Soil carbon stocks under grazed pasture and pasture-tree systems

Grant Douglas; Alec Mackay; Ronaldo Vibart; Mike Dodd; Ian McIvor; Catherine McKenzie

doi:10.1016/j.scitotenv.2020.136910

Soil carbon stocks under grazed pasture and pasture-tree systems

Sci Total Environ. 2020 May 1:715:136910. doi: 10.1016/j.scitotenv.2020.136910. Epub 2020 Jan 24.

Authors

Grant Douglas¹, Alec Mackay², Ronaldo Vibart¹, Mike Dodd¹, Ian McIvor³, Catherine McKenzie¹

Affiliations

¹ AgResearch, Grasslands Research Centre, Private Bag 11008, Palmerston North, New Zealand.
² AgResearch, Grasslands Research Centre, Private Bag 11008, Palmerston North, New Zealand. Electronic address: alec.mackay@agresearch.co.nz.
³ Plant and Food Research, Private Bag 11030, Palmerston North, New Zealand.

PMID: 32045969
DOI: 10.1016/j.scitotenv.2020.136910

Abstract

There is increasing interest in the potential of trees to sequester carbon (C) in above- and below-ground stocks to mitigate against increasing concentrations of greenhouse gases (GHG). This study determined whether pasture-tree (PT) systems influence soil C stocks compared with open pasture (OP) by sampling four sites with trees aged 14 to16 years. Poplars (Populus spp.) at Tikokino and Woodville and alders (Alnus spp.) at Poukawa and Ruakura were planted on contrasting soils (Haplustands, Endoaquepts, Durustalfs and Humaquepts, respectively). Trees at all four sites were arranged in partial-Nelder radial planting designs, with five stem densities ranging from 67 to 1276 stems ha^-¹. Soils were sampled at five stem density classes, along with adjacent OP areas in the same paddock, to a depth of 1 m (0-75, 75-150, 150-300, 300-600, 600-1000 mm). At three of the four sites, root mass density was greater (P < 0.05) in PT than in OP systems. At Woodville, estimates of total soil C mass to 1 m tended to be greater (P = 0.08) in the OP than in the PT system (200 vs. 163 Mg C ha^-¹, respectively), whereas no significant differences in total soil C masses between OP and PT were shown at the remaining sites (P > 0.10). Despite the limited statistical significance, estimates of total soil C mass at Tikokino and Woodville (sites with poplars) were 11 and 18% greater in OP than in PT systems, whereas estimates at Poukawa and Ruakura (sites with alders) were 2 and 6% greater in PT than in OP systems. Under the current conditions, our study suggests that tree species may be an additional factor influencing the C cycle and C accumulation in soils and need to be considered in the building of our soil C inventories.

Keywords: Alders (Alnus spp.); Grazing; Pasture-tree systems; Poplars (Populus spp.); Soil carbon.