The contribution of soil organic matter fractions to carbon and nitrogen mineralization and microbial community size and structure

The aims of this study were to: (i) assess the impact of hay and fertilizer application on organic matter (OM) fractions (dissolved organic matter (DOM), light fraction organic matter (LFOM, <1.0 g cm⁻³), heavy fraction OM (HFOM, <1.7 g cm⁻³)), carbon (C) and nitrogen (N) cycling processes and microbial community size and structure, and (ii) quantify the role of OM fractions to C and N cycling. Soil was collected in 2001 from a field experiment to which grass hay (1996) and/or fertilizer (1995 and 1999) had previously been applied. DOM-C (P<0.05) and DOM-N (P=0.07) were significantly higher in control and fertilized soil than hay and hay+fertilized soil. LFOM and HFOM C and N contents and C/N ratios were significantly (P<0.05) higher in hay+fertilized and hay amended soil than in control and fertilized soil. Potentially mineralizable-N (PMN), microbial biomass-C (MB-C), microbial biomass-N (MB-N) and microbial respiration (CO₂) were not affected by fertilizer and/or hay application. Gross N mineralization (Gross Min) and gross nitrification (Gross Nit) rates were significantly (P<0.05) higher in fertilized, hay, hay+fertilized soil than control soil. However, there was no significant difference between treatments in gross N immobilization rates. Results reported here highlight the importance of a labile fraction of the DOM pool to N and C cycling as its removal significantly (P<0.05) reduced PMN, MB-N, Gross Min and Gross Nit compared with whole soil in most or all treatments. In soil where DOM+LFOM were removed PMN was significantly (P<0.05) lower, but MB-C, Gross Min and Gross Nit was significantly (P<0.05) higher than in DOM removed soil. This suggests that LFOM plays an important role as a sink for mineral-N. Total soil phospholipid fatty acid (PLFA) concentration was significantly (P<0.05) higher in hay amended than control, fertilized and hay+fertilized soil. Principal components analysis was able to clearly discriminate between control, fertilized, hay+fertilized and hay amended soil. Soil amended with hay or fertilizer had a microbial community structure which differed from that of the control or hay+fertilized soils. Redundancy analysis with Monte Carlo permutation tests revealed that PLFA profiles were strongly correlated to differences in Gross Min, Gross Nit, MB-N, MB-C, MB-C/N ratio, total soil C and total soil C/N ratio. The results of this research suggest that changes in microbial structure are related to aspects of soil C and N pools and cycling.     

Labile, recalcitrant, and microbial carbon and nitrogen pools of a tallgrass prairie soil in the US Great Plains subjected to experimental warming and clipping

Carbon (C) and nitrogen (N) fluxes are largely controlled by the small but highly bio-reactive, labile pools of these elements in terrestrial soils, while long-term C and N storage is determined by the long-lived recalcitrant fractions. Changes in the size of these pools and redistribution among them in response to global warming may considerably affect the long-term terrestrial C and N storage. However, such changes have not been carefully examined in field warming experiments. This study used sulfuric acid hydrolysis to quantify changes in labile and recalcitrant C and N fractions of soil in a tallgrass prairie ecosystem that had been continuously warmed with or without clipping for about 2.5 years. Warming significantly increased labile C and N fractions in the unclipped plots, resulting in increments of 373 mg C kg⁻¹ dry soil and 15 mg N kg⁻¹ dry soil, over this period whilst clipping significantly decreased such concentrations in the warmed plots. Warming also significantly increased soil microbial biomass C and N in the unclipped plots, and increased ratios of soil microbial/labile C and N, indicating an increase in microbial C- and N-use efficiency. Recalcitrant and total C and N contents were not significantly affected by warming. For all measured pools, only labile and microbial biomass C fractions showed significant interactions between warming and clipping, indicating the dependence of the warming effects on clipping. Our results suggest that increased soil labile and microbial C and N fractions likely resulted indirectly from warming increases in plant biomass input, which may be larger than warming-enhanced decomposition of labile organic compounds.     

Turnover time of microbial biomass carbon in Japanese upland soils with different textures

We investigated the turnover time of microbial biomass-C in Japanese upland soils with various textures and examined the soil physicochemical properties influencing their turnover time. Samples from five different soil types (sand-dune regosol, light-colored Andosol, humic Andosol, brown forest soil, and dark red soil) were taken from upland concrete-frame plots in the experimental field of Chiba University. Each soil amended with [U -¹³C] glucose was incubated for 80 d at 25°C. Microbial biomass-C and -¹³C in soil were periodically determined by the fumigation-extraction method. The longest turnover time of microbial biomass-C was observed in the dark red soil (215 d) followed by the humic Andosol (134 d), brown forest soil (97 d), and light-colored Andosol (83 d) and the shortest in the sand-dune regosol (45 d). The turnover time of microbial biomass-C was significantly correlated with the value of soil clay (R: 0.917*), CEC (R: 0.921*), and macroaggregate (R: 0.907*) contents, but not with the total-C content. The amount of microbial biomass-C showed a close correlation with the turnover time of microbial biomass-C, suggesting that the turnover time of microbial biomass-C is an important factor influencing the accumulation of microbial biomass-C in soil.     

Role of soil drying in nitrogen mineralization and microbial community function in semi-arid grasslands of north-west Australia

We examined effects of wetting and then progressive drying on nitrogen (N) mineralization rates and microbial community composition, biomass and activity of soils from spinifex (Triodia R. Br.) grasslands of the semi-arid Pilbara region of northern Australia. We compared soils under and between spinifex hummocks and also examined impacts of fire history on soils over a 28 d laboratory incubation. Soil water potentials were initially adjusted to −100 kPa and monitored as soils dried. We estimated N mineralization by measuring changes in amounts of nitrate (NO₃⁻-N) and ammonium (NH₄⁺-N) over time and with change in soil water potential. Microbial activity was assessed by amounts of CO₂ respired. Phospholipid fatty acid (PLFA) analyses were used to characterize shifts in microbial community composition during soil drying. Net N mineralized under hummocks was twice that of open spaces between hummocks and mineralization rates followed first-order kinetics. An initial N mineralization flush following re-wetting accounted for more than 90% of the total amount of N mineralized during the incubation. Initial microbial biomass under hummocks was twice that of open areas between hummocks, but after 28 d microbial biomass was<2 μ g⁻¹ ninhydrin N regardless of position. Respiration of CO₂ from soils under hummocks was more than double that of soils from between hummocks. N mineralization, microbial biomass and microbial activity were negligible once soils had dried to −1000 kPa. Microbial community composition was also significantly different between 0 and 28 d of the incubation but was not influenced by burning treatment or position. Regression analysis showed that soil water potential, microbial biomass N, NO₃⁻-N, % C and δ¹⁵N all explained significant proportions of the variance in microbial community composition when modelled individually. However, sequential multiple regression analysis determined only microbial biomass was significant in explaining variance of microbial community compositions. Nitrogen mineralization rates and microbial biomass did not differ between burned and unburned sites suggesting that any effects of fire are mostly short-lived. We conclude that the highly labile nature of much of soil organic N in these semi-arid grasslands provides a ready substrate for N mineralization. However, process rates are likely to be primarily limited by the amount of substrate available as well as water availability and less so by substrate quality or microbial community composition.     

Soil CO2 flux in six monospecific forest plantations in Southern Rwanda

Forest soils contain the largest carbon stock of all terrestrial biomes and are probably the most important source of carbon dioxide (CO₂) to atmosphere. Soil CO₂ fluxes from 54 to 72-year-old monospecific stands in Rwanda were quantified from March 2006 to December 2007. The influences of soil temperature, soil water content, soil carbon (C) and nitrogen (N) stocks, soil pH, and stand characteristics on soil CO₂ flux were investigated. The mean annual soil CO₂ flux was highest under Eucalyptus saligna (3.92 μmol m⁻² s⁻¹) and lowest under Entandrophragma excelsum (3.13 μmol m⁻² s⁻¹). The seasonal variation in soil CO₂ flux from all stands followed the same trend and was highest in rainy seasons and lowest in dry seasons. Soil CO₂ flux was mainly correlated to soil water content (R² = 0.36-0.77), stand age (R² = 0.45), soil C stock (R² = 0.33), basal area (R² = 0.21), and soil temperature (R² = 0.06-0.17). The results contribute to the understanding of factors that influence soil CO₂ flux in monocultural plantations grown under the same microclimatic and soil conditions. The results can be used to construct models that predict soil CO₂ emissions in the tropics.     

Methane uptake in soils from Pinus radiata plantations, a reverting shrubland and adjacent pastures: Effects of land-use change, and soil texture, water and mineral nitrogen

Afforestation and reforestation of pastures are key land-use changes in New Zealand that help sequester carbon (C) to offset its carbon dioxide (CO₂) emissions under the Kyoto Protocol. However, relatively little attention has been given so far to associated changes in trace gas fluxes. Here, we measure methane (CH₄) fluxes and CO₂ production, as well as microbial C, nitrogen (N) and mineral-N, in intact, gradually dried (ca. 2 months at 20 °C) cores of a volcanic soil and a heavier textured, non-volcanic soil collected within plantations of Pinus radiata D. Don (pine) and adjacent permanent pastures. CH₄ fluxes and CO₂ production were also measured in cores of another volcanic soil under reverting shrubland (mainly Kunzea var. ericoides (A. Rich) J. Thompson) and an adjacent pasture. CH₄ uptake in the pine and shrubland cores of the volcanic soils at field capacity averaged about 35 and 14 μg CH₄-C m⁻² h⁻¹, respectively, and was significantly higher than in the pasture cores (about 21 and 6 μg CH₄-C m⁻² h⁻¹, respectively). In the non-volcanic soil, however, CH₄-C uptake was similar in most cores of the pine and pasture soils, averaging about 7-9 μg m⁻² h⁻¹, except in very wet samples. In contrast, rates of CO₂ production and microbial C and N concentrations were significantly lower under pine than under pasture. In the air-dry cores, microbial C and N had declined in the volcanic soil, but not in the non-volcanic soil; ammonium-N, and especially nitrate-N, had increased significantly in all samples. CH₄ uptake was, with few exceptions, not significantly influenced by initial concentrations of ammonium-N or nitrate-N, nor by their changes on air-drying. A combination of phospholipid fatty acid (PLFA) and stable isotope probing (SIP) analyses of only the pine and pasture soils showed that different methanotrophic communities were probably active in soils under the different vegetations. The C18 PLFAs (type II methanotrophs) predominated under pine and C16 PLFAs (type I methanotrophs) predominated under pasture. Overall, vegetation, soil texture, and water-filled pore space influenced CH₄-C uptake more than did soil mineral-N concentrations.     

Comparison of soil fungal/bacterial ratios in a pH gradient using physiological and PLFA-based techniques

We have compared the total microbial biomass and the fungal/bacterial ratio estimated using substrate-induced respiration (SIR) in combination with the selective inhibition technique and using the phospholipid fatty acid (PLFA) technique in a pH gradient (3.0-7.2) consisting of 53 mature broad-leaved forest soils. A fungal/bacterial biomass index using the PLFA technique was calculated using the PLFA 18:2ω6,9 as an indicator of fungal biomass and the sum of 13 bacterial specific PLFAs as indicator of the bacterial biomass. Good linear correlation (p<0.001) was found between the total microbial biomass estimated with SIR and total PLFAs (totPLFA), indicating that 1 mg biomass-C was equivalent to 130 nmol totPLFA. Both biomass estimates were positively correlated to soil pH. The fungal/bacterial ratio measured using the selective inhibition technique decreased significantly with increasing pH from about 9 at pH 3 to approximately 2 at pH 7, while the fungal/bacterial biomass index using PLFA measurements tended to increase slightly with increasing soil pH. Good correlation between the soil content of ergosterol and of the PLFA 18:2ω6,9 indicated that the lack of congruency between the two methods in estimating fungal/bacterial ratios was not due to PLFA 18:2ω6,9-related non-fungal structures to any significant degree. Several PLFAs were strongly correlated to soil pH (R² values >0.8); for example the PLFAs 16:1ω5 and 16:1ω7c increased with increasing soil pH, while i16:0 and cy19:0 decreased. A principal component analysis of the total PLFA pattern gave a first component that was strongly correlated to soil pH (R²=0.85, p<0.001) indicating that the microbial community composition in these beech/beech-oak forest soils was to a large extent determined by soil pH.     

Pasture type and fertilization effects on N2 fixation, N budgets and external energy inputs in western Canada

A grazing experiment was conducted in Brandon, Manitoba, Canada. The objectives were to examine the effects of including alfalfa and fertilizer management on N₂ fixation by alfalfa and plant N dynamics, and to compare N budgets in the four contrasting pasture systems and external energy inputs between fertilizer-N-based and legume-based pasture systems. Estimates of annual amounts of N₂ fixed, based on shoot herbage production in grazed mixed alfalfa/grass pastures, ranged from 40 to 118 kg N ha⁻¹ y⁻¹. The amounts would be in the range of 52-153 kg N ha⁻¹ y⁻¹, if the amounts of fixed N stored in the roots, were included. Compared to grass-only pastures, total amounts of N₂ fixed in the mixed pastures should be sufficient to improve total external N inputs, replace N fertilizer and sustain plant protein for grazing. The reliance of alfalfa (Medicago sativa L.) on N₂ fixation for growth was high (70-95%), and %N derived from the atmosphere by alfalfa (%Ndfa) was not affected by P fertilizer management. Thus, the amounts of N₂ fixed were predominantly regulated by alfalfa dry matter productivity. The data also indicated that alfalfa fixed 27 kg N t⁻¹ dry matter produced. In mixed alfalfa/grass pastures, high soil mineral N uptake by companion grasses, was essential to effectively utilize N that was fixed by alfalfa and returned to soils through the decomposition of alfalfa litter and roots. Compared to grass-only pastures with or without N fertilizer, alfalfa-based pastures could supply sufficient plant protein for grazing animals through N₂ fixation, and at same time, sustain animal productivity with only 28% of the external energy input of the grass-only pasture with N fertilizer.     

Microbial community response to nitrogen deposition in northern forest ecosystems

The productivity of temperate forests is often limited by soil N availability, suggesting that elevated atmospheric N deposition could increase ecosystem C storage. However, the magnitude of this increase is dependent on rates of soil organic matter formation as well as rates of plant production. Nonetheless, we have a limited understanding of the potential for atmospheric N deposition to alter microbial activity in soil, and hence rates of soil organic matter formation. Because high levels of inorganic N suppress lignin oxidation by white rot basidiomycetes and generally enhance cellulose hydrolysis, we hypothesized that atmospheric N deposition would alter microbial decomposition in a manner that was consistent with changes in enzyme activity and shift decomposition from fungi to less efficient bacteria. To test our idea, we experimentally manipulated atmospheric N deposition (0, 30 and 80 kg NO₃⁻-N) in three northern temperate forests (black oak/white oak (BOWO), sugar maple/red oak (SMRO), and sugar maple/basswood (SMBW)). After one year, we measured the activity of ligninolytic and cellulolytic soil enzymes, and traced the fate of lignin and cellulose breakdown products (¹³C-vanillin, catechol and cellobiose).In the BOWO ecosystem, the highest level of N deposition tended to reduce phenol oxidase activity (131±13 versus 104±5 μmol h⁻¹ g⁻¹) and peroxidase activity (210±26 versus 190±21 μmol h⁻¹ g⁻¹) and it reduced ¹³C-vanillin and ¹³C-catechol degradation and the incorporation of ¹³C into fungal phospholipids (p<0.05). Conversely, in the SMRO and SMBW ecosystems, N deposition tended to increase phenol oxidase and peroxidase activities and increased vanillin and catechol degradation and the incorporation of isotope into fungal phospholipids (p<0.05). We observed no effect of experimental N deposition on the degradation of ¹³C-cellulose, although cellulase activity showed a small and marginally significant increase (p<0.10). The ecosystem-specific response of microbial activity and soil C cycling to experimental N addition indicates that accurate prediction of soil C storage requires a better understanding of the physiological response of microbial communities to atmospheric N deposition.     

Measuring microbial uptake of nitrogen in nutrient-amended sandy soils—A mass-balance based approach

Soil microbial biomass N is commonly determined through fumigation-extraction (FE), and a conversion factor (K_EN) is necessary to convert extractable N to actual soil biomass N. Estimation of K_EN has been constrained by various uncertainties including potential microbial immobilisation. We developed a mass-balance approach to quantify changes in microbial N storage during nutrient-amended incubation, in which microbial uptake is determined as the residual in a ‘mass-balance’ based on soil-water N before and after amended incubation. The approach was applied to three sandy soils of southwestern Australia, to determine microbial N immobilisation during 5-day incubation in response to supply of 2.323 mg C g⁻¹, 100 μg N g⁻¹ and 20 μg P g⁻¹. The net N immobilisation was estimated to be 95-114 μg N g⁻¹ in the three soils, equivalent to 82.7-85.1% of soil-water N following the amendment. Such estimation for microbial uptake does not depend on fumigation and K_EN conversion, but for comparison purposes we estimated ‘nominal’ K_EN values (0.11-0.14) for the three soils, which were comparable to previously reported K_EN from soils receiving C and N amendment. The accuracy of our approach depends on the mass-balance equation and the integrated measurement errors of the multiple N pools, and was assessed practically through recoveries of added-N when microbial uptake can be minimised. Near-satisfactory recoveries were achieved under such conditions. Our mass-balance approach provides information not only about changes in the microbial biomass nitrogen storage, but also major N-pools and their fluxes in regulating soil N concentrations under substrate and nutrient amended conditions.     

Long-term effects of metal-containing farmyard manure and sewage sludge on soil organic matter in a fluvisol

Our aim was to establish the long-term effects of repeated applications after 20 y of organic amendments (farmyard manure at 10 t ha⁻¹ y⁻¹, and urban sewage sludge at two different rates, 10 t ha⁻¹ y⁻¹ and 100 t ha⁻¹ every 2 y) on the quality of a sandy and poorly buffered soil (Fluvisol, pH 6). Chemical characteristics and biodegradability of the labile organic matter, which is mainly derived from microbial biomass and biodegradation products of organic residues, were chosen as indicators for soil quality. The organic C content had reached a maximal value (30.6 g C kg⁻¹ in the 100 t sludge-treated soil), i.e. about 2.5 times that in the control. Six years after the last application, the organic C content and the microbial biomass content remained higher in sludge-treated soils than in the control. In contrast, the proportion of labile organic matter was significantly lower in sludge-treated soils than in manure-treated and control soils. The labile organic matter of sludge extracts appeared less humified than that of manure-treated and control soils.     

Effects of tree species and N additions on forest floor microbial communities and extracellular enzyme activities

Forest nitrogen (N) retention and soil carbon (C) storage are influenced by tree species and their associated soil microbial communities. As global change factors alter forest composition, predicting long-term C and N dynamics will require understanding microbial community structure and function at the tree species level. Because atmospheric N deposition is increasing N inputs to forested ecosystems across the globe, including the northeastern US, it is also important to understand how microbial communities respond to added N. While prior studies have examined these topics in mixed-species stands, we focused on the responses of different tree species and their associated microbial communities within a single forest type - a northern hardwood forest in the Catskills Mountains, NY. Based on prior studies, we hypothesized that N additions would stimulate extracellular enzyme activities in relatively labile litters, but suppress oxidative enzyme activities in recalcitrant litters, and tested for independent tree species effects within this context. During the 2007 growing season (May-June), we measured enzyme activities and microbial community composition (using phospholipid fatty acid analysis - PLFA) of the forest floor in single-species plots dominated by sugar maple (Acer saccharum), yellow birch (Betula alleghaniensis), red oak (Quercus rubra), American beech (Fagus grandifolia) and eastern hemlock (Tsuga canadensis), species whose litters range from relatively labile to recalcitrant. Half the plots were fertilized with N by adding NH₄NO₃ (50 kg ha⁻¹ y⁻¹) from 1997 to 2009. Non-metric multidimensional scaling (NMS) and multi-response permutation procedures (MRPP) were used to examine microbial community structure and relationship to enzyme activities.We found that in response to N additions, both microbial community composition and enzyme activities changed; however the strength of the changes were tree species-specific and the direction of these changes was and not readily predictable from prior studies conducted in mixed-species stands. For example, in contrast to other studies, we found that N additions caused a significant overall increase in fungal biomass that was strongest for yellow birch (24% increase) and weakest for sugar maple (1% increase). Contrary to our initial hypotheses and current conceptual models, N additions reduced hydrolytic enzyme activities in hemlock plots and reduced oxidative enzyme activity in birch plots, a species with relatively labile litter. These responses suggest that our understanding of the interactions between microbial community composition, enzyme activity, substrate chemistry, and nutrient availability as influenced by tree species composition is incomplete. NMS ordination showed that patterns in microbial community structure (PLFA) and function (enzyme activity) were more strongly influenced by tree species than by fertilization, and only partially agreed with the structure-function relationships found in other studies. This finding suggests that tree species-specific responses are likely to be important in determining the structure and function of northeastern hardwood forests in the future. Enhanced understanding of microbial responses to added N in single and mixed-species substrates with varying amounts of lignin and phenols may be needed for accurate predictions of future soil C and N dynamics.     

Subtropical plantations are large carbon sinks: Evidence from two monoculture plantations in South China

Quantifying the net carbon (C) storage of forest plantations is required to assess their potential to offset fossil fuel emissions. In this study, a biometric approach was used to estimate net ecosystem productivity (NEP) for two monoculture plantations in South China: Acacia crassicarpa and Eucalyptus urophylla. This approach was based on stand-level net primary productivity (NPP, based on direct biometric inventory) and heterotrophic respiration (R_h). In comparisons of R_h determination based on trenching vs. tree girdling, both trenching and tree girdling changed soil temperature and soil moisture relative to undisturbed control plots, and we assess the effects of corrections for disturbances of soil moisture and soil moisture on the estimation of soil CO₂ efflux partitioning. Soil microbial biomass and dissolved organic carbon were significantly lower in trenched plots than in tree girdled plots for both plantations. Annual soil CO₂ flux in trenched plots (R_h-t) was significantly lower than in tree-girdled plots (R_h-g) in both plantations. The estimates of R_h-t and R_h-g, expressed as a percentage of total soil respiration, were 58 ± 4% and 74 ± 6%, respectively, for A. crassicarpa, and 64 ± 3% and 78 ± 5%, respectively, for E. urophylla. By the end of experiment, the difference in soil CO₂ efflux between the trenched plots and tree-girdled plots had become small for both plantations. Annual R_h (mean of the annual R_h-t and R_h-g) and net primary production (NPP) were 470 ± 25 and 800 ± 118 g C m⁻² yr⁻¹, respectively, for A. crassicarpa, and 420 ± 35 and 2380 ± 187 g C m⁻² yr⁻², respectively, for E. urophylla. The two plantations in the developmental stage were large carbon sinks: NEP was 330 ± 76 C m⁻² yr⁻¹ for A. crassicarpa and 1960 ± 178 g C m⁻² yr⁻¹ for E. urophylla.     

Microbial colonisation of roots as a function of plant species

Fifteen plants species were grown in the greenhouse on the same soil and sampled at flowering to obtain rhizosphere soil and root material. In both fractions, the data on fungal and bacterial tissue obtained by amino sugar analysis were compared with the total microbial biomass based on fumigation-extraction and ergosterol data. The available literature on glucosamine concentrations in fungi and on muramic acid concentrations in bacteria was reviewed to prove the possibility of generating conversion values for general use in root material. All microbial properties analysed revealed strong species-specific differences in microbial colonisation of plant roots. The root material contained considerable amounts of microbial biomass C and biomass N, reaching mean levels of 10.9 and 1.4 mg g⁻¹ dry weight, respectively. However, the majority of CHCl₃ labile C and N, i.e. 89 and 55% was root derived. The average amount of ergosterol was 13 μg g⁻¹ dry weight and varied between 0.0 for Phacelia roots and 45.5 μg g⁻¹ dry weight for Vicia roots. The ergosterol content in root material of mycorrhizal and non-mycorrhizal plant species did not differ significantly. Fungal glucosamine was converted to fungal C by multiplication by 9 giving a range of 7.1-25.9 mg g⁻¹ dry weight in the root material. Fungal C and ergosterol were significantly correlated. Bacterial C was calculated by multiplying muramic acid by 45 giving a range from 1.7 to 21.6 mg g⁻¹ dry weight in the root material. In the root material of the 15 plant species, the ratio of fungal C-to-bacterial C ranged from 1.0 in mycorrhizal Trifolium roots to 9.5 in non-mycorrhizal Lupinus roots and it was on average 3.1. These figures mean that the microbial tissue in the root material consists on average of 76% fungal C and 24% bacterial C. The differences in microbial colonisation of the roots were reflected by differences in microbial indices found in the rhizosphere soil, most strongly for microbial biomass C and ergosterol, but to some extent also for glucosamine and muramic acid.     

Integrated diurnal soil respiration model during growing season of a typical temperate steppe: Effects of temperature, soil water content and biomass production

Soil respiration was measured with the enclosed chamber method in an ungrazed Leymus chinensis steppe during the growing seasons of 2001 and 2002. Soil respiration rate (R_S) was significantly influenced by air temperature (T) at the diurnal scale, and could be described by Van't Hoff's equation (R_S = R₁₀ exp(β(T − 10))). At the seasonal scale, the normalized soil respiration rate at 10 °C (R₁₀) was mainly controlled by soil water content (R² = 0.717, P < 0.001), while the sensitivity of soil respiration to temperature (Q₁₀) was partially affected by absolute growth rate (R² = 0.482, P = 0.004). Thus, soil respiration could be described as R_S = (20.015W − 84.085) (0.103AGR + 1.786)^(T−10)/10 during the growing seasons, integrating soil water content (W) and absolute growth rate (AGR) into the temperature-dependent soil respiration equation. It was validated by the observed soil respiration rates in this study (R² = 0.890, P < 0.001) and observations from near-field experiment (R² = 0.687, P = 0.011). It implied that accurately evaluating annual soil respiration should include the effects of plant biomass production and other abiotic factors besides air temperature.     

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.     

Increased N availability in grassland soils modifies their microbial communities and decreases the abundance of arbuscular mycorrhizal fungi

Two complementary studies were performed to examine (1) the effect of 18 years of nitrogen (N) fertilization, and (2) the effects of N fertilization during one growing season on soil microbial community composition and soil resource availability in a grassland ecosystem. N was added at three different rates: 0, 5.44, and 27.2 g N m⁻² y⁻¹. In both studies, Schizachyrium scoparium was the dominant plant species before N treatments were applied. Soil microbial communities from each experiment were characterized using fatty acid methyl ester (FAME) analysis. Discriminant analysis of the FAMEs separated the three N fertilizer treatments in both experiments, indicating shifts in the composition of the microbial communities. In general, plots that received N fertilizer at low or high application rates for 18 years showed increased proportions of bacterial FAMEs and decreased fungal FAMEs. In particular, control plots contained a significantly higher proportion of fungal FAMEs C18:1(cis9) and C18:2(cis9,12) and of the arbuscular mycorrhizal fungal (AMF) FAME, C16:1(cis11), than both of the N addition treatment plots. A significant negative effect of N fertilization on the AMF FAME, C16:1(cis11), was measured in the short-term experiment. Our results indicate that high rates of anthropogenic N deposition can lead to significant changes in the composition of soil microbial communities over short periods and can even disrupt the relationship between AMF and plants.     

Relationships between soil biota,nitrogen and phosphorus availability,and pasture growth under organic and conventional management

Legume-based pastures generally rely on soil biological activity to provide nitrogen (N) for plants. This study examined seasonal pasture growth in nine adjacent hill pastures, under sheep or beef, with different long-term managements, including certified organic, no fertilizer, and conventional fertilizer application, that formed a soil-fertility sequence. We determined relationships between net N mineralization, as a measure of soil biological activity and N availability, and microbial biomass, soil organic matter, and fauna. Net N mineralization generally explained differences in pasture production (r = 0.87). On an areal basis, net N mineralization was strongly related (r = 0.93) to total soil N (0–200 mm depth) and negatively related (r = −0.92) to soil C:N ratio, but not to soil C. Total N and C:N ratios were related to soil phosphorus (P) status and probably past N fixation by legumes. Where labile P was low, the N:P ratios of both soil microbes and enchytraeids were wide, and the organisms appeared to be P limited, possibly competing with plants for P. Faunal grazing on soil micro-organisms appeared to release P. We could find no convincing evidence that net N mineralization, pasture growth or soil biological diversity increased under organic farming. Rather, the data from organic pastures followed similar trend lines to data from pastures under conventional management.     

CO2 and inorganic N supply modify competition for N between co-occurring grass plants, tree seedlings and soil microorganisms

Plant-plant and plant-soil interactions play a key role in determining plant community structure and ecosystem function. However, the effects of global change on the interplay between co-occurring plants and soil microbes in successional communities are poorly understood. In this study, we investigated competition for nitrogen (N) between soil microorganisms, grass plants and establishing tree seedlings under factorial carbon dioxide (CO₂) and N treatments. Fraxinus excelsior seedlings were germinated in the presence or absence of grass competition (Dactylis glomerata) at low (380 μmol mol⁻¹) or high (645 μmol mol⁻¹) CO₂ and at two levels of N nutrition in a mesocosm experiment. Pulse ¹⁵N labelling was used to examine N partitioning among plant and soil compartments. Dactylis exerted a strong negative effect on Fraxinus biomass, N capture and ¹⁵N recovery irrespective of N and CO₂ treatment. In contrast, the presence of Dactylis had a positive effect on the microbial N pool. Plant and soil responses to N treatment were of a greater magnitude compared with responses to elevated CO₂, but the pattern of Fraxinus- and microbial-N pool response to N and CO₂ varied depending on grass competition treatment. Within the Dactylis competition treatment, decreases in Fraxinus biomass in response to N were not mirrored by decreases in tree seedling N content, suggesting a shift from below- to above-ground competition. In the Dactylis-sown pots, ¹⁵N recovery could be ranked Dactylis > microbial pool > Fraxinus in all N and CO₂ treatment combinations. Inequalities between Fraxinus and soil microorganisms in terms of ¹⁵N recovery were exacerbated by N addition. Contrary to expectations, elevated CO₂ did not increase plant-microbe competition. Nevertheless, microbial ¹⁵N recovery showed a small positive increase in the high CO₂ treatment. Overall, elevated CO₂ and N supply did not interact on plant/soil N partitioning. Our data suggest that the competitive balance between establishing tree seedlings and grass plants in an undisturbed sward is relatively insensitive to CO₂ or N-induced modifications in N competition between plant and soil compartments.