Drought alters respired δ13CO2 from autotrophic,but not heterotrophic soil respiration

Many researchers are interested in the variability of root-respired δ¹³CO₂ as an indication of linkages between belowground plant respiration and canopy processes. Most studies in this area have, however, relied upon the assumption that temporal variability of total soil respired δ¹³CO₂ reflects autotrophic soil processes, but in fact few supporting measurements of purely autotrophic soil respiration (partitioned from total soil respiration) are available. Here we use a combination of physical and isotopic partitioning methodologies to track the variability in autotrophic and heterotrophic soil δ¹³CO₂ at five sites in Eastern Canada during a very dry growing season. Three dimensional modeling of soil isotopic transport dynamics in the static sampling chambers allow us to constrain measurement bias and to eliminate non-steady-state effects as a potential driver of observed variability. We provide experimental results that support a pivotal assumption made in prior interpretations of soil δ¹³CO₂ dynamics: we observed minimal isotopic variability in soil microbial δ¹³CO₂ efflux, but appreciable temporal variability in root-respired δ¹³CO₂ at sites where near drought conditions were observed, suggesting that isotopic discrimination is likely linked to seasonal variations in transpirational demand.     

Quantifying the contribution of soil organic matter turnover to forest soil respiration, using natural abundance δC

Quantifying the loss of soil carbon through respiration has proved difficult, due to the challenge of measuring the losses associated with the turnover of soil organic matter (SOM) as distinct from autotrophic components. In forest ecosystems the δ¹³C value of respiration from turnover of SOM (δ¹³CR_SOM) is typically 2-4‰ enriched compared with that from roots and associated microbes (δ¹³CR_ROOTS), with that from the litter (δ¹³CR_LITTER) lying between the two. We measured soil respiration at 50 locations in a forest soil and then used differences in isotopic signatures to quantify the proportion of soil respiration arising from the turnover of SOM (fR_SOM) at a subset of 30 locations, chosen randomly. The soil surface CO₂ efflux was collected using an open chamber system supplied with CO₂-free air and the δ¹³C signature (δ¹³CR_S) measured, giving a mean (±SD) value across the site of −26.1 ± 0.58‰. The values of δ¹³CR_ROOTS, δ¹³CR_LITTER and δ¹³CR_SOM were measured at each location by incubation of roots, litter and root-free soil and collection of the CO₂ for isotopic analysis. δ¹³CR_SOM became progressively depleted with length of incubation (1.5‰ after 8 h), so CO₂ was collected after 20 min. The mean value of δ¹³CR_LITTER was −27.2 ± 0.68 ‰, which was indistinguishable from δ¹³CR_ROOTS of −27.6 ± 0.51‰, while δ¹³CR_SOM was −25.1 ± 0.88‰. δ¹³CR_ROOTS and δ¹³CR_SOM measured at each location were used as the end points of a two component mixing model to calculate fR_SOM, giving a mean value for fR_SOM of 0.61 ± 0.28. It was not possible to estimate fR_SOM using the total C contents of the roots and soil which were significantly depleted in ¹³C in comparison to their respired CO₂. However, at seven locations the δ¹³CR_S was slightly enriched compared with δ¹³CR_SOM (mean 0.3‰), which was not considered significantly different so fR_SOM was constrained to 1.0. If these seven rings were excluded mean fR_SOM was 0.49 ± 0.20. We have shown the possibility of using natural abundance ¹³C discrimination to quantify fR_SOM in a forest soil with an input of carbon only from C₃ photosynthesis.     

C signature of CO2 evolved from incubated maize residues and maize-derived sheep faeces

Analyses of the spatial and temporal variations in the natural abundance of ¹³C are frequently employed to study transformations of plant residues and soil organic matter turnover on sites where long continued vegetation with the C₃-type photosynthesis pathway has been replaced with a C₄-type vegetation (or vice versa). One controversial issue associated with such analyses is the significance of isotopic fractionation during the microbial turnovers of C in complex substrates. To evaluate this issue, C₃-soil and quartz sand were amended with maize residues and with faeces from sheep feed exclusively on maize silage. The samples were incubated at 15 °C for 117 days (maize residues) or 224 days (sheep faeces). CO₂ evolved during incubation was trapped in NaOH and analysed for C isotopic contents. At the end of incubation, 63 and 50% of the maize C was evolved as CO₂ in the soil and sand, respectively, while 32% of the faeces C incubated with soil and with sand was recovered as CO₂. Maize and faeces showed a similar decomposition pattern but maize decomposed twice as fast as faeces. The δ¹³C of faeces was 0.3‰ lower than that of the maize residue (δ¹³C −13.4‰), while the δ¹³C of the C₃-soil used for incubation was −31.6‰. The δ¹³C value of the CO₂ recovered from unamended C₃-soil was similar or slightly lower (up to −1.5‰) than that of the C₃-soil itself except for an initial flush of ¹³C enriched CO₂. The δ¹³C values of the CO₂ from sand-based incubations typically ranged −15‰ to −17‰, i.e. around −3‰ lower than the δ¹³C measured for maize and faeces. Our study clearly demonstrates that the decomposition of complex substrates is associated with isotopic fractionation, causing evolved CO₂ to be depleted in ¹³C relative to substrates. Consequently the microbial products retained in the soil must be enriched in ¹³C.     

Greater humification of belowground than aboveground biomass carbon into particulate soil organic matter in no-till corn and soybean crops

Quantifying the amount of carbon (C) incorporated from decomposing residues into soil organic carbon (C_S) requires knowing the rate of C stabilization (humification rate) into different soil organic matter pools. However, the differential humification rates of C derived from belowground and aboveground biomass into C_S pools has been poorly quantified. We estimated the contribution of aboveground and belowground biomass to the formation of C_S in four agricultural treatments by measuring changes in δ¹³C natural abundance in particulate organic matter (C_POM) associated with manipulations of C₃ and C₄ biomass. The treatments were (1) continuous corn cropping (C₄ plant), (2) continuous soybean cropping (C₃), and two stubble exchange treatments (3 and 4) where the aboveground biomass left after the grain harvest was exchanged between corn and soybean plots, allowing the separation of aboveground and belowground C inputs to C_S based on the different δ¹³C signatures. After two growing seasons, C_POM was primarily derived from belowground C inputs, even though they represented only ∼10% of the total plant C inputs as residues. Belowground biomass contributed from 60% to almost 80% of the total new C present in the C_POM in the top 10 cm of soil. The humification rate of belowground C inputs into C_POM was 24% and 10%, while that of aboveground C inputs was only 0.5% and 1.0% for soybean and corn, respectively. Our results indicate that roots can play a disproportionately important role in the C_POM budget in soils. Keywords Particulate organic matter; root carbon inputs; carbon isotopes; humification rate; corn; soybean.     

Three-source partitioning of CO2 efflux from soil planted with maize by C natural abundance fails due to inactive microbial biomass

A theoretical approach to the partitioning of carbon dioxide (CO₂) efflux from soil with a C₃ vegetation history planted with maize (Zea mays), a C₄ plant, into three sources, root respiration (RR), rhizomicrobial respiration (RMR), and microbial soil organic matter (SOM) decomposition (SOMD), was examined. The δ¹³C values of SOM, roots, microbial biomass, and total CO₂ efflux were measured during a 40-day growing period. A three-source isotopic mass balance based on the measured δ¹³C values and on assumptions made in other studies showed that RR, RMR, and SOMD amounted to 91%, 4%, and 5%, respectively. Two assumptions were thoroughly examined in a sensitivity analysis: the absence of ¹³C fractionation and the conformity of δ¹³C of microbial CO₂ and that of microbial biomass. This approach strongly overestimated RR and underestimated RMR and microbial SOMD. CO₂ efflux from unplanted soil was enriched in ¹³C by 2.0‰ compared to microbial biomass. The consideration of this ¹³C fractionation in the mass balance equation changed the proportions of RR and RMR by only 4% and did not affect SOMD. A calculated δ¹³C value of microbial CO₂ by a mass balance equation including active and inactive parts of microbial biomass was used to adjust a hypothetical below-ground CO₂ partitioning to the measured and literature data. The active microbial biomass in the rhizosphere amounted to 37% to achieve an appropriate ratio between RR and RMR compared to measured data. Therefore, the three-source partitioning approach failed due to a low active portion of microbial biomass, which is the main microbial CO₂ source controlling the δ¹³C value of total microbial biomass. Since fumigation-extraction reflects total microbial biomass, its δ¹³C value was unsuitable to predict δ¹³C of released microbial CO₂ after a C₃-C₄ vegetation change. The second adjustment to the CO₂ partitioning results in the literature showed that at least 71% of the active microbial biomass utilizing maize rhizodeposits would be necessary to achieve that proportion between RR and RMR observed by other approaches based on ¹⁴C labelling. The method for partitioning total below-ground CO₂ efflux into three sources using a natural ¹³C labelling technique failed due to the small proportion of active microbial biomass in the rhizosphere. This small active fraction led to a discrepancy between δ¹³C values of microbial biomass and of microbially respired CO₂.     

The stable carbon isotope composition of soil organic carbon and pedogenic carbonates along a bioclimatic gradient in the Palouse region,Washington State,USA

Isotopic signatures of soil components are commonly used to infer past ecologic and climatic shifts in the soil record. The theory behind the fractionation of isotopes that occurs during ecosystem processes is well understood; however, few isotopic studies have explored ecosystem relationships in modern soils. We discuss relationships of stable carbon isotopic signatures in plant tissue, soil organic carbon (SOC), laboratory-respired CO₂, and modern carbonates at 10 sites (seven containing pedogenic carbonates) along a C₃-dominated climatic gradient (mean annual precipitation (MAP) ranging from 200 to 1000 mm) in the Palouse region of eastern Washington state.A horizon soil organic carbon (SOC) δ¹³C values varied from −24.3‰ to −25.9‰ PDB. Values in the arid portion of the gradient (200 to approximately 500 mm MAP) generally decreased and linear regression of SOM ¹³C vs. MAP was significant (r²=0.71, p=0.02). Trends in plant-¹³C of two grass species (Agropyron spicatum and Festuca idahoensis) found throughout this portion of the gradient were similar to that of SOC. Mean pedogenic carbonate δ¹³C values varied from −4.1‰ to −10.8‰ PDB. Linear regression was significant for carbonate ¹³C vs. MAP (r²=0.79, p=0.007), estimated above-ground productivity (r²=0.88, p=0.002) and soil carbon content (r²=0.83, p=0.004). Carbonate δ¹³C values at the most arid site exhibited higher variability than other sites (presumably due to greater spatial variation in plant respiration vs. atmospheric diffusion). Our data suggest that carbon isotopic relationships among ecosystem components may prove useful in determining ecosystem level properties in modern systems, and potentially in ancient systems as well.     

Sensitivity analysis and quantification of uncertainty for isotopic mixing relationships in carbon cycle research

Quantifying and understanding the uncertainty in isotopic mixing relationships is critical to isotopic applications in carbon cycle studies at all spatial and temporal scales. Studies that depend on stable isotope approaches must also address quantification of uncertainty for parameters derived from isotopic studies. An important application of isotopic mixing relationships is determination of the isotopic content of ecosystem respiration (δ¹³C_S) via an inverse relationship (a Keeling plot) between atmospheric CO₂ concentrations ([CO₂]) and carbon isotope ratios of CO₂ (δ¹³C). Alternatively, a linear relationship between [CO₂] and the product of [CO₂] and δ¹³C (a Miller/Tans plot) can also be applied.We used three datasets of [CO₂] and δ¹³C in air to examine contrasting approaches to determine δ¹³C_S and its uncertainty. These datasets were from the Niwot Ridge, Colorado, AmeriFlux site, the Biosphere-Atmosphere Stable Isotope Network (BASIN), and from the Grünschwaige Grassland Research Station in Germany. The analysis of this data included Keeling plots and Miller/Tans plots fit with both Model I (ordinary least squares) and Model II regressions (geometric mean regression and orthogonal distance regression).Our analysis confirms previous observations that increasing the range of the measurements ([CO₂] range) used for a mixing line reduces the uncertainty associated with δ¹³C_S. Using a Model II regression technique to determine δ¹³C_S introduces a negatively skewed bias in δ¹³C_S which is especially significant for small [CO₂] ranges. This bias arises from comparatively greater variability in the dependent variable than the independent variable for a linear regression. For carbon isotope studies, uncertainty in the isotopic measurements has a greater effect on the uncertainty of δ¹³C_S than the uncertainty in [CO₂]. As a result, studies that estimate parameters via a Model II regression technique maybe biased in their conclusions. In contrast to earlier studies, we advocate Model I (ordinary least squares) regression to calculate δ¹³C_S and its uncertainty. Reducing the uncertainty of isotopic measurements reduces the uncertainty of δ¹³C_S, even when the [CO₂] range of samples is small (<20 ppm). As a result, improvement in isotope (rather than [CO₂]) measuring capability is presently needed to substantially reduce uncertainty in δ¹³C_S. We find for carbon isotope studies no inherent advantage or disadvantage to using either a Keeling or Miller/Tans approach to determine δ¹³C_S. We anticipate that the mathematical methods developed in this paper can be applied to other applications where linear regression is utilized.     

Spatial heterogeneity in the relocation of added C within the structure of an upland grassland soil

A pulse of ¹³CO₂ was added to the above ground vegetation in an upland grassland to determine the effects of faunal diversity on the flux of carbon to the surface horizons of the soil. Faunal diversity was manipulated by liming and biocide treatments for three years prior to the pulse addition. The relocation of ¹³C within roots and rhizosphere soil was determined by analysis of samples of bulk soil and of specific features identified on soil thin sections on four dates after the addition of the ¹³CO₂ pulse. Analysis of bulk soils showed only a small enrichment in ¹³C and no significant effects of the treatments. Analysis by isotope ratio mass spectrometry of the products of in situ laser combustion of root material and aggregates formed from faunal excrement showed that the distribution of the newly photosynthesised ¹³C is very localised, with large spatial variability in soil and root δ¹³C at scales of less than 1 mm. δ¹³C values ranged from the natural abundance level of around −28‰ to −4.9‰ in roots and to −8.4‰ in aggregates. The small pulse and large spatial variability masked any effects of the liming and biocide treatments in these soils. However, the variability in the relocation of newly photosynthesised carbon may help to explain the large spatial variability found in bacterial numbers at the sub-mm scale within soils and emphasises the importance of the accessibility of substrates to decomposers in undisturbed structured soils.     

Translocation of surface litter carbon into soil by Collembola

Soil invertebrates are important in nutrient cycling in soils, but the degree to which mesofauna such as Collembola are responsible for the direct movement of carbon (C) from the litter layer into soil has not yet been ascertained. We used naturally occurring stable C isotopic differences between a C₄ soil and alder leaves (C₃) to examine the effect of the collembolan Folsomia candida on C translocation into soil in laboratory microcosms. Collembolan numbers greatly increased in the presence of alder, but despite large collembolan populations there were no changes in decomposition rate (measured as litter mass loss, cumulative respired CO₂ and alder C:N ratios). Small changes in the δ¹³C values of bulk soil organic matter were detected, but could not be assigned to collembolan activity. However, mean δ¹³C values of soil microbial phospholipid fatty acids (PLFAs) were significantly lower in the presence of alder and Collembola together, demonstrating that collembolan activities resulted in greater availability of litter-derived C to the soil microbial community. Additionally, the presence of Collembola resulted in the translocation of alder-derived compounds (chlorophyll and its breakdown product pheophytin) into soil, demonstrating that Collembola modify soil organic matter at the molecular level. These results are consistent with deposition of collembolan faeces in underlying soil and demonstrate that despite their small size, Collembola contribute directly to C transport in the litter-soil environment.     

Tracking Collembola feeding strategies by the natural C signal of fatty acids in an arable soil with different fertilizer regimes

Compound specific stable isotope analysis (¹³C/¹²C ratio of fatty acids) was used to assess the allocation of plant carbon in soil microbiota, and to identify the trophic links to microbial grazers in an arable field with long-term mineral and organic fertilizer amendments. The feeding strategy of two dominant Collembola species, epedaphic Isotoma viridis and euedaphic Willemia anophthalma was determined. The investigation was conducted following a shift to amaranth, a C₄ plant, after 27 years of continuous C₃ crop rotation. The influence of new C₄ plant carbon was observed in microbial phospholipids (PLFAs) with higher δ¹³C recorded in C₄ amaranth than in C₃ clover soils. The strongest enrichment occurred in the fungal PLFA 18:2ω6,9c and bacterial PLFA 18:1ω9t with 11.2‰ and 6.6‰, respectively. However, other bacterial PLFAs showed no isotopic change, suggesting that the microbial community simultaneously utilized “new” and “old” plant carbon. The δ¹³C of Collembola fatty acids displayed species specific lipid pattern, which was affected by crop type, but not fertilizer amendments. Isotopic separation of Collembola lipids from amaranth and clover plots was more distinct in I. viridis than W. anophthalma. With up to 18‰, the enrichment in Collembola lipids was stronger than in microbial PLFAs, pointing to a distinct incorporation of carbon resources originating from the actual plant residues. The δ¹³C pattern in I. viridis indicated trophic links with bacteria, saprotrophic fungi and plant tissues, while saprotrophic fungi and plant tissues were accountable for the patterns observed in W. anophthalma.     

Natural abundance δN and δC of DNA extracted from soil

We report the first simultaneous measurements of δ¹⁵N and δ¹³C of DNA extracted from surface soils. The isotopic composition of DNA differed significantly among nine different soils. The δ¹³C and δ¹⁵N of DNA was correlated with δ¹³C and δ¹⁵N of soil, respectively, suggesting that the isotopic composition of DNA is strongly influenced by the isotopic composition of soil organic matter. However, in all samples DNA was enriched in ¹³C relative to soil, indicating microorganisms fractionated C during assimilation or preferentially used ¹³C enriched substrates. Enrichment of DNA in ¹⁵N relative to soil was not consistently observed, but there were significant differences between δ¹⁵N of DNA and δ¹⁵N of soil for three different sites, suggesting microorganisms are fractionating N or preferentially using N substrates at different rates across these contrasting ecosystems. There was a strong linear correlation between δ¹⁵N of DNA and δ¹⁵N of the microbial biomass, which indicated DNA was depleted in ¹⁵N relative to the microbial biomass by approximately 3.4‰. Our results show that accurate and precise isotopic measurements of C and N in DNA extracted from the soil are feasible, and that these analyses may provide powerful tools for elucidating C and N cycling processes through soil microorganisms.     

Organic matter turnover in soil physical fractions following woody plant invasion of grassland: Evidence from natural C and N

Soil physical structure causes differential accessibility of soil organic carbon (SOC) to decomposer organisms and is an important determinant of SOC storage and turnover. Techniques for physical fractionation of soil organic matter in conjunction with isotopic analyses (δ¹³C, δ¹⁵N) of those soil fractions have been used previously to (a) determine where organic C is stored relative to aggregate structure, (b) identify sources of SOC, (c) quantify turnover rates of SOC in specific soil fractions, and (d) evaluate organic matter quality. We used these two complementary approaches to characterize soil C storage and dynamics in the Rio Grande Plains of southern Texas where C₃ trees/shrubs (δ¹³C=−27‰) have largely replaced C₄ grasslands (δ¹³C=−14‰) over the past 100-200 years. Using a chronosequence approach, soils were collected from remnant grasslands (Time 0) and from woody plant stands ranging in age from 10 to 130 years. We separated soil organic matter into specific size/density fractions and determined their C and N concentrations and natural δ¹³C and δ¹⁵N values. Mean residence times (MRTs) of soil fractions were calculated based on changes in their δ¹³C with time after woody encroachment. The shortest MRTs (average=30 years) were associated with all particulate organic matter (POM) fractions not protected within aggregates. Fine POM (53-250 μm) within macro- and microaggregates was relatively more protected from decay, with an average MRT of 60 years. All silt+clay fractions had the longest MRTs (average=360 years) regardless of whether they were found inside or outside of aggregate structure. δ¹⁵N values of soil physical fractions were positively correlated with MRTs of the same fractions, suggesting that higher δ¹⁵N values reflect an increased degree of humification. Increased soil C and N pools in wooded areas were due to both the retention of older C₄-derived organic matter by protection within microaggregates and association with silt+clay, and the accumulation of new C₃-derived organic matter in macroaggregates and POM fractions.     

Contribution of crop residue carbon to soil respiration at a northern Prairie site using stable isotope flux measurements

Heterotrophic respiration from agricultural soils can be differentiated as originating from microbial decomposition of recent litter inputs or crop residue carbon (CRC) and resident soil organic carbon (SOC) pools of varying age and stages of decomposition. Our objective was to determine the relative contributions of these pools to respiration in a northern agroecosystem where the non-growing season is long. A tunable diode laser trace gas analyzer was used to determine atmospheric stable C isotope ratio (δ¹³C) values and ¹²CO₂ and ¹³CO₂ fluxes over an agricultural field in the Red River Valley of southern Manitoba, Canada. Measurement campaigns were conducted in the fall of 2006 and spring of 2007 following harvest of a maize (C₄) crop from soil having SOC derived from previous C₃ crops. Stable CO₂ isotopologue gradients were measured from the center of four 200 × 200 m experimental plots, and fluxes were calculated using the aerodynamic flux gradient method. The soil in two of the experimental plots underwent intensive tillage, while the other two plots were managed using a form of reduced tillage. Approximately 70% and 20-30% of the total respiration flux originated from the maize C₄-CRC during the fall of 2006 and spring of 2007, respectively. At least 25% of the maize residue was lost to respiration during this non-growing period. No difference in the partitioning of heterotrophic respiration into that derived from CRC and SOC was detected between the intensive tillage and recently established reduced tillage treatments at the site.     

Short-term C4 plant Spartina alterniflora invasions change the soil carbon in C3 plant-dominated tidal wetlands on a growing estuarine Island

Spartina alterniflora is an invasive C₄ perennial grass, native to North America, and has spread rapidly along the east coast of China since its introduction in 1979. Since its intentional introduction to the Jiuduansha Island in the Yangtze River estuary, Spartina alterniflora community has become one of the dominant vegetation types. We investigated the soil carbon in the Spartina alterniflora community and compared it with that of the native C₃Scirpus mariqueter community by measuring total soil carbon (TC), soil organic carbon (SOC), total soil nitrogen (TN), and the stable carbon isotope composition (δ¹³C) of various fractions. TC and SOC were significantly higher in Spartina alterniflora in the top 60 cm of soil. However, there was no significant difference in soil inorganic carbon (IC) between the two communities. Stable carbon isotopic analysis suggests that the fraction of SOC pool contributed by Spartina alterniflora varied from 0.90% to 10.64% at a soil depth of 0-100 cm with a greater percentage between 20 and 40 cm deep soils. The δ¹³C decreased with increasing soil depth in both communities, but the difference in δ¹³C among layers of the top 60 cm soil was significant (p<0.05), while that for the deeper soil layers (>60 cm) was not detected statistically. The changes in δ¹³C with depth appeared to be associated with the small contribution of residues from Spartina alterniflora at greater soil depth that was directly related to the vertical root distribution of the species.     

Effluxed CO2-C from sterilized and unsterilized treatments of a calcareous soil

Soil inorganic carbon (C) represents a substantial C pool in arid ecosystems, yet little data exist on the contribution of this pool to ecosystem C fluxes. A closed jar incubation study was carried out to test the hypothesis that CO₂-¹³C production and response to sterilization would differ in a calcareous (Mojave Desert) soil and a non-calcareous (Oklahoma Prairie) soil due to contributions of carbonate-derived CO₂. In addition to non-sterilized controls, soils were subjected to sterilization treatments (unbuffered HgCl₂ addition for Oklahoma soil and unbuffered HgCl₂ addition, buffered HgCl₂ addition, and autoclaving for Mojave Desert soil) to decrease biotic respiration and more readily measure abiotic CO₂ flux. Temperature and moisture treatments were also included with sterilization treatments in a factorial design.The rate of CO₂ production in both soils was significantly decreased (36-87%) by sterilization, but sterilization treatments differed in effectiveness. Sterilization had no significant effect on effluxed CO₂-¹³C values in the non-calcareous Oklahoma Prairie soil and autoclaved Mojave Desert soil as compared to their respective non-sterilized controls. However, sterilization significantly altered CO₂-¹³C values in Mojave Desert soil HgCl₂ sterilization treatments (both buffered and non-buffered). Plots of 1/CO₂ versus CO₂-δ¹³C (similar to Keeling plots) indicated that the source CO₂-δ¹³C value of the Oklahoma Prairie soil treatments was similar to the δ¹³C value of soil organic matter [(SOM); −17.76‰ VPDB] whereas the source for the (acidic) unbuffered-HgCl₂ sterilized Mojave Desert soil was similar to the δ¹³C value of carbonates (−0.93‰ VPDB). The source CO₂-δ¹³C value of non-sterilized and autoclaved (−18.4‰ VPDB) Mojave Desert soil treatments was intermediate between SOM (−21.43‰ VPDB) and carbonates and indicates up to 13% of total C efflux may be from abiotic sources in calcareous soils.     

Plant-biochar interactions drive the negative priming of soil organic carbon in an annual ryegrass field system

There is a knowledge gap on biochar carbon (C) longevity and its priming effects on soil organic carbon (SOC) and recent root-derived C under field conditions. This knowledge would allow the potential of biochar in long-term soil C sequestration to be established. However, most studies on biochar C longevity and its priming effect have been undertaken in plant-free laboratory incubations.A 388 d field study was carried out in the presence of an annual ryegrass (C₃) growing on a rhodic ferralsol with established C₃/C₄ plant-derived SOC (δ¹³C: −20.2‰) in a subtropical climate. A ¹³C-depleted hardwood biochar (δ¹³C: −35.7‰, produced at 450 °C) was applied at 0 and 30 dry t ha⁻¹ and mixed into the top 100-mm soil profile (equivalent to 3% w/w). We report on the differentiation and quantification of root respiration and mineralisation of soil-C and biochar-C in the field. Periodic ¹³CO₂ pulse labelling was applied to enrich δ¹³C of root respiration during two separate winter campaigns (δ¹³C: 151.5–184.6‰) and one summer campaign (δ¹³C: 19.8–31.5‰). Combined soil plus root respiration was separated from leaf respiration using a novel in-field respiration collar. A two-pool isotope mixing model was applied to partition three C sources (i.e. root, biochar and soil). Three scenarios were used to assess the sensitivity associated with the C source partitioning in the planted systems: 1) extreme positive priming of recent SOC derived from the current ryegrass (C₃) pasture; 2) equivalent magnitude of priming of SOC and labile root C; and 3) extreme positive priming of the native C4-dominant SOC.We showed that biochar induced a significant negative priming of SOC in the presence of growing plants but no net priming was observed in the unplanted soil. We also demonstrated the importance of experimental timeframe in capturing the transient nature of biochar-induced priming, from positive (day 0–62) to negative (day 62–388). The presence/absence of plants had no impact on biochar-C mineralisation in this ferralsol during the measurement period. Based on a two-pool exponential model, the mean residence time (MRT) of biochar varied from 351 to 449 years in the intensive pasture system to 415–484 years in the unplanted soils.     

Short-term effects of dairy slurry amendment on carbon sequestration and enzyme activities in a temperate grassland

Land application of animal wastes from intensive grassland farming has resulted in growing environmental problems relating to greenhouse gas emissions, ammonia volatilisation, and nitrate and phosphorus leaching into surface and groundwater. We examined the short-term effects of dairy slurry amendment on carbon sequestration and enzyme activities in a temperate grassland (Southwest England). Slurry was collected from cows fed either on perennial ryegrass (C₃) or maize (C₄) silages. Fifty m³ ha⁻¹ of each of the obtained C₃ or C₄ slurries (δ¹³C=−30.7 and −21.3‰, respectively) were applied to a C₃ pasture soil with δ¹³C of −30.0±0.2‰. We found that water soluble organic carbon (WSOC) content was two to three times higher in the slurry amended plots compared with the unamended control. No significant change in the soil microbial biomass (SMB) carbon content was observed in the four weeks (772 h) following slurry application. Natural abundance ¹³C isotope analysis suggested a rapid initial incorporation (>25% within 2 h of application) of slurry-derived C in the SMB-C and WSOC pools of the 0-2 cm layer. Linear relationships were found between slurry-derived C in the whole soil, SMB, and WSOC for the 0-2 cm depth in the soil. Applied slurry-derived C was sequestered in the SMB pool in two phases. The first phase (0-48 h) was dominated by the incorporation of labile slurry C from the liquid phase, whereas beyond 48 h slurry-derived C was mainly from less mobile particulate C. No significant differences between treatments were found for invertase and xylanase. Urease activity was always higher in slurry treatments. Cellobiohydrolase, β-N-acetyl-glucosamidase, β-glucosidase and acid phosphatase activities became significantly higher in slurry treatments after 336 h. However, the observed temporal changes in enzyme activities were not correlated with the amounts of slurry-C incorporated in the SMB and WSOC pool.     

Activity and composition of the methanogenic archaeal community in soil vegetated with wild versus cultivated rice

Wild rice (Oryza rufipogon) is a problematic weed in fields of cultivated rice (Oryza sativa). We hypothesized that the composition and/or the activity of the methanogenic microbial communities might be different in soil grown with cultivated versus wild rice. We used samples from Hainan, China, where wild rice grew on a field adjacent to cultivated rice. The composition of the methanogenic archaeal community was analyzed in samples of rice soil by targeting the 16S rRNA gene. Analysis of the terminal restriction fragment length polymorphism (T-RFLP) showed similar patterns in soil from wild versus cultivated rice. Sequences of archaeal 16S rRNA genes also showed similar composition in soil from wild versus cultivated rice, revealing the presence of Methanosarcinaceae, Methanosaetaceae, Methanobacteriales, Methanocellales (Rice Cluster I), Rice Cluster II, Crenarchaeota Group I.3 and Crenarchaeota Group I.1b. Incubation of soil samples under anoxic conditions generally resulted in vigorous CH₄ production after a lag phase of 7-8 days. Production of CH₄ was partially inhibited by methyl fluoride, a specific inhibitor of acetoclastic methanogenesis, resulting in nearly stoichiometric accumulation of acetate. CO₂ was produced without lag phase. The δ¹³C of the produced CO₂ was slightly lower in soil grown with cultivated rice versus wild rice, reflecting the δ¹³C of organic matter, which was about −29‰ for cultivated rice soil and about −24‰ for wild rice soil. The δ¹³C of the produced CH₄ and the acetate that accumulated in the presence of CH₃F was much more negative in cultivated versus wild rice soil, mainly since the isotopic fractionation factors for hydrogenotrophic methanogenesis were higher for soil from cultivated rice (α = 1.054) versus wild rice (α = 1.039). However, the percentage contribution of hydrogenotrophic methanogenesis to total CH₄ production was similar in both soils (27-35%). In conclusion, although the two soils exhibited different δ¹³C values of soil organic matter and derived products, they were similar with respect to rates and composition of the methanogenic communities.     

Turnover of soil organic matter and of microbial biomass under C3-C4 vegetation change: Consideration of C fractionation and preferential substrate utilization

Two processes contribute to changes of the δ¹³C signature in soil pools: ¹³C fractionation per se and preferential microbial utilization of various substrates with different δ¹³C signature. These two processes were disentangled by simultaneously tracking δ¹³C in three pools - soil organic matter (SOM), microbial biomass, dissolved organic carbon (DOC) - and in CO₂ efflux during incubation of 1) soil after C₃-C₄ vegetation change, and 2) the reference C₃ soil.The study was done on the Ap horizon of a loamy Gleyic Cambisol developed under C₃ vegetation. Miscanthus giganteus - a perennial C₄ plant - was grown for 12 years, and the δ¹³C signature was used to distinguish between ‘old’ SOM (>12 years) and ‘recent’ Miscanthus-derived C (<12 years). The differences in δ¹³C signature of the three C pools and of CO₂ in the reference C₃ soil were less than 1‰, and only δ¹³C of microbial biomass was significantly different compared to other pools. Nontheless, the neglecting of isotopic fractionation can cause up to 10% of errors in calculations. In contrast to the reference soil, the δ¹³C of all pools in the soil after C₃-C₄ vegetation change was significantly different. Old C contributed only 20% to the microbial biomass but 60% to CO₂. This indicates that most of the old C was decomposed by microorganisms catabolically, without being utilized for growth. Based on δ¹³C changes in DOC, CO₂ and microbial biomass during 54 days of incubation in Miscanthus and reference soils, we concluded that the main process contributing to changes of the δ¹³C signature in soil pools was preferential utilization of recent versus old C (causing an up to 9.1‰ shift in δ¹³C values) and not ¹³C fractionation per se.Based on the δ¹³C changes in SOM, we showed that the estimated turnover time of old SOM increased by two years per year in 9 years after the vegetation change. The relative increase in the turnover rate of recent microbial C was 3 times faster than that of old C indicating preferential utilization of available recent C versus the old C.Combining long-term field observations with soil incubation reveals that the turnover time of C in microbial biomass was 200 times faster than in total SOM. Our study clearly showed that estimating the residence time of easily degradable microbial compounds and biomarkers should be done at time scales reflecting microbial turnover times (days) and not those of bulk SOM turnover (years and decades). This is necessary because the absence of C reutilization is a prerequisite for correct estimation of SOM turnover. We conclude that comparing the δ¹³C signature of linked pools helps calculate the relative turnover of old and recent pools.     

Carbon stable isotope fractionation and trophic transfer of fatty acids in fungal based soil food chains

Stable isotope analysis has been used as a powerful tool in food web studies in terrestrial ecosystems. In addition the occurrence and abundance of fatty acids may serve as indicator for feeding strategies of soil animals. Here we combine both approaches and investigate the fatty acid composition, δ¹³C values of bulk tissues and individual fatty acids in soil organisms. The fungi Chaetomium globosum and Cladosporium cladosporioides were isotopically labelled by fructose derived from either C₃ or C₄ plants, and the fungal-feeding nematode Aphelenchoides sp. was reared on C. globosum. Fungi and nematodes were used as diet for the Collembolan Protaphorura fimata. The sugar source was fractionated differently by fungal lipid metabolism in a species-specific manner that points to a sensitivity of physiological processing to the non-random distribution of ¹³C/¹²C isotopes in the molecule. As a general trend stearic acid (18:0) was depleted in ¹³C compared to the precursor palmitic acid (16:0), whereas its desaturation to oleic acid (18:1 ω9) favoured the ¹³C-rich substrate.Fatty acid profiles of P. fimata varied due to food source, indicating incorporation of dietary fatty acids into Collembolan tissue. Individuals feeding on fungi had lower amounts in C20 fatty acids, with monoenoic C20 forms not present. This pattern likely separates primary consumers (fungivores) from predators (nematode feeders). The isotopic discrimination in ¹³C for bulk Collembola ranged between −2.6 and 1.4‰ and was dependent on fungal species and C₃/C₄ system, suggesting differences at metabolic branch points and/or isotope discrimination of enzymes. Comparison of δ¹³C values in individual fatty acids between consumer and diet generally showed depletion (i.e. de novo synthesis) or no changes (i.e. dietary routing), but the fractionation was not uniform and affected by the type of ingested food. Fatty acid carbon isotopes were more variable than those of bulk tissues, likely due to both the distrimination by enzymes and the different lipid origin (i.e. neutral or polar fraction).