Amino sugars as specific indices for fungal and bacterial residues in soil

Amino sugars are important indices for the contribution of soil microorganisms to soil organic matter. Consequently, the past decade has seen a great increase in the number of studies measuring amino sugars. However, some uncertainties remain in the interpretation of amino sugar data. The objective of the current opinion paper is to summarize current knowledge on amino sugars in soils, to give some advice for future research objectives, and to make a plea for the correct use of information. The study gives an overview on the origin of muramic acid (MurN), glucosamine (GlcN), galactosamine (GalN), and mannosamine (ManN). Information is also provided on measuring total amino sugars in soil but also on compound-specific δ¹³C and δ¹⁵N determination. Special attention is given to the turnover of microbial cell-wall residues, to the interpretation of the GlcN/GalN ratio, and to the reasons for converting fungal GlcN and MurN to microbial residue C. There is no evidence to suggest that the turnover of fungal residues generally differs from that of bacterial residues. On average, MurN contributes 7% to total amino sugars in soil, GlcN 60%, GalN 30%, and ManN 4%. MurN is highly specific for bacteria, GlcN for fungi if corrected for the contribution of bacterial GlcN, whereas GalN and ManN are unspecific microbial markers.     

Shifts in amino sugar and ergosterol contents after addition of sucrose and cellulose to soil

An incubation experiment with organic soil amendments was carried out with the aim to determine whether formation and use of microbial tissue (biomass and residues) could be monitored by measuring glucosamine and muramic acid. Living fungal tissue was additionally determined by the cell-membrane component ergosterol. The organic amendments were fibrous maize cellulose and sugarcane sucrose adjusted to the same C/N ratio of 15. In a subsequent step, spherical cellulose was added without N to determine whether the microbial residues formed initially were preferentially decomposed. In the non-amended control treatment, ergosterol remained constant at 0.44 μg g⁻¹ soil throughout the 67-day incubation. It increased to a highest value of 1.9 μg g⁻¹ soil at day 5 in the sucrose treatment and to 5.0 μg g⁻¹ soil at day 33 in the fibrous cellulose treatment. Then, the ergosterol content declined again. The addition of spherical cellulose had no further significant effects on the ergosterol content in these two treatments. The non-amended control treatment contained 48 μg muramic acid and 650 μg glucosamine g⁻¹ soil at day 5. During incubation, these contents decreased by 17% and 19%, respectively. A 33% increase in muramic acid and an 8% increase in glucosamine were observed after adding sucrose. Consequently, the ratio of fungal C to bacterial C based on bacterial muramic acid and fungal glucosamine was lowered in comparison with the other two treatments. No effect on the two amino sugars was observed after adding cellulose initially or subsequently during the second incubation period. This indicates that the differences in quality between sucrose and cellulose had a strong impact on the formation of microbial residues. However, the amino sugars did not indicate a preferential decomposition of microbial residues as N sources.     

Amino sugars and muramic acid—biomarkers for soil microbial community structure analysis

Characterizing functional and phylogenetic microbial community structure in soil is important for understanding the fate of microbially-derived compounds during the decomposition and turn-over of soil organic matter. This study was conducted to test whether amino sugars and muramic acid are suitable biomarkers to trace bacterial, fungal, and actinomycetal residues in soil. For this aim, we investigated the pattern, amounts, and dynamics of three amino sugars (glucosamine, mannosamine and galactosamine) and muramic acid in the total microbial biomass and selectively cultivated bacteria, fungi, and actinomycetes of five different soils amended with and without glucose. Our results revealed that total amino sugar and muramic acid concentrations in microbial biomass, extracted from soil after chloroform fumigation varied between 1 and 27 mg kg⁻¹ soil. In all soils investigated, glucose addition resulted in a 50-360% increase of these values. In reference to soil microbial biomass-C, the total amino sugar- and muramic acid-C concentrations ranged from 1-71 g C kg⁻¹ biomass-C. After an initial lag phase, the cultivated microbes revealed similar amino sugar concentrations of about 35, 27 and 17 g glucosamine-C kg⁻¹ TOC in bacteria, fungi, and actinomycetes, respectively. Mannosamine and galactosamine concentrations were lower than those for glucosamine. Mannosamine was not found in actinomycete cultures. The highest muramic acid concentrations were found in bacteria, but small amounts were also found in actinomycete cultures. The concentrations of the three amino sugars studied and muramic acid differed significantly between bacteria and the other phylogenetic microbial groups under investigation (fungi and actinomycetes). Comparison between the amino sugar and muramic acid concentrations in soil microbial biomass, extracted after chloroform fumigation, and total concentrations in the soil showed that living microbial biomass contributed negligible amounts to total amino sugar contents in the soil, being at least two orders of magnitude greater in the soils than in the soil inherent microbial biomass. Thus, amino sugars are significantly stabilized in soil.     

Optimisation of amino sugar quantification by HPLC in soil and plant hydrolysates

Amino sugars are increasingly used as indicators for the accumulation of microbial residues in soil and plant material. A reverse-phase high-performance liquid chromatography method was improved for the simultaneous determination of muramic acid, mannosamine, glucosamine and galactosamine in soil and plant hydrolysates via ortho-phthaldialdehyde (OPA) pre-column derivatisation and fluorescence detection. The retention time was reduced, and the separation of muramic acid and mannosamine was optimised by modifying the mobile phase. The effects of excitation wavelength, OPA reaction time, tetrahydrofuran concentration and pH value of the mobile phase on the amino sugar separation were tested. Quantification limits were in the range of 0.13 to 0.90 μg ml⁻¹. No interferences exist from amino acids or other primary amines, occurring in soil and plant hydrolysates.     

Dynamics of soil amino sugar pools during decomposition processes of corn residues as affected by inorganic N addition

Purpose  

Identifying the impact of inorganic-nitrogen (N) availability on soil amino sugar dynamics during corn (Zea mays L.) residue decomposition may advance our knowledge of microbial carbon (C) and N transformations and the factors controlling these processes in soils. Amino sugars are routinely used as microbial biomarkers to investigate C and N sequestration in microbial residues, and they are also involved in microbial-mediated soil organic matter (SOM) turnover. We conducted a 38-week incubation study using a Mollisol which was amended with corn residues and four levels of inorganic N (i.e., 0, 60.3, 167.2, and 701.9 mg N kg⁻¹ soil). The objective of this study was to examine the effects of inorganic-N availability on fungal and bacterial formation and stabilization of heterogeneous amino sugars during the corn residue decomposition in soil.     

Temporal responses of soil microorganisms to substrate addition as indicated by amino sugar differentiation

Amino sugars are one of the important microbial residue biomarkers which are associated with soil organic matter cycling. However, little is known about their transformation kinetics in response to available substrates because living biomass only contributes a negligible portion to the total mass of amino sugars. By using ¹⁵N tracing technique, the newly synthesized (labeled) amino sugars can be differentiated from the native portions in soil matrix, making it possible to evaluate, in quantitative manner, the transformation pattern of amino sugars and to interpret the past and ongoing changes of microbial communities during the assimilation of extraneous ¹⁵N. In this study, laboratory incubations of soil samples were conducted by using ¹⁵NH₄⁺ as nitrogen source with or without glucose addition. Both the ¹⁵N enrichment (expressed as atom percentage excess, APE) and the contents of amino sugars were determined by an isotope-based gas chromatography-mass spectrometry. The significant ¹⁵N incorporation into amino sugars was only observed in glucose plus ¹⁵NH₄⁺ amendment with the APE arranged as: muramic acid (MurN) > glucosamine (GlcN) > galactosamine (GalN). The dynamics of ¹⁵N enrichment in bacterial-derived MurN and fungal-derived GlcN were fitted to the hyperbolic equations and indicative for the temporal responses of different soil microorganisms. The APE plateau of MurN and fungal-derived GlcN represented the maximal extent of bacterial and fungal populations, respectively, becoming active in response to the available substrates. The different dynamics of the ¹⁵N enrichment between MurN and GlcN indicated that bacteria reacted faster than fungi to assimilate the labile substrates initially, but fungus growth was dominant afterward, leading to integrated microbial community structure over time. Furthermore, the dynamics of labeled and unlabeled portions of amino sugars were compound-specific and substrate-dependent, suggesting their different stability in soil. GlcN tended to accumulate in soil while MurN was more likely degraded as a carbon source when nitrogen supply was excessive.     

Land-use effects on amino sugars in particle size fractions of an Argiudoll

Identifying amino sugar pools from different land-use systems may advance our knowledge of land-use effects on the fate of microbially-derived substances. Surface soils (0–10 cm) from (1) native pasture, (2) a >80-years-arable site, and (3) a >80-years-afforested site were fractionated into clay, silt, fine-, and coarse-sand fractions. Then, soil organic carbon, N, glucosamine, galactosamine, mannosamine, and muramic acid were analyzed.Afforestation did not influence the amino sugar content in bulk soil, whereas cultivation reduced the content by 54%. The concentrations of amino sugars in g kg⁻¹ SOM declined after both long-term cropping and afforestation by 6% and 13%, respectively, relative to that in the grassland. The amino sugar depletion at the forest site occurred mainly from the silt fraction (by 25%), while that in the cultivated site was mainly due to preferential loss of amino sugars from clay (by 19% compared with the grassland). Both ratios of glucosamine to galactosamine and glucosamine to muramic acid increased when the prairie was converted to forest or cultivated land, suggesting that bacterial N especially is better preserved than fungal N under prairie conditions.     

无机氮素加入量对玉米秸秆分解过程中棕壤氨基糖含量的影响

作物秸秆还田作为一种调控土壤养分循环、减少氮肥损失、维持和提高土壤有机质水平的有力措施,越来越受到人们的重视[1-2]。还田的秸秆是微生物的碳源和能源,常导致土壤微生物量迅速增加,相应的微生物死亡率和微生物残体积累量也提高[3-4]。根据Appuhn等[5]对微生物细胞壁组分的     

无机氮素加入量对玉米秸秆分解过程中棕壤氨基糖含量的影响

在室内恒温(25℃)培养条件下,通过气相色谱法研究高C/N比玉米秸秆降解过程中微生物来源的氨基糖含量及其占有机质比例的变化及其对无机氮素添加水平(0, 60.3, 167.2, 701.9 mg•N•kg-1土,依次标记为N0, Nlow, Nmed, Nhigh)的响应情况。结果表明：在玉米秸秆分解过程中,土壤中的氨基糖含量及其对有机质贡献的比例随着无机氮素供应水平的增加而增加,即以微生物代谢物形式截获的有机碳/氮相应增多。Nmed和Nhigh处理中氨基糖积累量显著高于Nlow和N0处理。不同微生物来源的氨基糖受外源氮素的影响情况不同,胞壁酸比氨基葡萄糖更易于受到土壤中碳氮供给的影响,具有相对较快的转化速率;而在数量上氨基葡萄糖对土壤有机质的贡献比例显著高于前者;氨基半乳糖在土壤中的积累过程较为缓慢,受外源无机氮素添加水平的影响并不明显。可见,在高C/N比作物残体分解过程中,无机氮素的供应水平是影响土壤中氨基糖积累转化的重要因素之一。但是,过多的无机氮素施入并不能被微生物完全同化利用,因此秸秆还田的土壤中必须要考虑有效氮素的水平问题。     

Microbial residues were increased by film mulching with manure amendment in a semiarid agroecosystem

Amino sugars, as a kind of microbial residue, are strongly associated with cycling of microbial-derived soil organic matter. However, responses of amino sugars to agricultural practices on the Loess Plateau in North-western China are poorly known. The objective was to evaluate effects of film mulching (no film mulching + NPK fertilizers, CK; film mulching + NPK fertilizers, PF; film mulching + NPK fertilizers + cow manure, FM) on accumulations of amino sugars in this region. FM significantly increased total amino sugar by 190.46 mg kg^?1 in 0–10 cm layer and 214.66 mg kg^?1 in 10–20 cm layer relative to CK, but PF significantly decreased it by 139.28 mg kg^?1 in 0–10 cm layer. Ratios of glucosamine to muramic acid were markedly decreased by 2.50 in 0–10 cm layer and 2.28 in 10–20 cm layer in FM than CK, suggesting a tendency of microbial residues pool shift towards bacterial residues in this agroecosystem. These results indicated film mulching alone was not benefitial to accumulation of amino sugar while organic manure contributed to the build-up of amino sugar partly due to manure contained microbial residues. The different patterns of amino sugars suggested significant changes in the quality of microbial-derived organic matter.     

Plant-N incorporation into microbial amino sugars as affected by inorganic N addition: A microcosm study of N-labeled maize residue decomposition

Carbon (C) and/or nitrogen (N) in plant residues can be assimilated into microbial biomass during the plant residue decomposition before incorporation into SOM in the form of microbial residues. Yet, microbial transformation of plant residue-N into microbial residues and the effects of inorganic N inputs on this process have not been well documented. Here, we undertook a 38-week incubation with a silt loam soil amended with a ¹⁵N-labeled maize (Zea mays L.) residue to determine how the transformation of maize residue-N into soil amino sugars was affected by rates of inorganic N addition. The newly metabolized amino sugars derived from maize residue-N were differentiated and quantified by using an isotope-based gas chromatography-mass spectrometry technique. We found that greater amounts of maize residue-N were transformed into amino sugars with lower inorganic N addition at the early stages of the plant residue degradation. However, the trend was reversed during later stages of decay as greater percentage of maize residue-N (8.6-9.4%) were enriched in amino sugars in the N_med and N_high soils, as compared with N₀ and N_low (7.5-8.2%). This indicated that higher availability of inorganic N could delay the transformation process of plant-N into microbial residues during the mineralization of plant residues. The dynamic transformations of the plant residue-N into individual amino sugars were compound-specific, with very fast incorporation into bacterial MurA_M-new found during the initial weeks, while the dynamics of maize residue-derived GluN exhibited a delayed response to assimilate plant-N into fungal products. The findings indicated differential contributions of maize residue decomposing microorganisms over time. Moreover, we found no preferential utilization of inorganic N over plant residue-N into amino sugars during the incubation course, but inorganic N inputs altered the rate of plant-N accumulation in microbial-derived organic matters. Our results indicated that higher N availability had a positive impact on the accumulation or stabilization of newly-produced microbial residues in the long term.     

Net microbial amino sugar accumulation process in soil as influenced by different plant material inputs

Identifying the impact of plant material inputs on soil amino sugar synthesis may advance our knowledge of microbial transformation processes in soils. In a 12-week laboratory microcosm incubation, 1, 2, 4, and 6% (w/w) soybean leaf or maize stalk were initially added to soil, respectively, whereas soil without plant addition was used as a control. The results showed that adding organic materials to the soil led to a net accumulation of amino sugars, because of greater microbial synthesis. The ratios of glucosamine to galactosamine and of glucosamine to muramic acid, two indicators differentiating the relative contribution to soil organic matter of fungi and bacteria, showed substantial variance across the gradient of substrate addition. Our results suggest that the amount of nutrients in a given substrate is the primary attribute determining microbial net accumulation of soil amino sugars, especially in the relatively short term, whereas the composition of nutrients might be more important in the relatively long term when nutrients are not sufficient. The use of the two ratios (glucosamine to galactosamine and glucosamine to muramic acid) reflects different dynamics of galactosamine and muramic acid during the decomposition of organic substrates in soils. Muramic acid, compared with galactosamine, is more likely to accumulate in the soil active organic fraction under abundant nutrient conditions, whereas it would be decomposed along with active organic matter when the nutrients are scarce and remain in minor quantities in the clay fraction without being attacked by microbes.     

外源氮添加对土壤微生物残体积累动态的影响－基于Meta分析

微生物残体是土壤有机碳库的重要贡献者。为明确外源氮添加对土壤微生物残体积累动态的影响,本文收集整理了1980—2020年已发表的文献,共选取122组试验观测数据,利用整合分析方法（Meta-analysis）,以微生物残体标识物－氨基糖为目标组分,定量分析了不同种类和数量的外源氮添加对土壤中微生物来源细胞残体积累数量和组成比例的影响,并系统解析其主要影响因素。结果表明:外源氮添加（0 ~ 6000 kg hm?1）对微生物细胞残体的积累有显著的促进作用,并能引起土壤中真菌和细菌来源细胞残体相对比例发生明显变化。与不加氮对照相比,氮添加使土壤氨基糖总量增加27%,其中氨基葡萄糖、氨基半乳糖和胞壁酸含量分别增加22.5%、29.8%和19.0%。同时,不同种类外源氮素添加对氨基糖积累特征的影响也有所不同,表现为有机氮（如动物厩肥）比无机氮添加对氨基糖积累的促进作用更大。此外,氮添加对氨基糖的影响程度还与土壤自身的碳氮比、土地利用类型和自然降雨量等环境因子密切相关。其中是否添加碳源对微生物残体的响应有较大影响,表现为:无碳源添加会降低土壤氨基糖葡萄糖和胞壁酸对氮添加的响应,削弱了微生物残体对土壤有机质的贡献比例;而氮源同时配合碳源添加条件下,土壤氨基糖积累量显著高于单一氮源添加的处理,说明氮添加对微生物残体积累的影响存在着碳氮耦合效应。     

Amino sugars in soils of the North American cultivated prairie

Characterizing amino sugar dynamics in cultivated soils helps to further understand the influence of cultivation on soil organic matter turnover. This study was designed to evaluate accumulations and patterns of four amino sugars in 17 surface (0–10 cm) soil samples along a climosequence in the North American long-term cultivated prairie from Saskatoon, Candada, to Texas, USA. Mean annual temperature (MAT) ranged from 0.9 to 22.2°C and mean annual precipitation (MAP) from 300 to 1308 mm. Samples were analyzed for glucosamine, mannosamine, galactosamine, and muramic acid. Amino sugar contents (mg kg^?1 soil) varied markedly among the 17 sites and were controlled by mean annual temperature (MAT) and clay and silt contents, mainly. The relationship between amino sugar-N proportions to total N (%) and MAT followed parabolic regression models. Compared with native sites, amino sugars were depleted by 53% and the amino sugar-N by 18% of the total, on average, after long-term cropping. The intensity of amino sugar-N depletion correlated positively with MAT (r = 0.77^***). Bacterially-derived galactosamine and muramic acid declined preferentially to mainly chitin-derived glucosamine after long-term cropping. The glucosamine-to-galactosamine and glucosamine-to-muramic acid ratios can be used, therefore, as indicators for the identification of land use effects on microbially-derived SOM residues.     

A novel GC/MS technique to assess N and C incorporation into soil amino sugars

Amino sugars have been used as biomarker to indicate microorganism contribution to soil organic matter turnover and sequestration. However, there is no direct gas chromatograph mass spectrometry (GC/MS) approach to assess microbial synthesis of amino sugars in soil. We developed a novel method which combines laboratory incubation of substrate containing ¹⁵N or ¹³C and a GC/MS technique to trace ¹⁵N or ¹³C isotope changes in three amino sugars, glucosamine, galactosamine, and muramic acid. Sample preparation followed the procedure of Zhang and Amelung (1996) [Zhang, X., Amelung, W., 1996. Gas chromatographic determination of muramic acid, glucosamine, galactosamine, and mannosamine in soils. Soil Biology and Biochemistry 28, 1201-1206.]. The GC/MS determination was conducted using a full scan mode with both electronic ionization (EI) and chemical ionization (CI) sources. The CI source was suitable for all of the three amino sugars, while the EI source was not applicable to muramic acid due to its low sensitivity in the determination as well as low concentration of muramic acid in soil. The enrichment of ¹⁵N or ¹³C in amino sugars during incubation was estimated by calculating the atom percentage excess (APE). ¹⁵N incorporation was evaluated according to fragment (F) abundance ratio of mass F+1 to F, whilst ¹³C incorporation was estimated according to the ratio of mass F+n to F (n is skeleton carbon number in the fragment). This novel method was assessed by using two soil samples (a Kandiudult and a Udoll) incubated with either ¹⁵N-amonium or U-¹³C-glucose. The results indicate that the GC/MS determination is reproducible, thus this technique is useful in detecting the microbial synthesis of amino sugars in soil, and especially it should be possible when looking at the position or how much labeled carbon and nitrogen atoms have been incorporated.     

Soil organic matter and water-stable aggregates under different tillage and residue conditions in a tropical dryland agroecosystem

Changes in the proportions of water-stable soil aggregates, organic C, total N and soil microbial biomass C and N, due to tillage reduction (conventional, minimum and zero tillage) and crop residue manipulation (retained or removed) conditions were studied in a tropical rice–barley dryland agroecosystem. The values of soil organic C and total N were the highest (11.1 and 1.33 g kg⁻¹ soil, respectively) in the minimum tillage and residue retained (MT+R) treatment and the lowest (7.8 and 0.87 g kg⁻¹, respectively) in conventional tillage and residue removed (CT−R) treatment. Tillage reduction from conventional to minimum and zero conditions along with residue retention (MT+R,ZT+R) increased the proportion of macroaggregates in soil (21–42% over control). The greatest increase was recorded in MT+R treatment and the smallest increase in conventional tillage and residue retained (CT+R) treatment. The lowest values of organic C and total N (7.0–8.9 and 0.82–0.88 g kg⁻¹ soil, respectively) in macro- and microaggregates were recorded in CT−R treatment. However, the highest values of organic C and total N (8.6–12.6 and 1.22–1.36 g kg⁻¹, respectively) were recorded in MT+R treatment. The per cent increase in the amount of organic C in macroaggregates was greater than in microaggregates. In all treatments, macroaggregates showed wider C/N ratio than in microaggregates. Soil microbial biomass C and N ranged from 235 to 427 and 23.9 to 49.7 mg kg⁻¹ in CT−R and MT+R treatments, respectively. Soil organic C, total N, and microbial biomass C and N were strongly correlated with soil macroaggregates. Residue retention in combination with tillage reduction (MT+R) resulted in the greatest increase in microbial biomass C and N (82–104% over control). These variables showed better correlations with macroaggregates than other soil parameters. Thus, it is suggested that the organic matter addition due to residue retention along with tillage reduction accelerates the formation of macroaggregates through an increase in the microbial biomass content in soil.     

玉米不同残体添加对棕壤团聚体中氨基糖分配的影响

[目的]秸秆残体还田能引起土壤微生物残留物氨基糖的变化,然而不同部位秸秆残体因含碳氮化学组分差异,还田到不同肥力土壤后对氨基糖在团聚体中分配的影响尚不明析.因此,研究添加玉米不同残体对不同肥力棕壤团聚体中氨基糖分配的影响,并利用微生物标识物氨基葡萄糖与胞壁酸比值变化指示棕壤团聚体真菌和细菌群落组成动态变化,对深入阐明秸...     

Free amino sugar reactions in soil in relation to soil carbon and nitrogen cycling

Amino sugars represent a major constituent of microbial cell walls and hydrolyzed soil organic matter. Despite their potential importance in soil nitrogen cycling, comparatively little is known about their dynamics in soil. The aim of this study was therefore to quantify the behaviour of glucosamine in two contrasting grassland soil profiles. Our results show that both free amino sugars and amino acids represented only a small proportion of dissolved organic N and C pool in soil. Based upon our findings we hypothesize that the low concentrations of free amino sugars found in soils is due to rapid removal from the soil solution rather than slow rates of production. Further, we showed that glucosamine removal from solution was a predominantly biotic process and that its half-life in soil solution ranged from 1 to 3 h. The rates of turnover were similar to those of glucose at low substrate concentrations, however, at higher glucosamine concentrations its microbial use was much less than for glucose. We hypothesized that this was due to the lack of expression of a low affinity transport systems in the microbial community. Glucosamine was only weakly sorbed to the soil's solid phase (K_d=6.4±1.0) and our results suggest that this did not limit its bioavailability in soil. Here we showed that glucosamine addition to soil resulted in rapid N mineralization and subsequent NO₃⁻ production. In contrast to some previous reports, our results suggest that free amino sugars turn over rapidly in soil and provide a suitable substrate for both microbial respiration and new biomass formation.     

Land use effects on amino sugar signature of chromic Luvisol in the semi-arid part of northern Tanzania

 Characterizing amino sugar signature in particle size separates of tropical soils is important for further understanding the fate of microbial-derived compounds during the decomposition of soil organic matter (SOM) in tropical agroecosystems. We investigated the impact of land-use changes on the nature, amount and dynamics of amino sugars in soil of the semi-arid northern Tanzania. Samples were collected from the uppermost 10 cm of native woodland, degraded woodland, fields cultivated for 3 and 15 years and homestead fields fertilized with animal manure. The amount of glucosamine, galactosamine, mannosamine and muramic acid were determined in bulk soil and size separates. Compared to the native woodland, a 68% and 72% reduction in total amino sugar contents were found in the 3- and 15-year cultivated fields, respectively. Moreover, 39% of the total amino sugar was lost from the degraded woodland. This may be attributed to accelerated decomposition of amino sugars and/or decreasing microbial biomass input under the semi-arid environment following clear-cutting and cultivation. In contrast, only a 20% decline was found from the fields where animal manure had been applied. Most of the amino sugar depletion occurred from the coarse and fine sand-associated SOM. The decline from the silt and clay-bound amino sugar was relatively small, indicating the importance of organo-mineral associations in the stabilization of microbial-derived sugars in this tropical soil. After 15 years of continuous cultivation, the ratio of glucosamine:galactosamine increased from 1.44 to 2.23, while the ratio of glucosamine:muramic acid increased from 14.5 to 26.5 (P<0.05). These results suggest that cultivation may have led to preferential depletion of bacterial-derived amino sugars (muramic acid and galactosamine) compared with fungal-derived glucosamine. Received: 22 June 2000     

Preferential sequestration of microbial carbon in subsoils of a glacial-landscape toposequence, Dane County, WI, USA

Microorganisms participate in soil carbon storage by contributing biomass in the form of refractory microbial cell components. However, despite the important contribution of microbial biomass residues to the stable carbon pool, little is known about how the contribution of these residues to soil carbon storage varies as a function of depth. In this study, we evaluated microbial residue biomarkers (amino sugars) in varied pedogenic horizons from six soil profiles of two geographic sites on a glacial-landscape toposequence in Dane County, WI. We found that the amino sugars appeared to preferentially accumulate in subsoil. Specifically, although total amounts of amino sugars decreased downward through the profile as even as total organic carbon did, the rate of decrease was significantly lower, suggesting that these compounds are more refractory than general soil organic carbon. The proportion of amino sugars to soil organic carbon increased along the depth gradient (from top to bottom), with the exception of Bg horizons associated with high water tables. We also observed that microbial residue patterns measured by amino sugar ratio (e.g., glucosamine to muramic acid) showed different dynamic tendencies in the two different geographic sites, suggesting that residue carbon contribution by fungi and bacteria is likely site-specific and complex. In summary, regardless of the redox microenvironment created by groundwater dynamics in a given soil, our study supports the hypothesis that microbial residues are refractory and that they contribute to terrestrial carbon sequestration.