Cover Crops to Reduce Nitrate Leaching. Effect on Water and Nitrogen Balance and other Ecosystem Services

Keywords: Catch crops, nitrogen, mineralisation, trapping nitrate, arable agriculture, Nitrates Directive, nitrate vulnerable zones, fallow periods

The presence of excess nitrate in surface water and groundwater is a double challenge, both in terms of public health (WHO standard for drinking water <50 mg NO₃/L) and environmental protection (eutrophication of coastal zones). It is the subject of a European public policy that has been adapted by each Member State at national level and includes action plans for reducing nitrate as part of the ‘Nitrates Directive’.

High nitrate levels are due to the excessive use of nitrogen fertilisers, and the natural production of nitrate by the mineralisation of soil organic matter. Cover crops as catch crops trap the residual mineral nitrogen in the soil before the drainage period, thereby reducing nitrogen leaching and the nitrate concentration in the drainage water.

As part of the preparation for the fifth action program (beginning in 2013), the public authorities have requested an updated set of information and agronomic data relating to the effectiveness of cover crops. To this end, the Ministries of Ecology and Agriculture have asked the INRA to conduct a study whose objective is to take stock of the knowledge and research, as well as the uncertainties and questions requiring further investigation on this issue.

The main objective of this study is therefore to review the available knowledge on nitrogen management during fallow periods for arable crop systems in each of the different agricultural French regions, as well as the conditions influencing the effectiveness of catch crops in reducing nitrate leaching. As there is insufficient scientific literature available when it comes to understanding the diversity of situations in mainland France, the contracting body accepted INRA’s proposal to supplement this corpus of knowledge by a simulation study. Moreover, the examination of the effects of catch crops was also extended to other potential ecosystem services, i.e. protecting the soil against water erosion, carbon sequestration in soils, contributing to the control of weeds, pests and diseases, etc. In addition, the study also examined other plant covers such as oilseed rape and winter wheat volunteers, as well as other fallow period practices such as mulch residues from the previous crop, either buried or left on the surface of the soil. In this document, the term ‘cover crops’ refers both to catch crops and volunteers. When examining specifically the function of trapping nitrate, the term ‘catch crops’ is used; when referring to other functions, the term ‘cover crop’ is used.

Eric Justes, N Beaudoin, P Bertuzzi, R Charles, J Constantin, C Dürr, C Hermon, A Joannon, C Le Bas, B Mary, C Mignolet, F Montfort and O Réchauchère.

INRA, 24 Chemin de Borde Rouge – Auzeville CS 52627, F-31326 Castanet Tolosan cedex France.

27 pages, 3 figures, 119 references.