Microbial Decomposition Of Soil Organic Matter Research Articles

Wildfire effects on BVOC emissions from boreal forest floor on permafrost soil in Siberia

One of the effects of climate change on boreal forest will be more frequent forest wildfires and permafrost thawing. These will increase the availability of soil organic matter (SOM) for microorganisms, change the ground vegetation composition and ultimately affect the emissions of biogenic volatile organic compounds (BVOCs), which impact atmospheric chemistry and climate. BVOC emissions from boreal forest floor have been little characterized in southern boreal region, and even less so in permafrost soil, which underlies most of the northern boreal region. Here, we report the long-term effects of wildfire on forest floor BVOC emission rates along a wildfire chronosequence in a Larix gmelinii forest in central Siberia. We determined forest floor BVOC emissions from forests exposed to wildfire 1, 23 and > 100 years ago. We studied how forest wildfires and the subsequent succession of ground vegetation, as well as changes in the availability of SOM along with the deepened and recovered active layer, influence BVOC emission rates. The forest floor acted as source of a large number of BVOCs in all forest age classes. Monoterpenes were the most abundant BVOC group in all age classes. The total BVOC emission rates measured from the 23- and >100-year-old areas were ca. 2.6 times higher than the emissions from the 1-year-old area. Lower emissions were related to a decrease in plant coverage and microbial decomposition of SOM after wildfire. Our results showed that forest wildfires play an important indirect role in regulating the amount and composition of BVOC emissions from post-fire originated boreal forest floor. This could have a substantial effect on BVOC emissions if the frequency of forest wildfires increases in the future as a result of climate warming.

Experimental test of temperature and moisture controls on the rate of microbial decomposition of soil organic matter: preliminary results

Soil organic matter (SOM) is a major reservoir of carbon derived from the biosphere that is returned to the atmosphere largely via microbial decomposition. The potential for feedbacks between climate change and SOM decomposition makes a full understanding of the controls on SOM decomposition rates essential to modeling future climate changes. We measured soil CO2 flux in a laboratory setting using pots containing a uniform mix of soil in which we varied both temperature and moisture. Following initial desiccation, a strong CO2 pulse was measured within two hours of rewetting and a return to equilibrium conditions obtained within 168 hours, with the magnitude of the initial pulse varying by soil temperature and moisture addition. At equilibrium conditions, no correlation was found between CO2 flux and temperature across all moisture levels, although a weak positive correlation (r2=0.1 to 0.2) was seen at moderate to high moisture levels. A much stronger correlation (r2 > 0.4) was found between CO2 flux and soil moisture across the full range of temperatures and at both low and high temperatures. Thus, we conclude that when all other variables were constrained, soil moisture fluctuations appeared to have greater impact than temperature variations on the rate of microbial decomposition of SOM. These preliminary results suggest directions for future research examining the relationships between soil moisture, temperature and CO2 flux for soils in which clay mineral and/or SOM composition are varied.

Short and Long‐Term Controls on Active Layer and Permafrost Carbon Turnover Across the Arctic

AbstractDecomposition of soil organic matter (SOM) in permafrost terrain and the production of greenhouse gases is a key factor for understanding climate change‐carbon feedbacks. Previous studies have shown that SOM decomposition is mostly controlled by soil temperature, soil moisture, and carbon‐nitrogen ratio (C:N). However, focus has generally been on site‐specific processes and little is known about variations in the controls on SOM decomposition across Arctic sites. For assessing SOM decomposition, we retrieved 241 samples from 101 soil profiles across three contrasting Arctic regions and incubated them in the laboratory under aerobic conditions. We assessed soil carbon losses (Closs) five times during a 1 year incubation. The incubated material consisted of near‐surface active layer (ALNS), subsurface active layer (ALSS), peat, and permafrost samples. Samples were analyzed for carbon, nitrogen, water content, δ13C, δ15N, and dry bulk density (DBD). While no significant differences were observed between total ALSS and permafrost Closs over 1 year incubation (2.3 ± 2.4% and 2.5 ± 1.5% Closs, respectively), ALNS samples showed higher Closs (7.9 ± 4.2%). DBD was the best explanatory parameter for active layer Closs across sites. Additionally, results of permafrost samples show that C:N ratio can be used to characterize initial Closs between sites. This data set on the influence of abiotic parameter on microbial SOM decomposition can improve model simulations of Arctic soil CO2 production by providing representative mean values of CO2 production rates and identifying standard parameters or proxies for upscaling potential CO2 production from site to regional scales.

Soil C and N availability determine the priming effect: microbial N mining and stoichiometric decomposition theories

The increasing input of anthropogenically derived nitrogen (N) to ecosystems raises a crucial question: how does available N modify the decomposer community and thus affects the mineralization of soil organic matter (SOM). Moreover, N input modifies the priming effect (PE), that is, the effect of fresh organics on the microbial decomposition of SOM. We studied the interactive effects of C and N on SOM mineralization (by natural (13) C labelling adding C4 -sucrose or C4 -maize straw to C3 -soil) in relation to microbial growth kinetics and to the activities of five hydrolytic enzymes. This encompasses the groups of parameters governing two mechanisms of priming effects - microbial N mining and stoichiometric decomposition theories. In sole C treatments, positive PE was accompanied by a decrease in specific microbial growth rates, confirming a greater contribution of K-strategists to the decomposition of native SOM. Sucrose addition with N significantly accelerated mineralization of native SOM, whereas mineral N added with plant residues accelerated decomposition of plant residues. This supports the microbial mining theory in terms of N limitation. Sucrose addition with N was accompanied by accelerated microbial growth, increased activities of β-glucosidase and cellobiohydrolase, and decreased activities of xylanase and leucine amino peptidase. This indicated an increased contribution of r-strategists to the PE and to decomposition of cellulose but the decreased hemicellulolytic and proteolytic activities. Thus, the acceleration of the C cycle was primed by exogenous organic C and was controlled by N. This confirms the stoichiometric decomposition theory. Both K- and r-strategists were beneficial for priming effects, with an increasing contribution of K-selected species under N limitation. Thus, the priming phenomenon described in 'microbial N mining' theory can be ascribed to K-strategists. In contrast, 'stoichiometric decomposition' theory, that is, accelerated OM mineralization due to balanced microbial growth, is explained by domination of r-strategists.

Using pore space 3D geometrical modelling to simulate biological activity: Impact of soil structure

This study is the follow-up to a previous one devoted to soil pore space modelling. In the previous study, we proposed algorithms to represent soil pore space by means of optimal piecewise approximation using simple 3D geometrical primitives: balls, cylinders, cones, etc. In the present study, we use the ball-based piecewise approximation to simulate biological activity. The basic idea for modelling pore space consists in representing pore space using a minimal set of maximal balls (Delaunay spheres) recovering the shape skeleton. In this representation, each ball is considered as a maximal local cavity corresponding to the “intuitive” notion of a pore as described in the literature. The space segmentation induced by the network of balls (pores) is then used to spatialise biological dynamics. Organic matter and microbial decomposers are distributed within the balls (pores). A valuated graph representing the pore network, organic matter and microorganism distribution is then defined. Microbial soil organic matter decomposition is simulated by updating this valuated graph. The method has been implemented and tested on real data. As far as we know, this approach is the first one to formally link pore space geometry and biological dynamics. The long-term goal is to define geometrical typologies of pore space shape that can be attached to specific biological dynamic properties. This paper is a first attempt to achieve this goal.

3D geometric structures and biological activity: Application to microbial soil organic matter decomposition in pore space

During the past 10 years, soil scientists have started to use 3D Computed Tomography in order to gain a clearer understanding of the geometry of soil structure and its relationships with soil properties. We propose a geometric model for the 3D representation of pore space and a practical method for its computation. Our basic idea consists in representing pore space using a minimal set of maximal balls (Delaunay spheres) recovering the shape skeleton. In this representation, each ball could be considered as a maximal local cavity corresponding to the “intuitive” notion of a pore as described in the literature. The space segmentation induced by the network of balls (pores) was then used to spatialize biological dynamics. Organic matter and microbial decomposers were distributed within the balls (pores). A valuated graph representing the pore network, organic matter and distribution of micro-organisms was then defined. Microbial soil organic matter decomposition was simulated by updating this valuated graph. The method was implemented and tested using real CT images. The model produced realistic simulated results when compared with data in the literature in terms of the water retention curve and carbon mineralization. A decrease in water pressure decreased carbon mineralization, which is also in accordance with findings in the literature. From our results we showed that the influence of water pressure on decomposition is a function of organic matter distribution in the pore space. As far as we know, this is the approach to have linked pore space geometry and biological dynamics in a formal way. Our next goal will be to compare the model with experimental data of decomposition using different soil structures, and to define geometric typologies of pore space shape that can be attached to specific biological and dynamic properties.

Three-source partitioning of CO 2 efflux from soil planted with maize by 13C natural abundance fails due to inactive microbial biomass

A theoretical approach to the partitioning of carbon dioxide (CO 2) efflux from soil with a C 3 vegetation history planted with maize ( Zea mays), a C 4 plant, into three sources, root respiration (RR), rhizomicrobial respiration (RMR), and microbial soil organic matter (SOM) decomposition (SOMD), was examined. The δ 13C values of SOM, roots, microbial biomass, and total CO 2 efflux were measured during a 40-day growing period. A three-source isotopic mass balance based on the measured δ 13C 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 δ 13C of microbial CO 2 and that of microbial biomass. This approach strongly overestimated RR and underestimated RMR and microbial SOMD. CO 2 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 δ 13C value of microbial CO 2 by a mass balance equation including active and inactive parts of microbial biomass was used to adjust a hypothetical below-ground CO 2 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 CO 2 source controlling the δ 13C value of total microbial biomass. Since fumigation–extraction reflects total microbial biomass, its δ 13C value was unsuitable to predict δ 13C of released microbial CO 2 after a C 3–C 4 vegetation change. The second adjustment to the CO 2 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 CO 2 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 δ 13C values of microbial biomass and of microbially respired CO 2.

Soil wettability, aggregate stability, and the decomposition of soil organic matter

In well-structured topsoils, part of the soil organic matter (SOM) is located in the interior of the soil aggregates. Because of its location, this part of the SOM is little accessible to micro-organisms, and consequently not readily mineralised. Additionally, the physico-chemical conditions on the aggregate surfaces, being the main habitat of the organisms, control the accessibility, and hence, the rate of mineralisation. We hypothesise that hydrophobic conditions on aggregate surfaces reduce the rate of mineralisation of inside SOM, and simultaneously enhance the aggregate stability. The objectives of this study were therefore to study the significance of soil wettability with respect to both SOM mineralisation and aggregate stability. We used soil material from a loess-derived Gleyic Luvisol, either used as cropland or as grassland. Wettability was measured in terms of both the advancing soil–water contact angle and the solid surface free energy. Aggregate stability was assessed by immersion of aggregates in water–ethanol mixtures of varying surface tension. The impact of aggregation on SOM mineralisation was determined by respiration experiments that measured the CO 2-release both from the aggregates and from the corresponding homogenised soil material. It was found that the contact angle of the soil samples ranged from 17° to 79°, and the solid surface free energy from 34 to 68 mJ m −2. Aggregates showed increasing stability with decreasing surface tension of the testing liquid. With increasing contact angle, the initial aggregate breakdown was decreased, which we attribute to the wettability-dependence of the liquid adsorption rates of the aggregates. Soil respiration measurements showed that microbial SOM decomposition was affected by the aggregation status, i.e., the homogenised samples released significantly more CO 2 than the aggregates. We conclude that even subcritical soil water repellency (with contact angles <90°) can have a significant impact on the protection of SOM against microbial decomposition.

Soil carbon cycling in a temperate forest: radiocarbon-based estimates of residence times, sequestration rates and partitioning of fluxes

Temperate forests of North America are thought to be significant sinks of atmospheric CO2. We developed a below-ground carbon (C) budget for well-drained soils in Harvard Forest Massachusetts, an ecosystem that is storing C. Measurements of carbon and radiocarbon ( 14 C) inventory were used to determine the turnover time and maximum rate of CO2 production from heterotrophic respiration of three fractions of soil organic matter (SOM): recognizable litter fragments (L), humified low density material (H), and high density or mineral-associated organic matter (M). Turnover times in all fractions increased with soil depth and were 2-5 years for recognizable leaf litter, 5-10 years for root litter, 40-100+ years for low density humified material and >100 years for carbon associated with minerals. These turnover times represent the time carbon resides in the plant + soil system, and may underestimate actual decomposition rates if carbon resides for several years in living root, plant or woody material. Soil respiration was partitioned into two components using 14 C: recent photosynthate which is metabolized by roots and microorganisms within a year of initial fixation (Recent- C), and C that is respired during microbial decomposition of SOM that resides in the soil for several years or longer (Reservoir-C). For the whole soil, we calculate that decomposition of Reservoir-C contributes approximately 41% of the total annual soil respiration. Of this 41%, recognizable leaf or root detritus accounts for 80% of the flux, and 20% is from the more humified fractions that dominate the soil carbon stocks. Measurements of CO 2 and 14 CO2 in the soil atmosphere and in total soil respiration were combined with surface CO2 fluxes and a soil gas diffusion model to determine the flux and isotopic signature of C produced as a function of soil depth. 63% of soil respiration takes place in the top 15 cm of the soil (O + A + Ap horizons). The average residence time of Reservoir-C in the plant + soil system is 81 years and the average age of carbon in total soil respiration (Recent-C + Reservoir-C) is 41 years. The O and A horizons have accumulated 4.4 kgC m -- 2 above the plow layer since abandon- ment by settlers in the late-1800's. C pools contributing the most to soil respiration have short enough turnover times that they are likely in steady state. However, most C is stored as humified organic matter within both the O and A horizons and has turnover times from 40 to