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• Tree water uptake does not necessarily correspond to soil water content. • Deferred wet season onset by two months increases tree's groundwater uptake. • 25% increase in wet season precipitation reduces tree's shallow soil water uptake. • Trees adapt to precipitation changes through water uptake and stomatal regulation. Gaining insights into the impacts of altered precipitation patterns on tree species' water uptake sources and water usage strategies is crucial for water resource management, but a comprehensive understanding is lacking. We explored water uptake patterns and water-related traits of four dominant tree species (Evergreen: Carallia brachiate, Symplocos poilanei, Schefflera heptaphylla ; Deciduous: Tetradium glabrifolium) in two altered seasonal precipitation patterns (deferred wet season [DW] and wetter wet season [WW]) and a control (CK) in a secondary tropical forest. Using stable isotopes (δ2H and δ18O) and MixSIAR models, we determined water uptake sources of the target species from a 60 cm soil profile and groundwater. Our results showed that the DW treatment increased (2.3% to 11.5%) groundwater utilization by trees in June and November compared to the CK, and promoted greater water uptake by trees from 0 to 40 cm soil layer in March. However, the WW treatment had a mild impact on water utilization in plants, leading to a reduction in water uptake by 1% - 3.8% from the 0–40 cm soil layer for the four tree species in September. The four studied tree species primarily rely on soil water, with the deciduous T. glabrifolium exhibited more flexible water uptake strategies compared to the other three evergreen species. For plant traits, S. heptaphylla exhibited larger stomatal size under DW treatment, paralleling the elongated stomatal in C. brachiate under the DW treatment. Conversely, S. poilanei and T. glabrifolium exhibit greater flexibility in balancing midday water potential. Under altered precipitation patterns, these plants adopt different morphological and/or physiological strategies, along with shift their water uptake sources to adapt to fluctuating water conditions. Overall, this study improves our understanding of plant water use in secondary tropical forests and emphasizes the importance of considering species-specific responses in forest management under potential precipitation pattern changes. [ABSTRACT FROM AUTHOR]

Missing data is a ubiquitous problem in evaluating long-term experimental measurements, such as those associated with the FluxNet project, due to the equipment failures, system maintenance, power-failure, and lightning strikes among other things. To estimate annual values of net ecosystem carbon exchange (NEE), latent heat flux (LE) and sensible heat flux (H), such gaps in the measured data must be filled or imputed. So far, no standardized method has been accepted and the imputation methods used are largely dependent on the researchers’ choice. Here, we used multiple imputation (MI) to gap-fill the missing data for annual estimations of NEE, LE and H at three flux sites associated with the FluxNet effort. MI is a Monte Carlo technique in which the missing values are replaced by several simulated values. Each data set imputed is a complete one where the observed values are the same as those in the original data set; only the missing values are different. Thus, the normal statistical analysis (e.g. annual total calculation) can be applied to each data set separately. The results of each analysis can be recombined into one summary. We applied the MI method to eddy covariance measurements collected from Walker Branch Watershed (WBW) site (a deciduous forest), Duke site (a coniferous forest) and Niwot site (a subalpine forest). Results showed that annual estimations of NEE, LE and H by MI were comparable to other imputation methods but MI was much easier to apply because of readily available software and standard algorithms. Besides the normal statistical analyses, MI also provided confidence intervals for each estimated parameter. This confidence interval is most useful when assessing energy, water, and carbon balance closures at a given tower site. Significant differences in annual NEE, LE and H were found among years at the three AmeriFlux sites. NEE at the Niwot Ridge site was lower and LE and H were higher than at the other two sites. With the available software and realistic gap-filling capability, MI has the potential to become a standardized method to gap-fill eddy covariance flux data for annual estimations and to improve the analysis of uncertainties associated with annual estimations of NEE, LE and H from regional and global flux networks. [Copyright &y& Elsevier]

• Atmospheric nitrogen (N) deposition enhanced soil heterotrophic respiration (R H). • Biochar mitigated the stimulatory effect of N deposition on soil R H. • R H was positively related to β-glucosidase and cellobiohydrolase (CBH) activities. • Reduced R H by biochar was linked to decreased CBH activity and cbh I gene abundance. Both atmospheric nitrogen (N) deposition and biochar application can markedly impact soil heterotrophic respiration (R H), an important component of the global carbon cycle. However, the interactive effects of N deposition and biochar application on soil R H in subtropical forest ecosystems remain unclear. Here, we conducted a three-year (2019–2022) field trial within a bamboo forest in subtropical China to examine the responses of soil physicochemical and microbial properties to N deposition and biochar application, and to elucidate how biochar regulates N deposition-induced change in soil R H. Nitrogen deposition stimulated soil R H by 8.1–9.8 % annually over three years compared to the control, and this stimulation was mitigated (by 8.1–8.9 % annually) with biochar addition. In the context of N deposition, the decrease of soil R H by biochar application was not through changing soil temperature, moisture or labile organic carbon content. Biochar treatment reduced the abundances of bacterial glycoside hydrolase family 48 gene (GH48) and fungal glycoside hydrolase family 7 cellobiohydrolase I gene (cbh I) and the activities of β-glucosidase and cellobiohydrolase (CBH), but increased the abundance of cbbL gene and activity of RubisCO enzyme. Furthermore, the R H was correlated positively (P < 0.01) with β-glucosidase and CBH activities and negatively (P < 0.01) with RubisCO enzyme activity. Structural equation modeling revealed that the biochar-induced reduction of soil R H under N deposition was associated with decreases in the abundance of cbh I gene and the activity of CBH in soils. We highlight that management practices can mitigate soil carbon loss in forests through modulating soil microbial functions under atmospheric N deposition, and that biochar application in Moso bamboo forests has the potential to reduce R H by approximately 7.6 × 106 t CO 2 yr−1 under atmospheric N deposition. [Display omitted] [ABSTRACT FROM AUTHOR]

• Night warming had contrasting impacts on flowering date in 3 temperate steppes. • Divergent precipitation impacts on flowering date were found in 3 temperate steppes. • The findings highlight the vital role of plant growth in modulating flowering date. Our understanding of how simultaneous climate warming and changing precipitation influence plant phenology in grasslands is still limited. As part of a field transplant experiment, this study was conducted to explore the flowering phenology of the dominant species in three temperate steppes (i.e. a desert, typical, and meadow steppe) along a precipitation gradient in response to simulated night warming and precipitation manipulation (i.e. decreased, ambient, and increased precipitation). Of all monitored species and across three growing seasons (2015–2017), night warming advanced flowering date in the desert and typical steppes, but delayed it in the meadow steppe. Decreased precipitation postponed flowering date but increased precipitation advanced it in all the three steppes. Plant growth largely determined the changes in flowering date under night warming and changing precipitation across and in each of the three steppes. The dominant role of plant growth in modulating reproductive phenology provides mechanistic understanding in interpreting phenological response and facilitate the projections of response of temperate grasslands under future climate change scenarios. [ABSTRACT FROM AUTHOR]

The balance between ecosystem carbon dioxide (CO 2 ) uptake and release determines the level of carbon (C) sequestration in terrestrial ecosystems and its potential impact on CO 2 concentration in the atmosphere. However, how changes in temperature and precipitation will affect the relationships of net ecosystem productivity (NEP) with gross primary productivity (GPP) and ecosystem respiration (ER) remains unclear. In this study, a nine-year field manipulative experiment was conducted with elevated temperature and increased precipitation in a semiarid steppe of Inner Mongolia, China. Experimental warming reduced GPP and ER by almost the same amount, leading to a slight change in NEP (−0.16 μmol m −2 s −1 ), whereas increased precipitation stimulated GPP more than ER during the growing seasons, resulting in an enhanced NEP (+0.63 μmol m −2 s −1 ). In addition, seasonal patterns of ecosystem C fluxes and the NEP-GPP or NEP-ER relationships were not altered by experimental warming. However, increased precipitation delayed the peak of GPP during the growing seasons and enhanced the correlation between NEP and GPP in the steppe ecosystem. The enhanced control of GPP over NEP under the increased precipitation suggests that ecosystem C sequestration is attributed more to C uptake than C release when water availability is improved in the semiarid grassland. Our findings provide an insight into the response mechanism of ecosystem C flux to warming and precipitation change in semiarid grasslands, and facilitate the projection of terrestrial ecosystem C dynamics and climate feedbacks in the future. [ABSTRACT FROM AUTHOR]

• Symmetric flowering sensitivity to precipitation shift was found in temperate steppes. • Climate warming stimulated precipitation sensitivity (S p) in the desert steppe only. • Warming enhanced dependences of s p on abiotic and biotic factors during flowering stage. The phenological sensitivity of terrestrial plants to precipitation change (precipitation sensitivity, S p) and whether S p is affected by climatic warming remain largely unknown. A four-year (2015–2018) field experiment with warming and increased/decreased precipitation was conducted to investigate the impacts of climate change on S p in three dominant temperate grasslands (i.e., desert, typical, and meadow steppes) on the Mongolian Plateau of Northern China. Results showed that S p of flowering phenology to reduced and increased precipitation was symmetric in each of the three steppes. Experimental warming stimulated S p by 0.30 day/(10 mm· °C) over the three steppes, however, by 1.20 day/(10 mm· °C) in the desert steppe, which could be primarily attributed to enhanced dependences of S p on soil moisture and normalized difference vegetation index (NDVI) during the flowering stage. These findings suggest that warming has the greater potential to stimulate S p of phenological events in arid rather than semiarid and mesic grasslands and that S p is jointly controlled by soil water availability and community development indicated by NDVI in different flowering stages in temperate grasslands. Considering the above two factors in phenology models will promote robust prediction of grassland reproductive phenology in temperate regions under concurrent climate warming and changing precipitation. [ABSTRACT FROM AUTHOR]

• Symmetric flowering sensitivity to precipitation shift was found in temperate steppes. • Climate warming stimulated precipitation sensitivity (S p) in the desert steppe only. • Warming enhanced dependences of s p on abiotic and biotic factors during flowering stage. The phenological sensitivity of terrestrial plants to precipitation change (precipitation sensitivity, S p) and whether S p is affected by climatic warming remain largely unknown. A four-year (2015–2018) field experiment with warming and increased/decreased precipitation was conducted to investigate the impacts of climate change on S p in three dominant temperate grasslands (i.e., desert, typical, and meadow steppes) on the Mongolian Plateau of Northern China. Results showed that S p of flowering phenology to reduced and increased precipitation was symmetric in each of the three steppes. Experimental warming stimulated S p by 0.30 day/(10 mm· °C) over the three steppes, however, by 1.20 day/(10 mm· °C) in the desert steppe, which could be primarily attributed to enhanced dependences of S p on soil moisture and normalized difference vegetation index (NDVI) during the flowering stage. These findings suggest that warming has the greater potential to stimulate S p of phenological events in arid rather than semiarid and mesic grasslands and that S p is jointly controlled by soil water availability and community development indicated by NDVI in different flowering stages in temperate grasslands. Considering the above two factors in phenology models will promote robust prediction of grassland reproductive phenology in temperate regions under concurrent climate warming and changing precipitation. [ABSTRACT FROM AUTHOR]

• Altered precipitation (PPT) changed SR (soil respiration) in a coastal wetland. • SR was reduced by a decline in PPT and enhanced by an increase in PPT. • The response of SR to altered PPT was positive asymmetric. • Soil EC variations to altered PPT were correlated with the asymmetric response of SR. • Asymmetric response of SR relied on the acclimation of AGB and LAI to high soil EC. Intensified interannual variability in precipitation is anticipated to alter soil salinization in coastal wetlands, which may significantly impact the structure and function of ecosystems, especially soil carbon cycling. However, the response patterns and mechanisms of soil respiration to precipitation changes in coastal wetlands remain uncertain. To assess the response pattern of soil respiration to altered precipitation, a field manipulation experiment with five precipitation treatments (-60%, -40%, +0%, +40%, and +60% of ambient precipitation) was conducted from 2015 to 2019 in a coastal wetland in the Yellow River Delta, China. Over five years, the responses of soil respiration along this experimental precipitation gradient were positive asymmetric, with a greater increase in soil respiration under wet treatments (by 24.3% and 27.7% under the +40% and +60% treatments, respectively) than reduction under dry treatments (by 10.3% and 4.0% under the -60% and -40% treatments, respectively). The observed positive asymmetric responses of soil respiration were associated with the decreased response of belowground biomass and leaf area index to high soil electric conductivity under decreased precipitation. Our study demonstrated that the acclimation of vegetation to salinization was a major determining factor in the asymmetric response of soil respiration along an experimental precipitation gradient in coastal wetlands. Our findings emphasize that soil salinity will play an essential role in regulating soil carbon cycles in coastal wetlands. The positive asymmetric response of soil respiration to altered precipitation indicates that the intensification of precipitation variability in the future may negatively affect the stability of the soil carbon stock. [ABSTRACT FROM AUTHOR]

Abstract: Nitrogen (N) addition enhances primary productivity of terrestrial ecosystems. However, the effects of N fertilization and/or deposition on net ecosystem CO2 exchange (NEE) are not fully understood. The effects of N on NEE were investigated in two experimental cheatgrass ecosystems in Ecologically Controlled Enclosed Lysimeter Laboratories (EcoCELLs), Reno, Nevada. In this experiment, no N fertilization was added to the two EcoCELLs in the first year and two different N fertilization regimes were applied in the second year. N fertilizer was applied once to one EcoCELL (pulse fertilization, PF), and the same total amount of N in biweekly increments to the other EcoCell (gradual fertilization, GF). NEE, photosynthetically active radiation (PAR) and canopy green leaf area index (LAI) were continuously measured in the two EcoCELLs during the pretreatment and N-fertilized years. Plant N content and biomass were measured at the end of the growing season in each year. Radiation-use efficiency (RUECO2) was calculated as the ratio of gross ecosystem photosynthesis (GEP) to the intercepted photosynthetically active radiation (IPAR). The responses of NEE to IPAR were used to estimate the maximum ecosystem photosynthetic capacity (F max). N fertilization stimulated canopy LAI, plant N content, F max, RUECO2, NEE and biomass in both methods of N supply applications. PF led to higher LAI, F max and NEE than GF, but both had a similar RUECO2 during the early growing season. GF maintained higher LAI, F max, RUECO2 and NEE than PF during the late growing season. At the ecosystem level, N fertilization stimulated daily NEE directly by increasing canopy LAI, plant N content, shoot/root ratio and the maximum ecosystem photosynthetic capacity, and increased the seasonally accumulated NEE indirectly by extending the growing season. PF differed significantly from GF in its effects on NEE and RUECO2, possibly due to differential rates and timing of N availability. Our study suggested that these changes in the canopy RUECO2 and growing season under N fertilization or N deposition regimes should be considered in modeling studies of ecosystem C sequestration. [Copyright &y& Elsevier]

• Salt marsh functioned as CO2 sink over 8 years in the Yellow River Delta. • Annual NEE was mainly regulated by early growth precipitation. • Early growth precipitation regulated maximal biomass. • Early growth precipitation drives annual NEE by water-salt stress. Salt marshes are significant contributors to global "blue carbon" resources, and these habitats are sensitive to precipitation events due to periodically dry-wet alternation induced by tides. However, whether annual marsh-atmospheric CO 2 flux responds more to annual or seasonal precipitation remains unclear. Here, eight years (2012-2019) of eddy covariance data were evaluated to determine typical CO 2 budgets, and we assessed the effect of annual and seasonal precipitation on interannual net ecosystem CO 2 exchange (NEE) in a salt marsh of the Yellow River Delta, China. The salt marsh was a sink for atmospheric CO 2 in each of the eight study years, with an 8-year average NEE of -51.7 ± 9.7 g C m−2 a−1 varying from -8 to -85 g C m−2 a−1. Annual NEE was mainly regulated by precipitation levels during the early growth stage of plants, which modulated maximal plant biomass accumulation via water-salt transport. Besides, a spring precipitation distribution experiment showed that wetter early growth stage of plants enhanced carbon assimilation capacity in the salt marsh by decreasing soil salinity and promoting plant biomass accumulation. These findings suggest that soil water-salt conditions induced by precipitation during the onset of the growing season are crucial for interannual variations of NEE in salt marshes. [ABSTRACT FROM AUTHOR]

Abstract: We review 15 techniques for estimating missing values of net ecosystem CO2 exchange (NEE) in eddy covariance time series and evaluate their performance for different artificial gap scenarios based on a set of 10 benchmark datasets from six forested sites in Europe. The goal of gap filling is the reproduction of the NEE time series and hence this present work focuses on estimating missing NEE values, not on editing or the removal of suspect values in these time series due to systematic errors in the measurements (e.g., nighttime flux, advection). The gap filling was examined by generating 50 secondary datasets with artificial gaps (ranging in length from single half-hours to 12 consecutive days) for each benchmark dataset and evaluating the performance with a variety of statistical metrics. The performance of the gap filling varied among sites and depended on the level of aggregation (native half-hourly time step versus daily), long gaps were more difficult to fill than short gaps, and differences among the techniques were more pronounced during the day than at night. The non-linear regression techniques (NLRs), the look-up table (LUT), marginal distribution sampling (MDS), and the semi-parametric model (SPM) generally showed good overall performance. The artificial neural network based techniques (ANNs) were generally, if only slightly, superior to the other techniques. The simple interpolation technique of mean diurnal variation (MDV) showed a moderate but consistent performance. Several sophisticated techniques, the dual unscented Kalman filter (UKF), the multiple imputation method (MIM), the terrestrial biosphere model (BETHY), but also one of the ANNs and one of the NLRs showed high biases which resulted in a low reliability of the annual sums, indicating that additional development might be needed. An uncertainty analysis comparing the estimated random error in the 10 benchmark datasets with the artificial gap residuals suggested that the techniques are already at or very close to the noise limit of the measurements. Based on the techniques and site data examined here, the effect of gap filling on the annual sums of NEE is modest, with most techniques falling within a range of ±25gCm−2 year−1. [Copyright &y& Elsevier]

• Night warming did not affect soil respiration, but changing precipitation influenced it in three temperate steppes. • There was an asymmetric sensitivity of soil respiration to increased and decreased precipitation. • The findings highlight diverse limiting factors and mechanisms in affecting soil respiration among various steppes. Responses and feedbacks of ecosystem carbon (C) cycling in the terrestrial biosphere to climate change pose the major challenge for Earth System Model projections. However, whether ecosystem C cycling among diverse vegetation types shows different sensitivities and the associated underlying mechanisms remain elusive. As part of a 5-year (2014–2018) soil monolith transplant experiment, this study was designed to examine the influences of night warming, decreased precipitation, and increased precipitation on soil respiration in the three temperate steppes (i.e. desert, typical, and meadow steppes) along a precipitation gradient on the Mongolian Plateau. Over the 5 years, night warming had no effect on soil respiration in any of the three steppes. Decreased precipitation reduced soil respiration, whereas increased precipitation stimulated it in each steppe. In addition, the sensitivity of soil respiration to increased precipitation was larger than that to decreased precipitation, suggesting a positive asymmetry of soil respiration in response to changing precipitation. The soil respiration responses were predominately determined by both soil water availability and belowground net primary productivity in the desert and typical steppes, but by soil total C alone in the meadow steppe. These findings not only improve mechanistic understanding on the responses of soil respiration in various grassland ecosystems, but also facilitate the extrapolation from local to regional scale for the robust projections of terrestrial C-climate change feedbacks. Image, graphical abstract [ABSTRACT FROM AUTHOR]