Biomass responses in a temperate European grassland through 17 years of elevated CO₂

Abstract

Future increase in atmospheric CO concentrations will potentially enhance grassland biomass production and shift the functional group composition with consequences for ecosystem functioning. In the "GiFACE" experiment (Giessen Free Air Carbon dioxide Enrichment), fertilized grassland plots were fumigated with elevated CO (eCO ) year-round during daylight hours since 1998, at a level of +20% relative to ambient concentrations (in 1998, aCO was 364 ppm and eCO 399 ppm; in 2014, aCO was 397 ppm and eCO 518 ppm). Harvests were conducted twice annually through 23 years including 17 years with eCO (1998 to 2014). Biomass consisted of C3 grasses and forbs, with a small proportion of legumes. The total aboveground biomass (TAB) was significantly increased under eCO (p = .045 and .025, at first and second harvest). The dominant plant functional group grasses responded positively at the start, but for forbs, the effect of eCO started out as a negative response. The increase in TAB in response to eCO was approximately 15% during the period from 2006 to 2014, suggesting that there was no attenuation of eCO effects over time, tentatively a consequence of the fertilization management. Biomass and soil moisture responses were closely linked. The soil moisture surplus (c. 3%) in eCO manifested in the latter years was associated with a positive biomass response of both functional groups. The direction of the biomass response of the functional group forbs changed over the experimental duration, intensified by extreme weather conditions, pointing to the need of long-term field studies for obtaining reliable responses of perennial ecosystems to eCO and as a basis for model development.

Elevated CO2 does not stimulate carbon sink in a semi-arid grassland

“…Theoretical analyses further suggest that plant community composition may be a potential mediator of CFEs under multifactorial global change (Kardol et al 2012;Langley & Hungate 2014), partly due to the diverse CO 2 responses of photosynthesis among plant species and related variations in interspecific competition (Poorter & Navas 2003). For example, growth stimulation of C 3 forbs under eCO 2 is higher than that of C 3 /C 4 grasses in two FACE experiments (Lee et al 2011;Newton et al 2014), in contrast to other work Andresen et al 2017). In addition, a 20-year FACE experiment has revealed that eCO 2 can promote growth of C 3 grasses greater than C 4 grasses during the first 12 years of treatment, but the pattern reverses during the subsequent 8 years .…”

“…; Andresen et al . ). In addition, a 20‐year FACE experiment has revealed that eCO 2 can promote growth of C 3 grasses greater than C 4 grasses during the first 12 years of treatment, but the pattern reverses during the subsequent 8 years (Reich et al .…”

Explaining the doubling of N2O emissions under elevated CO2 in the Giessen FACE via in‐field 15N tracing

“…The grassland has not been ploughed for at least 100 years, being managed as a hay meadow with two cuts per year, with granular mineral calcium‐ammonium‐nitrate fertilizer applied at the rate of 40 kg N ha −1 year −1 in mid‐April. Before 1996, fertilizer was applied at a rate of 50–100 kg N ha −1 year −1 (Andresen et al., ; Kammann et al., ). Meteorological data were available from meteorological stations at the field site.…”

“…Three equally sized control plots were maintained at ambient atmospheric CO 2 levels (aCO 2 ). The soil of the study site is classified as a Fluvic Gleysol (FAO classification) with a sandy clay loam texture overlying a subsoil clay layer (J€ ager et al, 2003 (Andresen et al, 2017;Kammann et al, 2008). Meteorological data were available from meteorological stations at the field site.…”

“…The rising atmospheric CO 2 concentration, which has recently reached 400 ppm (Dlugokencky & Tans, ), is unprecedented in the last 800,000 years (IPCC, ). This increase in CO 2 concentration stimulates plant growth (Andresen et al., ; Obermeier et al., ) and is expected to affect soil nitrogen (N) cycling and the production pathways of nitrous oxide (N 2 O; van Groenigen, Osenberg, & Hungate, ). Microbial N transformations via nitrification and denitrification contribute about 70% of the annual N 2 O emissions worldwide (IPCC, ; Mosier, Delgado, & Keller, ) and anthropogenic contributions to N 2 O emissions are triggered by N fertilizer application in agriculture (Singh, Bardgett, Smith, & Reay, ).…”