Drivers of soil respiration of root and microbial origin in grasslands
Author(s)
Gavrichkova, Olga
Date Issued
February 10, 2009
Type
Doctoral Thesis
Abstract
Soil historically has been a major source of atmospheric enrichment of CO2 and in the same
time is one of the biggest storing reservoirs of carbon on the global scale. In fact, soils hold three times as much carbon as the terrestrial biosphere and about twice as much as the atmosphere and exert a large influence on the cycling of carbon between different pools. Soil respiration, which is
the flux of CO2 from soils to the atmosphere, is thus an important component of the ecosystem C budgets and is a major source of CO2 released by terrestrial ecosystems. Soil respiration is the result
of the production of CO2 from the biological activity of roots and associated microorganisms and the activity of heterotrophic bacteria and fungi living on litter and in the root-free soil. Different
sources of soil CO2 efflux are known to experience high spatial and temporal variation with different controlling factors involved on different time-scales. However, up to now not so many studies have deal with the interannual variability of soil respiration and its components and only
few of them were performed in grassland ecosystems despite the fact that it is one of the world’s most widespread vegetation types which comprises 32% of the earth’s area of natural vegetation.
This study aimed to advance the understanding of the processes and factors controlling the behaviour of different soil respiration sources in grassland ecosystems. It provides the analysis of the response of soil CO2 efflux and its components: root- and microbial-derived respiration to
different biotic and abiotic factors as well as to widely diffused management activities over a period of three years in a mediterranean grassland site and integrates also different laboratory and in situ
methodological approaches for deeper studying of the contribution of various respiration sources to total CO2 efflux from soil and the speed of C cycling within the plant community.
Soil respiration was partitioned in the field using micro (1mm) and macro (1 cm) pore
meshes. Soil respiration obtained from the cores with different pore-sized meshes and from the control undisturbed soil were used to calculate values of root-derived and microbial-derived respiration sources. These fluxes were then related to canopy photosynthetic activity, soil temperature, soil moisture and some soil biochemical parameters.
Methodological approach based on pulse labeling of plants in artificial 13CO2 or 14CO2 atmosphere was used to found out the speed of the cycling of C in grassland ecosystem (in situ) as well as to study the effect of different plant species, plant growing stages, and different nutrient
supply on the magnitude of root respiration and on the speed of translocation and respiration of recently assimilated C through roots (on a single species, in laboratory).
The obtained results showed an importance of C assimilate supply in the determination of
the variability of root component of soil respiration. It was closely related to gross primary production with a time lag of circa 20h for time scales from daily to annual. Soil temperature which often masks the direct relationship between root respiration and photosynthetic C supply failed to
explain diurnal and seasonal changes in root-derived respiration. Laboratory experiments with a single plant species have shown however that the observed time lag is not stable during the plant ontogenesis, and vary depending on the plant growing stage. The same photosynthetic activity could also result in different magnitude of root respiration, depending on the type of nutrient supply (ex: N in form of NH+
4 or NO-3). All these finings suggest that root respiration is a complex process, tightly coupled to plant canopy activity and could not be explained simply by changes in
soil temperature and moisture. Further studies are needed to verify the bonds between aboveground and belowground processes for different species and vegetation types, as well as for various plant growing stages.
Soil temperature and soil water content exerted a significant effect on microbial component of soil respiration. Being a larger part of total CO2 efflux from soil at Amplero (» 70%), these factors influenced also total soil respiration dynamic on different time scales. Introduction of a
management regime have modified however the activity of microbial community by an increase of the quantity of easily available C substrates from the rhizodeposition process, resulting in a general suppression of microbial enzymatic activity and further decrease C mineralization rates.
Combination of laboratory studies and in situ measurements is necessary for understanding of the effect of changing substrate quality, nutrient and moisture conditions on microbial activity and its C
use efficiency.
Additional information
Dottorato di ricerca in Ecologia forestale
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