Root Biomass under Range of Pasture Swards -Implications for Soil Carbon Stocks

Abstract

Soil carbon (C) is the largest terrestrial store of carbon, greater than all other pools combined. Root biomass is recognised as an important contributor to the accumulation of soil C and recent studies have suggested that a higher diversity of plant species with higher root biomass may increase soil C content. Maintenance of soil C stocks is important as losses can contribute to increased greenhouse gas emissions. Furthermore soil C is important for maintaining soil quality. Much of New Zealand agricultural land is under pastoral grazing and is the greatest contributor to national greenhouse gas emissions from land. The potential for storing additional soil C in pasture soils is poorly understood. This study has assessed root biomass and carbon contents of soils under a range of pasture diversities and species composition to determine whether (i) there is greater root biomass under increasing plant diversity, (ii) changes in root properties between diversity treatments are evident and (iii) there is variation in root biomass between seasons. Soil samples were collected from an existing small plot trial managed by DairyNZ and Landcare Research containing 14 different mixtures of pasture species (three replicates per treatment). From each plot, fifteen soil cores were taken to a depth of 60cm, and bulked by 10cm depth increments. Root characteristics were determined after removing soil via scanning and analysing with WinRHIZO©, and subsequently root biomass determined following oven drying and weighing. A sub-sample of soil was also taken for C and nitrogen (N) analysis by combustion furnace. While there were no significant differences in root biomass with individual treatments, plots which had a presence of herb species chicory and plantain, resulted in a lesser root biomass when compared with plots without herb species. This would imply that while the use of herb species is desirable for above-ground production and drought tolerance, they provide limited potential for increasing C input to soil. Root characteristics determined that the use of tall fescue as a base grass resulted in a reduced root length density (RLD, root length per volume of soil to 60 cm), specific root length (SRL) and an increased average diameter compared with ryegrass based pastures. These characteristics promote increased root longevity in soil.