Bioavailability Of Soil Organic Matter Research Articles

Soil Quality Evaluation of Hydrocarbon Polluted Parcels

For hydrocarbon polluted soils that underwent bioremediation it is important to assess its condition after a period of time, but it is more useful when there is an opportunity of comparison against an unpolluted soil and an untreated polluted one. This paper provides a comparison of three adjacent parcels, being the first clean, the second polluted and bioremediated, the third polluted and left to natural attenuation. Study was conducted determining pH, electrical conductivity, carbonates, soil organic matter, chemical oxygen demand, eight anions, and twelve metals. Data were compared against those references for agricultural soils found in the Mexican NOM-021. A Pearson correlation was applied to find coincidences between the three parcels, obtained results allowed to say that bioremediated parcel allowed for most uniform pH, negligible salinity risk, and medium content of soil organic matter, but treatment has enabled heavy metal accumulation since its values are higher in respect to the other parcels. Natural attenuated parcel has some spots with lower pH, a moderately saline risk, a high content of soil organic matter, and lower content of heavy metals. The clean or unaffected parcel exhibit the higher pH values, a slightly saline condition, soil organic matter ranges from high to very high content, heavy metals content is medium, but no reaching dangerous levels. An important assessment is that bioremediation has enhanced the bioavailability of soil organic matter but it is not similar to the values in the unpolluted parcel.

Temperature sensitivity of SOM decomposition is linked with a K-selected microbial community.

Temperature sensitivity (Q10 ) of soil organic matter (SOM) decomposition is a crucial parameter to predict the fate of soil carbon (C) under global warming. Nonetheless, the response pattern of Q10 to continuous warming and the underlying mechanisms are still under debate, especially considering the complex interactions between Q10 , SOM quality, and soil microorganisms. We examined the Q10 of SOM decomposition across a mean annual temperature (MAT) gradient from -1.9 to 5.1°C in temperate mixed forest ecosystems in parallel with SOM quality and bioavailability, microbial taxonomic composition, and functional genes responsible for organic carbon decomposition. Within this temperature gradient of 7.0°C, the Q10 values increased with MAT, but decreased with SOM bioavailability. The Q10 values increased with the prevalence of K-strategy of soil microbial community, which was characterized by: (i) high ratios of oligotrophic to copiotrophic taxa, (ii) ectomycorrhizal to saprotrophic fungi, (iii) functional genes responsible for degradation of recalcitrant to that of labile C, and (iv) low average 16S rRNA operon copy number. Because the recalcitrant organic matter was mainly utilized by the K-strategists, these findings independently support the carbon quality-temperature theory from the perspective of microbial taxonomic composition and functions. A year-long incubation experiment was performed to determine the response of labile and recalcitrant C pools to warming based on the two-pool model. The decomposition of recalcitrant SOM was more sensitive to increased temperature in southern warm regions, which might attribute to the dominance of K-selected microbial communities. It implies that climate warming would mobilize the larger recalcitrant pools in warm regions, exacerbating the positive feedback between increased MAT and CO2 efflux. This is the first attempt to link temperature sensitivity of SOM decomposition with microbial eco-strategies by incorporating the genetic information and disentangling the complex relationship between Q10 and soil microorganisms.

Distinguishing functional pools of soil organic matter based on solubility in hot water

Evidence is emerging that the solubility of soil organic matter (SOM) in water is a key factor regulating the turnover of carbon (C) and nitrogen (N). We used data from a field trial with a wide range of treatments in a case study to: (1) examine the link between SOM solubility and bioavailability and (2) evaluate whether low water-solubility is a factor contributing to the persistence of refractory SOM. The trial was established in 2000 on a silt loam (Udic Dystocrept) at Lincoln, New Zealand to identify management practices that maintain SOM following the conversion of long-term pasture to arable cropping. The following land use treatments were sampled (0–7.5, 7.5–15 and 15–25 cm) in 2013: (1) long-term ryegrass-white clover pasture; (2) arable cropping rotation, managed using either intensive, minimum, or no tillage; and (3) continuous bare fallow (plots maintained plant-free using herbicide; not cultivated). The bioavailability of SOM was determined by measuring C and N mineralisation in a 98-day incubation at 25°C (soil maintained near field capacity) and water solubility was assessed by measuring hot-water-extractable C and N (16-h extraction at 80°C). After 13 years of arable cropping, C stocks (to 25 cm) were 11 t ha–1 less than in pasture soil (decrease of 14%). Tillage ‘intensity’ had no effect on C stocks in the top 25 cm. Large losses of C were observed in the bare fallow treatment (19 t C ha–1 less than pasture soil). The bioavailability of SOM (CO2-C mineralised as a proportion of soil C) also declined under arable cropping and bare fallow. The relationship between total C and mineralised C had a significant (P < 0.001) intercept, indicating that part of the organic matter (13 g C kg–1) did not contribute to C mineralisation (it was biologically inert). Across treatments and sampling depths, SOM mineralised in 98 days generally corresponded well with that extracted in hot water. A significant fraction of SOM (~9 g C kg–1) did not release C to hot water. Water-insoluble organic matter, including compounds that are strongly bonded to mineral surfaces, may comprise a significant part of the refractory SOM.

Mycorrhizal Atriplex nummularia promote revegetation and shifts in microbial properties in saline Brazilian soil

Soil microbial attributes are the most sensitive indicators of changes in soil quality caused by land use. The restoration of areas with salinity and sodicity problems via revegetation has a significant impact on soil attributes. Understanding the response of microbial indicators is essential to maintain the bioavailability of soil organic matter and provides an alternative to land reclamation. Therefore, the objective of this study was to evaluate the efficiency of revegetation on the regeneration of saline soil in a Brazilian semiarid area through evaluating the presence of arbuscular mycorrhizal fungi (AMF) and changes in the soil microbial structure and chemical attributes. A field experiment was performed with a randomized block design of eight treatments: a control (without plant cultivation), monocultures (Atriplex nummularia [Atri], Mimosa caesalpiniifolia [Sab], Leucaena leucocephala [Leu], and Azadirachta indica [Neem]), and intercrops (Atri and Sab, Atri and Neem, and Atri and Leu). Soil composite samples were collected from each plot during the dry and rainy seasons when the plants had completed 24 and 30 months of growth after transplanting, respectively. The cultivation of the saline soil allowed an increase in microbial biomass carbon in both seasons. In the rainy season, the soil basal respiration and metabolic quotient were lower in the cultivated area than in the control area. Spore density and mycorrhizal colonization were higher in the cultivated area than in the control area in both seasons. We provide the first data related to the beneficial effects of revegetation with naturally mycorrhized plants of A. nummularia in monoculture or intercrop on the soil quality of a semiarid area.

Biotic versus Abiotic Controls on Bioavailable Soil Organic Carbon

Processes controlling microbial access to soil organic matter are critical for soil nutrient cycling and C stabilization. The bioavailability of soil organic matter partly depends on the rate that substrates become water-soluble, which is determined by some combination of biological, biochemical, and purely abiotic processes. Our goal was to unravel these biotic and abiotic processes to better understand mechanisms controlling the dynamics of bioavailable soil organic carbon (SOC). We sampled soils in a California annual grassland from manipulated plots with and without plants to help distinguish bioavailable SOC generated from mineral-associated organic matter versus from plant detritus (i.e., the “light fraction”). In the laboratory, soils were incubated for 8 months under all possible combinations of three levels of moisture and two levels of microbial biomass using continuous chloroform sterilization. We measured cumulative carbon dioxide (CO2) production and the net change in soil water-extractable organic C (WEOC) to quantify C that was accessed biologically or biochemically. Under the driest conditions, microbes appeared to primarily access WEOC from recent plant C, with the other half of CO2 production explained by extracellular processes. These results suggest that dry, uncolonized conditions promote the adsorption of WEOC onto mineral surfaces. Under wetter conditions, microbial access increased by two orders of magnitude, with a large concomitant decrease in WEOC, particularly in soils without plant inputs from the previous growing season. The largest increase in WEOC occurred in wet sterilized soil, perhaps because exoenzymes and desorption continued solubilizing C but without microbial consumption. A similar amount of WEOC accumulated in wet sterilized soil whether plants were present or not, suggesting that desorption of mineral-associated C was the abiotic WEOC source. Based on these results, we hypothesize that dry-live and wet-uncolonized soil microsites are sources of bioavailable SOC, whereas wet-live and dry-uncolonized microsites are sinks.

Carbon and nitrogen mineralization in hierarchically structured aggregates of different size

Soil organic matter pools are turned over at different rates, but uncertainty persists regarding how far the hierarchically organized soil structure controls the mineralization dynamics. To better understand carbon and nitrogen mineralization in undisturbed aggregate classes (coarse aggregates: 2–6.3mm, fine aggregates: <2mm), we conducted a long-term (224-days) laboratory incubation experiment. A grassland soil (Haplic Cambisol) was chosen since its aggregates were not disturbed by tillage. The field-moist aggregate size classes were separated by a gentle dry-sieving method. We monitored the CO2–C and NH3–N emissions, nitrogen mineralization, pool sizes of total and salt extractable (0.5MK2SO4) organic carbon and nitrogen, and microbial biomass carbon and nitrogen. By this approach, we could distinguish between the carbon and nitrogen mineralization processes of two soil aggregate size classes, relative to undisturbed bulk soil. The classes showed different aggregate architectures with variously sized subunits, but confirmed the aggregate hierarchy. For both aggregate classes, the recombined sum of respired CO2–C per unit of soil organic carbon equaled the bulk soil, proving that our aggregate separation preserved the original aggregates as intact functional units. Both aggregate size classes and the bulk soil respired only 4% soil organic carbon throughout the incubation period. The coarse aggregates, which mostly comprised small macroaggregates, mineralized more carbon per unit soil organic carbon than the fine aggregates (composed of microaggregates), indicating a higher bioavailability of soil organic matter in the coarse aggregates. Accordingly microbial metabolic efficiency was higher in coarse than in fine aggregates. Nitrogen mineralization was higher in fine aggregates than in coarse aggregates, but was impaired by carbon limitation as the incubation experiment proceeded. We conclude that bioavailability of soil organic matter was affected by different aggregate architectures in the different aggregate size classes. The lower bioavailability of soil organic matter in fine aggregates is due to an enhanced stabilization through cation bridging of the dolomitic soil material at the microaggregate level, whereas the higher soil organic matter bioavailability in coarse aggregates is explained by labile organic components at the macroaggregate level.

Insights into complexation of dissolved organic matter and Al(III) and nanominerals formation in soils under contrasting fertilizations using two-dimensional correlation spectroscopy and high resolution-transmission electron microscopy techniques

Understanding the organomineral associations in soils is of great importance. Using two-dimensional correlation spectroscopy (2DCOS) and high resolution-transmission electron microscopy (HRTEM) techniques, this study compared the binding characteristics of organic ligands to Al(III) in dissolved organic matter (DOM) from soils under short-term (3-years) and long-term (22-years) fertilizations. Three fertilization treatments were examined: (i) no fertilization (Control), (ii) chemical nitrogen, phosphorus and potassium (NPK), and (iii) NPK plus swine manure (NPKM). Soil spectra detected by the 2DCOS Fourier transform infrared (FTIR) spectroscopy showed that fertilization modified the binding characteristics of organic ligands to Al(III) in soil DOM at both short- and long- term location sites. The CH deformations in aliphatic groups played an important role in binding to Al(III) but with minor differences among the Control, NPK and NPKM at the short-term site. While at the long-term site both C–O stretching of polysaccharides or polysaccharide-like substances and aliphatic O–H were bound to Al(III) under the Control, whereas only aliphatic O–H, and only polysaccharides and silicates, were bound to Al(III) under NPK and NPKM, respectively. Images from HRTEM demonstrated that crystalline nanominerals, composed of Fe and O, were predominant in soil DOM under NPK, while amorphous nanominerals, predominant in Al, Si, and O, were dominant in soil DOM under Control and NPKM. In conclusion, fertilization strategies, especially under long-term, could affect the binding of organic ligands to Al(III) in soil DOM, which resulted in alterations in the turnover, reactivity, and bioavailability of soil organic matter. Our results demonstrated that the FTIR-2DCOS combined with HRTEM techniques could enhance our understanding in the binding characteristics of DOM to Al(III) and the resulted nanominerals in soils.

Bioavailability of soil organic matter and microbial community dynamics upon permafrost thaw

Amplified Arctic warming could thaw 25% of the permafrost area by 2100, exposing vast amounts of currently fixed organic carbon to microbially mediated decomposition and release of greenhouse gasses through soil organic matter (SOM) respiration. We performed time-series incubation experiments with Holocene permafrost soils at 4°C for up to 11 days to determine changes in exoenzyme activities (EEAs) (i.e. phosphatase, β-glucosidase, aminopeptidase) as a measure for the bioavailability of SOM in response to permafrost thaw. We also profiled SSU rRNA transcripts to follow the qualitative and quantitative changes in viable prokaryotes and eukaryotes during incubation. EEA, amount of rRNA transcripts and microbial community structures differed substantially between the various soil intervals in response to thaw: after 11 days of incubation, the active layer became slightly depleted in C and P and harboured bacterial phyla indicative of more oligotrophic conditions (Acidobacteria). A fast response in phosphatase and β-glucosidase upon thaw, and a predominance of active copiotrophic Bacteroidetes, showed that the upper permafrost plate serves as storage of easily degradable carbon derived from the overlying thawed active layer during summer. EEA profiles and microbial community dynamics furthermore suggest that the deeper and older permafrost intervals mainly contain recalcitrant SOM, and that extracellular soil-bound exoenzymes play a role in the initial cleavage of biopolymers, which could kick-start microbial growth upon thaw. Basidiomycetous fungi and Candidate Subdivision OP5 bacteria were the first to respond in freshly thawed deeper permafrost intervals, and might play an important role in the decomposition of recalcitrant SOM to release more labile substrates to support the major bacterial phyla (β-Proteobacteria, Actinobacteria, Firmicutes), which predominated thereafter.

Amino sugars in arctic soils

Amino sugars in soils are assumed to be of microbial origin. Their levels in soil, can provide insights into the microbial contribution to soil organic matter (SOM) and might serve as an indicator of SOM quality. Four amino sugars, glucosamine, galactosamine, mannosamine, and muramic acid were determined by gas chromatography in five soils from the tundra Alaska before and after incubation for 4 months at 4 and 25°C. Glucosamine and muramic acid analyses were utilized to represent fungal biomass and bacterial biomass, respectively. Total amino sugar N represented 0.8 to 3.3% of the total soil N in these five tundra soils, and the proportion of soil N as amino sugars increased with depth. Total amino sugar and fungal glucosamine N increased or remained constant during incubation at 4 and 25°C (except site 3 Cf horizon which decreased after incubation at 4°C), suggesting that total microbial biomass and fungal biomass increased. Bacterial biomass had an increase after incubation at 4°C, and a relative decrease at 25°C (except for Cf horizon). Fungal glucosamine to muramic acid ratios demonstrated that fungal biomass increased faster than bacterial biomass at 25°C, and slower at 4°C, indicating that at higher temperature, fungi dominated the microbial biomass in the soils studied. The mechanisms involved in the accumulation and decomposition of amino sugars in soil during incubation were not clear. No correlations were observed between the amino sugar contents and CO2 evolution during incubation, indicating that amino sugar abundance may not be an indicator of the bioavailability of SOM in soils. However, they can provide insights into the microbial contribution to SOM.

Comparing bioavailability in five Arctic soils by pyrolysis-gas chromatography/mass spectrometry

The bioavailability of soil organic matter (SOM) from five Arctic soils was studied using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and laboratory incubation techniques. Soil samples were incubated concurrently at 4 and 25 °C for approx. 4 months. Py-GC/MS was used to determine the relative percentage of a suite of chemical components in the SOM before and after incubation at both temperatures. Following incubation at 4 °C, the relative percentage of polysaccharides and phenols in SOM decreased compared to that of amino carbohydrates and lipids. The change in the relative percentage of lignin was uncorrelated to any other fraction of the soil organic matter. Following incubation at 25 °C, the relative percentage of polysaccharides and lignin in SOM decreased compared to that of lipids. The changes in relative percentage of amino carbohydrates and phenols were uncorrelated to any other fraction of the SOM. The cumulative mass of carbon dioxide respired during incubation was found to be strongly correlated to the initial relative abundance of polysaccharides for incubations at 4 °C.