水分含量对秸秆还田土壤碳矿化和微生物特性的影响

An 80-d incubation experiment was conducted to investigate straw decomposition,the priming effect and microbial characteristics in a non-fertilized soil(soil 1) and a long-term organic manure-fertilized soil(soil 2) with and without13 C-labeled maize straw amendment under different moisture levels. The soil 2 showed a markedly higher priming effect,microbial biomass C(Cmic),and β-glucosidase activity,and more abundant populations of bacteria and fungi than the soil 1. Also,soil CO2 emission,Cmic,β-glucosidase activity,and bacterial and fungal population sizes were substantially enhanced by straw amendment. In the presence of straw,the amount of straw mineralization and assimilation by microbes in the soil at 55% of water holding capacity(WHC) were significantly higher by 31% and 17%,respectively,compared to those at 25% of WHC. In contrast,β-glucosidase activity and fungal population size were both enhanced as the moisture content decreased. Cmicdecreased as straw availability decreased,which was mainly attributed to the reduction of straw-derived Cmic. Amended soils,except the amended soil 2 at 25% of WHC,had a more abundant fungal population as straw availability decreased,indicating that fungal decomposability of added straw was independent of straw availability. Non-metric multidimensional scaling analysis based on fungal denatured gradient gel electrophoresis band patterns showed that shifts in the fungal community structure occurred as water and straw availability varied. The results indirectly suggest that soil fungi are able to adjust their degradation activity to water and straw availability by regulating their community structure.     

Potential importance of bacteria and fungi in nitrate assimilation in soil

Soil microorganisms can use a wide range of N compounds but are thought to prefer NH₄⁺. Nevertheless, ¹⁵N isotope dilution studies have shown that microbial immobilization of NO₃⁻ can be an important process in many soils, particularly relatively undisturbed soils. Our objective was to develop a method for measuring NO₃⁻ immobilization potential so that the relative contributions of bacteria and fungi could be determined. We modified and optimized a soil slurry method that included amendments of KNO₃, glucose, and methionine sulfoximine (an inhibitor of N assimilation) in the presence of two protein synthesis inhibitors: chloramphenicol, which inhibits bacteria, or cycloheximide, which inhibits fungi. By adding ¹⁵N-labeled KNO₃, we were able to measure gross rates of NO₃⁻ production (i.e., gross nitrification) and consumption (i.e., gross NO₃⁻ immobilization). We found that bacteria, not fungi, had the greatest potential for assimilating, or immobilizing, NO₃⁻ in these soils. This is consistent with their growth habit and distribution in the heterogeneous soil matrix.     

Effect of climate and soil type on the immobilization of nitrogen by decomposing straw in northern and southern Europe

 Wheat straw enclosed in mesh bags was buried for periods up to 1 year over two seasons in Scottish, Danish and Portuguese soils treated with ¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃. Scottish soils were: Terryvale, a poorly drained sandy loam; and Tipperty, an imperfectly drained brown forest soil with a higher clay content. The Danish soil (Foulum) was a freely drained sandy loam and the Portuguese soils were a sandy soil (Evora) and a clay soil (Beja). During the first month, ¹⁵N was being incorporated into the straw in the Tipperty, Terryvale and Foulum soils simultaneously as the total N content was decreasing. Subsequently, the straws began to show net immobilization and the total N content of the original straw was exceeded in Tipperty and Foulum soils after 4 months and 8 months, respectively. Net immobilization in Terryvale was detected only in the second season and did not occur in the first because of high soil moisture content. The rates of ¹⁵N incorporation were similar in the two Portuguese soils, and a loss of N was only detected after 8 months. After 1 month, in the two clay soils, Beja and Tipperty, ¹⁵NO₃ ^– was incorporated into straw to a greater extent than ¹⁵NH₄ ⁺ and this was attributed to ¹⁵NH₄ ⁺ fixation by clay minerals. In contrast, ¹⁵NH₄ ⁺ was more efficiently incorporated than ¹⁵NO₃ ^– under waterlogged conditions (Terryvale) and NO₃ ^– loss could be attributed to denitrification. The proportion of added ¹⁵N in the straw residue after 1 month varied between 6% and 18% for ¹⁵NH₄ ⁺ and 2% and 23% for ¹⁵NO₃ ^– and immobilization of N in the longer term tended to be greater in soils from northern Europe than from Portugal. Received: 19 January 1998     

Decomposition of wheat straw differing in nitrogen content in soils under conventional and organic farming management

An incubation experiment was carried out to investigate the interactions of two straw qualities differing in N content and two soils differently accustomed to straw additions. One soil under conventional farming management (CFM) regularly received straw, the other soil under organic farming management (OFM) only farmyard manure. The soils of the two sites were similar in texture, pH, cation‐exchange capacity, and glucosamine content. The soil from the OFM site had higher contents of organic C, total N, muramic acid, microbial biomass C and N (C_mic and N_mic), but a lower ergosterol content and lower ratios ergosterol to C_mic and fungal C to bacterial C. The straw from the CFM had threefold higher contents of total N, twofold higher contents of ergosterol and glucosamine, a 50% higher content of muramic acid, and a 30% higher fungal C–to–bacterial C ratio. The straw amendments led to significant net increases in C_mic, N_mic, and ergosterol. Microbial biomass C showed on average a 50% higher net increase in the organic than in the CFM soil. In contrast, the net increases in N_mic and ergosterol differed only slightly between the two soils after straw amendment. The CO₂ evolution from the CFM soil always exceeded that from the OFM, by 50% or 200 µg (g soil)^–1 in the nonamended control soil and by 55% or additional 600 µg (g soil)^–1 in the two straw treatments. In both soils, 180 µg g^–1 less was evolved as CO₂‐C from the OFM straw. The metabolic quotient qCO₂ was nearly twice as high in the control and in the straw treatments of the CFM soil compared with that of the OFM. In contrast, the difference in qCO₂ was insignificant between the two straw qualities. Differences in the fungal‐community structure may explain to a large extent the difference in the microbial use of straw in the two soils under different managements.     

Conversion of forest to agricultural land affects the relative contribution of bacteria and fungi to nitrification in humid subtropical soils

Land-use and management practices can affect soil nitrification. However, nitrifying microorganisms responsible for specific nitrification process under different land-use soils remains unknown. Thus, we investigated the relative contribution of bacteria and fungi to specific soil nitrification in different land-use soils (coniferous forest, upland fields planted with corn and rice paddy) in humid subtropical region in China. ¹⁵N dilution technique in combination with selective biomass inhibitors and C₂H₂ inhibition method were used to estimate the relative contribution of bacteria and fungi to heterotrophic nitrification and autotrophic nitrification in the different land-use soils in humid subtropical region. The results showed that autotrophic nitrification was the predominant nitrification process in the two agricultural soils (upland and paddy), while the nitrate production was mainly from heterotrophic nitrification in the acid forest soil. In the upland soils, streptomycin reduced autotrophic nitrification by 94%, whereas cycloheximide had no effect on autotrophic nitrification, indicating that autotrophic nitrification was mainly driven by bacteria. However, the opposite was true in another agricultural soil (paddy), indicating that fungi contributed to the oxidation of NH₄⁺ to NO₃^?. In the acid forest soil, cycloheximide, but not streptomycin, inhibited heterotrophic nitrification, demonstrating that fungi controlled the heterotrophic nitrification. The conversion of forest to agricultural soils resulted in a shift from fungi-dominated heterotrophic nitrification to bacteria- or fungi-dominated autotrophic nitrification. Our results suggest that land-use and management practices, such as the application of N fertilizer and lime, the long-term waterflooding during rice growth, straw return after harvest, and cultivation could markedly influence the relative contribution of bacteria and fungi to specific soil nitrification processes.     

Do growth yield efficiencies differ between soil microbial communities differing in fungal:bacterial ratios? Reality check and methodological issues

Soil communities dominated by fungi such as those of no-tillage (NT) agroecosystems are often associated with greater soil organic matter (SOM) storage. This has been attributed in part to fungi having a higher growth yield efficiency (GYE) compared to bacteria. That is, for each unit of substrate C utilized, fungi invest a greater proportion into biomass and metabolite production than do bacteria. The assumption of higher fungal efficiency may be unfounded because results from studies in which fungal and bacterial efficiencies have been characterized are equivocal and because few studies have measured microbial GYE directly. In this study, we measured microbial GYE in agricultural soils by following ¹³C-labeled glucose loss, total CO₂-C, and ¹³CO₂-C evolution at 2 h intervals for 20 h in two experiments (differing in N amendment levels) in which the fungal:bacterial biomass ratios (F:B) were manipulated. No differences in efficiency were observed for communities with high versus low F:B in soils with or without added inorganic N. When calculated using ¹³CO₂-C (in contrast to total CO₂-C) evolution, growth yield efficiencies of soils having high and low F:B were 0.69±0.01 and 0.70±0.01, respectively. When soils were amended with N, soils with high and low F:B had growth yield efficiencies of 0.78±0.01 and 0.76±0.01, respectively. Our experiments do not support the widely held assumption that soil fungi have greater growth efficiency than soil bacteria. Thus, claims of greater fungal efficiency may be unsubstantiated and should be evoked cautiously when explaining the mechanisms underlying greater C storage and slower C turnover in fungal-dominated soils.     

土壤腐殖质氮结构的X-光电能谱研究

X－ray photoelectron spectroscopy(XPS) was applied to examine the N structures of soil humic substances and some of their analogues.It was found that for soil humic substances XPS method gave similar results as those obtained by ^15N CPMAS NMR (cross-polarization magic-angle spinning nuclear magnetic resonance) method.70%-86% of total N in soil humic substances was in the form of amide,and 6%-13% was presented as ammes,with the remaining part as heterocyclic N.There was no difference in the distribution of the forms of N between the humic substances from soils formed over hundreds or thousands of years and the newly formed ones.For fulvic acid from weathered coal and benzoquinone-(NH4)2SO4 polymer the XPS results deviated significantly from the ^15N CPMAS NMR data.     

Immobilization of 15N in forest litter studied by 15N CPMAS NMR spectroscopy

Solutions labelled with ¹⁵N were applied as (¹⁵NH₄)₂SO₄ or K¹⁵NO₃ to isolated microplots in the floor of mountain beech forest (Nothofagus solandri var. cliffortioides) and incubated for 135 days under field conditions of moisture and temperature. Solid state ¹⁵N CPMAS NMR spectra of the forest litter layer showed that more than 80% of the total signal intensity was attributable to the secondary amide-peptide peak. The degree of ¹⁵N enrichment or form of N did not alter the relative intensity of signals attributable to ¹⁵N in peptides, nucleic acids and aliphatic amine groups (amino sugars and free NH₂ on amino acids). Combinations of ¹³C and ¹⁵N-NMR spectra, edited by a process that exploited differences in proton spin properties between distinct categories of organic matter, indicated incorporation of ¹⁵N in humified organic matter rather than partly degraded plant material. This application demonstrated that solid state ¹⁵N CPMAS NMR has potential for use in studies of N immobilization under field conditions and with materials containing little N and small ¹⁵N enrichment.     

Nitrogen losses from two grassland soils with different fungal biomass

Nitrogen losses from agricultural grasslands cause eutrophication of ground- and surface water and contribute to global warming and atmospheric pollution. It is widely assumed that soils with a higher fungal biomass have lower N losses, but this relationship has never been experimentally confirmed. With the increased interest in soil-based ecosystem services and sustainable management of soils, such a relationship would be relevant for agricultural management. Here we present a first attempt to test this relationship experimentally. We used intact soil columns from two plots from a field experiment that had consistent differences in fungal biomass (68 ± 8 vs. 111 ± 9 μg C g⁻¹) as a result of different fertilizer history (80 vs. 40 kg N ha⁻¹ y⁻¹ as farm yard manure), while other soil properties were very similar. We performed two greenhouse experiments: in the main experiment the columns received either mineral fertilizer N or no N (control). We measured N leaching, N₂O emission and denitrification from the columns during 4 weeks, after which we analyzed fungal and bacterial biomass and soil N pools. In the additional ¹⁵N experiment we traced added N in leachates, soil, plants and microbial biomass. We found that in the main experiment, N₂O emission and denitrification were lower in the high fungal biomass soil, irrespective of the addition of fertilizer N. Higher ¹⁵N recovery in the high fungal biomass soil also indicated lower N losses through dentrification. In the main experiment, N leaching after fertilizer addition showed a 3-fold increase compared to the control in low fungal biomass soil (11.9 ± 1.0 and 3.9 ± 1.0 kg N ha⁻¹, respectively), but did not increase in high fungal biomass soil (6.4 ± 0.9 after N addition vs. 4.5 ± 0.8 kg N ha⁻¹ in the control). Thus, in the high fungal biomass soil more N was immobilized. However, the ¹⁵N experiment did not confirm these results; N leaching was higher in high fungal biomass soil, even though this soil showed higher immobilization of ¹⁵N into microbial biomass. However, only 3% of total ¹⁵N was found in the microbial biomass 2 weeks after the mineral fertilization. Most of the recovered ¹⁵N was found in plants (approximately 25%) and soil organic matter (approximately 15%), and these amounts did not differ between the high and the low fungal biomass soil. Our main experiment confirmed the assumption of lower N losses in a soil with higher fungal biomass. The additional ¹⁵N experiment showed that higher fungal biomass is probably not the direct cause of higher N retention, but rather the result of low nitrogen availability. Both experiments confirmed that higher fungal biomass can be considered as an indicator of higher nitrogen retention in soils.     

Fungal development and the transformation of 15N-labelled amino- and ammonium-nitrogen in forest soils under several management regimes

The effect of glucose on the transformation of ¹⁵N-labelled glycine in soils from three different vegetation types, viz. exotic pine plantation, protected eucalypt and burnt eucalypt forest, was studied in the laboratory during a 10-week incubation. There was considerable interchange between organic and inorganic N throughout the incubation, and the various N transformations could be interpreted in terms of fungal development as measured by the nylon mesh technique. The fungi in the pine and burnt eucalypt soils were particularly active but produced different end results, those in the pine soils maintaining greater amounts of the added ¹⁵N in hydrolysable forms whereas the fungi in the burnt eucalypt soil caused incorporation of most of the label into the nonhydrolysable fraction. Both management regimes however resulted in a loss of hydrolysable native-soil N. Addition of glucose caused rapid net immobilization of exchangeable ¹⁵NH₄⁺-N, some of which was re-mineralized in the later stages of incubation. Glucose also resulted in a depletion of hydrolysable ¹⁵NH₄⁺-N and a corresponding accretion in the total hydrolysable-N fraction. Increased demand for amino compounds by fungi in the presence of glucose is considered responsible.     

Nitrogen transformations in cold and frozen agricultural soils following organic amendments

Though microbial activity is known to occur in frozen soils, little is known about the fate of animal manure N applied in the fall to agricultural soils located in areas with prolonged winter periods. Our objective was to examine transformations of soil and pig slurry N at low temperatures. Loamy and clay soils were either unamended (Control), amended with ¹⁵NH₄-labeled pig slurry, or amended with the pig slurry and wheat straw. Soils were incubated at −6, −2, 2, 6, and 10 °C. The amounts of NH₄, NO₃ and microbial biomass N (MBN), and the presence of ¹⁵N in these pools were monitored. Total mineral N, NO₃ and ¹⁵NO₃ increased at temperature down to −2 °C in the loam soil and −6 °C in the clay soil, indicating that nitrification and mineralization proceeded in frozen soils. Nitrification and mineralization rates were 1.8-4.9 times higher in the clay than in the loamy soil, especially below freezing point (3.2-4.9), possibly because more unfrozen water remained in the clay than in the loamy soil. Slurry addition increased nitrification rates by 3-14 times at all temperatures, indicating that this process was N-limited even in frozen soils. Straw incorporation caused significant net N immobilization only at temperatures ≥2 °C in both soils; the rates were 1.4-3.4 higher in the loam than in the clay soil. Nevertheless, up to 30% of the applied ¹⁵N was present in MBN at all temperatures. These findings indicate that microbial N immobilization occurred in frozen soils, but was not strong enough to induce net immobilization below the freezing point, even in the presence of straw. The Q₁₀ values for estimated mineralization and nitrification rates were one to two orders-of-magnitude larger below 2 °C than above this temperature (13-208 versus 1.5-6.9, respectively), indicating that these processes are highly sensitive to a small increase in soil temperature around the freezing point of water. This study confirms that net mineralization and nitrification can occur at potentially significant rates in frozen agricultural soils, especially in the presence of organic amendments. In contrast, net N immobilization could be detected essentially above the freezing point. Our results imply that fall-applied N could be at risk of overwinter losses, particularly in fine-textured soils.     

15N标记秸秆在太湖地区水稻土上的氮素矿化特征研究

采用室内恒温培养试验研究了在太湖地区乌栅土和黄泥土上添加15N标记秸秆后,秸秆15N在矿质氮、微生物氮和不同粒径土壤组分中的分配情况,并应用氮同位素库稀释法测定了秸秆在两种土壤上的氮总矿化速率。结果表明:两种土壤添加秸秆后,土壤矿质氮量在7～28 d之间迅速下降,微生物氮在前7 d逐渐升高,随后维持稳定。随着秸秆的分解,秸秆15N进入矿质氮库和微生物氮库,矿质15N在第7天时最高,占添加秸秆15N的9.24%～12.3%,微生物15N在第14天时最高,占添加秸秆15N的21.3%～40.5%,随后矿质15N和微生物15N量均下降。在培养的第7～28天之间,矿质15N和微生物15N出现下降,可能存在秸秆氮的损失。培养56 d时,10.5%～13.3%的秸秆15N进入土壤53μm～2 mm组分,24.5%～26.5%进入2～53μm组分,30%进入<2μm组分,有5.7%～14.9%的秸秆氮损失掉,仍有15.4%～29.1%的秸秆未分解,秸秆在乌栅土上分解的更多,但损失也更多。添加秸秆后0.5 d时,秸秆在乌栅土和黄泥土上的氮总矿化速率分别为1.61 mg kg-1d-1和1.48 mg kg-1d-1;56 d时,秸秆在乌栅土和黄泥土上的氮总矿化速率分别为0.26 mg kg-1 d-1和0.36 mg kg-1 d-1。     

Decoupled stoichiometric,isotopic, and fungal responses of an ectomycorrhizal black spruce forest to nitrogen and phosphorus additions

Many northern forests are limited by nitrogen (N) availability, slight changes in which can have profound effects on ecosystem function and the activity of ectomycorrhizal (EcM) fungi. Increasing N and phosphorus (P) availability, an analog to accelerated soil organic matter decomposition in a warming climate, could decrease plant dependency on EcM fungi and increase plant productivity as a result of greater carbon use efficiency. However, the impact of altered N and P availability on the growth and activity of EcM fungi in boreal forests remains poorly understood despite recognition of their importance to host plant nutrition and soil carbon sequestration. To address such uncertainty we examined above and belowground ecosystem properties in a boreal black spruce forest following five years of factorial N and P additions. By combining detailed soil, fungal, and plant δ¹⁵N measurements with in situ metrics of fungal biomass, growth, and activity, we found both expected and unexpected patterns. Soil nitrate isotope values became ¹⁵N enriched in response to both N and P additions; fungal biomass was repressed by N yet both biomass and growth were stimulated by P; and, black spruce dependency on EcM derived N increased slightly when N and P were added alone yet significantly declined when added in combination. These findings contradict predictions that N fertilization would increase plant P demands and P fertilization would further exacerbate plant N demands. As a result, the prediction that EcM fungi predictably respond to plant N limitation was not supported. These findings highlight P as an under appreciated mediator of the activity of denitrifying bacteria, EcM fungi, and the dynamics of N cycles in boreal forests. Further, use of δ¹⁵N values from bulk soils, plants, and fungi to understand how EcM systems respond to changing nutrient availabilities will often require additional ecological information.     

Bacterial and fungal contributions to soil nitrogen cycling under Douglas fir and red alder at two sites in Oregon

A study was conducted at two experimental tree plantations in the Pacific Northwest to assess the roles of bacteria and fungi in nitrogen (N) cycling. Soils from red alder (Alnus rubra) and Douglas-fir (Pseudotsuga menziesii) plots in low- (H.J. Andrews) and high- (Cascade Head) productivity stands were sampled in 2005 and 2006. Fungal:bacterial ratios were determined using phospholipid fatty acid (PLFA) profiles and quantitative (Q)-PCR. Ratios from these two molecular methods were highly correlated and showed that microbial biomass varied significantly between the two experimental sites and to a lesser extent between tree types with fungal:bacterial biomass ratios lower in more N-rich plots. ¹⁵N isotope dilution experiments, with ammonium (NH₄⁺) and nitrate (NO₃^?), were paired with antibiotics that blocked bacterial (bronopol) and fungal (cycloheximide) protein synthesis. This modified isotope dilution technique was used to determine the relative contribution of bacteria and fungi to net N mineralization and gross rates of ammonification and nitrification. When bacterial protein synthesis was blocked NH₄⁺ consumption and nitrification rates decreased in all treatments except for NH₄⁺ consumption in the Douglas-fir plots at H.J. Andrews, suggesting that prokaryotic nitrifiers are a major sink for mineral NH₄⁺ in forest soils with higher N availability. Cycloheximide consistently increased NH₄⁺ consumption, however the trend was not statistically significant. Both antibiotics additions also significantly increased gross ammonification, which may have been due to continued activity of extra- and intracellular enzymes involved in producing NH₄⁺ combined with the inhibition of NH₄⁺ assimilation into proteins. The implication of this result is that microorganisms are likely a major sink for soil dissolved organic N (DON) in soils.     

Influence of nitrogen on atrazine and 2, 4 dichlorophenoxyacetic acid mineralization in blackwater and redwater forested wetland soils

 Microcosms were used to determine the influence of N additions on active bacterial and fungal biomass, atrazine and dichlorophenoxyacetic acid (2,4-D) mineralization at 5, 10 and 15 weeks in soils from blackwater and redwater wetland forest ecosystems in the northern Florida Panhandle. Active bacterial and fungal biomass was determined by staining techniques combined with direct microscopy. Atrazine and 2,4-D mineralization were measured radiometrically. Treatments were: soil type, (blackwater or redwater forested wetland soils) and N additions (soils amended with the equivalent of 0, 200 or 400 kg N ha^–1 as NH₄NO₃). Redwater soils contained higher concentrations of C, total N, P, K, Ca, Mn, Fe, B and Zn than blackwater soils. After N addition and 15 weeks of incubation, active bacterial biomass in redwater soils was lower when N was added. Active bacterial biomass in blackwater soils was lower when 400 kg N ha^–1, but not when 200 kg N ha^–1, was added. Active fungal biomass in blackwater soils was higher when 400 kg N ha^–1, but not when 200 kg N ha^–1, was added. Active fungal biomass in redwater soils was lower when 200 kg N ha^–1, but not when 400 kg N ha^–1, was added. After 15 weeks of incubation 2,4-D degradation was higher in redwater wetland soils than in blackwater soils. After 10 and 15 weeks of incubation the addition of 200 or 400 kg N ha^–1 decreased both atrazine and 2,4-D degradation in redwater soils. The addition of 400 kg N ha^–1 decreased 2,4-D degradation but not atrazine degradation in blackwater soils after 10 and 15 weeks of incubation. High concentrations of N in surface runoff and groundwater resulting from agricultural operations may have resulted in the accumulation of N in many wetland soils. Large amounts of N accumulating in wetlands may decrease mineralization of toxic agricultural pesticides. Received: 26 June 1998     

The number and biomass of microorganisms in ancient buried and recent chernozems under different land uses

The size, number, and biomass of bacteria and microscopic fungi were studied in chernozems of different land uses (forest, fallow, pasture, and cropland), in paleosols under mounds of different ages in the territories adjacent to the background recent chernozems; and in the cultural layer of an ancient settlement of the Bronze Age, Early Iron Age, and Early Middle Age (4100–1050 years ago). The method of cascade filtration revealed that bacterial cells had a diameter from 0.1 to 1.85 μm; their average volume varied from 0.2 to 1.1 μm³. Large bacterial cells predominated in the soils of natural biocenoses; fine cells were dominants in the arable soils and their ancient analogues. The bacterial biomass counted by the method of cascade filtration was first found to be 10–380 times greater than that determined by luminescence microscopy. The maximal bacterial biomass (350–700 μg/g) was found in the soils of the birch forest edge (~80-year-old) and under the 80-year-old fallow. In the soils of the 15–20 year-old fallows and pastures, the bacterial biomass was 110–180 μg/g; in the arable soils and soils under the mounds, it was 80–130 and 30–130 μg/g, respectively. The same sequence was recorded in soils for the content of fungal mycelium and spores, which predominated over the bacterial mass. With the increasing age of the buried paleosols from 1100 to 3900 years, the share of the biomass of fungal spores increased in the total fungal and total microbial biomasses. In the cultural layer of the Berezovaya Luka (Altai region) settlement that had been functioning about 4000 years ago, the maximal biomass and number of fungal spores and the average biomass of bacteria and fungal mycelium comparable to that in the studied soils were revealed. In this cultural layer, the organic matter content was low (Corg, 0.4%), and the content of available phosphorus was high (P₂O₅, 17 mg/g). These facts attest to the significant saturation of this layer with microbial cenoses 4000 years ago and to their partial preservation up to now owing to the high concentration of ancient human wastes there.     

Mineralization of bacteria and fungi in chloroform-fumigated soils

It has been suggested by others that the size of the flush of mineralization caused by CHC1₃ fumigation can be used to estimate the amount of microbial biomass in soils. Calculation of biomass from the flush requires that the proportion of CHCl₃-killed cell C mineralized be known. To determine this proportion, 15 species of [¹⁴C]labelled fungi and 12 species of [¹⁴C]labelled bacteria were added to four types of soil and these were fumigated for 24 h with CHC1₃, reinoculated with unfumigated soil, and incubated at 22°C for 10 days. The average percentage mineralization of the fungi was 43.7 ± 5.3, while the average for the bacteria was 33.3 ± 9.9. Using a 1:3 ratio for distribution of total biomass between the bacterial and fungal populations, respectively, it was calculated that the average mineralization of both types of cells was 41.1%. In experiments conducted to determine if CHC1₃ vapour alters stabilized microbial metabolites or dead microbial cells in a manner which makes them more susceptible to degradation, it was found that both fumigated and unfumigated dead fungal materials mineralized to the same extent in soil during 10 days of incubation.     

Influence of nitrogen on cellulose and lignin mineralization in blackwater and redwater forested wetland soils

 Microcosms were used to determine the influence of N additions on active bacterial and active fungal biomass, cellulose degradation and lignin degradation at 5, 10 and 15 weeks in soils from blackwater and redwater wetlands in the northern Florida panhandle. Blackwater streams contain a high dissolved organic C concentration which imparts a dark color to the water and contain low concentrations of nutrients. Redwater streams contain high concentrations of suspended clays and inorganic nutrients, such as N and P, compared to blackwater streams. Active bacterial and fungal biomass was determined by direct microscopy; cellulose and lignin degradation were measured radiometrically. The experimental design was a randomized block. Treatments were: soil type (blackwater or redwater forested wetlands) and N additions (soils amended with the equivalent of 0, 200 or 400 kg N ha^–1 as NH₄NO₃). Redwater soils contained higher concentrations of C, total N, P, K, Ca, Mn, Fe, B and Zn than blackwater soils. After N addition and 15 weeks of incubation, the active bacterial biomass in redwater soils was lower than in blackwater soils; the active bacterial biomass in blackwater soils was lower when 400 kg N ha^–1, but not when 200 kg N ha^–1, was added. The active fungal biomass in blackwater soils was higher when 400 kg N ha^–1, but not when 200 kg N ha^–1, was added. The active fungal biomass in redwater wetland soils was lower when 200 kg N ha^–1, but not when 400 kg N ha^–1, was added. Cellulose and lignin degradation was higher in redwater than in blackwater soils. After 10 and 15 weeks of incubation, the addition of 200 or 400 kg N as NH₄NO₃ ha^–1 decreased cellulose and lignin degradation in both wetland soils to similar levels. This study indicated that the addition of N may slow organic matter degradation and nutrient mineralization, thereby creating deficiencies of other plant-essential nutrients in wetland forest soils. Received: 7 April 1999     

Gross nitrogen mineralization-, immobilization-, and nitrification rates as a function of soil C/N ratio and microbial activity

A laboratory experiment was designed to challenge the idea that the C/N ratio of forest soils may control gross N immobilization, mineralization, and nitrification rates. Soils were collected from three deciduous forests sites varying in C/N ratio between 15 and 27. They were air-dried and rewetted to induce a burst of microbial activity. The N transformation rates were calculated from an isotope dilution and enrichment procedure, in which ¹⁵NH₄Cl or Na¹⁵NO₃ was repeatedly added to the soils during 7 days of incubation. The experiments suggested that differences in gross nitrogen immobilization and mineralization rates between the soils were more related to the respiration rate and ATP content than to the C/N ratio. Peaks of respiration and ATP content were followed by high rates of mineralization and immobilization, with 1-2 days of delay. The gross immobilization of NH₄⁺ was dependent on the gross mineralization and one to two orders of magnitude larger than the gross NO₃⁻ immobilization. The gross nitrification rates were negatively related to the ATP content and the C/N ratio and greatly exceeding the net nitrification rates. Taken together, the observations suggest that leaching of nitrate from forest soils may be largely dependent on the density and activity of the microbial community.     

The use of phospholipid fatty acid analysis to estimate bacterial and fungal biomass in soil

The cell content of 12 bacterial phospholipid fatty acids (PLFA) was determined in bacteria extracted from soil by homogenization/centrifugation. The bacteria were enumerated using acridine orange direct counts. An average of 1.40×10^-17 mol bacterial PLFA cell^-1 was found in bacteria extracted from 15 soils covering a wide range of pH and organic matter contents. With this factor, the bacterial biomass based on PLFA analyses of whole soil samples was calculated as 1.0–4.8 mg bacterial C g^-1 soil C. The corresponding range based on microscopical counts was 0.3–3.0 mg bacterial C g^-1 soil C. The recovery of bacteria from the soils using homogenization/centrifugation was 2.6–16% (mean 8.7%) measured by PLFA analysis, and 12–61% (mean 26%) measured as microscopical counts. The soil content of the PLFA 18:26 was correlated with the ergosterol content (r=0.92), which supports the use of this PLFA as an indicator of fungal biomass. The ratio 18:26 to bacterial PLFA is therefore suggested as an index of the fungal:bacterial biomass ratio in soil. An advantage with the method based on PLFA analyses is that the same technique and even the same sample is used to determine both fungi and bacteria. The fungal:bacterial biomass ratio calculated in this way was positively correlated with the organic matter content of the soils (r=0.94).