Partitioning European grassland net ecosystem CO2 exchange into gross primary productivity and ecosystem respiration using light response function analysis

TowerCO2 fluxmeasurements from 20 European grasslands in theEUROGRASSFLUXdata set covering awide range of environmental and management conditions were analyzed with respect to their ecophysiological characteristics and CO2 exchange (gross primary production, Pg, and ecosystem respiration, Re) using light-response function analysis. Photosynthetically active radiation (Q) and top-soil temperature (Ts) were identified as key factors controllingCO2 exchange between grasslands and the atmosphere at the 30-min scale.Anonrectangular hyperbolic light-response model P(Q) and modified nonrectangular hyperbolic light–temperature-response model P(Q, Ts) proved to be flexible tools for modelingCO2 exchange in the light. At night, it was not possible to establish robust instantaneous relationships betweenCO2 evolution rate rn and environmental drivers, though under certain conditions, a significant relationship rn ¼ r0 ekT Ts was found using observation windows 7–14 days wide. Principal light-response parameters—apparent quantum yield, saturated gross photosynthesis, daytime ecosystem respiration, and gross ecological light-use efficiency, e = Pg/Q, display patterns of seasonal dynamics which can be formalized and used for modeling. Maximums of these parameters were found in intensively managed grasslands of Atlantic climate. Extensively used semi-natural grasslands of southern and central Europe have much lower production, respiration, and light-use efficiency, while temperate and mountain grasslands of central Europe ranged between these two extremes. Parameters from light–temperature-response analysis of tower data are in agreement with values obtained using closed chambers and free-air CO2 enrichment. Correlations between light-response and productivity parameters provides the possibility to use the easier to measure parameters to estimate the parameters that are more difficult to measure. Gross primary production (Pg) of European grasslands ranges from 1700 g CO2 m_2 year_1 in dry semi-natural pastures to 6900 g CO2 m_2 year_1 in intensively managed Atlantic grasslands. Ecosystemrespiration (Re) is in the range 1800 < Re < 6000 g CO2 m_2 year_1. Annual net ecosystem CO2 exchange (NEE) varies from significant net uptake (>2400 g CO2 m_2 year_1) to significant release (<_600 g CO2 m_2 year_1), though in 15 out of 19 cases grasslands performed as net CO2 sinks. The carbon sourcewas associated with organic rich soils, grazing, and heat stress. Comparison of Pg, Re, and NEE for tower sites with the same characteristics from previously published papers obtained with other methods did not reveal significant discrepancies. Preliminary results indicate relationships of grasslandPg andRe to macroclimatic factors (precipitation and temperature), but these relationships cannot be reduced to simple monofactorial models.