Reliable models predicting soil organic carbon (SOC) evolution are required to better manage cropping systems with the objectives of mitigating climate change and improving soil quality.In this study, data from 60 selected long-term field trials conducted in arable systems in France were used to evaluate a revised version of AMG model integrating a new mineralization submodel. The drivers of SOC evolution identified using Random Forest analysis were consistent with those considered in AMG. The model with its default parameterization simulated accurately the changes in SOC stocks over time, the relative model error (RRMSE = 5.3%) being comparable to the measurement error (CV = 4.3%). Model performance was little affected by the choice of plant C input estimation method, but was improved by a site specific optimization of SOC pool partitioning. AMG shows a good potential for predicting SOC evolution in scenarios varying in climate, soil properties and crop management. Highlights:• SOC evolution in 60 long-term French trials was analyzed and simulated • On average, SOC stocks declined between 1970 and 2015 at a rate of -0.20 t C/ha/yr • Drivers of SOC evolution were similar in Random Forest analysis and AMG model • Implementing a new mineralization module in AMG improved SOC evolution predictions • Optimizing site-specific stable C pool could further improve model performance

Abstract. Implementation of the Nitrates Directive (NiD) and its environmental impacts were compared for member states in the northwest of the European Union (Ireland, United Kingdom, Denmark, the Netherlands, Belgium, Northern France and Germany). The main sources of data were national reports for the third reporting period for the NiD (2004NiD ( -2007 and results of the MITERRA-EUROPE model. Implementation of the NiD in the considered member states is fairly comparable regarding restrictions for where and when to apply fertilizer and manure, but very different regarding application limits for N fertilization. Issues of concern and improvement of the implementation of the NiD are accounting for the fertilizer value of nitrogen in manure, and relating application limits for total nitrogen (N) to potential crop yield and N removal. The most significant environmental effect of the implementation of the NiD since 1995 is a major contribution to the decrease of the soil N balance (N surplus), particularly in Belgium, Denmark, Ireland, the Netherlands and the United Kingdom. This decrease is accompanied by a modest decrease of nitrate concentrations since 2000 in fresh surface waters in most countries. This decrease is less prominent for groundwater in view of delayed response of nitrate in deep aquifers. In spite of improved fertilization practices, the southeast of the Netherlands, the Flemish Region and Brittany remain to be regions of major concern in view of a combination of a high nitrogen surplus, high leaching fractions to groundwater and tenacious exceedance of the water quality standards. On average the gross N balance in 2008 for the seven member states in EU-ROSTAT and in national reports was about 20 kg N ha −1 yr −1 lower than by MITERRA. The major cause is higher estimates of N removal in national reports which can amount to more than 50 kg N ha −1 yr −1 . Differences between procedures in member states to assess nitrogen balances and waterPublished by Copernicus Publications on behalf of the European Geosciences Union. H. J. M. van Grinsven et al.: Benchmarking the Nitrates Directive in northwestern Europequality and a lack of cross-boundary policy evaluations are handicaps when benchmarking the effectiveness of the NiD. This provides a challenge for the European Commission and its member states, as the NiD remains an important piece of legislation for protecting drinking water quality in regions with many private or small public production facilities and controlling aquatic eutrophication from agricultural sources.

The hydrological and biogeochemical monitoring of catchments has become a common approach for studying the effect of the evolution of agricultural practices on water resources. In numerous studies, the catchment is used as a "mega-lysimeter" to calculate annual input-output budgets. However, the literature reflects two opposite interpretations of the trends of nitrate concentration in streamwater. For some authors, essentially in applied studies, the mean residence time of leached nitrate in shallow groundwater systems is much less than one year and river loads reflect annual land use while for others, nitrate is essentially transport limited, independent of soil nitrate supply in the short term and annual variations reflect changes in climatic conditions. This study tests the effect of agricultural land-use changes on inter-annual nitrate trends on stream water of six small adjacent catchments from 0.10 to 0.57 km² in area, on granite bedrock, at Kerbernez, in Western Brittany (France). Nitrate concentrations and loads in streamwater have been monitored for nine years (1992 to 2000) at the outlet of the catchments. An extensive survey of agricultural practices from 1993 to 1999 allowed assessment of the nitrogen available for leaching through nitrogen budgets. For such small catchments, year-to-year variations of nitrate leaching can be very important, even when considering the 'memory effect' of soil, while nitrate concentrations in streamwater appear relatively steady. No correlation was found between the calculated mean nitrate concentration of drainage water and the mean annual concentration in streams, which can even exhibit opposite trends in inter-annual variations. The climatic conditions do not affect the mean concentration in streamwater significantly. These results suggest that groundwater plays an important role in the control of streamwater nitrate concentration.

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Diversification of cropping systems has been proposed as a major mechanism to move towards sustainable cropping systems. To date, a diversification option that has received little attention is introduction of ley pastures into cropping systems, but the use of ley pastures is challenged by most future-oriented scenarios aiming to feed the world sustainably. In these scenarios, ruminant livestock feed only on permanent pastures, while cropping systems focus completely on production of crop-based human food. Diversification of cropping systems with ley pastures is thus compromised by knowledge gaps and future-oriented policy options. Here, we review ecosystem services provided by introducing ley pastures into cropping systems to increase sustainability of agriculture, discuss types of ley pastures and their management liable to promote these services, and raise future challenges related to introducing ley pastures into cropping systems. We conclude that (1) ley pastures provide a large set of input (soil conservation, nutrient provision and recycling, soil water retention, biological control of pests) and output (water purification, climate regulation, habitat provision for biodiversity conservation, forage production) ecosystem services of primary importance to cropping systems and society, respectively, as long as their spatial and temporal insertion within cropping systems is well-managed; otherwise, disservices may be produced. (2) To benefit from ecosystem services provided by ley pastures in cropping systems while limiting their disservices, it appears necessary to define a safe operating space for ley pastures in cropping systems. Moving towards this space requires changing plant breeding programs towards multiservice ley pastures, producing knowledge about emerging ways of introducing ley pastures into cropping systems (e.g., living mulch, green manure) and better quantifying the bundles of ecosystem services provided by ley pastures in cropping systems.