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With the rapid development of underground rail transportation in China, the demand for precast concrete pipe sheets in metro construction continues to increase, and the quality and safety of precast concrete pipe sheets profoundly affect the safety of tunnels. The quality and safety of precast concrete pipe sheets have a profound impact on the safety of tunnels. The maintenance of pipe sheets plays an important role in ensuring the strength and durability of concrete pipe sheets. Therefore, it is important to study the maintenance methods of concrete pipe sheets. The method of substituting immersion curing with spray curing is proposed in this paper. It has been shown that spray curing can overcome the challenges of high pollution and high cost caused by the immersion curing of precast concrete pipe (PCP) sheets. Experimental tests investigate the influence of macro-mechanical properties and microstructure of pipe sheet materials under various ages of the two curing conditions. The results highlighted that the conventional methods of water immersion for PCP have minimal effect on concrete's macro-mechanical characteristics or microstructure after a certain curing age. The comparative studies on both curing processes showed that after seven days of curing, the degree of moisture attained by the spray-cured samples was relatively lower than that of the water immersion approach. In addition, samples with the water spray curing process had larger pore sizes (which could degrade faster) with lower mechanical strength. Interestingly, after 28 days of curing age, both approaches attained complete hydration levels, resulting in a dense pore structure with reduced median pore size and higher mechanical performance. Investigations on two distinct methods indicated that the early strength properties (after seven days of curing) acquired with water immersion curing were higher than the mechanical properties obtained with spray curing. This difference in mechanical properties, in addition, was not significant after 28 days of curing age. As a result, the suggested technique of water spray curing is an appropriate method with minimum water use to some extent. [ABSTRACT FROM AUTHOR]

Plant stoichiometry and nutrient allocation can reflect a plant's adaptation to environmental nutrient changes. However, the allocation strategies of carbon (C), nitrogen (N), and phosphorus (P) between leaf and fine root in response to wildfire have been poorly studied. Our primary objective was to elucidate the trade-off of elemental allocation between above- and belowground parts in response to the soil nutrient changes after a wildfire. We explored the allocation sloping exponents of C, N, and P between leaf and fine root at the species and community levels at four recovery periods (year 2, 10, 20, and 30) after moderately severe wildfire and one unburned treatment in boreal forests in Great Xing'an Mountains, northeast China. Compared with the unburned treatment, leaf C concentration decreased and fine root C increased at year 2 after recovery. The leaf N concentration at year 10 after recovery was higher than that of unburned treatment. Plant growth tended to be limited by P concentration at year 10 after recovery. Nutrient allocation between leaf and fine root differed between species and community levels, especially in the early recovery periods (i.e., 2 and 10 years). At the community level, the nutrient concentrations of the leaf changed more as compared to that of the fine root at year 2 after recovery when the fine root nutrients changed more than those of the leaf. The different C, N, and P allocation strategies advanced the understanding of plant adaptation to soil nutrient changes during the postfire ecosystem restoration. [ABSTRACT FROM AUTHOR]

Wildfire is crucial in the regulation of nutrient allocation during the succession of boreal forests. However, the allocation strategies of carbon (C), nitrogen (N) and phosphorus (P) between leaves and fine roots in response to wildfire severities remain poorly studied. We aimed to explore the allocation strategies of C, N and P between leaves and fine roots among different fire severities. We selected four wildfire severities (unburned, low, moderate and high severity) after 10 years recovery in the Great Xing'an Mountains, northeast China, and compared C, N and P concentrations in leaves and fine roots of all species among fire severities using stoichiometry theory and allometric growth equations. Compared with unburned treatment, C concentrations in leaves and fine roots increased at low severity, and leaf N concentration was the greatest at high severity, but the lowest fine root N concentration occurred at high severity. Plant nutrient utilization tended to be P-limited at high fire severity according to the mean value of N:P ratio > 16. More importantly, C, N and P allocation strategies between fine roots and leaves changed from allometry to isometry with increasing fire severities, which showed more elements allocated to leaves than to fine roots with increasing fire severities. These changes in patterns suggest that the allocation strategies of elements between leaves and fine roots are of imbalance with the wildfire severity. This study deepens our understanding of nutrient dynamics between plant and soil in ecosystem succession. [ABSTRACT FROM AUTHOR]

Background: Plant nitrogen use efficiency (NUE) is an important ecological indicator that reflects the capacity of a plant to transform nitrogen into production, which is essential for further elucidating plant growth and terrestrial ecosystem productivity. Although there are a growing number of studies that address NUE changes at local scales, the variations of NUE over large spatial scales remain unclear. In this study, we analyzed the geographic patterns of NUE and explored its phylogenic and environmental drivers across 1452 species at 1102 sites in China. Results: NUE tended to decrease with latitude (r = − 0.56), whereas it increased with longitude (r = 0.54), and varied widely in different ecosystems and plant life forms. Furthermore, NUE was negatively correlated with plant foliar phosphorus concentration (r = − 0.53), soil pH (r = − 0.10), soil total phosphorus (r = − 0.13) and available phosphorus (r = − 0.05), but positively with the mean annual temperature (r = 0.32), annual precipitation (r = 0.27), and aridity index (r = 0.26). NUE was significantly altered with phylogeny and evolved toward a lower value (r = − 0.28), which may have been due to increasing nitrogen deposition and fixation in biogeochemical evolution. Overall, the combination of foliar phosphorus concentration, phylogeny, climate, and soil properties accounted for 52.7% of the total variations of NUE. In particular, foliar phosphorus concentration was the most important factor, whereas plant evolutionary history was second in contributing to NUE variations. Conclusions: Our study emphasizes the pivotal role of plant stoichiometry and phylogeny in nitrogen cycling and suggests incorporating them into earth system models to better understanding plant growth and nitrogen cycling in the context of environmental changes. [ABSTRACT FROM AUTHOR]

Aims Although many studies have reported net gains of soil organic carbon (SOC) after afforestation on croplands, this is uncertain for Chinese paddy rice croplands. Here, we aimed to evaluate the effects of afforestation of paddy rice croplands on SOC sequestration and soil respiration (Rs). Such knowledge would improve our understanding of the effectiveness of various land use options on greenhouse gas mitigation in China. Methods The investigation was conducted on the Chongming Island, north subtropical China. Field sites were reclaimed from coastal salt marshes in the 1960s, and soils were homogeneous with simple land use histories. SOC stocks and Rs levels were monitored over one year in a paddy rice cropland, an evergreen and a deciduous broad-leaved plantation established on previous paddy fields and a reference fallow land site never cultivated. Laboratory incubation of soil under fast-changing temperatures was used to compare the temperature sensitivity (Q10) of SOC decomposition across land uses. Important Findings After 15–20 years of afforestation on paddy fields, SOC concentration only slightly increased at the depth of 0–5cm but decreased in deeper layers, which resulted in a net loss of SOC stock in the top 40cm. Seasonal increase of SOC was observed during the rice-growing period in croplands but not in afforested soils, suggesting a stronger SOC sequestration by paddy rice cropping. However, SOC sequestered under cropping was more labile, as indicated by its higher contents of dissolved organic carbon and microbial biomass. Also, paddy soils had higher annual Rs than afforested soils; Rs abruptly increased after paddy fields were drained and plowed and remained distinctively high throughout the dry farming period. Laboratory incubation revealed that paddy soils had a much higher Q10 of SOC decomposition than afforested soils. Given that temperature was the primary controller of Rs in this region, it was concluded that despite the stronger SOC sequestration by paddy rice cropping, its SOC was less stable than in afforested systems and might be more easily released into the atmosphere under global warming. [ABSTRACT FROM AUTHOR]

Soil bacterial succession under intensive anthropogenic disturbances is not well known. Using terminal restriction fragment length polymorphisms and 454 pyrosequencing of 16S rRNA genes, this study investigated how soil bacterial diversity and community structure changed under two agricultural land uses (paddy rice and upland cropping) in relation to soil development along a 500-year chronosequence created by intermittent reclamation of estuarine salt marshes. Multivariate analysis revealed orderly changes in soil physicochemical properties and bacterial community structure with time, confirming the occurrence of soil development and bacterial succession. Patterns of soil development and bacterial succession resembled each other, with recent land uses affecting their trajectories but not the overall direction. Succession of bacterial community structure was mainly associated with changes in α- Proteobacteria and Verrucomicrobia. Two stages of bacterial succession were observed, a dramatic-succession stage during the first several decades when bacterial diversity increased evidently and bacterial community structure changed rapidly, and a long gradual-succession stage that lasted for centuries. Canonical correspondence analysis identified soil Na, potentially mineralizable nitrogen, total phosphorous, and crystallinity of iron oxyhydrates as potential environmental drivers of bacterial succession. To conclude, orderly succession of soil bacterial communities occurred along with the long-term development of agroecosystems, which in turn was associated with soil physicochemical changes over time. [ABSTRACT FROM AUTHOR]

Drought and extreme precipitation events can have major environmental and socioeconomic impacts. Yet, how drought and wetness are changing in China in the context of climate change is still under debate. Here, the standardized precipitation evapotranspiration index (SPEI) was calculated based on high-quality and more densely distributed daily meteorological observation data from 655 stations across China during the period of 1965–2017. National and regional trends in drought and wetness and their various characteristics, including intensity, duration, frequency, and percentage of area affected, were investigated at multiple timescales. We found that (1) China as a whole has undergone a significant (p < 0.01, trend significant at the level of 0.01) wetting trend, with an annual SPEI increase of 0.5 per decade from 1965 to 2017. A seasonal wetting trend was also observed, with summer being particularly significant (p < 0.01). (2) Regionally, each subregion also showed a wetting trend during the study period except for southwest China, and these wetting trends were significant in the western region of northwest China (p < 0.05, trend significant at the level of 0.05), the Tibetan Plateau (p < 0.05), and eastern China (p = 0.06). (3) Decadal trends in drought and wetness intensity, frequency, duration, and affected areas indicated that the drought events also became more severe and more frequent in the last two decades, and the areas showing drying trends were mainly located in southwest China (especially for the autumn drought) and the southwestern parts of eastern northwest China (spring drought). Our results highlight the fact that although a wetting trend was observed in most regions of China, the frequent occurrence of severe drought in southwest China and the southwestern parts of eastern northwest China still present a considerable threat to both the environment and society. Therefore, how to effectively coordinate the allocation of regional water resources to cope with drought risk under future climate change will be particularly important. [ABSTRACT FROM AUTHOR]

Reference evapotranspiration (ET0) is important to the global energy balance and to hydrological cycling. However, the extent to which ET0 changes, the main driving factors, and especially the implications of its shift for drought in Southwest China are not clear. In this study, trends in Penman–Monteith ET0 and other climatic parameters at 79 stations in Southwest China from 1960 to 2018 were investigated by using the Mann–Kendall test. Furthermore, partial correlation analysis and multiple linear regression were used to determine the dominant climate driving factors in changes in ET0. The relative contribution of precipitation and ET0 to drought duration was also quantified based on spatial multiple linear regression. Results revealed that annual ET0 decreased significantly (p < 0.01) at a rate of 14.1 mm per decade from 1960 to 2000, and this decrease disappeared around 2000. For the entire study period, the sunshine duration (Tsun) was the most closely correlated with and played a dominant role in the variations in ET0 at both annual and seasonal (summer and autumn) timescales, whereas the relative humidity was the most dominant factor in the spring and winter. Trends in the Standardized Precipitation Evapotranspiration Index revealed that drought has become more serious in Southwest China, and ET0 has made a greater contribution to the duration of drought than precipitation. Our findings highlight that more attention should be paid to the impacts of ET0 changes on drought in Southwest China. Furthermore, these results can provide a reference for the allocation of water resources and the implementation of countermeasures to climate change. [ABSTRACT FROM AUTHOR]

Abstract: Dynamics of reclaimed coastal wetland soils under cultivation has not been well understood, especially at temporal scales longer than a century. In this study, we analyzed major chemical properties of plow-layer soils extensively sampled under two cropping systems (paddy rice vs. upland cropping) along a 500-year soil chronosequence created by intermittent reclamation of coastal salt marshes. The results suggested a rapid desalinization of soil immediately after reclamation. During 500 years of cultivation, the decalcification process lowered soil pH from >8 to nearly neutral. Soil organic carbon (SOC) contents markedly declined in the initial 16 years, but then rapidly recovered within 30 years and thereafter slowly accumulated with cultivation duration. Meanwhile, the recalcitrance of SOC increased. Soil nutrient status was enhanced after centuries of cultivation as indicated by the improved total nitrogen (TN) and phosphorous (TP). Amorphous Fe oxyhydrates progressively decreased, but the crystallinity of Fe oxyhydrates increased with cultivation time. Cropping system greatly affected plow-layer soil properties, with paddy soils having higher SOC, MBC, NH4OAc-extractable Ca2+, but lower TP, NH4OAc-extractable K+, potentially mineralizable nitrogen and Fe crystallinity than upland soils. Most soil properties revealed clear temporal patterns with more remarkable changes occurring in the first several decades after reclamation than in the following centuries. In conclusion, there was a transition phase of the soil system within the first several decades after reclamation of coastal wetlands, possibly harmful to agricultural production, but centuries of cultivation seemed to have significantly improved overall soil fertility. [Copyright &y& Elsevier]

• Stable SOC was mainly distributed at hillside and valley in the tropical rainforest. • Terrain explained the most spatial variations of SOC and its stability fractions. • Abiotic factors explained more variations of SOC fractions than biotic factors. • Terrain should be considered in the SOC estimation and related biogeochemical model. Estimation of soil organic carbon (SOC) storage and its dynamics in tropical montane forests is crucial for climate change prediction, which calls for further investigation into the spatial variation in SOC and its stability mechanisms. In this study, 60 subplots (20 m × 20 m) were randomly selected within a 60 ha tropical montane rainforest dynamic monitoring plot located in southern China. The physical (HFC and LFC) and chemical fractions of SOC (alkyl C, O-alkyl C, aromatic C and carbonyl C), microbial biomass carbon and other soil properties at depths of 0–10 cm, plant species and root biomass (0–10 cm and 10–20 cm) were investigated. Geostatistical methods, multiple regression trees and redundant analysis were used to reveal that the spatial variations in SOC and its stability mechanisms with the terrain. The results show that the spatial variations in HFC and the ratios of alkyl carbon/O-alkyl carbon had a moderate spatial dependence due to the complex terrain. High SOC and its physical stability fractions (HFC) were distributed along ridgelines, while the chemical stability index of SOC (alkyl C/O-alkyl C) was the highest on hillsides and in valleys. Terrain convexity best explained the spatial variations in SOC and HFC, while total nitrogen and convexity together best explained the spatial variation in the chemical fractions of SOC. Abiotic factors explained more of the variation in SOC and its fractions than biotic factors, while abiotic and biotic factors were covariant. The specific factors controlling the distribution of SOC and its fractions differed with the types of micro-terrain. These results highlight the influence of terrain on the distribution and accumulation of SOC in tropical forest ecosystems. Hence, terrain should be considered a key factor in biogeochemical models. [ABSTRACT FROM AUTHOR]

• There was a diurnal hysteresis between GPP and CH 4 flux in an alpine meadow. • Similar hysteresis was observed between CH 4 and PAR, and between PAR and Ta. • The hysteresis between CH 4 and GPP attributes to the hysteresis of PAR and Ta. The relationship between methane (CH 4) and gross primary productivity (GPP) remains elusive and the underlying mechanism is even far from clear. To explore this relationship, we continually monitored CH 4 flux and CO 2 flux by eddy covariance technique in an alpine meadow on the Qinghai–Tibetan Plateau, China, during June 2015–December 2017. We found a parabolic-like relationship of net CH 4 flux with GPP change in both the morning and afternoon at diurnal scale. However, with similar GPP, CH 4 uptake was stronger in the morning than that in the afternoon, forming an anti-clockwise hysteretic relationship between CH 4 and GPP in all the three years. Meanwhile, similar hysteresis loop was also observed between CH 4 flux and photosynthetically active radiation (PAR), and between PAR and air temperature (Ta) during all the three years, with stronger CH 4 uptake (or lower Ta) in the morning than that in the afternoon at the same PAR. Results from our study suggested that the hysteretic relationship between CH 4 flux and GPP might be mainly induced by the hysteresis between PAR and Ta. The similar hysteresis pattern between CH 4 flux (or Ta) and PAR in the non-growing season further confirmed our hypothesis. The findings provide new insights into the coupling of plant photosynthetic activity and net CH 4 flux and help to improve model parameterization for simulating CO 2 –CH 4 interaction processes. [ABSTRACT FROM AUTHOR]

The changing climate is shifting the seasonality of photosynthesis in vegetation, including the start (SOS), end (EOS), and length (LOS) of the growing season, and the peak photosynthesis timing (PPT). While the SOS, EOS, and LOS have been widely investigated, the PPT of grasslands – as a proxy for the response of seasonal plant photosynthesis to climate change – has been overlooked. In this study, we propose a hybrid generalized additive model (HGAM) method to extract PPT using the Vegetation Photosynthesis Model (VPM)-based gross primary production (GPP) product, and we examine the dynamics, drivers, and consequences of PPT changes in temperate and alpine grasslands in China over 2000–2016. We found that the PPTs in temperate and alpine grasslands have exhibited advancing (with −0.68 days yr−1, p < 0.05) and delaying (with 0.29 days yr−1, p = 0.158) trends, respectively. In addition, preseason precipitation and soil moisture were positively correlated with the PPT in temperate and alpine grasslands, respectively, while the preseason temperature consistently controlled the PPT changes in both grasslands. Furthermore, we found that an earlier PPT was associated with higher annual production in the temperate grasslands but not in the alpine grasslands in China. The divergent PPT patterns indicated the varied adaptation characteristics to climatic constraints in the temperate and alpine grasslands and also caused different consequences on carbon uptake. This study highlights the importance of PPT in understanding the spatiotemporal dynamics of vegetation photosynthesis and the carbon cycle under a changing climate. • Identification of peak photosynthesis timing (PPT) by a hybrid generalized additive model (HGAM). • Divergent PPT trends exist between temperate and alpine grasslands in China. • Different climatic factors affect the PPTs in temperate and alpine grasslands. • The correlation between an earlier peak and higher production exists only in temperate grasslands. [ABSTRACT FROM AUTHOR]

Wildfire is the primary natural disturbance in boreal forest ecosystems. It substantially changes soil nutrient conditions and plant nutrient dynamics. After a wildfire, various plant strategies of nutrient utilization are fundamental to ecosystem recovery processes. Stability of plant nutrients reflects the ability of plants possessing relatively constant elemental concentrations in the face of nutrient changes, which can be calculated by the value of "nutrient homeostasis". However, the mechanism of how nutrient homeostasis mediates plant community recovery in post-fire ecosystems remains unknown. The dominant tree species that survived after fire and the new emergence of regenerated tree species are the important components of a plant community during the recovery process. Our primary objective was to elucidate the nutrient homeostasis trade-off between dominant and regenerated species over years after recovery. Five treatments, namely, 2 year, 10 year, 20 year, 30 years after moderate burning severity, and unburned forests, were designed in the boreal forests of Great Xing'an Mountains, Northeast China. Compared with unburned forests, wildfire lowered the average value of homeostasis of plant nutrients (N and P). Moreover, the mean homeostasis value of the dominant species (i.e., Larix gmelinii) was higher than that of the regenerated species (i.e., Betula platyphylla). The slope of relationship between nutrient homeostasis and recovery years of the regenerated species was higher than that of the dominant species, suggesting that the nutrient homeostasis in the regenerated species recovered more quickly than dominant species after recovery. Compared with the dominant species, changes in the regenerated species' homeostasis can explained more to the changes of species diversity during the years after recovery. This study revealed plant nutrient adaptation in different species and different plant organs with years after wildfire and highlighted the importance of nutrient homeostasis in plant adaptation strategies and the recovery of plant community., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

The effects of grazing on methane (CH 4 ) budgets are important for understanding the balance of greenhouse gas emissions and removals in grassland ecosystems. However, the CH 4 budgets of grazing systems, that is simultaneously considering CH 4 uptake by grassland soils and emissions from ruminant enteric fermentation, livestock folds and animal feces, are poorly investigated, particularly for Chinese grasslands, and thus, remained unclear currently. Here, a synthesis of 43 individual studies was carried out to assess the grazing season/annual CH 4 budgets and their responses to grazing in grassland ecosystems of China. The results showed that heavy grazing (HG) significantly decreased, while light grazing (LG) and moderate grazing (MG) had no significant effects soil CH 4 uptake, as compared to un-grazing sites. Grazing has shifted Chinese grasslands from a sink to source for atmospheric CH 4 , and the grazing season/annual CH 4 budgets increased with increasing grazing intensity, while the offset of CH 4 uptake by grassland soils to total CH 4 emissions from sheep, sheepfolds and feces were exponentially decreased with increasing grazing intensity. Moreover, the herbage biomass (HBM), organic matter intake (OMI) and live weight gain (LWG) were decreased while CH 4 emission intensities (i.e., CH 4 emission per HBM, OMI, and LWG) were linearly increased with increasing grazing intensity. Our results demonstrate that mediating grazing intensity, e.g., from HG to LG, could yield the optimal balance between maintaining productive grasslands and meanwhile mitigating CH 4 emissions. This study could help for building strategies with implications for grassland management in China with similar CH 4 emission problems., (Copyright © 2018 Elsevier B.V. All rights reserved.)