Seasonal drought will become more intense and frequent in some regions due to global climate change, which may lead to significant changes in the competitive interactions and uptake depths of plant ...water sources. Based on delta 18O and delta D in xylem and soil water, we investigated the impacts of seasonal drought on plant water sources from July 2011 to October 2013 for a subtropical coniferous plantation in southeastern China. Our results indicated that the studied tree species of Pinus massoniana, Pinus elliottii and Cunninghamia lanceolata appeared to have inter-specific competition for water resources from similar depths. There was a switch of the major water source from shallow soil during the non-drought periods (July to October) to deep soil during the drought periods (November-June). Similar seasonal trend of water uptake was estimated by delta 18O and delta D. However, relative to the results of delta D, water sources predicted by delta 18O showed that trees seemed to derive more deep soil water during the drought periods and more shallow soil water during the non-drought periods. The differences of plant water source partitioning based on delta 18O and delta D were significant during both the drought and non-drought periods, which probably resulted from the artifacts of the cryogenic vacuum distillation and spectral contamination correction. These findings will have important implications for further studies when just one of the dual stable isotopes of delta 18O and delta D is applied.
•In situ measurements of δv and δET were made during the maize growing season.•δT was close to δx between 13:00 and 15:00, indicating isotopic steady state (ISS).•T/ET was 0.87±0.052 for the growing ...season according to the isotopic labeling.•δET should be balanced by enhanced δR according to 18O mass conservation.
The oxygen isotope compositions of ecosystem water pools and fluxes are useful tracers in the water cycle. As part of the Heihe Watershed Allied Telemetry Experimental Research (HiWATER) program, high-frequency and near-continuous in situ measurements of 18O composition of atmospheric vapor (δv) and of evapotranspiration (δET) were made with the flux-gradient method using a cavity ring-down spectroscopy water vapor isotope analyzer. At the sub-daily scale, we found, in conjunction with intensive isotopic measurements of other ecosystem water pools, that the differences between 18O composition of transpiration (δT) and of xylem water (δx) were negligible in early afternoon (13:00–15:00 Beijing time) when ET approached the daytime maximum, indicating isotopic steady state. At the daily scale, for the purpose of flux partitioning, δT was approximated by δx at early afternoon hours, and the 18O composition of soil evaporation (δE) was obtained from the Craig-Gordon model with a moisture-dependent soil resistance. The relative contribution of transpiration to evapotranspiration ranged from 0.71 to 0.96 with a mean of 0.87±0.052 for the growing season according to the isotopic labeling, which was good agreement with soil lysimeter measurements showing a mean transpiration fraction of 0.86±0.058. At the growing season scale, the predicted 18O composition of runoff water was within the range of precipitation and irrigation water according to the isotopic mass conservation. The 18O mass conservation requires that the decreased δ18O of ET should be balanced by enhanced δ18O of runoff water.
Intrinsic water use efficiency (iWUE) is a critical eco-physiological function allowing plants to adapt to water- and nutrient-limited habitats in arid and semi-arid regions. However, the ...distribution of iWUE in coexisting species along aridity gradients and its controlling factors are unknown. We established two transects along an aridity gradient in the grasslands of Losses Plateau (LP) and Inner Mongolia Plateau (MP) to elucidate the patterns and underlying mechanisms of iWUE distribution in coexisting species along aridity gradient. We determined leaf carbon (δ
13
C) and oxygen (δ
18
O) stable isotopes, functional traits related to carbon fixation, and limiting resources. Bulk leaf δ
13
C and δ
18
O were used as proxies for time-integrated iWUE and stomatal conductance (gs) during the growing season. Our results showed that variability in iWUE within transect was primarily controlled by species, sampling sites and an interactive effect between species and sampling sites. Mean values of iWUE (iWUE
Mean
) increased and coefficient of variation (CV) in iWUE (iWUE
CV
) decreased with an increase in aridity, demonstrating that increases in aridity lead to conservative and convergent water use strategies. Patterns of iWUE
Mean
and iWUE
CV
were controlled primarily by the ratio of soil organic carbon to total nitrogen in LP and soil moisture in MP. This revealed that the most limited resource drove the distribution patterns of iWUE along aridity gradients. Interspecific variation in iWUE within transect was positively correlated with Δ
18
O, indicating that interspecific variation in iWUE was primarily regulated by gs. Furthermore, relationship between iWUE and multi-dimensional functional trait spectrum indicated that species evolved species-specific strategies to adapt to a harsh habitat by partitioning limiting resources. Overall, these findings highlighted the interactive effects of limiting resources and leaf functional traits on plant adaptation strategies for iWUE, and emphasized the importance of considering biological processes in dissecting the underlying mechanisms of plant adaptation strategies at large regional scales.
•Overall species abundance decreased to cope with climatic stress.•Abundance of water-saving species increased to cope with climatic stress.•Climatic stress drove divergence of stomatal conductance ...in coexisting species.
Convergence of eco-physiological function in harsh habitats due to environmental filtering is well-known, however, some evidence of eco-physiological divergence in coexisting species also exists. Experimental data to verify whether divergence of eco-physiological function in coexisting species increases with increasing environmental stress remain limited. Here, we measured bulk leaf oxygen isotope and functional traits along a climatic stress gradient in a grassland in Tibetan Plateau where drought, excess radiation, and temperature increased from east to west. Enrichment (Δ18O) in bulk leaf oxygen isotope was used as proxy for time-integrated gs. We found that the leaf water use (Δ18O and leaf area LA) spectrum was independent of the leaf economic spectrum, indicating that coexisting species shifted from high nitrogen use efficiency (high ratio of leaf carbon to nitrogen) and water loss (high LA) in low-stress habitats to high photosynthetic capacity (high leaf nitrogen) and low water loss (low gs) in high-stress habitats. As the stress intensity of climate increases, higher mean and maximum and lower skewness (varying from positive to negative values) of Δ18O in coexisting species confirmed that the abundance of water-saving species increased to cope with climatic stress, while the greater standard deviation and kurtosis further indicated that climatic stress drove the divergence in gs in coexisting species. This study revealed that coexisting species adopt more diversified water use strategies and the abundance of water-saving species increases to cope with climatic stress, regardless of decreases in overall species abundance.
The interannual variation of the Bowen ratio, through its effect on the warming extent of available energy to the ecosystem land surface air, heavily influences the ecosystem microclimate and affects ...the hydrological cycle at both regional and global scales. Although the precipitation amount in southeast China is not expected to change greatly as a result of climate change, the precipitation frequency may be altered in the future. We explored the interannual variation of the Bowen ratio and its affecting mechanisms based on eddy covariance measurements in a subtropical plantation in southeast China during 2003-2012. The results indicated that the annual mean Bowen ratio was 0.35 ± 0.06, with a range of 0.29-0.45. The Bowen ratio during the dry season (July-October) positively correlated with the annual Bowen ratio (R(2) = 0.85, p<0.001). The effective precipitation frequency during the dry season, through its positive effect on shallow soil water content, indirectly and negatively affected the annual Bowen ratio. Between 2003 and 2012, the annual Bowen ratio exhibited a marginally significant decreasing trend (p = 0.061), meanwhile the effective precipitation frequency and shallow soil water content during the dry season increased significantly (p<0.001). The annual Bowen ratio may decrease further if the effective precipitation frequency and shallow soil water content during the dry season follow similar trends in the future. The warming effect of available energy to the surface air of our studied plantation may decline with the decreasing annual Bowen ratio.
Environmental conditions (EV) changes not only affect temporal variations in carbon fluxes directly, but affect them indirectly by impacting plant biotic traits. Investigating the extent of the ...effects of EV and biotic changes can help deepen our understanding of ecosystem carbon cycling. Therefore, we partitioned and quantified the contributions of EV and biotic changes’ effects on seasonal variations in carbon fluxes (net ecosystem carbon exchange (NEE), and its components, i.e., gross ecosystem carbon exchange (GEE) and ecosystem respiration (RE)) in a (winter) wheat–(summer) maize rotation ecosystem from 2010 to 2012. A path analysis accompanied by Granger causality tests (GCTs), which filtered out several variables that were not causal for dependent variables, was used to calculate their respective contributions by integrating path coefficients. The seasonal variations in NEE, RE, and GEE were significantly and jointly affected by EV and the leaf area index (LAI) with
R
2
values ranging from 0.63 to 0.94 after the GCT. The path analysis indicated that the seasonal variations of carbon fluxes were dominated by the effects of EV changes (induced from varying EV for different fluxes, crops, and years), which contributed 60.7% (mean of two years), 64.5%, and 58.2% to wheat NEE, RE, and GEE, respectively, and 62.5%, 82.3%, and 58.1% to maize NEE, RE, and GEE, respectively. Overall, our study provided a new basis that future climatic changes may have important impacts on carbon exchanges in this rotation cropland.
Enhancing the terrestrial ecosystem carbon sink (referred to as terrestrial C sink) is an important way to slow down the continuous increase in atmospheric carbon dioxide (CO
) concentration and to ...achieve carbon neutrality target. To better understand the characteristics of terrestrial C sinks and their contribution to carbon neutrality, this review summarizes major progress in terrestrial C budget researches during the past decades, clarifies spatial patterns and drivers of terrestrial C sources and sinks in China and around the world, and examines the role of terrestrial C sinks in achieving carbon neutrality target. According to recent studies, the global terrestrial C sink has been increasing from a source of (-0.2±0.9) Pg C yr
(1 Pg=10
g) in the 1960s to a sink of (1.9±1.1) Pg C yr
in the 2010s. By synthesizing the published data, we estimate terrestrial C sink of 0.20-0.25 Pg C yr
in China during the past decades, and predict it to be 0.15-0.52 Pg C yr
by 2060. The terrestrial C sinks are mainly located in the mid- and high latitudes of the Northern Hemisphere, while tropical regions act as a weak C sink or source. The C balance differs much among ecosystem types: forest is the major C sink; shrubland, wetland and farmland soil act as C sinks; and whether the grassland functions as C sink or source remains unclear. Desert might be a C sink, but the magnitude and the associated mechanisms are still controversial. Elevated atmospheric CO
concentration, nitrogen deposition, climate change, and land cover change are the main drivers of terrestrial C sinks, while other factors such as fires and aerosols would also affect ecosystem C balance. The driving factors of terrestrial C sink differ among regions. Elevated CO
concentration and climate change are major drivers of the C sinks in North America and Europe, while afforestation and ecological restoration are additionally important forcing factors of terrestrial C sinks in China. For future studies, we recommend the necessity for intensive and long term ecosystem C monitoring over broad geographic scale to improve terrestrial biosphere models for accurately evaluating terrestrial C budget and its dynamics under various climate change and policy scenarios.
•DIC fluxes of input and output the catchment were estimated under water balance.•Sources of DIC fluxes in stream were quantified with an isotope mixing model.•Soil biological CO2 contributed ...47.3 ± 6.2% of DIC flux in the headwater stream.•CO2 and carbonate dissolution controlled DIC variation in rainfall-runoff processes.
Dissolved inorganic carbon (DIC) in headwater streams significantly contributes to the riverine CO2 emission to the atmosphere and oceans. However, understanding of the sources and mechanisms of DIC generation and mass balances of DIC flux in headwater streams at the scale of catchments remain poor and add to uncertainty in regional carbon estimates. In this study, the input and output of water fluxes and their chemical and isotopic compositions in a headwater stream were investigated from 2016 to 2019 in a subtropical plantation catchment. Our results showed that the headwater stream catchment experienced a net carbon loss of 262–4352 kg C km−2 yr−1 based on the water balance of annual precipitation, evapotranspiration, headwater stream and groundwater discharge, and change in soil water storage. The headwater stream exhibited comparable DIC flux (3561 ± 933 kg C km−2 yr−1) and higher DIC concentration (7.91 ± 3.13 mg L−1) than precipitation (3662 ± 682 kg C km−2 yr−1 and 2.63 ± 1.22 mg L−1, respectively). Differences in DIC concentration between precipitation and headwater stream were mainly controlled by soil CO2 dissolution, carbonate weathering, and their synergistic effects. Relative contributions to DIC fluxes in this headwater stream were 25.3 ± 5.2% by precipitation, 47.3 ± 6.2% by soil biological CO2, and 27.4 ± 7.3% by carbonate weathering. Our results indicated that the DIC-generating processes may result in an underestimation of soil biological CO2 efflux to the atmosphere. Further, soil carbonate weathering, and a net carbon loss at the scale of catchments may be overlooked in acid soils.
Soil net nitrogen (N) mineralization (Nmin) is a pivotal process in the global N cycle regulating the N availability of plant growth. Understanding the spatial patterns of Nmin, its temperature ...sensitivity (Q10) and regulatory mechanisms is critical for improving the management of soil nutrients. In this study, we evaluated 379 peer‐reviewed scientific papers to explore how Nmin and the Q10 of Nmin varied among different ecosystems and regions at the global scale. The results showed that Nmin varied significantly among different ecosystems with a global average of 2.41 mg N soil kg−1 day−1. Furthermore, Nmin significantly decreased with increasing latitude and altitude. The Q10 varied significantly among different ecosystems with a global average of 2.21, ranging from the highest found in forest soils (2.43) and the lowest found for grassland soils (1.67) and significantly increased with increasing latitude. Path analyses indicated that Nmin was primarily affected by the content of soil organic carbon (C), soil C:N ratio, and clay content, where Q10 was primarily influenced by the soil C:N ratio and soil pH. Furthermore, the activation energy (Ea) of soil N mineralization was significantly and negative correlated with the substrate quality index among all ecosystems, indicating the applicability of the carbon quality temperature hypothesis to soil N mineralization at a global scale. These findings provided empirical evidence supporting that soil N availability, under global warming scenarios, is expected to increase stronger in colder regions as compared with that low‐latitude regions due to the higher Q10. This may alleviate the restriction of N supply for increased primary productivity at higher latitudes.
Water use efficiency is among the most important eco-physiological strategies for species adaptation to water-limiting habitats. However, understanding of how co-occurring species adopt diverse water ...use strategies to cope with water- and resource-limited soil habitats via the adaptation of leaf functional traits remains limited. Here, leaf δ13C and δ18O were used as proxies for water use efficiency (WUE) and stomatal conductance (gs) across a soil limiting-resource gradient from a non-karst primary forest to karst communities including primary forest, secondary forest, shrubland, and grassland. Leaf apparent morphology, stomatal morphology, macro- and micro-nutrients and stoichiometry were measured. We found that there were significant differences in soil physical and chemical properties across five communities, forming a soil limiting-resource gradient. Co-occurring species in each community segregated along a continuous and wide range of WUE and gs, revealing a wide spectrum of stomatal regulation intensity and contrasting water use strategies. Multiple leaf functional traits separated into two principal components: leaf economics spectrum (LES) and leaf limiting-resource spectrum (LLRS). As the stress intensity of soil limiting-resource increases, the number of composition traits of LLRS increased and the dominant traits of LLRS shifted from leaf-thickness to macro-nutrients. The major driver of gs variability in each community was LES. Variability in WUE in forests was mainly determined by LES, in shrubland by LLRS, and in grassland by neither LES nor LLRS. Our study illustrated for the first time how soil limiting-resource induced changes in multi-dimensional leaf traits with effects on water use strategies of co-occurring species: the variability in WUE was mainly controlled by the leaf economic spectrum in mild soil limiting-resource habitats; however, that effect was replaced or offset by leaf limiting-resource spectrum in moderate or severe soil limiting-resource habitats.
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