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The dynamics of sugar (hexose) concentration in ripening grape berries (Vitis vinifera L.) were simulated with a refined mechanistic model. Changes in sugar concentration were reproduced by the sum of sugar import (S), sugar metabolism (M) and water budget (W). S and W were derived from model inputs of fresh and dry mass, and M was simulated with a relative metabolism rate describing the depletion of hexose. The relative metabolism rate was associated with the relative growth rate of dry mass with a coefficient (k) that was constant for a given cultivar under various growth conditions (temperature, water supply, and source-sink ratio) but varied with genotype. The k value was~20% higher for cv. Merlot than for cv. Cabernet Sauvignon, indicating more imported sugars would be depleted by Merlot than Cabernet Sauvignon. The model correctly simulated the negative effect of lowered leaf-to-fruit ratio and the positive effect of water shortage on sugar concentration. Sensitivity analysis revealed that the present model was weakly sensitive to k because of sugar accumulation being predominantly controlled by S, with M relatively small (~20%) with respect to the increment of sugar concentration. Model simulation indicated that the decreasing leaf-to-fruit ratio reduced S more than M and W, causing a net decrease in sugar concentration. In contrast, the water shortage decreased S less than M and W, resulting in a net increase in sugar concentration.

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

In this study, we evaluated trimethylsilyl (TMS) derivatives as derivatization reagents for the compound-specific stable carbon isotope analysis of soil amino acids by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). We used non-proteinogenic amino acids to show that the extraction-derivatization-analysis procedure provides a reliable method to measure δ(13)C values of amino acids extracted from soil. However, we found a number of drawbacks that significantly increase the final total uncertainty. These include the following: production of multiple peaks for each amino acid, identified as di-, tri- and tetra-TMS derivatives; a number of TMS-carbon (TMS-C) atoms added lower than the stoichiometric one, possibly due to incomplete combustion; different TMS-C δ(13)C for di-, tri- and tetra-TMS derivatives. For soil samples, only four amino acids (leucine, valine, threonine and serine) provide reliable δ(13)C values with a total average uncertainty of 1.3 ‰. We conclude that trimethylsilyl derivatives are only suitable for determining the (13)C incorporation in amino acids within experiments using (13)C-labelled tracers but cannot be applied for amino acids with natural carbon isotope abundance until the drawbacks described here are overcome and the measured total uncertainty significantly decreased.

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