Cereal yield and quality as affected by nitrogen availability in organic and conventional arable crop rotations: A combined modeling and experimental approach

Abstract

The effects of nitrogen (N) availability related to fertilizer type, catch crop management, and rotation composition on cereal yield and grain N were investigated in four organic and one conventional cropping systems in Denmark using the FASSET model. The four-year rotation studied was: spring barley–(faba bean or grass-clover)–potato–winter wheat. Experiments were done at three locations representative of the different soil types and climatic conditions in Denmark. The three organic systems that included faba bean as the N fixing crop comprised a system with manure (stored pig slurry) and undersowing catch crops (OF + C + M), a system with manure but without undersowing catch crops (OF − C + M), and a system without manure and with catch crops (OF + C − M). A grass-clover green manure was used as N fixing crop in the other organic system with catch crops (OG + C + M). Cuttings of grass-clover were removed from the plots and an equivalent amount of total-N in pig slurry was applied to the cropping system. The conventional rotation included mineral fertilizer and catch crops (CF + C + F), although only non-legume catch crops were used. Measurements of cereal dry matter (DM) at harvest and of grain N contents were done in all plots. On average the FASSET model was able to predict the yield and grain N of cereals with a reasonable accuracy for the range of cropping systems and soil types studied, having a particularly good performance on winter wheat. Cereal yields were better on the more loamy soil. DM yield and grain N content were mainly influenced by the type and amount of fertilizer-N at all three locations. Although a catch crop benefit in terms of yield and grain N was observed in most of the cases, a limited N availability affected the cereal production in the four organic systems. Scenario analyses conducted with the FASSET model indicated the possibility of increasing N fertilization without significantly affecting N leaching if there is an adequate catch crop management. This would also improve yields of cereal production of organic farming in Denmark.

Introduction

The proportion of organic farms in Denmark has been stable during this century after a rapid increase during the previous two decades (Plantedirektoratet, 2009). Nevertheless, there is an increasing demand for organic products by consumers, who are increasingly concerned about food quality and safety. Furthermore, from an environmental perspective, organic farming may lower the environmental impacts of the cropping system and improve biodiversity compared with conventional agriculture (Lampkin, 1998, Hole et al., 2005).

One of the important obstacles to the conversion from conventional to organic farming in cereal production is the possible reduction in yields (Berry et al., 2002). The yields in organic farming are restricted by a higher proliferation of weeds and diseases, and are dependent on the availability of N mineralized from organic manure and plant debris. The adoption of adequate rotations and management practices, such as weed control, crop residue treatment, use of catch crops, or an appropriate timing and amount of manure application determine the degree to which yields and nutrient losses are affected (Thorup-Kristensen et al., 2003, Rasmussen et al., 2006, Olesen et al., 2009).

Spring barley and winter wheat constitutes about 24% and 6%, respectively, of the area under organic farming in Denmark (Plantedirektoratet, 2009). In a long-term field experiment initiated in 1997 (Olesen et al., 2000a) these two crops are part of a rotation in combination with grass-clover, pulse and row crops in a variety of cropping systems and tested at three different sites with varying soil types. Long-term field studies enables direct and residual treatment effects on soil fertility to be compared taking yearly crop yield variability due to different weather conditions into account, and their importance has been addressed in previous works (Peterson et al., 1993, Wei et al., 2001).

The investment of time and resources required by long-term experiments limits the range of agronomic practices studied in this type of research. Agronomic models are complementary tools that, when properly used, enable similar information to be derived. An additional and unique feature of modeling is the possibility to assess the effects and interactions under alternative environmental and management scenarios, as for example those related to the effect of policy implementations (Nendel, 2009) or climate change (Olesen, 2005). The crop submodel of the FASSET dynamic whole-farm model (Berntsen et al., 2003) has been calibrated and validated for winter wheat (Olesen et al., 2002a, Olesen et al., 2002b) and assessed for intercrops of pea and barley (Berntsen et al., 2004) under Danish conditions. FASSET can simulate the effects of soil management and those of nitrogen (N) and water availability on growth and yield of a range of crops, including cereals and catch crops, under a variety of soil types and cropping systems. The objective of this work was to investigate the effects of N availability on winter wheat and spring barley yield and quality in conventional and organic cropping systems that differed in the type and amount of fertilizer received and in the composition of the rotation including the presence or absence of catch crops. The FASSET model is used to discuss the results from the field experiment and to suggest possible management strategies that may improve organic cereal production in Denmark.