Analisi del ciclo del carbonio di steppe ed aree agricole abbandonate dell' Asia centrale
Author(s)
Belelli Marchesini, Luca
Date Issued
August 27, 2008
Type
Doctoral Thesis
Abstract
Eurasian steppes cover a vast area of about 8·106 km2, of which a fraction between 60% and 70% was converted to agricultural use in the past, and have been poorly investigated in respect with the exchanges of CO2 with the atmosphere while they could play a relevant role in the carbon uptake located over the northern hemisphere lands at temperate latitudes. In particular, old agricultural fields abandoned after 1990 in former Soviet Union could contribute actively to the sequestration of atmospheric carbon, since after land use change they tend to restore the original stock of carbon of steppe ecosystems, that went partly lost with the agricultural use of the land. This study provides a quantitative characterization of the carbon pools, of the patterns of carbon allocation within the ecosystems, of the CO2 exchanges between the biosphere and the atmosphere, as well as of the response of CO2 fluxes to environmental drivers of true bunchgrass steppes and neighbouring old agricultural fields hosting different temporal stages of recovering grassland located in the Republic of Hakasia and in the Republic of Tuva (Russian Federation).
Within about 40 years, cultivations caused a loss of organic carbon not lower than 46%-52% in respect with the level of steppes (average carbon sotck: 140.9 tCha-1 in Hakasia and 45.5 tCha-1 in Tuva) and affected both the soil organic carbon pool, 80-92% of the lost carbon, and the belowground biomass pool for the remaining part.
The biomass carbon stock of steppes of Hakasia and Tuva was found to be stored mainly in the belowground pool, with a proportion ranging 88-94%, due to the highly developed root system of perennial species. In old agricultural fields, belowground biomass was still predominant but the share of biomass found in the aboveground pool, from 12% to 42%, was larger than in steppes.
The assessment of net primary productivity (NPP) at sites of Hakasia by different biometric methods ranged between 6.0 ad 13.1 t d.m. ha-1, and organicated carbon was allocated primarily to the root system, with a fraction varying between 70% and 90% for both steppe and old field ecosystems.
CO2 fluxes at ecosystem scale were monitored by eddy covariance technique over three stages of an old field succession in Hakasia represented by an early stage (5 years since land use change), an intermediate stage (10 years) and a mature stage (steppe). Yearly CO2 exchange estimates disclosed all the sites to act as carbon sinks, with a strenght declining from the early stage (2.1 tC ha-1) to the steppe ecosystem (1.1 tC ha-1). Also the amounts of carbon assimilated by photosynthesis (GPP) and released to the atmosphere through ecosystem respiration (Reco) decreased over the stages of the succession, yielding the observed trend in the net ecosystem productivity (NEP).
The steppe ecosystem, even if characterized by lower carbon assimilation than old field ecosystems, displayed a less pronounced reduction of the photosynthetic process in response to extreme high air temperature and air dryness conditions, maintaining its efficiency in carbon uptake.
The steppe ecosystem was also found resilient to disturbances since after a fire burst before the onset of the growing season, it showed an enhanced carbon sequestration that could completely offset the amount of carbon lost from the burnt biomass.
The carbon budget of the true steppe, estimated additionally using the ecological inventory methodology based on the difference between net primary productivity and heterotrophic respiration (NEP=NPP-Rh), agreed with the result obtained with the micrometeorological technique.
The spatialization of the carbon balance observed at sites in the steppe region of the Republic of Hakasia, to assess the magnitude of the sink of steppes and old fields ecosystems in the Russian Federation, suggests a carbon sequestration of 0.17 PgC yr-1.
Additional information
Dottorato di ricerca in Ecologia forestale
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