植物、土壤及土壤管理对土壤微生物群落结构的影响

土壤微生物是土壤生态系统的重要组成部分,对土壤微生物群落结构多样性的研究是近年来土壤生态学研究的热点。本文综述了有关植物、土壤类型以及土壤管理措施对土壤微生物群落结构影响的最新研究结果,指出植物的作用因植物群落结构多样性、植物种类、同种植物不同的基因型,甚至同一植物不同根的区域而异;而土壤的作用与土壤质地和有机质含量等因素有关;植物和土壤类型在对土壤微生物群落结构影响上的作用存在互作关系。不同的土壤管理措施对土壤微生物群落结构影响较大,长期连作、大量的外援化学物质的应用降低了土壤微生物的多样性;而施用有机肥、免耕可以增加土壤微生物群落结构多样性,有利于维持土壤生态系统的功能。     

Evaluation of soil saturation, soil chemistry, and early spring soil and air temperatures as risk factors in yellow-cedar decline

Yellow‐cedar (Callitropsis nootkatensis (D. Don) Oerst.) is a valuable tree species that is experiencing a widespread decline and mortality in southeast Alaska. This study evaluated the relative importance of several potential risk factors associated with yellow‐cedar decline: soil saturation, soil aluminum (Al) toxicity or calcium (Ca) deficiency, and air and soil temperature. Data were collected from permanent vegetation plots established in two low‐elevation coastal forests exhibiting broad ranges of cedar mortality. Measurements of each risk factor were contrasted among classified forest zones to indicate if there were strong links with decline. Hydrology alone is weakly associated with yellow‐cedar decline, but could have a predisposing role in the decline by creating exposed conditions because of reduced forest productivity. Yellow‐cedar decline is not strongly associated with soil pH and extractable Al and Ca, but there appears to be Ca enrichment of surface soils by feedback from dead yellow‐cedar foliage. Air and soil temperature factors are strongly associated with decline. Based on these results, an hypothesis is presented to explain the mechanism of tree injury where exposure‐driven tree mortality is initiated in gaps created by soil saturation and then expands in gaps created by the tree‐mortality itself. The exposure allows soils to warm in early spring causing premature dehardening in yellow‐cedar trees and subsequent freezing injury during cold events. Yellow‐cedars growing in the protection of shade or snow are not preconditioned by this warming, and thus not as susceptible to cold injury. Yellow‐cedar decline appears to be associated with regional climate changes, but whether the cause of these changes is related to natural or human‐induced climate shifts remains uncertain. Management implications, the possible role of climate, and recommended research are discussed.     

Estimating respiration of roots in soil: Interactions with soil CO2, soil temperature and soil water content

Little information is available on the variability of the dynamics of the actual and observed root respiration rate in relation to abiotic factors. In this study, we describe I) interactions between soil CO₂ concentration, temperature, soil water content and root respiration, and II) the effect of short-term fluctuations of these three environmental factors on the relation between actual and observed root respiration rates. We designed an automated, open, gas-exchange system that allows continuous measurements on 12 chambers with intact roots in soil. By using three distinct chamber designs with each a different path for the air flow, we were able to measure root respiration over a 50-fold range of soil CO₂ concentrations (400 to 25000 ppm) and to separate the effect of irrigation on observed vs. actual root respiration rate. All respiration measurements were made on one-year-old citrus seedlings in sterilized sandy soil with minimal organic material.Root respiration was strongly affected by diurnal fluctuations in temperature (Q₁₀ = 2), which agrees well with the literature. In contrast to earlier findings for Douglas-fir (Qi et al., 1994), root respiration rates of citrus were not affected by soil CO₂ concentrations (400 to 25000 ppm CO₂; pH around 6). Soil CO₂ was strongly affected by soil water content but not by respiration measurements, unless the air flow for root respiration measurements was directed through the soil. The latter method of measuring root respiration reduced soil CO₂ concentration to that of incoming air. Irrigation caused a temporary reduction in CO₂ diffusion, decreasing the observed respiration rates obtained by techniques that depended on diffusion. This apparent drop in respiration rate did not occur if the air flow was directed through the soil. Our dynamic data are used to indicate the optimal method of measuring root respiration in soil, in relation to the objectives and limitations of the experimental conditions.     

Restoration,soil organisms,and soil processes: emerging approaches

Better understanding of the connection between aboveground plant communities and belowground soil organisms and processes has led to an explosion in recent research on the applications of this link to the field of ecological restoration. Research is only beginning to have the capacity to link soil organisms and specific ecosystem functions. Establishing general ecological principles of the role microbial communities have during ecological restoration is also still in its infancy. As such, the literature is at a critical point to generate a Special Feature that brings together novel approaches of linking soil and restoration to promote more regular inclusion and consideration of soil organisms and soil‐based processes in ecological restoration. In this special feature, we bring together nine research articles from different ecosystems that study the relationship between restoration activities, soil microbial communities, and soil properties. From these research articles, we describe two primary themes: (1) research on the impacts of ecosystem‐specific restoration activities on soil organisms and processes and (2) research testing methods of soil manipulation to improve restoration outcomes. We hope to inspire readers and restoration practitioners to consider soil microbes and soil processes in their research, restoration projects, and world views.     

Harvester ant nests, soil biota and soil chemistry

Many ant species accumulate organic debris in the vicinity of their nests. These organic materials should provide a rich resource base for the soil biota. We examined the effect of harvester ant nests (Pogonomyrmex barbatus) on the soil community and soil chemistry. Ant nest soils supported 30-fold higher densities of microarthropods and 5-fold higher densities of protozoa than surrounding, control soils. The relative abundances of the major groups of protozoa differed as well: amoebae and ciliates were relatively overrepresented, and flagellates underrepresented, in ant nest versus control soils. Densities of bacteria and fungi were similar in the two soil types. Concentrations of nitrate, ammonium, phosphorus, and potassium were significantly higher in ant nest soils, while concentrations of magnesium, calcium, and water were similar in nest and control soils. Ant nest soils were marginally more acidic than controls. The results demonstrate that P. barbatus nests constitute a significant source of spatial heterogeneity in soil biota and soil chemistry in arid grasslands. Received: 17 March 1997 / Accepted: 10 June 1997     

Interpreting,measuring, and modeling soil respiration

This paper reviews the role of soil respiration in determining ecosystem carbon balance, and the conceptual basis for measuring and modeling soil respiration. We developed it to provide background and context for this special issue on soil respiration and to synthesize the presentations and discussions at the workshop. Soil respiration is the largest component of ecosystem respiration. Because autotrophic and heterotrophic activity belowground is controlled by substrate availability, soil respiration is strongly linked to plant metabolism, photosynthesis and litterfall. This link dominates both base rates and short-term fluctuations in soil respiration and suggests many roles for soil respiration as an indicator of ecosystem metabolism. However, the strong links between above and belowground processes complicate using soil respiration to understand changes in ecosystem carbon storage. Root and associated mycorrhizal respiration produce roughly half of soil respiration, with much of the remainder derived from decomposition of recently produced root and leaf litter. Changes in the carbon stored in the soil generally contribute little to soil respiration, but these changes, together with shifts in plant carbon allocation, determine ecosystem carbon storage belowground and its exchange with the atmosphere. Identifying the small signal from changes in large, slow carbon pools in flux dominated by decomposition of recent material and autotrophic and mycorrhizal respiration is a significant challenge. A mechanistic understanding of the belowground carbon cycle and of the response of different components to the environment will aid in identifying this signal. Our workshop identified information needs to help build that understanding: (1) the mechanisms that control the coupling of canopy and belowground processes; (2) the responses of root and heterotrophic respiration to environment; (3) plant carbon allocation patterns, particularly in different forest developmental stages, and in response to treatments (warming, CO₂, nitrogen additions); and (4) coupling measurements of soil respiration with aboveground processes and changes in soil carbon. Multi-factor experiments need to be sufficiently long to allow the systems to adjust to the treatments. New technologies will be necessary to reduce uncertainty in estimates of carbon allocation, soil carbon pool sizes, and different responses of roots and microbes to environmental conditions.     

Microbial soil respiration and its dependency on carbon inputs, soil temperature and moisture

This experiment was designed to study three determinant factors in decomposition patterns of soil organic matter (SOM): temperature, water and carbon (C) inputs. The study combined field measurements with soil lab incubations and ends with a modelling framework based on the results obtained. Soil respiration was periodically measured at an oak savanna woodland and a ponderosa pine plantation. Intact soils cores were collected at both ecosystems, including soils with most labile C burnt off, soils with some labile C gone and soils with fresh inputs of labile C. Two treatments, dry‐field condition and field capacity, were applied to an incubation that lasted 111 days. Short‐term temperature changes were applied to the soils periodically to quantify temperature responses. This was done to prevent confounding results associated with different pools of C that would result by exposing treatments chronically to different temperature regimes. This paper discusses the role of the above‐defined environmental factors on the variability of soil C dynamics. At the seasonal scale, temperature and water were, respectively, the main limiting factors controlling soil CO₂ efflux for the ponderosa pine and the oak savanna ecosystems. Spatial and seasonal variations in plant activity (root respiration and exudates production) exerted a strong influence over the seasonal and spatial variation of soil metabolic activity. Mean residence times of bulk SOM were significantly lower at the Nitrogen (N)‐rich deciduous savanna than at the N‐limited evergreen dominated pine ecosystem. At shorter time scales (daily), SOM decomposition was controlled primarily by temperature during wet periods and by the combined effect of water and temperature during dry periods. Secondary control was provided by the presence/absence of plant derived C inputs (exudation). Further analyses of SOM decomposition suggest that factors such as changes in the decomposer community, stress‐induced changes in the metabolic activity of decomposers or SOM stabilization patterns remain unresolved, but should also be considered in future SOM decomposition studies. Observations and confounding factors associated with SOM decomposition patterns and its temperature sensitivity are summarized in the modeling framework.     

Variation of microbial communities in soil, rhizosphere, and rhizoplane in response to crop species, soil type, and crop development.

We investigated the influence of plant species, soil type, and plant development time on the shaping of microbial communities in soil and in association with roots. The sample group consisted of a total of 32 microcosms in three habitats: soil, rhizosphere, and rhizoplane. Communities were represented by the patterns of a sequence-specific separation of rRNA target sequences. Effects of experimental parameters were classified by a cluster analysis of pattern similarities. The type of plant species (clover, bean, or alfalfa) had the greatest effect in plant-associated habitats and also affected soil patterns. Plant development had a minor habitat-dependent effect that was partly obscured by replicate variation. The results stress the applicability of biased community representations in an analysis of induced variation.     

森林植物多样性、树种重要值与土壤理化性质对球囊霉素相关土壤蛋白的影响

球囊霉素相关土壤蛋白(glomalin-related soil protein, GRSP)在土壤物理结构调节和土壤碳库稳定性中发挥着重要作用,但植物多样性和优势种如何影响GRSP还缺乏系统性研究。本研究依托东北林业大学哈尔滨实验林场的72块样地, 对1 m深土壤剖面分5层采样, 测定土壤易提取球囊霉素(easily extractable GRSP, EEG)、总提取球囊霉素(total GRSP, TG)及土壤理化性质, 并同时计算植物多样性指数及优势种重要值(importance value, IV), 进一步通过相关分析和冗余排序分析判断影响GRSP的主要因素与贡献。结果表明: (1)在整个土壤剖面上均表现为TG和EEG与土壤有机碳(SOC)正相关, 在部分土层深度与全氮(total nitrogen, TN)和含水量(moisture content, MC)正相关, 而与电导率(electrical conductivity, EC)和pH值负相关。(2)部分土层TG和EEG与黑皮油松(Pinus tabuliformis var. mukdensis)、樟子松(P. sylvestris var. mongolica)、胡桃楸(Juglans mandshurica)、黄檗(Phellodendron amurense)、榆树(Ulmus pumila)优势种重要值显著相关, 表现为黑皮油松重要值越高, 而黄檗、榆树重要值越小, 越有利于EEG的积累, 并且伴随EEG-C/SOC (EEG中C占SOC比例)增加、EEG/TG增大; 群落中胡桃楸、黄檗、榆树更有利于TG积累, 黑皮油松、落叶松(Larix gmelinii)、樟子松不利于TG的积累。(3)植物Simpson指数、Shannon-Wiener指数、物种丰富度与EEG、TG、EEG/TG无显著相关性, 而与EEG-C/SOC、EEG-N/TN (EEG中N占TN的比例)、TG-C/SOC (TG中C占SOC比例)、TG-N/TN (TG中N占TN的比例)显著负相关; 土壤EEG/TG和EEG-N/TN与植物均匀度指数显著正相关, 在1 m土壤不同土层趋势类似。(4)方差分解分析表明: 生物因子对GRSP变化的解释率是20.2%, 土壤理化因子解释率为7.8%, 而生物因子中植物优势种重要值的解释率最大(16.4%), 而植物物种多样性指数解释率仅为0.4%。冗余排序发现常绿针叶树种(黑皮油松和樟子松)越多且阔叶树种越少时, GRSP含量和GRSP对土壤碳氮的贡献越高(P < 0.01), 其机制可能与树种菌根类型有关: 外生菌根树种重要值与TG显著负相关, 丛枝菌根树种重要值与TG显著正相关。本研究解析了植物物种多样性对GRSP含量的重要影响, 并强调未来土壤管理和评估可以通过调整优势物种而不是树种多样性来促进GRSP积累。     

The effects of mycorrhizal roots on litter decomposition, soil biota, and nutrients in a spodosolic soil

We studied the effects of mycorrhizal pitch pine (Pinus rigida) roots on litter decomposition, microbial biomass, nematode abundance and inorganic nutrients in the E horizon material of a spodosolic soil, using field microcosms created in a regenerating pitch pine stand in the New Jersey Pinelands. Pine roots stimulated litter decomposition by 18.7% by the end of the 29 month study. Both mass loss and N and P release from the litter were always higher in the presence of roots than in their absence. Nutrient concentrations in decomposing litter were similar, however, in the presence and absence of roots, which suggests that the roots present in the with-root treatment did not withdraw nutrients directly from the litter. The soil was slightly drier in the presence of roots, but there was no discernible effect on soil microbial biomass. The effects of roots on soil extractable inorganic nutrients were inconsistent. Roots, however, were consistently associated with higher numbers of soil nematodes. These results suggest that, in soils with low total C and N contents, roots stimulate greater activity of the soil biota, which contribute, in turn, to faster litter decomposition and nutrient release.Contribution No. 95-22 from the Institute of Marine and Coastal Sciences.Contribution No. 95-22 from the Institute of Marine and Coastal Sciences.     

Effects of young Artemisia rothrockii shrubs on soil moisture,soil nitrogen cycling,and resident herbs

Questions: How do young sagebrush shrubs (Artemisia rothrockii, Asteraceae) affect soil moisture availability? How do young sagebrush shrubs affect soil nitrogen cycling? How does the resident herb community respond to shrub removal in the early stages of sagebrush encroachment? Location: Mulkey and Bullfrog Meadows on the Kern Plateau in the Golden Trout Wilderness, Sierra Nevada Mountains, Inyo National Forest, Inyo County, California, USA. Methods: We removed young encroaching sagebrush shrubs from 3.5 m × 3.5 m plots and compared soil moisture, net mineralization, net nitrification, and herb cover with paired control plots over four growing seasons. Results: On average throughout the experiment, the difference between removal plots and control plots in soil moisture was small. Removal plots were wetter by 1.3 ± 2.0% at 0–30 cm depth, 2.1 ± 3.1% at 30–60 cm depth and 3.1 ± 5.8% at 60–90 cm depth. By contrast, after four years, net mineralization was 32 ± 26% (mean ± 95% CI) lower in sagebrush removal plots, suggesting that sagebrush encroachment increases rates of N‐cycling. Total herb cover was 13.0 ± 6.4% (mean ± 95% CI) higher in plots where young sagebrush shrubs were removed. This difference in cover appeared during the first season in which sagebrush shrubs were removed. Conclusions: Our results suggest that while young sagebrush shrubs do not contribute substantially to meadow drying, they alter N cycling rates, and may indirectly increase the rate of their own encroachment by competitively reducing resident herbs.     

Effects of soil frost on nitrogen net mineralization, soil solution chemistry and seepage losses in a temperate forest soil

Freezing and thawing may alter element turnover and solute fluxes in soils by changing physical and biological soil properties. We simulated soil frost in replicated snow removal plots in a mountainous Norway spruce stand in the Fichtelgebirge area, Germany, and investigated N net mineralization, solute concentrations and fluxes of dissolved organic carbon (DOC) and of mineral ions (NH₄⁺, NO₃⁻, Na⁺, K⁺, Ca²⁺, Mg²⁺). At the snow removal plots the minimum soil temperature was −5 °C at 5 cm depth, while the control plots were covered by snow and experienced no soil frost. The soil frost lasted for about 3 months and penetrated the soil to about 15 cm depth. In the 3 months after thawing, the in situ N net mineralization in the forest floor and upper mineral soil was not affected by soil frost. In late summer, NO₃⁻ concentrations increased in forest floor percolates and soil solutions at 20 cm soil depth in the snow removal plots relative to the control. The increase lasted for about 2–4 months at a time of low seepage water fluxes. Soil frost did not affect DOC concentrations and radiocarbon signatures of DOC. No specific frost effect was observed for K⁺, Ca²⁺ and Mg²⁺ in soil solutions, however, the Na⁺ concentrations in the upper mineral soil increased. In the 12 months following snowmelt, the solute fluxes of N, DOC, and mineral ions were not influenced by the previous soil frost at any depth. Our experiment did not support the hypothesis that moderate soil frost triggers solute losses of N, DOC, and mineral ions from temperate forest soils.     

棉花秸秆还田对土壤团聚体有机碳及氮磷钾含量的影响

试验设置棉花秸秆还田和不还田两个处理,还田年限为3年.于第3年11月采集0～5、5～10、10～20和20～30 cm的原状土,采用干筛法将土壤团粒结构分级,研究棉花秸秆还田对土壤团聚体组成和不同粒径土壤团聚体中有机碳和速效氮、磷、钾含量的影响.结果表明: 在0～5 cm土层,棉花秸秆还田显著提高了>5和5～2 mm团聚体含量,显著降低了<0.25 mm微团聚体含量;团聚体有机碳、碱解氮和速效钾含量显著提高,平均提高幅度分别为19.2%、14.2%和17.3%.在5～10 cm土层中,棉花秸秆还田显著提高了>5和5～2 mm团聚体含量,显著降低了<0.25 mm微团聚体含量;团聚体有机碳、碱解氮和速效钾显著提高,平均提高幅度分别为19.6%、12.6%和23.4%.在10～20 cm土层中,棉花秸秆还田显著降低了<0.25 mm微团聚体含量;团聚体有机碳、碱解氮和速效钾含量显著提高,平均提高幅度分别为8.4%、10.9%和11.5%.在20～30 cm土层中,棉花秸秆还田仅显著提高了团聚体速效钾含量,比对照提高了12.0%.棉花秸秆还田显著提高了0～5、5～10 cm土层>5和2～5 mm团聚体各养分贡献率及0.25～0.5 mm团聚体有机碳、碱解氮贡献率,显著降低了<0.25 mm微团聚体各养分贡献率.棉花秸秆还田能增加大团聚体百分含量,降低微团聚体百分含量;显著提高各粒径土壤团聚体中有机碳、碱解氮和速效钾的含量,对速效磷含量无显著影响;提高了大团聚体对土壤养分的贡献率,有利于土壤结构及其养分状况的改善和棉花籽棉、皮棉产量的提高.     

Litter decay controlled by temperature,not soil properties,affecting future soil carbon

Widespread global changes, including rising atmospheric CO₂ concentrations, climate warming and loss of biodiversity, are predicted for this century; all of these will affect terrestrial ecosystem processes like plant litter decomposition. Conversely, increased plant litter decomposition can have potential carbon‐cycle feedbacks on atmospheric CO₂ levels, climate warming and biodiversity. But predicting litter decomposition is difficult because of many interacting factors related to the chemical, physical and biological properties of soil, as well as to climate and agricultural management practices. We applied ¹³C‐labelled plant litter to soil at ten sites spanning a 3500‐km transect across the agricultural regions of Canada and measured its decomposition over five years. Despite large differences in soil type and climatic conditions, we found that the kinetics of litter decomposition were similar once the effect of temperature had been removed, indicating no measurable effect of soil properties. A two‐pool exponential decay model expressing undecomposed carbon simply as a function of thermal time accurately described kinetics of decomposition. (R² = 0.94; RMSE = 0.0508). Soil properties such as texture, cation exchange capacity, pH and moisture, although very different among sites, had minimal discernible influence on decomposition kinetics. Using this kinetic model under different climate change scenarios, we projected that the time required to decompose 50% of the litter (i.e. the labile fractions) would be reduced by 1–4 months, whereas time required to decompose 90% of the litter (including recalcitrant fractions) would be reduced by 1 year in cooler sites to as much as 2 years in warmer sites. These findings confirm quantitatively the sensitivity of litter decomposition to temperature increases and demonstrate how climate change may constrain future soil carbon storage, an effect apparently not influenced by soil properties.     

The sensitivity of soil respiration to soil temperature,moisture, and carbon supply at the global scale

Soil respiration (Rs) is a major pathway by which fixed carbon in the biosphere is returned to the atmosphere, yet there are limits to our ability to predict respiration rates using environmental drivers at the global scale. While temperature, moisture, carbon supply, and other site characteristics are known to regulate soil respiration rates at plot scales within certain biomes, quantitative frameworks for evaluating the relative importance of these factors across different biomes and at the global scale require tests of the relationships between field estimates and global climatic data. This study evaluates the factors driving Rs at the global scale by linking global datasets of soil moisture, soil temperature, primary productivity, and soil carbon estimates with observations of annual Rs from the Global Soil Respiration Database (SRDB). We find that calibrating models with parabolic soil moisture functions can improve predictive power over similar models with asymptotic functions of mean annual precipitation. Soil temperature is comparable with previously reported air temperature observations used in predicting Rs and is the dominant driver of Rs in global models; however, within certain biomes soil moisture and soil carbon emerge as dominant predictors of Rs. We identify regions where typical temperature‐driven responses are further mediated by soil moisture, precipitation, and carbon supply and regions in which environmental controls on high Rs values are difficult to ascertain due to limited field data. Because soil moisture integrates temperature and precipitation dynamics, it can more directly constrain the heterotrophic component of Rs, but global‐scale models tend to smooth its spatial heterogeneity by aggregating factors that increase moisture variability within and across biomes. We compare statistical and mechanistic models that provide independent estimates of global Rs ranging from 83 to 108 Pg yr^?1, but also highlight regions of uncertainty where more observations are required or environmental controls are hard to constrain.     

Influence of rocks on soil temperature,soil water potential,and rooting patterns for desert succulents

Summary At a site in the Sonoran Desert, subterranean rocks and exposed boulders affected soil water potential as well as root morphology and distribution. For Agave deserti, the number of lateral roots per unit length of main root was 11 times higher under rocks and six times higher alongside rocks than in rock-free regions. Total root length per unit soil volume for Echinocereus engelmannii averaged 3-fold higher within 1 cm of boulders than 5 cm away, where the soil was drier. The total length of lateral roots per unit length of main root for Ferocactus acanthodes was 4.2 m m^–1 under rocks but only 0.8 m m^–1 in rock-free regions. The number of lateral roots per unit length of main root for Opuntia acanthocarpa was 7-fold higher alongside rocks than in rock-free regions and even higher under rocks. For transplanted and watered A. deserti, the number of new main roots produced per 1–2 month interval averaged 13 for five plants on the north side of boulders, 8 on the south side, 11 for five plants with half of their roots under rocks, 2 for those with half of their roots over rocks, and 3 for the control plants without rocks. Laboratory experiments showed that the soil water potential under rocks for 10 and 30 mm waterings stayed above –0.5 MPa for 13 and 19 d longer, respectively, than for regions away from rocks. The shortwave absorptance of granitic rocks from the field site was 0.82, the thermal conductivity coefficient was 1.50 W m^–1 °C^–1, and the volumetric heat capacity was 1.75 MJ m^–3 °C^–1. Field measurements indicated that 5-cm-thick buried rocks decreased the diel variation in soil temperatures on their undersurface by only 0.4° C compared with soil. Thus, the primary influence of rocks at the field site on root proliferation and branching for the four species was apparently caused by influences on soil water content.     

Nutritional disorders between potassium,magnesium, calcium,and phosphorus in soil

A new soil testing procedure has been used to demonstrate the effect of an overfertilization by potassium during the preceding years. The total concentration of cations was governed by the amount of soluble anions and the proportion between the different cations was dependent on exchange reactions and is described by activity ratio. High activity ratio between potassium and calcium induced Ca-deficiency, which resulted in a restricted root functioning shown by periodic decreases of nutrient uptake rates and plant growth rate. P-deficiency restricted root growth, but although ammonium phosphate was most effective to increase P-concentration in soil extracts and P-absorption by plants, ordinary superphosphate gave the highest yield and the best utilization of the absorbed phosphorus, magnesium, and calcium.     

Decomposition of cellulose,soil organic matter and plant litter in a temperate grassland soil

The formation of mor humus in an experimental grassland plot, which has been acidified by long-term fertiliser treatment, has been studied by comparing the rates of cellulose, soil organic matter and plant litter decay with those in an adjacent plot with near-neutral pH and mull humus. The decomposition of cellulose filter paper in litter bags of 5 mm, 1-mm and 45-μm mesh size buried at 3 to 4 cm depth the plots was followed by measuring the weight loss and changes in glucose content over a 6 month period. Soil pH was either 5.3 or 4.3. Decomposition of native soil organic matter and plant litter in soil from the same plots were followed using CO₂ evolution in laboratory microcosms. Cellulose weight loss at pH 5.3 was greatest from the 5-mm mesh bags and least from the 45-um mesh bags. At pH 4.3 there was little weight loss from bags and no significant differences in weight loss between bags with different sized mesh. There was, however, a reduction in the glucose content of the hydrolysed and derivatised filter paper with time. The decomposition rate of native soil organic matter in the low pH soil was increased to that observed in the less acid soil when the pH of the former was increased from 4.3 to 5.3. The increase in decomposition rate of added plant litter in the more acid soil as a result of CA(OH)₂ addition was only 60% of that observed in the soil with pH 5.3. These data support the hypothesis that the absence of soil animals and the restricted microbial decomposition in the acidic soil was responsible for mor humus formation.     

The critical soil P levels for crop yield, soil fertility and environmental safety in different soil types

Background and aims

Sufficient soil phosphorus (P) is important for achieving optimal crop production, but excessive soil P levels may create a risk of P losses and associated eutrophication of surface waters. The aim of this study was to determine critical soil P levels for achieving optimal crop yields and minimal P losses in common soil types and dominant cropping systems in China.

Methods

Four long-term experiment sites were selected in China. The critical level of soil Olsen-P for crop yield was determined using the linear-plateau model. The relationships between the soil total P, Olsen-P and CaCl₂-P were evaluated using two-segment linear model to determine the soil P fertility rate and leaching change-point.

Results

The critical levels of soil Olsen-P for optimal crop yield ranged from 10.9 mg kg^?1 to 21.4 mg kg^?1, above which crop yield response less to the increasing of soil Olsen-P. The P leaching change-points of Olsen-P ranged from 39.9 mg kg^?1 to 90.2 mg kg^?1, above which soil CaCl₂-P greatly increasing with increasing soil Olsen-P. Similar change-point was found between soil total P and Olsen-P. Overall, the change-point ranged from 4.6 mg kg^?1 to 71.8 mg kg^?1 among all the four sites. These change-points were highly affected by crop specie, soil type, pH and soil organic matter content.

Conclusions

The three response curves could be used to access the soil Olsen-P status for crop yield, soil P fertility rate and soil P leaching risk for a sustainable soil P management in field.     

Visualization, modelling and prediction in soil microbiology

The introduction of new approaches for characterizing microbial communities and imaging soil environments has benefited soil microbiology by providing new ways of detecting and locating microorganisms. Consequently, soil microbiology is poised to progress from simply cataloguing microbial complexity to becoming a systems science. A systems approach will enable the structures of microbial communities to be characterized and will inform how microbial communities affect soil function. Systems approaches require accurate analyses of the spatio-temporal properties of the different microenvironments present in soil. In this Review we advocate the need for the convergence of the experimental and theoretical approaches that are used to characterize and model the development of microbial communities in soils.