower in CA2 (Fig. 1, upper plot). The organic arable system (OA) had the lowest cereal yields
Yields
The cereal yield levels were relatively high for the region, reflecting the fertile soil of the experimental site. The total N content of the soil is more than twice the national average in agricultural soils (Korsaeth and Eltun, 2000).
Cereal yields differed most among the arable systems (Fig. 1, upper plot). The low yields of organically cropped barley and wheat (OA) may be partly due to P and K deficits. Nutrient budgets for OA over the first 10 years of the experiment (1990–1999), showed
Conclusions
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Relating N losses to the amount of food produced, instead of evaluating N losses on an area basis, increases both the relative difference between systems and affects the ranking order in terms of environmental impact.
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Organic arable cropping based on green manure as the only nutrient source has enhanced risks for soil N mining, N losses and reduced productivity, whereas conventional arable cropping may sustain the soil N content and achieve a low N loss-to-production ratio by using catch crops,
Acknowledgements
H. Riley and R. Eltun are gratefully acknowledged for critically reading the manuscript and T. Gaardløs for his skilled technical assistance. Two anonymous referees are also acknowledged for valuable comments on an earlier version of the manuscript. The project was funded jointly by the Norwegian Institute for Agricultural and Environmental Research and Yara International ASA.
References (64)
Contributions of animal agriculture to meeting global human food demand
Livestock Prod. Sci.
(1999)- et al.
The dynamic North Florida dairy farm model: a user-friendly computerized tool for increasing profits while minimizing N leaching under varying climatic conditions
Comput. Electron. Agric.
(2005) - et al.
Integrated evaluation of greenhouse gas emissions (CO2, CH4, N2O) from two farming systems in southern Germany
Agric. Ecosyst. Environ.
(2002) - et al.
Evaluating pea and barley cultivars for complementarity in intercropping at different levels of soil N availability
Field Crops Res.
(2001) - et al.
A model of ammonia volatilization from a grazing livestock farm
Atmos. Environ.
(1996) - et al.
Effect of protein and energy supplements on milk production in organic farming
Anim. Feed. Sci. Technol.
(2002) - et al.
Nitrogen mass balances in conventional, integrated and ecological cropping systems and the relationship between balance calculations and nitrogen runoff in an 8-year field experiment
Agric. Ecosys. Environ.
(2000) - et al.
Temporal changes in mineralization and immobilization of N during degradation of plant material: implications for the plant N supply and nitrogen losses
Soil Biol. Biochem.
(2002) - et al.
Modeling cropping strategies to improve human nutrition in Uganda
Agric. Syst.
(2001) - et al.
Energy and labor efficiency for three pairs of conventional and alternative mixed cropping (pasture arable) farms in Canterbury, New-Zealand
Agric. Ecosyst. Environ.
(1995)