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1.a. 1.b. 2.a. 2.b. 3.a. 3.b. 2.c. 2.d. LABORATORIO DI ECOLOGIA FORESTALE. Carbon and water exchanges over a natural steppe and a fallow field in southern Siberia: preliminary results.

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Carbon and water exchanges over a natural steppe and a fallow field in southern Siberia: preliminary results.

Luca Belelli Marchesini (1), Giovanni Manca (1), Elena N. Parfenova (2), Tatiana Otnuykova (2),

Nadejda Tchebakova (2) and Riccardo Valentini (1)

(1) University of Tuscia, DiSAFRi, Viterbo, Italy.

(2) Sukachev Institute of Forest, Krasnoyarsk, Russia.

Temperate grassland ecosystems extent ranges, according to estimates, from approximately 41 to 56 million km2 comprising 31 to 43 percent of the earth surface. The Russian Federation hosts 6256,52 km2 of grasslands (WRI-PAGE) being the second country in the world for area of this biome. Short term studies have shown that grasslands may be a sink for atmospheric CO2 during their peak biomass accumulation period , although data of continuous fluxes monitoring along different seasons are limited. Most of carbon is allocated in the extensive fibrous root system and stored in the soil, which may turn into a source of carbon during the rest of the year.

Carbon dioxide, latent heat and sensible heat fluxes were measured over a natural steppe and a fallow field in Hakassia region (southern Siberia) from 30 June and 4 July 2002 respectively to 5 January 2003 using the eddy covariance technique. Assessment of carbon pools and primary productivity of these ecosystems were made by periodical samplings from 15 Mat to 15 October 2002.

The sites:

Study sites are located in Hakassia, an autonomous republic of Russia, near the town of Shira. Hakasian steppes are in the Khakas-Minusinsk depression surrounded by mountains of Kuznetsk Altai in the west, West Sayan in the south and East Sayan in the east. Zonal types of steppes are in the Iyus-Shira region are classified as true steppes or tall bunchgrass steppes with Stipa krilovii and Helictotrichium desertorum dominant species. The climate of this region is extremely continental (January mean temperature –18.8 °C; July mean temperature 17.6 °C) with an average annual precipitation of 360 mm mostly distributed in summer.

The natural steppe site (Hakas 1; 54.72 N, 90.00 E) is mainly composed of Stipakrilovii and Festuca valesiaca in the dominant vegetation storey. (Fig 1.a)

The fallow field site (Hakas 2; 54.77 N; 89.95 E) is a patch of former agricultural field left in 1999 over which Melilotus spp.and Avena spp. were cultivated (fig 1.b). Actual vegetation is represented by the first succesional stage of recovering grassland and it is composed of Artemisia spp.(A.sieversiana, A.scoparia, A.Jacutica). Comparison to other neighbouring fallow lands showed the same Artemisia successions with significant but temporal partecipation of grasses (Agropyron repens, Setaria viridis) or typical weeds (Chenopodium album, Cannabis ruderalis); with time these temporal communities will be replaced by steppe plant species which will form zonal steppe communities in the absence of land use.

Carbon and energy fluxes :

Fluxes of CO2(Fc), latent heat (LE) and sensible heat (H) have been measured according to the eddy covariance technique. A three-dimensional ultrasonic anemometer (Gill R3, Gill Instruments Ltd, UK) was placed on the top of a 4,5 m tower in order to measure wind speed, direction and air temperature. CO2 and H2O concentrations were measured with an open path fast response infra red gas analyser (LI-7500, Li-Cor Inc., USA). Data collection included physical variables of the ecosystems (air and soil temperature, net radiation, global radiation and photosynthetic active radiation, air and soil moisture, soil heat flux, atmospheric pressure, precipitations).

Fluxes computation, given by the mean covariance of the vertical wind speed and a scalar ( T, CO2 and H2O concentration) on a 30 min. base, have been corrected for air densities fluctuations (Web et al., 1980). Further corrections to account for spectral losses and night-time fluxes underestimations have been applied according to the Euroflux methodology (Aubinet et al., 2000). Gaps in Fc data series were filled through non linear regressions modelling the response of net ecosystem exchange (NEE) to photosyntetic photon flux density (PPFD) for day-time and to soil temperature for night-time data.

During the growing season both sites showed a variable behaviour as daily carbon sinks or sources depending on sun radiation availability (fig.2a). Hakas 2 site was the most sensitive to radiation regime changes, showing larger fluctuations in daily NEE which ranged between -2.24 and +1.50 g C m-2 (the negative sign indicates an uptake of carbon from the atmosphere); the higher respiration rates of Hakas 2 made this ecosystem a direct source of carbon to the atmosphere when phototsyntetic activity was low. Net ecosystem productivity, referred to the same period, was -34.7 g C m-2 and -40.3 g C m-2 for the natural steppe site and the fallow field respectively, and evidenced the similar weak capacity of both ecosystems to sequester atmospheric CO2 (fig.2b). Carbon effluxes from the natural steppe due to respiration processes were 0.1÷0.2 g C d-1 during winter season, determining not relevant changes in the carbon balance (fig.2c).

The analysis of energy balance during the months of July, August and September 2002 revealed that, for a long period of the summer, the energy input of the two ecosystems was allocated more to latent heat flux (LE) rather than sensible heat flux (H); day-time soil heat fluxes constituted about 10 % of net radiation (fig.2d).

Net primary production and carbon pools:

Above ground phytomass (AG) was clipped every 15 days over 20 randomly chosen sampling plots of (0.5 x 0.5) m2. Vegetal biomass was separated into live and dead, which included litter and dried in a stove at 70°C until it reached its dry weight. Below ground phytomass (BG) was sampled in the same sampling plots and at the same frequency of AG phytomass, collecting 1 soil core for each sampling plot reaching the depth of 30 cm. Roots were cleaned from soil, separated into fine (ø < 2mm)and medium(2mm <ø> 5mm), then dried and weighed. Ecosystem net primary production (NPP) was estimated summing the positive periodical increments in biomass dry weight using the following algorithm: 

NPP =Σ (Δ AG biomass+ Δ AGTot dead ) (Singh et al.,1975) 

Where:AG biomass= AG live biomass; AGTot dead= AG dead phytomass (including litter).

Below-ground NPP will be similarly calculated.

The growth trend of phytomass (fig.3a et 3b) exposes the temporal shift of about 30 days between the peak value of AG live phytomass , reached within the second and the third weeks of August in the natural steppe and in the fallow field respectively, and the maximum quantity of BG phytomass. In Hakas 1 roots form a thick fibrous system and constituted 78% of total biomass at the beginning of the samplings (15 May) while represented up to 90 % in September. Root biomass in the fallow field was less developed but still ranged between 50% and 80% of the total biomass along the summer season. Above ground primary production between 15 may and 15 October , expressed as tons of dry phytomass per hectare, was extremely similar for both sites (Hak1: 1.73 t/ha; Hak 2: 1.70 t/ha), while belowground production differed remarkably being 17.83 t/ha and 9.07 t/ha respectively. Roots were thus responsible for 91% of NPP of the natural steppe and 84% of the fallow field.

Chemical analysis of the soil evidenced its role as the largest carbon pool of these ecosystems: the soil carbon content of the natural steppe patch was 89 t C ha-1, while as consequence of the former agricultural use, the soil of the fallow field contained 32 t C ha-1, about 64% less.


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