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Compound-specific isotopic analyses were used to assess the dynamics and origin of organic matter in soils across a 30 year chronosequence where native savanna (C) had been replaced with eucalyptus (C). Apolar lipid fractions were recovered from plants and soils planted with Eucalyptus 0, 7.5, 17 and 30 years ago. The molecular composition of lipids in plants and soils identified three major pentacyclic triterpene methyl esters (PTMEs) specific to savanna, and three odd n-alkanes common to both vegetation types. Savanna-derived PTMEs and δC remained at similar levels in soils after 30 years of eucalyptus growth. C content of PTMEs under savanna was lower than that of bulk soil and displayed the presence of older (1,000s of years) PTMEs in soils. However, the C content of soil PTMEs under eucalyptus was higher than that of the soil PTMEs under savanna. This suggests both recent (10s of years) and 'pre-bomb' mineralization of savanna-derived PTMEs. Compound-specific C analyses of soil n-alkanes showed a trend typical of a C/C vegetation shift, with more depleted δC values with increasing time under Eucalyptus. C analyses of n-alkanes suggest that n-alkanes have high turnover rates in the first few years after land conversion, and n-alkane composition thereafter is dominated by eucalyptus inputs. Compound-specific analyses reveal that lipids from past vegetations remained in soils. Compound-specific analyses allow a more nuanced description of carbon turnover in soils, and may improve our mechanistic understanding of soil organic carbon dynamics. [ABSTRACT FROM AUTHOR]

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 (879-1042mg 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. [ABSTRACT FROM AUTHOR]

Soils contain the largest carbon (C) reservoir on Earth, but the mean residence time (MRT) of soil C is often poorly estimated, despite its importance for assessing the efficiency with which soils may serve as a sink for atmospheric C. The objective of this study was to evaluate how the structure of simple models of soil C dynamics affects the MRT determined from isotope-mixing experiments, using data from field studies with either artificial labelling (FACE) or C3/C4 vegetation change. We first highlighted theoretically how non-steady-state conditions and the model structure (one single, two successive, or two parallel C pools) can have an impact on the MRT assessment. We then tested these different model structures against published data on the dynamics of different soil organic matter separates and their constituents. Our findings indicated that many of the reviewed studies assumed wrongly that the system was at steady state or could be described by a single-pool approach. To select the correct model, exact knowledge of C input rates and several data points are needed from the beginning of the experiment. For steady-state conditions an apparent temporal change of MRT computed from a single-pool model is thus a clear indicator that a two-pool approach must be chosen. The errors made by the wrong choice of model varied with the length of the experiment and usually resulted in an over-estimate of MRT by a factor of 1.15 for some data published on physical size separates, but by a factor of up to 11 for individual microbial biomarkers such as muramic acid. [ABSTRACT FROM AUTHOR]