Response of soil microflora to changes in nematode abundance — evidence for large scale effects in grassland soil+

The effect of increasing nematode abundance on microbial biomass and activity in a temperate grassland soil was investigated in a microcosm experiment. The experiment lasted for 33 days. The natural nematode diversity, as well as relevant aspects of the spatial heterogeneity of the soil microhabitat in a 80 m² sampling area were maintained in the microcosms. No correlation was found between nematode abundance and microbial biomass (CFE) or ergosterol content (as a measure of active fungal biomass). However, a doubling of nematode abundance reduced CO₂ production by 11 % and increased bacterial substrate utilization (BIOLOG) by 18 %. A possible explanation is that fungal activity was strongly reduced at higher nematode density, overcompensating the simultaneous increase in bacterial activity. The results show that the nematode community in a grassland soil is capable of causing a considerable shift in soil microbial activities towards an increased bacterial metabolism, overriding the spatial heterogeneity of the soil habitat and the taxonomic diversity of the community itself, and thereby producing functional effects relevant at spatial scales that far exceed the activity domains of the organisms involved.     

The initial rate of C substrate utilization and longer-term soil C storage

The initial reaction of microbial transformation and turnover of soil carbon inputs may influence the magnitude of longer-term net soil C storage. The objective of this study was to test the merit of the hypothesis that the more rapid substrates are initially utilized, the longer the residual products remain in the soil. We used simple model C compounds to determine their decomposition rates and persistence over time. Pure ¹⁴C compounds of glucose, acetate, arginine, oxalate, phenylalanine, and urea were incubated in soil for 125 days at 24°C. Total respired CO₂ and ¹⁴CO₂ was quantitatively measured every day for 15 days and residual soil ¹⁴C after 125 days. The percent ¹⁴C remaining in the soil after 125 days of incubation was positively and significantly correlated with the percent substrate utilized in the first day of incubation. The ¹⁴C in the microbial biomass ranged from 4–15% after 15 days and declined through day 125, contributing significantly to the ¹⁴C that evolved over the longer time period. Priming of ¹²C soil organic matter (SOM) was negative at day 3 but became positive, reaching a maximum on day 12; the total increase in soil C from added substrates was greater than the primed C. The primed C came from ¹²C SOM rather than the microbial biomass. This data supports the concept that the more rapidly a substrate is initially mineralized, the more persistent it will be in the soil over time.     

Contrasting grain crop and grassland management effects on soil quality properties for a north-central Missouri claypan soil landscape

Abstract

Crop management has the potential to either enhance or degrade soil quality, which in turn impacts on crop production and the environment. Few studies have investigated how crop management affects soil quality over different landscape positions. The objective of the present study was to investigate how 12 years of annual cropping system (ACS) and conservation reserve program (CRP) practices impacted soil quality indicators at summit, backslope and footslope landscape positions of a claypan soil in north-central Missouri. Claypan soils are particularly poorly drained because of a restrictive high-clay subsoil layer and are vulnerable to high water erosion. Three replicates of four management systems were established in 1991 in a randomized complete block design, with landscape position as a split-block treatment. The management systems were investigated: (1) annual cropping system 1 (ACS1) was a mulch tillage (typically ≥ 30% of soil covered with residue after tillage operations) corn (Zea mays L.)–soybean (Glycine max (L.) Merr.) rotation system, (2) annual cropping system 2 (ACS2) was a no-till corn–soybean rotation system, (3) annual cropping system 3 (ACS3) was a no-till corn–soybean–wheat (Triticum aestivum L.) rotation system, with a cover crop following wheat, (4) CRP was a continuous cool-season grass and legume system. In 2002, soil cores (at depths of 0–7.5, 7.5–15 and 15–30 cm) were collected by landscape position and analyzed for physical, chemical and biological soil quality properties. No interactions were observed between landscape and crop management. Relative to management effects, soil organic carbon (SOC) significantly increased with 12 years of CRP management, but not with the other management systems. At the 0–7.5-cm soil depth in the CRP system, SOC increased over this period by 33% and soil total nitrogen storage increased by 34%. Soil aggregate stability was approximately 40% higher in the no-till management systems (ACS2 and ACS3) than in the tilled system (ACS1). Soil aggregation under CRP management was more than double that of the three grain-cropping systems. Soil bulk density at the shallow sampling depth was greater in ACS3 than in ACS1 and ACS2. In contrast to studies on other soil types, these results indicate only minor changes to claypan soil quality after 12 years of no-till management. The landscape had minor effects on the soil properties. Of note, SOC was significantly lower in the 7.5–15-cm soil depth at the footslope compared with the other landscape positions. We attribute this to wetter and more humid conditions at this position and extended periods of high microbial activity and SOC mineralization. We conclude that claypan soils degraded by historical cropping practices will benefit most from the adoption of CRP or CRP-like management.     

Dissipation of bacterially derived C and N through the meso- and macrofauna of a grassland soil

Feeding relationships between organisms may be determined by observations of behaviour in manipulative experiments or, as in more recent times, by the use of stable isotope labelling to trace the passage of ¹³C and ¹⁵N through food webs. Here we introduce living bacteria, labelled with both ¹³C and ¹⁵N into intact soil cores to understand further the movement of bacterially sourced C and N into the meso- and macrofauna of a grassland soil. We found that these groups showed a range of isotope levels which relate to their feeding strategies. Some had no label (e.g. dipterous larvae), whilst others were highly labelled which may indicate a preference for the added bacteria. This latter group included Collembola, generally perceived as being predominantly fungal feeders. This work describes a novel technique which has the potential to provide critical information about the dissipation of bacterially derived C and N through the soil food web.     

Conversion of grassland to coniferous woodland has limited effects on soil nitrogen cycle processes

In the last century, conversion of native North American grasslands to Juniperus virginiana forests or woodlands has dramatically altered ecosystem structure and significantly increased ecosystem carbon (C) stocks. We compared soils under recently established J. virginiana forests and adjacent native C₄-dominated grassland to assess changes in potential soil nitrogen (N) transformations and plant available N. Over a 2-year period, concentrations of extractable inorganic N were measured in soils from forest and grassland sites. Potential gross N ammonification, nitrification, and consumption rates were determined using ¹⁵N isotope-dilution under laboratory conditions, controlling for soil temperature and moisture content. Potential nitrification rates (V_max) and microbial biomass, as well as soil physical and chemical properties were also assessed. Extractable NH₄⁺ concentrations were significantly greater in grassland soils across the study period (P ≤ 0.01), but analysis by date indicated that differences in extractable inorganic N occurred more frequently in fall and winter, when grasses were senescent but J. virginiana was still active. Laboratory-based rates of gross N mineralization (ammonification) and nitrification were greater in grassland soils (P ≤ 0.05), but only on one of four dates. Potential nitrification rates (V_max) were an order of magnitude greater than gross nitrification rates in both ecosystems, suggesting that nitrification is highly constrained by NH₄⁺ availability. Differences in plant uptake of N, C inputs, and soil microclimate as forests replace grasslands may influence plant available N in the field, as evidenced by seasonal differences in soil extractable NH₄⁺, and total soil C and N accumulation. However, we found few differences in potential soil N transformations under laboratory conditions, suggesting that this grassland-to-forest conversion caused little change in mineralizable organic N pools or potential microbial activity.     

Effects of tillage on soil microrelief, surface depression storage and soil water storage

Conservation of soil water is an important management objective for crop production in the semi-arid tropics where droughts are persistent. Identification of the best tillage methods to achieve this objective is thus imperative. The integrated effects of conservation tillage on soil micro topography and soil moisture on a sandy loam soil were evaluated. The field experiment consisted of five tillage treatments, namely tied ridging (TR), no till (NT), disc plough (DP), strip catchment tillage (SCT) and hand hoe (HH). Data measured in the field included soil moisture content, surface roughness, infiltration and sorghum grain yield. A depth storage model was used to estimate depression storage TR treatment and the higher the surface roughness, the greater the depression storage volume. Regression analysis showed that random roughness decreased exponentially with increase in cumulative rainfall. Higher moisture contents were associated with treatments having higher depressional storage. Infiltration rate was significantly higher in the tilled soils than the untilled soils. The DP treatment had the highest cumulative infiltration while NT had the lowest. The Infiltration model which was fitted to the infiltration data gave good fit. Grain yield was highest in TR and least in NT, whereas DP and HH had similar yields.     

Response of soil chemical and biological variables to small and large scale changes in climatic factors

This paper studies the effect of large- and small-scale changes of soil temperature and humidity on soil microbial biomass C and N, ergosterol, carbon utilization potential, organic and inorganic N and rate of C and N mineralization at 25°C. Large-scale variations are identified with seasonal changes in temperature and humidity. To simulate small-scale changes, soil temperature and humidity were manipulated in the field. The treatment resulted in damping of temperature fluctuations and a decrease of soil humidity.The majority of the studied variables exhibit pronounced seasonality, showing a clear-cut distinction between summer (July–August) and winter (December). In summer, C mineralization rate and carbon utilization potential was high but microbial and fungal biomass (ergosterol) was low.C and N mineralization rate and microbial and fungal biomass were only affected by sampling date, demonstrating that gross parameters of biomass and activity of microorganisms are not affected by small-scale changes in temperature and humidity. In contrast, variables relating to N availability (organic N, NH₄⁺ and NO₃⁻, microbial biomass N) and carbon utilization potential of the microbial community were highly affected by small-scale changes in soil abiotic conditions. The results suggest that changes in N dynamics induced by small-scale changes of temperature and humidity are caused by shifts in the structure of the microbial community rather than by variations in microbial biomass.     

Effects of desertification on soil organic C and N content in sandy farmland and grassland of Inner Mongolia

Desertification is one of the most serious types of land degradation. A field experiment was conducted during 2002 and 2003 in Horqin Sand Land, China to investigate changes in soil C and N contents in relation to land desertification. Four primary results were derived from this work. First, land desertification characterized by wind erosion resulted in a significant decrease in soil fine particles (clay + silt) with a corresponding increase in sand content. In comparison to non-desertified land, soil fine particle content decreased by up to 89.2%, and sand content increased by up to 47.2%, in the severely desertified land. Second, the organic C and total N in soil were mainly associated with the soil fine particles, and decreased significantly with desertification development. Organic C decreased by 29.2% and total N by 31.5% in the severely desertified land compared to the non-desertified land. Third, the decrease in organic C and N content was greater in desertified grassland than in desertified farmland. Fourth, the changes in organic C and total N content had a significant positive correlation with the soil fine particle content (P < 0.01) and a significant negative correlation with coarse sand content (P < 0.01), indicating that land desertification by wind erosion is mediated through a loss of soil fine particles, with a resultant decrease in soil organic C and total N.     

Spatial heterogeneity in the relocation of added C within the structure of an upland grassland soil

A pulse of ¹³CO₂ was added to the above ground vegetation in an upland grassland to determine the effects of faunal diversity on the flux of carbon to the surface horizons of the soil. Faunal diversity was manipulated by liming and biocide treatments for three years prior to the pulse addition. The relocation of ¹³C within roots and rhizosphere soil was determined by analysis of samples of bulk soil and of specific features identified on soil thin sections on four dates after the addition of the ¹³CO₂ pulse. Analysis of bulk soils showed only a small enrichment in ¹³C and no significant effects of the treatments. Analysis by isotope ratio mass spectrometry of the products of in situ laser combustion of root material and aggregates formed from faunal excrement showed that the distribution of the newly photosynthesised ¹³C is very localised, with large spatial variability in soil and root δ¹³C at scales of less than 1 mm. δ¹³C values ranged from the natural abundance level of around −28‰ to −4.9‰ in roots and to −8.4‰ in aggregates. The small pulse and large spatial variability masked any effects of the liming and biocide treatments in these soils. However, the variability in the relocation of newly photosynthesised carbon may help to explain the large spatial variability found in bacterial numbers at the sub-mm scale within soils and emphasises the importance of the accessibility of substrates to decomposers in undisturbed structured soils.     

气候变化对红松气候生产力的影响研究

根据植物生长的Logistic曲线推导出计算树木气候生产力的模式,分别利用不同年代气象资料计算分析东北地区红松气候生产力及气候变化对气候生产力的影响结果表明,该区红松气候生产力最高值达288m~3/hm~2,最低值仅为115m~3/hm~2,二者相差1倍以上,不同气候对红松气候生产力的影响明显不同。     

Nine years of enriched CO2 changes the function and structural diversity of soil microorganisms in a grassland

To gain insight into microbial function following increased atmospheric CO₂ concentration, we investigated the influence of 9 years of enriched CO₂ (600 μl litre⁻¹) on the function and structural diversity of soil microorganisms in a grassland ecosystem under free air carbon dioxide enrichment (FACE), as affected by plant species (Trifolium repens L. and Lolium perenne L. in monocultures and mixed culture) and nitrogen (N) supply. We measured biomass and activities of enzymes covering cycles of the most important elements (C, N and P). The microbial community was profiled by molecular techniques of phospholipid fatty acid (PLFA) and denaturing gradient gel electrophoresis (DGGE) analysis. The enrichment in CO₂ increased soil microbial biomass (+48.1%) as well as activities of invertase (+36.2%), xylanase (+22.9%), urease (+23.8%), protease (+40.2%) and alkaline phosphomonoesterase (+54.1%) in spring 2002. In autumn, the stimulation of microbial biomass was 25% less and that of enzymes 3–12% less than in spring. Strong correlations between activities of invertase, protease, urease and alkaline phosphomonoesterase and microbial biomass were found. The stimulation of microbial activity in the enriched atmosphere was probably caused by changes in the quantity and kind of root litter and rhizodeposition. The response of soil microorganisms to enriched CO₂ was most pronounced under Trifolium monoculture and under greater N supply. The PLFA analysis revealed that total PLFA contents were greater by 24.7% on average, whereby the proportion of bioindicators representative of Gram‐negative bacteria increased significantly in the enriched CO₂ under less N‐fertilized Lolium culture. Discriminant analysis showed marked differences between the PLFA profiles of the three plant communities. Shannon diversity indices calculated from DGGE patterns were greater (+12.5%) in the enriched CO₂, indicating increased soil bacterial diversity. We conclude that greater microbial biomass and enzyme activity buffer the potential increase in C sequestration occurring from greater C addition in enriched CO₂ due to greater mineralization of soil organic matter.     

Burning causes long-term changes in soil organic matter content of a South African grassland

The effects of burning a native grassland on soil organic matter status was investigated on a long-term (50 years) field experiment where different times and frequencies of burning were compared. Significant decreases in organic C were observed only in the surface 0-2 cm layer and only under annual and biennial winter burning and biennial and triennial autumn burning. Burning in spring did not significantly affect organic C content presumably because substantial amounts of litter decomposed and/or were incorporated into the soil by faunal activity prior to burning. Total N content was decreased substantially to a depth of 6 cm by all burning treatments and as a result, the C:N ratio of soil organic matter was widened. In addition, the amount of potentially mineralizable N, as measured by either aerobic incubation or plant N uptake in a pot experiment, was much reduced. Burning also induced a decrease in light fraction and hot water-extractable C in the 0-2 cm layer but an increase in these parameters, and in microbial biomass C and root density, in the 4-10 cm layer. This was attributed to burning causing a decrease in above-ground litter inputs but increased turnover of root material below the surface. Despite the decrease in organic C and total N content with increasing soil depth, potentially mineralizable N showed the opposite trend. This unexpected finding was confirmed at a nearby site under native grassland and contrasted with decreasing potentially mineralizable N with depth which was measured under a fertilized kikuyu grass dairy pasture. The wide C:N ratio of litter from native grassland, in association with the decreasing size and activity of the microbial biomass with depth results in greater N immobilization (thus less net mineralization) occurring in soil samples taken from close to the soil surface.     

Trends of climatic changes in the Kamennaya Steppe

The main trends of local climatic changes in the Kamennaya Steppe area are described on the basis of a great pool of data. The global climatic changes have resulted in the increase of the mean annual temperature and annual precipitation, especially during the last 15–20 years     

Impacts of soil erosion on organic carbon and nutrient dynamics in an alpine grassland soil

The impact of soil erosion on the nutrient dynamics in alpine grassland soils is still an essential problem. Selecting a grass-covered hillslope in eastern Tibet Plateau, the cesium-137 (¹³⁷Cs) technique was used to determine the impacts of soil erosion on soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and total potassium (TK). The ¹³⁷Cs data revealed that there were distinct soil redistribution patterns in different hillslope positions because of the influences of slope runoff, plant coverage and grazing activity. For the upper slope, soil erosion first decreased downward, followed by soil deposition in its lower part. In contrast, for middle and toe slopes, there was an increasing soil erosion along a downslope transect. Across the lower slope, soil erosion showed an irregular variation. Influenced by the selective transport of water erosion, SOC, TN and TP storage decreased with increasing soil erosion in upper, middle and toe slopes. In contrast, SOC, TN and TP storage varied little with soil erosion in the lower slope. On the whole hillslope, TK storage also varied little with soil erosion due to the large amount of potassium elements derived from soil parent materials. Particularly noteworthy was the greatest storage of SOC, TN and TP in the lower slope where most obvious net soil erosion occurred, which is closely related to the humus accumulation combined with gravel separation as well as weathering and pedogenesis of parent rocks induced by soil freeze-thaw.     

Sources of soil CO2 in calcareous grassland with woody plant encroachment

Purpose

The objective of this study is to estimate the contribution of various sources that influence soil CO₂ concentrations in calcareous grassland.

Materials and methods

The research was performed at the Podgorski Kras plain (45?°33?? N, 13?°55?? E, 400?C430?m.a.s.l.) in the sub-Mediterranean region of Slovenia (SW Slovenia), where many meadows and pastures have been abandoned. In parallel to the measurement of soil respiration R _s, soil gas was sampled for stable isotope analysis. Samples were taken biweekly at two sites, Grassland and Invaded, from July 2008 until November 2010. In addition, daily variations in concentration and stable isotope composition of soil CO₂ were determined in May 2009. The partitioning of soil CO₂ concentrations was performed using stable isotope mass balance calculation.

Results and discussion

The concentration and isotope composition of soil CO₂ exhibited similar seasonal variations at both sites. Lower ??¹³C_CO2 values, ranging from ?28.2 to ?15.2 ??, which occurred during warm periods and higher values, up to ?12.1 ??, were typical of cold winter periods, from December to March. Organic sources were estimated to constitute between 78 and 99?% of total soil CO₂ during warmer periods from May until October. This contribution was lower during the winter, ranging from 46 to 77?%. In winter, the atmospheric component to soil CO₂ dominated, constituting up to 60?%. On average, the inorganic contribution was estimated to comprise 12?% of the soil CO₂ at all sampling locations. The contribution of this source to soil CO₂ concentration, at up to 41?%, was highest in Grassland during the growing season. The inorganic source of soil CO₂ was also an important component during daily variations. The highest contribution was observed during the day, in parallel to the highest respiration rates.

Conclusions

The inorganic pool is shown to be an important part of soil CO₂ in calcareous areas and should be considered as equal to organic CO₂ as a source in soil CO₂ partitioning.     

退耕驱动的近地表特性变化对土壤侵蚀的潜在影响

黄土高原是我国、乃至全球土壤侵蚀最严重的区域之一,侵蚀强度及时空分布特征受近地表特性的显著影响.退耕还林(草)工程大面积的有效实施,势必会引起近地表特性(土壤理化性质、植被茎秆、枯落物、生物结皮、根系系统)的显著变化,进而对坡面径流的水动力学特性及侵蚀过程产生影响.本文从退耕驱动的近地表特性变化、近地表特性变化对坡面径流水动力学特性的影响、近地表特性变化对土壤侵蚀过程(土壤分离、泥沙输移、泥沙沉积)的影响及其机制、区域土壤侵蚀对退耕的响应4个方面较为系统地总结近几十年国内外的研究成果,并提出了目前亟待强化的研究领域.这对理解退耕坡面土壤侵蚀过程及其动力机制、建立植被覆盖坡面的土壤侵蚀过程模型、评价退耕坡面的水土保持效益,具有重要的理论意义.     

Interacting effects of temperature, soil moisture and plant biomass production on ecosystem respiration in a northern temperate grassland

Chamber measurements of total ecosystem respiration (TER) in a native Canadian grassland ecosystem were made during two study years with different precipitation. The growing season (April–September) precipitation during 2001 was less than one-half of the 30-year mean (1971–2000), while 2002 received almost double the normal growing season precipitation. As a consequence soil moisture remained higher in 2002 than 2001 during most of the growing season and peak aboveground biomass production (253.9 g m⁻²) in 2002 was 60% higher than in 2001. Maximum respiration rates were approximately 9 μmol m⁻² s⁻¹ in 2002 while only approximately 5 μmol m⁻² s⁻¹ in 2001. Large diurnal variation in TER, which occurred during times of peak biomass and adequate soil moisture, was primarily controlled by changes in temperature. The temperature sensitivity coefficient (Q₁₀) for ecosystem respiration was on average 1.83 ± 0.08, and it declined in association with reductions in soil moisture. Approximately 94% of the seasonal and interannual variation in R₁₀ (standardized rate of respiration at 10 °C) data was explained by the interaction of changes in soil moisture and aboveground biomass, which suggested that plant aboveground biomass was good proxy for accounting for variations in both autotrophic and heterotrophic capacity for respiration. Soil moisture was the dominant environmental factor that controlled seasonal and interannual variation in TER in this grassland, when variation in temperature was held constant. We compared respiration rates measured with chambers and that determined from nighttime eddy covariance (EC) measurements. Respiration rates measured by both techniques showed very similar seasonal patterns of variation in both years. When TER was integrated over the entire growing season period, the chamber method produced slightly higher values than the EC method by approximately 4.5% and 13.6% during 2001 and 2002, respectively, much less than the estimated uncertainty for both measurement techniques. The two methods for calculating respiration had only minor effects on the seasonal-integrated estimates of net ecosystem CO₂ exchange and ecosystem gross photosynthesis.     

Effects of long-term grassland management on the chemical nature and bioavailability of soil phosphorus

Relationships between the relative solubility of soil phosphorus (P) and short-term plant P uptake were investigated using soils obtained from a field trial that had been maintained under contrasting mowing regimen (no mowing, mowing with clippings left, mowing with clippings removed) for 15?years. In a glasshouse pot experiment, P uptake by red clover and Italian ryegrass was found to be 40% lower for the clippings removed treatment compared with the no mowing treatment, which was consistent with the fact that concentrations of readily extracted inorganic P were 42% lower in the clippings removed treatment soil. However, P uptake was 51–54% higher for the clippings left treatment soil compared with no mowing, despite the fact that levels of readily extracted soil inorganic P were similar in both treatments. This indicated that biological and biochemical processes associated with enhanced mineralisation of organic P and turnover of P through the microbial biomass made a greater contribution to increased plant P uptake in the clippings left soil compared with the other treatments. These findings highlight the importance of soil biological processes in determining the P nutrition and productivity of managed grasslands.     

北京地区下凹式绿地土壤渗透能力及蓄水对土壤物理性质的影响

采用自制的模拟下凹式绿地,通过模拟不同设计暴雨强度条件下下凹式绿地的进水负荷,动态监测下凹式绿地在蓄水过程中土壤水分的渗透量和平均入渗速率,研究北京地区下凹式绿地在蓄积周边外来雨水径流过程中土壤水分的渗透规律以及蓄积雨水径流后对土壤孔隙及土壤密度的影响。结果表明:1)在土壤含水率基本相同的条件下,下凹式绿地土壤渗透性能在1、3、5年一遇暴雨情况下会随着暴雨强度的增加而增大;2)在设计暴雨强度一致时,土壤水分的渗透量和平均入渗速率没有明显差异,土壤密度小、总孔隙度大的渗透量和平均入渗速率更大;3)随着绿地蓄水次数的增多,土壤密度和总孔隙度变化较大,蓄水试验后,土壤密度由最初的1.33 g/cm3变为1.65g/cm3、总孔隙度由原来的50.06%变为39.18%。     

Model analysis of the effects of soil age, fires and harvesting on the carbon storage of boreal forest soils

Potential causes for changes in the amounts of carbon (C) stored in the soils of boreal forests were studied by measuring the C in the soil along a 5000-year chronosequence in coastal western Finland and using a simple dynamic model of decomposition. The amount of soil C stabilized at an age of about 2000 years. This suggests that the youth of many boreal soils does not make them sinks for atmospheric C. Simulated repeated fires kept the amount of soil C reduced by about 25%, but if fires were prevented then the C in the soil increased. Stored C may thus be less than the potential storage where fires are frequent, and it could be increased by preventing fires. Simulated clear-cutting caused a temporary 5–10% decrease in the amount of soil C over a 20-year period after the harvesting. It also caused a long-term decrease in the amount of soil C such that, after two 100-year rotations, the amount had been decreased by 14%. Stored C is almost certainly less than the potential storage and decreasing where forests are harvested.