Biogas cropping systems: Short term response of yield performance and N use efficiency to biogas residue application

The promotion of electricity generation from renewable energy sources has led to a substantial expansion of biogas production in Germany, with most biogas plants co-digesting slurry and crops, and maize being by far the most dominant substrate. At present only limited information is available on the agronomic and environmental performance of biogas cropping systems, and on the fertilizer value of biogas residues. In a two-year field experiment at two sites in northern Germany differing in soil quality, we analyzed the dry matter (DM) yield, N yield, methane yield, and the N balance of different cropping systems: (i) maize monoculture (R1), (ii) maize-whole crop wheat followed by Italian ryegrass as a double crop (R2), (iii) maize-grain wheat followed by mustard as a catch crop (R3), and (iv) perennial ryegrass ley (R4). Rotations R1, R2 and R3 were grown at Site 1 (sandy loam), whereas R1 and R4 were tested at Site 2 (humus sand). Crops were supplied with varying amounts (0-360 kg N ha−1, ryegrass: 0-480 kg N ha−1) of biogas residue, cattle slurry, pig slurry, or mineral N fertilizer, which allowed quantifying their N use efficiency in terms of apparent N recovery (ANR) and relative N fertilizer value (RNFV).No significant interactions of crop rotation and fertilizer type on DM and methane yield, plant N recovery, as well as N balance, were detected at Site 1, where R1 (19.3 t DM ha−1; 6750 m3N CH4 ha−1; N: norm conditions, i.e. 273 K, 1024 hPa) clearly out-yielded R2 (16.8 t DM ha−1; 5351 m3N CH4 ha−1) and R3 (12.8 t DM ha−1), while R2 had a higher plant N recovery. At Site 2, the perennial ryegrass ley achieved substantially lower maximum DM and methane yield (13.9 t DM ha−1; 4251 m3N CH4 ha−1) than maize monoculture (17.3 t DM ha−1; 6038 m3N CH4 ha−1). Fertilizer type affected grass performance: application of biogas residue resulted in lower yield and N recovery, but higher N balance than mineral fertilizer and cattle slurry. The N use efficiency of biogas residue application depended on the crop rotation. Biogas residue resulted in higher ANR and RNFV compared to pig slurry in R1, whilst in R2 the reverse occurred, and effects were similar to cattle slurry in R4. The results revealed that, except for grassland, substrate cropping based on biogas residues can exploit the yield potential without causing high N surplus.

► Maize monoculture outperformed the maize-wheat-grass rotation in terms of DM and methane yield. ► N use efficiency of biogas residue is affected by the cropping system. ► Except for grassland, biogas residue based substrate production allows negative or balanced N budgets.