Microbially available carbon in buried riparian soils in a glaciated landscape

Buried horizons and lenses in riparian soil profiles harbor large amounts of carbon relative to the surrounding soil horizons. Because these buried soil horizons, as well as deep surface horizons, frequently lie beneath the water table, their impact on nitrogen transport across the terrestrial–aquatic interface depends upon their frequency and spatial distribution, and upon the lability of associated organic matter. We collected samples of 51 soil horizons from 14 riparian zones Rhode Island, USA, where soil profiles are characterized by glacial outwash and alluvial deposits. These soil samples came from as deep as 2 m and ranged in carbon content from <1% to 44% in a buried O horizon 54–74 cm deep. We used these samples to: (1) determine the extent to which carbon in buried horizons, and deep surface horizons, is potentially microbially available; (2) identify spatial patterns of carbon mineralization associated with surface and buried horizons; and (3) evaluate likely relationships between soil horizon types, chemical characteristics and carbon mineralization. Carbon mineralization rates associated with buried horizons during anaerobic incubations ranged from 0.0001 to 0.0175 μmol C kg soil^?1 s^?1 and correlated positively with microbial biomass (R=0.89, P<0.0001, n=21). Excluding surface O horizons from the analysis, carbon mineralization varied systematically with horizon type (surface A, buried A, buried O, lenses, A/C, B, C) (P<0.05) but not with depth or depth x horizon interaction (overall R²=0.59, P<0.0005, n=47). In contrast to this result and to most published data sets, ¹³C-to-¹²C and ¹⁵N-to-¹⁴N ratios of organic matter declined with depth (¹³C?26.9 to ?29.3 per mil, ¹⁵N+5.6 to ?0.8 per mil). The absence of a relationship between horizon depth and C availability suggests that carbon availability in these buried horizons may be determined by the abundance and quality of organic matter at the time of horizon formation or burial, rather than by duration since burial, and implies that subsurface microbial activity is largely disconnected from surface ecosystems. Our results contribute to the emerging view that buried horizons harbor microbially available C in quantities relevant to ecosystem processes, and suggest that buried C-rich soil horizons need to be incorporated into assessments of the depth of the biologically active zone in near-stream subsurface soils.     

Spatial variability of enzyme activities and microbial biomass in the upper layers of Quercus petraea forest soil

Extracellular lignocellulose-degrading enzymes are responsible for the transformation of organic matter in hardwood forest soils. The spatial variability on a 12 × 12 m plot and vertical distribution (0–8 cm) of the ligninolytic enzymes laccase and Mn-peroxidase, the polysaccharide-specific hydrolytic enzymes endoglucanase, endoxylanase, cellobiohydrolase, 1,4-β-glucosidase, 1,4-β-xylosidase and 1,4-β-N-acetylglucosaminidase and the phosphorus-mineralizing acid phosphatase were studied in a Quercus petraea forest soil profile. Activities of all tested enzymes exhibited high spatial variability in the L and H horizons. Acid phosphatase and 1,4-β-N-acetylglucosaminidase exhibited low variability in both horizons, while the variability of Mn-peroxidase activity in the L horizon, and endoxylanase and cellobiohydrolase activities in the H horizon were very high. The L horizon contained 4× more microbial biomass (based on PLFA) and 7× fungal biomass (based on ergosterol content) than the H horizon. The L horizon also contained relatively more fungi-specific and less actinomycete-specific PLFA. There were no significant correlations between enzyme activities and total microbial biomass. In the L horizon cellulose and hemicellulose-degrading enzymes correlated with each other and also with 1,4-β-N-acetylglucosaminidase and acid phosphatase activities. Laccase, Mn-peroxidase and acid phosphatase activities correlated in the H horizon. The soil profile showed a gradient of pH, organic carbon and humic compound content, microbial biomass and enzyme activities, all decreasing with soil depth. Ligninolytic enzymes showed preferential localization in the upper part of the H horizon. Differences in enzyme activities were accompanied by differences in the microbial community composition where the relative amount of fungal biomass decreased and actinomycete biomass increased with soil depth. The results also showed that the vertical gradients occur at a small scale: the upper and lower parts of the H horizon only 1 cm apart were significantly different with respect to seven out of nine activities, microbial biomass content and community composition.     

Revisiting the definitions of the sombric horizon in soil taxonomy and world reference base for soil resources

A review of the literature suggests that the sombric horizon (from French sombre, dark) was established in Soil Taxonomy (ST) and the World Reference Base for Soil Resources (WRB) from limited data and without a clear understanding of how this horizon forms. This paper reviews data on sombric horizons, evaluates four hypotheses regarding their origin, and offers suggestions for improving the identification of sombric horizons. Of the 30 pedons recognized in the literature as having sombric or sombric-like horizons, 12 fully satisfied the existing criteria in ST and the WRB. Soils with a true sombric horizon may be restricted to the highlands of central Africa (Burundi, Rwanda, Congo) on relatively cool (mean annual air temperature 16–20 °C), moist (mean annual precipitation 1450–2000 mm) plateaus and mountains at elevations ranging from 1450 to 2000 m. Soils with a sombric horizon occur primarily on highly weathered materials from a variety of crystalline rocks. The surface of the sombric horizon occurs at depths of 40 to 110 cm from the surface (average = 76 cm) and ranges from 27 to 100 cm in thickness (average = 63 cm). The sombric horizon commonly is dark reddish brown (5YR 3/3), acidic (average pH = 4.7), low in exchangeable bases (average base saturation = 4%), high in organic C (average = 1.3%), and despite abundant clay (average = 56%) has a low cation-exchange capacity (average = 12 cmol(+)/kg soil). Based on existing data, the sombric horizon contains humus that has migrated downward in the soil, possibly in response to climate and vegetation change. Sombric horizons are not to be confused with sombric-like horizons which may contain andic soil properties or spodic materials. In Soil Taxonomy, soils with sombric horizons are classified primarily as Sombriudoxes (8 pedons) and Sombrihumults (4 pedons). In the World Reference Base for Soil Resources, sombric horizons occur primarily in Umbric Ferralsols (Sombric).     

Land-use/cover change effects and carbon controls on volcanic soil profiles in highland temperate forests

The stabilization of SOM by Al–humus complexes and non-crystalline minerals is a key issue to explain the soil-C variability and the biogeochemical processes that determine the fate of soil C following land-use/cover change (LUCC) in volcanic landscapes. In an altitudinal gradient of volcanic soils (2550–3500 masl), we quantified the total soil C (C_T) concentrations and stocks in soil pits sampled by genetic horizons. We performed analyses at landscape and local scales in order to identify and integrate the underlying environmental controls on C_T and the effects of LUCC. We selected four sites, two on the upper piedmont, one on the lower mountain slope and one on the middle mountain slope at Cofre de Perote volcano (eastern central Mexico) where temperate forests are the natural vegetation. At each site we selected three to five units of use/cover as a chronosequence of the LUCC pathways. In each soil horizon chemical characteristics (i.e. N, C/N ratio, pH, exchangeable bases) were determined and mineralogical properties were estimated from selective Al, Fe and Si oxalate and pyrophosphate extractions (i.e. the Alp/Alo ratio, the active Al related to non-crystalline minerals as Al_o ? Al_p, the allophane concentration, and the non-crystalline Al and Fe minerals as Al_o + 1/2Fe_o). At landscape scale, the Al–humus complexes were strongly related to the C_T concentration in topsoil (A horizons) but this relationship decreased with depth. In turn, the non-crystalline minerals and the C/N ratio explained the variability of the C_T concentrations in C horizons. At local scale, C_T concentrations and stocks were depleted after conversion of forest to agriculture in Vitric Andosols at the upper piedmont but this was not observed in Silandic Andosols. However, in Vitric Andosols the reduction of the C_T stocks is partially recovered throughout the regeneration/reforestation processes. The results suggest that the lower vulnerability of Silandic Andosols than Vitric Andosols to changes in the C_T after LUCC is due to the higher levels of SOM stabilized by Al–humus complexes and non-crystalline minerals in the Silandic soils. Furthermore, the importance of the allophane to explain the C_T stocks in the Silandic Andosols of the middle slopes suggests that the C_T stabilized by this mineral fraction in the subsoil adds an important fraction of the C_T to the estimates of the stocks.     

Tag-encoded pyrosequencing analysis of bacterial diversity in a single soil type as affected by management and land use

Impacts of management and land use on soil bacterial diversity have not been well documented. Here we present the application of the bacterial tag encoded FLX amplicon pyrosequencing (bTEFAP) diversity method, which will promote studies in soil microbiomes. Using this modified FLX pyrosequencing approach we evaluated bacterial diversity of a soil (Pullman soil; fine, mixed, thermic Torrertic Paleustolls) with 38% clay and 34% sand (0–5 cm) under four systems. Two non-disturbed grass systems were evaluated including a pasture monoculture (Bothriochloa bladhii (Retz) S.T. Blake) [P] and a diverse mixture of grasses in the Conservation Reserve Program (CRP). Two agricultural systems were evaluated including a cotton (Gossypium hirsutum L.) -winter wheat (Triticum aestivum L.)-corn (Zea mays L.) rotation [Ct–W–Cr] and the typical practice of the region, which is continuous monoculture cotton (Ct–Ct). Differences due to land use and management were observed in soil microbial biomass C (CRP > P = Ct–W–Cr > Ct–Ct). Using three estimators of diversity, the maximum number of unique sequences operational taxonomic units (OTU; roughly corresponding to the species level) never exceeded 4500 in these soils at the 3% dissimilarity level. The following trend was found using the most common estimators of bacterial diversity: Ct–W–Cr > P = CRP > Ct–Ct. Predominant phyla in this soil were Actinobacteria, Bacteriodetes and Fermicutes. Bacteriodetes were more predominant in soil under agricultural systems (Ct–W–Cr and Ct–Ct) compared to the same soil under non-disturbed grass systems (P and CRP). The opposite trend was found for the Actinobacteria, which were more predominant under non-disturbed grass systems (P and CRP). Higher G? bacteria and lower G+ bacteria were found under Ct–W–Cr rotation and highest abundance of actinomycetes under CRP. The bTEFAP technique proved to be a powerful method to characterize the bacterial diversity of the soil studied under different management and land use in terms not only on the presence or absence, but also in terms of distribution.     

Nitrogen fertilization and cropping systems effects on soil organic carbon and total nitrogen pools under chisel-plow tillage in Illinois

Agricultural soils can be a major sink for atmospheric carbon (C) with adoption of recommended management practices (RMPs). Our objectives were to evaluate the effects of nitrogen (N) fertilization and cropping systems on soil organic carbon (SOC) and total N (TN) concentrations and pools. Replicated soil samples were collected in May 2004 to 90 cm depth from a 23-year-old experiment at the Northwestern Illinois Agricultural Research and Demonstration Center, Monmouth, IL. The SOC and TN concentrations and pools, soil bulk density (ρ_b) and soil C:N ratio were measured for five N rates [0 (N₀), 70 (N₁), 140 (N₂), 210 (N₃) and 280 (N₄) kg N ha⁻¹] and two cropping systems [continuous corn (Zea mays L.) (CC), and corn–soybean (Glycine max (L.) Merr.) rotation (CS)]. Long-term N fertilization and cropping systems significantly influenced SOC concentrations and pools to 30 cm depth. The SOC pool in 0–30 cm depth ranged from 68.4 Mg ha⁻¹ for N₀ to 75.8 Mg ha⁻¹ for N₄. Across all N treatments, the SOC pool in 0–30 cm depth for CC was 4.7 Mg ha⁻¹ greater than for CS. Similarly, TN concentrations and pools were also significantly affected by N rates. The TN pool for 0–30 cm depth ranged from 5.36 Mg ha⁻¹ for N₀ to 6.14 Mg ha⁻¹ for N₄. In relation to cropping systems, the TN pool for 0–20 cm depth for CC was 0.4 Mg ha⁻¹ greater than for CS. The increase in SOC and TN pools with higher N rates is attributed to the increased amount of biomass production in CC and CS systems. Increasing N rates significantly decreased ρ_b for 0–30 cm and decreased the soil C:N ratio for 0–10 cm soil depth. However, none of the measured soil properties were significantly correlated with N rates and cropping systems below 30 cm soil depth. We conclude that in the context of developing productive and environmentally sustainable agricultural systems on a site and soil specific basis, the results from this study is helpful to strengthening the database of management effects on SOC storage in the Mollisols of Midwestern U.S.     

A soil chronosequence in the semi-arid environment of Patagonia (Argentina)

Soil development with time was investigated on beach ridges with ages ranging from about 1380 to 6240 ¹⁴C-years BP at the eastern coast of central Patagonia. The main pedogenic processes are accumulation of organic matter and carbonate leaching and accumulation within the upper part of the soils. Soil formation is strongly influenced by incorporation of eolian sediments into the interstitial spaces between the gravel of which the beach ridges are composed. Different amounts of eolian material in the soils lead to differentiation into Leptosols (containing ≤ 10% fine earth in the upper 75 cm) and Regosols (containing > 10% fine earth). Soil depth functions and chronofunctions of organic carbon, calcium carbonate, pH, Ca:Zr, Mg:Zr, K:Zr, Na:Zr, Fe:Zr, Mn:Zr, and Si:Al (obtained from X-ray fluorescence analysis) were evaluated. To establish soil chronofunctions mean values of the horizon data of 0–10 cm below the desert pavement were used, which were weighted according to the horizon thicknesses. The depth function of pH shows a decrease towards the surface, indicating leaching of bases from the upper centimeters. Chronofunctions of pH show that within 6000 radiocarbon years of soil development pH drops from 7.0 to 6.6 in the Leptosols and from 8.1 to 7.5 in the Regosols. The higher pH of the Regosols is due to input of additional bases from the eolian sediments. Chronofunctions of Ca:Zr and K:Zr indicate progressive leaching of Ca and K in the Regosols, showing close relationships to time (R² = 0.972 and 0.995). Na leaching as indicated by decreasing Na:Zr ratios shows a strong correlation to time only in the Leptosols (R² = 0.999). Both, Leptosols and Regosols show close relationships to time for Fe:Zr (R² = 0.817 and 0.824), Mn:Zr (R² = 0.940 and 0.803), and Si:Al (0.971 and 0.977), indicating enrichment of Fe and Mn and leaching of Si. Leaching of mobile elements takes place on a higher level in the Regosols than in the Leptosols from the beginning of soil formation. Hence, a significant part of the eolian sediments must have been incorporated into the beach ridges very soon after their formation.     

The structure of microbial communities in redoximorphic microsites of Gleysol

Microbial communities were studied in redoximorphic microsites of highly heterogeneous Gleysol at a mm scale using 16S and 18S amplicon sequencing to demonstrate if the composition of soil microbes reflects the differences in ferric and ferrous micro-sites. In both explored gley horizons with redoximorphic features (Bg2 and Cg), ferric mottles were significantly enriched with total P and Fe and depleted of O, Si, Al, K and Ca compared with the adjacent ferrous groundmass (SEM–EDS). Ferric mottles were determined as Fe oxide coatings and hypocoatings. In Bg2, both prokaryotic and micro-eukaryotic communities differed significantly between mottles and groundmass in composition of operational taxonomic units (OTUs) and in proportions of phyla, reflecting heterogeneities in the soil properties there. Mottles in Bg2 were characterized by increased proportion of Proteobacteria, decreased proportion of Acidobacteriota among prokaryotes and by dominance of a single proteobacterial OTU from Anaplasmataceae compared to all other samples. The composition of micro-eukaryotes showed an opposite trend, as micro-eukaryotes of Bg2 groundmass were unique among the other horizons, while micro-eukaryotes of Bg2 mottles had similar composition to neighbouring horizons. Microbial communities of adjacent samples were not more similar to each other than communities of randomly selected ones in Bg2 horizon. That suggests that at mm scale, the sample distance does not represent the driving factor of microbial community composition and that the adjacent samples differ rather due to physicochemical factors. The spatial organization of microbial communities revealed in Bg2 has not reappeared in similarly organized Cg horizon, probably due to other overriding factors. The differences revealed between Bg2 and Cg horizons, including granulometric composition, content of crystalline Fe, exchangeable Al, and organic carbon, as well as exposition to groundwater, were discussed as possible reasons of the distinct organization in Cg. The similarity of pro−/eukaryotic communities of adjacent and non-adjacent couples suggests no distance decay pattern at a mm scale. The agreement between patchiness in soil properties and microbial communities was revealed for the first time and confirms the importance of microscale patterns in soil.     

Influence of carbon amendments on soil denitrifier abundance in soil microcosms

It is known that carbon (C) amendments increase microbial activity in anoxic soil microcosm studies, however the effects on abundance of total and denitrifier bacterial communities is uncertain. Quantitative PCR was used to target the 16S rRNA gene for the total bacterial community, the nosZ functional gene to reflect a broad denitrifier community, and functional genes from narrow denitrifier communities represented by Pseudomonas mandelii and related species (cnorB_P) and Bosea/Bradyrhizobium/Ensifer spp. (cnorB_B). Repacked soil cores were amended with varying amounts of glucose and red clover plant tissue (0–1000 mg C kg^? 1 of soil) and incubated for 96 h. Carbon amendment significantly increased respiration as measured by cumulative CO₂ emissions. Inputs of red clover or glucose at 1000 mg C kg^? 1 of soil caused increased abundance in the total bacteria under the conditions used. There was about an approximate 2-fold increase in the abundance of bacteria bearing the nosZ gene, but only in treatments receiving 500 or 1000 mg C kg^? 1 of soil of glucose or red clover, respectively. Additions of ≥ 500 mg C kg^? 1 soil of red clover and ≥ 250 mg C kg^? 1 of glucose increased cnorB_P-gene bearing denitrifiers. Changes in abundance of the targeted communities were related to C availability in soil, as indicated by soil respiration, regardless of C source. Applications of C amendments at rates that would occur in agricultural soils not only increase microbial activity, but can also induce changes in abundance of total bacterial and denitrifier communities in studies of anoxic soil microcosms.     

Testing the stability of carbon pools stored in tussock sedge meadows

Tussocks formed by Carex stricta are a relatively large carbon (C) pool in sedge meadows, but the stability of organic matter in these ecosystems is not well understood. We initiated year-long incubation experiments (22.5 °C) to evaluate the CO₂ and CH₄ production potentials of sedge meadow substrates under field moist and inundated treatments from five sites in the Upper Midwest, USA (4 reference, 1 restored). C mineralization potentials decreased with depth (tussocks > underlying soil), and were positively correlated with macro-organic matter content and negatively with lignin. Across sites, C stored in tussocks and soil at the restoration was the least stable, suggesting that the restoration of C-storage function may take decades. Mineralization potentials were similar between field moist and inundated treatments, but inundation resulted in higher methane production, accounting for 24–51% of total carbon mineralized from tussocks. In the field however, C. stricta tussocks emitted less methane (393 ± 76 mg CH₄ m⁻² d⁻¹) than tussock interspaces (1362 ± 371 mg CH₄ m⁻² d⁻¹) early in the growing season; we suggest that tussock tops oxidized methane produced from deeper anoxic horizons. Our results highlight the importance of considering how microtopography modulates greenhouse gas flux from wetlands and suggests that the C stored in the older, more decomposed C. stricta tussock sedge meadow substrates (both within and between sites) is relatively stable.     

Effects of municipal solid waste compost,farmyard manure and chemical fertilizers on wheat growth,soil composition and soil bacterial characteristics under Tunisian arid climate

The use of municipal solid waste compost (MSWC) as soil organic amendment is of an economic and environmental interest. However, little is known about the effectiveness of MSWC application on agricultural soil in northern Africa arid climate. We assessed the impact of five years' applications of different organic and mineral fertilizers on wheat grain yields and soil chemical and microbial characteristics. Soils were treated with MSWC at rates of 40 (C1) and 80 (C2) Mg ha^?1, farmyard manure at a rate of 40 Mg ha^?1 (M), chemical fertilizers (Cf) and the combinations (C1Cf, C2Cf, MCf). Wheat grain yield was enhanced with all amendments. Parallel increases of heavy metal levels and faecal coliform were also recorded except for Cf treatments. Based on wheat grain yield, heavy metal and faecal coliform data, we determined the treatment effectiveness index (E_xx), calculated by dividing the pollutant increase ratio by the grain yield increase ratio. The treatment effectiveness index E_C1 indicated lower faecal and heavy metal pollution with positive gains in wheat yields. Despite polluting effects on soil determined by the different treatments, no significant differences between treatments were observed in total bacterial count and soil bacterial community structure, as shown by 16S rRNA gene PCR-denaturing gradient gel electrophoresis banding patterns and 16S rRNA gene Length Heterogeneity-PCR analysis. According to the collected data, the use of MSWC at a rate of 40 Mg ha^?1 might be recommended.     

Bacterial community structure and microbial activity in different soils amended with biogas residues and cattle slurry

Anaerobic digestion of organic materials generates residues of differing chemical composition compared to undigested animal manures, which may affect the soil microbial ecosystem differently when used as fertilizers. This study investigated the effects of two biogas residues (BR-A and BR-B) and cattle slurry (CS) applied at rates corresponding to 70 kg NH₄⁺-N ha⁻¹ on bacterial community structure and microbial activity in three soils of different texture (a sandy, a clay and an organic clay soil). 16S rRNA genes were targeted in PCR reactions and bacterial community profiles visualized using terminal restriction fragment length polymorphism. General microbial activity was measured as basal respiration (B-resp), substrate-induced respiration (SIR), specific growth rate (μ_SIR), metabolic quotient (qCO₂) and nitrogen mineralization capacity (NMC). Non-metric multidimensional scaling analysis visualized shifts in bacterial community structure related to microbial functions. There were significant differences in bacterial community structure after 120 days of incubation (+20 °C at 70% of WHC) between non-amended (control) and amended soils, especially in the sandy soil, where CS caused a more pronounced shift than biogas residues. Terminal-restriction fragment (TRF) 307, the predominant peak in CS-amended sandy soil, was identified as possibly Bacillus or Streptococcus. TRF 226, the dominant peak in organic soil amended with BR-B, was classified as Rhodopseudomonas. B-resp significantly increased and SIR decreased in all amendments to organic soil compared with the control, potentially indicating decreased efficiency of heterotrophic microorganisms to convert organic carbon into microbial biomass. This was also reflected in an elevated qCO₂ in the organic soil. The μ_SIR level was higher in the sandy soil amended with BR-A than with BR-B or CS, indicating a shift toward species capable of rapidly utilizing glucose. NMC was significantly elevated in the clay and organic soils amended with BR-A and BR-B and in the sandy soil amended with BR-B and CS. Thus, biogas residues and cattle slurry had different effects on the bacterial community structure and microbial activity in the three soils. However, the effects of biogas residues on microbial activities were comparable in magnitude to those of cattle slurry and the bacterial community structure was less affected. Therefore, we do not see any reason not to recommend using biogas residues as fertilizers based on the results presented.     

Soil microbial community response to land-management and depth,related to the degradation of organic matter in English wetlands: Implications for the in situ preservation of archaeological remains

Wetlands are important habitats not only for their unique ecological value but also because they contain organic material that is fundamental to our understanding of precedent landscape and human past. This study compares the effects of two different land-management regimes on metabolic diversity and bacterial community structure with depth in order to relate them to the process of organic matter degradation and the potential for preservation in situ of organic archaeological artefacts in wetland soils. Soil cores were collected at five depths down to 100 cm from two wetlands sites in England. Environmental variables were monitored and the metabolic capabilities of the microbial community were studied using Biolog Ecoplates^®. DNA was extracted from soil, and the bacterial community structure was examined by polymerase chain reaction followed by denaturing gradient gel electrophoresis (PCR-DGGE). To determine compositional changes in the bacterial community with depth, information about specific groups of bacteria at the site with higher water table (Hatfield Moor) was obtained by cloning and sequencing of 16S rRNA genes. Biolog and DGGE analyses showed depth variation and between-site variation. Carbon substrate utilization and bacterial diversity decreased with increasing depth. The wetland soil under an arable regime in which the water levels were kept elevated, showed higher metabolic capability and bacterial richness when compared with the soil under pasture and subjected to long-standing drainage. Cloning and sequencing showed that Proteobacteria and Acidobacteria were the predominant taxa within the soil profile, but there was a clear shift in bacterial community composition with increasing depth as several taxonomic groups (δ-Proteobacteria and Spirochaetes) were only detectable at 50 cm depth. Because the site with a high and stable water table presented higher metabolic activity and bacterial diversity, it may be that saturated conditions and a high water table are not sufficient to guarantee the preservation in situ of organic material such as archaeological artefacts.     

Incongruous variation of denitrifying bacterial communities as soil N level rises in Canadian canola fields

Soil N fertilization stimulates the activity of the soil bacterial species specialized in performing the different steps of the denitrification processes. Different responses of these bacterial denitrifiers to soil N management could alter the efficiency of reduction of the greenhouse gas N₂O into N₂ gas in cultivated fields. We used next generation sequencing to show how raising the soil N fertility of Canadian canola fields differentially modifies the diversity and composition of nitrite reductase (nirK and nirS) and nitrous oxide reductase (nosZ) gene-carrying denitrifying bacterial communities, based on a randomized complete blocks field experiment. Raising soil N levels increased up to 60% the ratio of the nirK to nirS genes, the two nitrite reductase coding genes, in the Brown soil and up to 300% in the Black soil, but this ratio was unaffected in the Dark Brown soil. Raising soil N levels also increased the diversity of the bacteria carrying the nitrite reductase gene nirK (Simpson index, P = 0.0417 and Shannon index, 0.0181), and changed the proportions of the six dominant phyla hosting nirK, nirS, and nosZ gene-carrying bacteria. The level of soil copper (Cu) and the abundance of nirK gene, which codes for a Cu-dependent nitrite reductase, were positively related in the Brown (P = 0.0060, R² = 0.48) and Dark Brown (0.0199, R² = 0.59) soils, but not in the Black soil. The level of total diversity of the denitrifying communities tended to remain constant as N fertilization induced shifts in the composition of these denitrifying communities. Together, our results indicate that higher N fertilizer rate increases the potential risk of nitrous oxide (N₂O) emission from canola fields by promoting the proliferation of the mostly adaptive N₂O-producing over the less adaptive N₂O-reducing bacterial community.     

Influence of mouldboard plough and rotary harrow tillage on microbial biomass and nutrient stocks in two long-term experiments on loess derived Luvisols

The nutrient-specific effects of tillage on microbial activity (basal respiration), microbial biomass (C, N, P, S) indices and the fungal cell-membrane component ergosterol were examined in two long-term experiments on loess derived Luvisols. A mouldboard plough (30 cm tillage depth) treatment was compared with a rotary harrow (8 cm tillage depth) treatment over a period of approximately 40 years. The rotary harrow treatment led to a significant 8% increase in the mean stocks of soil organic C, 6% of total N and 4% of total P at 0–30 cm depth compared with the plough treatment, but had no main effect on the stocks of total S. The tillage effects were identical at both sites, but the differences between the sites of the two experiments were usually stronger than those between the two tillage treatments. The rotary harrow treatment led to a significant increase in the mean stocks of microbial biomass C (+18%), N (+25%), and P (+32%) and to a significant decrease in the stocks of ergosterol (−26%) at 0–30 cm depth, but had no main effect on the stocks of microbial biomass S or on the mean basal respiration rate. The mean microbial biomass C/N (6.4) and C/P (25) ratios were not affected by the tillage treatments. In contrast, the microbial biomass C/S ratio was significantly increased from 34 to 43 and the ergosterol-to-microbial biomass C ratio significantly decreased from 0.20% to 0.13% in the rotary harrow in comparison with the plough treatment. The microbial biomass C-to-soil organic C ratio varied around 2.1% in the plough treatment and declined from 2.6% at 0–10 cm depth to 2.0 at 20–30 cm depth in the rotary harrow treatment. The metabolic quotient qCO₂ revealed exactly the inverse relationships with depth and treatment to the microbial biomass C-to-soil organic C ratio. Rotary harrow management caused a reduction in the microbial turnover in combination with an improved microbial substrate use efficiency and a lower contribution of saprotrophic fungi to the soil microbial community. This contrasts the view reported elsewhere and points to the need for more information on tillage-induced shifts within the fungal community in arable soils.     

Effects of tillage and nitrogen management on soil chemical and physical properties after 23 years of continuous sorghum

Long-term tillage and nitrogen (N) management practices can have a profound impact on soil properties and nutrient availability. A great deal of research evaluating tillage and N applications on soil chemical properties has been conducted with continuous corn (Zea Mays L.) throughout the Midwest, but not on continuous grain sorghum (Sorghum bicolor (L.) Moench). The objective of this experiment was to examine the long-term effects of tillage and nitrogen applications on soil physical and chemical properties at different depths after 23 years of continuous sorghum under no-till (NT) and conventional till (CT) (fall chisel-field cultivation prior to planting) systems. Ammonium nitrate (AN), urea, and a slow release form of urea were surface broadcast at rates of 34, 67, and 135 kg N ha⁻¹. Soil samples were taken to a depth of 15 cm and separated into 2.5 cm increments. As a result of lime applied to the soil surface, soil pH in the NT and CT plots decreased with depth, ranging from 6.9 to 5.7 in the NT plots and from 6.5 to 5.9 in the CT plots. Bray-1 extractable P and NH₄OAc extractable K was 20 and 49 mg kg⁻¹ higher, respectively, in the surface 2.5 cm of NT compared to CT. Extractable Ca was not greatly influenced by tillage but extractable Mg was higher for CT compared to NT below 2.5 cm. Organic carbon (OC) under NT was significantly higher in the surface 7.5 cm of soil compared to CT. Averaged across N rates, NT had 2.7 Mg ha⁻¹ more C than CT in the surface 7.5 cm of soil. Bulk density (Δ_b) of the CT was lower at 1.07 g cm⁻³ while Δ_b of NT plots was 1.13 g cm⁻³. This study demonstrated the effect tillage has on the distribution and concentration of certain chemical soil properties.     

Adsorption of “real” dissolved organic matter on the clay and fine silt fractions of a forested Stagnic Gleysol

Dissolved organic matter (DOM) in soils is partially adsorbed when passing through a soil profile. In most adsorption studies, water soluble organic matter extracted by water or dilute salt solutions is used instead of real DOM gained in situ by lysimeters or ceramic suction cups. We investigated the adsorption of DOM gained in situ from three compartments (forest floor leachate and soil solution from 20 cm (Bg horizon) and 60 cm depth (2Bg horizon)) on the corresponding clay and fine silt fractions (< 6.3 μm, separated together from the bulk soil) of the horizons Ah, Bg, and 2Bg of a forested Stagnic Gleysol by batch experiments. An aliquot of each clay and fine silt fraction was treated with H₂O₂ to destroy soil organic matter. Before and after the experiments, the solutions were characterized by ultra‐violet and fluorescence spectroscopy and analyzed for sulfate, chloride, nitrate, and fluoride. The highest affinity for DOM was found for the Ah samples, and the affinity decreased in the sequence Ah > Bg > 2Bg. Dissolved organic matter in the 2Bg horizon can be regarded as slightly reactive, because adsorption was low. Desorption of DOM from the subsoil samples was reflected more realistically with a non‐linear regression approach than with initial mass isotherms. The results show that the extent of DOM adsorption especially in subsoils is controlled by the composition and by the origin of the DOM used as adsorptive rather than by the mineralogical composition of the soil or by contents of soil organic matter. We recommend to use DOM gained in situ when investigating the fate of DOM in subsoils.     

Bacterial community structure of nirK-bearing denitrifiers and the development of properties of soils in created mitigation wetlands

We investigated the abundance and genetic heterogeneity of bacterial nitrite reductase genes (nir) and soil structural properties in created and natural freshwater wetlands in the Virginia piedmont. Soil attributes included soil organic matter (SOM), total organic carbon (TOC), total nitrogen (TN), pH, gravimetric soil moisture (GSM), and bulk density (D_b). A subset of soil attributes were analyzed across the sites, using euclidean cluster analysis, resulting in three soil condition (SC) groups of increasing wetland soil development (i.e., SC1 < SC2 < SC3; less to more developed or matured) as measured by accumulation of TOC, TN, the increase of GSM, and the decrease of D_b. There were no difference found in the bacterial community diversity between the groups (p = 0.4). NirK gene copies detected ranged between 3.6 × 10⁴ and 3.4 × 10⁷ copies g⁻¹ soil and were significantly higher in the most developed soil group, SC3, than in the least developed soil group, SC1. However, the gene copies were lowest in SC2 that had a significantly higher soil pH (~6.6) than the other two SC groups (~5.3). The same pattern was found in denitrifying enzyme activity (DEA) on a companion study where DEA was found negatively correlated with soil pH. Gene fragments were amplified and products were screened by terminal restriction fragment length polymorphism (T-RFLP) analysis. Among 146 different T-RFs identified, fourteen were dominant and together made up more than 65% of all detected fragments. While SC groups did not relate to whole nirK communities, most soil properties that identified SC groups did significantly correlate to dominant members of the community.     

Biochar soil amendment increased bacterial but decreased fungal gene abundance with shifts in community structure in a slightly acid rice paddy from Southwest China

Biochar’s role on greenhouse gas emission and plant growth has been well addressed. However, there have been few studies on changes in soil microbial community and activities with biochar soil amendment (BSA) in croplands. In a field experiment, biochar was amended at rates of 0, 20 and 40 t ha⁻¹ (C0, C1 and C2, respectively) in May 2010 before rice transplantation in a rice paddy from Sichuan, China. Topsoil (0–15 cm) was collected from the rice paddy while rice harvest in late October 2011. Soil physico-chemical properties and microbial biomass carbon (MBC) and nitrogen (MBN) as well as selected soil enzyme activities were determined. Based on 16S rRNA and 18S rRNA gene, bacterial and fungal community structure and abundance were characterized using terminal-restriction fragment length polymorphism (T-RFLP) combined with clone library analysis, denaturing gradient gel electrophoresis (DGGE) and quantitative real-time PCR assay (qPCR). Contents of SOC and total N and soil pH were increased but bulk density decreased significantly. While no changes in MBC and MBN, gene copy numbers of bacterial 16S rRNA was shown significantly increased by 28% and 64% and that of fungal 18S rRNA significantly decreased by 35% and 46% under BSA at 20 and 40 t ha⁻¹ respectively over control. Moreover, there was a significant decrease by 70% in abundance of Methylophilaceae and of Hydrogenophilaceae with an increase by 45% in Anaerolineae abundance under BSA at 40 t ha⁻¹ over control. Whereas, using sequencing DGGE bands of fungal 18S rRNA gene, some bands affiliated with Ascomycota and Glomeromycota were shown inhibited by BSA at rate of 40 t ha⁻¹. Significant increases in activities of dehydrogenase, alkaline phosphatases while decreased β-glucosidase were also observed under BSA. The results here indicated a shift toward a bacterial dominated microbial community in the rice paddy with BSA.     

Soil tillage effects on monovalent cations (Na+ and K+) in vertisols soil solution

Potassium is an essential macronutrient for plants; it is characterized by increased photosynthetic activity by ensuring a better utilization of light energy, also acts as a regulator of cell osmotic pressure, decreasing transpiration and helping to maintain cell turgidity. However, the sodium is not an essential element for plants, although it is beneficial to certain crops, in some instances can replace the potassium and osmotic regulation making and turgidity of the cells, this effect is greatest when the supply of potassium is deficient (Wild, 1992). Both elements, in periods of aridity, delayed the wilting of plants to maintain cellular osmotic potential and in cold periods, they lower the freezing point of sap (Navarro and Navarro, 2000).This is an experiment to study the influence of soil management techniques on the monovalent cations in soil solutions at different depths. The cropping systems studied are conventional tillage, minimum tillage and direct drilling.Conventional tillage releases more Na⁺ and K⁺ to the soil solution than the conservative techniques. In the case of Na⁺, the conventional tillage soil solution has an average concentration of 0.563 meq/L compared to 0.303 meq/L of minimum tillage and 0.340 meq/L of direct drilling. As for the K⁺, the soil solution concentration of conventional tillage is 0.097 meq/L, compared to 0.079 meq/L of the solution of minimum tillage and 0.056 meq/L of direct drilling.The behavior for the two cations studied is distinct at different depths. The Na⁺ is more abundant in water samples of soil taken in depth. Therefore, the salinization risk may take place in the subsoil, especially in conventional tillage where the Bw₁ horizon values are three times higher than in the Ap horizon, while the K⁺ is more abundant in the surface horizon. Conventional tillage and minimum tillage techniques, in the Ap horizon have a similar pattern with a K⁺ concentration average of 0.15 meq/L and 0.14 meq/L, respectively, resulting in lower values for direct drilling.Studies on clay soils have not been performed previously because of the difficulty presented by these soils when soil solution extracted for analysis. We analyzed the monovalent cations (sodium and potassium) from soil solution; because the soil solution is the immediate source of sodium and potassium for plants.