Nitrate leaching in organic arable crop rotations
By Margrethe Askegaard and Jørgen E. Olesen, Department of Agroecology, Danish Institute of Agricultural Sciences.
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Nitrate leaching from crop rotations for organic grain production were investigated from 1997 to 2000 on three locations in Denmark. The locations were Jyndevad, Foulum and Flakkebjerg, respectively, with soil clay contents of 5, 9 and 14%, and average precipitation of 952, 713 and 624 mm. |
Three experimental factors were included in the experiment in a factorial design:
- proportion of grass-clover and pulses in the rotation,
- catch crop (with and without),
- manure (with and without
Two four-course rotations were established at each locality. In this way, the experiment allowed three crop rotations to be compared (see Table 1).
Design and management is further described in Table 1. Suction cups were installed at 0.8 m depth (Jyndevad) and at 1.0 m depth (Foulum and Flakkebjerg) in selected plots.
Effect of the treatments
The effects of crop rotations and catch crops are presented in Table 2. There were no significant differences in nitrate leaching between crop rotations at the three locations except for the interaction between crop rotation and catch crop at Foulum, where the nitrate leaching in the treatment without catch crops was 42% larger in rotation 2 than in rotation 4.
There were significant effects of catch crops on nitrate leaching at Jyndevad in rotation 1 and 2 and at Foulum in rotation 2. The effect of catch crops on the nitrate leaching at Jyndevad was based on stubble-harrowed plots in the treatment without catch crops because of the need to control couch grass. This has probably increased the relative effect of catch crops due to a larger risk of nitrate leaching in the harrowed plots.
No effects of manure application on the estimated nitrate leaching were found at the three locations.
Absence of slurry effects
It was expected that the nitrate leaching losses would have been larger in the treatments with manure application than without, because manure application increases the N mineralisation in soils during autumn/winter. However, we did not observe such differences. The reasons were probably:
1) that the effect of ammonia-N in the slurry on grain yields was similar to the effect of N in conventional fertilizers (Olesen et al. 2002), indicating that the crop N-uptake had been effective, and
2) that manure input to a crop rotation contributes little to nitrate leaching when the application is below the N-optimum for the crops.
In the long term, measurable differences are expected to build up due to larger input of manure-derived organic matter and plant residues. However, the magnitude of such differences will probably depend on the effect of manure application on the biological N fixation in the system.
Time lag of effects on the coarse sand
The nitrate concentrations in the treatments with and without catch crops can be compared in all the fields at Jyndevad. Here, the catch crops did not reduce the nitrate concentrations immediate after harvest, but after one to two months the concentrations were effectively reduced in the treatments with catch crops relative to the treatments without (Figure 1). In several of the fields and years this reduction continued after spring ploughing and it lasted in some cases until harvest of the following crop.
At Foulum and Flakkebjerg the time-lag effect after harvest was not recognised but there seemed to be the same positive effect of the catch crops on nitrate concentrations after spring ploughing.
Drinking water quality with catch crops
Estimated year-averaged nitrate concentrations in rotation 2 without catch crop showed similar values at the three locations, between 12.2 and 14.4 mg nitrate-N per L as an average of the rotation (data not shown). The use of catch crops reduced the nitrate concentrations to below 10 mg nitrate-N per L in rotation 1 and 2 at all locations. In comparison the WHO limit for drinking water is 11.3 mg NO3-N per L.
The estimated year-averaged nitrate-N concentration in rotation 4 without catch crops was 8.9 mg per L at Foulum and 9.4 mg per L at Flakkebjerg. The use of the legume catch crops maintained this level with a tendency for an increase.
Green manure crops may increase nitrate leaching
Rotation 4 was not included at Jyndevad, because the coarse sandy soil at Jyndevad was considered unsuitable for organic rotations without a fertility-building green manure crop. However, the applicability of this strategy can be questioned, because the nitrate leaching losses were large.
At Foulum the problem could be handled by inclusion of catch crops, which reduced the leaching losses to the same level as the crop rotation without a green manure crop. At Flakkebjerg the green manure crop did not increase the nitrate leaching.
On the Jyndevad coarse sand it could be relevant to replace the green manure crop with N2-fixing catch crops used strategic in the rotation. This would give a smaller but more frequent N-input to the crop rotation.
Further information
Olesen, J.E., Askegaard, M. and Rasmussen, I.A., 2000. Design of an organic farming crop rotation experiment. Acta Agriculturae Scandinavica, Section B, Soil and Plant Science 50, 13-21.
Olesen, J.E., Rasmussen, I.A., Askegaard, M. and Kristensen, K. 2002. Whole-rotation dry matter and nitrogen grain yields from the first course of an organic farming crop rotation experiment. Journal of Agricultural Science 129, 361-370.
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