Biomass yield, crude protein yield and nitrogen use efficiency over nine years in annual and perennial cropping systems

Emerging biorefinery technologies can lead to new applications and new markets for various types of crop biomass. This may allow significant changes in agricultural production from crop rotations dominated by annual grain and seed crops towards annual or perennial cropping systems composed with the aims of higher biomass yield and environmental sustainability. In this study, we investigated 7 annual and 7 perennial cropping systems on a sandy loam soil, with large differences in N fertilization. Yield of dry matter (DM) and crude protein (CP) was measured over nine growing seasons from 2013 to 2021. A conventional four-year cash crop rotation with cereals and winter oil seed rape served as a reference and achieved mean annual yields of DM and CP of 10.5 and 0.85 Mg ha⁻¹ y⁻¹, respectively, across the nine years. Continuous maize and triticale had significantly higher DM and CP yields, with 57 and 15 % increases in DM yield compared to the reference crop rotation, respectively. Optimized four-year crop rotations with various annual crops including triticale, maize, beet, hemp or faba bean and various intermediate crops achieved 51–84 % and 42–78 % higher yield of DM and CP, respectively. Perennial cropping systems with festulolium and tall fescue with three or four harvests per year achieved 63–65 % higher DM yield and 192–200 % higher CP yield (2.47–2.55 Mg ha⁻¹ y⁻¹) compared to the cash crop rotation. Perennial cropping systems with miscanthus and willow had high DM but low CP yield. As a measure of nitrogen use efficiency, partial factor productivity of DM yield (PFP_DM) and CP yield (PFP_CP) were calculated, and both varied significantly between cropping systems, with highest PFP_DM for M. × giganteus and willow (114–192 kg DM kg N⁻¹) and lowest for festulolium and tall fescue (38–40 kg DM kg N⁻¹). PFP_CP was highest for the optimized crop rotations (6.88–7.94 kg CP kg N⁻¹) and lowest for miscanthus (2.94–4.98 kg CP kg N⁻¹). Across 12 of the cropping systems, which included both protein crops and lignocellulosic crops, there was a non-linear DM yield response to N fertilization rate with PFP_DM decreasing from 134.9 to 37.2 kg DM kg N⁻¹ when increasing the N rate from 50 to 500 kg ha⁻¹ y⁻¹. On the other hand, there was a linear CP yield response and, therefore, a constant PFP_CP of 5.94 kg CP kg⁻¹ N across N fertilization rates. The results clearly indicate that cropping systems can be modified to achieve higher DM and CP yields but also that choice of cropping system and optimal N fertilization may need adjustment depending on the use of the harvested biomass, the possibilities for biorefining into various components and products as well as the economic value of the components.