In Europe the controls of S emission have decreased the concentrations of sulphur dioxide in the atmosphere dramatically over the last 20-30 years leading to decreased inputs of S to agricultural land. Today, therefore, S has become a limiting nutrient in many European countries. To maintain a sufficient S supply in the future when further reductions in the atmospheric deposition are expected, it is important to reduce leaching losses of sulphate (Eriksen et al. 2002).

Catch crops absorb sulphate

It has been demonstrated that a catch crop succeeding the main crop can absorb sulphate from the root zone during autumn and winter and thereby reduce sulphate leaching (Eriksen and Thorup-Kristensen, 2002). Especially cruciferous crops, having a high S-demand and vigorous root growth, efficiently depleted the soil sulphate pool (figure 1).

In a crop rotation, including both low-S-demanding cereals and high-S-demanding main crops, e.g., crucifers or vegetables, a suitable catch crop strategy may prevent excess sulphate from leaching in years with low-S-demanding crops and instead transfer S to the following high-S-demanding crop.

For such a strategy to work it is important that S immobilised in the catch crop is mineralised quickly after incorporation. Therefore an experiment was established to investigate the net release of plant-available S following incorporation of a wide range of catch crops differing in chemical composition.

Sequestration and mineralization of S

Plant material from two catch crop field experiments was collected in early November in two years and used for incorporation in pot experiments with barley in the following years. The collected catch crop material showed a very wide range in composition (table 1).

At both sites, the legumes performed well and despite low S content the S-uptake in legumes in the autumn was considerable especially at the sandy loam site (Aarslev) where the best catch crops sequestered 10 to 12 kg S per ha, whereas the poorest catch crops sequestered less than 3 kg S per ha. None of the catch crops at the coarse sand site (Jyndevad) were able to sequester more than 5 kg S per ha.

The total S uptake in aboveground biomass in barley following incorporation of catch crops or application of inorganic S was used to estimate the relative mineralisation of catch crop S available for barley during one growth season (figure 2). Both experimental years there was a huge spread in S mineralization between the catch crops with crucifers as high S-release crops (57 to 85 pct mineralised of total S added) and legumes as low S-release crops (-2 to 46 pct of total S added). These differences could be partly attributed to differences in the C/S ratio of the catch crops.

Management of S catch crops

When deciding on a strategy for catch crops the farmer must prioritise between optimisation of N effects, control of soil borne diseases, improvement of soil structure, erosion control, minimising the cost of growing a catch crop, or import of N through fixation by legumes. Usually, highest priority is given to maximising the N efficiency in the system. And it is in this context the effect of catch crops on sulphur nutrition should be seen.

Non-legume catch crops will often follow crops leaving high amounts of N rich residues or after ploughing in of perennial crops. Such catch crops may be incorporated in late winter or early spring - late enough to avoid leaching from mineralisation of catch crop residues and early enough to avoid the catch crop from taking up nutrients that would normally be directly available for the succeeding crop. In this case there is no conflict between N and S nutrition, as most species within the non-legume category will release both N and S soon after incorporation, especially the crucifers having low C/N and C/S ratios.

Legumes as S catch crops

In the case of legumes, they are generally not as effective as non-legumes to deplete soil inorganic N and should be grown where little inorganic N is left in the soil. With legume species incorporation may be postponed to allow more time for biological N fixation and thus increased N mineralisation after incorporation. However, the late incorporation of materials with high C/S ratios may retard S mineralisation considerably or even lead to immobilisation of S.

In S-deficient crop rotations, the release of S from incorporated catch crops will not be able to fulfil the needs of S-demanding crops such as oilseed rape, grass ley, kale or onion. These crops require 30 to 70 kg S per ha compared to only about 10 to 20 kg per ha required by cereals and they will probably always need supplemental S fertiliser. Following the incorporation of legume catch crops it may even be advisable to use supplemental S fertiliser.

Generally, if the mineralisation of catch crop S is not synchronised with plant S uptake, there is a risk of leaching losses prior to or after the growing season of the following crop. Therefore, any catch crop strategy should focus on the entire crop rotation. Even when the catch crop is not directly contributing to the nutrition of the following crop, as in the case of some legumes, sulphate, that might otherwise be leached, is retained in the system.

References

Eriksen J., Olesen J.E. and Askegaard M. (2002) Sulphate leaching and sulphur balances of an organic crop rotation on three Danish soils. European Journal of Agronomy 17: 1-9.

Eriksen J. & Thorup-Kristensen K. (2002) The effect of catch crops on sulphate leaching and availibility of S in the succeeding crop on sandy loam soil in Denmark. Agriculture, Ecosystems and Environment 90: 247-254.

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