We investigated the relationship between the δC signal in current-year and 1-year-old needle bulk material, starch extracts, and early- or late-wood in mature spruce trees (Picea abies) to identify the modifying influence of climatic conditions on the different δC signals. Seasonal patterns of δC were determined in total bulk needle material from 1998 to 2000, and in acid soluble starch extracts in 1999 and 2000, and δC values of early- and late-wood were measured for the years 1991-2000. δC of bulk needle material was most enriched in spring with a trend towards depletion in the course of the season. Current-year needles showed a more distinct seasonal pattern in δC compared to 1-year-old needles. Seasonal trends in bulk material and starch were similar, but the highly enriched signal in spring could not be fully explained by the influence of the δC values of starch (weighted with the corresponding starch amounts). δC of starch in 1-year-old needles, and to a lesser extent of current-year needles, correlated with δC of early-wood, indicating a transfer of the isotopic signal. In addition, early-wood δC corresponded weakly to winter precipitation. In the summer, δC of total bulk needle material and starch showed no relation to the late-wood δC signature. Late-wood δC, however, related to global radiation, relative humidity and temperature, with more enriched values corresponding to warmer and drier conditions. We conclude that the signature of early-wood is determined more by biochemical fractionation, e.g. during starch formation, than by climatic conditions, which exert only a minor influence and are reflected in the isotopic signal of late-wood.

Stable C isotope signals in plant tissues became a key tool in explaining growth responses to the environment. The technique is based on the fundamental assumption that the isotopic composition of a given unit of tissue (e.g. a tree ring) reflects the specific C uptake conditions in the leaf at a given time. Beyond the methodological implications of any deviation from this assumption, it is of physiological interest whether new C is transferred directly from sources (a photosynthesizing leaf) to structural sinks (e.g. adjacent stem tissue), or inherently passes through existing (mobile) C pools, which may be of variable (older) age. Here, we explore the fate of 13 C-labelled photosynthates in the crowns of a 30-35 m tall, mixed forest using a canopy crane. In all nine study species labelled C reached woody tissue within 2-9 h after labelling. Four months later, very small signals were left in branch wood of Tilia suggesting that low mixing of new, labelled C with old C had taken place. In contrast, signals in Fagus and Quercus had increased, indicating more intense mixing. This species-specific mixing of new with old C pools is likely to mask year-or season-specific linkages between tree ring formation and climate and has considerable implications for climate reconstruction using stable isotopes as proxies for past climatic conditions.

Starch and soluble sugars are the major photosynthetic products, and their carbon isotope signatures reflect external versus internal limitations of CO(2) fixation. There has been recent renewed interest in the isotope composition of carbohydrates, mainly for use in CO(2) flux partitioning studies at the ecosystem level. The major obstacle to the use of carbohydrates in such studies has been the lack of an acknowledged method to isolate starch and soluble sugars for isotopic measurements. We here report on the comparison and evaluation of existing methods (acid and enzymatic hydrolysis for starch; ion-exchange purification and compound-specific analysis for sugars). The selectivity and reproducibility of the methods were tested using three approaches: (i) an artificial leaf composed of a mixture of isotopically defined compounds, (ii) a C(4) leaf spiked with C(3) starch, and (iii) two natural plant samples (root, leaf). Starch preparation methods based on enzymatic or acid hydrolysis did not yield similar results and exhibited contaminations by non-starch compounds. The specificity of the acidic hydrolysis method was especially low, and we therefore suggest terming these preparations as HCl-hydrolysable carbon, rather than starch. Despite being more specific, enzyme-based methods to isolate starch also need to be further optimized to increase specificity. The analysis of sugars by ion-exchange methods (bulk preparations) was fast but produced more variable isotope compositions than compound-specific methods. Compound-specific approaches did not in all cases correctly reproduce the target values, mainly due to unsatisfactory separation of sugars and background contamination. Our study demonstrates that, despite their wide application, methods for the preparation of starch and soluble sugars for the analysis of carbon isotope composition are not (yet) reliable enough to be routinely applied and further research is urgently needed to resolve the identified problems.

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Natural abundance (δ) of the stable isotopes 13C and 15N has gained acceptance for studying C and N cycling in forests. In most studies, bulk soil samples are collected to determine isotope abundance. Such sampling overlooks the potential impact of preferential flow on isotope distribution. The objective of this study was to investigate the effects of preferential flow on the distribution of soil organic carbon (SOC), total N, δ13C, and δ15N in a forest soil in Central Switzerland. Preferential flow paths in the soil were identified with a dye tracer, Brilliant Blue (Plüss‐Staufer AG, Oftringen, Switzerland), that was homogeneously applied to the soil surface. In the stained preferential flow paths, concentrations of SOC and total N were 15 to 75% higher than in the soil matrix. The total increase of SOC in preferential flow paths ranged from 740 to 960 g C m−2 in four individual soil plots. Values of δ13C and δ15N were lowest in tree leaves and in the forest floor, and increased with soil depth, thus with the degree of decomposition of SOC. In the mineral soil, preferential flow paths were significantly depleted in 13C by 0.15 to 0.4‰ as compared with the soil matrix. The δ15N values increased with soil depth from 0.9 to 4.7‰ in the preferential flow paths and from 0.5 to 6‰ in the soil matrix. Adding a highly enriched 15N‐tracer homogeneously to the soil surface showed a higher recovery of 15N in the soil and in the fine roots sampled from preferential flow paths than in those sampled from the soil matrix. Our results suggest that in preferential flow paths, SOC is younger and N cycling is more rapid than in the soil matrix.