ALBERT

Description: The Russian boreal forest, which mainly consists of extensive forests in Siberia, is the largest continuous forest region on Earth and represents 70 % of the world's boreal forest. Siberian forest is a tremendous repository of terrestrial organic carbon (C), which may increase owing to climate change, potential increases in ecosystem productivity and hence C sequestration. Phosphorus (P) availability could limit the C sequestration potential, but tree roots may mine the soil deeper to increase access to mineral P. Improved understanding and quantification of the processes controlling P availability in surface and deep soil layers of forest ecosystems are thus required. Relative contributions of organic and inorganic P and, consequently, P availability in forest ecosystems depend on decomposition processes, which could be strongly affected by vegetation composition, temperature, precipitation, and their changes due to a warming climate. The objectives of the present study were to (1) evaluate P status of surface and deep forest soil horizons from two contrasted biomes in Southwestern Siberia (i.e. forest steppe in the West Siberian plain and blackish ("chernevaya" in Russian) taiga in the low Salair mountains) and (2) assess the effects of vegetation (siberian fir stand, common aspen stand and herbs in a forest gap) and local climate on soil P fractions. Results revealed high contents in total P (645–1042 mg kg−1 in the surface mineral soils) and available inorganic P (diffusive phosphate ions in one week = 83–126 mg kg−1). In addition, there was an accumulation of diffusive phosphate ions in the subsoils resulting from differences between soil horizons in total inorganic P and soil properties. Consequently, deeper root systems may mine substantial amounts of available P for the trees and the potential enhanced growth and C sequestration due to climate change should thus a~priori not be P-limited. High proportions of total organic P (47–56 % of total P in the surface mineral soils) show that decomposition processes potentially play a significant role in P availability. Results show that decomposition processes are affected by vegetation (deciduous broadleaved trees, evergreen coniferous, herbs) and local climate (precipitations; snow cover with its isolating effect on soil). Results on the effects of plant species and local climate improved our understanding of the potential effects of climate change on P availability through warming and vegetation redistribution.

Description: The Siberian forest is a tremendous repository of terrestrial organic carbon (C), which may increase owing to climate change, potential increases in ecosystem productivity and hence C sequestration. Phosphorus (P) availability could limit the C sequestration potential, but tree roots may mine the soil deep to increase access to mineral P. Improved understanding and quantification of the processes controlling P availability in surface and deep soil layers of Siberian forest ecosystems are thus required. The objectives of the present study were to (1) evaluate P status of surface and deep soil horizons from different forest plots in southwestern Siberia and (2) assess the effects of physicochemical soil properties, microbiological activity and decomposition processes on soil P fractions and availability. Results revealed high concentrations of total P (879–1042 mg kg−1 in the surface mineral soils) and plant-available phosphate ions. In addition, plant-available phosphate ions accumulated in the subsoil, suggesting that deeper root systems may mine sufficient available P for the trees and the potentially enhanced growth and C sequestration, may not be P-limited. Because the proportions of total organic P were large in the surface soil layers (47–56% of total P), we concluded that decomposition processes may play a significant role in P availability. However, microbiological activity and decomposition processes varied between the study plots and higher microbiological activity resulted in smaller organic P fractions and consequently larger available inorganic P fractions. In the studied Siberian soils, P availability was also controlled by the physicochemical soil properties, namely Al and Fe oxides and soil pH.