Mature Caragana Shelterbelts on Prairie Soils: Are They Environmental Assets?.

ABSTRACT

Cultivation of shelterbelts has been promoted as having potential for mitigating atmospheric CO₂; however, the impact of N-fixing tree species on N₂O emissions has not been well studied. Caragana arborescens (caragana) is a favoured shelterbelt tree species in the Prairies, with an N-fixation rate of about 335 µg N g soil^-1 h^-1 and about 80% of total N derived from N-fixation. Nitrogen inputs derived from N-fixation can exceed plant N requirements, which may lead to N losses via N₂O emissions.  The objective of the study was to investigate the contribution of caragana to soil N₂O emissions in shelterbelts. Soil NO₃-N  and N₂O emissions were quantified in three caragana and coniferous shelterbelts across the Boreal and Prairies ecozones of Saskatchewan, Canada. The age of the shelterbelts ranged between 32 and 40 years. The coniferous shelterbelts were used as non N-fixing reference. At all sites, soil NO₃-N was greater in caragana shelterbelts (9.2–29.9 Mg ha^-1) than in the non N-fixing shelterbelt (0.7–16.1 mg L^-1). Seasonal cumulative N₂O emissions from caragana shelterbelt plots were greater (183–409 g N₂O-N ha^-1 yr^-1) than the non N-fixing plots (22–121 g N₂O-N ha^-1 yr^-1).  The increased NO₃-N in caragana shelterbelts was attributed to accrual and microbial decomposition of N-rich plant residues, roots and old nodules. The success of agroforestry systems in mitigating climate change will depend on proper understanding of trade-offs between C sequestration and the emission of trace gases such as N₂O. Further research is needed on sustainable ways of designing shelterbelt tree species in such a way as to maximize the potential benefits of N-fixation feature of caragana trees while reducing potentials for N₂O emissions.

Keywords: Caragana shelterbelt, carbon sequestration, nitrous oxide emissions