Net ecosystem carbon dioxide exchange over grazed steppe in central Mongolia

Abstract

This paper presents results of 1 year (from March 25, 2003 to March 24, 2004 of continuous measurements of net ecosystem CO 2 exchange (NEE) above a steppe in Mongolia using the eddy covariance technique. The steppe, typical of central Mongolia, is dominated by C 3 plants adapted to the continental climate. The following two questions are addressed: (1) how do NEE and its components: gross ecosystem production (GEP) and total ecosystem respiration (R eco ) vary seasonally? (2) how do NEE, GEP, and R eco respond to biotic and abiotic factors? The hourly minimal NEE and the hourly maximal R eco were À3.6 and 1.2 lmol m À2 s À1 , respectively (negative values denoting net carbon uptake by the canopy from the atmosphere). Peak daily sums of NEE, GEP, and R eco were À2.3, 3.5, and 1.5 g C m À2 day À1 , respectively. The annual sums of GEP, R eco , and NEE were 179, 138, and À41 g C m À2 , respectively. The carbon removal by sheep was estimated to range between 10 and 82 g C m À2 yr À1 using four different approaches. Including these estimates in the overall carbon budget yielded net ecosystem productivity of À23 to 1 20 g C m À2 yr À1 . Thus, within the remaining experimental uncertainty the carbon budget at this steppe site can be considered to be balanced. For the growing period (from April 23 to October 21, 2003), 26% and 53% of the variation in daily NEE and GEP, respectively, could be explained by the changes in leaf area index. Seasonality of GEP, R eco , and NEE was closely associated with precipitation, especially in the peak growing season when GEP and R eco were largest. Water stress was observed in late July to early August, which switched the steppe from a carbon sink to a carbon source. For the entire growing period, the light response curves of daytime NEE showed a rather low apparent quantum yield (a 5À0.0047 lmol CO 2 lmol À1 photons of photosynthetically active radiation). However, the a values varied with air temperature (T a ), vapor pressure deficit, and soil water content.

Grazing intensity and soil depth effects on soil properties in alpine meadow pastures of Qilian Mountain in northwest China

“…In this way, soil cores were collected from 10 points and mixed to obtain a composite sample, and three replicate samples from each of four soil depths were collected from the different 100 m × 100 m plots in each sampling area. Soil was sub-sampled from the evenly mixed composite sample by quartering, then brought to the laboratory, air-dried at room temperature and ground and passed through 1-mm sieves for chemical analyses (Jastrow et al 1998;Wang et al 2002;Li et al 2005). Soil bulk density (BD) was determined by the core method (Blake & Hartge 1986).…”

Seasonal Dynamics in Carbon Dioxide Flux Es of the Herbaceous Layer of a Moist Kenyan Savannah

“…This is typical of most grassland, where gross and net photosynthesis are strongly correlated to green biomass (leaf area index) Li et al, 2005;Otieno et al, 2009;Beringer et al, 2011). Given that shoot N also peaked in the wet season, the combined increase in green biomass (photosynthetic surface area), shoot N and light utilization efficiency facilitated rapid CO 2 uptake resulting into the high GPP and NEE.…”

“…During drought, the daily trends of GPP and NEE were reversed and they both decreased as VPD and PPFD increased. The decline in GPP and NEE during drought is attributed to either stomatal regulation by VPD (Eamus et al, 2001;Maherali et al, 2003;Li et al, 2005) or their direct limitation by soil moisture (Kanniah et al, 2010). Under drought with relatively high VPD, increasing VPD decreases stomatal conductance and hence GPP and NEE (Maherali et al, 2003;Li et al, 2005) in order to restrict plant water loss.…”

“…The decline in GPP and NEE during drought is attributed to either stomatal regulation by VPD (Eamus et al, 2001;Maherali et al, 2003;Li et al, 2005) or their direct limitation by soil moisture (Kanniah et al, 2010). Under drought with relatively high VPD, increasing VPD decreases stomatal conductance and hence GPP and NEE (Maherali et al, 2003;Li et al, 2005) in order to restrict plant water loss. Stomatal sensitivity to VPD is also known to increase with dropping leaf water potential in response to low soil moisture (Jones, 1998;Li et al, 2005).…”

“…Under drought with relatively high VPD, increasing VPD decreases stomatal conductance and hence GPP and NEE (Maherali et al, 2003;Li et al, 2005) in order to restrict plant water loss. Stomatal sensitivity to VPD is also known to increase with dropping leaf water potential in response to low soil moisture (Jones, 1998;Li et al, 2005). Kanniah et al (2010) however, reported canopy light use efficiency to be more strongly correlated to SWC at 10 cm depth than the atmospheric VPD.…”