Root production and turnover and carbon budgets of two contrasting grasslands under ambient and elevated atmospheric carbon dioxide concentrations

Abstract

Sl'MMARYMonoliths of two contrasting vegetation types, a species-rich grassland on a brown earth soil over limestone and a species-poor community on a peat>' gley, were transferred to solardomes and grown under ambient (350 /d 1"') and elevated (600//I i"^) CO^ for 2 yr. Shoot biomass was unaltered but root biomass increased by 40-50% under elevated CO^. Root production was increased by elevated CO^ in the peat soil, measured both as instantaneous and cumulative rates, but only the latter measure was increased in the limestone soil. Root growth was stimulated more at 6 cm depth than at 10 cm in the limestone soil. Turnover was faster under elevated COî n the peat soil, but there was only a small effect on turnover in the limestone soil. Elevated CO^ reduced nitrogen concentration in roots and might have increased mycorrhizal colonization. Respiration rate was correlated with N concentration, and was therefore lower in roots grown at elevated CO^-Estimates of the C budget of the two communities, based upon root production and on net C uptake, suggest that C sequestration in the peat soil increases by c. 02 kg C m"^ yr"^ (= 2 t ha yr"^) under elevated CO^.

Elevated atmospheric CO2, fine roots and the response of soil microorganisms: a review and hypothesis

“…A few studies have observed changes in root demography under elevated CO # , and there seems to be a commonality among them. Elevated CO # increases fine-root production in most species, but this response can be dampened by low soil fertility (Pregitzer et al, 1995 ;Berntson & Bazzaz, 1996 ;Day et al, 1996 ;Tingey et al, 1996 ;Fitter et al, 1997 ;Kubiske et al, 1998 ;Pregitzer et al, 2000). Increases in fine-root production under elevated CO # undoubtedly give rise to a greater standing crop of fine roots, which has been documented in a large number of studies (Rogers et al, , 1997.…”

“…In tree species, 40-50% of fine roots die and disappear within a single year (Hendrick & Pregitzer, 1992 ;Burke & Raynal, 1994 ;Fahey & Hughes, 1994), and this proportion can be even greater in grasslands (Fitter et al, 1997). Moreover, elevated CO # can disproportionately increase net fine-root production over mortality (Pregitzer et al, 1995 ;Day et al, 1996 ;Pregitzer et al, 2000), although such a response is not universal (Berntson & Bazazz, 1996 ;Fitter et al, 1997). In contrast, above-ground litter production in most experiments occurs as a seasonal pulse deposited at the soil surface.…”

“…For example, fine-root and mycorrhizal mortality occur throughout the growing season (Hendrick & Pregitzer, 1992 ;Fitter et al, 1997) and these organic matter inputs enter mineral soil directly. In tree species, 40-50% of fine roots die and disappear within a single year (Hendrick & Pregitzer, 1992 ;Burke & Raynal, 1994 ;Fahey & Hughes, 1994), and this proportion can be even greater in grasslands (Fitter et al, 1997).…”

“…For example, fine-root and mycorrhizal mortality occur throughout the growing season (Hendrick & Pregitzer, 1992 ;Fitter et al, 1997) and these organic matter inputs enter mineral soil directly. In tree species, 40-50% of fine roots die and disappear within a single year (Hendrick & Pregitzer, 1992 ;Burke & Raynal, 1994 ;Fahey & Hughes, 1994), and this proportion can be even greater in grasslands (Fitter et al, 1997). Moreover, elevated CO # can disproportionately increase net fine-root production over mortality (Pregitzer et al, 1995 ;Day et al, 1996 ;Pregitzer et al, 2000), although such a response is not universal (Berntson & Bazazz, 1996 ;Fitter et al, 1997).…”

Root dynamics and global change: seeking an ecosystem perspective

“…Grassland systems, on the other hand, can be studied as intact ecosystems in manipulative experiments (e.g. Fitter et al, 1997), and the arbitrary separation of responses by fine roots and coarse roots is unnecessary. In an annual crop system, the important research questions are more likely to revolve around root deployment and resource capture rather than equilibrium responses and C flux (Fitter et al, 1991).…”