In vegetable production it is usual to leave crop residues on the field. Measurements carried out by Agroscope researchers show that removing these residues significantly reduces nitrate leaching.
Excessively high nitrate concentrations measured in many places throughout Switzerland, including the Gäu-Olten region in the Canton of Solothurn, led to the implementation of the ‘NitroGäu’ research project, one of whose main aims was to develop measures for reducing nitrate leaching. The project was supported by the Federal Office for Agriculture, the Canton of Solothurn and the Federal Office for the Environment.
In a subproject, nitrate leaching in vegetable crops was measured in a trial conducted in a lysimeter facility at Agroscope’s Zurich-Reckenholz site. Lysimeters are vessels filled with soil at the bottom of which the percolating soil water is collected. Over the course of the five-year lysimeter trial, two methods were compared in terms of nitrate leaching: (1) the leaving of crop residues on the field, which is usual in vegetable production, and (2) the removal of these residues.
Removing crop residues reduces nitrate leaching
The trial showed that leaving crop residues on the fields resulted in the leaching of considerably more nitrate, viz., an average 20% more than when plant residues are removed (127 versus 106 kg nitrate per hectare and year).
However, the total surplus of the nitrogen balance when crop residues were left on the field as opposed to being removed was much higher than the additionally leached amount: 146 kg instead of 21 kg per hectare and year. The majority of this nitrogen is stored in the soil and is mineralised over the following years and decades, with subsequent repeated leaching of nitrogen being possible. Leaching losses therefore increase over the years, although at a slower and slower pace over time.
Nitrogen input via soil blocks
It turns out that the soil blocks in which commercial vegetable producers start off and plant their seeds are a further source of nitrogen. Soil-block content analyses showed that they release non-negligible amounts of N into soils (9−35 kg nitrogen per hectare of a set). Hence, this input quantity should be taken into account in farm nutrient balancing – something that has previously not been done as part of the Proof of Ecological Performance.
Excessive amounts of nitrate contained in leachate
In general and over all years and trials, lysimeter leachate contained on average more than four times more nitrate than that required by the Swiss standard for groundwater used as drinking water: 110 versus 25 mg per litre. The trial was fertilised with mineral fertilisers according to Agroscope recommendations (PRIF, 2017), which currently advise reducing the nitrogen requirement of the following crop by 20% when leaving crop residues on the field.
Rethinking fertilisation recommendations and the handling of plant residues
In view of the sometimes-very-high amounts of nitrogen in crop residues, the handling of these plant residues should be improved. Practical methods for removing and further utilising the residues or for increasing the nitrogen utilisation of plant material left on the field need to be developed. For the fertiliser recommendations, we must consider whether the amount of nitrogen imputed in the residues of the previous crop should be increased.
- When vegetable producers leave crop residues on the field, large amounts of nitrogen enter the soil which over the years are leached in the form of nitrate.
- Commonly used in intensive field vegetable production, the soil blocks in which the crops are sown and planted are an additional source of nitrogen that should be taken into account during farm nitrogen balancing.
- Practical methods for removing and further utilising residues and for increasing the nitrogen utilisation of the plant material left on the field should be developed.
- The fertiliser recommendations should take better account of the nitrogen input from the plant residues of the previous crop.