Metabolic and anabolic responses of arable and forest soils to nutrient addition§

The increase in microbial C content, cumulative respiration and changes in ”︁available” C were determined after adding glucose (2 mg glucose-C (g soil)^—1, ”︁C”), glucose + nitrogen (”︁C+N”) or glucose + nitrogen + phosphorus (”︁C+N+P”) to four soils. In two sandy soils, one agricultural and the other from a beech forest in Germany, available C was still present approximately 7 days after C addition. The supplement N and N+P decreased the content of available C and stimulated respiration rate and microbial growth. In two loamy forest soils from Italy, which had a high native content of microbial C, available C was present in the beech soil but not in a silver fir soil treated with C+N. In the Italian beech and fir soil, microbial growth was highest with C+N+P and C+N addition respectively. Available C remaining in the soil was related to some extent to the native microbial C content. However, microbial growth and respiration response varied between soil and treatment. The respiratory coefficient, that is the ratio of assimilated to respired C, varied between 0.0 and 1.45 μg C_mic (μg CO₂-C)^—1 and was generally higher when a large amount of native biomass was present. The eco-physiological strategy of the soil microbiota in using C seemed to shift according to the biomass content, the added concentration and composition of available substrates, and emergent system properties.     

Energetic eco‐physiology of the soil microbiota in two landscapes of southern and northern Germany

Interactions between microbial communities and organic matter were analyzed for soils from the project regions ’︁Ecosystem Research in the Agricultural Landscape/FAM, Munich’ in southern Germany and ’︁Ecosystem Research in the Bornhöved Lake district’ from northern Germany using ratios between microbial biomass content (C_mic), microbial metabolic quotient (qCO₂) and organic carbon content (C_org). In the agricultural soils in southern Germany, the qCO₂/C_org ratio differed significantly with respect to agricultural management in contrast to ecophysiological C_mic/C_org ratio. In addition, C_mic/C_org ratio decreased from 39 to 21 mg C_mic g^—1 C_org and qCO₂/C_org ratio increased from 72 to 180 mg CO₂‐C g^—1 C_mic h^—1 (g C_org g^—1 soil)^—1 with increasing soil depth. For the upper soil horizons from the landscape in northern Germany the two quotients differed significantly with reference to land use showing highest microbial colonization under grassland and lowest under beech forest. In contrast, C use efficiency was lowest in arable field under maize monoculture and highest in a wet grassland having a high organic C content.     

The Origin of Soil Organic C,Dissolved Organic C and Respiration in a Long‐Term Maize Experiment in Halle,Germany, Determined by 13C Natural Abundance

For a quantitative analysis of SOC dynamics it is necessary to trace the origins of the soil organic compounds and the pathways of their transformations. We used the ¹³C isotope to determine the incorporation of maize residues into the soil organic carbon (SOC), to trace the origin of the dissolved organic carbon (DOC), and to quantify the fraction of the maize C in the soil respiration. The maize‐derived SOC was quantified in soil samples collected to a depth of 65 cm from two plots, one ’︁continuous maize’ and the other ’︁continuous rye’ (reference site) from the long‐term field experiment ’︁Ewiger Roggen’ in Halle. This field trial was established in 1878 and was partly changed to a continuous maize cropping system in 1961. Production rates and δ¹³C of DOC and CO₂ were determined for the Ap horizon in incubation experiments with undisturbed soil columns. After 37 years of continuous maize cropping, 15% of the total SOC in the topsoil originated from maize C. The fraction of the maize‐derived C below the ploughed horizon was only 5 to 3%. The total amount of maize C stored in the profile was 9080 kg ha⁻¹ which was equal to about 31% of the estimated total C input via maize residues (roots and stubble). Total leaching of DOC during the incubation period of 16 weeks was 1.1 g m⁻² and one third of the DOC derived from maize C. The specific DOC production rate from the maize‐derived SOC was 2.5 times higher than that from the older humus formed by C₃ plants. The total CO₂‐C emission for 16 weeks was 18 g m⁻². Fifty‐eight percent of the soil respiration originated from maize C. The specific CO₂ formation from maize‐derived SOC was 8 times higher than that from the older SOC formed by C₃ plants. The ratio of DOC production to CO₂‐C production was three times smaller for the young, maize‐derived SOC than for the older humus formed by C₃ plants.     

Three‐source partitioning of CO2 efflux from maize field soil by 13C natural abundance

A natural‐¹³C‐labeling approach—formerly observed under controlled conditions—was tested in the field to partition total soil CO₂ efflux into root respiration, rhizomicrobial respiration, and soil organic matter (SOM) decomposition. Different results were expected in the field due to different climate, site, and microbial properties in contrast to the laboratory. Within this isotopic method, maize was planted on soil with C₃‐vegetation history and the total CO₂ efflux from soil was subdivided by isotopic mass balance. The C₄‐derived C in soil microbial biomass was also determined. Additionally, in a root‐exclusion approach, root‐ and SOM‐derived CO₂ were determined by the total CO₂ effluxes from maize (Zea mays L.) and bare‐fallow plots. In both approaches, maize‐derived CO₂ contributed 22% to 35% to the total CO₂ efflux during the growth period, which was comparable to other field studies. In our laboratory study, this CO₂ fraction was tripled due to different climate, soil, and sampling conditions. In the natural‐¹³C‐labeling approach, rhizomicrobial respiration was low compared to other studies, which was related to a low amount of C₄‐derived microbial biomass. At the end of the growth period, however, 64% root respiration and 36% rhizomicrobial respiration in relation to total root‐derived CO₂ were calculated when considering high isotopic fractionations between SOM, microbial biomass, and CO₂. This relationship was closer to the 50% : 50% partitioning described in the literature than without fractionation (23% root respiration, 77% rhizomicrobial respiration). Fractionation processes of ¹³C must be taken into account when calculating CO₂ partitioning in soil. Both methods—natural ¹³C labeling and root exclusion—showed the same partitioning results when ¹³C isotopic fractionation during microbial respiration was considered and may therefore be used to separate plant‐ and SOM‐derived CO₂ sources.     

Soil microbial biomass and activity: the effect of site characteristics in humid temperate forest ecosystems

Microbial biomass, respiratory activity, and in‐situ substrate decomposition were studied in soils from humid temperate forest ecosystems in SW Germany. The sites cover a wide range of abiotic soil and climatic properties. Microbial biomass and respiration were related to both soil dry mass in individual horizons and to the soil volume in the top 25 cm. Soil microbial properties covered the following ranges: soil microbial biomass: 20 µg C g^–1–8.3 mg C g^–1 and 14–249 g C m^–2, respectively; microbial C–to–total organic C ratio: 0.1%–3.6%; soil respiration: 109–963 mg CO₂‐C m^–2 h^–1; metabolic quotient (qCO₂): 1.4–14.7 mg C (g C_mic)^–1 h^–1; daily in‐situ substrate decomposition rate: 0.17%–2.3%. The main abiotic properties affecting concentrations of microbial biomass differed between forest‐floor/organic horizons and mineral horizons. Whereas microbial biomass decreased with increasing soil moisture and altitude in the forest‐floor/organic horizons, it increased with increasing N_tot content and pH value in the mineral horizons. Quantities of microbial biomass in forest soils appear to be mainly controlled by the quality of the soil organic matter (SOM), i.e., by its C : N ratio, the quantity of N_tot, the soil pH, and also showed an optimum relationship with increasing soil moisture conditions. The ratio of C_mic to C_org was a good indicator of SOM quality. The quality of the SOM (C : N ratio) and soil pH appear to be crucial for the incorporation of C into microbial tissue. The data and functional relations between microbial and abiotic variables from this study provide the basis for a valuation scheme for the function of soils to serve as a habitat for microorganisms.     

Initial results from a long-term, multi-site field study of the effects on soil fertility and microbial activity of sludge cakes containing heavy metals

In a long‐term study of the effects on soil fertility and microbial activity of heavy metals contained in sewage sludges, metal‐rich sludge cakes each with high Zn, Cu or Cd concentrations were applied annually for 4 years (1994–1997) to nine sites throughout Britain. These sites were selected to represent agricultural soils with a range of physical and chemical properties, typical of those likely to be amended with sewage sludge. The aim was to establish individual total Zn (approx. 60–450 mg kg^?1), total Cu (approx. 15–200 mg kg^?1) and total Cd (approx. 0.2–4 mg kg^?1) metal dose–response treatments at each site. Sludges with low metal concentrations were added to all treatments to achieve as constant an addition of organic matter as possible. Across the nine sites, soil pH was the single most important factor controlling Zn (P < 0.001; r² = 92%) and Cd extracted with 1 m NH₄NO₃ (P < 0.001; r² = 72%), and total iron content the most important factor controlling Cu extracted with 1 m NH₄NO₃ (P < 0.001; r² = 64%). There were also positive relationships (P < 0.001) between soil organic carbon (C) concentrations and soil biomass C and respiration rates across the nine sites. Oxidation of sludge C following land application resulted in approximately 45% of the digested sludge cake C and approximately 64% of the ‘raw’ sludge cake C being lost by the end of the 4‐year application period. The sludge cake applications generally increased soil microbial biomass C and soil respiration rates, whilst most probable numbers of clover Rhizobium were generally unchanged. Overall, there was no evidence that the metal applications were damaging soil microbial activity in the short term after the cessation of sludge cake addition.     

Oily waste containing natural radionuclides: does it cause stimulation or inhibition of soil bacterial community?

Contamination with oily wastes containing natural radionuclides is a potential hazard for soil health and function. Our study aimed to reveal both structural and functional changes of the microbial community resistant to and able to decompose oily wastes in soil. To do this, we determined CO₂ efflux, microbial biomass (by the extraction‐fumigation method), and community structure (by PCR‐SSCP) for 120 d after application of radioactive oily wastes to the soil at the ratio 1:4. The addition of the waste resulted in an increase of the activity concentration of ²²⁶Ra by 130 times (up to 643 Bq kg^?1) and of ²³²Th by 29 times (up to 254 Bq kg^?1). The calculated weighted dose for the radionuclide ²²⁶Ra was found to be below the values that are known to affect microorganisms. However, the cumulative effect of a repeated deposition of radioactive oily waste may result in an increase of the weighted dose up to an effective level. During the incubation, the hydrocarbon (HC) content of the waste‐treated soil decreased from 156 to 54 g kg^?1 of soil indicating intensive decomposition of added organics by soil microorganisms. The waste application, however, led to an inhibition of soil microbial biomass compared with the control (by 26–47%). Microbial respiration was stimulated in the first month of incubation and then decreased until the end of the incubation period (by up to 74% compared to the control). The qCO₂ was estimated to be 3‐fold higher than the control on day 1 of incubation and equal to the control on day 120 of incubation. The bacterial diversity decreased in the contaminated soil compared with the control soil. The bacterial community structure was altered by domination of new oil degrader species belonging to the genera Dyella, Pseudoxanthomonas, Sinobacter, and Parvibaculum. Thus, disposal of radioactive petroleum waste strongly altered the structure of the microbial community resulting in the selection of resistant species able to decompose pollutants and also affected the community function (inhibition of microbial biomass and stimulation of respiration) which tended to stabilize after long‐term incubation.     

Net nitrogen immobilization in soil induced by small additions of energy sources

Abstract

This study investigated whether small additions to soil of primary paper-mill sludge, a wood fibre residue from paper production (fibre sludge), caused temporary N immobilization and thereby reduced the amount of inorganic nitrogen leached from agricultural land. This was achieved by measuring respiration and immobilization of N in incubation studies at 8°C, with fibre sludge added at rates varying from 63 to 1000?mg?C?kg^?1 soil. Glucose added at rates of 63–250?mg?C?kg^?1 soil was used as a reference. Respiration in soil after glucose addition followed an exponential course with the highest rates on days 2–4. During this period maximum peaks of net N immobilization were measured. Even addition of only 63?mg glucose-C?kg^?1 soil caused significant immobilization of N in soil. Fibre sludge additions to soil caused lower respiration activities, characterized by two initial peaks followed by somewhat higher respiration rates during the remaining incubation than for glucose. It was likely that hemicellulose, which amounted to 14% of the total C, was the initial available energy source in the sludge as concentrations of water-soluble C were very low. Addition of at least 250?mg?C?kg^?1 soil as fibre sludge was required to cause significant N immobilization in soil corresponding to 5?kg?N?ha^?1. Both nitrate and ammonium were immobilized. Relating maximum N immobilization data during days 2 to 10 to corresponding respiration data for glucose and fibre sludge revealed that microbes utilised similar amounts of C per unit N immobilized. On average, 175.6±74.8?mg CO₂-C were respired to immobilize 1?mg?N and the relationship between C respiration and N immobilization was linear (R ²=0.984). To make soil application of fibre sludge a realistic counter-measure against N leaching from agricultural soils, pre-treatment is necessary to increase the content of energy readily available to microbes.     

Microbial Respiration in Soils of the Argentine Pampas after Metsulfuron Methyl, 2,4‐D,and Glyphosate Treatments

Abstract

Short‐term response of microbial respiration after treatment with different doses of the herbicides metsulfuron methyl (MET), 2,4‐D, and glyphosate (GLY) was studied in microcosms of soils collected in three agricultural sites of the Southern Pampas region, Buenos Aires, Argentina. The influence of diammonium phosphate [(NH₄)₂PO₄] on carbon dioxide (CO₂) evolution, when applied with the highest doses of the herbicides, was also investigated. MET had no effect on microbial respiration of an acidic soil of San Román (pH 6.06), even at the highest rate. However, MET inhibited microbial respiration in soils of Bordenave (pH 7.44), at a rate of 0.1 mg kg^?1 soil. Low application rates of GLY and 2,4‐D produced only transitory effects on CO₂ evolution, whereas the addition of high doses of these herbicides stimulated microbial activity. On the other hand, the addition of fertilizer to soil treated with a high dose of GLY temporarily inhibited CO₂ release.     

Initial results from long-term field studies at three sites on the effects of heavy metal-amended liquid sludges on soil microbial activity

In a long‐term study of the effects on soil fertility and microbial activity of heavy metals contained in sewage sludges, metal‐amended liquid sludges each with elevated Zn, Cu or Cd concentrations were applied over a 3‐year period (1995–1997) to three sites in England. The experiments were sited adjacent to experimental plots receiving metal‐rich sludge cakes enabling comparisons to be made between the effects of heavy metal additions in metal‐amended liquid sludges and sludge cakes. The liquid sludge additions were regarded as ‘worst case’ treatments in terms of likely metal availability, akin to a long‐term situation following sewage sludge additions where organic matter levels had declined and stabilised. The aim was to establish individual Zn (50–425 mg kg^?1), Cu (15–195 mg kg^?1) and Cd (0.3–4.0 mg kg^?1) metal dose–response treatments at each site, but with significantly smaller levels of organic matter addition than the corresponding sludge cake experiments. There were no differences (P > 0.05) in soil respiration rates, biomass carbon concentrations or most probable numbers of clover Rhizobium between the treatments at any of the sites at the end of the liquid sludge application programme. Soil heavy metal extractability differed between the metal‐amended liquid sludge and metal‐rich sludge cake treatments; Zn and Cd extractabilities were higher from the liquid sludge additions, whereas Cu extractability was higher from the sludge cake application. These differences in metal extractability in the treated soil samples reflected the contrasting NH₄NO₃ extractable metal contents of the metal‐amended liquid sludges and sludge cakes that were originally applied.     

Root and Microbial Contributions to Soil Respiration during a Soybean Growing Season

Soil respiration is an important process for carbon geochemical cycling. Based on our five long‐term fertilizer experiments, soil respiration was measured using pot experiments with or without planting soybean. Soil respiration rates and soybean root biomass were determined at different observation times. Soil respiration rates due to soil microbial activity could be estimated by extrapolating a newly derived regressive equation at zero root biomass. Soil microbial respiration rates in the control were also observed directly, ranging from 16.0 to 42.7 mg carbon (C) m^?2 h^?1. Average soil microbial respiration rates from the regression analyses and direct observations were 32.9 and 27.8 mg C m^?2 h^?1, respectively. The average proportions of soil respiration rates due to the soybean growth were 63.0% using the regressive equation and 69.8% from direct observation. Therefore, the application of these two methods could provide new insight for separating plant root respiration from soil microbial respiration, which is important for estimating their individual contributions to atmospheric carbon dioxide.     

Microbial CO2 production, CH4 dynamics and nitrogen in a wetland soil (New York State, USA) associated with three plant species (Typha, Lythrum, Phalaris)

Plants furnish soil with organic carbon (OC) compounds that fuel soil microorganisms, but whether individual plant species – or plants with unique traits – do so uniquely is uncertain. We evaluated soil microbial processes within a wetland in which areas dominated by a distinct plant species (cattail –Typha sp.; purple loosestrife –Lythrum salicaria L.; reed canarygrass –Phalaris arundinacea L.) co‐mingled. We also established an experimental plot with plant shoot removal. The Phalaris area had more acidic soil pH (7.08 vs. 7.27–7.57), greater amount of soil organic matter (19.0% vs. 9.0–11.5%), and the slowest production rates of CO₂ (0.10 vs. 0.21–0.46 μmol kg⁻¹ s⁻¹) and CH₄ (0.040 vs. 0.054–0.079 nmol kg⁻¹ s⁻¹). Nitrogen cycling was dominated by net nitrification, with similar rates (17.2–18.9 mg kg⁻¹ 14 days⁻¹) among the four sampling areas. In the second part of the study, we emplaced soil cores that either allowed root in‐growth or excluded roots to evaluate how roots directly affect soil CO₂ and CH₄. The three plant species had similar amounts of root growth (ca 290 g m⁻² year⁻¹). Fungal biomass was similar in soils with root in‐growth versus root exclusion, regardless of dominant plant species. Rates of soil CO₂ production did not differ with root in‐growth versus root exclusion, and added glucose increased CO₂ production rates by only 35%. Root in‐growth did lead to greater rates of CH₄ production; albeit, addition of glucose had much greater effect on CH₄ production (1.24 nmol kg⁻¹ s⁻¹) compared with controls without added glucose (0.058 nmol kg⁻¹ s⁻¹). Our data revealed relatively few subtle differences in soil characteristics and processes associated with different plant species; albeit, roots had little effect, even inhibiting some microbial processes. This research highlights the need for both field and experimental studies in long‐established monocultures of plant species to understand the role of plant biodiversity in soil function.     

Soil respiration rate and its sensitivity to temperature in pasture systems of dry-tropics

Abstract

Tree clearing is a topical issue the world over. In Queensland, the high rates of clearing in the past were mainly to increase pasture production. The present research evaluates the impact of clearing on some soil biological properties, i.e. total soil respiration, root respiration, microbial respiration, and microbial biomass (C and N), and the response of soil respiration to change in temperature.

In-field and laboratory (polyhouse) experiments were undertaken. For in-field studies, paired cleared and uncleared pasture plots were selected to represent three major tree communities of the region, i.e. Eucalyptus populnea, E. melanophloia, and Acacia harpophylla. The cleared sites were chosen to represent three different time-since-clearing durations (5, 11–13, and 33 years; n=18 for cleared and uncleared plots) to determine the temporal impact of clearing on soil biological properties. Experiments were conducted in the polyhouse to study in detail the response of soil respiration to changes in soil temperature and soil moisture, and to complement in-field studies for estimating root respiration.

The average rate of CO₂ emission was 964 g CO₂/m²/yr, with no significant difference (P<0.05) among cleared and uncleared sites. Microbial respiration and microbial biomass were greater at uncleared compared with those at cleared sites. The Q ₁₀-value of 1.42 (measured for different seasons in a year) for in-field measurements suggested a small response of soil respiration to soil temperature, possibly due to the limited availability of soil moisture and/or organic matter. However, results from the polyhouse experiment suggested greater sensitivity of root respiration to temperature change than for total soil respiration. Since root biomass (herbaceous roots) was greater at the cleared than at uncleared sites, and root respiration increased with an increase in temperature, we speculate that with rising ambient temperature and consequently soil temperature, total soil respiration in cleared pastures will increase at a faster rate than that in uncleared pastures.     

Root and rhizomicrobial respiration: A review of approaches to estimate respiration by autotrophic and heterotrophic organisms in soil

Partitioning the root‐derived CO₂ efflux from soil (frequently termed rhizosphere respiration) into actual root respiration (RR, respiration by autotrophs) and rhizomicrobial respiration (RMR, respiration by heterotrophs) is crucial in determining the carbon (C) and energy balance of plants and soils. It is also essential in quantifying C sources for rhizosphere microorganisms and in estimation of the C contributing to turnover of soil organic matter (SOM), as well as in linking net ecosystem production (NEP) and net ecosystem exchange (NEE). Artificial‐environment studies such as hydroponics or sterile soils yield unrealistic C‐partitioning values and are unsuitable for predicting C flows under natural conditions. To date, several methods have been suggested to separate RR and RMR in nonsterile soils: 1) component integration, 2) substrate‐induced respiration, 3) respiration by excised roots, 4) comparison of root‐derived ¹⁴CO₂ with rhizomicrobial ¹⁴CO₂ after continuous labeling, 5) isotope dilution, 6) model‐rhizodeposition technique, 7) modeling of ¹⁴CO₂ efflux dynamics, 8) exudate elution, and 9) δ¹³C of CO₂ and microbial biomass. This review describes the basic principles and assumptions of these methods and compares the results obtained in the original papers and in studies designed to compare the methods. The component‐integration method leads to strong disturbance and non‐proportional increase of CO₂ efflux from different sources. Four of the methods (5 to 8) are based on the pulse labeling of shoots in a ¹⁴CO₂ atmosphere and subsequent monitoring of ¹⁴CO₂ efflux from the soil. The model‐rhizodeposition technique and exudate‐elution procedure strongly overestimate RR and underestimate RMR. Despite alternative assumptions, isotope dilution and modeling of ¹⁴CO₂‐efflux dynamics yield similar results. In crops and grasses (wheat, ryegrass, barley, buckwheat, maize, meadow fescue, prairie grasses), RR amounts on average to 48±5% and RMR to 52±5% of root‐derived CO₂. The method based on the ¹³C isotopic signature of CO₂ and microbial biomass is the most promising approach, especially when the plants are continuously labeled in ¹³CO₂ or ¹⁴CO₂ atmosphere. The “difference” methods, i.e., trenching, tree girdling, root‐exclusion techniques, etc., are not suitable for separating the respiration by autotrophic and heterotrophic organisms because the difference methods neglect the importance of microbial respiration of rhizodeposits.     

Carbon flows in the rhizosphere of ryegrass (Lolium perenne)

This study addresses the issue of carbon (C) fluxes through below ground pools within the rhizosphere of Lolium perenne using the ¹⁴C pulse labeling. Lolium perenne was grown in plexiglas chambers on topsoil of a Haplic Luvisol under controled laboratory conditions. ¹⁴C‐CO₂ efflux from soil, as well as ¹⁴C content in shoots, roots, soil, dissolved organic C (DOC), and microbial biomass were monitored for 11 days after the pulsing. Lolium allocates about 48 % of the total assimilated ¹⁴C below the soil surface, and roots were the primary sink for this C. Maximum ¹⁴C content in the roots was observed 12 hours after the labeling and it amounts to 42 % of the assimilated C. Only half of the ¹⁴C amount was found in the roots at the end of the monitoring period. The remainder was lost through root respiration, root decomposition, and rhizodeposition. Six hours after the ¹⁴C pulse labeling soil accounted for 11 %, DOC for 1.1 %, and microbial biomass for 4.9 % of assimilated C. ¹⁴C in CO₂ efflux from soil was detected as early as 30 minutes after labeling. The maximum ¹⁴C‐CO₂ emission rate (0.34 % of assimilated ¹⁴C h^—1) from the soil occurred between four and twelve hours after labeling. From the 5^th day onwards, only insignificant changes in carbon partitioning occurred. The partitioning of assimilated C was completed after 5 days after assimilation. Based on the ¹⁴C partitioning pattern, we calculated the amount of assimilated C during 47 days of growth at 256 g C m^—2. Of this amount 122 g C m^—2 were allocated to below ground, shoots retained 64 g C m^—2, and 70 g C m^—2 were lost from the shoots due to respiration. Roots were the main sink for below ground C and they accounted for 74 g C m^—2, while 28 g C m^—2 were respired and 19 g C m^—2 were found as residual ¹⁴C in soil and microorganisms.     

Attempts to determine available carbon in soils

Summary The size of the C pool that is readily available to microorganisms affects important N transformations that occur in soils, e.g., denitrification and N immobilization. In the present work, the C content of soil extracts, the C in water displaced from soil and biomass C were compared as indicators of available C. A comparison of C measured by the anthrone method and a total organic-C analysis of extracts indicated that only a small portion of C was in carbohydrates. The substrate-induced respiration method of measuring biomass in conjunction with the Wright-Hobbie model was used as an additional determination of available C. Total organic-C analysis of a 1 N H₂SO₄ extract gave the highest C values (500–1700 mg C kg^-1 soil) and the substrate-induced respiration method gave the lowest (1–5 mg glucose equivalents kg^-1 soil). The C values closest to the C turnover measured in long-term incubation studies were obtained by the substrate-induced respiration method using the Wright-Hobbie model.Contribution from the Soil-Microbial Systems Laboratory, NRI, USDA-ARS, Beltsville, MD 20705, USA     

Contributions of root respiration to total soil respiration before and after frost in Populus euphratica forests

Temporal changes in soil CO₂‐efflux rate was measured by a canopy‐gap method in a Populus euphratica forest located at the both sides of Tarim River banks (W China). Soil CO₂‐efflux rates in situ were correlated with key soil biotic (e.g., fungal, bacterial, and actinomycetes populations) and abiotic (e.g., soil moisture, temperature, pH, organic C) variables. Two kinds of measurement plots were selected: one under the crown of a living Populus euphratica tree and the other under a dead standing Populus euphratica tree. Diurnal variations in soil respiration in these plots were measured both before and after the occurrence of the first frost. Soil respiration of the dead standing Populus euphratica (Rd) was assumed to be a measure of heterotrophic respiration rate (Rh), and root respiration rate (Rr) was estimated as the difference between soil respiration under living (Rl) minus soil respiration under dead standing Populus euphratica. Daily variation of Rr contribution to the total soil respiration in Populus euphratica forests were analyzed before and after the frost. The contribution of root respiration to total soil respiration before and after frost varied from 22% to 45% (mean 30%) and from 38% to 50% (mean 45%), respectively. In addition, Rh was significantly correlated with soil temperature both before and after frost. In contrast, Rr was not significantly correlated with soil temperature. Change in Q₁₀ of Rr was different from that of Rh from before the frost to after the frost. Variation of Q₁₀ of Rr from before the frost to after the frost was larger than that of Q₁₀ of Rh. Thus, the results indicate that different soil respiration models are needed for Rr and Rh because different factors control the two components of soil respiration.     

Impact of sulfadiazine and chlorotetracycline on soil bacterial community structure and respiratory activity

Veterinary medicines enter agricultural soils by the use of animal excrements as fertilizers. To study their impact on soil bacterial communities, microcosms containing orthic luvisol soil were spiked with the antimicrobial agents sulfadiazine (SDZ) and chlorotetracycline (CTC) at three different concentrations (1, 10, 50 mg kg⁻¹ soil) and incubated for 48 days at 20 °C. The impact on the microbial respiratory activity was measured continuously in a respirometer (Sapromat). Changes in bacterial community structure were visualized by means of PCR-denaturing gradient gel electrophoresis (DGGE) of 16S rDNA derived from soil samples after 1, 7, 11 and 48 days. Additionally, growth inhibitory effects of SDZ and CTC on bacteria previously isolated from the same soil were tested in agar diffusion tests. In microcosms with soil and antibiotics only, no effects could be observed, either on respiratory activity or on bacterial population structure. Therefore, further incubations were conducted in the presence of an additional assimilable carbon source (5 g glucose kg⁻¹ soil). In the presence of glucose, SDZ affected soil respiration as well as the bacterial community structure: Additional bands appeared and some bands already visible at the beginning of incubations increased in intensity. A clear relationship between SDZ concentrations and changes in DGGE patterns became visible. During 48 days of incubation, changes in DGGE patterns were minimal in microcosms with 50 mg SDZ kg⁻¹soil indicating an inhibition of strains, which were capable of growing on glucose in the presence of lower SDZ concentrations. Only a few soil bacterial isolates (5 out of 47 strains tested) were weakly inhibited by SDZ in agar diffusion disk tests. Contrastingly, CTC inhibited growth of 12 soil bacterial isolates significantly in disk tests, but no effects on soil respiration and bacterial community structure could be observed. In the presence of the soil matrix the growth inhibitory potential of CTC decreased due to adsorption or complexation. This was confirmed in growth inhibition experiments with soil suspensions and time-dependent sampling.     

重复添加植物残体后土壤微生物活性和生物量对盐度的响应特征

Microbial adaptation to salinity can be achieved through synthesis of organic osmolytes,which requires high amounts of energy;however,a single addition of plant residues can only temporarily improve energy supply to soil microbes.Therefore,a laboratory incubation experiment was conducted to evaluate the responses of soil microbes to increasing salinity with repeated additions of plant residues using a loamy sand soil with an electrical conductivity in saturated paste extract（EC_e） of 0.6 dS m^-1.The soil was kept non-saline or salinized by adding different amounts of NaCl to achieve EC_e of 12.5,25.0 and 50.0 dS m^-1.The non-saline soil and the saline soils were amended with finely ground pea residues at two rates equivalent to 3.9 and 7.8 g C kg^-1 soil on days 0,15 and29.The soils receiving no residues were included as a control.Cumulative respiration per g C added over 2 weeks after each residue addition was always greater at 3.9 than 7.8 g C kg^-1 soil and higher in the non-saline soil than in the saline soils.In the saline soils,the cumulative respiration per g C added was higher after the second and third additions than after the first addition except with3.9 g C kg^-1 at EC_e of 50 dS m^₁.Though with the same amount of C added(7.8 g C kg^-1),salinity reduced soil respiration to a lesser extent when 3.9 g C kg^-1 was added twice compared to a single addition of 7.8 g C kg^-1.After the third residue addition,the microbial biomass C concentration was significantly lower in the soils with EC_e of 25 and 50 dS m^₁ than in the non-saline soil at3.9 g C kg^-1,but only in the soil with EC_e of 50 dS m^-1 at 7.8 g C kg^-1.We concluded that repeated residue additions increased the adaptation of soil microbial community to salinity,which was likely due to high C availability providing microbes with the energy needed for synthesis of organic osmolytes.     

Biodegradation kinetics of monosaccharides and their contribution to basal respiration in tropical forest soils

Low molecular weight (LMW) organic compounds in soil solution are easily biodegradable and could fuel respiration by soil microorganisms. Our main aim was to study the mineralization kinetics of monosaccharides using ¹⁴C-radiolabelled glucose. Based on these data and the soil solution concentrations of monosaccharides, we evaluated the contribution of monosaccharides to basal respiration for a variety of tropical forest soils. Further, the factors controlling the mineralization kinetics of monosaccharides were examined by comparing tropical and temperate forest soils. Monosaccharides comprised on average 5.2 to 47.7% of dissolved organic carbon in soil solution. Their kinetic parameters (V _max and K_M ), which were described by a single Michaelis-Menten equation, varied widely from 11 to 152?nmol?g^?1?h^?1 and 198 to 1294?µmol?L^?1 for tropical soils, and from 182 to 400?nmol?g^?1?h^?1 and 1277 to 3150?µmol?L^?1 for temperate soils, respectively. The values of V _max increased with increasing microbial biomass-C in tropical and temperate soils, while the K_M values had no correlations with soil biological or physicochemical properties. The positive correlation between V _max values and microbial biomass-C indicates that microbial biomass-C is an essential factor to regulate the V _max values in tropical and temperate forest soils. The biodegradation kinetics of monosaccharides indicate that the microbial capacity of monosaccharide mineralization far exceeds its rate at soil solution concentration. Monosaccharides in soil solution are rapidly mineralized, and their mean residence times in this study were very short (0.4–1.9?h) in tropical forests. The rates of monosaccharide mineralization at actual soil solution concentrations made up 22–118% of basal respiration. Probably because of the rapid and continuous production and consumption of monosaccharides, monosaccharide mineralization is shown to be a dominant fraction of basal respiration in tropical forest soils, as well as in temperate and boreal forest soils.