Partitioning European grassland net ecosystem CO 2 exchange into gross primary productivity and ecosystem respiration using light response function analysis

Abstract

Tower CO 2 flux measurements from 20 European grasslands in the EUROGRASSFLUX data set covering a wide range of environmental and management conditions were analyzed with respect to their ecophysiological characteristics and CO 2 exchange (gross primary production, P g, and ecosystem respiration, R e) using light-response function analysis. Photosynthetically active radiation ( Q) and top-soil temperature ( T s) were identified as key factors controlling CO 2 exchange between grasslands and the atmosphere at the 30-min scale. A nonrectangular hyperbolic light-response model P( Q) and modified nonrectangular hyperbolic light–temperature-response model P( Q, T s) proved to be flexible tools for modeling CO 2 exchange in the light. At night, it was not possible to establish robust instantaneous relationships between CO 2 evolution rate r n and environmental drivers, though under certain conditions, a significant relationship r n = r 0 e k T T s 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, ɛ = P g/ 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 CO 2 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 ( P g) of European grasslands ranges from 1700 g CO 2 m −2 year −1 in dry semi-natural pastures to 6900 g CO 2 m −2 year −1 in intensively managed Atlantic grasslands. Ecosystem respiration ( R e) is in the range 1800 < R e < 6000 g CO 2 m −2 year −1. Annual net ecosystem CO 2 exchange (NEE) varies from significant net uptake (>2400 g CO 2 m −2 year −1) to significant release (<−600 g CO 2 m −2 year −1), though in 15 out of 19 cases grasslands performed as net CO 2 sinks. The carbon source was associated with organic rich soils, grazing, and heat stress. Comparison of P g, R e, 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 grassland P g and R e to macroclimatic factors (precipitation and temperature), but these relationships cannot be reduced to simple monofactorial models.