Radiation and Water Use Efficiencies of Bioenergy Cropping Systems.

The production of electricity and heat from anaerobic digestion of biomass has become an important part of the bioenergy sector in Germany. About 600000 ha of arable land are currently used for producing substrates used in biogas plants. The by far dominating crop for this purpose is silage maize. Alternative cropping systems, such as double- and inter-cropping or crop rotation systems, have been proposed in order to increase the productivity and to obtain a higher diversity of crops and rotations and to reduce unwanted environmental impacts.

Under the climatic conditions of Northern Germany crop rotations including winter intercrops and combining C₄- and C₃-crops might provide an adequate alternative to maize monoculture. This is because relatively low temperatures at least during the early growth period are comparatively more unfavourable for the thermopile C₄-plant maize and secondly, the water availability in this region is high, so that the prolonged leaf area duration of crop rotations and therefore their higher water demand might not cause suffering from strong drought stress. Instead, the prolonged leaf area duration may cause a high cumulative uptake of photosynthetic active radiation (PAR) which leads, depending on the light use efficiency (LUE), to a high biomass production.

To investigate dry matter productivity and resource use efficiency of energy cropping systems for the Northern part of Germany, a two year field experiment was conducted on two sites with contrasting soil conditions. Thereby we established on a site with loamy sand soil texture three cropping systems, including a maize monoculture, an intensive biomass crop rotation (silage maize – silage wheat - grass winter intercrop) and a mixed crop rotation (silage maize – grain wheat – mustard winter intercrop) and on a sandy soil site a maize monoculture and a four-cutting perennial grassland. All cropping systems were cultivated under four different N-supply-levels.

The objective of the presented study was to analyse the water and radiation use efficiency of these cropping systems. For this purpose a simple model, which includes soil water and evapotranspiration processes, in combination with frequent measurements of canopy parameters was used.