1.1. Changing demands on food production systems

Until now food production kept pace with population growth, despite the worries by Malthus, Attenborough and many others during the last centuries. Increased use of synthetic fertilizer has been a major driver in increasing crop yields. Agricultural policies has kept and still keeps focus on increasing agricultural productivity by increasing external inputs, both in the developing and the developed world. In the developing economies agriculture is a dominant economic sector, in some African countries contributing up to 50% to the gross domestic product (GDP), while in developed countries it generally is less than 5% (World Bank 2014). In some developed countries the agro- and food sector is still a dominant sector. For example, in The Netherlands agricultural production and innovation has always been a major basis for the national economy with an agro–food sector contributing 20% to the export of products and 10% to GDP (Eurostat 2014). Together with the enormous boost in global agricultural production and land productivity and the associated improvement of human nutrition and well-being, the efficiency of the use of critical natural resources, like biodiversity, phosphate and fossil fuel in the global food production system (the system that feeds the global population), has decreased and impacts on environment, climate and human health have increased. The efficiency in terms of what eventually is consumed per use of these resources decreased because of (i) food waste and losses in the whole chain, which currently reach 30–40% (Gustavson et al 2011), (ii) increased production and consumption of animal products, but also (iii) the increased use of agricultural land for the production of luxurious products, such as tea, coffee, sugar, etc. Food production currently contributes about 25% to all GHG emissions (Vermeulen et al 2012), is responsible for 60% of loss of land based biodiversity (inferred from Ten Brink 2010) and is the major source of global eutrophication (Sutton et al 2013). The increasing world population in combination with a more affluent diet is projected to demand an increase in crop production of 50% and in livestock production of 70% by 2050 (FAO 2012). According to Tilman et al (2011), most likely the food industry even has to double their productivity to satisfy people's demand by 2050. The larger part of the increase of demand for cereals would be to feed livestock to meet the increase in diets rich in animal protein. Many projections indicate that such an increase can be met by yield gap closure (e.g. Foley et al 2011, Mueller et al 2012). However, currently the annual yield increase for staple food crops is slowing down at the global scale (Grassini et al 2013). This will lead to expansion of agriculture area (notably in developing countries). For the first time since 1980 harvested areas for wheat and corn have started to increase (Grassini et al 2013). This increase in part was caused by increased demand for biofuels. This land extension is a direct threat to biodiversity (Alkemade et al 2009, Van Vuuren 2012). Without additional efforts, current agricultural practices also will increase global emissions of greenhouse gasses and lead to higher losses of nitrogen and phosphorus to the environment (Tilman et al 2011, Garnett and Godfray 2012). The main challenge is therefore to guarantee future food security, while reducing environmental pollution and biodiversity loss (Sutton et al 2013).

1.2. Sustainable intensification and sustainable extensification

Currently, there is an ongoing debate in society on how to accommodate increasing demand for livestock products and global food security for a larger and wealthier population with more sustainable agriculture (Lang and Heasman 2004, Roberts 2008). Leading scientists and politicians hold contrasting views on the potential contribution of intensification and extensification of agriculture and of changing diets to a more sustainable food system. Here we define: Sustainable intensification of agriculture by: Maintaining or increasing food production per hectare without compromising the environment and depleting natural resources. This objective generally translates to increasing external inputs, such as nitrogen (N), while decreasing resource losses to the environment; we define sustainable extensification of agriculture by: Decreasing the depletion of natural resources and the environmental impacts while limiting the decrease of food production per hectare. This objective translates to reducing external inputs, such as N, and livestock densities while minimizing food loss. Agriculture and food include crops and animal products. Both sustainable intensification and extensification require improved management of nutrients, such as nitrogen and phosphorus, but also water, pesticides, seed, plant and livestock diversity and by that increase resource efficiencies. While both strategies focus on agricultural production, they have implications for natural resource efficiencies of food processing and consumption. Sustainable intensification with increased food production provides more room for resource demanding food choices, with a larger share of animal products, and for food loss in retail and consumption to satisfy consumer preferences. When accompanied by decreasing food production, sustainable extensification asks for less resource demanding food choices, different diets and minimizing food wastes. Sustainable intensification appears to be a good strategy for improving food security in areas where there is a large yield gap, such as in Africa (Garnett and Godfray 2012), under the condition that smallholders profit from it. According to Phalan et al (2011), intensification is also the best global strategy to spare land and halt biodiversity loss. The current dominant strategy for closure of crop yield gaps is intensification of land use by the combination of increasing external inputs and use of high yielding crop and animal varieties (van Grinsven et al 2014). This form of intensification may be regarded as unsustainable in view of risks for the environment, such as soil degradation and losses of nutrients and pesticides which pose threats for biodiversity and human health (e.g. Garnett et al 2013, Sutton et al 2011). These threats are particularly manifest in countries or regions where current inputs are already high. In situations with current low crop yields and low levels of fertilization, as in various regions in Africa, the external cost of land extension for increased crop production would likely outweigh cost of intensification for biodiversity and health, but many African small holder farmers lack resources to purchase fertilizers, pesticides and technology. For Europe sustainable intensification is currently the dominant strategy to improve food security given the small potential to increase the area of agricultural land. In view of concerns about nutrient losses to air and water, one could, however, hypothesize that sustainable extensification could also be a strategy for Europe to meet both the global and regional demand for food and biodiversity. A necessary condition for both strategies would be to maintain farm income and in the case of the many poor smallholder farmers, to increase income. While extensification holds the risk of loss of income because of reduced yield (at constant market prices), intensification holds the risk of income loss due to a decrease of commodity prices combined with increased cost of fertilizers, pesticides, seeds, machinery etc. Intensification and extensification can create both risks and opportunities for biodiversity, farmers and consumers in different parts of the world. The challenge is to find the optimal combination of intensification and extensification of agricultural production in various parts of the world that reduce risks of environmental pollution and resource depletion in both areas of extensification and of intensification, while increasing the net global production of food. Achieving this optimal combination is facilitated by a globally operating food system with intensive shipping of agricultural products and resources over the globe (Galloway et al 2007). Therefore in Europe and particularly The Netherlands, there is room for improving the balance between food production and environmental pressure.

1.3. Characteristics of low input/organic farming and impacts on crop yield

Key to sustainable intensification is closing the yield and harvest gap while minimizing losses of nutrients and pesticides to the environment. The yield gap is the difference between the highest possible yield using best management and all the technology and inputs available relative to the current yield (Foley et al 2011), whereas the harvest gap is the difference between current and maximum cropping frequency (Ray and Foley 2013). Yield gaps depend on crop type, genotype, management and also on regional edaphic physical and environmental conditions (van Grinsven et al 2014). Yield gaps are largest for poorly managed and low input farms. For the developed agricultural countries the yield gaps are largest for organic production methods and amount up to 20–30% (De Ponti et al 2012, Seufert et al 2012, Ponisio et al 2015). A problem is to establish good reference cases, both for organic and conventional methods and yield gaps therefore show a large range with several organic farms even showing higher yields than conventional farms. The global harvest gap is estimated at 57% (Ray and Foley 2013), as compared to a global yield gap of 45 to 70% for major cereals (Mueller et al 2012). While there is huge potential to close the harvest gap in the tropics, the harvest gap in European countries is generally less than 10%. Organic agriculture is a case of extensification not allowing inputs like synthetic fertilizers and pesticides, nor allowing the use of genetically modified organisms (GMOs). There is no consensus on the relative contribution of various factors to the yield gap between organic and conventional production methods, but lack of nitrogen fertilizer is claimed to be an important reason (Smil 2002). Most agricultural systems are nitrogen limited and therefore nitrogen is important when closing the yield gap. Both conventional and organic systems show nitrogen leakages. Although organic farmers are forced to better manage nutrients, they may occasionally overuse organic sources of N to boost production of valuable crops, in case there is ample availability of nitrogen from N fixing crops, external manure or compost. The main argument against organic farming is that the amount of naturally available nitrogen, including biological nitrogen fixation, is not sufficient to sustain and increase production to feed the world. Smil (2002) estimates that synthetic nitrogen fertilizer feeds about 40% of the world population, Erisman et al (2008) estimated that nitrogen fertilizers fed 48% of the worldʼs population in 2008. Arable organic systems depend on N-fixing crops and animal manure to provide nutrients, which would mean that effectively about one third of the available arable land would not be available to directly produce food (Schröder and Sørensen 2011). Estimates of the yield gap between conventional and organic cultivation of cereals of 20 to 40% by Seufert et al (2012) and De Ponti et al (2012). This yield gap reflects the combined effect of the use of synthetic fertilizer, pesticides, GMO and improved management of nutrients, soils and crops. This implies that the effect of synthetic N fertilizer on cereal yields would be less than 40%. Ponisio et al (2015) found a yield gap of 9% between organic and conventional treatments when N inputs were similar, and a gap of 30% when taking into account the effect of higher N input in conventional systems. This implies that by adopting practices from organic agriculture the effect of synthetic N fertilizer on global yields could be reduced to 20% and much more people can be fed by agriculture without synthetic N fertilizer than the 50% estimated by Smil (2002) and Erisman et al (2008). The challenge for both conventional and organic systems is to use nitrogen/nutrients and other resources, such as water, as efficiently as possible. Organic farming focuses on what it can do using the natural processes to increase the production of quality food, with sustainable soil management in a clean environment (e.g. Tomich et al 2011). Strict organic production systems demand sophisticated management skills to conserve nutrients and control pests and diseases. Organic arable agriculture typically requires more drought resistance systems with higher soil organic matter and more biodiversity. The organic system forces the community to create resilient systems while boosting production, in that sense aiming for sustainable intensification, as is the case in conventional systems (e.g. Mäder et al 2002). Conventional farms using best management practices to reduce external input and to control disease, aim for the same with similar results (Oenema et al 2011). Conventional farming also depends on manure to maintain organic carbon in the soil, especially when crop residues are not returned to the field. However, intensification through conventional farming has shown a reduction in soil quality and increased concern about resistance of pests and diseases against antibiotics and pesticides.

1.4. Scope and objective