Response of plant production to growing/non-growing season asymmetric warming in an alpine meadow of the Northern Tibetan Plateau

Response of plant diversity and soil microbial diversity to warming and increased precipitation in alpine grasslands on the Qinghai-Xizang Plateau - A review

Plant diversity and soil microbial diversity are closely related, and they maintain the health and stability of terrestrial ecosystems. As a hotspot region of global biodiversity research, both air temperature and precipitation of the Qinghai-Xizang Plateau tend to increase in future. Based on an overview of the responses of grassland/alpine ecosystems to seasonal asymmetric warming and increased precipitation worldwide, we elaborated the advancements and uncertainties on the responses of plant diversity and soil microbial diversity to warming and increased precipitation in alpine grasslands on the Qinghai-Xizang Plateau. The future research focus of plant diversity and soil microbial diversity in the alpine grasslands of the Qinghai-Xizang Plateau under climate warming and increased precipitation was proposed. Generally, previous studies found that the responses of plant species diversity and soil microbial species diversity to warming and increased precipitation differed between alpine meadows and alpine steppes, but few studies focused on their responses to warming and increased precipitation in alpine desert steppes. Previous studies mainly focused on species diversity, although phylogenetic and functional diversities are also important aspects of biodiversity. Previous studies mainly explained responses of plant diversity and soil microbial diversity to warming and increased precipitation based on niche theory, although neutral theory is also the other important mechanism in regulating biodiversity. Moreover, previous studies almost ignored the coupling relationship between plant diversity and soil microbial diversity. Therefore, the following four aspects need to be strengthened, including the responses of plant diversity and soil microbial diversity to warming and increased precipitation in alpine desert steppes, the responses of plant and soil microbial phylogenetic diversity and functional diversity to warming and increased precipitation, combining the niche theory and neutral theory to examining the mechanism of biodiversity, and the coupling relationships between plant diversity and soil microbial diversity under warming and increased precipitation.

Asymmetric pre-growing season warming may jeopardize seed reproduction of the sand-stabilizing shrub Caragana microphylla

Our current understanding of the processes and mechanisms by which seasonal asymmetric warming affects seed reproduction in semiarid regions, which are essential in preserving the stability of both vegetation ecosystem structure and function, remains poorly understood. Here, we conducted a field warming experiment, including pre-growing season warming (W1), in-growing season warming (W2), and combined pre- and in-growing season warming (W3) treatments, to investigate the seed reproductive strategy of Caragana microphylla, an important sand-stabilizing shrub, from the perspective of reproductive phenology, reproductive effort, and reproductive success. Results show that the warming treatments advanced the initial stages of reproductive phenology, prolonged its duration, and decreased its synchrony (magnitude = W3 > W2 > W1). Additionally, flowering phenology was more sensitive to warming than podding phenology. The W1 treatment inclined seed reproduction towards the conservative strategy with low reproductive effort and success. The W3 treatment tended to increase seed reproductive effort and success. While the W2 treatment did not affect reproductive success, it did increase reproductive effort. Changes in reproductive phenology explained 20 % of the variation in reproductive effort and 38 % of the variation in reproductive success. However, these changes also directly hindered reproductive success (direct effect = -0.57) while indirectly promoting reproductive success (indirect effect = 0.27) by increasing reproductive efforts. Our results reveal that the seasonal asymmetry of warming altered the seed reproduction strategy of sand-stabilizing shrubs, with warmer winters and springs before the growing season decreasing seed fecundity.

A bibliometric analysis of research trends and hotspots in alpine grassland degradation on the Qinghai-Tibet Plateau

A bibliometric analysis of current research, hotspots, and development trends was used to develop an overall framework of mechanisms of alpine grassland degradation on the Qinghai-Tibet Plateau. This investigation includes data from 1,330 articles on alpine grassland degradation on the Qinghai-Tibet Plateau, acquired from the Chinese Science Citation Database (CSCD) and Web of Science Core Collection (WOS). Research was divided into three themes: spatial scope and management of typical grassland degradation problems, dynamic mechanisms of grassland degradation and effects of ecological engineering, and grassland degradation risk based on remote sensing technology. The results of the analysis showed that the research can be summarized into three aspects: typical grassland degradation identification, dynamic mechanism analysis of grassland degradation, and grassland ecosystem stability strategy. The main findings can summarized, as follows: (1) Ecological analyses using the river source as a typical region defined the formation of "black soil beach" type degraded grasslands in the region, and promoted the ecological environment management and protection of the alpine grassland by discussing the causes of regional ecological environment changes; (2) Dynamic mechanism analyses of climate change and characteristics analyses of grassland vegetation-soil degradation revealed that alpine grassland degradation is the result of multiple main factors; and (3) Risk prediction methods for grassland degradation, methods of grassland management and sustainable countermeasures for agriculture and animal husbandry development, and the development of a comprehensive index of influencing factors on grassland degradation all indicate that selecting the right grassland restoration measures is the key to grassland restoration. Remote sensing monitoring and high-throughput sequencing technology should be used in future research on grassland ecosystems. In addition, multiscale, multidimensional, and multidisciplinary systematic research methods and long-term series data mining could help identify the characteristics and causes of alpine grassland system degradation. These findings can help identify a more effective coordination of landscape, water, lake, field, forest, grass, and sand management for the prevention of alpine grassland degradation.

Response of Aboveground Net Primary Production, Species and Phylogenetic Diversity to Warming and Increased Precipitation in an Alpine Meadow

The uncertain responses of aboveground net primary productivity (ANPP) and plant diversity to climate warming and increased precipitation will limit our ability to predict changes in vegetation productivity and plant diversity under future climate change and further constrain our ability to protect biodiversity and ecosystems. A long-term experiment was conducted to explore the responses of ANPP, plant species, phylogenetic α-diversity, and community composition to warming and increased precipitation in an alpine meadow of the Northern Tibet from 2014 to 2019. Coverage, height, and species name were obtained by conventional community investigation methods, and ANPP was obtained using observed height and coverage. Open-top chambers with two different heights were used to simulate low- and high-level climate warming. The low- and high-level increased precipitation treatments were achieved by using two kinds of surface area funnel devices. The high-level warming reduced sedge ANPP (ANPPsedge) by 62.81%, species richness (SR) by 21.05%, Shannon by 13.06%, and phylogenetic diversity (PD) by 14.48%, but increased forb ANPP (ANPPforb) by 56.65% and mean nearest taxon distance (MNTD) by 33.88%. Species richness, Shannon, and PD of the high-level warming were 19.64%, 9.67%, and 14.66% lower than those of the low-level warming, respectively. The high-level warming-induced dissimilarity magnitudes of species and phylogenetic composition were greater than those caused by low-level warming. The low- rather than high-level increased precipitation altered species and phylogenetic composition. There were significant inter-annual variations of ANPP, plant species, phylogenetic α-diversity and community composition. Therefore, climate warming and increased precipitation had non-linear effects on ANPP and plant diversity, which were due to non-linear changes in temperature, water availability, and/or soil nutrition caused by warming and increased precipitation. The inter-annual variations of ANPP and plant diversity were stronger than the effects of warming and especially increased precipitation on ANPP and plant diversity. In terms of plant diversity conservation and related policy formulation, we should pay more attention to regions with greater warming, at least for the northern Tibet grasslands. Besides paying attention to the responses of ANPP and plant diversity to climate change, the large inter-annual changes of ANPP and plant diversity should be given great attention because the large inter-annual variation indicates the low temporal stability of ANPP and plant diversity and thus produces great uncertainty for the development of animal husbandry.

Competitive interactions in two different plant species: Do grassland mycorrhizal communities and nitrogen addition play the same game?

In the Tibetan Plateau grassland ecosystems, nitrogen (N) availability is rising dramatically; however, the influence of higher N on the arbuscular mycorrhizal fungi (AMF) might impact on plant competitive interactions. Therefore, understanding the part played by AMF in the competition between Vicia faba and Brassica napus and its dependence on the N-addition status is necessary. To address this, a glasshouse experiment was conducted to examine whether the grassland AMF community's inocula (AMF and NAMF) and N-addition levels (N-0 and N-15) alter plant competition between V. faba and B. napus. Two harvests took day 45 (1^st harvest) and day 90 (2^nd harvest), respectively. The findings showed that compared to B. napus, AMF inoculation significantly improved the competitive potential of the V. faba. In the occurrence of AMF, V. faba was the strongest competitor being facilitated by B. napus in both harvests. While under N-15, AMF significantly enhanced tissue N:P ratio in B. napus mixed-culture at 1^st harvest, the opposite trend was observed in 2^nd harvest. The mycorrhizal growth dependency slightly negatively affected mixed-culture compared to monoculture under both N-addition treatments. The aggressivity index of AMF plants was higher than NAMF plants with both N-addition and harvests. Our observation highlights that mycorrhizal associations might facilitate host plant species in mixed-culture with non-host plant species. Additionally, interacting with N-addition, AMF could impact the competitive ability of the host plant not only directly but also indirectly, thereby changing the growth and nutrient uptake of competing plant species.

Non-growing/growing season non-uniform-warming increases precipitation use efficiency but reduces its temporal stability in an alpine meadow

There are still uncertainties on the impacts of season-non-uniform-warming on plant precipitation use efficiency (PUE) and its temporal stability (PUE_stability) in alpine areas. Here, we examined the changes of PUE and PUE_stability under two scenes of non-growing/growing season non-uniform-warming (i.e., GLNG: growing-season-warming lower than non-growing-season-warming; GHNG: growing-season-warming higher than non-growing-season-warming) based on a five-year non-uniform-warming of non-growing/growing season experiment. The GLNG treatment increased PUE by 38.70% and reduced PUE_stability by 50.47%, but the GHNG treatment did not change PUE and PUE_stability. This finding was mainly due to the fact that the GLNG treatment had stronger influences on aboveground biomass (AGB), non-growing-season soil moisture (SM_NG), temporal stability of AGB (AGB_stability), temporal stability of non-growing-season air temperature (T _{a_NG_stability}), temporal stability of growing-season vapor pressure deficit (VPD_{G_stability}) and temporal stability of start of growing-season (SGS_stability). Therefore, the warming scene with a higher non-growing-season-warming can have greater influences on PUE and PUE_stability than the warming scene with a higher growing-season-warming, and there were possibly trade-offs between plant PUE and PUE_stability under season-non-uniform-warming scenes in the alpine meadow.

Geo-Distribution Patterns of Soil Fungal Community of Pennisetum flaccidum in Tibet

Pennisetum flaccidum can be used as a pioneer species for the restoration of degraded grasslands and as a high-quality forage for local yak and sheep in alpine regions. The geographical distribution pattern of soil fungal community can modify that of P. flaccidum. A field survey along 32 sampling sites was conducted to explore the geo-distribution patterns of soil fungal community of P. flaccidum in Tibet. Soil fungal species, phylogenetic and function diversity generally had a closer correlation with longitude/elevation than latitude. The geo-distribution patterns of soil fungal species, phylogenetic and function diversity varied with soil depth. Soil fungal species, phylogenetic and function diversity had dissimilar geo-distribution patterns. Precipitation had stronger impacts on total abundance, species α-diversity, phylogenetic α-diversity, and function β-diversity than temperature for both topsoil (0-10 cm depth) and subtopsoil (10-20 cm depth). Furthermore, precipitation had stronger impacts on function α-diversity for topsoil, species β-diversity for topsoil, and phylogenetic β-diversity for subtopsoil than temperature. The combination of species, phylogenetic and function diversity can better reflect geo-distribution patterns of soil fungal community. Compared to global warming, the impact of precipitation change on the variation in soil fungal community of P. flaccidum should be given more attention.

Climate Controls on the Spatial Variability of Vegetation Greenup Rate across Ecosystems in Northern Hemisphere

Variations in individual phenological events in response to global change have received considerable attentions. However, the development of phenological stages is relatively neglected, especially based on in situ observation data. In this study, the rate of vegetation greenup (Vgreenup) across the Northern Hemisphere was examined for different plant functional types (PFTs) by using eddy covariance flux data from 40 sites (417 site-years). Then, the controls of climatic variables on the spatial distribution of Vgreenup across PFTs were further investigated. The mean Vgreenup was 0.22 ± 0.11 g C m^-2 day^-2 across all sites, with the largest and lowest values observed in cropland and evergreen needle-leaf forest, respectively. A strong latitude dependence by Vgreenup was observed in both Europe and North America. The spatial variations of Vgreenup were jointly regulated by the duration of greenup (Dgreenup) and the amplitude of greenup (Agreenup). However, the predominant factor was Dgreenup in Europe, which changed to Agreenup in North America. Spring climatic factors exerted significant influences on the spatial distribution of Vgreenup across PFTs. Specifically, increasing temperature tended to shorten Dgreenup and promote Agreenup simultaneously, resulting in an acceleration of Vgreenup. Dryness had a depression effect on Vgreenup for the whole study area, as exhibited by a lower Vgreenup with increasing vapor pressure deficit or decreasing soil moisture. However, Vgreenup in North America was only significantly and positively correlated with temperature. Without the limitation of other climatic factors, the temperature sensitivity of Vgreenup was higher in North America (0.021 g C m^-2 day^-2 °C^-1) than in Europe (0.015 g C m^-2 day^-2 °C^-1). This study provides new cognitions for Vgreenup dynamics from in situ observations in complement to satellite observations, which can improve our understanding of terrestrial carbon cycles.

Modeling Nutrition Quality and Storage of Forage Using Climate Data and Normalized-Difference Vegetation Index in Alpine Grasslands

Quantifying forage nutritional quality and pool at various spatial and temporal scales are major challenges in quantifying global nitrogen and phosphorus cycles, and the carrying capacity of grasslands. In this study, we modeled forage nutrition quality and storage using climate data under fencing conditions, and using climate data and a growing-season maximum normalized-difference vegetation index under grazing conditions based on four different methods (i.e., multiple linear regression, random-forest models, support-vector machines and recursive-regression trees) in the alpine grasslands of Tibet. Our results implied that random-forest models can have greater potential ability in modeling forage nutrition quality and storage than the other three methods. The relative biases between simulated nutritional quality using random-forest models and the observed nutritional quality, and between simulated nutrition storage using random-forest models and the observed nutrition storage, were lower than 2.00% and 6.00%, respectively. The RMSE between simulated nutrition quality using random-forest models and the observed nutrition quality, and between simulated nutrition storage using random-forest models and the observed nutrition storage, were no more than 0.99% and 4.50 g m−2, respectively. Therefore, random-forest models based on climate data and/or the normalized-difference vegetation index can be used to model forage nutrition quality and storage in the alpine grasslands of Tibet.

Characteristics of Picea neoveitchii tree growth in mountain areas of central China: insights from isotopic compositions and satellite-derived indices

Leaf nitrogen (N) status and stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) were used to study environmental factors that control mountain individuals of Picea neoveitchii trees, a coniferous species endemic and endangered in China. From May to September 2016, we carried out observations at four different altitude locations extending southeast of Daba Mountain in western Hubei Province. Needle-shaped leaf δ¹³C was positively correlated with needle N and C content calculated from the needle area (N_area and C_area content), needle δ¹⁵N, needle mass, and leaf mass per area (LMA), respectively. Needle δ¹⁵N was also positively correlated with monthly temperature and precipitation for the current month and last month. The seasonal normalised difference vegetation index (NDVI) was highest in June at the lowest altitude and August at the highest altitude. We found that N availability as an important driving factor for tree growth is controlled by surface soil temperature, while in summer, air temperatures above 23 °C exceed the physiological threshold of trees and limit the growth of trees. We concluded that the negative effect of higher temperature on tree growth is greater than the positive effect of higher nitrogen.

Asymmetrical warming of growing/non-growing season increases soil respiration during growing season in an alpine meadow

Soil respiration (R_s) is an important carbon flux in the global carbon cycle, and understanding the influence of global warming on R_s is critical for precise prediction future climate change. Actually, global warming is expected to be seasonally asymmetric, however, it is still unclear on the response of R_s to asymmetrical warming of growing/non-growing season in alpine regions. In this study, an experiment with asymmetrical warming of growing/non-growing season (including three treatments, CK: control; GLNG: warming magnitude of growing season lower than non-growing season; GHNG: warming magnitude of growing season higher than non-growing season) was performed in an alpine meadow of the Northern Tibet since June 2015. The 'GLNG' and 'GHNG' treatments increased mean R_s by 71.22% (1.89 μmol CO₂ m^-2 s^-1) and 34.32% (0.91 μmol CO₂ m^-2 s^-1) during growing season in 2019, respectively. However, the 'GLNG' and 'GHNG' treatments did not significantly affect mean R_s during growing season in 2015, 2016, 2017 and 2018, respectively. The variation coefficient of growing season mean R_s was 32.95% under the CK treatment in 2015-2019. Therefore, warming may have a lagging effect on R_s. The warming scene with a greater warming during non-growing season may have a stronger effect on R_s than the warming scene with a greater warming during growing season. Inter-annual variation of R_s may be greater than the warming effect on R_s in alpine meadows.

Stability response of alpine meadow communities to temperature and precipitation changes on the Northern Tibetan Plateau

Biomass temporal stability plays a key role in maintaining sustainable ecosystem functions and services of grasslands, and climate change has exerted a profound impact on plant biomass. However, it remains unclear how the community biomass stability in alpine meadows responds to changes in some climate factors (e.g., temperature and precipitation). Long‐term field aboveground biomass monitoring was conducted in four alpine meadows (Haiyan [HY], Henan [HN], Gande [GD], and Qumalai [QML]) on the Qinghai‐Tibet Plateau. We found that climate factors and ecological factors together affected the community biomass stability and only the stability of HY had a significant decrease over the study period. The community biomass stability at each site was positively correlated with both the stability of the dominant functional group and functional groups asynchrony. The effect of dominant functional groups on community stability decreased with the increase of the effect of functional groups asynchrony on community stability and there may be a ‘trade‐off’ relationship between the effects of these two factors on community stability. Climatic factors directly or indirectly affect community biomass stability by influencing the stability of the dominant functional group or functional groups asynchrony. Air temperature and precipitation indirectly affected the community stability of HY and HN, but air temperature in the growing season and nongrowing season had direct negative and direct positive effects on the community stability of GD and QML, respectively. The underlying mechanisms varied between community composition and local climate conditions. Our findings highlighted the role of dominant functional group and functional groups asynchrony in maintaining community biomass stability in alpine meadows and we highlighted the importance of the environmental context when exploring the stability influence mechanism. Studies of community stability in alpine meadows along with different precipitation and temperature gradients are needed to improve our comprehensive understanding of the mechanisms controlling alpine meadow stability.

Differential Responses of Plant Primary Productivity to Nutrient Addition in Natural and Restored Alpine Grasslands in the Qinghai Lake Basin

Climate, land-use changes, and nitrogen (N) deposition strongly impact plant primary productivity, particularly in alpine grassland ecosystems. In this study, the differential responses of plant community primary productivity to N and phosphorus (P) nutrient application were investigated in the natural (NG) and "Grain for Green" restored (RG) alpine grasslands by a continuous 3-year experiment in the Qinghai Lake Basin. N addition only significantly promoted plant aboveground biomass (AGB) by 42% and had no significant effect on belowground biomass (BGB) and total biomass (TB) in NG. In comparison with NG, N addition elevated AGB and BGB concurrently in RG by 138% and 24%, respectively, which further significantly increased TB by 41% in RG. Meanwhile, N addition significantly decreased BGB and the AGB ratio (R/S) both in NG and RG. Compared with N addition, P addition did not perform an evident effect on plant biomass parameters. Additionally, AGB was merely negatively influenced by growing season temperatures (GST) under the N addition treatment in NG. AGB was negatively associated with GST but positively related to growing season precipitation (GSP) in RG. By contrast, changes in the R/S ratio in RG were positively correlated with GST and negatively related to GSP. In sum, the results revealed that plant community biomass exhibited convergent (AGB and R/S) and divergent (BGB and TB) responses to N addition between NG and RG. In addition, the outcomes suggested that climate warming would enhance plant biomass allocation to belowground under ongoing N deposition, and indicated the significance of precipitation for plant growth and AGB accumulation in this restored alpine grassland ecosystem.

Permafrost response to temperature rise in carbon and nutrient cycling: Effects from habitat-specific conditions and factors of warming

Permafrost is experiencing climate warming at a rate that is two times faster than the rest of the Earth's surface. However, it is still lack of a quantitative basis for predicting the functional stability of permafrost ecosystems in carbon (C) and nutrient cycling. We compiled the data of 708 observations from 89 air-warming experiments in the Northern Hemisphere and characterized the general effects of temperature increase on permafrost C exchange and balance, biomass production, microbial biomass, soil nutrients, and vegetation N dynamics through a meta-analysis. Also, an investigation was made on how responses might change with habitat-specific (e.g., plant functional groups and soil moisture status) conditions and warming variables (e.g., warming phases, levels, and timing). The net ecosystem C exchange (NEE) was found to be downregulated by warming as a result of a stronger sensitivity to warming in respiration (15.6%) than in photosynthesis (6.2%). Vegetation usually responded to warming by investing more C to the belowground, as belowground biomass increased much more (30.1%) than aboveground biomass (2.9%). Warming had a minor effect on microbial biomass. Warming increased soil ammonium and nitrate concentrations. What's more, a synthesis of 70 observations from 11 herbs and 9 shrubs revealed a 2.5% decline of N in green leaves. Compared with herbs, shrubs had a stronger response to respiration and had a decline in green leaf N to a greater extent. Not only in dry condition did green leaf N decline with warming but also in wet conditions. Warming in nongrowing seasons would negatively affect soil water, C uptake, and biomass production during growing seasons. Permafrost C loss and vegetation N decline may increase with warming levels and timing. Overall, these findings suggest that besides a positive C cycling-climate feedback, there will be a negative feedback between permafrost nutrient cycling and climate warming.

Asymmetrical Warming Between Elevations May Result in Similar Plant Community Composition Between Elevations in Alpine Grasslands

Asymmetrical warming between elevations is a common phenomenon and warming magnitude increases with increasing elevations on the Tibetan Plateau, which in turn may reduce temperature differences between elevations. However, it is still unclear how such phenomenon will affect plant community composition in alpine grasslands on the Tibetan Plateau. Therefore, in this study, we performed an experiment at three elevations (i.e., 4,300 m, 4,500 m, and 4,700 m) in alpine grasslands, the Northern Tibetan Plateau since May, 2010. Open top chambers were established at the elevations 4,500 m and 4,700 m. Plant species and phylogenetic composition were investigated in August, 2011–2019. There were no significant differences in plant species and phylogenetic composition, environmental temperature and moisture conditions between the elevation 4,300 m under non-warming conditions and the elevation 4,500 m under warming conditions in 2019. There were also no significant differences in plant species composition, environmental temperature and moisture conditions between the elevation 4,500 m under non-warming conditions and the elevation 4,700 m under warming conditions in 2019. Therefore, the narrowing temperature differences between elevations may result in plant community composition between elevations tending to be similar in alpine grasslands on the Tibetan Plateau under future elevational asymmetrical warming.

Nitrogen deposition magnifies the positive response of plant community production to precipitation: Ammonium to nitrate ratio matters

The impacts of atmospheric nitrogen (N) deposition amount on plant communities have been extensively explored. However, the responses of plant communities to the ratio of reduced (NH₄⁺-N) and oxidized (NO₃^--N) forms remain unclear in natural ecosystems. A field N enrichment experiment using different NH₄⁺-N/NO₃^--N ratios was conducted in a natural semi-arid grassland in northern China from 2014 to 2019. Nitrogen addition tended to reduce plant species richness and significantly enhanced plant community aboveground net primary productivity (ANPP). Neither plant species richness nor plant ANPP at species and community levels was significantly affected by NH₄⁺-N/NO₃^--N ratios. At the plant functional group level, ANPP of grasses was not significantly affected by the NH₄⁺-N/NO₃^--N ratios examined, whereas ANPP of forbs was significantly increased at 1:1 NH₄⁺-N/NO₃^--N. Regardless of N supplied using the different ratios of NH₄⁺-N/NO₃^--N examined, plant community ANPP was positively associated with growing season precipitation. Unexpectedly, 1:1 NH₄⁺-N/NO₃^--N (NH₄NO₃) significantly improved the positive response of plant community ANPP to precipitation (it had the biggest slope value). Our results suggest that precipitation was the main determinant of the influence of NH₄⁺-N/NO₃^--N ratios on plant community ANPP. Therefore, the results of our study showed that without referring to NH₄⁺-N/NO₃^--N ratios and precipitation, models using NH₄NO₃ enrichment may overestimate the positive effect of atmospheric N deposition on ecosystem ANPP in semi-arid ecozones.

Effect of precipitation on respiration of different reconstructed soils

Respiration and hydrothermal characteristics of four reconstructed soils in barren gravel land at a site in Shaanxi Province were monitored before, during, and after two precipitation events. Both precipitation events significantly reduced soil temperature but there were great fluctuations in temperature after the second precipitation event. Moreover, precipitation increased the moisture content of the reconstructed soils. Before the first precipitation event, the soil volumetric water content was relatively stable, while it gradually decreased before the second precipitation event. The first precipitation event significantly stimulated the respiration rate for all reconstructed soils, while the second precipitation event generally inhibited it, especially during the precipitation event. The key factors influencing respiration for different reconstructed soils were different between the precipitation events. When soil volumetric water content showed persistent variation before precipitation, soil moisture was the most influential factor. In contrast, if water content was stable, soil temperature was more influential. Soil moisture and temperature jointly influenced soil respiration before, during, and after the precipitation event, while soil moisture was always the most influential factor after precipitation.

Responses of vegetation activity to the daytime and nighttime warming in Northwest China

Though temperature over the past three decades has shown an asynchronous warming trend between daytime and nighttime, the response of vegetation activity to such non-uniform warming is still not very clear. In this study, the least squares linear trend analysis and geographic information system spatial analysis were conducted to analyze the spatiotemporal patterns of the daytime and nighttime warming based on the daily temperature data from 1982 to 2015 in Northwest China. The normalized difference vegetation index (NDVI) from Global Inventory Monitoring and Modeling System and vegetation type data were used to investigate the responses of vegetation activity to the daytime and nighttime warming using the partial correlation analysis. Our results suggested that (1) there was a very significant increasing trend in both daytime and nighttime temperatures in Northwest China from 1982 to 2015; night temperatures increased about 1.2 times faster than daytime temperatures, showing diurnal asymmetric warming; (2) the responses of vegetation activity to daytime and nighttime warming in Northwest China showed a distinct spatial pattern; the change in night temperatures had a more significant (positive in most regions) effect on vegetation; (3) various types of vegetation responded differently to asymmetric daytime and nighttime warming. Grassland NDVI, broad-leaved, and coniferous forest NDVI significantly responded to daytime warming. Shrub NDVI and desert NDVI significantly responded to night warming. These findings can deepen the understanding of the effects of the daytime and nighttime warming on vegetation activities in arid regions in the context of the current asymmetric warming.

Impact of diurnal unsymmetrical warming on soil respiration in an agroecological system of the Lhasa region

Purpose

The impact of diurnal unsymmetrical rise in temperature on soil respiration (R_s) is not fully understood; thus, we explored such a warming influence on R_s in an agroecological system of the Lhasa.

Materials and methods

A field warming experiment (C: control; DW: daytime warming; NW: nighttime warming; DW+NW: daytime plus nighttime warming) was carried out in a naked barley ecological system.

Results and discussion

The DW, NW and DW+NW treatments dramatically increased soil temperature and decreased soil moisture but did not markedly modify R_s. The effects of DW and NW on soil respiration sensitivity (Q₁₀) during the daytime and nighttime were different; they had no effects on daytime Q₁₀ of R_s, but a significant inhibitory effect on nighttime Q₁₀ of R_s.

Conclusions

A diurnal unsymmetrical rise in temperature brought about different results for the Q₁₀ of R_s but did not cause changes in R_s under different experimental treatments in agroecological systems of the Lhasa.