Shrub Soil Research Articles

Development of an establishment scheme for a DGVM

Environmental changes are expected to shift the distribution and abundance of vegetation by determining seedling establishment and success. However, most current ecosystem models only focus on the impacts of abiotic factors on biogeophysics (e.g., global distribution, etc.), ignoring their roles in the population dynamics (e.g., seedling establishment rate, mortality rate, etc.) of ecological communities. Such neglect may lead to biases in ecosystem population dynamics (such as changes in population density for woody species in forest ecosystems) and characteristics. In the present study, a new establishment scheme for introducing soil water as a function rather than a threshold was developed and validated, using version 1.0 of the IAP-DGVM as a test bed. The results showed that soil water in the establishment scheme had a remarkable influence on forest transition zones. Compared with the original scheme, the new scheme significantly improved simulations of tree population density, especially in the peripheral areas of forests and transition zones. Consequently, biases in forest fractional coverage were reduced in approximately 78.8% of the global grid cells. The global simulated areas of tree, shrub, grass and bare soil performed better, where the relative biases were reduced from 34.3% to 4.8%, from 27.6% to 13.1%, from 55.2% to 9.2%, and from 37.6% to 3.6%, respectively. Furthermore, the new scheme had more reasonable dependencies of plant functional types (PFTs) on mean annual precipitation, and described the correct dominant PFTs in the tropical rainforest peripheral areas of the Amazon and central Africa.

Temperature sensitivity of mineral soil carbon decomposition in shrub and graminoid tundra, west Greenland

Shrub expansion is transforming Arctic tundra landscapes, but the impact on the large pool of carbon stored in high-latitude soils is poorly understood. Soil carbon decomposition is a potentially important source of greenhouse gases, which could create a positive feedback to atmospheric temperature. Decomposition is temperature sensitive, but the response to temperature can be altered by environmental variables. We focus on mineral soils, which can comprise a substantial part of the near-surface carbon stock at the landscape scale and have physiochemical characteristics that influence temperature sensitivity. We conducted a soil incubation experiment to measure carbon dioxide (CO2) emissions from tundra soils collected from west Greenland at two depths of mineral soils (0-20 cm and 20-40 cm below the surface organic horizon) incubated at five temperatures (4, 8, 12, 16, 24 °C) and two moisture levels (40 % and 60 % water holding capacity). We used an information theoretic model comparison approach to evaluate temperature, moisture and depth effects, and associated interactions, on carbon losses through respiration and to determine the temperature sensitivity of decomposition in shrub- and graminoid-dominated soils. We measured ecologically important differences in heterotrophic respiration and temperature sensitivity of decomposition between vegetation types. Graminoid soils had 1.8 times higher cumulative respiration and higher temperature sensitivity (expressed as Q-10) in the shallow depths (Q-10graminoid = 2.3, Q-10shrub = 1.8) compared to shrub soils. Higher Q-10 in graminoid soils was also observed for the initial incubation measurements (Q-10graminoid = 2.4, Q-10shrub = 1.9). Cumulative respiration was also higher for shallow soils, increased with moisture level, and had a temperature-depth interaction. Increasing soil moisture had a positive effect on temperature sensitivity in graminoid soils, but not in shrub soils. Mineral soil associated with graminoid-dominated vegetation had greater carbon losses from decomposition and a higher temperature sensitivity than shrub-dominated soils. An extrapolation of our incubation study suggests that organic carbon decomposition in western Greenland soils will likely increase with warming and with an increase in soil moisture content. Our results indicate that landscape level changes in vegetation and soil heterogeneity are important for understanding climate feedbacks between tundra and the atmosphere.

晋西北不同年限小叶锦鸡儿灌丛土壤氮矿化和硝化作用

PDF HTML阅读 XML下载 导出引用 引用提醒 晋西北不同年限小叶锦鸡儿灌丛土壤氮矿化和硝化作用 DOI: 10.5846/stxb201506101175 作者: 作者单位: 山西省林业调查规划院,山西省农业科学院农业环境与资源研究所,山西省农业科学院农业环境与资源研究所,美国弗吉尼亚理工大学作物土壤环境系 作者简介: 通讯作者: 中图分类号: 基金项目: 山西省农业科学院博士基金项目（YBSJJ1309）；公益性行业（农业）科研专项经费（201103003）；山西省百人计划项目 Soil nitrogen mineralization and nitrification under Caragana microphylla shrubs of different ages in the northwestern Shanxi Loess Plateau Author: Affiliation: Shanxi Academy of Forestry Survey,Planning and Design,Taiyuan,Institute of Agricultural Environment and Resources,Shanxi Academy of Agricultural Sciences,Institute of Agricultural Environment and Resources,Shanxi Academy of Agricultural Sciences,Department of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA 24061 Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:利用PVC管顶盖埋管法研究了晋西北黄土高原区小叶锦鸡儿人工灌丛不同定植年限（5，10，20，30，40a）土壤氮矿化与硝化速率的动态和净矿化与硝化总量。结果表明，⑴小叶锦鸡儿灌丛土壤无机氮主要以NO3--N形式存在，不同生长年限相同月份的土壤硝态氮（NO3--N）含量分别是铵态氮（NH4+-N）含量的1.5-15.4倍；⑵土壤氮素硝化速率和矿化速率随生长年限延长而加快，30年生时达到高峰，数值达40.2，44.1 mg m-2 d-1。从季节性变化看，7-8月份是硝化速率和矿化速率快速增长期，30年生小叶锦鸡儿灌丛土壤硝化速率和矿化速率分别达到86.9，93.1 mg m-2 d-1，显著高于其它生长年限（P < 0.05）；（3）土壤氮素硝化与矿化总量同样随小叶锦鸡儿生长年限延长而增加，30年生时达到最高，与5年生相比，分别增加了3.7和3.1倍。（4）5-10月份小叶锦鸡儿生长期内，各年限土壤全氮量的2.3%被矿化成无机氮，其中87%最终被转化成NO3--N形式存在于土体中。 Abstract:Nitrogen (N) is a main limiting factor for plant growth and development in many regions. Soil organic N, accounting for up to 95% of the soil N pool, cannot be used by plants directly, but can be absorbed and utilized by plants after being mineralized into nitrate N (NO3--N) and ammonium N (NH4+-N). Therefore, soil net N nitrification (Nnit) and net N mineralization (Nmin) rates have been considered two important indicators for evaluating the stability and function of an ecosystem. Caragana microphylla, a leguminous and sand-binding shrub, is a dominant species widely distributed in the Loess Plateau of northwestern Shanxi. Previous studies on soil N dynamics have mainly focused on forest, grassland, and farmland ecosystems to study the effect of environmental factors such as temperature and humidity on soil N mineralization and nitrification. However, few researchers have focused on soil N dynamic changes under shrub lands in arid and semiarid regions. The objective of this study was to explore the content of NO3--N and NH4+-N and the rate of soil net N nitrification and net N mineralization under Caragana microphylla shrubs as a function of plant age. We used a closed-top PVC tube in situ incubation method to detect the soil net N mineralization and net N nitrification in Caragana microphylla shrubs of different ages (5, 10, 20, 30, and 40 a) in the northwest Shanxi Loess Plateau. The results indicated that NO3--N accounted for a larger proportion of mineral N in Caragana microphylla shrub soil. The NO3--N concentration was 1.5-15.4 times higher than NH4+-N in the same month of different years. Soil net N nitrification and net N mineralization rates increased as the Caragana microphylla shrubs' ages increased, with the greatest average rates from May to October (40.2 and 44.1 mg m-2 d-1, respectively) being observed under 30-year-old shrubs. The net N nitrification and mineralization rate increased faster in July and August relative other months during the growing season. The net N nitrification and mineralization rates were 86.9 and 93.1 mg m-2 d-1, respectively, at 30-year-old shrubs, which had the highest rates of all age classes (P < 0.05). The total net N nitrification and mineralization also increased with increased age of Caragana microphylla. The total net N nitrification and mineralization from May to October increased by 3.7- and 3.1-fold at 30-year-old shrubs, respectively, relative to 5-year-old shrubs. During the Caragana microphylla growing season from May to October, 2.3% of soil total N was mineralized into inorganic N, 87% of which was eventually transformed into NO3--N. The results of this study provide great insight for guiding shrub utility and land management in the establishment and development of shrub ecosystems in the Loess plateau. 参考文献 相似文献 引证文献

Revegetation impacts soil nitrogen dynamics in the water level fluctuation zone of the Three Gorges Reservoir, China

Revegetations in riparian ecosystem are important in regulating soil nitrogen (N) dynamics. However, impacts of revegetation on soil N cycling and thereby on ecosystem functioning are not fully understood. We conducted an in situ incubation in the water level fluctuation zone (WLFZ) of the Three Gorges Reservoir region to evaluate soil N transformation including net N mineralization rate, net ammonification rate, net nitrification rate, net denitrification rate, N leaching and plant N uptake as well as the soil inorganic N (NH4+-N and NO3−-N) concentration in the top soils (0–20cm) following revegetations (implementing tree, shrub and herb plantations) over two years. The soil inorganic N concentration and N leaching were lower in the tree soils than in herb and shrub soils. Tree plantations decreased net N mineralization rate and net ammonification rate compared to herb and shrub soils, possibly due to lower soil organic carbon (SOC) input and soil temperatures. Whereas tree plantations increased soil net denitrification rate compared to herb and shrub soils because of higher tree NO3−-N uptake together with higher net nitrification rate. The inorganic N in the tree and shrub soils were lower in fall and summer, respectively, which was dependent on the seasonal variations in plant N uptake, soil N transformation, and N leaching. Thus, our results suggest that tree plantations could decrease soil inorganic N concentration and N leaching by altering both the quantity and quality of SOC and thereby potentially improve water quality in the riparian zone.

Disentangling direct and indirect effects of a legume shrub on its understorey community

Direct and indirect interactions among plants contribute to shape community composition through above‐ and belowground processes. However, we have not disentangled yet the direct and indirect soil and canopy effects of dominants on understorey species. We addressed this issue in a semi‐arid system from southeast Spain dominated by the legume shrub Retama sphaerocarpa. During a year with an exceptionally dry spring, we removed the shrub canopy to quantify aboveground effects and compared removed‐canopy plots to open plots between shrubs to quantify soil effects, both with and without watering. We added a grass removal treatment in order to separate direct from indirect shrub effects and quantified biomass, abundance, richness and composition of the forb functional group. With watering, changes in forb biomass were primarily driven by indirect shrub effects, with contrasting negative soil and positive aboveground indirect effects; changes in forb abundance and composition were more influenced by direct shrub soil effects with contrasting species composition between open and Retama patches. As community composition was different between open and Retama patches the indirect effects of Retama on forb species did not concern forbs from the open community but forbs from Retama patches. Indirect effects are, thus, important at the functional group level rather than at the species level. Without watering, there were no significant interactions. Changes in species richness between treatments were weak and seldom significant. We conclude that shrub effects on understorey forbs are primarily due to their influence on soil properties, directly affecting forb species composition but indirectly affecting the biomass of the forbs of the Retama patches, and only with sufficient water.

Soil organic and inorganic carbon and stable carbon isotopes in the Yanqi Basin of northwestern China

SummarySoil organic carbon (SOC) and soil inorganic carbon (SIC) are important reservoirs of carbon in terrestrial ecosystems. Assessments of both SOC and SIC are lacking in arid regions. We carried out a survey in central Xinjiang, the Yanqi Basin, to evaluate the dynamics of SOC and SIC. Twenty‐one soil profiles were sampled from three land types: desert, shrub and agricultural soils. We determined SOC and SIC and the stable carbon isotope compositions (δ13C). Our data showed a decrease with depth for SOC in all soil profiles but an increase for SIC except in the desert soils. Both SOC and SIC stocks (over the 0–30‐cm depth) were least in the desert soils (1.0 ± 0.3 and 4.0 ± 0.8 kg C m−2 for SOC and SIC, respectively) but greatest in the agricultural soils (4.6 ± 0.6 and 11.0 ± 2.0 kg C m−2 for SOC and SIC, respectively). Total soil carbon stocks at 0–100 cm were 11.6 ± 4.8, 45.1 ± 10.4 and 51.2 ± 5.6 kg C m−2 in the desert soils, shrub soils and agricultural soils, respectively. On average, SIC accounted for more than 80% of the total carbon in this region. There were no significant differences in δ13C of SOC between land‐use types. In contrast, the δ13C of SIC was different: desert soils (−0.6‰) &gt; shrub soils (−2.2‰) &gt; agricultural soils (−3.4‰). The depleted 13C in SIC in the agricultural soils indicates enhancement of pedogenic carbonate by cropping. Our study suggests that converting shrub land to agricultural land in arid regions may lead to an increase not only in SOC stock, but also in SIC stock.

Variations of soil water isotopes and effective contribution times of precipitation and throughfall to alpine soil water, in Wolong Nature Reserve, China

Alpine shrubs and soils in catchments of the headwaters of the Yangtze River in west China are very important for reducing flooding and sustaining base flow during the summer. However, the contributions of precipitation, throughfall, and interflow to soil water and their effective contribution times in alpine shrub soil are not fully understood. In this study, we investigated a time series of stable isotopes in soil water, precipitation, canopy throughfall, interflow, litter, and humus water in a hillslope area. In addition, the spatial variation of water isotopes in soil profile in the Wolong Valley, located in the upper watershed of the Yangtze River, Sichuan, China, was considered. We found that (1) precipitation and throughfall significantly affected water isotopes in litter, humus, and shallow soil (0cm to 50cm deep) by affecting preferential flows. (2) Rainwater from a small precipitation event (about 4.0mm d−1) also penetrated soil to depths of 40cm to 50cm. (3) Interflow could comprise as much as 96% of water in soil columns during non-rainy days, but the proportion would decrease quickly after the precipitation amount reached more than 3mm d−1. (4) Mean effective contribution times of recharge in soil (0cm to 50cm deep) occurred 3 to 5days despite the occurrence of large precipitation events (15.0mm and 18.9mm). Therefore, preferential flows composed of precipitation/throughfall and interflow were dominant in hillslope hydrology in the southeast edge of Tibet, which caused runoff to increase during the rainy season.

Variation of soil nutrients and particle size under different vegetation types in the Yellow River Delta

The Yellow River Delta wetland, located at the southern coast of Bohai Gulf, provides important ecosystem services such as flood control, water purification, biodiversity conservation, nutrient removal and carbon sequestration, shoreline stabilization, tourism attraction and wetland products maintains in the Yellow River Delta. This study assessed how agricultural activities in a reclamation wetland changed soil pH, soil electric conductivity, soil nutrient and soil particle size as compared to natural vegetation by using a combination of field experiments and lab analysis. The vegetation type included adjacent alfalfa field (Medicago sativa), cotton field (Gossypium spp.), Chinese tamarisk shrub (Tamarix chinensis), and reed marsh (Phragmites sage). The results indicated that the soil pH was higher (pH>8) in alfalfa field and cotton field, and alfalfa field and reed marsh had significant function in reducing soil salt content, soil electric conductivity of alfalfa field at 0–30cm were 140.38±14.36, 114.48±14.36, 125.30±11.37μs/cm. The effect of different vegetation types on soil nutrient was significant (P<0.05). Soil organic matter at 0–10cm in Chinese tamarisk shrub and reed marsh was 21.66±3.82g/kg and 16.51±4.60g/kg, which was higher than that of alfalfa field (10.47±2.36g/kg) and cotton field (9.82±1.27g/kg), but soil total nitrogen content in alfalfa field was the highest, which is significantly higher than that of cotton field, Chinese tamarisk shrub and reed marsh(P<0.05), the content of soil total nitrogen at 0–10cm and 10–20cm was 7.67±0.38g/kg and 5.97±0.51g/kg, respectively, while the content of available P and available K was reversed. The difference of soil particle size between layers was not significant (P>0.05), the sand content of Chinese tamarisk shrub soils in 0–10cm was the highest, the next was alfalfa field and cotton field, and the content of silt and clay in reed marsh was higher than the others. The correlation and significant degree between soil particle size and soil nutrient was related with vegetation types, and soil organic matter was significantly positively correlated with soil silt and clay content on the alfalfa field. The results demonstrated that wetland cultivation was one of the most important factors influencing on the nutrient fate and reserves in soil, which could lead to rapid nutrient release and slow restoration through abandon cultivation. Consequently, compared with cotton field, alfalfa field is more favorable to sustainable management of wetland cultivation in the Yellow River Delta. It should be considered in wetland restoration projects planning.

Diversity of ground-dwelling beetles (Coleoptera) in restored habitats in the hill and gully area of Loess Plateau, Ningxia Hui Autonomous Region

The aim of this study was to probe into changes in diversity of ground-dwelling beetles (Coleoptera) and their relation to environmental variables within restored habitats in the hill and gully area of Loess Plateau, Ningxia Hui Autonomous Region, China. An investigation of ground-dwelling beetle communities was carried out using pitfall traps in six habitats including shrub/grassland, tree/shrub mix, fuel-wood forest, naturally restored grassland, plantation forest, and level terraced field in July and August, 2013. We found that species richness and abundance of individual ground-dwelling beetles was higher (P<0.05) in the shrub/grassland, tree/shrub mix, and fuel-wood forest compared to the other three stands. Species richness of ground-dwelling beetles was markedly lower (P<0.05) in naturally restored grassland, plantation forest and level terraced field, whereas the abundance of individual ground-dwelling beetles were higher in plantation forest and level terraced field than in naturally restored grassland. Jaccard indices revealed a high level of similarity amongst shrub/grassland, tree/shrub mix, and fuel-wood forest stands, and then between plantation forest stands and level terraced fields. Multiple linear regression analyses showed that herbaceous biomass, cover of shrub species and soil water content were the key factors affecting species richness of ground-dwelling beetles, while it was canopy cover and leaf litter depth that most affected total abundance. Canonical correspondence analysis (CCA) showed that the cover and depth of leaf litter, cover of canopy and herbaceous layers were important determinants of species abundance and composition of beetle communities. 第 4 期 杭佳等: 宁夏黄土丘陵区不同生态恢复生境地表甲虫多样性 517 In conclusion, conversion to shrub/grasslands may be one option for raising ground-dwelling beetle diversity in the hill and gully area of Loess Plateau, Ningxia.

Soil water repellency in Mediterranean rangelands under contrasted climatic, slope and patch conditions in southern Spain

Abstract This study evaluates the spatial and temporal variability in soil water repellency (SWR) in two Mediterranean rangelands (with similar tree and shrub species) and its relationships to different eco-geomorphologic variables (climate, aspect, soil cover and some soil properties). Every month from October 2008 to May 2009 (rainy season), soil moisture and SWR was measured in field conditions by means of gravimetric method and Water Drop Penetration Test, respectively. Also, disturbed and undisturbed soil samples were taken to analyze some soil properties: bulk density, texture, organic matter, pH and electrical conductivity. The entire tests were performed in north and south aspect hillslopes and beneath shrub and bare soil in every of them. The results indicated that: i) climatic conditions seem to be more transcendent than the vegetal species for explaining the variability in SWR; ii) thus, SWR appears to be controlled by the antecedent rainfall and soil moisture; iii) more severe SWR were observed in patches characterized by sandier soils and/or higher organic matter contents; and iv) the factor ‘hillslope aspect’ was not found so crucial as it was expected.

A MULTIDISCIPLINARY MODEL FOR ASSESSING DEGRADATION IN MEDITERRANEAN RANGELANDS

ABSTRACTThis paper presents an annual multidisciplinary, non‐spatial model which formalizes the relationships linking the dynamics of shrubs, herbs, soil, livestock and farmers' behaviour with possible exogenous drivers of degradation, such as weather and prices. The model does not represent a pasture–livestock system but a shrub–soil one and is applied to a rangeland in Lagadas County (Northern Greece). A sensitivity analysis of the model is also presented. It shows that livestock, in general, and factors increasing farmers' profits, in particular, are currently helping to combat shrub invasion in Lagadas while having low impacts on erosion rates. Copyright © 2012 John Wiley &amp; Sons, Ltd.

A new entomopathogenic nematode, Steinernema tielingense n. sp. (Rhabditida: Steinernematidae), from north China

A new species of Steinernema was isolated from shrub soils in Liaoning province during a survey for entomopathogenic nematodes in north China. This nematode was obtained by the insect-baiting technique using last instar larvae of Galleria mellonella (Lepidoptera: Pyralidae). This new species is described herein as S. tielingense n. sp. It belongs to the ‘feltiae-kraussei-oregonense’ group and is characterised by infective third-stage juvenile (IJ) with a body length of 915 (824-979) μm, distance from head to excretory pore of 69 (64-73) μm, tail length of 81 (74-85) μm, ratio E=88 (85-94)%, lateral field with eight ridges at mid-body, first generation male spicule 88 (79-98) μm and gubernaculum 61 (49-70) μm long, and first generation female with a vulval protrusion and ratio D=41 (32-49)%. The new species distinctly differs from the related species S. kraussei, S. silvaticum, S. oregonense and S. cholashanense in the different number of ridges in the lateral fields and hyaline tail length as % of total tail length of IJ and male body length and distance from head to excretory pore. Cross hybridisation tests showed that these species were reproductively isolated. The sequences analyses of the internal transcribed spacers (ITS) and D2-D3 regions of the ribosomal DNA confirm this to be a new species.

Soil nitrogen dynamics following short-term revegetation in the water level fluctuation zone of the Three Gorges Reservoir, China

With the completion of the Three Gorges project, native vegetation in the water level fluctuation zone between the elevations of 145 m to175 m disappeared due to the inundation with a hydrological regime with flooding in winter instead of summer, a reversal of the natural one. We conducted a field experiment in the riparian ecosystem in the Three Gorges Reservoir to study nitrogen (N) dynamics under short-term revegetation and flooding during the period from 2008 to 2009. Soil chemical and physical characteristics, net mineralization potential rate (NMPR), net nitrification potential rate (NNPR), and denitrification potential rate (DPR) were determined in the laboratory. These results showed a significant decrease in inorganic N (NH 4 +-N and NO 3 −-N) following the short-term revegetation and flooding, which was probably related to the interactions between surface flow, flooding, plant N uptake, and N transformation. Plants in conjunction with flooding increased the NMPR and NNPR, and increased the DPR only at the beginning of the revegetation and then decreased DPR after the flooding by regulating the concentration of soil organic carbon (SOC) and C:N ratios in soil, and decreasing the soil bulk density. Vegetation types affected N dynamics by changing SOC, soil N availability, and C:N ratios. The inorganic N, NMPR, and NNPR were higher in shrub soil than those in herb and tree soils due to higher SOC, whereas the DPR in tree soils was lower compared to shrub and herb soils because of lower C:N ratios together with the lower SOC. These results imply that soil inorganic N declined following the revegetation and flooding, and the revegetation in the riparian zone could potentially improve water quality.

A cross-seasonal comparison of active and total bacterial community composition in Arctic tundra soil using bromodeoxyuridine labeling

Arctic soil microorganisms remain active at ecologically relevant rates in frozen soils. We used bromodeoxyuridine (BrdU) labeling and terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA gene amplicons to examine active bacterial communities in two Alaskan tundra soils collected in summer and winter of 2005. Active community T-RFLP profiles were compared to total community profiles to determine if active bacteria were a subset of the total community. In shrub soils, active bacteria communities differed in composition between summer and winter, and winter-active bacterial taxa were not detected in the total community, suggesting that they are likely rare within the overall community. In contrast, tussock tundra soil contained more bacterial taxa that were active in both summer and winter and also represented a large portion of the total community. Using in silico digest of a sequence library from this site, we attempted to identify the dominant organisms in our samples. Our previous research suggested that the total microbial community was stable throughout the year, but this new study suggests that the active community is more dynamic seasonally. In general, only a subset of the total community was growing at a given time. This temporal niche partitioning may contribute to the high diversity of microbial communities in soils. Understanding which taxa contribute to microbial function under different conditions is the next frontier in microbial ecology and linking composition to biogeochemical cycling.

Microbial growth rate measurements reveal that land-use abandonment promotes a fungal dominance of SOM decomposition in grazed Mediterranean ecosystems

The present study investigated the effects of land-use abandonment on the soil decomposer community of two grazed Mediterranean ecosystems (an annual grassland with scattered holm oaks and a low-density shrubland). To test the influence of grazing abandonment, a set of plots within each site were fenced and kept undisturbed during 4–5 years, during which above-ground plant community structure was monitored. After that, soil samples were collected from grazed and abandoned plots corresponding to the three different soil conditions: away from (“grass”) and below tree canopies (“oak”) within the annual grassland, and from the shrubland (“shrub”). Soil samples were split into two different layers (0–5 and 5–15 cm) and then analyzed for saprotrophic fungal (acetate into ergosterol incorporation) and bacterial (leucine incorporation) growth rates. Ergosterol content (as a fungal biomass estimator) and a standard set of soil chemistry variables were also measured. After 5 years of grazing exclusion, saprotrophic fungal growth rate clearly increased in both grass and oak surface layers whereas bacterial growth rate was not altered. This translated into significantly higher fungal-to-bacterial (F/B) growth rate ratios within the ungrazed plots. Similar trends were observed for the shrub soils after 4 years of exclusion. On the contrary, abandonment of grazing had negligible effects on the ergosterol content, as well as on the soil chemical variables (soil organic carbon, total N, C/N ratio, and pH), in all the three soil conditions assessed. These results indicated a shift toward a more fungal-dominated decomposer activity in soils following cessation of grazing and highlighted the sensitivity of the microbial growth rate parameters to changes associated with land use. Moreover, there were evidences of a faster fungal biomass turnover in the ungrazed plots, which would reflect an accelerated, though not bigger, fungal channel in soil organic matter mineralization.

Temperature-Dependable Microwave Dielectric Model for an Arctic Soil

Dielectric measurements of an organic-rich permafrost soil over the range from 1.0 to 16 GHz and from -30°C to +25°C are presented. The measured shrub soil contains up to 90% organic matter and is the first soil of this composition for which the soil dielectric has been characterized. The measurements were fitted to the generalized refractive mixing dielectric model (GRMDM) recently proposed by Mironov et al., which combines the complex refractive indexes for the major components of the soil. These components were found to be the solid content, bound water, transient bound water, liquid capillary water, and moistened ice water. The dielectric properties of the frequency-dispersive components are each described by their own Debye relaxation spectrum. The GRMDM has been modified to incorporate the temperature dependence of the Debye parameters. The phase transformation of the soil water components at the freezing temperature is taken into account. As a result, a temperature-dependable GRMDM (TD GRMDM) has been developed, including model parameters which have a physical interpretation. This TD GRMDM predicts the dielectric for this soil in the whole range of moistures, frequencies, and temperatures measured. The model prediction errors are on the same order as that of dielectric measurements. The model proposed is the first of its kind to provide a physical basis for radar and radiothermal remote sensing algorithms that retrieve the freeze/thaw state and the volumetric moisture in the upper layer of an Arctic soil.

Seasonal variation in enzyme activities and temperature sensitivities in Arctic tundra soils

AbstractArctic soils contain large amounts of organic matter due to very slow rates of detritus decomposition. The first step in decomposition results from the activity of extracellular enzymes produced by soil microbes. We hypothesized that potential enzyme activities are low relative to the large stocks of organic matter in Arctic tundra soils, and that enzyme activity is low at in situ temperatures. We measured the potential activity of six hydrolytic enzymes at 4 and 20 °C on four sampling dates in tussock, intertussock, shrub organic, and shrub mineral soils at Toolik Lake, Alaska. Potential activities of N‐acetyl glucosaminidase, β‐glucosidase, and peptidase tended to be greatest at the end of winter, suggesting that microbes produced enzymes while soils were frozen. In general, enzyme activities did not increase during the Arctic summer, suggesting that enzyme production is N‐limited during the period when temperatures would otherwise drive higher enzyme activity in situ. We also detected seasonal variations in the temperature sensitivity (Q10) of soil enzymes. In general, soil enzyme pools were more sensitive to temperature at the end of the winter than during the summer. We modeled potential in situβ‐glucosidase activities for tussock and shrub organic soils based on measured enzyme activities, temperature sensitivities, and daily soil temperature data. Modeled in situ enzyme activity in tussock soils increased briefly during the spring, then declined through the summer. In shrub soils, modeled enzyme activities increased through the spring thaw into early August, and then declined through the late summer and into winter. Overall, temperature is the strongest factor driving low in situ enzyme activities in the Arctic. However, enzyme activity was low during the summer, possibly due to N‐limitation of enzyme production, which would constrain enzyme activity during the brief period when temperatures would otherwise drive higher rates of decomposition.

Soil biota, but not soil nutrients, facilitate the invasion of Bidens pilosa relative to a native species Saussurea deltoidea

Cui Q‐G &amp; He W‐M (2009). Soil biota, but not soil nutrients, facilitate the invasion of Bidens pilosa relative to a native species Saussurea deltoidea. Weed Research49, 201–206.SummaryExisting evidence has shown that soil biota and nutrient availability in the introduced ranges are drivers of plant invasions. Bidens pilosa, an invasive species, grows better in rich soil from under shrubs than in poor soil from spaces between the shrubs. Here, we hypothesise that soil biota will benefit B. pilosa more than the native herb Saussurea deltoidea and that B. pilosa will also be favoured over S. deltoidea in nutrient‐rich soil compared with soil lower in nutrients. To test both hypotheses, we conducted an experiment in which B. pilosa and S. deltoidea were subjected to non‐sterile and sterile soils from both shrubs and gaps. Sterilisation had greater negative effects on the growth of B. pilosa than S. deltoidea indicating that mutualists appear to have stronger effects on the invasive than on the native plant. Although shrub soil was significantly richer in nutrients than gap soil, this difference did not affect biomass production of B. pilosa. In contrast, B. pilosa had greater total biomass in non‐sterile shrub soil than in non‐sterile gap soil. These findings suggest that successful invasions of B. pilosa are because of the shrub‐dependent effect of soil biota in the introduced ranges and that its habitat association appears to be closely linked to soil biota, but not soil nutrients.

Copper Sorption Behavior of Selected Soils of the Oasis in the Middle Reaches of Heihe River Basin, China

The mobility and bioavailability of copper (Cu) depends on the Cu sorption capacity of soil and also on the chemical form of Cu in soils. Laboratory batch experiments were carried out to study the sorption and distribution of Cu in nine soils differing in their physicochemical properties from the oasis in the middle reaches of Heihe river basin, China: desert soil (S-1), agricultural soils (S-2, S-3, S-8, and S-9), marshland soil (S-4), and hungriness shrub soils (S-5 and S-6). Copper sorption behavior was studied using the sorption isotherm and sequential extraction procedure. In general, the sorption capacity for Cu decreased in the order: S-4 > S-9 > S-2 > S-8 > S-3 > S-6 > S-5 > S-7 > S-1. The correlation results suggest that soils with higher CEC, silt, clay, CaCO3, and organic matter will retain Cu more strongly and in greater amounts than soils that are sandy with lower CEC, CaCO3, and organic matter. pH is not an important impact factor to Cu sorption in experimental soil samples because pH in soils used in this study had a narrow range. The distribution of sorbed Cu varied between nine soils studied and depended on both soil properties and initial added Cu concentration. There are significant differences in the distribution of Cu in each soil with the increase of initial Cu concentration. The predominance of Cu associated with the available fraction, which was over 50% of the total sorbed Cu in most cases, indicates that the change of geochemical conditions might promote the release of Cu back into soil solution thus impacting organisms in the soils. The added Cu has also the tendencies to locate in the residual fraction, which was larger than 5% of the total amount extracted from the four fractions in most soils.

Shrub-interspace dynamics alter relationships between microbial community composition and belowground ecosystem characteristics

In desert ecosystems, belowground characteristics are influenced chiefly by the formation and persistence of “shrub-islands of fertility” in contrast to barren plant interspaces. If soil microbial communities are exclusively compared between these two biogeochemically distinct soil types, the impact of characteristics altered by shrub species, especially soil C and N, are likely to be overemphasized and overshadow the role of other characteristics in structuring microbial composition. To determine how belowground characteristics influence microbial community composition, and if the relative importance of these characteristics shifts across the landscape (i.e., between and within shrub and interspace soils), changes in microbial communities across a 3000-year cold desert chronosequence were related to 27 belowground characteristics in surface and subsurface soils. When shrub and interspace communities in surface and subsurface soils were combined across the entire chronosequence, communities differed and were primarily influenced by soil C, NO 3 − concentrations, bulk density, pH, and root presence. Within shrub soils, microbial communities were shrub species-specific, especially in surface soils, highlighting differences in soil characteristics created by specific shrub species and/or similarity in stresses structuring shrub species and microbial communities alike. Microbial communities in shrub soils were not influenced by soil C, but by NO 3 − and NH 4 + concentrations, pH, and silt in surface soils; and Cl, P, soil N, and NO 3 − concentrations in subsurface soils. Interspace soil communities were distinct across the chronosequence at both depths and were strongly influenced by dune development. Interspace communities were primarily associated with soil stresses (i.e., high B and Cl concentrations), which decreased with dune development. The distribution of Gram-positive bacteria, Actinobacteria, and fungi highlighted community differences between and within shrub and interspace soils, while Gram-negative bacteria were common in all soils across the chronosequence. Of the 27 belowground characteristics investigated, 13 separated shrub from interspace communities, and of those, only five emerged as factors influencing community composition within shrub and interspace soils. As dunes develop across this cold desert chronosequence, microbial community composition was not regulated primarily by soil C, but by N and P availability and soil stresses in shrub soils, and exclusively by soil stresses in interspace soils.