Trading tree growth for soil degradation: effects at 10 years of black plastic mulch on fine roots, earthworms, organic matter and nitrate in a multi-species riparian buffer.
Jones, A., J. Fortier, D. Gagnon and B. Truax. 2020.
Abstract
This study assessed the effects of two vegetation management treatments (black polyethylene mulch and a control
treatment) on tree growth, fine root biomass of trees and of competing vegetation, earthworms, and physiochemical soil properties in a 10 year-old agricultural riparian buffer planted in southern Québec (Canada) with five trees
species: Populus × canadensis Moench (hybrid poplar), Fraxinus pennsylvanica Marsh. (red or green ash) , Quercus
macrocarpa Michx. (bur oak) , Quercus rubra L. (red oak) , and Pinus strobus L. (eastern white pine). Vegetation
management treatments had a significant effect on tree growth, fine root biomass, earthworms and on many soil
properties. Overall, the use of plastic mulch increased tree growth in all species (not to the same extent), but
locally decreased soil organic matter, total soil carbon, total soil nitrogen, earthworm biomass and abundance,
competing vegetation fine root biomass and the biomass of the total pool of fine roots. Also, more than three times
more soil nitrate was measured under mulch despite the higher tree growth. Lower soil phosphorus availability
(in some species), base cation saturation and moisture content under the mulch suggested higher uptake of these
soil resources to sustain high tree growth. Across treatments and species, significant and strong correlations were
observed between total fine root biomass and soil carbon, between competing vegetation fine root biomass and
earthworm biomass, and between total soil carbon and earthworm biomass. These highlight the central role of
the total pool of fine roots in maintaining or enhancing soil carbon, the importance of understory herbaceous
vegetation for earthworm populations and the importance of organic matter as a factor limiting earthworm ac-
tivity. Species × Treatment interactions were observed on tree growth, fine roots, and earthworms. Pine growth
was little enhanced by the mulch and its fine root biomass peaked in the control treatment, suggesting plastic-
ity in allocation to fine roots in response to resource limitations. Pine had similar earthworm biomass across
both vegetation management treatments, with two to three times less earthworm biomass than other tree species
observed in control plots, reflecting its low needle litter quality. Only poplar and ash had higher tree fine root
biomass under the mulch. Red oak was the species the most negatively affected by herbaceous competition. In
practice, plastic mulch is rarely removed from tree plantation sites. This should change in a context where plastic
mulch is increasingly used as an alternative to herbicide, and where riparian agroforestry projects need to meet
both timber and non-timber objectives (carbon sequestration in the soil and in tree biomass, N and P storage in
woody biomass, soil quality restoration), while minimizing their impacts in terms of nitrate leaching. Identifying
alternative mulching materials and strategies that would provide high growth benefits without degrading the soil
or water quality is a priority to optimize ecosystem services provision in tree plantations and riparian buffers
Key Words
Soil carbon Soil microclimate Competing vegetation Polyethylene mulch Ecosystem services Agroforestry
