Simultaneous measurements of CO<sub>2</sub> and water exchanges over three agroecosystems in South-West France

Abstract

Abstract.During the last few decades, many studies have been performed to determine water and carbon budgets of broadleaf and deciduous forests, crops and grasslands. However, since most measurements have been made in different regions and at different periods, it is difficult to compare the results directly. In order to evaluate accurately the respective contribution of various agroecosystems to global water and carbon exchanges, it is necessary to compare data obtained in similar climatic and weather conditions. To address this question, we present the results from simultaneous measurements carried out during one year over three typical agroecosystems of the Les Landes region in South-West France: an agricultural field with maize from 29 May to 18 October, a young (5 year-old) pine forest and a mature (37 year-old) pine forest. All measurements were collected as part of the Regional Experiment component of the CarboEurope-IP project. During most of the year, the agricultural field without vegetation is a source of CO 2 , but from late June to early September the maize crop becomes a stronger carbon sink than the forests. Over the whole measurement period the three agroecosystems behave as CO 2 sinks with carbon storage of about 335, 210 and 160 g C m −2 for the young forest, the mature forest and the agricultural field, respectively. We investigated the influence of climatic conditions on Gross Primary Production (GPP) of the three ecosystems and observed a predominant role of vapour pressure deficit (VPD) for forests and of photosynthetic photon flux density (F P P ) for maize. Daily Water Use Efficiencies (WUE) of the three ecosystems were evaluated and expressed as functions of the mean daily vapour pressure deficit (VPD). Similar trends were observed for the two forests, which suggests that for a given species WUE is independent of stand age. The WUE of the maize crop at maturity was also found to Correspondence to: E. Lamaud (lamaud@bordeaux.inra.fr) depend upon VPD, but it is about twice as large as for the forests, owing to the physiological advantages of C 4 species.

Importance of the vegetation-groundwater-stream continuum to understand transformation of biogenic carbon in aquatic systems – A case study based on a pine-maize comparison in a lowland sandy watershed (Landes de Gascogne, SW France)

“…We hypothesize that this difference was a consequence of a less intense soil respiration in croplands during summer. From simultaneous eddy covariance measurements over pine forests and maize croplands of the study area, Stella et al (2009) confirmed that ecosystem respiration was lower in croplands than in forests over the whole year. However, Stella et al (2009) also showed that ecosystem respiration was larger during the growing season of the maize, because of increased of soil respiration in response to the higher soil water content caused by irrigation.…”

“…From simultaneous eddy covariance measurements over pine forests and maize croplands of the study area, Stella et al (2009) confirmed that ecosystem respiration was lower in croplands than in forests over the whole year. However, Stella et al (2009) also showed that ecosystem respiration was larger during the growing season of the maize, because of increased of soil respiration in response to the higher soil water content caused by irrigation. In forest sites, groundwater pCO 2 increases during the summer because soil CO 2 diffuses downward and then is dissolved into the water table Tsypin and Macpherson, 2012).…”

“…At the studied catchment scale, lithology, topography, soils, and precipitation are more or less uniform (Augusto et al, 2010;Jolivet et al, 2003). At the plot scale, spatial variability of water table depth in relation to land use was thus necessarily dependent on how water outputs (drainage, evapotranspiration or groundwater storage) of the water mass balance were human-affected (Govind et al, 2012;Stella et al, 2009). Local forests are never irrigated, conversely, irrigation with extraction of groundwater (that decreases groundwater storage) in local croplands could strongly bias the water mass balance at the plot scale since about half of the water diverted for irrigation is rapidly consumed through evapotranspiration (e.g., Jackson et al, 2001).…”

“…Local forests are never irrigated, conversely, irrigation with extraction of groundwater (that decreases groundwater storage) in local croplands could strongly bias the water mass balance at the plot scale since about half of the water diverted for irrigation is rapidly consumed through evapotranspiration (e.g., Jackson et al, 2001). Additionally, evapotranspiration in maize croplands is typically higher than in forests owing to the larger stomatal conductance that makes the exchange of C and water between the biosphere and the atmosphere much easier (Govind et al, 2012;Stella et al, 2009). Other studies have shown that the combination of subsoiling practices (increasing soil permeability) with deep agricultural ditches in croplands also affected water mass balance at the plot scale by enhancing lateral drainage of groundwater (Evans et al, 1996;Robinson et al, 1985).…”

Detecting regional variability in sources and sinks of carbon dioxide: a synthesis

“…The flux data both for surface energy balance and CO 2 exchange, obtained from several vegetation types proved essential data for calibrating the land surface schemes (e.g. Ahmadov et al, 2007) and for investigating and understanding fundamental differences in sequestration capacities Jarosz et al (2009) and Stella et al (2009). While observations on additional sites were only obtained during the experimental campaigns, the backbone of sites was part of the CarboEurope network of sites and these potentially allow for further studies involving question of representability and interannual variation.…”