C and N natural abundance of the soil microbial biomass

Stable isotope analysis is a powerful tool in the study of soil organic matter formation. It is often observed that more decomposed soil organic matter is ¹³C, and especially ¹⁵N-enriched relative to fresh litter and recent organic matter. We investigated whether this shift in isotope composition relates to the isotope composition of the microbial biomass, an important source for soil organic matter. We developed a new approach to determine the natural abundance C and N isotope composition of the microbial biomass across a broad range of soil types, vegetation, and climates. We found consistently that the soil microbial biomass was ¹⁵N-enriched relative to the total (3.2 ‰) and extractable N pools (3.7 ‰), and ¹³C-enriched relative to the extractable C pool (2.5 ‰). The microbial biomass was also ¹³C-enriched relative to total C for soils that exhibited a C3-plant signature (1.6 ‰), but ¹³C-depleted for soils with a C4 signature (−1.1 ‰). The latter was probably associated with an increase of annual C3 forbs in C4 grasslands after an extreme drought. These findings are in agreement with the proposed contribution of microbial products to the stabilized soil organic matter and may help explain the shift in isotope composition during soil organic matter formation.     

Leaf litter C and N cycling from a deciduous permanent crop

Our understanding of leaf litter carbon (C) and nitrogen (N) cycling and its effects on N management of deciduous permanent crops is limited. In a 30-day laboratory incubation, we compared soil respiration and changes in mineral N [ammonium (NH₄⁺-N) + nitrate (NO₃^–-N)], microbial biomass nitrogen (MBN), total organic carbon (TOC) and total non-extractable organic nitrogen (TON) between a control soil at ¹⁵N natural abundance (δ¹⁵N = 1.08‰) without leaf litter and a treatment with the same soil, but with almond (Prunus dulcis (Mill.) D.A. Webb) leaf litter that was also enriched in ¹⁵N (δ¹⁵N = 213‰). Furthermore, a two-end member isotope mixing model was used to identify the source of N in mineral N, MBN and TON pools as either soil or leaf litter. Over 30 d, control and treatment TOC pools decreased while the TON pool increased for the treatment and decreased for the control. Greater soil respiration and significantly lower (p < 0.05) mineral N from 3 to 15 d and significantly greater MBN from 10 to 30 d were observed for the treatment compared to the control. After 30 d, soil-sourced mineral N was significantly greater for the treatment compared to the control. Combined mineral N and MBN pools derived from leaf litter followed a positive linear trend (R² = 0.75) at a rate of 1.39 μg N g^?1 soil day^?1. These results suggest early-stage decomposition of leaf litter leads to N immobilization followed by greater N mineralization during later stages of decomposition. Direct observations of leaf litter C and N cycling assists with quantifying soil N retention and availability in orchard N budgets.     

Seasonal dynamics of leaf- and root-derived C in arctic tundra mesocosms

We investigated contributions of leaf litter, root litter and root-derived organic material to tundra soil carbon (C) storage and transformations. ¹⁴C-labeled materials were incubated for 32 weeks in moist tussock tundra soil cores under controlled climate conditions in growth chambers, which simulated arctic fall, winter, spring and summer temperatures and photoperiods. In addition, we tested whether the presence of living plants altered litter and soil organic matter (SOM) decomposition by planting shoots of the sedge Eriophorum vaginatum in half of the cores. Our results suggest that root litter accounted for the greatest C input and storage in these tundra soils, while leaf litter was rapidly decomposed and much of the C lost to respiration. We observed transformations of ¹⁴C between fractions even when total C appeared unchanged, allowing us to elucidate sources and sinks of C used by soil microorganisms. Initial sources of C included both water soluble (WS) and acid-soluble (AS) fractions, primarily comprised of carbohydrates and cellulose, respectively. The acid-insoluble (AIS) fraction appeared to be a sink for C when conditions were favorable for plant growth. However, decreases in ¹⁴C activity from the AIS fraction between the fall and spring harvests in all treatments indicated that microorganisms consumed recalcitrant C compounds when soil temperatures were below 0 °C. In planted leaf litter cores and in both planted and unplanted SOM cores, the greatest amounts of ¹⁴C at the end of the experiment were found in the AIS fraction, suggesting a high rate of humification or accumulation of decay-resistant plant tissues. In unplanted leaf litter cores and planted and unplanted root litter cores most of the ¹⁴C remaining at the end of the experiment was in the AS fraction suggesting less extensive humification of leaf and root detritus. Overall, the presence of living plants stimulated decomposition of leaf litter by creating favorable conditions for microbial activity at the soil surface. In contrast, plants appeared to inhibit decomposition of root litter and SOM, perhaps because of microbial preferences for newer, more labile inputs from live roots.     

Budgets for root-derived C and litter-derived C: comparison between conventional tillage and no tillage soils

Placement of plant residues in conventional tillage (CT) and no-tillage (NT) soils affects organic matter accumulation and the organization of the associated soil food webs. Root-derived C inputs can be considerable and may also influence soil organic matter dynamics and soil food web organization. In order to differentiate and quantify C contributions from either roots or litter in CT and NT soils, a ¹⁴C tracer method was used.To follow root-derived C, maize plants growing in the field were ¹⁴C pulse-labeled, while the plant litter in those plots remained unlabeled. The ¹⁴C was measured in NT and CT soils for the different C pools (shoots, roots, soil, soil respiration, microbial biomass). Litter-derived C was followed by applying ¹⁴C labeled maize litter to plots which had previously grown unlabeled maize plants. The ¹⁴C pools measured for the litter-derived CT and NT plots included organic matter, microbial biomass, soil respiration, and soil organic C.Of the applied label in the root-derived C plots, 35–55, 6–8, 3, 1.6, and 0.4–2.4% was recovered in the shoots, roots, soil, cumulative soil respiration, and microbial biomass, respectively. The ¹⁴C recovered in these pools did not differ between CT and NT treatments, supporting the hypothesis that the rhizosphere microbial biomass in NT and CT may be similar in utilization of root-derived C. Root exudates were estimated to be 8–13% of the applied label. In litter-derived C plots, the percentage of applied label recovered in the particulate organic matter (3.2–82%), microbial biomass (4–6%), or cumulative soil respiration (12.5–14.7%) was the same for CT and NT soils. But the percentage of ¹⁴C recovered in CT soil organic C (18–69%) was higher than that in NT (12–43%), suggesting that particulate organic matter (POM) leaching and decomposition occurred at a higher rate in CT than in NT. Results indicate faster turnover of litter-derived C in the CT plots.     

Variable use of plant- and soil-derived carbon by microorganisms in agricultural soils

In this study we used compound specific ¹³C and ¹⁴C isotopic signatures to determine the degree to which recent plant material and older soil organic matter (SOM) served as carbon substrates for microorganisms in soils. We determined the degree to which plant-derived carbon was used as a substrate by comparison of the ¹³C content of microbial phospholipid fatty acids (PLFA) from soils of two sites that had undergone a vegetation change from C3 to C4 plants in the past 20-30 years. The importance of much older SOM as a substrate was determined by comparison of the radiocarbon content of PLFA from soils of two sites that had different ¹⁴C concentrations of SOM.The ¹³C shift in PLFA from the two sites that had experienced different vegetation history indicated that 40-90% of the PLFA carbon had been fixed since the vegetation change took place. Thus PLFA were more enriched in ¹³C from the new C4 vegetation than it was observed for bulk SOM indicating recent plant material as preferentially used substrate for soil microorganisms. The largest ¹³C shift of PLFA was observed in the soil that had high ¹⁴C concentrations of bulk SOM. These results reinforce that organic carbon in this soil for the most part cycles rapidly. The degree to which SOM is incorporated into microbial PLFA was determined by the difference in ¹⁴C concentration of PLFA derived from two soils one with high ¹⁴C concentrations of bulk SOM and one with low. These results showed that 0-40% of SOM carbon is used as substrate for soil microorganisms. Furthermore a different substrate usage was identified for different microorganisms. Gram-negative bacteria were found to prefer recent plant material as microbial carbon source while Gram-positive bacteria use substantial amounts of SOM carbon. This was indicated by ¹³C as well as ¹⁴C signatures of their PLFA. Our results find evidence to support ‘priming’ in that PLFA indicative of Gram-negative bacteria associated with roots contain both plant- and SOM-derived C. Most interestingly, we find PLFA indicative of archeobacteria (methanothrophs) that may indicate the use of other carbon sources than plant material and SOM to a substantial amount suggesting that inert or slow carbon pools are not essential to explain carbon dynamics in soil.     

Carbon input belowground is the major C flux contributing to leaf litter mass loss: Evidences from a C labelled-leaf litter experiment

Partitioning of the quantities of C lost by leaf litter through decomposition into (i) CO₂ efflux to the atmosphere and (ii) C input to soil organic matter (SOM) is essential in order to develop a deeper understanding of the litter-soil biogeochemical continuum. However, this is a challenging task due to the occurrence of many different processes contributing to litter biomass loss. With the aim of quantifying different fluxes of C lost by leaf litter decomposition, a field experiment was performed at a short rotation coppice poplar plantation in central Italy. Populus nigra leaf litter, enriched in ¹³C (δ¹³C ∼ +160‰) was placed within collars to decompose in direct contact with the soil (δ¹³C ∼ −26‰) for 11 months. CO₂ efflux from within the collars and its isotopic composition were determined at monthly intervals. After 11 months, remaining litter and soil profiles (0-20 cm) were sampled and analysed for their total C and ¹³C content. Gas chromatography (GC), GC-mass spectrometry (MS) and GC-combustion-isotope ratio (GC/C/IRMS) were used to analyse phospholipid fatty acids (PLFA) extracted from soil samples to identify the groups of soil micro-organisms that had incorporated litter-derived C and to determine the quantity of C incorporated by the soil microbial biomass (SMB). By the end of the experiment, the litter had lost about 80% of its original weight. The fraction of litter C lost as an input into the soil (67 ± 12% of the total C loss) was found to be twice as much as the fraction released as CO₂ to the atmosphere (30 ± 3%), thus demonstrating the importance of quantifying litter-derived C input to soils, in litter decomposition studies. The mean δ¹³C values of PLFAs in soil (δ¹³C = −12.5‰) showed sustained incorporation of litter-derived C after one year (7.8 ± 1.6% of total PLFA-C). Thus, through the application of stable ¹³C isotope analyses, we have quantified two major C fluxes contributing to litter decomposition, at macroscopic and microscopic levels.     

Microbial immobilisation of C rhizodeposits in rhizosphere and root-free soil under continuous C labelling of oats

A greenhouse experiment was conducted by growing oats (Avenasativa L.) in a continuously ¹³CO₂ labeled atmosphere. The allocation of ¹³C-labeled photosynthates in plants, microbial biomass in rhizosphere and root-free soil, pools of soil organic C, and CO₂ emissions were examined over the plant's life cycle. To isolate rhizosphere from root-free soil, plant seedlings were placed into bags made of nylon monofilament screen tissue (16 μm mesh) filled with soil. Two peaks of ¹³C in rhizosphere pools of microbial biomass and dissolved organic carbon (DOC), as well as in CO₂ emissions at the earing and ripeness stages were revealed. These ¹³C maxima corresponded to: (i) the end of rapid root growth and (ii) beginning of root decomposition, respectively. The δ¹³C values of microbial biomass were higher than those of DOC and of soil organic matter (SOM). The microbial biomass C accounted for up to 56 and 39% of ¹³C recovered in the rhizosphere and root-free soil, respectively. Between 4 and 28% of ¹³C assimilated was recovered in the root-free soil. Depending on the phenological stage, the contribution of root-derived C to total CO₂ emission from soil varied from 61 to 92% of total CO₂ evolved, including 4-23% attributed to rhizomicrobial respiration. While 81-91% of C substrates used for microbial growth in the root-free soil and rhizosphere came from SOM, the remaining 9-19% of C substrates utilized by the microbial biomass was attributable to rhizodeposition. The use of continuous isotopic labelling and physical separation of root-free and rhizosphere soil, combined with natural ¹³C abundance were effective in gaining new insight on soil and rhizosphere C-cycling.     

Microbial community utilization of added carbon substrates in response to long-term carbon input manipulation

The chemical composition and quantity of plant inputs to soil are primary factors controlling the size and structure of the soil microbial community. Little is known about how changes in the composition of the soil microbial community affect decomposition rates and other ecosystem functions. This study examined the degradation of universally ¹³C-labeled glucose, glutamate, oxalate, and phenol in soil from an old-growth Douglas-fir (Pseudotsuga menziesii)—western hemlock (Tsuga heterophylla) forest in the Oregon Cascades that has experienced 7 y of chronic C input manipulation. The soils used in this experiment were part of a larger Detritus Input and Removal Treatment experiment and have received normal C inputs (control), doubled wood inputs, or root and litter input exclusion (no inputs). Soil from the doubled wood treatment had a higher fungal:bacterial ratio, and soil from the no inputs treatment had a lower fungal:bacterial ratio, than the control soil. Differences in the utilization of the compounds added to the field-manipulated soils were assessed by following the ¹³C tracer into microbial biomass and respiration. In addition, ¹³C-phospholipid fatty acids (PLFA) analysis was used to examine differential microbial utilization of the added substrates. Glucose and glutamate were metabolized similarly in soils of all three litter treatments. In contrast, the microbial community in the double wood soil respired more added phenol and oxalate, whereas microbes in the no inputs soil respired less added phenol and oxalate, than the control soil. Phenol was incorporated primarily into fungal PLFA, especially in soil of the double wood treatment. The addition of all four substrates led to enhanced degradation of soil organic matter (priming) in soils of all three litter treatments, and was greater following the addition of phenol and oxalate as compared to glucose and glutamate. Priming was greater in the no inputs soil as compared to the control or doubled wood soils. These results demonstrate that altering plant inputs to soil can lead to changes in microbial utilization of C compounds. It appears that many of these changes are the result of alteration in the size and composition of the microbial community.     

Carbon fluxes in soil food webs of increasing complexity revealed by C labelling and C natural abundance

Soil food webs are mainly based on three primary carbon (C) sources: root exudates, litter, and recalcitrant soil organic matter (SOM). These C sources vary in their availability and accessibility to soil organisms, which could lead to different pathways in soil food webs. The presence of three C isotopes (¹²C, ¹³C and ¹⁴C) offers an unique opportunity to investigate all three C sources simultaneously. In a microcosm experiment we studied the effect of food web complexity on the utilization of the three carbon sources. We choose an incomplete three factorial design with (i) living plants, (ii) litter and (iii) food web complexity. The most complex food web consisted of autochthonous microorganisms, nematodes, collembola, predatory mites, endogeic and anecic earthworms. We traced C from all three sources in soil, in CO₂ efflux and in individual organism groups by using maize grown on soil developed under C₃ vegetation and application of ¹⁴C labelled ryegrass shoots as a litter layer. The presence of living plants had a much greater effect on C pathways than food web complexity. Litter decomposition, measured as ¹⁴CO₂ efflux, was decreased in the presence of living plants from 71% to 33%. However, living plants increased the incorporation of litter C into microbial biomass and arrested carbon in the litter layer and in the upper soil layer. The only significant effect of food web complexity was on the litter C distribution in the soil layers. In treatments with fungivorous microarthropods (Collembola) the incorporation of litter carbon into mineral soil was reduced. Root exudates as C source were passed through rhizosphere microorganisms to the predator level (at least to the third trophic level). We conclude that living plants strongly affected C flows, directly by being a source of additional C, and indirectly by modifying the existing C flows within the food web including CO₂ efflux from the soil and litter decomposition.     

Production of root-derived material and associated microbial growth in soil at different nutrient levels

Summary Maize plants were grown for 42 days in a sandy soil at two different mineral nutrient levels, in an atmosphere containing ¹⁴CO₂. The ¹⁴C and total carbon contents of shoots, roots, soil and soil microbial biomass were measured 28, 35 and 42 days after germination. Relative growth rates of shoots and roots decreased after 35 days at the lower nutrient level, but were relatively constant at the higher nutrient level. In the former treatment, 2% of the total ¹⁴C fixed was retained as a residue in soil at all harvests while at the higher nutrient level up to 4% was retained after 42 days. Incorporation of ¹⁴C into the soil microbial biomass was close to its maximum after 35 days at the lower nutrient level, but continued to increase at the higher level. Generally a good agreement existed between microbial biomass, ¹⁴C contents and numbers of fluorescent pseudomonads in the rhizosphere. Numbers of fluorescent pseudomonads in the rhizosphere were maximal after 35 days at the lower nutrient level and continued to increase at the higher nutrient level. The proportions of the residual ¹⁴C in soil, incorporated in the soil microbial biomass, were 28% to 41% at the lower nutrient level and 20%6 – 30% at the higher nutrient level. From the lower nutrient soil 18%6 – 52%6 of the residual soil ¹⁴C could be extracted with 0.5 N K₂SO₄, versus 14%6 – 16% from the higher nutrient soil.Microbial growth in the rhizosphere seemed directly affected by the depletion of mineral nutrients while plant growth and the related production of root-derived materials continued.     

Rhizospheric influence on soil respiration and decomposition in a temperate Norway spruce stand

Assessments of terrestrial carbon fluxes require a thorough understanding of links between primary production, soil respiration and carbon loss through drainage. In this study, stem girdling was used to terminate autotrophic soil respiration including rhizosphere respiration and root exudation in a temperate Norway spruce stand. Rates of soil respiration and dissolved organic carbon (DOC) formation were measured in the second year after girdling, comparing an intact plant-rhizosphere continuum with an exclusive decomposer system. The molecular and isotopic composition of DOC in the soil solution was analysed with a coupled Py-GC/MS-C-IRMS system to distinguish between the carbon sources of dissolved carbon. Pyrolysis products were grouped according to their precursor origins: polysaccharides, proteins or of mixed origin (mainly derivates of lignins and proteins). When dead roots became available for decomposition, rates of heterotrophic soil respiration in girdling plots peaked at 6.5 μmol m⁻² s⁻¹, comparable to peak rates of total soil respiration (autotrophic and heterotrophic) in control plots, 6.1 μmol m⁻² s⁻¹. A significant response of soil respiration to temperature was found in control plots only, showing that an unlimiting supply of organic substrates for microbial respiration may mask any temperature effects. The enhanced decomposition in girdled plots was further supported by the isotopic composition of DOC in soil solution; all three precursor groups became isotopically enriched as the growing season progressed (polysaccharides by 2.3‰, proteins by 1.9‰, mixed origin group by 2.2‰). This indicates a trophic level shift due to incorporation of organic substrate into the microbial food chain. In the control plots’ mixed origin fraction, the isotopic composition changed over time from a signature resembling that of lignin (−28.9‰) to one similar of the protein fraction (−25.7‰). Significant temporal changes of structural DOC composition occurred in the girdling plots only. These results suggest that changes in the microbial community and in decomposition rates occurred in both girdled and control plots in the following ways: (i) increased substrate availability (dead roots) gave rise to generally enhanced performance of the decomposer community in girdled plots, (ii) root-derived exudates probably contributed to enhanced decomposition of recalcitrant lignin in the control plots and (iii) the structural composition of DOC seemed to be more a result of decomposition than of plant root exudation in all plots.     

C fractionation at the root-microorganisms-soil interface: A review and outlook for partitioning studies

Natural variations of the ¹³C/¹²C ratio have been frequently used over the last three decades to trace C sources and fluxes between plants, microorganisms, and soil. Many of these studies have used the natural-¹³C-labelling approach, i.e. natural δ¹³C variation after C₃-C₄ vegetation changes. In this review, we focus on ¹³C fractionation in main processes at the interface between roots, microorganisms, and soil: root respiration, microbial respiration, formation of dissolved organic carbon, as well as microbial uptake and utilization of soil organic matter (SOM). Based on literature data and our own studies, we estimated that, on average, the roots of C₃ and C₄ plants are ¹³C enriched compared to shoots by +1.2 ± 0.6‰ and +0.3 ± 0.4‰, respectively. The CO₂ released by root respiration was ¹³C depleted by about −2.1 ± 2.2‰ for C₃ plants and −1.3 ± 2.4‰ for C₄ plants compared to root tissue. However, only a very few studies investigated ¹³C fractionation by root respiration. This urgently calls for further research. In soils developed under C₃ vegetation, the microbial biomass was ¹³C enriched by +1.2 ± 2.6‰ and microbial CO₂ was also ¹³C enriched by +0.7 ± 2.8‰ compared to SOM. This discrimination pattern suggests preferential utilization of ¹³C-enriched substances by microorganisms, but a respiration of lighter compounds from this fraction. The δ¹³C signature of the microbial pool is composed of metabolically active and dormant microorganisms; the respired CO₂, however, derives mainly from active organisms. This discrepancy and the preferential substrate utilization explain the δ¹³C differences between microorganisms and CO₂ by an ‘apparent’ ¹³C discrimination. Preferential consumption of easily decomposable substrates and less negative δ¹³C values were common for substances with low C/N ratios. Preferential substrate utilization was more important for C₃ soils because, in C₄ soils, microbial respiration strictly followed kinetics, i.e. microorganisms incorporated heavier C (? = +1.1‰) and respired lighter C (? = −1.1‰) than SOM. Temperature and precipitation had no significant effect on the ¹³C fractionation in these processes in C₃ soils. Increasing temperature and decreasing precipitation led, however, to increasing δ¹³C of soil C pools.Based on these ¹³C fractionations we developed a number of consequences for C partitioning studies using ¹³C natural abundance. In the framework of standard isotope mixing models, we calculated CO₂ partitioning using the natural-¹³C-labelling approach at a vegetation change from C₃ to C₄ plants assuming a root-derived fraction between 0% and 100% to total soil CO₂. Disregarding any ¹³C fractionation processes, the calculated results deviated by up to 10% from the assumed fractions. Accounting for ¹³C fractionation in the standard deviations of the C₄ source and the mixing pool did not improve the exactness of the partitioning results; rather, it doubled the standard errors of the CO₂ pools. Including ¹³C fractionations directly into the mass balance equations reproduced the assumed CO₂ partitioning exactly. At the end, we therefore give recommendations on how to consider ¹³C fractionations in research on carbon flows between plants, microorganisms, and soil.     

Microbial community abundance and structure are determinants of soil organic matter mineralisation in the presence of labile carbon

Altered rates of native soil organic matter (SOM) mineralisation in the presence of labile C substrate (‘priming’), is increasingly recognised as central to the coupling of plant and soil-biological productivity and potentially as a key process mediating the C-balance of soils. However, the mechanisms and controls of SOM-priming are not well understood. In this study we manipulated microbial biomass size and composition (chloroform fumigation) and mineral nutrient availability to investigate controls of SOM-priming. Effects of applied substrate (¹³C-glucose) on mineralisation of native SOM were quantified by isotopic partitioning of soil respiration. In addition, the respective contributions of SOM-C and substrate-derived C to microbial biomass carbon (MBC) were quantified to account for pool-substitution effects (‘apparent priming’). Phospholipid fatty acid (PLFA) profiles of the soils were determined to establish treatment effects on microbial community structure, while the ¹³C-enrichment of PLFA biomarkers was used to establish pathways of substrate-derived C-flux through the microbial communities. The results indicated that glucose additions increased SOM-mineralisation in all treatments (positive priming). The magnitude of priming was reduced in fumigated soils, concurrent with reduced substrate-derived C-flux through putative SOM-mineralising organisms (fungi and actinomycetes). Nutrient additions reduced the magnitude of positive priming in non-fumigated soils, but did not affect the distribution of substrate-derived C in microbial communities. The results support the view that microbial community composition is a determinant of SOM-mineralisation, with evidence that utilisation of labile substrate by fungal and actinomycete (but not Gram-negative) populations promotes positive SOM-priming.     

Earthworms increase soil microbial biomass carrying capacity and nitrogen retention in northern hardwood forests

Earthworms have been shown to produce contrasting effects on soil carbon (C) and nitrogen (N) pools and dynamics. We measured soil C and N pools and processes and traced the flow of ¹³C and ¹⁵N from sugar maple (Acer saccharum Marsh.) litter into soil microbial biomass and respirable C and mineralizable and inorganic N pools in mature northern hardwood forest plots with variable earthworm communities. Previous studies have shown that plots dominated by either Lumbricus rubellus or Lumbricus terrestris have markedly lower total soil C than uncolonized plots. Here we show that total soil N pools in earthworm colonized plots were reduced much less than C, but significantly so in plots dominated by contain L. rubellus. Pools of microbial biomass C and N were higher in earthworm-colonized (especially those dominated by L. rubellus) plots and more ¹³C and ¹⁵N were recovered in microbial biomass and less was recovered in mineralizable and inorganic N pools in these plots. These plots also had lower rates of potential net N mineralization and nitrification than uncolonized reference plots. These results suggest that earthworm stimulation of microbial biomass and activity underlie depletion of soil C and retention and maintenance of soil N pools, at least in northern hardwood forests. Earthworms increase the carrying capacity of soil for microbial biomass and facilitate the flow of N from litter into stable soil organic matter. However, declines in soil C and C:N ratio may increase the potential for hydrologic and gaseous losses in earthworm-colonized sites under changing environmental conditions.     

Rhizosphere priming of soil organic matter by bacterial groups in a grassland soil

Plants often impact the rate of native soil organic matter turnover through root interactions with soil organisms; however the role of root-microbial interactions in mediation of the “priming effect” is not well understood. We examined the effects of living plant roots and N fertilization on belowground C dynamics in a California annual grassland soil (Haploxeralf) during a two-year greenhouse study. The fate of ¹³C-labeled belowground C (roots and organic matter) was followed under planted (Avena barbata) and unplanted conditions, and with and without supplemental N (20 kg N ha⁻¹ season⁻¹) over two periods of plant growth, each followed by a dry, fallow period of 120 d. Turnover of belowground ¹³C SOM was followed using ¹³C-phospholipid fatty acid (PLFA) biomarkers. Living roots increased the turnover and loss of belowground ¹³C compared with unplanted soils. Planted soils had 20% less belowground ¹³C present than in unplanted soils after 2 cycles of planting and fallow. After 2 treatment cycles, unlabeled soil C was 4.8% higher in planted soils than unplanted. The addition of N to soils decreased the turnover of enriched belowground ¹³C during the first treatment season in both planted and unplanted soils, however no effect of N was observed thereafter. Our findings suggest that A. barbata may increase soil C levels over time because root and exudate C inputs are significant, but that increase will be moderated by an overall faster C mineralization rate of belowground C. N addition may slow soil C losses; however, the effect was minor and transient in this system. The labeled root-derived ¹³C was initially recovered in gram negative (highest enrichment), gram positive, and fungal biomarkers. With successive growing seasons, the labeled C in the gram negative and fungal markers declined, while gram positive markers continued to accumulate labeled belowground C. The rhizosphere of A. barbata shifted the microbial community composition, resulting in greater abundances of gram negative markers and lower abundances of gram positive, actinobacteria and cyclopropyl PLFA markers compared to unplanted soil. However, the longer-term utilization of labeled belowground C by gram positive bacteria was enhanced in the rhizosphere microbial community compared with unplanted soils. We suggest that the activities of gram positive bacteria may be major controllers of multi-year rhizosphere-related priming of SOM decomposition.     

Is the soil microbial community related to the basal area of trees in a Scots pine stand?

The main energy sources of soil microorganisms are litter fall, root litter and exudation. The amount on these carbon inputs vary according to basal area of the forest stand. We hypothesized that soil microbes utilizing these soil carbon sources relate to the basal area of trees. We measured the amount of soil microbial biomass, soil respiration and microbial community structure as determined by phospholipid fatty acid (PLFA) profiles in the humus layer (FH) of an even-aged stand of Scots pine (Pinus sylvestris L.) with four different basal area levels ranging from 19.9 m² ha⁻¹ in the study plot Kasper 1 to 35.7 m² ha⁻¹ in Kasper 4. Increasing trend in basal respiration, total PLFAs and fungal-to-bacterial ratio was observed from Kasper 1 to Kasper 3 (basal area 29.2 m² ha⁻¹). The soil microbial community structure in Kasper 3 differed from that of the other study plots.     

Microbial community structure mediates response of soil C decomposition to litter addition and warming

Microbial activity has been highlighted as one of the main unknowns controlling the fate and turnover of soil organic matter (SOM) in response to climate change. How microbial community structure and function may (or may not) interact with increasing temperature to impact the fate and turnover of SOM, in particular when combined with changes in litter chemistry, is not well understood. The primary aim of this study was to determine if litter chemistry impacted the decomposition of soil and litter-derived carbon (C), and its interaction with temperature, and whether this response was controlled by microbial community structure and function. Fresh or pre-incubated eucalyptus leaf litter (¹³C enriched) was added to a woodland soil and incubated at 12, 22, or 32 °C. We tracked the movement of litter and soil-derived C into CO₂, water-extractable organic carbon (WEOC), and microbial phospholipids (PLFA). The litter additions produced significant changes in every parameter measured, while temperature, interacting with litter chemistry, predominately affected soil C respiration (priming and temperature sensitivity), microbial community structure, and the metabolic quotient (a proxy for microbial carbon use efficiency [CUE]). The direction of priming varied with the litter additions (negative with fresh litter, positive with pre-incubated litter) and was related to differences in the composition of microbial communities degrading soil-C, particularly gram-positive and gram-negative bacteria, resulting from litter addition. Soil-C decomposition in both litter treatments was more temperature sensitive (higher Q₁₀) than in the soil-only control, and soil-C priming became increasingly positive with temperature. However, microbes utilizing soil-C in the litter treatments had higher CUE, suggesting the longer-term stability of soil-C may be increased at higher temperature with litter addition. Our results show that in the same soil, the growth of distinct microbial communities can alter the turnover and fate of SOM and, in the context of global change, its response to temperature.     

Soil microbial dynamics in maize-growing soil under different tillage and residue management systems

The long-term impact of tillage and residue management on soil microorganisms was studied over the growing season in a sandy loam to loamy sand soil of southwestern Quebec, growing maize (Zea mays L.) monoculture. Tillage and residue treatments were first imposed on plots in fall 1991. Treatments consisted of no till, reduced tillage, and conventional tillage with crop residues either removed from (−R) or retained on (+R) experimental plots, laid out in a randomized complete block design. Soil microbial biomass carbon (SMB-C), soil microbial biomass nitrogen (SMB-N) and phospholipid fatty acid (PLFA) contents were measured four times, at two depths (0-10 and 10-20 cm), over the 2001 growing season. Sample times were: May 7 (preplanting), June 25, July 16, and September 29 (prior to corn harvest). The effect of time was of a greater magnitude than those attributed to tillage or residue treatments. While SMB-C showed little seasonal change (160 μg C g⁻¹ soil), SMB-N was responsive to post-emergence mineral nitrogen fertilization, and PLFA analysis showed an increase in fungi and total PLFA throughout the season. PLFA profiles showed better distinction between sampling time and depth, than between treatments. The effect of residue was more pronounced than that of tillage, with increased SMB-C and SMB-N (61 and 96%) in +R plots compared to −R plots. This study illustrated that measuring soil quality based on soil microbial components must take into account seasonal changes in soil physical and chemical conditions.     

Do same-aged but different height Norway spruce (Picea abies) clones affect soil microbial community?

The influence of individual trees in monocrop forests on soil microbial communities is poorly understood. We measured basal respiration, substrate-induced respiration and phospholipid fatty acids (PLFA), bacterial growth rate with the ³H-thymidine incorporation technique and fungal growth rate as ¹⁴C-acetate incorporation into ergosterol to investigate whether slow- and fast-growing 12-year-old Norway spruce (Picea abies) clones have affected differently on their associated soil microbial communities. Understorey vegetation, soil chemical properties and elemental concentrations of needles were also determined. The slow- and fast-growing spruce clones differed in PLFA profiles, understorey vegetation and elemental concentrations in needles suggesting that spruce clones have directly or indirectly affected soil microbes.     

Distribution and fate of 13C-labeled root and straw residues from ryegrass and crimson clover in soil under western Oregon field conditions

Annual ryegrass (Lolium multiflorum Lam.) and crimson clover (Trifolium incarnatum L.) were pulse-labeled with ¹³C-CO₂ in the field between the initiation of late winter growth (mid-February) and through flowering and seed formation (late May). Straw was harvested after seed maturation (July), and soil containing ¹³C-labeled roots and root-derived C was left in the field until September. ¹³C-enriched and ¹³C-unenriched straw residues of each species were mixed in factorial combinations with soil containing either ¹³C-enriched or ¹³C-unenriched root-derived C and incubated in the field for 10 months. The contributions of C derived from straw, roots, and soil were measured in soil microbial biomass C, respired C, and soil C on five occasions after residue incorporation (September, October, November, April, and June). At straw incorporation (September), 25–30% of soil microbial biomass C was derived from root C in both ryegrass and clover treatments, and this value was sustained in the ryegrass treatment from September to April but declined in the clover treatment. By October, between 20 and 30% of soil microbial biomass C was derived from straw, with the percentage contribution from clover straw generally exceeding that from ryegrass straw throughout the incubation. By June, ryegrass root-derived C contributed 5.5% of the soil C pool, which was significantly greater than the contributions from any of the three other residue types (about 1.5%). This work has provided a framework for more studies of finer scale that should focus on the interactions between residue quality, soil organic matter C, and specific members of the soil microbial community.