[show abstract][hide abstract] ABSTRACT: This study compares a traditional agricultural approach to minimise N pollution of groundwater (incorporation of crop residues) with applications of small amounts of biodiesel co-product (BCP) to arable soils. Loss of N from soil to the aqueous phase was shown to be greatly reduced in the laboratory, mainly by decreasing concentrations of dissolved nitrate-N. Increases in soil microbial biomass occurred within 4 days of BCP application-indicating rapid adaptation of the soil microbial community. Increases in biomass-N suggest that microbes were partly mechanistic in the immobilisation of N in soil. Straw, meadow-grass and BCP were subsequently incorporated into experimental soil mesocosms of depth equal to plough layer (23 cm), and placed in an exposed netted tunnel to simulate field conditions. Leachate was collected after rainfall between the autumn of 2009 and spring of 2010. Treatment with BCP resulted in less total-N transferred from soil to water over the entire period, with 32.1, 18.9, 13.2 and 4.2 mg N kg(-1) soil leached cumulatively from the control, grass, straw and BCP treatments, respectively. More than 99 % of nitrate leaching was prevented using BCP. Accordingly, soils provided with crop residues or BCP showed statistically significant increases in soil N and C compared to the control (no incorporation). Microbial biomass, indicated by soil ATP concentration, was also highest for soils given BCP (p < 0.05). These results indicate that field-scale incorporation of BCP may be an effective method to reduce nitrogen loss from agricultural soils, prevent nitrate pollution of groundwater and augment the soil microbial biomass.
Water Air and Soil Pollution 01/2014; 225:1831. · 1.75 Impact Factor
[show abstract][hide abstract] ABSTRACT: Many soil microbes exist in biofilms. These biofilms are typified by variable quantities of extracellular polymeric substances (EPS: predominantly polysaccharides, glycoconjugates, and proteins) and the embedded microbial cells. A method to measure soil-EPS (the biofilm exclusive of microbial cells) has not yet been described. The present work investigates the potential of five extraction methods to estimate changes in soil-EPS content. A rationale for selection of appropriate EPS extraction and methodology is discussed, including the crucial consideration of both intracellular and extracellular contamination.
EPS was developed in situ by provision of labile C (glycerol) to the microbial biomass of a moist soil and then applying desiccation stress. Only two out of the five extraction methods showed statistically significant increases in polysaccharide production responding to substrate addition. Humified organic matter, estimated by its humic acid equivalent (HAE) was used to indicate the degree of extracellular contamination, and/or creation of humic artefacts – both of which affect detection of changes in EPS. The HAE concentration was very high when applying original and modified methods designed to extract glomalin related soil protein (GRSP). Extraction methods involving heating with dilute sulphuric acid appeared to overestimate EPS-polysaccharide. Using microbial ATP as an indicator of cell-lysis, confidence could only be ascribed to EPS extraction with cation exchange resin. Using this method, the expected increases in EPS-polysaccharide were clearly apparent. The HAE/protein ratios of EPS extracts were also lowest with cation exchange – indicating this method did not cause excessive contamination from humified soil organic matter or create related artefacts.
Soil Biology and Biochemistry 01/2014; · 3.65 Impact Factor
[show abstract][hide abstract] ABSTRACT: Elevated CO2 respiration rates have been observed in soils treated with complex mixtures versus single low molecular weight (LMW) organic substrates, and it has been postulated that a more diverse range of soil microorganisms responds to increasingly complex mixtures of LMW organic substrates. To test this hypothesis, 13C-labelled substrates (glycine, an amino acid mixture and an extract of water soluble compounds from plant roots) were applied at 15 μg C g−1 soil to an arable top soil. The soils were incubated and destructively sampled after 8, 24, 48, 120 and 240 h, and the 13C content of biomarker PLFA for Gram negative bacteria, Gram positive bacteria, Actinobacteria and fungi was determined. There was no significant increase in the concentration of the biomarker PLFA, apart from Actinobacteria at the end of the incubation (120 and 240 h). However there were significant changes in total PLFA concentration due to increases in the 16:0 and 18:0 PLFA, which cannot be assigned to specific functional groups of microorganisms. 13C incorporation into the biomarker PLFA of all microbial groups was significant at every time point, but more 13C was determined in the biomarker PLFA of all microbial groups after the application of the amino acid mixture compared to glycine. Calculations of the proportion of the incorporated of 13C in the different biomarker PLFA suggested the routing of substrate 13C between the microbial groups over time. This was related to the broad functional ecology (‘r’ or ‘K’ strategy) of the different microbial groups. In conclusion, we observed that the response of all microbial groups (detected as 13C incorporation) was increased by the addition of more complex mixtures of LMW organic substrates, but that different microbial groups responded differently over time.
Soil Biology and Biochemistry 01/2013; 34:57-64. · 3.65 Impact Factor
[show abstract][hide abstract] ABSTRACT: The aim of this work was to determine the magnitude of the priming effect, i.e. short-term changes in the rate (negative or positive) of mineralisation of native soil organic carbon (C), following addition of biochars. The biochars were made from Miscanthus giganteus, a C4 plant, naturally enriched with 13C. The biochars were produced at 350 °C (biochar350) and 700 °C (biochar700) and applied with and without ryegrass as a substrate to a clay-loam soil at pH 3.7 and 7.6. A secondary aim was to determine the effect of ryegrass addition on the mineralisation of the two biochars.After 87 days, biochar350 addition caused priming effects equivalent to 250 and 319 μg CO2–C g−1 soil, in the low and high pH soil, respectively. The largest priming effects occurred at the start of the incubations. The size of the priming effect was decreased at higher biochar pyrolysis temperatures, which may be a way of controlling priming effects following biochar incorporation to soil, if desired. The priming effect was probably induced by the water soluble components of the biochar. At 87 days of incubation, 0.14% and 0.18% of biochar700 and 0.61% and 0.84% of biochar350 were mineralized in the low and high pH soil, respectively. Ryegrass addition gave an increased biochar350 mineralisation of 33% and 40%, and increased biochar700 at 137% and 70%, in the low and high pH soils, respectively. Certainly, on the basis of our results, if biochar is used to sequester carbon a priming effect may occur, increasing CO2–C evolved from soil and decreasing soil organic C. However, this will be more than compensated for by the increased soil C caused by biochar incorporation. A similar conclusion holds for accelerated mineralisation of biochar due to incorporation of fresh labile substrates. We consider that our results are the first to unequivocally demonstrate the initiation, progress and termination of a true positive priming effect by biochar on native soil organic C.
Soil Biology and Biochemistry. 01/2011; 43(11):2304-2314.
[show abstract][hide abstract] ABSTRACT: The efficiency of the fumigation extraction method on the determination of soil microbial biomass carbon and ninhydrin-N was tested in three different soils (UK grassland, UK arable, Chinese arable) amended with black carbon (biochar or activated charcoal). Addition of activated charcoal to soil resulted in a significant decrease in K2SO4 extractable carbon and ninhydrin-N in all three soils, whereas the addition of biochar generally did not. A lower concentration of the extraction reagent (0.05 M vs. 0.5 M K2SO4) resulted in a significantly lower extraction efficiency in the grassland soil. The extraction efficiency of organic carbon was more affected by black carbon than that of ninhydrin-N, which resulted in a decreased biomass C/ninhydrin-N ratio. The impact of black carbon on the extraction efficiency of soil microbial biomass depended on the type of black carbon, on the concentration of the extraction medium and on soil type.
[show abstract][hide abstract] ABSTRACT: The advantages of no-tillage (NT) over conventional tillage (CT) systems in improving soil quality are generally accepted, resulting from benefits in soil physical, chemical and biological properties. However, most evaluations have only considered surface soil layers (maximum 0–30 cm depth), and values have not been corrected to account for changes in soil bulk density. The objective of this study was to estimate a more realistic contribution of the NT to soil fertility, by evaluating C- and N-related soil parameters at the 0–60 cm depth in a 20-year experiment established on an oxisol in southern Brazil, with a soybean (summer)/wheat (winter) crop succession under NT and CT. At full flowering of the soybean crop, soil samples were collected at depths of 0–5, 5–10, 10–20, 20–30, 30–40, 40–50 and 50–60 cm. For the overall 0–60 cm layer, correcting the values for soil bulk density, NT significantly increased the stocks of C (18%) and N (16%) and microbial biomass C (35%) and N (23%) (MB-C and -N) in comparison to CT. Microbial basal respiration and microbial quotient (qMic) were also significantly increased under NT. When compared with CT, NT resulted in gains of 0.8 Mg C ha−1 yr−1 (67% of which was in the 0–30 cm layer) and 70 kg N ha−1 yr−1 (73% in the 0–30 cm layer). In the 0–5-cm layer, MB-C was 82% higher with NT than with CT; in addition, the 0–30 cm layer accumulated 70% of the MB-C with NT, and 58% with CT. In comparison to CT, the NT system resulted in total inputs of microbial C and N estimated at 38 kg C ha−1 yr−1 and 1.5 kg N ha−1 yr−1, respectively. Apparently, N was the key nutrient limiting C and N stocks, and since adoption of NT resulted in a significant increase of N in soils which were deficient in N, efforts should be focused on increasing N inputs on NT systems.
[show abstract][hide abstract] ABSTRACT: The drying and re-wetting of soils can result in the modification of the amounts and forms of nutrients which can transfer,
via leachate, from the soil to surface waters. We tested, under laboratory conditions, the hypothesis that the rate of re-wetting
of a dried soil affects the solubilisation and concentrations of different forms of phosphorus (P) in leachate. A portion
of grassland pelostagnogley soil (sieved moist <2mm) was dried at 35°C and another portion maintained at approximately 40%
water-holding capacity. Water (25ml) was added at ten regularly spaced time intervals in 2.5-ml aliquots to the surfaces
of both soils over periods of 0, 2, 4, 24 and 48h, resulting in different rates of application. The leachate was collected
and analysed for dissolved (<0.45μm) and particulate total P and molybdate reactive and unreactive P. The rate of re-wetting
significantly changed the concentrations of P, especially dissolved forms, in the leachate. Dissolved P concentrations were
highest in leachate from the 2-h treatment, while particulate P concentrations were highest in the 0-h treatment leachate.
In all cases, most P was unreactive and, therefore, likely to be in an organic form. Soil drying decreased microbial biomass,
but this could not be directly linked to an increase of P in leachate. These results suggest that changes in patterns of rainfall
frequency and intensity predicted by climate change scenarios could significantly affect the quantities of P leached from
Biology and Fertility of Soils 01/2009; 45(6):635-643. · 2.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: Our aim was to determine whether the smaller biomasses generally found in low pH compared to high pH arable soils under similar management are due principally to the decreased inputs of substrate or whether some factor(s) associated with pH are also important. This was tested in a soil incubation experiment using wheat straw as substrate and soils of different pHs (8.09, 6.61, 4.65 and 4.17). Microbial biomass ninhydrin-N, and microbial community structure evaluated by phospholipid fatty acids (PLFAs), were measured at 0 (control soil only), 5, 25 and 50 days and CO2 evolution up to 100 days. Straw addition increased biomass ninhydrin-N, CO2 evolution and total PLFA concentrations at all soil pH values. The positive effect of straw addition on biomass ninhydrin-N was less in soils of pH 4.17 and 4.65. Similarly total PLFA concentrations were smallest at the lowest pH. This indicated that there is a direct pH effect as well as effects related to different substrate availabilities on microbial biomass and community structure. In the control soils, the fatty acids 16:1ω5, 16:1ω7c, 18:1ω7c&9t and i17:0 had significant and positive linear relationships with soil pH. In contrast, the fatty acids i15:0, a15:0, i16:0 and br17:0, 16:02OH, 18:2ω6,9, 17:0, 19:0, 17:0c9,10 and 19:0c9,10 were greatest in control soils at the lowest pHs. In soils given straw, the fatty acids 16:1ω5, 16:1ω7c, 15:0 and 18:0 had significant and positive linear relationships with pH, but the concentration of the monounsaturated 18:1ω9 PLFA decreased at the highest pHs. The PLFA profiles indicative of Gram-positive bacteria were more abundant than Gram-negative ones at the lowest pH in control soils, but in soils given straw these trends were reversed. In contrast, straw addition changed the microbial community structures least at pH 6.61. The ratio: [fungal PLFA 18:2w6,9]/[total PLFAs indicative of bacteria] indicated that fungal PLFAs were more dominant in the microbial communities of the lowest pH soil. In summary, this work shows that soil pH has marked effects on microbial biomass, community structure, and response to substrate addition.
[show abstract][hide abstract] ABSTRACT: The preparation of soil for measurement of properties such as microbial biomass P involves the removal of plant roots. Any soil attached to the roots (root-attached soil) is also removed. In a very poorly drained silty clay loam under grassland we found that the root-attached soil contained more than twice the quantity of bicarbonate extractable P than the bulk soil. Discarding this root-attached soil could potentially result in underestimation of bicarbonate extractable P. We also showed that preferential inclusion of deeper soil due to variability of root density with depth is likely to result in underestimation of soil bicarbonate extractable P in fumigated and unfumigated soil samples. Additionally we investigated a conventional and alternative (rapid) sod preparation technique that might affect the accuracy of measurement of sod bicarbonate extractable P as part of a microbial biomass P measurement. Preparation technique made no significant difference to the quantity of P recovered.
Soil Science Society of America Journal 01/2009; 73(6):1861-1863. · 1.82 Impact Factor
[show abstract][hide abstract] ABSTRACT: Soil biological processes contribute stability against physical disruption. We present an approach of step-wise fragmentation to assess the role that microbes and organic matter have on soil aggregate stabilisation. Compared to slaking and ultrasound procedures, the approach has a low impact on the microbial biomass. It also does not impose a severe drying stress. Grassland soil was found to be more stable than arable soil. Further examination of the arable soil revealed that increased disruption by shaking caused unstable microaggregates 53–250 μm in size to fragment, leaving a higher proportion of stable microaggregates in this size range. Carbohydrates, C:N, and basal respiration were found to be higher in the stable microaggregates than the other size fractions. Our results indicate that a distinct size range of soil aggregates exists in which microbial stabilisation dominates. This contradicts other research and questions the usefulness of measuring the biological properties of aggregate size fractions without understanding the physical effects of the fractionation procedure.
[show abstract][hide abstract] ABSTRACT: Effects of changing pH along a natural continuous gradient of a UK silty-loam soil were investigated. The site was a 200 m soil transect of the Hoosfield acid strip (Rothamsted Research, UK) which has grown continuous barley for more than 100 years. This experiment provides a remarkably uniform soil pH gradient, ranging from about pH 8.3 to 3.7. Soil total and organic C and the ratio: (soil organic C)/(soil total N) decreased due to decreasing plant C inputs as the soil pH declined. As expected, the CaCO3 concentration was greatest at very high pH values (pH > 7.5). In contrast, extractable Al concentrations increased linearly (R2 = 0.94, p < 0.001) from below about pH 5.4, while extractable Mn concentrations were largest at pH 4.4 and decreased at lower pHs. Biomass C and biomass ninhydrin-N were greatest above pH 7. There were statistically significant relationships between soil pH and biomass C (R2 = 0.80, p < 0.001), biomass ninhydrin-N (R2 = 0.90, p < 0.001), organic C (R2 = 0.83, p < 0.001) and total N (R2 = 0.83, p < 0.001), confirming the importance of soil organic matter and pH in stimulating microbial biomass growth. Soil CO2 evolution increased as pH increased (R2 = 0.97, p < 0.001). In contrast, the respiratory quotient (qCO2) had the greatest values at either end of the pH range. This is almost certainly a response to stress caused by the low p. At the highest pH, both abiotic (from CaCO3) and biotic Co2 will be involved so the effects of high pH on biomass activity are confounded. Microbial biomass and microbial activity tended to stabilise at pH values between about 5 and 7 because the differences in organic C, total N and Al concentrations within this pH range were small. This work has established clear relationships between microbial biomass and microbial activity over an extremely wide soil pH range and within a single soil type. In contrast, most other studies have used soils of both different pH and soil type to make similar comparisons. In the latter case, the effects of soil pH on microbial properties are confounded with effects of different soil types, vegetation cover and local climatic conditions.
[show abstract][hide abstract] ABSTRACT: Soil organic matter comprises all dead plant and animal residues, from the most recent inputs to the most intensively humified. We have found that traces of fresh substrates at microg g(-1) soil concentrations (termed 'trigger molecules') activate the biomass to expend more energy than is contained in the original 'trigger molecules'. In contrast, we suggest that the rate limiting step in soil organic matter mineralisation is independent of microbial activity, but is governed by abiological processes (which we term the Regulatory Gate theory). These two findings have important implications for our understanding of carbon mineralisation in soil, a fundamental process in the sequestration of soil organic matter.
[show abstract][hide abstract] ABSTRACT: Biological and chemical stabilization of organic C was assessed in soils sampled from the long-term experiments at Rothamsted (UK), representing a wide range of carbon inputs and managements by extracting labile, non-humified organic matter (NH) and humic substances (HS). Four sequentially extracted humic substances fractions of soil organic matter (SOM) were extracted and characterized before and after a 215-day laboratory incubation at 25 degrees C from two arable soils, a woodland soil and an occasionally stubbed soil. The fractions corresponded to biochemically stabilised SOM extracted in 0.5M NaOH (free fulvic acids (FA) and humic acids (HA)) and chemically plus biochemically stabilised SOM extracted from the residue with 0.1M Na4P2O7 plus 0.1M NaOH (bound FA and HA). Our aim was to investigate the effects of chemical and biochemical stabilization on carbon sequestration. The non-humic to humic (NH/H) C ratio separated the soils into two distinct groups: arable soils (unless fertilised with farmyard manure) had an NH/H C ratio between 1.05 and 0.71, about twice that of the other soils (0.51-0.26). During incubation a slow, but detectable, decrease in the NH/H C ratio occurred in soils of C input equivalent or lower to 4Mgha(-1)y(-1), whereas the ratio remained practically constant in the other soils. Before incubation the free to bound humic C ratio increased linearly (R2=0.91) with C inputs in the soils from the Broadbalk experiment and decreased during incubation, showing that biochemical stabilization is less effective than chemical stabilization in preserving humic C. Changes in delta13C and delta15N after incubation were confined to the free FA fractions. The delta13C of free FA increased by 1.48 and 0.80 per thousand, respectively, in the stubbed and woodland soils, indicating a progressive biological transformation. On the contrary, a decrease was observed for the bound FA of both soils. Concomitantly, a Deltadelta15N of up to +3.52 per thousand was measured after incubation in the free FA fraction and a -2.58 Deltadelta15N in the bound FA. These changes, which occurred during soil incubation in the absence of C inputs, indicate that free FA fractions were utilised by soil microorganisms, and bound FA were decomposed and replaced, in part, by newly synthesized FA. The 13CPMAS-TOSS NMR spectra of free HA extracted before and after 215 days of incubation were mostly unchanged. In contrast, changes were evident in bound HA and showed an increase in aromatic C after incubation.
[show abstract][hide abstract] ABSTRACT: Soil organic matter is extensively humified; some fractions existing for more than 1000 years. The soil microbial biomass is surrounded by about 50 times its mass of soil organic matter, but can only metabolize it very slowly. Paradoxically, even if more than 90% of the soil microbial biomass is killed, the mineralization of soil organic matter proceeds at the same rate as in an unperturbed soil. Here we show that soil organic matter mineralization is independent of microbial biomass size, community structure or specific activity. We suggest that the rate limiting step is governed by abiological processes (which we term the Regulatory Gate hypothesis), which convert non-bioavailable soil organic matter into bioavailable soil organic matter, and cannot be affected by the microbial population. This work challenges one of the long held theories in soil microbiology proposed by Winogradsky, of the existence of autochthonous and zymogenous microbial populations. This has significant implications for our understanding of carbon mineralization in soils and the role of soil micro-organisms in the global carbon cycle. Here we describe experiments designed to determine if the Regulatory Gate operates. We conclude that there is sufficient experimental evidence for it to be offered as a working hypothesis.
Soil Biology and Biochemistry 01/2008; 40(1):61–73. · 3.65 Impact Factor
[show abstract][hide abstract] ABSTRACT: Most investigations into the effects of changing soil pH on microbial activity use, from necessity, soils taken from different sites so that soil physical and chemical properties are confounded. Studies along continuous gradients of soil pH within a single soil type are rare, simply because so few exist, in UK or even worldwide. Here we report measurements of mineralisation of native organic matter and added arginine along a continuous soil pH gradient (range about pH 3.7–8.3) of a UK silty clay loam soil (Chromic Luvisol or Typic Paleudalf). The soil has been maintained under constant management for more than 100 years, with winter wheat sown annually. The soil NH4+-N concentration was maximal at the lowest pH (pH 3.7), declining exponentially until pH 5.5 and remaining negligible thereafter. However, unexpectedly, soil NO3−-N concentration was also maximal at pH 3.7 and was significantly negatively correlated with increasing pH thereafter. To investigate these unexpected NO3−-N results, arginine was added as a labile source of organic N and its extent of ammonification and nitrification measured at soil pHs 3.79, 4.42, 6.08 and 7.82. While arginine ammonification was apparently greatest at pHs 3.79 and 4.42, similar to mineralisation of soil organic N, nitrification of this added N was greatest at soil pH 7.82 and least at pH 3.79, the reverse of the situation with soil organic N, but much more in line with what was expected. It was concluded that the decline in soil NO3−-N with increasing pH in the unamended soils was an artefact, caused by increasing plant uptake of NO3−-N as yield increased, rather than a true effect of low pH increasing nitrification of soil organic N. Our results differ from most previous studies, which showed poor correlations between soil pH and arginine mineralisation. This was attributed to our use of much longer incubation times (up to 50 days) than usually employed. Under our conditions, arginine was therefore shown to be a useful model for mineralisation of labile soil organic N.
Soil Biology and Biochemistry 01/2008; 40(3):797-802. · 3.65 Impact Factor