Productivity, Respiration, and Light-Response Parameters of World Grassland and Agro-Ecosystems Derived From Flux-Tower Measurements

Grasslands and agroecosystems occupy nearly a third of the land surface area, but their quantitative contribution to the global carbon cycle remains uncertain. We used a set of 316 site-years of year-round net CO2 exchange (Fc) measurements to quantitatively analyze gross primary productivity, ecosystem respiration, and light-response parameters of extensively and intensively managed grasslands, shrublands/savanna, wetlands, and cropland ecosystems worldwide. Analyzed data set included data from 72 flux-tower sites worldwide partitioned into gross photosynthesis (Pg) and ecosystem respiration (Re) components using the light-response functions method (Gilmanov et al. 2003, Bas. Appl. Ecol. 4:167-183) from the RANGEFLUX and WorldGrassAgriflux data sets supplemented by data from 46 sites partitioned using the temperature-response method (Reichstein et al. 2005, Gl. Change. Biol. 11:1424-1439) from the FLUXNET La Thuile data set. Maximum values of the apparent quantum yield (α = 75 mmol mol-1), photosynthetic capacity (Amax = 3.4 mg CO2 m-2 s-1), maximum daily gross photosynthesis (Pg,max = 116 g CO2 m-2 d-1), and gross ecological light-use efficiency (εecol = 59 mmol mol-1) of intensively managed grasslands and high-productive croplands exceed those for forest ecosystems, indicating high potential of non-forest ecosystems for uptake and sequestration of atmospheric CO2. Maximum values of annual gross primary production (8600 g CO2 m-2 yr-1), total ecosystem respiration (7900 g CO2 m-2 yr-1), and net CO2 exchange (2400 g CO2 m-2 yr-1) for non-forest ecosystems are observed in intensively managed grasslands and high-yield crops, and are comparable or higher than in forest ecosystems (excluding tropical forests). On the average, 80% of the non-forest sites were sinks for atmospheric CO2, with mean annual net CO2 uptake 848 g CO2 m-2 yr-1 for intensively managed grasslands and 933 g CO2 m-2 yr-1 for croplands. The new flux-tower data indicate the need to revise substantially previous views of grassland and agricultural ecosystems as being predominantly a source of carbon, or having a neutral role, in the regional and continental carbon budgets.