Global warming and a changing precipitation regime could have a profound impact on ecosystem carbon fluxes, especially in arid and semiarid grasslands where water is limited. A field experiment manipulating temperature and precipitation has been conducted in a temperate steppe in northern China since 2005. A paired, nested experimental design was used, with increased precipitation as the primary factor and warming simulated by infrared radiators as the secondary factor. The results for the first 2 yr showed that gross ecosystem productivity (GEP) was higher than ecosystem respiration, leading to net C sink (measured by net ecosystem CO(2) exchange, NEE) over the growing season in the study site. The interannual variation of NEE resulted from the difference in mean annual precipitation. Experimental warming reduced GEP and NEE, whereas increased precipitation stimulated ecosystem C and water fluxes in both years. Increased precipitation also alleviated the negative effect of experimental warming on NEE. The results demonstrate that water availability plays a dominant role in regulating ecosystem C and water fluxes and their responses to climatic change in the temperate steppe of northern China.

Water-mediated responses of ecosystem carbon fluxes to climatic change in a temperate steppe

. Closed (non-steady state) chambers are widely used for quantifying carbon dioxide (CO2) fluxes between soils or low-stature canopies and the atmosphere. It is well recognised that covering a soil or vegetation by a closed chamber inherently disturbs the natural CO2 fluxes by altering the concentration gradients between the soil, the vegetation and the overlying air. Thus, the driving factors of CO2 fluxes are not constant during the closed chamber experiment, and no linear increase or decrease of CO2 concentration over time within the chamber headspace can be expected. Nevertheless, linear regression has been applied for calculating CO2 fluxes in many recent, partly influential, studies. This approach has been justified by keeping the closure time short and assuming the concentration change over time to be in the linear range. Here, we test if the application of linear regression is really appropriate for estimating CO2 fluxes using closed chambers over short closure times and if the application of nonlinear regression is necessary. We developed a nonlinear exponential regression model from diffusion and photosynthesis theory. This exponential model was tested with four different datasets of CO2 flux measurements (total number: 1764) conducted at three peatlands sites in Finland and a tundra site in Siberia. Thorough analyses of residuals demonstrated that linear regression was frequently not appropriate for the determination of CO2 fluxes by closed-chamber methods, even if closure times were kept short. The developed exponential model was well suited for nonlinear regression of the concentration over time c(t) evolution in the chamber headspace and estimation of the initial CO2 fluxes at closure time for the majority of experiments. However, a rather large percentage of the exponential regression functions showed curvatures not consistent with the theoretical model which is considered to be caused by violations of the underlying model assumptions. Especially the effects of turbulence and pressure disturbances by the chamber deployment are suspected to have caused unexplainable curvatures. CO2 flux estimates by linear regression can be as low as 40% of the flux estimates of exponential regression for closure times of only two minutes. The degree of underestimation increased with increasing CO2 flux strength and was dependent on soil and vegetation conditions which can disturb not only the quantitative but also the qualitative evaluation of CO2 flux dynamics. The underestimation effect by linear regression was observed to be different for CO2 uptake and release situations which can lead to stronger bias in the daily, seasonal and annual CO2 balances than in the individual fluxes. To avoid serious bias of CO2 flux estimates based on closed chamber experiments, we suggest further tests using published datasets and recommend the use of nonlinear regression models for future closed chamber studies.

https://bg.copernicus.org/articles/4/1005/2007/bg-4-1005-2007.pdf

CO 2 flux determination by closed-chamber methods can be seriously biased by inappropriate application of linear regression

A large remaining source of uncertainty in global model predictions of future climate is how ecosystem carbon (C) cycle feedbacks to climate change. We conducted a field manipulative experiment of warming and nitrogen (N) addition in a temperate steppe in northern China during two contrasting hydrological growing seasons in 2006 [wet with total precipitation 11.2% above the long-term mean (348 mm)] and 2007 (dry with total precipitation 46.7% below the long-term mean). Irrespective of strong intra- and interannual variations in ecosystem C fluxes, responses of ecosystem C fluxes to warming and N addition did not change between the two growing seasons, suggesting independence of warming and N responses of net ecosystem C exchange (NEE) upon hydrological variations in the temperate steppe. Warming had no effect on NEE or its two components, gross ecosystem productivity (GEP) and ecosystem respiration (ER), whereas N addition stimulated GEP but did not affect ER, leading to positive responses of NEE. Similar responses of NEE between the two growing seasons were due to changes in both biotic and abiotic factors and their impacts on ER and GEP. In the wet growing season, NEE was positively correlated with soil moisture and forb biomass. Negative effects of warming-induced water depletion could be ameliorated by higher forb biomass in the warmed plots. N addition increased forb biomass but did not affect soil moisture, leading to positive effect on NEE. In the dry growing season, NEE showed positive dependence on grass biomass but negative dependence on forb biomass. No changes in NEE in response to warming could result from water limitation on both GEP and ER as well as little responses of either grass or forb biomass. N addition stimulated grass biomass but reduced forb biomass, leading to the increase in NEE. Our findings highlight the importance of changes in abiotic (soil moisture, N availability) and biotic (growth of different plant functional types) in mediating the responses of NEE to climatic warming and N enrichment in the semiarid temperate steppe in northern China.

R. Kanber

Papers

Automated closed-system canopy-chamber for continuous field-crop monitoring of CO2 and H2O fluxes