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

Authors: GILMANOV, TAGIR - South Dakota State University; Johnson, Douglas; Svejcar, Anthony

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/12/2009
Publication Date: 1/8/2010
Citation: Gilmanov, T.G., Johnson, D.A., Svejcar, A.J. 2010. Productivity, Respiration, and Light-Response Parameters of World Grassland and Agro-Ecosystems Derived from Flux-Tower Measurements. Rangeland Ecology and Management. 63: 16-39.

Interpretive Summary: Quantifying the contribution of different ecosystems to total regional, continental and global stocks and exchanges of carbon is an important task in understanding the global carbon cycle. Generalizations concerning the role of forests, wetlands, and tundra ecosystems in the global carbon budget have been provided recently. Although grasslands and agroecosystems occupy nearly a third of the Earth's land surface area, their contribution to the global carbon cycle remains uncertain. Available estimates tyically characterize grasslands as weak sinks (take up carbon dioxide), while croplands are considered moderate to strong sources (give off carbon dioxide). These assessments, however, are not based on direct measurements of carbon exchange but instead are based on indirect measurements such as inventories of biomass and soil organic matter. Although these estimates are useful as initial approximations, direct measurements are needed to previsely understand the contributions of grasslands and agroecosystems to regional and global atmospheric carbon dioxide exchange. In this publication, we present the first synthesis of results from tower carbon dioxide flux measurements at 118 tower site locations, which represent various grassland, cropland, shrubland, savanna, and wetland ecosystems of the world. We first describe the methodology used to derive the terms of the carbon balance used in the global ecosystem comparison, then compare global ecosystem carbon cycle characteristics such as seasonal light responses and carbon dioxide uptake with a focus on ecosystems without trees compared to forests. On average, 80% of the non-forest sites were sinks for atmospheric carbon dioxide. However, part of this apparent sink is accumulated in crop and forage harvests, which are products decomposed off site. Therefore, although agricultural fields are predominantly carbon sinks for atmospheric carbon dioxide, this does not necessarily imply that the carbon stocks at the measurement site are accumulating. These data indicate that grasslands and agricultural croplands play a significant role in the uptake of atmospheric CO2 and its transformation to biomass and soil organic matter. These results underscore the need to examine the fate of carbon transported from grasslands and agroecosystems, and metabolized in and/or transported to other geographic locations.

Technical Abstract: Grasslands and agroecosystems occupy nearly a third of the terrestrial area, but their contribution to the global carbon cycle remains uncertain. We used a set of 316 site-years of net carbon dioxide (CO2)) exchange measurements to quantify gross primary productivity, ecosystem respiration, and light-response parameters of grasslands, shrublands/savanna, wetlands, and cropland ecosystems worldwide. We analyzed data from 72 global flux-tower sites partitioned into gross photosynthesis and ecosystem respiration using the light-response method from data sets of RANGEFLUX and WorldGrassAgriflux supplemented by 46 sites from the FLUXNET La Thuile data set partitioned using the temperature-response method. Maximum values of the quantum yield (a=75 mmol mol-1), photosynthetic capacity (Amax=3.4 mg CO2 m-2 s-1), gross photosyntheses (Pg.max=116 g CO2 m-2 d-1), and ecological light-use efficiency (Eecol=59 mmol mol-1) of intensively managed grasslands and high-production croplands exceeded those of most forest ecosystems, indicating the potential of non-forest ecosystems for uptake of atmospheric CO2. Maximum values of 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) were observed for intensively managed grasslands and high-yield crops and are comparable or higher than those for forest ecosystems, excluding some tropical forests. On average, 80% of the non-forest sites were sinks for atmospheric CO2, with a mean net uptake of 700 g CO2 m-2 yr-1 for intensively managed grasslands and 933 g CO2 m-2 yr-1 for croplands. However, part of the apparent sink is accumulated in crop and forage harvests, which are labile carbon pools decomposed off site. Therefore, although agricultural fields are predominantly carbon sinks for atmospheric CO2, this does not necessarily imply that the carbon stock at the measurement site is increasing.