Microbial carbon use efficiency in different ecosystems: A meta-analysis based on a biogeochemical equilibrium model

Soil microbial carbon use efficiency (CUE) is a crucial parameter that can be used to evaluate the partitioning of soil carbon (C) between microbial growth and respiration. However, general patterns of microbial CUE among terrestrial ecosystems (e.g., farmland, grassland, and forest) remain controversial. To address this knowledge gap, data from 41 study sites (n = 197 soil samples) including 58 farmlands, 95 forests, and 44 grasslands were collected and analyzed to estimate microbial CUEs using a biogeochemical equilibrium model. We also evaluated the metabolic limitations of microbial growth using an enzyme vector model and the drivers of CUE across different ecosystems. The CUEs obtained from soils of farmland, forest, and grassland ecosystems were significantly different with means of 0.39, 0.33, and 0.42, respectively, illustrating that grassland soils exhibited higher microbial C sequestration potentials (p < .05). Microbial metabolic limitations were also distinct in these ecosystems, and carbon limitation was dominant exhibiting strong negative effects on CUE. Exoenzyme stoichiometry played a greater role in impacting CUE values than soil elemental stoichiometry within each ecosystem. Specifically, soil exoenzymatic ratios of C:phosphorus (P) acquisition activities (EEAC:P ) and the exoenzymatic ratio of C:nitrogen (N) acquisition activities (EEAC:N ) imparted strong negative effects on soil microbial CUE in grassland and forest ecosystems, respectively. But in farmland soils, EEAC:P exhibited greater positive effects, showing that resource constraints could regulate microbial resource allocation with discriminating patterns across terrestrial ecosystems. Furthermore, mean annual temperature (MAT) rather than mean annual precipitation (MAP) was a critical climate factor affecting CUE, and soil pH as a major factor remained positive to drive the changes in microbial CUE within ecosystems. This research illustrates a conceptual framework of microbial CUEs in terrestrial ecosystems and provides the theoretical evidence to improve soil microbial C sequestration capacity in response to global change.

Relationship between Satsuma mandarin yield and meteorological factors in Shizuoka and distribution of alternative flowering indices in Mikkabi production area, Japan

BACKGROUND

Biennial bearing, the biggest factor destabilizing citrus fruit production, is primarily attributed to unfavorable weather changes. However, the relationship between the yield of Satsuma mandarin for more than 50 years and the weather remains poorly understood. Therefore, the present study investigated the effect of meteorological factors, such as average temperature, precipitation, and sunshine duration, on the yield of Satsuma mandarin in Shizuoka Prefecture, Japan. We analyzed fruit production statistics and meteorological observation data from 1961 to 2018. Furthermore, we investigated the alternative indices of flowering and sprouting in each of the 20 districts of Mikkabi Town, Hamamatsu City, Shizuoka Prefecture.

RESULTS

We detected a positive correlation between the average temperatures in March and February and yields in Shizuoka and Hamamatsu Cities. All alternative indices, except flowering in the early-maturing cultivar, were significantly lower in the southern than in the northern districts (P < 0.01 or 0.05). Particularly, the alternative index of flowering in the late-maturing cultivar was significantly different between the northern and southern districts (P < 0.01). Furthermore, we found a negative correlation between the mean annual temperature or average temperature from January to April and alternative indices of flowering and sprouting.

CONCLUSION

The findings of our field survey in Mikkabi production area suggest that mean annual temperature may affect the biennial bearing of Satsuma mandarin. © 2023 Society of Chemical Industry.

Analyzing trait-climate relationships within and among taxa using machine learning and herbarium specimens

PREMISE

Continental-scale leaf trait studies can help explain how plants survive in different environments, but large data sets are costly to assemble at this scale. Automating the measurement of digitized herbarium collections could rapidly expand the data available to such studies. We used machine learning to identify and measure leaves from existing, digitized herbarium specimens. The process was developed, validated, and applied to analyses of relationships between leaf size and climate within and among species for two genera: Syzygium (Myrtaceae) and Ficus (Moraceae).

METHODS

Convolutional neural network (CNN) models were used to detect and measure complete leaves in images. Predictions of a model trained with a set of 35 randomly selected images and a second model trained with 35 user-selected images were compared using a set of 50 labeled validation images. The validated models were then applied to 1227 Syzygium and 2595 Ficus specimens digitized by the National Herbarium of New South Wales, Australia. Leaf area measurements were made for each genus and used to examine links between leaf size and climate.

RESULTS

The user-selected training method for Syzygium found more leaves (9347 vs. 8423) using fewer training masks (218 vs. 225), and found leaves with a greater range of sizes than the random image training method. Within each genus, leaf size was positively associated with temperature and rainfall, consistent with previous observations. However, within species, the associations between leaf size and environmental variables were weaker.

CONCLUSIONS

CNNs detected and measured leaves with levels of accuracy useful for trait extraction and analysis and illustrate the potential for machine learning of herbarium specimens to massively increase global leaf trait data sets. Within-species relationships were weak, suggesting that population history and gene flow have a strong effect at this level. Herbarium specimens and machine learning could expand sampling of trait data within many species, offering new insights into trait evolution.

Topoclimate effect on treeline elevation depends on the regional framework: A contrast between Southern Alps (New Zealand) and Apennines (Italy) forests

Deciphering the spatial patterns of alpine treelines is critical for understanding the ecosystem processes involved in the persistence of tree species and their altitudinal limit. Treelines are thought to be controlled by temperature, and other environmental variables but they have rarely been investigated in regions with different land-use change legacies. Here, we systematically investigated treeline elevation in the Apennines (Italy) and Southern Alps (New Zealand) with contrasting human history but similar biogeographic trajectories, intending to identify distinct drivers that affect their current elevation and highlight their respective peculiarities. Over 3622 km of Apennines, treeline elevation was assessed in 302 mountain peaks and in 294 peaks along 4504 km of Southern Alps. The major difference between the Southern Alps and Apennines treeline limit is associated with their mountain aspects. In the Southern Alps, the scarcely anthropized Nothofagus treeline elevation was higher on the warmer equator-facing slopes than on the pole-facing ones. Contrary to what would be expected based on temperature limitation, the elevation of Fagus sylvatica treelines in the Apennines was higher on colder, pole-facing slopes than on human-shaped equator-facing, warmer mountainsides. Pervasive positive correlations were found between treeline elevation and temperature in the Southern Alps but not in the Apennines. While the position of the Fagus and Nothofagus treelines converge on similar isotherms of annual average temperature, a striking isothermal difference between the temperatures of the hottest month on which the two taxonomic groups grow exists. We conclude that actual treeline elevation reflects the ecological processes driven by a combination of local-scale topoclimatic conditions, and human disturbance legacy. Predicting dynamic processes affecting current and future alpine treeline position requires further insight into the modulating influences that are currently understood at a regional scale.

Spatial Distribution and Estimation Model of Soil pH in Coastal Eastern China

Soil pH is an essential indicator for assessing soil quality and soil health. In this study, based on the Chinese farmland soil survey dataset and meteorological dataset, the spatial distribution characteristics of soil pH in coastal eastern China were analyzed using kriging interpolation. The relationships between hydrothermal conditions and soil pH were explored using regression analysis with mean annual precipitation (MAP), mean annual temperature (MAT), the ratio of precipitation to temperature (P/T), and the product of precipitation and temperature (P*T) as the main explanatory variables. Based on this, a model that can rapidly estimate soil pH was established. The results showed that: (a) The spatial heterogeneity of soil pH in coastal eastern China was obvious, with the values gradually decreasing from north to south, ranging from 4.5 to 8.5; (b) soil pH was significantly correlated with all explanatory variables at the 0.01 level. In general, MAP was the main factor affecting soil pH (r = -0.7244), followed by P/T (r = -0.6007). In the regions with MAP < 800 mm, soil pH was negatively correlated with MAP (r = -0.4631) and P/T (r = -0.7041), respectively, and positively correlated with MAT (r = 0.6093) and P*T (r = 0.3951), respectively. In the regions with MAP > 800 mm, soil pH was negatively correlated with MAP (r = -0.6651), MAT (r = -0.5047), P/T (r = -0.3268), and P*T (r = -0.5808), respectively. (c) The estimation model of soil pH was: y = 23.4572 - 6.3930 × lgMAP + 0.1312 × MAT. It has been verified to have a high accuracy (r = 0.7743, p < 0.01). The mean error, the mean absolute error, and the root mean square error were 0.0450, 0.5300, and 0.7193, respectively. It provides a new path for rapid estimation of the regional soil pH, which is important for improving the management of agricultural production and slowing down soil degradation.

Interannual variation in rainfall modulates temperature sensitivity of carbon allocation and flux in a tropical montane wet forest

Tropical forests exert a disproportionately large influence on terrestrial carbon (C) balance but projecting the effects of climate change on C cycling in tropical forests remains uncertain. Reducing this uncertainty requires improved quantification of the independent and interactive effects of variable and changing temperature and precipitation regimes on C inputs to, cycling within and loss from tropical forests. Here, we quantified aboveground litterfall and soil-surface CO₂ efflux ("soil respiration"; F_S ) in nine plots organized across a highly constrained 5.2°C mean annual temperature (MAT) gradient in tropical montane wet forest. We used five consecutive years of these measurements, during which annual rainfall (AR) steadily increased, in order to: (a) estimate total belowground C flux (TBCF); (b) examine how interannual variation in AR alters the apparent temperature dependency (Q₁₀ ) of above- and belowground C fluxes; and (c) quantify stand-level C allocation responses to MAT and AR. Averaged across all years, F_S , litterfall, and TBCF increased positively and linearly with MAT, which accounted for 49, 47, and 46% of flux rate variation, respectively. Rising AR lowered TBCF and F_S , but increased litterfall, with patterns representing interacting responses to declining light. The Q₁₀ of F_S , litterfall, and TBCF all decreased with increasing AR, with peak sensitivity to MAT in the driest year and lowest sensitivity in the wettest. These findings support the conclusion that for this tropical montane wet forest, variations in light, water, and nutrient availability interact to strongly influence productivity (litterfall+TBCF), the sensitivity of above- and belowground C fluxes to rising MAT (Q₁₀ of F_S , litterfall, and TBCF), and C allocation patterns (TBCF:[litterfall+TBCF]).

Mean annual temperature influences local fine root proliferation and arbuscular mycorrhizal colonization in a tropical wet forest

Mean annual temperature (MAT) is an influential climate factor affecting the bioavailability of growth-limiting nutrients nitrogen (N) and phosphorus (P). In tropical montane wet forests, warmer MAT drives higher N bioavailability, while patterns of P availability are inconsistent across MAT. Two important nutrient acquisition strategies, fine root proliferation into bulk soil and root association with arbuscular mycorrhizal fungi, are dependent on C availability to the plant via primary production. The case study presented here tests whether variation in bulk soil N bioavailability across a tropical montane wet forest elevation gradient (5.2°C MAT range) influences (a) morphology fine root proliferation into soil patches with elevated N, P, and N+P relative to background soil and (b) arbuscular mycorrhizal fungal (AMF) colonization of fine roots in patches. We created a fully factorial fertilized root ingrowth core design (N, P, N+P, unfertilized control) representing soil patches with elevated N and P bioavailability relative to background bulk soil. Our results show that percent AMF colonization of roots increased with MAT (r 2 = .19, p = .004), but did not respond to fertilization treatments. Fine root length (FRL), a proxy for root foraging, increased with MAT in N+P-fertilized patches only (p = .02), while other fine root morphological parameters did not respond to the gradient or fertilized patches. We conclude that in N-rich, fine root elongation into areas with elevated N and P declines while AMF abundance increases with MAT. These results indicate a tradeoff between P acquisition strategies occurring with changing N bioavailability, which may be influenced by higher C availability with warmer MAT.

Impact of Mean Annual Temperature on Nutrient Availability in a Tropical Montane Wet Forest

Despite growing understanding of how rising temperatures affect carbon cycling, the impact of long-term and whole forest warming on the suite of essential and potentially limiting nutrients remains understudied, particularly for elements other than N and P. Whole ecosystem warming experiments are limited, environmental gradients are often confounded by variation in factors other than temperature, and few studies have been conducted in the tropics. We examined litterfall, live foliar nutrient content, foliar nutrient resorption efficiency (NRE), nutrient return, and foliar nutrient use efficiency (NUE) of total litterfall and live foliage of two dominant trees to test hypotheses about how increasing mean annual temperature (MAT) impacts the availability and ecological stoichiometry of C, N, P, K, Ca, Mg, Mn, Fe, Zn, and Cu in tropical montane wet forests located along a 5.2°C gradient in Hawaii. Live foliage responded to increasing MAT with increased N and K concentrations, decreased C and Mn concentrations, and no detectable change in P concentration or in foliar NRE. Increases in MAT increased nutrient return via litterfall for N, K, Mg, and Zn and foliar NUE for Mn and Cu, while decreasing nutrient return for Cu and foliar NUE for K. The N:P of litterfall and live foliage increased with MAT, while there was no detectable effect of MAT on C:P. The ratio of live foliar N or P to base cations and micronutrients was variable across elements and species. Increased MAT resulted in declining N:K and P:K for one species, while only P:K declined for the other. N:Ca and N:Mn increased with MAT for both species, while N:Mg increased for one and P:Mn increased for the other species. Overall, results from this study suggest that rising MAT in tropical montane wet forest: (i) increases plant productivity and the cycling and availability of N, K, Mg, and Zn; (ii) decreases the cycling and availability of Mn and Cu; (iii) has little direct effect on P, Ca or Fe; and (iv) affects ecological stoichiometry in ways that may exacerbate P-as well as other base cation and micronutrient - limitations to tropical montane forest productivity.

Belowground biomass of alpine shrublands across the northeast Tibetan Plateau

Although belowground biomass (BGB) plays an important role in global cycling, the storage of BGB and climatic effects on it are remaining unclear. With data from 49 sites, we aimed to investigate BGB and its climatic controls in alpine shrublands in the Tibetan Plateau. Our study showed that the BGB (both grass-layer and shrub-layer biomass) storage in the alpine shrublands was 67.24 Tg, and the mean BGB density and shrublands area were 1,567.38 g/m2 and 4.29 × 104 km2, respectively. Shrub layer had a larger BGB stock and accounted for 66% of total BGB this area, while only 34% was accumulated in the grass layer. BGB of the grass layer in the Tibetan Plateau shrublands was larger than that of Tibetan alpine grasslands, indicating that shrubland ecosystem played a critical importance role in carbon cycle on the Tibetan Plateau. The BGB in the grass layer and shrub layer demonstrated different correlations with climatic factors. Specifically, the effects from mean annual temperature on shrub-layer BGB were not significant, similarly to the relationship between mean annual precipitation and grass-layer BGB. But shrub-layer BGB had a significantly positive relationship with mean annual precipitation (p < .05), while grass-layer BGB showed a trend of decrease with increasing mean annual temperature (p < .05). Consequently, the actual and potential increases of BGB varied due to different increases of mean annual precipitation and temperature among different areas of the Tibetan Plateau. Therefore, in the warmer and wetter scenario, due to contrary relationships from mean annual precipitation and temperature on shrub-layer BGB and grass-layer BGB, it is necessary to conduct a long-term monitoring about dynamic changes to increase the precision of assessment of BGB carbon sequestration in the Tibetan Plateau alpine shrublands.

A rural-urban latitudinal study of the distributions of Culex quinquefasciatus and Culex pipiens bioforms in their southernmost sympatric fringe

Mosquitoes grouped in the complex Culex pipiens L. (Diptera: Culicidae) are important vectors of medical and veterinary diseases. In the South American sympatric region, Cx. pipiens and Culex quinquefasciatus Say coexist and potentially hybridize. To identify key drivers of their geographical distribution, mosquito immatures were collected from flower vases of eight urban/rural cemetery pairs within a 5° latitudinal transect along Buenos Aires Province, Argentina. The specimens were identified by molecular methods and their relative proportion modelled as a function of environmental variables. At the beginning of the warm season, northern and southern cemeteries presented exclusively Cx. quinquefasciatus and Cx. pipiens, respectively, with different proportions of both at mid latitudes. By the end of the summer, Cx. quinquefasciatus was present throughout the study area, exclusively in 11 of the 16 cemeteries both rural and urban, whereas Cx. pipiens was predominant only in the southernmost pair. Mean annual temperature, photoperiod variability and time of the season were key drivers of their distributions. All specimens of Cx. pipiens were identified as form molestus and no hybrids were recognized. The reported distribution patterns and the potential absence of Cx. pipiens f. pipiens and hybrids are discussed, along with their implications in disease transmission.

Aboveground carbon sequestration in dry temperate forests varies with climate not fire regime

The storage of carbon in plant tissues and debris has been proposed as a method to offset anthropogenic increases in atmospheric [CO₂ ]. Temperate forests represent significant above-ground carbon (AGC) "sinks" because their relatively fast growth and slow decay rates optimise carbon assimilation. Fire is a common disturbance event in temperate forests globally that should strongly influence AGC because: discrete fires consume above-ground biomass releasing carbon to the atmosphere, and the long-term application of different fire-regimes select for specific plant communities that sequester carbon at different rates. We investigated the latter process by quantifying AGC storage at 104 sites in the Sydney Basin Bioregion, Australia, relative to differences in components of the fire regime: frequency, severity and interfire interval. To predict the potential impacts of future climate change on fire/AGC interactions, we stratified our field sites across gradients of mean annual temperature and precipitation and quantified within- and between-factor interactions between the fire and climate variables. In agreement with previous studies, large trees were the primary AGC sink, accounting for ~70% of carbon at sites. Generalised additive models showed that mean annual temperature was the strongest predictor of AGC storage, with a 54% near-linear decrease predicted across the 6.1°C temperature range experienced at sites. Mean annual precipitation, fire frequency, fire severity and interfire interval were consistently poor predictors of total above-ground storage, although there were some significant relationships with component stocks. Our results show resilience of AGC to frequent and severe wildfire and suggest temperature mediated decreases in forest carbon storage under future climate change predictions.

[Characteristics of soil organic carbon mineralization in low altitude and high altitude forests in Wuyi Mountains, southeastern China]

Examining the variations of soil organic carbon mineralization at different altitudes is crucial for better understanding of soil organic carbon (SOC) dynamics. We selected the low altitude and high altitude broad-leaved forest soils in Wuyi Mountains as the research object, and incubated them under particular annual average temperature (17 and 9 ℃, respectively) in laboratory to investigate the difference of SOC mineralization characteristics. The results showed that the cumulative SOC mineralization had no significant difference between forest soils at low and high altitude in a 126-day incubation period under ambient temperature. Soil organic carbon content of high altitude soil was significantly higher than that from low altitude. The dynamics of SOC mineralization could fitted by the first-order kinetics. Both mineralization potential (C_P) and mineralization rate constant (K) values of two soils had no significant difference, but C_P/SOC value and mineralization ratio were significantly higher at low altitude, indicating that the carbon sequestration capacity of low altitude soil was relatively lower than that of high altitude under ambient temperature. Soil microbial biomass carbon and microbial quotients were significantly higher than that of low altitude with the increase of incubation time, indicating that the ability of microbial carbon assimilation was greater at high altitude. On the other hand, the activities of β-1,4-glucosidase and cellobiohydrolase in high altitude soil were higher, suggesting that more labile carbon would be decomposed by soil microbes. The carbon sequestration capacity and microbial carbon utilization efficiency in high altitude soil would be reduced and thus result in a decline of soil organic carbon storage under the scenarios of climate warming.

A global analysis of parenchyma tissue fractions in secondary xylem of seed plants

Parenchyma is an important tissue in secondary xylem of seed plants, with functions ranging from storage to defence and with effects on the physical and mechanical properties of wood. Currently, we lack a large-scale quantitative analysis of ray parenchyma (RP) and axial parenchyma (AP) tissue fractions. Here, we use data from the literature on AP and RP fractions to investigate the potential relationships of climate and growth form with total ray and axial parenchyma fractions (RAP). We found a 29-fold variation in RAP fraction, which was more strongly related to temperature than with precipitation. Stem succulents had the highest RAP values (mean ± SD: 70.2 ± 22.0%), followed by lianas (50.1 ± 16.3%), angiosperm trees and shrubs (26.3 ± 12.4%), and conifers (7.6 ± 2.6%). Differences in RAP fraction between temperate and tropical angiosperm trees (21.1 ± 7.9% vs 36.2 ± 13.4%, respectively) are due to differences in the AP fraction, which is typically three times higher in tropical than in temperate trees, but not in RP fraction. Our results illustrate that both temperature and growth form are important drivers of RAP fractions. These findings should help pave the way to better understand the various functions of RAP in plants.

Weak tradeoff between xylem safety and xylem-specific hydraulic efficiency across the world's woody plant species

The evolution of lignified xylem allowed for the efficient transport of water under tension, but also exposed the vascular network to the risk of gas emboli and the spread of gas between xylem conduits, thus impeding sap transport to the leaves. A well-known hypothesis proposes that the safety of xylem (its ability to resist embolism formation and spread) should trade off against xylem efficiency (its capacity to transport water). We tested this safety-efficiency hypothesis in branch xylem across 335 angiosperm and 89 gymnosperm species. Safety was considered at three levels: the xylem water potentials where 12%, 50% and 88% of maximal conductivity are lost. Although correlations between safety and efficiency were weak (r(2)  < 0.086), no species had high efficiency and high safety, supporting the idea for a safety-efficiency tradeoff. However, many species had low efficiency and low safety. Species with low efficiency and low safety were weakly associated (r(2)  < 0.02 in most cases) with higher wood density, lower leaf- to sapwood-area and shorter stature. There appears to be no persuasive explanation for the considerable number of species with both low efficiency and low safety. These species represent a real challenge for understanding the evolution of xylem.

Different responses of soil respiration and its components to nitrogen addition among biomes: a meta-analysis

Anthropogenic activities have increased nitrogen (N) deposition by threefold to fivefold over the last century, which may considerably affect soil respiration (Rs). Although numerous individual studies and a few meta-analyses have been conducted, it remains controversial as to how N addition affects Rs and its components [i.e., autotrophic (Ra) and heterotrophic respiration (Rh)]. To reconcile the difference, we conducted a comprehensive meta-analysis of 295 published studies to examine the responses of Rs and its components to N addition in terrestrial ecosystems. We also assessed variations in their responses in relation to ecosystem types, environmental conditions, and experimental duration (DUR). Our results show that N addition significantly increased Rs by 2.0% across all biomes but decreased by 1.44% in forests and increased by 7.84% and 12.4% in grasslands and croplands, respectively (P < 0.05). The differences may largely result from diverse responses of Ra to N addition among biomes with more stimulation of Ra in croplands and grasslands compared with no significant change in forests. Rh exhibited a similar negative response to N addition among biomes except that in croplands, tropical and boreal forests. Methods of partitioning Rs did not induce significant differences in the responses of Ra or Rh to N addition, except that Ra from root exclusion and component integration methods exhibited the opposite responses in temperate forests. The response ratios (RR) of Rs to N addition were positively correlated with mean annual temperature (MAT), with being more significant when MAT was less than 15 °C, but negatively with DUR. In addition, the responses of Rs and its components to N addition largely resulted from the changes in root and microbial biomass and soil C content as indicated by correlation analysis. The response patterns of Rs to N addition as revealed in this study can be benchmarks for future modeling and experimental studies.