Abstract. Isotope-based approaches to study plant water sources rely on the assumption that root water uptake and within-plant water transport are non-fractionating processes. However, a growing number of studies have reported offsets between plant and source water stable isotope composition for a wide range of ecosystems. These isotopic offsets can result in the erroneous attribution of source water used by plants and potential overestimations of groundwater uptake by the vegetation. We conducted a global meta-analysis to quantify the magnitude of these plant source water isotopic offsets and explored whether their variability could be explained by either biotic or abiotic factors. Our database compiled 112 studies spanning arctic to tropical biomes that reported the dual water isotope composition (δ2H and δ18O) of plant (stem) and source water, including soil water (sampled following various methodologies and along a variable range of depths). We calculated plant source 2H offsets in two ways: a line conditioned excess (LC-excess) that describes the 2H deviation from the local meteoric water line and a soil water line conditioned excess (SW-excess) that describes the deviation from the soil water line, for each sampling campaign within each study. We tested for the effects of climate (air temperature and soil water content), soil class, and plant traits (growth form, leaf habit, wood density, and parenchyma fraction and mycorrhizal habit) on LC-excess and SW-excess. Globally, stem water was more depleted in 2H than in soil water (SW-excess <  0) by 3.02±0.65 ‰ (P < 0.05 according to estimates of our linear mixed model and weighted by sample size within studies). In 95 % of the cases where SW-excess was negative, LC-excess was negative, indicating that the uptake of water that had not undergone evaporative enrichment (such as groundwater) was unlikely to explain the observed soil–plant water isotopic offsets. Soil class and plant traits did not have any significant effect on SW-excess. SW-excess was more negative in cold and wet sites, whereas it was more positive in warm sites. The climatic effects on SW-excess suggest that methodological artefacts are unlikely to be the sole cause of observed isotopic offsets. Our results would imply that plant source water isotopic offsets may lead to inaccuracies when using the isotopic composition of bulk stem water as a proxy to infer plant water sources.

Abstract. Isotope-based approaches to study plant water sources rely on the assumption that root water uptake and within-plant water transport are non-fractionating processes. However, a growing number of studies have reported offsets between plant and source water stable isotope composition, for a wide range of ecosystems. These isotopic offsets can result in the erroneous attribution of source water used by plants and potential overestimations of groundwater uptake by the vegetation. We conducted a global meta-analysis to quantify the magnitude of these plant-source water isotopic offsets and explore whether their variability could be explained by either biotic or abiotic factors. Our database compiled 112 studies, spanning arctic to tropical biomes that reported the dual water isotope composition (δ2H and δ18O) of plant (stem) and source water, including soil water. We calculated 2H offsets in two ways: a line conditioned excess (LC-excess) that describes the 2H deviation from the local meteoric water line, and a soil water line conditioned excess (SW-excess), that describes the deviation from the soil water line, for each sampling campaign within each study. We tested for the effects of climate (air temperature and soil water content), soil class and plant traits (growth form, leaf habit, wood density and parenchyma fraction and mycorrhizal habit) on LC-excess and SW-excess. Globally, stem water was more depleted in 2H than soil water (SW-excess < 0) by 3.02 ± 0.65 ‰. In 95 % of the cases where SW-excess was negative, LC-excess was negative, indicating that the uptake of water from mobile pools was unlikely to explain the observed soil-plant water isotopic offsets. SW-excess was more negative in cold and wet sites, whereas it was more positive in warm sites. Soil class and plant traits did not have any significant effect on SW-excess. The climatic effects on SW-excess suggest that methodological artefacts are unlikely to be the sole cause of observed isotopic offsets. Instead, our results support the idea that these offsets are caused by isotopic heterogeneity within plant stems whose relative importance will depend on soil and plant water status and evaporative demand. Our results would imply that plant-source water isotopic offsets may lead to inaccuracies when using the isotopic composition of bulk stem water as a proxy to infer plant water sources.

No abstract

The cycling of nutrients from plant litter has relevant implications on the functioning of terrestrial ecosystems by controlling the availability of nutrients and net primary production. The effects of global change have been widely studied for most aspects of ecosystem functioning, but the direct implications on the stoichiometry and nutrient dynamics of litter decomposition are still poorly understood. We conducted a meta-analysis to determine how warming, changes of water availability, and enrichments of N and P interfere with the immobilisation/mineralisation of nutrients and the stoichiometric relationships during litter decomposition. Our database consisted of 194 experiments from 43 studies that experimentally simulated (i) warming, (ii) drought, (iii) irrigation, (iv) N enrichment, (v) P enrichment, and (vi) combined N and P (N + P) enrichment. We compared the early stages (approximately half a year) and late stages (approximately one year) of decomposition; and the specific effects taking into account the climate, the species used and the ecosystem type. We explored the different effects of all treatments and highlight three of them. (i) Warming and nutrient enrichment induce a faster release of C during decomposition, prominent in grassland and coniferous forests of continental climates, and with a potential feedback loop to climate change. (ii) C:P and overall C:N ratios generally decreased in most of the scenarios of global change analyzed at short- and long-term during litter decomposition, while the N:P ratios are more resilient to change. (iii) P limitation might be exacerbated in in warming continental climates; arid environments experiencing droughts; temperate environments with increasing water availability; and temperate broadleaved forest experiencing N and P enrichment. Our results provide information about the fate of litter decomposition and its nutrient and stoichiometric dynamics in response to drivers of global change. However, further experimentation and analysis considering all interacting drivers are warranted.