Integration of biogas production into organic arable farming systems: crop yield response and economic effects

Abstract

There are several important differences between organic and conventional biogas production. One difference concerns the biomass production system (e.g., legume-grass instead of maize), and another difference is the way in which biogas plants are integrated into nutrient cycles, which results in different effects on soil fertility, crop yields, and the environment. The focus of this study is an analysis of the yield response and economic impact of biogas plant integration into an organic arable farming system. We compare two organic farming systems: the reference system (an arable farming system without biogas production) and the biogas system (an arable farming system with biogas production). Both farming systems underwent the same crop rotation, and the cash crops produced in the biogas system were sold identically to those in the reference system. In the reference system, the grass–legume mixture of the green fallow was mulched and left as green manure on the fields. In the biogas system, the grass–legume mixture was used for biogas production; therefore, electricity, heat, and the biogas slurry for fertilization of the cash crops were produced. The comparisons are based on a long-term field experiment and additional modeling data. This research approach was chosen to achieve reliable results on the basis of measured yield effects under comparable site and management conditions. Within the investigated period (2010–2014), the cereal yields of the biogas system were significantly (27–47%) higher than those of the reference system (arable farming). Case studies and model calculations on the basis of the experimental data showed higher entrepreneurial profits from the biogas systems as a result of the yield effects. The entrepreneurial profit of the arable farming in the biogas system was approximately 300 € ha−1 higher than that of the reference system. The higher revenues from the cash crops also compensated for the financial losses induced by the biogas plants. The dimensions of a biogas plant are important for improving the overall profit of a farm; the size must be adapted to the on-site supply of substrates and the need for biogas slurry. This study shows that suitable organic biogas production leads to synergistic effects between bioenergy and food production. The integration of a biogas plant into an organic farming system can ensure energy supply while increasing food output.