Soil Microbial Residues Research Articles

Dynamic contribution of microbial residues to soil organic matter accumulation influenced by maize straw mulching

Soil degradation is a serious problem in northeast China due to the routine removal of aboveground crop residues under conventional tillage systems; thus, crop residue retention becomes an essential approach for maintaining and improving soil organic matter (SOM) in this area. However, the impact of returning crop residue on microbial-driven SOM accumulation remains unclear. In this context, an 8-year field experiment with maize straw mulching (SM) was conducted in northeast China to evaluate how maize straw return may influence the dynamic contribution of microbial residues to SOM accumulation. Conventional cultivation was used as a control (CR), in which only 10 cm of aboveground maize biomass remained after the annual harvest. Soil samples (0–10 cm) were collected after the annual harvest for the measurement of soil organic carbon (SOC) and amino sugars (AS). Based on a first-order model, we found that soil microbial residue accumulation in this temperate arable soil might reach a steady state within decades under a specific agricultural management practice. Maize straw mulching both strengthened the retention ability of soil microbial residues and expanded their accumulation capacity. According to the glucosamine to muramic acid ratio, maize straw mulching facilitated the accumulation of fungal residues more than bacterial ones. Compared to maize residue removal, maize straw mulching accelerated the accumulation of microbial residues in SOM and enhanced the contribution of microbial residues to SOM sequestration in the surface soil, likely elucidating the pivotal mechanisms by which the “4/1000” initiative goal for agricultural soil will be met, at least for the first 10 years. Because bacterial residues are actively involved in SOM turnover while fungal residues dominantly contribute to SOM accrual enhanced by maize straw return compared to maize residue removal, we conclude that SOM in this arable field might be stabilized at a higher sequestration capacity under management with maize straw mulching.

Effects of N-fixing tree species (Alnus sibirica)on amino sugars in soil aggregates of Larix kaempferi plantation in eastern Liaoning Province, China.

Soil microbial residues are important components of soil stable carbon (C) pools. How N-fixing tree species influence microbial residues in soil aggregates in larch plantations is still unclear. To determine the effects of N-fixing tree species on the distribution of microbial residues in different aggregates, we compared the distribution of amino sugars, biomarker of microbial residues, among aggregates in a pure larch (Larix kaempferi) plantation and a mixed plantation of larch (Larix kaempferi) and alder (Alnus sibirica) in eastern Liaoning Province. The results showed that alder did not affect the distribution of amino sugars, but significantly increased amino sugars content in soil aggregates. The total amino sugars in different soil aggregates were enriched by 130%-170% in the mixed larch plantation compared with those in pure larch plantation. The contributions of glucosamine, galactosamine and muramic acid to the increases of total amino sugars caused by alder introduction were 66.5%-66.9%, 30.0%-30.6% and 2.5%-3.2%, respectively. Alder introduction significantly accelerated the glucosamine/muramic acid ratios in >2000 μm and <250 μm aggregates, but not in 250-2000 μm aggregates. Moreover, alder introduction increased the microbial contribution to soil organic C in all aggregates, but did not influence this contribution among aggregates, indicating that the effects of alder introduction on microbial contribution to aggregates were homogeneous.

Effects of drying and wetting cycles on the transformations of extraneous inorganic N to soil microbial residues

The incorporation of extraneous nitrogen (N) into amino sugars (AS) could reflect the contribution of microbial residues to soil N transformation. Investigating the impact of drying-wetting (DW) on dynamics of newly-produced AS is critical because this represents microbial-driven N retention/losses in soil. A 36-day incubation of soil samples was conducted under different drying intensities, using 15N-labelled-(NH4)2SO4 as an N source together with/without glucose addition. There were multiple DW periods and they ranged from a constant moisture content treatment, to a one day drying (low-drying-intensity, LD), a two day drying (medium-drying-intensity, MD), or a three day drying event (severe-drying-intensity, SD). The immobilization of added-N was restricted in DW when available carbon was not added, thus glucose addition increased the effect of DW on the incorporation of added-N into AS. The response of total 15N-AS to DW varied depending on drying intensities. The MD was beneficial to the incorporation of added-N into total 15N-AS, while total 15N-AS contents were low in SD treatment. The effect of DW on contribution of bacterial and fungal residues to N transformation was also related to drying intensities. Our study indicated that DW altered microbial transformation of added-N, and the effect was drying intensity-specific, and available carbon-dependent.

Specific Responses of Soil Microbial Residue Carbon to Long-Term Mineral Fertilizer Applications to Reddish Paddy Soils

Mineral nutrient inputs to soil may alter microbial activity and consequently influence the accumulation of microbial residues. In this study, we investigated the effects of application rates and ratios of mineral fertilizers on the microbial residue carbon (MRC) of reddish paddy soils after long-term (15-year) fertilizer applications in southern China. Contents of three soil amino sugars as microbial residue contents were determined and MRC were calculated based on amino sugars. Results showed that three individual amino sugar contents increased as fertilizer application rates increased until maximum values were reached at a rate of 450-59-187 kg ha−1 year−1 (N-P-K). The three amino sugar contents then declined significantly under the highest mineral fertilizer application rate of 675-88-280 kg ha−1 year−1 (N-P-K). In addition, to enhance the microbial residue contents, it was more beneficial to double P (N:P:K = 1:0.26:0.41) in fertilizers applied to the P-deficient reddish paddy soils than to double either N (N:P:K = 2:0.13:0.41) or K (N:P:K = 1:0.13:0.82). The contents of the three individual amino sugars and microbial residues under different fertilizer application rates and ratios were significantly and positively correlated with soil organic carbon (SOC), total N, total P, and pH. Increases in values of the fungal C to bacterial C ratios showed that soil organic matter (SOM) stability increased because of the fertilizer applications over the past 15 years. The contents and ratios of amino sugars can be used as indicators to evaluate the impact of mineral fertilizer applications on SOM dynamics in subtropical paddy soils. The results indicated that fertilizer applications at a rate of 450-59-187 kg ha−1 year−1 (N-P-K) may improve crop yields, SOC contents, and SOC stability in subtropical paddy soils.

Parent material and conifer biome influence microbial residue accumulation in forest soils

Microbial residues are a significant component of soil organic carbon (C), yet their distribution and function remain understudied. We evaluated changes in microbial residues and their contribution to organic C along a soil development sequence on three contrasting parent materials (granite, basalt and andesite) and three conifer biomes (ponderosa pine (Pinus ponderosa Laws.), PP; white fir (Abies concolor Lindl.), WF; and red fir (Abies magnifica A. Murr.), RF) at different elevations in the Sierra Nevada of California. Soil samples were taken from both A and B horizons and microbial residues were determined by amino sugar analysis. The effect of conifer biome on amino sugars was complex and dependent on parent material and horizon. We found parent material significantly influenced soil amino sugars which exhibited a pattern of andesite > basalt > granite in both A and B horizons. Both correlation and redundancy analyses indicated a significant correlation of amino sugars with the amount of short-range-order materials. This suggests soil mineralogy plays an important role in influencing amino sugar accumulation. This is further supported by larger differences among parent materials than between conifer biomes in ratios of fungal-to bacterial-derived amino sugars. The proportion of amino sugars to soil organic C was significantly influenced by parent material in the B horizon following the pattern of basalt > andesite > granite, but not affected by conifer biome. Our results suggest that mineralogy strongly influences the degree to which soil microbial residues persist in temperate forest soils.

Microorganisms and their residues under restored perennial grassland communities of varying diversity

Rising atmospheric CO2 concentration and global mean temperatures have stimulated interest in managing terrestrial systems to sequester more carbon and mitigate climate change. In a restored prairie experiment, we compared high diversity (HD, 25 species) with low diversity (LD, 6 species) prairies to investigate the effect of plant diversity on soil microbial communities and their residues with soil depth. We assayed lipid and amino sugar biomarkers for soil samples, taken after 9 years following the establishment of the prairie treatment, at 5 depth increment layers: 0–2 cm, 12–15 cm, 25–27 cm, 50–52 cm, and 98–100 cm. We found that the microbial biomass and residues decreased considerably with depth in both diversity treatments. Ordination analysis of lipid profiles indicated soil microbial communities were consistently distinct between the deeper and the upper layers, regardless of treatment, and also differed between the LD and HD treatments. Plant diversity effects on soil microbial communities strongly correlated with arbuscular mycorrhizal fungi (AMF), as indicated by the lipid marker 16:1ω5c. Soil microbial residues in deeper horizons were relatively more enriched in HD than LD treatments, suggesting that greater plant diversity might sustain higher soil carbon storage through relatively recalcitrant necromass inputs in the long term. Decreasing glucosamine/muramic acid (GluN/MurA) ratio in LD and increasing in HD with depth suggested that the new microbially-accumulated carbon was positively contributed by fungal-derived residues. Our results indicate that plant diversity drives soil microbial carbon sequestration through changes in AMF abundance in restored native tallgrass ecosystems. These findings have implications for understanding how the management of plant diversity can improve soil quality and sustainability in grasslands, and how efforts to conserve and restore diverse grasslands could mitigate greenhouse gas emissions.

Soil microbial community and microbial residues respond positively to minimum tillage under organic farming in Southern Germany

In a field trial comprising organic farming and minimum tillage management strategies in Scheyern, Germany, we evaluated the long-term (21-year) effects of organic farming (use of a diverse crop rotation with legume cover crop and without application of synthetic fertilizer or pesticides) and minimum tillage (6–8cm depth) on the microbial community structure and microbial residues in Cambisols. Organic farming had a positive effect on microbial biomass, total phospho-lipid fatty acids (PLFA), Gram (+) bacteria, Gram (−) bacteria and the arbuscular mycorrhizal fungi (AMF) indicator PLFA 16:1ω5 and amino sugars. The increase in presence of Gram (+) bacteria when compared to integrated farming was also reflected by increased content of bacterial muramic acid (MurN), i.e. an increased formation of bacterial residues. Minimum tillage significantly increased microbial biomass N and the fungal PLFA 18:2ω6,9, averaging the values of upper (0–8cm) and deeper (12–25cm) soil, but had no effects on PLFA 16:1ω5. Minimum tillage generally resulted in a negative depth gradient of almost all microbial properties analyzed. The only important exception was fungal galactosamine (GlcN), which led to increases in the fungal C/bacterial C ratio and in the contribution of microbial residue C to SOC in the deeper soil. Significant second order tillage×management interactions indicated that minimum tillage effects on microbial biomass and PLFA indices (Gram (+) and (i15:0+i17:0)/(a15:0+a17:0)) were much stronger in the organic farming system than in the integrated farming system. Redundancy analysis (RDA) showed SOC and H2O content predominantly affected the microbial community structure in the present study. Minimum tillage in combination with organic farming appears to be an effective agricultural strategy that enhances soil microbial biomass, microbial residues and bacterial and fungal abundances. The results indicate that the positive effects of minimum tillage on microbial community can be enhanced by organic farming. Microbial residues as a fraction of SOC respond faster to farming management than to tillage.

Effects of quebracho tannin extract (Schinopsis balansae) and activated charcoal on feed intake and digestibility by goats and their faecal microbial biomass

Faecal microorganisms directly influence autochthonous soil microbial biomass, microbial residues and subsequent soil C and N cycling. This study investigated the effects of feeding activated charcoal (AC) and quebracho tannin extract (QTE) on feed and nutrient intake, apparent nutrient digestibility and faecal excretion of C, N and fibre fractions in growing male goats, and on faecal microbial biomass. The completely randomised feeding trial comprised of three subsequent periods with 12 male Boer goats (age 4.8 ± 0.59 months, weight 28 ± 3.9 kg) assigned to five treatments and a control. Experimental diets contained 2 and 4% QTE (QTE2, QTE4), 1.5 and 3.0% AC (AC1.5, AC3.0) and a mixture of 2% QTE plus 1.5% AC (QTEAC). Faecal microbial biomass was measured using ergosterol and amino sugars as indices for fungi and bacteria. There were no significant effects of QTE and AC on intake of feed and nutrients. Faecal concentration of slowly decomposable carbohydrates increased with QTE as well as with AC feeding (p < 0.001), whereas faecal N concentration only increased with QTE feeding (p < 0.001). Although total microbial C concentration in faeces was not affected by QTE and AC feeding, QTE shifted the microbial community towards fungi (p < 0.01). The results demonstrate that including up to 4% QTE in goat diets enhances the concentration of slowly decomposable N and C fractions in the faeces, whereas feeding up to 3% AC increases the faecal concentration of slowly decomposable C fractions and the faecal C/N ratio.

Response of soil fertility indices to long-term application of biogas and raw slurry under organic farming

The long-term effects of biogas slurry application on soil fertility indices were compared with raw slurry in biodynamic organic farming systems. An on-farm soil and slurry sampling was carried out to quantify the effects on stocks of soil organic matter, microbial biomass and microbial residues. Five fields with biogas slurry and five neighbouring fields with raw slurry amendments were selected at 5 different sites in the north-east of Baden-Württemberg, Germany. The application of biogas slurry ranged from 15 to 25 years and did not affect SOC, total N stocks or the soil C/N ratio. Biogas slurry application decreased the soil microbial biomass to SOC ratio, which indicates a reduced availability of the biogas slurry C input to soil microorganisms compared with raw slurry. At some sites, differences in clay content masked any slurry effects on the microbial activity, biomass, and residue indices. There were no general effects of biogas slurry on the ratios of ergosterol to microbial biomass C or amino sugar-based fungal C to bacterial C, whereas an increasing clay content caused a significant shift towards bacteria according to the latter ratio. Since the soils had been farmed organically in diverse crop rotations for at least 40 years, chemical differences in slurry composition were not great enough to result in different biochemical properties. The consistency in the data of all approaches strongly indicates the validity of the current on-farm study by comparing neighbouring fields.

The influence of multiwalled carbon nanotubes on polycyclic aromatic hydrocarbon (PAH) bioavailability and toxicity to soil microbial communities in alfalfa rhizosphere

Carbon nanotubes (CNTs) may affect bioavailability and toxicity of organic contaminants due to their adsorption properties. Recent studies have observed the influence of multiwalled carbon nanotubes (MWNTs) on the fate of polycyclic aromatic hydrocarbons (PAHs) and other organic contaminants. Greenhouse studies (49d) were conducted with alfalfa plants in two different soil types. Four treatment conditions (0, 25, 50, or 100mg/kg MWNTs+100mg/kg PAHs mixture-pyrene and phenanthrene) were tested in order to determine their effects on soil microbial community composition and PAH residues. Microbial community structure in the two highest treatments (50mg/kg and 100mg/kg MWNTs) showed a dramatic shift in the presence of MWNTs in sandy loam soil (1% organic matter) in comparison to the control (0mg/kg MWNTs). Many microbial fatty acid methyl ester (FAMEs) markers (i15:0, 16:1ω5c, 10Me17:0, 10Me16:0) were missing in the control soil. However, there was a lower abundance of these FAMEs in the 25mg/kg MWNT treatment (except 10Me17:0) and a higher presence of these FAMEs in the 50mg/kg and 100mg/kg MWNT treatments compared to control. In contrast, microbial community composition was not influenced by the MWNT treatments in sandy clay loam soil (5.9% organic matter). However, pyrene degradation in sandy clay loam soil significantly increased by 21% in the highest MWNT treatment group (100mg/kg) and 9.34% in 50mg/kg MWNT treatment. Under the conditions tested in this study, MWNTs significantly impacted the soil microbial community distribution and PAH degradation and effects were dependent on soil types, specifically organic matter content.

Higher rates of manure application lead to greater accumulation of both fungal and bacterial residues in macroaggregates of a clay soil

Microbial residues represent a significant soil organic matter pool and participate in soil aggregation. The addition of organic manure is known to modify soil aggregation and strongly influence soil microbial residues. How manure application influences the spatial distribution of microbial residues in soil aggregates is largely unknown. This study attempts to determine the effect of manure application at various rates on the content and distribution of microbial residues among aggregates of different sizes. We used a long-term manure application experiment in a Mollisol in northeastern China, where manure has been applied since 2001 at rates of 0, 7.5, 15, and 22.5 Mg ha−1 yr−1 (dry weight). The abundance of microbial residues was indicated by amino sugar analysis. Glucosamine and muramic acid were used as biomarkers for fungal and bacterial residues, respectively. Amino sugars were examined within four aggregate fractions: large macroaggregate (>2000 μm), small macroaggregate (250–2000 μm), large microaggregate (53–250 μm) and small microaggregate (<53 μm). Application of manure at 15 and 22.5 Mg ha−1 yr−1 provided significantly higher proportions of macroaggregates and mean weight diameter (MWD) than non-manure treatment and manure applied at 7.5 Mg ha−1 yr−1. Manure application, especially at higher rates, significantly stimulated the accumulation of total amino sugars in both macroaggregates and large microaggregates and more amino sugars were found in >250 μm macroaggregates compared with microaggregates. However, effects of manure application rates on amino sugar accumulation in larger aggregates were limited when manure rate was increased from 15 to 22.5 Mg ha−1 yr−1. The response of fungal- and bacterial-derived amino sugars to manure application rates differed among aggregate fractions, i.e., glucosamine associated with macroaggregates increased more than that of microaggregates, whereas the enhancement of muramic acid was prominent in both macroaggregates and large microaggregates. The mass proportions of macroaggregates and MWD showed significant positive correlations with amino sugar contents, indicating that these microbial residues are involved in the formation and stabilization of aggregates. Manure applications greatly increased the contribution of microbial residues to soil organic C (SOC) in small macroaggregates and large microaggregates (P ≤ 0.05). We conclude that higher manure input may promote soil aggregation and higher SOC storage, which is closely related to a greater microbial residues-mediated improvement of soil aggregate stability. Our results also suggest that measurement of amino sugar content is a useful approach to assess fungal and bacterial contributions to soil aggregation.

Detritusphere effects on P availability and C mineralization in soil

SummaryThis paper investigates two possible processes in the detritusphere: the effect of various fresh plant materials on extractable phosphorus in soil and the effect of decomposing litter on mineralization of soil microbial residues. The objective was to quantify these two poorly‐studied processes. The work was done in a soil at two different fertility levels following 30 years of different fertilizer practice, as we have previously observed effects of fertilizer history on soil carbon dynamics. We incubated fresh leaf material from Lupinus, Raphanus and Rumex separated from the soil by a fine mesh. At 4, 7, 14 and 28 days after incubation, extractable phosphate was measured in 1‐mm soil layers at increasing distance from the litter. In order to establish a labelled microbial residue pool, the soil had been labelled with 14C‐glucose 3 weeks prior to incubation, and respiration was measured throughout the incubation. Extractable soil phosphorus (P) increased up to 3 mm from the litter layer, most in Rumex and least in Lupinus. Extractable soil P increased throughout the sampling period, corresponding to a maximum of 20% of the P lost from the plant litter layer. Measurement of residue‐derived organic acids in the soil showed no specific pattern. The study did not indicate any solubilization of soil P as the increase in soluble P can be fully accounted for by leakage from the litter. Mineralization of soil 14C in the form of microbial biomass and microbial residues was significantly accelerated by the addition of litter. The magnitude was correlated with decomposition of the plant litter, and corresponded to a maximum of 5% of the soil 14C. Because of the localized impact of litter on the detritusphere soil layer, the impact on soil 14C in this soil fraction was potentially even greater. In conclusion, the decomposition of litter had significant effects on soil carbon mineralization, although we could not demonstrate an effect of plant‐derived compounds on soil P availability, beyond the direct contribution of litter P.

Soil microbial residue dynamics after 3-year elevated O3 exposure are plant species-specific

Background and aims Rising ozone (O3) concentrations poses a great threat to crop growth and ecosystem carbon storage, but the underlying mechanism remains unclear. Identifying the impact of elevated O3 on soil microbial residues may advance our knowledge of microbial-mediated soil organic matter (SOM) turnover. In this paper, we aimed to investigate the effects of elevated O3 on the accumulation of amino sugars in the soil of the two wheat cultivars (Tritcium aestivum L.) with different ozone-tolerances.

Nitrogen deposition promotes the production of new fungal residues but retards the decomposition of old residues in forest soil fractions

Atmospheric nitrogen (N) deposition has frequently been observed to increase soil carbon (C) storage in forests, but the underlying mechanisms still remain unclear. Changes in microbial community composition and substrate use are hypothesized to be one of the key mechanisms affected by N inputs. Here, we investigated the effects of N deposition on amino sugars, which are used as biomarkers for fungal- and bacterial-derived microbial residues in soil. We made use of a 4-year combined CO2 enrichment and N deposition experiment in model forest ecosystems, providing a distinct (13) C signal for 'new' and 'old' C in soil organic matter and microbial residues measured in density and particle-size fractions of soils. Our hypothesis was that N deposition decreases the amount of fungal residues in soils, with the new microbial residues being more strongly affected than old residues. The soil fractionation showed that organic matter and microbial residues are mainly stabilized by association with soil minerals in the heavy and fine fractions. Moreover, the bacterial residues are relatively enriched at mineral surfaces compared to fungal residues. The (13) C tracing indicated a greater formation of fungal residues compared to bacterial residues after 4years of experiment. In contradiction to our hypotheses, N deposition significantly increased the amount of new fungal residues in bulk soil and decreased the decomposition of old microbial residues associated with soil minerals. The preservation of old microbial residues could be due to decreased N limitation of microorganisms and therefore a reduced dependence on organic N sources. This mechanism might be especially important in fine heavy fractions with low C/N ratios, where microbial residues are effectively protected from decomposition by association with soil minerals.

Long-term influence of different tillage intensities on soil microbial biomass, residues and community structure at different depths

The site-specific contribution of bacterial and especially fungal residues to a tillage-induced C sequestration is largely unknown, although microbial residues contribute a significant percentage to the soil organic C (SOC) pool. In the current study, the co-accumulation of microbial residues and organic matter was investigated in a mature, 15-year-old on-farm tillage experiment (mouldboard plough (MBT), grubber (GRT), no tillage (NT)) on four different arable silt-loam sites in central and southern Germany at 0 to 40 cm soil depth. The GRT and NT treatments increased the stocks of SOC (+7 %) and microbial biomass C (+20 %) in comparison with the MBT treatment. The differences between the GRT and NT were small, but there were more positive effects for the GRT treatment in most cases. Our results indicate significant tillage effects in loess-derived silt loams suitable for rain-fed sugar beet production, although strong site-specific differences occurred for most of the soil chemical and microbiological properties analysed. In the GRT and NT treatments, the increased stocks of SOC were not caused by the preferential accumulation of fungal residues at 0–5 cm depth, whereas ergosterol-free biotrophic arbuscular mycorrhizal fungi (AMF) was promoted at the expense of saprotrophic fungi at 30–40 cm depth. Our results suggest that the relationship between saprotrophic fungi and AMF is an important factor for tillage-induced changes in microbial turnover of SOC.

Specific response of fungal and bacterial residues to one-season tillage and repeated slurry application in a permanent grassland soil

The dynamics of fungal and bacterial residues to a one-season tillage event in combination with manure application in a grassland soil are unknown. The objectives of this study were (1) to assess the effects of one-season tillage event in two field trials on the stocks of microbial biomass, fungal biomass, microbial residues, soil organic C (SOC) and total N in comparison with permanent grassland; (2) to determine the effects of repeated manure application to restore negative tillage effects on soil microbial biomass and residues. One trial was started 2 years before sampling and the other 5 years before sampling. Mouldboard ploughing decreased the stocks of SOC, total N, microbial biomass C, and microbial residues (muramic acid and glucosamine), but increased those of the fungal biomarker ergosterol in both trials. Slurry application increased stocks of SOC and total N only in the short-term, whereas the stocks of microbial biomass C, ergosterol and microbial residues were generally increased in both trials, especially in combination with tillage. The ergosterol to microbial biomass C ratio was increased by tillage, and decreased by slurry application in both trials. The fungal C to bacterial C ratio was generally decreased by these two treatments. The metabolic quotient qCO2 showed a significant negative linear relationship with the microbial biomass C to SOC ratio and a significant positive relationship with the soil C/N ratio. The ergosterol to microbial biomass C ratio revealed a significant positive linear relationship with the fungal C to bacterial C ratio, but a negative one with the SOC content. Our results suggest that slurry application in grassland soil may promote SOC storage without increasing the role of saprotrophic fungi in soil organic matter dynamics relative to that of bacteria.

Variations in Soil Microbial Communities and Residues Along an Altitude Gradient on the Northern Slope of Changbai Mountain, China

Altitudinally-defined climate conditions provide specific vegetation types and soil environments that could influence soil microbial communities, which in turn may affect microbial residues. However, the knowledge is limited in terms of the degree to which microbial communities and residues present and differ along altitude. In this study, we examined the soil microbial communities and residues along the northern slope of Changbai Mountain, China using phospholipid fatty acid (PLFA) and amino sugar analysis, respectively. Soil samples were taken from five different vegetation belts defined by climates. Principal component analysis (PCA) revealed substantial differences in soil microbial community composition among study sites, appeared to be driven primarily by soil pH and C/N ratio on the first principal component (PC1) which accounted for 50.7% of the total sample variance. The alpine tundra was separated from forest sites on the second principal component (PC2) by a signifiscantly higher amount of fungal PLFA (18:2ω6,9). Soil pH and C/N ratio were also correlated with the ratios of Gram-positive to Gram-negative bacteria (Gm+/Gm−), glucosamine to galactosamine (GluN/GalN), and glucosamine to muramic acid (GluN/MurA). Both total PLFAs and amino sugars were positively correlated with soil organic carbon, inorganic nitrogen, available phosphorus and potassium. We concluded that soil pH and C/N ratio were the most important drivers for microbial community structure and amino sugar pattern, while substrate availability was of great importance in determining the concentrations of microbial communities and residues. These findings could be used to facilitate interpretation of soil microbial community and amino sugar data derived from measurements in latitude or managed forests.

Soil microbial residue storage linked to soil legacy under biofuel cropping systems in southern Wisconsin, USA

Microbial residues can be a significant component of soil organic matter, and their component amino sugars are integrative indicators of biologically relevant ecosystem properties. We evaluate the impact of soil attributes, microbial functional group biomass, and cropping system type on soil amino sugar profiles in three model biofuel cropping systems in southern Wisconsin, USA. Total soil carbon and clay content explained differences in soil amino sugar profiles, with glucosamine and galactosamine more strongly related to soil carbon and muramic acid associated with clay content. Amino sugars were not correlated to cropping system or to microbial functional group lipid abundance, suggesting amino sugar differences among locations were due to differences in soil legacy properties such as total carbon, clay content and culture age rather than to current biotic drivers. The disconnect between the current biotic composition and legacies of past microbial activity suggests that microbial residues should be considered as trajectories over time rather than static system properties.

Continuous manuring combined with chemical fertilizer affects soil microbial residues in a Mollisol

In this study, the influence of 10 years’ continuous application of organic manure at various rates combined with chemical fertilizer on microbial residues was evaluated in a highly fertile temperate soil. The presence and origin of microbial residues were indicated by amino sugar analysis. The treatments were: (1) CK, unfertilized control; (2) OM0, only chemical fertilizer, no manure added; (3) OM1, organic manure added at 7.5 Mg ha−1 year−1 plus chemical fertilizer; (4) OM2, organic manure added at 15 Mg ha−1 year−1 plus chemical fertilizer; and (5) OM3, organic manure added at 22.5 Mg ha−1 year−1 plus chemical fertilizer. Fertilization significantly increased the total amino sugar concentrations, especially in the plots with higher manure addition rates (OM2 and OM3 plots, P < 0.05). This suggests a positive effect of organic manure combined with chemical fertilizer on the accumulation of microbial residues in soil. However, the highest manure rate (OM3) did not lead to further increase in the total amino sugar pool as compared with the moderate manure rate (OM2). This suggests manure addition “saturates” in its effect on microbial residue build-up. The different patterns of individual amino sugars suggest a change in the quality of microbial-derived soil organic matter after 10 years.

Formation and remobilisation of soil microbial residue. Effect of clay content and repeated additions of cellulose and sucrose

In two studies, we assessed the mass balance of added 14C-labelled sucrose and 15NH415NO3 by measuring 14CO2, 14C and 15N in soil microbial biomass (SMB) and 14C and 15N in soil solution. Specifically, we assessed the potential of recently added 14C to be re-mobilised by cryptic growth using subsequent additions of sucrose and cellulose and the effect of physical protection on the stabilisation of the labelled substrate. We used both a constructed soil with low soil organic matter content and varied the clay content as well as a natural soil. We observed a substantial initial as well as a later stage transfer of 14C into unidentifiable form, hypothesised to be microbial residues. When using a standard kEC value of 0.45, only roughly 50% of the added labelled substrates were accountable and therefore we explored the full range of reported kEC values to assess the mass balance. Subsequent application of unlabelled sucrose and cellulose did not substantially increase turnover 14C and 15N. Contrary to our expectation, there was no effect of clay content on the amount of unidentified 14C and 15N. The unidentified 14C and 15N is ascribed to formation of soil microbial residue. The low recovery of added isotope suggests that our mechanistic models are missing a large and important pool in order to realistically simulate organic matter turnover in soil.