Perennial grasslands as benchmarks for agricultural sustainability

Background/Question/Methods Concerns about the negative environmental impacts of annual crops are stimulating interest in developing perennial alternatives such as herbaceous perennial grain and biofuel crops. Advances in plant breeding and post-harvest processing make successful development of perennial alternatives more feasible. However, studies that assess the long-term impacts of annually harvested perennial systems on ecosystem functioning are needed. We used five pairs of harvested perennial grass and annual crop fields located in five counties in Kansas, USA to determine differences in land management practices on ecosystem functions. Perennial grass fields had been annually harvested for 75 or more years and had not received fertilizer inputs. Annual crop fields, each located immediately adjacent to a perennial grass field, had been in production for similar periods of time and were used primarily or exclusively for winter wheat production. We assessed management impacts on soil physical, chemical and biological properties; above- and below-ground food web structures; above- and belowground plant biomass; and energy inputs of the perennial fields and wheat fields. We also used historical county and watershed data to determine the impact on water quality of converting grassland watersheds to annual crops. .
Results/Conclusions
Perennial grass fields maintained 4 Mg ha^-1 more root carbon (C), 43 Mg ha^-1 more soil C and 4 Mg ha^-1 more soil nitrogen (N) than annual crop fields in the surface 1 m. Perennial grass fields also supported populations of soil organisms correlated with soil chemical, physical, and biological properties indicative of higher soil quality. Aboveground, perennial fields harboured greater numbers and/or diversity of insect pollinators, herbivores and detritivores. Despite the lack of mineral fertilizer applications, the aboveground harvests of perennial fields yielded similar levels of N compared to those of conventional high-input wheat (Triticum aestivum) fields and at only 8% of the in-field energy costs. The cumulative N harvested per ha from the perennial fields over the past 75 years was approximately 23% greater than that harvested from the region’s wheat fields. At the watershed scale, increased annual crop cover was correlated with higher riverine nitrate-nitrogen (NO₃-N) levels. We conclude that harvested perennial grasslands provide important ecological benchmarks for agricultural sustainability.