The Influence of Spatial Patterns of Soil Moisture on the Grass and Shrub Responses to a Summer Rainstorm in a Chihuahuan Desert Ecotone

The cycling of surface water, energy, nutrients, and carbon is different between semiarid grassland and shrubland ecosystems. Although differences are evident when grasslands are compared to shrublands, the processes that contribute to this transition are more challenging to document. We evaluate how surface redistribution of precipitation and plant responses to the resulting infiltration patterns could contribute to the changes that occur during the transition from grassland to shrubland. We measured soil water potential under grasses (Bouteloua eriopoda), shrubs (Larrea tridentata) and bare soil and changes in plant water relations and gas exchange following a 15 mm summer storm in the grassland–shrubland ecotone at the Sevilleta National Wildlife Refuge in central New Mexico USA. Following the storm, soil water potential (Ψ_s) increased to 30 cm depth beneath both grass and shrub canopies, with the greatest change observed in the top 15 cm of the soil. The increase in Ψ_s was greater beneath grass canopies than beneath shrub canopies. Ψ_s under bare soil increased only to 5 cm depth. The substantial redistribution of rainfall and different rooting depths of the vegetation resulted in high Ψ_s throughout most of the rooting volume of the grasses whereas soil moisture was unchanged throughout a large portion of the shrub rooting volume. Consistent with this pattern, predawn water potential (Ψ_PD) of grasses increased more than 5 MPa to greater than −1 MPa whereas Ψ_PD of shrubs increased to −2.5 MPa, a change of less than 2 MPa. Transpiration increased roughly linearly with Ψ_PD in both grasses and shrubs. In grasses, assimilation was strongly correlated with Ψ_PD whereas there was no relationship in shrubs where assimilation showed no significant response to the pulse of soil moisture following the storm. These data show that preferential redistribution of water to grass canopies enhances transpiration and assimilation by grasses following large summer storms. This process may inhibit shrubland expansion at the ecotone during periods without extreme drought.     

Sodium Hypochlorite Stress in Enterococcus faecalis: Influence of Antecedent Growth Conditions and Induced Proteins

Compared with exponential growing bacteria, carbohydrate-starved cells of Enterococcus faecalis exhibit a high level of resistance to sodium hypochlorite with maximal resistance observed in cultures entering stationary phase. Chloramphenicol treatment, at various stages of growing phase, does not abolish the hypochlorite resistance of starved cells. However, Enterococcus faecalis conditioned by low sodium hypochlorite concentrations does not develop tolerance towards a lethal dose of the disinfectant. Two-dimensional gel analysis shows that protein synthesis is drastically turned off by hypochlorite treatment, whereas synthesis of a few proteins is enhanced by a low concentration of this chemical agent. Received: 5 September 1996 / Accepted: 29 October 1996     

Seasonal Differences in Photochemical Efficiency and Chlorophyll and Carotenoid Contents in Six Mediterranean Shrub Species under Field Conditions

The effects of summer and winter stress on the chlorophyll and carotenoid contents and photosystem 2 efficiency were examined in six Mediterranean scrub species. These six species belong to two different plant functional types: drought semi-deciduous (Halimium halimifolium L., Rosmarinus officinalis L., Erica scoparia L.) and evergreen sclerophylls (Juniperus phoenicea L., Pistacia lentiscus L., Myrtus communis L.). Two sites with different water availability were chosen. In the xerophytic site, despite they belong to two different functional types, R. officinalis and J. phoenicea showed a similar response. These were the most affected species in summer. H. halimifolium showed optimal values of F_v/F_m and non-significant seasonal changes in xanthophyll content. In the mesic site, E. scoparia and M. communis were apparently the most affected species by winter climatic conditions. P. lentiscus presented a pattern similar to H. halimifolium, except for elevated F₀ values. In all the studied species, lutein plus zeaxanthin content was negatively correlated with F_v/F_m in summer and with leaf water potential, thus indicating that the thermal dissipation of energy was a general pattern for all species. Under stress, plant response is more species-specific than dependent on its functional type.     

Long-Time Respiration of Contaminated Soil Samples during Laboratory Storage under Different Conditions

Soil samples have to be stored during transportation and investigation in the laboratory if they are not analyzed directly at the site. Existing standards and investigations give no recommendations on handling and storage of soil materials for (eco-)toxicological investigations. The objective of this investigation was to determine if microbial turnovers and losses of volatile organic compounds mainly cause storage-dependent changes in soil samples. Furthermore, recommendations are given for storage of soil reserve samples for toxicological investigations. During 18 months of storage, the microbial respiration of six highly contaminated soil samples was determined. Physicochemical characteristics, such as contaminant and nutrient content, were analyzed before and after storage. From the investigations it can be concluded that the oxygen consumption depends on the storage temperature, organic matter content, nutrient content, and total content of toxic substances. Based on the results, a flow scheme was derived that could be a useful tool for a sequential approach to determine the storage capacity of contaminated soil samples and sites for toxicological investigations.     

Influence of Soil Moisture on Litter Respiration in the Semiarid Loess Plateau

Understanding the response mechanisms of litter respiration to soil moisture in water-limited semi-arid regions is of vital importance to better understanding the interplay between ecological processes and the local carbon cycle. In situ soil respiration was monitored during 2010–2012 under various conditions (normal litter, no litter, and double litter treatments) in a 30-year-old artificial black locust plantation (Robinia pseudoacacia L.) on the Loess Plateau. Litter respiration with normal and double litter treatments exhibited similar seasonal variation, with the maximum value obtained in summer (0.57 and 1.51 μmol m⁻² s⁻¹ under normal and double litter conditions, respectively) and the minimum in spring (0.27 and 0.69 μmol m⁻² s⁻¹ under normal and double litter conditions, respectively). On average, annual cumulative litter respiration was 115 and 300 g C m⁻² y⁻¹ under normal and double litter conditions, respectively. Using a soil temperature of 17°C as the critical point, the relationship between litter respiration and soil moisture was found to follow quadratic functions well, whereas the determination coefficient was much greater at high soil temperature than at low soil temperature (33–35% vs. 22–24%). Litter respiration was significantly higher in 2010 and 2012 than in 2011 under both normal litter (132–165 g C m⁻² y⁻¹ vs. 48 g C m⁻² y⁻¹) and double litter (389–418 g C m⁻² y⁻¹ vs. 93 g C m⁻² y⁻¹) conditions. Such significant interannual variations were largely ascribed to the differences in summer rainfall. Our study demonstrates that, apart from soil temperature, moisture also has significant influence on litter respiration in semi-arid regions.     

Germination of Echinochloa crus-galli (Barnyard Grass) Seeds under Anaerobic Conditions : Respiration and Response to Metabolic Inhibitors

Echinochloa crus-galli L. Beauv., a rice-field weed, can germinate and grow for extended periods of time in an anaerobic environment. Compared to pea, which does not germinate under anaerobiosis, the evolution of CO₂ in Echinochloa and rice is lower and the peak rate of CO₂ evolution is delayed when germinated without oxygen. The plants studied also differ with respect to their respiration ratio ([CO₂] N₂/[CO₂] air) and metabolism used during the early stages of germination. Echinochloa does not increase its glycolytic rate under anaerobiosis, whereas pentose phosphate pathway activity appears to increase during the first 40 to 50 hours of germination.

Based on its response to metabolic inhibitors (NaF, dinitrophenol, and malonate), anaerobic metabolism in Echinochloa proceeds primarily through glycolysis, with partial operation of the tricarboxylic acid cycle and little or no oxidative phosphorylation. Also, Echinochloa is sensitive to CN during aerobic germination, whereas rice appears to be able to shift to CN-insensitive electron transport. Finally, the effectiveness of cyanide and azide on inhibiting germination of Echinochloa in N₂, but not CO, suggests that cytochrome oxidase is not used to reoxidize pyridine nucleotides in the absence of oxygen. The possible existence of an alternate electron acceptor is discussed.

     

Influence of Shrub Encroachment on the Soil Microbial Community Composition of Remnant Hill Prairies

Hill prairies are remnant grasslands perched on the bluffs of major river valleys, and because their steep slopes make them unsuitable for traditional row crop agriculture, they have some of the lowest levels of anthropogenic disturbance of any prairie ecosystems in the Midwestern USA. However, many decades of fire suppression have allowed for shrub encroachment from the surrounding forests. While shrub encroachment of grasslands can modify soil respiration rates and nutrient storage, it is not known whether shrubs also alter the community composition of soil microorganisms. We conducted transect sampling of nine different hill prairie remnants showing varying degrees of shrub encroachment, and we used DNA-based community profiling (automated ribosomal intergenic spacer analysis) to characterize the composition of bacterial and fungal communities in the open prairie habitat, the shrub-encroached border, and the surrounding forest. While both bacterial and fungal communities showed statistically significant variation across these habitats, their predominant patterns were different. Bacterial communities of forest soils were distinct from those of the open prairie and the shrub-encroached areas, while fungal communities of the open prairie were distinct from those of the forest and the shrub-encroached border. Shrub encroachment significantly altered the community composition of soil fungal communities. Furthermore, fungal communities of heavily encroached prairie remnants more closely resembled those of the surrounding forest than those of lightly encroached prairies. Thus, shrub encroachment can cause soil fungi to shift from a “grassland” community to a “woody” community, with potential consequences for soil processes and plant-microbe interactions.     

Feedbacks of Soil Inoculum of Mycorrhizal Fungi Altered by N Deposition on the Growth of a Native Shrub and an Invasive Annual Grass

Anthropogenic nitrogen (N) deposition causes shifts in vegetation types as well as species composition of arbuscular mycorrhizal (AM) fungi and other soil microorganisms. A greenhouse experiment was done to determine whether there are feedbacks between N-altered soil inoculum and growth of a dominant native shrub and an invasive grass species in southern California. The region is experiencing large-scale loss of Artemisia californica shrublands and replacement by invasive annual grasses under N deposition. Artemisia californica and Bromus madritensis ssp. rubens were grown with soil inoculum from experimental plots in a low N deposition site that had (1) N-fertilized and (2) unfertilized soil used for inoculum, as well as (3) high-N soil inoculum from a site exposed to atmospheric N deposition for four decades. All treatments plus a nonmycorrhizal control were given two levels of N fertilizer solution. A. californica biomass was reduced by each of the three inocula compared to uninoculated controls under at least one of the two N fertilizer solutions. The␣inoculum from the N-deposition site caused the greatest growth depressions. By contrast, B.␣madritensis biomass increased with each of the three inocula under at least one, or both, of the N solutions. The different growth responses of the two plant species may be related to the types of AM fungal colonization. B. madritensis was mainly colonized by a fine mycorrhizal endophyte, while A. californica had primarily coarse endophytes. Furthermore, A. californica had a high level of septate, nonmycorrhizal root endophytes, while B. madritensis overall had low levels of these endophytes. The negative biomass response of A. californica seedlings to high N-deposition inoculum may in part explain its decline; a microbially-mediated negative feedback may occur in this system that causes poor␣seedling growth and establishment of A.␣californica in sites subject to N deposition and B. madritensis invasion.     

Soil Respiration and Belowground Carbon Allocation in Mangrove Forests

Mangrove forests cover large areas of tropical and subtropical coastlines. They provide a wide range of ecosystem services that includes carbon storage in above- and below ground biomass and in soils. Carbon dioxide (CO₂) emissions from soil, or soil respiration is important in the global carbon budget and is sensitive to increasing global temperature. To understand the magnitude of mangrove soil respiration and the influence of forest structure and temperature on the variation in mangrove soil respiration I assessed soil respiration at eleven mangrove sites, ranging from latitude 27°N to 37°S. Mangrove soil respiration was similar to those observed for terrestrial forest soils. Soil respiration was correlated with leaf area index (LAI) and aboveground net primary production (litterfall), which should aid scaling up to regional and global estimates of soil respiration. Using a carbon balance model, total belowground carbon allocation (TBCA) per unit litterfall was similar in tall mangrove forests as observed in terrestrial forests, but in scrub mangrove forests TBCA per unit litter fall was greater than in terrestrial forests, suggesting mangroves allocate a large proportion of their fixed carbon below ground under unfavorable environmental conditions. The response of soil respiration to soil temperature was not a linear function of temperature. At temperatures below 26°C Q10 of mangrove soil respiration was 2.6, similar to that reported for terrestrial forest soils. However in scrub forests soil respiration declined with increasing soil temperature, largely because of reduced canopy cover and enhanced activity of photosynthetic benthic microbial communities.     

草地群落的土壤呼吸

通过土壤呼吸作用向大气释放ＣＯ２是陆地生态系统碳循环的一个最主要的环节,也是人类活动影响下陆地生态系统对大气中ＣＯ２浓度产生影响,从而导致全球气候变化的关键生态学过程,因而成为全球碳循环研究中倍受关注的核心问题［３３,３７,３８］。１土壤呼吸的测定方...     

Water Organic Pollution and Eutrophication Influence Soil Microbial Processes,Increasing Soil Respiration of Estuarine Wetlands: Site Study in Jiuduansha Wetland

Undisturbed natural wetlands are important carbon sinks due to their low soil respiration. When compared with inland alpine wetlands, estuarine wetlands in densely populated areas are subjected to great pressure associated with environmental pollution. However, the effects of water pollution and eutrophication on soil respiration of estuarine and their mechanism have still not been thoroughly investigated. In this study, two representative zones of a tidal wetland located in the upstream and downstream were investigated to determine the effects of water organic pollution and eutrophication on soil respiration of estuarine wetlands and its mechanism. The results showed that eutrophication, which is a result of there being an excess of nutrients including nitrogen and phosphorus, and organic pollutants in the water near Shang shoal located upstream were higher than in downstream Xia shoal. Due to the absorption and interception function of shoals, there to be more nitrogen, phosphorus and organic matter in Shang shoal soil than in Xia shoal. Abundant nitrogen, phosphorus and organic carbon input to soil of Shang shoal promoted reproduction and growth of some highly heterotrophic metabolic microorganisms such as β-Proteobacteria, γ-Proteobacteria and Acidobacteria which is not conducive to carbon sequestration. These results imply that the performance of pollutant interception and purification function of estuarine wetlands may weaken their carbon sequestration function to some extent.     

Soil Ripping and Herbicides Enhance Tree and Shrub Restoration on Stripmines

I evaluated responses by 16 native woody species to differential soil compaction and density of ground cover. The trees and shrubs studied represent sites in southern Illinois that commonly have restrictions to root growth from soil or drainage conditions. The study site was a restored surface coal mine in southern Illinois with a rooting medium compacted by grading and a dense ground cover of pasture species. Soil compaction was alleviated in half the study area before tree planting by mechanically ripping the soil to a depth of 1.2 m. Roots of half the trees and shrubs were dipped in a Terra® slurry before planting, and the ground cover around all planting spots was afterwards sprayed with herbicide. In year 2 after planting the ground cover in half of the unripped and half of the ripped area was further controlled by repeated application of herbicides. Ripping significantly increased height growth of all trees combined and all species individually in each year of the study. Second-year control of ground cover increased height growth of all trees combined and of seven species individually. Some species were damaged by herbicides. Terra® had little evident effect on species performance. Animal damage reduced early survival and growth, especially of Acer (maple) and Cornus (dogwood) species, and later growth of Quercus rubra (red oak). Removal of ground cover with herbicides tended to increase deer browse. Soil ripping, herbicide application, and choosing tree species unattractive to deer can be recommended to increase success in planting trees for forest restoration.     

中国农田土壤呼吸速率及驱动因子

土壤呼吸在全球碳收支中具有重要地位.研究中国典型农业区土壤呼吸的时空格局及影响因素,有助于构建区域尺度土壤呼吸定量评价模型,能够为评估中国乃至全球农业生态系统碳/源汇特征提供依据.本研究整合了2000~2012年中国农田生态系统土壤呼吸的主要研究成果,分析了华南、西南、华北、西北和东北5个典型农业区土壤呼吸的季节变化和区域差异,以及影响土壤呼吸的主要驱动因子.结果表明,5个典型农业区的土壤呼吸均存在明显的季节变化特征;中国农田生态系统年均土壤呼吸速率为(682.8±18.3)g C m?2.5个典型农业区年均土壤呼吸速率大小表现为华南区西南区华北区东北区西北区.全国农业土壤的年呼吸通量为(0.90±0.02)Pg C;水作和旱作两种土地利用类型间土壤呼吸速率差异显著(P0.05),旱作土壤呼吸速率约为水作的1.3倍;不同作物类型间土壤呼吸速率差异显著(P0.05),其排序为棉花玉米大豆水稻小麦;农田土壤呼吸与年均气温、土壤温度、土壤含水量和净初级生产力等影响因素呈显著正相关(P0.01),而与年均降水量的相关性不显著.     

Short- and Long-term Influence of Litter Quality and Quantity on Simulated Heterotrophic Soil Respiration in a Lowland Tropical Forest

Heterotrophic soil respiration (SR_H) alone can contribute up to 50% of total ecosystem respiration in tropical forests. Whereas the abiotic controls of SR_H have been extensively studied, the influence of plant traits is less well characterised. We used field experiments and a modelling approach to test the relative influence of plant traits on SR_H in lowland tropical forest in French Guiana. We measured leaf- and root litter traits for five common tree species and conducted a root decomposition experiment to evaluate the influence of root chemistry on decay rates. We measured SR_H in trenched plots and used our field measurements to parameterize and test the Century model of soil C dynamics. Overall, the Century model performed well in simulating SR_H, and species-specific root decomposition in Century corresponded well to decomposition rates measured in situ. Root litter characterized by low lignin-to-nitrogen ratios decomposed more rapidly than low-quality root litter during the first 6 months. Model runs over different time scales revealed that litter quality substantially influenced SR_H on an annual time-scale by determining the rates of root- and leaf litter decomposition. However, litter mass had an overriding influence on SR_H over the longer term in 20-year model runs. Synthesis Using simple plant trait data to parameterise the Century model, we were able to accurately simulate changes in SR_H in a lowland tropical forest. Our results suggest that this approach could be used to predict changes in tropical soil C dynamics under global change scenarios by including data on changes in plant productivity and C inputs to the soil (for example litterfall and root turnover).     

Soil Respiration along Environmental Gradients in Olympic National Park

Although mountainous landscapes dominate large areas of the Earth, our understanding of how elevation and aspect influence soil respiration in complex mountainous terrain is very limited. Therefore, we measured soil respiration throughout the growing season in 1999 and 2000 at 11 forested sites in Olympic National Park, Washington, USA along elevation-climatic gradients. The study sites ranged from temperate rain forest to alpine forests near tree line. Soil temperature was a significant predictor of soil respiration at all sites, and soil moisture explained additional variability at three sites (R2 from 0.42 to 0.90, P ≤ 0.01). Soil temperatures at the highest-elevation sites were 4.5°C cooler than those at the lowest elevation, but there were no relationships between soil respiration rates at a given temperature and elevation or mean annual temperature that would indicate acclimation of soil respiration to the cooler temperatures at high-elevation sites. Experimental urea additions (1.0 and 2.0 g N m-2 y-1) made at seven of the sites had no consistent effect on soil respiration. Total soil carbon dioxide (CO2) efflux during the growing season (May-September) varied from 0.34 to 0.75 kg C/m2 and was greater at low-elevation sites with warmer soil temperatures and longer growing seasons. Elevation and the length of the frost-free season could both be used to predict growing season (r2 = 0.53) and annual (r2 = 0.81) soil CO2 efflux for the 10 sites located in steep mountainous terrain. Significant correlations also existed with mean annual temperature. These results suggest that warmer soils and a longer snow-free season associated with climatic warming could cause the mountainous ecosystems of the Olympic peninsula to evolve increasing amounts of CO2 from all elevations and aspects.     

草鱼TRAP-PCR反应体系的建立

目的:通过优化草鱼TRAP-PCR反应体系,将新型分子标记-靶位区域扩增多态性(target region amplified polymorphism,TRAP)引用到草鱼遗传多样性研究中。方法:以草鱼DNA为材料,分析了模板DNA、Mg2 、dNTPs、引物浓度,以及循环参数、退火温度对TRAP-PCR扩增结果的影响。结果:确立了稳定性强、重复性好的草鱼TRAP-PCR最佳反应体系和扩增参数:在25μl的PCR反应体系中,含约50ng模板DNA,1UTaq酶,1×PCR缓冲液,2.0mmol/L MgCl2,4种dNTPs各0.2mmol/L,固定引物与随机引物各15pmol;首先使模板在94℃变性3min;然后94℃变性1min,38℃退火1min,72℃延伸lmin进行5个循环;接着94℃变性45s,55℃退火45s,72℃延伸lmin再进行35个循环,最后72℃延伸7min。结论:TRAP-PCR反应体系稳定可靠,该新型分子标记可应用于草鱼遗传多样性研究中。     

Sensitivity of Soil Respiration to Variability in Soil Moisture and Temperature in a Humid Tropical Forest

Precipitation and temperature are important drivers of soil respiration. The role of moisture and temperature are generally explored at seasonal or inter-annual timescales; however, significant variability also occurs on hourly to daily time-scales. We used small (1.54 m²), throughfall exclusion shelters to evaluate the role soil moisture and temperature as temporal controls on soil CO₂ efflux from a humid tropical forest in Puerto Rico. We measured hourly soil CO₂ efflux, temperature and moisture in control and exclusion plots (n = 6) for 6-months. The variance of each time series was analyzed using orthonormal wavelet transformation and Haar-wavelet coherence. We found strong negative coherence between soil moisture and soil respiration in control plots corresponding to a two-day periodicity. Across all plots, there was a significant parabolic relationship between soil moisture and soil CO₂ efflux with peak soil respiration occurring at volumetric soil moisture of approximately 0.375 m³/m³. We additionally found a weak positive coherence between CO₂ and temperature at longer time-scales and a significant positive relationship between soil temperature and CO₂ efflux when the analysis was limited to the control plots. The coherence between CO₂ and both temperature and soil moisture were reduced in exclusion plots. The reduced CO₂ response to temperature in exclusion plots suggests that the positive effect of temperature on CO₂ is constrained by soil moisture availability.     

Soil Respiration and Student Inquiry: A Perfect Match

This activity explores the cycling of carbon between the atmosphere (primarily as CO₂) and biomass in plants, animals, and microscopic organisms. Students design soil respiration experiments using a protocol that resembles current practice in soil ecology. Three methods for measuring soil respiration are presented. Student-derived questions direct soil research projects.     

Soil Respiration in Different Agricultural and Natural Ecosystems in an Arid Region

The variation of different ecosystems on the terrestrial carbon balance is predicted to be large. We investigated a typical arid region with widespread saline/alkaline soils, and evaluated soil respiration of different agricultural and natural ecosystems. Soil respiration for five ecosystems together with soil temperature, soil moisture, soil pH, soil electric conductivity and soil organic carbon content were investigated in the field. Comparing with the natural ecosystems, the mean seasonal soil respiration rates of the agricultural ecosystems were 96%–386% higher and agricultural ecosystems exhibited lower CO₂ absorption by the saline/alkaline soil. Soil temperature and moisture together explained 48%, 86%, 84%, 54% and 54% of the seasonal variations of soil respiration in the five ecosystems, respectively. There was a significant negative relationship between soil respiration and soil electrical conductivity, but a weak correlation between soil respiration and soil pH or soil organic carbon content. Our results showed that soil CO₂ emissions were significantly different among different agricultural and natural ecosystems, although we caution that this was an observational, not manipulative, study. Temperature at the soil surface and electric conductivity were the main driving factors of soil respiration across the five ecosystems. Care should be taken when converting native vegetation into cropland from the point of view of greenhouse gas emissions.