Nutrient accumulation and carbon sequestration in 6-year-old hybrid poplars in multiclonal agricultural riparian buffer strips

Riparian vegetation has a critical role to play in non-point source pollution abatement and water quality protection within watersheds in agricultural areas. In addition to their water quality function, riparian buffers also have the potential to sequester large amounts of carbon (C). In this study, the C and nutrient sequestration by five unrelated hybrid poplar (Populus spp.) clones growing in the riparian zone of four southern Quebec (Canada) agroecosystems is measured aboveground after 6 years of growth and compared to free-growing (unmanaged) herbaceous buffer strips. Very large differences in C sequestration and nutrient accumulation in hybrid poplar buffers were observed across the four agricultural riparian sites. For all variables measured in this study, the largest effect detected by the ANOVA was the Site effect. While C sequestration, N and P accumulation in total aboveground biomass were, respectively 52 t ha−1, 770 kg ha−1 and 82 kg ha−1 at the Bromptonville site, these values were as low as 6.4 t ha−1, 90 kg ha−1 and 10 kg ha−1 at the Magog site. Site fertility, in terms of NO3 supply rate, was the main factor controlling biomass growth, and consequently C sequestration and nutrient accumulation in hybrid poplars. Although Site effect was by far the largest effect in this study, the Clone effect was also important, with clone 3729 (Populus nigra L. × Populus maximowiczii A. Henry) being the most effective for C and nutrient sequestration. Across the four study sites, total aboveground C sequestration, N and P accumulation at the site level could be, respectively enhanced by 31–37%, 29–41% and 30–38% with the sole use of clone 3729. Site × Clone interactions for the variables tested in this study were generally not significant, or significant but small in magnitude compared to main effects (Site and Clone). Important C and nutrient accumulation differences exist after 6 years of growth between hybrid poplar buffers and unmanaged herbaceous buffers. Compared to an unmanaged herbaceous buffer, the distinct advantage of hybrid poplar buffers is that C and nutrient sequestration greatly increase with increasing site fertility, whereas they do so to a much lesser extent in herbaceous buffers.