Catch crops are grown in the period between two main crops, especially in the winter season. The aim of their cultivation is to retain nutrients, especially nitrates, in the root zone, to prevent the nitrates from leaching and so to conserve them for the following crop. It is therefore important to evaluate the amount of nitrogen incorporated into the biomass of the catch crop, its release to the soil during mineralisation and to reduce the applied nitrogen fertiliser for the subsequent crop accordingly (Beisecker et al., 2015, Tendler and Beisecker, 2015).
In France (Brittany), a wheat-(catch crop)-corn and a wheat-corn rotation were compared tested on experimental plots. During several years, nitrogen losses through leaching were measured in autumn/winter by means of porous cups. The losses in the plots with catch crops (7 kg N ha/yr) were ten times less than in the plots without catch crops (85 kg N ha/yr) (Besnard and Kerveillant, 2006). Other studies have shown the importance of catch crops in the reduction of nitrates leaching, too (Martinez, 1990).
In France, sowing catch crops is compulsory in nitrate-vulnerable zones since the last nitrate action programme. In the Netherlands, growing catch crops after maize on sandy soils is compulsory. In Gemany, in cooperations of water suppliers and farmers, water suppliers often subsidise catch crops cultivation over the winter season (Oelmann et al., 2017).
As catch crop growth is related to the retention of nitrates in the root zone, the proportion of arable land being covered by catch crops (or reverse, being left as bare soil) could be a simple indicator for nitrates leaching reductions.
Catch crops also are one element of soil cover, and in this function reduce erosion and thus pesticide run-off into surface waters.
The cultivation of catch crops is one element of the Greening part of CAP direct payments. Catch crop areas are part of EFAs (Ecological focus area). In 2015 and 2016, the EFA measure requirements applied to 68 % of arable land in the EU (excluding France) or 59.7 million hectares. Although the requirements are, that 5 % of this arable land must be under one or more of the different EFA elements, in practice, in 2016 8.5 million hectares (14 %) was covered by EFA elements (before weighting factors). In 2015, 27.7 % of the total EFA area was declared by farmers (before weighting factors) as catch crops. In 2016, an increase in catch/cover crops took place in BG, EE, HR, LV, LT, HU, SI (Hart et al., 2017).
Although detailed data on catch crop cultivation are collected for the Greening payments, these are not available publicly at Eurostat. Furthermore, catch crop cultivation may take place under different other regimes, like CAP pillar II-measures or private farmers-waterworks cooperations (Oelmann et al., 2017).
The degree of catch crop-cultivation may be used as an indicator (in reverse order) for groundwater pollution due to nitrates.