Comparing annual and perennial crops for bioenergy production – influence on nitrate leaching and energy balance

Production of energy crops is promoted as a means to mitigate global warming by decreasing dependency on fossil energy. However, agricultural production of bioenergy can have various environmental effects depending on the crop and production system. In a field trial initiated in 2008, nitrate concentration in soil water was measured below winter wheat, grass‐clover and willow during three growing seasons. Crop water balances were modelled to estimate the amount of nitrate leached per hectare. In addition, dry matter yields and nitrogen (N) yields were measured, and N balances and energy balances were calculated. In willow, nitrate concentrations were up to approximately 20 mg l −1 nitrate‐N during the establishment year, but declined subsequently to <5 mg l −1 nitrate‐N, resulting in an annual N leaching loss of 18, 3 and 0.3 kg ha −1  yr −1  N in the first 3 years after planting. A similar trend was observed in grass‐clover where concentrations stabilized at 2–4 mg l −1 nitrate‐N from the beginning of the second growing season, corresponding to leaching of approximately 5 kg ha −1  yr −1 N. In winter wheat, an annual N leaching loss of 36–68 kg ha −1  yr −1 was observed. For comparison, nitrate leaching was also measured in an old willow crop established in 1996 from which N leaching ranged from 6 to 27 kg ha −1  yr −1 . Dry matter yields ranged between 5.9 and 14.8 Mg yr −1 with lowest yield in the newly established willow and the highest yield harvested in grass‐clover. Grass‐clover gave the highest net energy yield of 244  GJ  ha −1  yr −1 , whereas old willow, winter wheat and first rotation willow gave net energy yields of 235, 180 and 105  GJ  ha −1  yr −1 . The study showed that perennial crops can provide high energy yields and significantly reduce N losses compared to annual crops.