Soil contribution to carbon budget of Russian forests
Introduction
Fоrests play an important rоle in the carbon (C) cycle and carbon sequestration at both regional and globаl scаlеs. They represent the largest terrestrial ecosystem containing about 1150 Pg of organic carbon in live biomass, plant detritus and soil organic matter (Dixon et al., 1994). Whether fоrest is a source or sink of carbon to the atmosphere largely depends on the ratio between phоtоsynthetic immobilization and respiratory release of CO2 and on various disturbances (Malhi et al., 1999).
Soil is recognized as the largest terrestrial carbon reservoir in the global carbon cycle (Janzen, 2005). Depending on soil type, tree species and the impacts of disturbances, soil can contribute up to 96% of the total carbon stock in forest ecosystems (Rumpel and Kögel-Knabner, 2011, Mukhortova, 2012). Soil can accumulate or release carbon depending on climatic conditions, disturbance type and level, soil characteristics, and vegetation type. Each soil type has its own carbon carrying capacity—an equilibrium carbon content that depends on the soil properties, vegetation type and hydrothermal conditions (Guo and Gifford, 2002). This equilibrium C content is the outcome of a bаlance between input and output fluxes to the pool of soil C (Fearnside and Barbosa, 1998, Guo and Gifford, 2002). The main source of organic matter input into the soil is vegetation and the amount of this input depends on ecosystems’ productivity. The output flux includes mineralization of organic matter, losses due to disturbances and leaching of dissolved organic carbon from the ecosystem (Guo and Gifford, 2002).
The mineralization efflux of CO2 from the soil surface (soil respiration—RS) is a key component of the carbon cycle of terrestrial ecosystems, which can contribute 50–95% of total ecosystem respiration (e.g. Xu and Qi, 2001). RS is the sum of such processes as autotrophic root respiration (RA) and plant residues decomposition (respiration of heterotrophic organisms). It can vary significantly across both time and spаcе according to changes in vegetation and soil properties (e.g. Rochette et al., 1991, Stoyan et al., 2000, Xu and Qi, 2001, Raich et al., 2002, Hibbard et al., 2005). However, on short time scales, variation in soil CO2 flux is mainly driven by soil temperature and moisture (e.g. Raich and Schlesinger, 1992, Peng and Thomas, 2006). The CO2 emission from soil increases exponentially with increasing temperature when any other factors and resources are not limiting (Lloyd and Taylor, 1994) that often is modelled through Q10 coefficient. However, many studies report large variability of Q10 for the same sites during the growth season, e.g. from 1.98 to 5.00 for sod-poszolic soils and from 1.72 to 6.20 for gray forest soils (Kurganova, 2010), that may generate uncontrolled biases in the results. The relationship between intensity of soil CO2 flux and soil moisture can be described by an upward convex curve (Peng et al., 2008).
Russian forest is a significant element of the global carbon budget (Pan et al., 2011), and hence they can play an important role in climate change mitigation. They comprise about 23% of the entire world's forest area. Forest land and forested area (closed forests) cover 51.6% and 45.3% of the total land area of the country respectively (Onuchin et al., 2009). These forest areas contain 21% of the world's growing stock, and 13% of the live forest biomass of the globe (FAO, 2009). They keep about 43 Pg C in terrestrial vegetation including 35 Pg C in live biomass (Shvidenko et al., 2007, Shvidenko et al., 2009).
Current science on climate change has been coming to understanding of need of a terrestrial ecosystems full and verified carbon account (FCA). Uncertainty of the FCA is crucially driven by uncertainty of RS and particularly its heterotrophic part (Shvidenko et al., 2010a, Shvidenko et al., 2010b). The major objective of this study is assessing the soil contribution to the current carbon budget of Russian forests aiming at uncertainty's level that would not exceed some certain levels acceptable for policy makers. The latter still remains a topic of discussions. Some studies indicate a presumptive level of ±20–25% (CI 0.9) for net ecosystem carbon budget at the continental scale (Nilsson et al., 2007). It means that uncertainty of Rh should not exceed this threshold. While a wide range of climatic conditions, diversity of landscapes and forest ecosystems and other drivers over the vast territory of Russian forests results in a large temporal and spatial variety of soil respiration, this study attempted to understand whether the FCA of forest ecosystems is achievable under proper organization of the information background and consistent application of systems (holistic) analysis. The paper also includes some results on forest soil carbon stock that have been earlier published in an aggregated form (Schepaschenko et al., 2013) taking into account relevance of consideration of links between the amount of carbon and heterotrophic respiration.
Section snippets
Assessment of soil carbon pool
The soil organic carbon (SOC) pool was calculated separately for the topsoil organic O horizon (FAO, 2006) and for 1 m of soil below. The soil map of the Russian Federation at a scale of 1:2.5 million and a reference soil profiles’ database (modified by authors from Stolbovoi and McCallum, 2002) were used to calculate the SOC pool for typical soil profiles and their distribution over the country. A database of soil carbon measurements (1068 records) was collected from published papers. It was
Forest soil carbon pool
Our estimates of carbon content of soils of Russian forests (SOC) is 144.5 Pg C, and 94.2% of this amount is allocated in the 1 m soil layer and the rest (5.8%) is contributed by the on-ground organic layer (Table 3)—the SOC pool with a fast turnover rate. About 81% of the total SOC pool of Russian forests is located in the Asian part of Russia that comprises 79.5% of the total forested area of the country. A majority of the SOC stock is related to boreal forests (83% and 93% in European and Asian
Forest soil carbon pool
The major part of Russian forests are represented by boreal ecosystems (92% of the total forested area) that generally contain more carbon than the temperate forest biome and have the total carbon density close to that in tropical forests (238 Mg C ha−1) (Pan et al., 2011). About one-fourth of boreal forest carbon is stored in vegetation. The rest, ∼75%, is in the forest soil. Carbon accumulation in boreal forest soils is driven by slow decomposition rates, in part due to a short vegetation period
Conclusions
The system of carbon stock accounting and estimation of respiration activity of forest soils described here allows for improvements over the previously reported results. The calculated SOC pool in Russian forests (limited to the top 1 m depth) makes up 144.5 Pg C that is about 46% of the total SOC stock of terrestrial ecosystems of Russia (Schepaschenko et al., 2013). Mean annual heterotrophic soil respiration flux from Russian forests is estimated to be about 1688 ± 135 Tg C yr−1, or about 1.2% of the
Acknowledgments
This research was partially supported by National Institute for Environmental Studies, Japan, by the European Community's Framework Programme (FP7) via GEOCarbon (No. 283080), and by the Russian Government Megagrant Project No. 14.В25.31.0031.
The authors gratefully acknowledge financial support from the Scholarship Council of the Scholarship Foundation of the Republic of Austria and Austrian Agency for International Cooperation in Education and Research (OeAD-GmbH), Centre for International
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