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  • 1
    Language: English
    In: Soil Biology and Biochemistry, September 2010, Vol.42(9), pp.1363-1371
    Description: In this re-evaluation of our 10-year old paper on priming effects, I have considered the latest studies and tried to identify the most important needs for future research. Recent publications have shown that the increase or decrease in soil organic matter mineralization (measured as changes of CO efflux and N mineralization) actually results from interactions between living (microbial biomass) and dead organic matter. The priming effect (PE) is not an artifact of incubation studies, as sometimes supposed, but is a natural process sequence in the rhizosphere and detritusphere that is induced by pulses or continuous inputs of fresh organics. The intensity of turnover processes in such hotspots is at least one order of magnitude higher than in the bulk soil. Various prerequisites for high-quality, informative PE studies are outlined: calculating the budget of labeled and total C; investigating the dynamics of released CO and its sources; linking C and N dynamics with microbial biomass changes and enzyme activities; evaluating apparent and real PEs; and assessing PE sources as related to soil organic matter stabilization mechanisms. Different approaches for identifying priming, based on the assessment of more than two C sources in CO and microbial biomass, are proposed and methodological and statistical uncertainties in PE estimation and approaches to eliminating them are discussed. Future studies should evaluate directions and magnitude of PEs according to expected climate and land-use changes and the increased rhizodeposition under elevated CO as well as clarifying the ecological significance of PEs in natural and agricultural ecosystems. The conclusion is that PEs – the interactions between living and dead organic matter – should be incorporated in models of C and N dynamics, and that microbial biomass should regarded not only as a C pool but also as an active driver of C and N turnover.
    Keywords: Priming Effect ; Microbial Activity ; Hotspots ; Rhizosphere ; Detritusphere ; C and N Modeling ; Soil Organic Matter Turnover ; Enzyme Activities ; 14c ; 13c ; 15n ; Substrate Availability ; C Sequestration ; Elevated Co2 ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 2
    Language: English
    In: Soil Biology and Biochemistry, March, 2014, Vol.70, p.229(8)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.soilbio.2013.12.021 Byline: Yakov Kuzyakov, Irina Bogomolova, Bruno Glaser Abstract: Stability and transformation products of incomplete combustion of vegetation or fossil fuel, frequently called pyrogenic or black carbon and of biochar in soil, remains unknown mainly because of their high recalcitrance compared to other natural substances. Therefore, direct estimations of biochar decomposition and transformations are difficult because 1) changes are too small for any relevant experimental period and 2) due to methodological constraints (ambiguity of the origin of investigated compounds). We used.sup.14C-labeled biochar to trace its decomposition to CO.sub.2 during 8.5 years and transformation of its chemical compounds: neutral lipids, glycolipids, phospholipids, polysaccharides and benzenepolycarboxylic acids (BPCA). .sup.14C-labeled biochar was produced by charring.sup.14C-labeled Lolium residues. We incubated the.sup.14C-labeled biochar in a Haplic Luvisol and in loess for 8.5 years under controlled conditions. In total only about 6% of initially added biochar were mineralized to CO.sub.2 during the 8.5 years. This is probably the slowest decomposition obtained experimentally for any natural organic compound. The biochar decomposition rates estimated by.sup.14CO.sub.2 efflux between the 5th and 8th years were of 7 x 10.sup.-4 % per day. This corresponds to less than 0.3% per year under optimal conditions and is about 2.5 times slower as reported from the previous shorter study (3.5 years). After 3.5 years of incubation, we analyzed.sup.14C in dissolved organic matter, microbial biomass, and sequentially extracted neutral lipids, glycolipids, phospholipids, polysaccharides and BPCA. Biochar-derived C (.sup.14C) in microbial biomass ranged between 0.3 and 0.95% of the.sup.14C input. Biochar-derived C in all lipid fractions was less than 1%. Over 3.5 years, glycolipids and phospholipids were decomposed 1.6 times faster (23% of their initial content per year) compared to neutral lipids (15% year.sup.-1). Polysaccharides contributed ca. 17% of the.sup.14C activity in biochar. The highest portion of.sup.14C in the initial biochar (87%) was in BPCA decreasing only 7% over 3.5 years. Condensed aromatic moieties were the most stable fraction compared to all other biochar compounds and the high portion of BPCA in biochar explains its very high stability and its contribution to long-term C sequestration in soil. Our new approach for analysis of biochar stability combines.sup.14C-labeled biochar with.sup.14C determination in chemical fractions allowed tracing of transformation products not only in released CO.sub.2 and in microbial biomass, but also evaluation of decomposition of various biochar compounds with different chemical properties. Author Affiliation: (a) Department of Soil Science of Temperate Ecosystems, University of Gottingen, 37077 Gottingen, Germany (b) Department of Agricultural Soil Science, University of Gottingen, 37077 Gottingen, Germany (c) Department of Soil Biogeochemistry, Institute of Agronomy and Nutritional Sciences, Martin-Luther University Halle-Wittenberg, Von-Seckendorff-Platz 3, 06120 Halle, Germany Article History: Received 5 September 2013; Revised 20 December 2013; Accepted 23 December 2013
    Keywords: Polysaccharides -- Chemical Properties ; Polysaccharides -- Analysis ; Lipids -- Chemical Properties ; Lipids -- Analysis ; Fossil Fuels -- Chemical Properties ; Fossil Fuels -- Analysis ; Loess -- Chemical Properties ; Loess -- Analysis ; Soil Carbon -- Chemical Properties ; Soil Carbon -- Analysis
    ISSN: 0038-0717
    Source: Cengage Learning, Inc.
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  • 3
    Language: English
    In: Soil Biology and Biochemistry, April 2015, Vol.83, pp.184-199
    Description: Soils are the most heterogeneous parts of the biosphere, with an extremely high differentiation of properties and processes within nano- to macroscales. The spatial and temporal heterogeneity of input of labile organics by plants creates microbial hotspots over short periods of time – the hot moments. We define microbial hotspots as small soil volumes with much faster process rates and much more intensive interactions compared to the average soil conditions. Such hotspots are found in the rhizosphere, detritusphere, biopores (including drilosphere) and on aggregate surfaces, but hotspots are frequently of mixed origin. Hot moments are short-term events or sequences of events inducing accelerated process rates as compared to the average rates. Thus, . For this hotspot concept we extensively reviewed and examined the localization and size of hotspots, spatial distribution and visualization approaches, transport of labile C to and from hotspots, lifetime and process intensities, with a special focus on process rates and microbial activities. The fraction of active microorganisms in hotspots is 2–20 times higher than in the bulk soil, and their specific activities (i.e. respiration, microbial growth, mineralization potential, enzyme activities, RNA/DNA ratio) may also be much higher. The duration of hot moments in the rhizosphere is limited and is controlled by the length of the input of labile organics. It can last a few hours up to a few days. In the detritusphere, however, the duration of hot moments is regulated by the output – by decomposition rates of litter – and lasts for weeks and months. Hot moments induce succession in microbial communities and intense intra- and interspecific competition affecting C use efficiency, microbial growth and turnover. The faster turnover and lower C use efficiency in hotspots counterbalances the high C inputs, leading to the absence of strong increases in C stocks. Consequently, the . Maintenance of stoichiometric ratios by accelerated microbial growth in hotspots requires additional nutrients (e.g. N and P), causing their microbial mining from soil organic matter, i.e. priming effects. Consequently, . We estimated the contribution of the hotspots to the whole soil profile and suggested that, irrespective of their volume, the hotspots are mainly responsible for the ecologically relevant processes in soil. By this review, we raised the importance of concepts and ecological theory of distribution and functioning of microorganisms in soil.
    Keywords: C and N Cycles ; Microbial Activities ; Priming Effects ; Microbial Successions ; Subsoil Processes ; Active Microorganisms ; Microbiota-Habitat Interactions ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 4
    Language: English
    In: Soil Biology and Biochemistry, Dec, 2013, Vol.67, p.192(20)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.soilbio.2013.08.024 Byline: Evgenia Blagodatskaya, Yakov Kuzyakov Abstract: Microbial functioning refers to microbial activity because only the active microorganisms drive biogeochemical processes. Despite the importance of active microorganisms, most methods focus on estimating total microbial biomass and fail to evaluate its active fraction. At first, we have described the differences among the active, potentially active, and dormant microbial states in soil and suggested threshold values of parameters for their identification. Secondly, we critically reviewed the ability of a broad range of approaches to estimate and characterize the active and the potentially active microorganisms in soil. Following approaches were evaluated: plate count and microbial cultures; direct microscopy combined with cell staining; ATP, PLFA, DNA and RNA content; microarray analyses; PCR-based approaches; stable isotope probing; soil proteomics, enzymes activity; and various approaches based on respiration and substrate utilization. The "static" approaches, mainly based on the single-stage determination of cell components (ATP, DNA, RNA, and molecular biomarkers), detect well the presence of microorganisms and total biomass, but they fail to evaluate the active part and consequently the functions. In contrast, the dynamic approaches, estimating the changes of these parameters during microbial growth and based on process rates: substrate utilization and product formation, e.g., respiration, help to evaluate active microbial biomass and relate it to specific process rates. Based on a comparison of all approaches for their universality (possibility to analyze active, potentially active and dormant microorganisms), we concluded that 1) direct microscopy with complementary stains, 2) a combination of RNA-based FISH with staining of total microbial biomass, and 3) approaches based on microbial growth were the most advantageous and allowed simultaneous quantitative estimation of active, potentially active, and dormant microorganisms in soil. The active microorganisms compose only about 0.1-2% of the total microbial biomass and very seldom exceed 5% in soils without input of easily available substrates. Nonetheless, the fraction of potentially active microorganisms (ready to start utilization of available substrates within few hours) is much higher, contributing between 10 and 40% (up to 60%) of the total microbial biomass. Therefore, we emphasize the role of potentially active microorganisms with quick response to fluctuating substrate input in soil microhabitats and hotspots. The transition from the potentially active to the active state occurs in minutes to hours, but the shift from dormant to active state takes anywhere from hours to days. Despite very fast activation, the reverse process - fading to the potentially active and dormant stage - requires a much longer period and is very different for individual criteria: ATP, DNA, RNA, enzyme production, respiration rates. This leads to further difficulties in the estimation of the active part of microbial community by methods based on these parameters. Consequently, the standardization, further elaboration, and broad application of approaches focused on the portion of active microorganisms in soil and their functions are urgently needed. We conclude that because active microorganisms are the solely microbial drivers of main biogeochemical processes, analyses of the active and potentially active fractions are necessary in studies focused on soil functions. Author Affiliation: (a) Dept. of Soil Science of Temperate Ecosystems, University of Gottingen, Germany (b) Institute of Physicochemical and Biological Problems in Soil Science, Russian Academy of Sciences, Pushchino, Russia (c) Dept. of Agricultural Soil Science, University of Gottingen, Germany Article History: Received 16 May 2013; Revised 6 August 2013; Accepted 30 August 2013
    Keywords: Soil Microbiology -- Analysis
    ISSN: 0038-0717
    Source: Cengage Learning, Inc.
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  • 5
    Language: English
    In: Soil Biology and Biochemistry, 2015, Vol.90, p.87(14)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.soilbio.2015.07.021 Byline: Anna Gunina, Yakov Kuzyakov Abstract: Sugars are the most abundant organic compounds in the biosphere because they are monomers of all polysaccharides. We summarize the results of the last 40 years on the sources, content, composition and fate of sugars in soil and discuss their main functions. We especially focus on sugar uptake, utilization and recycling by microorganisms as this is by far the dominating process of sugar transformation in soil compared to sorption, leaching or plant uptake. Moreover, sugars are the most important carbon (C) and energy source for soil microorganisms. Two databases have been created. The 1st database focused on the contents of cellulose, non-cellulose, hot-water and cold-water extractable sugars in soils (348 data, 32 studies). This enabled determining the primary (plant-derived) and secondary (microbially and soil organic matter (SOM) derived) sources of carbohydrates in soil based on the galactose + mannose/arabinose + xylose (GM/AX) ratio. The 2nd database focused on the fate of sugar C in soils (734 data pairs, 32 studies using.sup.13C or.sup.14C labeled sugars)..sup.13C and.sup.14C dynamics enabled calculating the: 1) initial rate of sugar mineralization, 2) mean residence time (MRT) of C of the applied sugars, and 3) MRT of sugar C incorporated into 3a) microbial biomass and 3b) SOM. The content of hexoses was 3-4 times higher than pentoses, because hexoses originate from plants and microorganisms. The GM/AX ratio of non-cellulose sugars revealed a lower contribution of hexoses in cropland and grassland (ratio 0.7-1) compare to forest (ratio 1.5) soils. .sup.13C and.sup.14C studies showed very high initial rate of glucose mineralization (1.1% min.sup.-1) and much higher rate of sugars uptake by microorganisms from the soil solution. Considering this rate along with the glucose input from plants and its content in soil solution, we estimate that only about 20% of all sugars in soil originate from the primary source - decomposition of plant litter and rhizodeposits. The remaining 80% originates from the secondary source - microorganisms and their residues. The estimated MRT of sugar C in microbial biomass was about 230 days, showing intense and efficient internal recycling within microorganisms. The assessed MRT of sugar C in SOM was about 360 days, reflecting the considerable accumulation of sugar C in microbial residues and its comparatively slow external recycling. The very rapid uptake of sugars by microorganisms and intensive recycling clearly demonstrate the importance of sugars for microbes in soil. We speculate that the most important functions of sugars in soil are to maintain and stimulate microbial activities in the rhizosphere and detritusphere leading to mobilization of nutrients by accelerated SOM decomposition - priming effects. We conclude that the actual contribution of sugar C (not only whole sugar molecules, which are usually determined) to SOM is much higher than the 10 [+ or -] 5% commonly measured based on their content. Author Affiliation: (a) Department of Agricultural Soil Science, Georg-August-University of Gottingen, Germany (b) Department of Soil Biology and Biochemistry, Dokuchaev Soil Science Institute, Russian Federation (c) Department of Soil Science of Temperate Ecosystems, Georg-August-University of Gottingen, Germany Article History: Received 21 April 2015; Revised 24 July 2015; Accepted 25 July 2015
    Keywords: Soil Microbiology ; Glucose ; Microorganisms ; Cellulose
    ISSN: 0038-0717
    Source: Cengage Learning, Inc.
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  • 6
    Language: English
    In: Soil Biology and Biochemistry, Dec, 2013, Vol.67, p.106(8)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.soilbio.2013.08.015 Byline: Marie Spohn, Yakov Kuzyakov Abstract: Despite its importance for terrestrial nutrient and carbon cycling, the spatial organization of microbial activity in soil and in the rhizosphere is poorly understood. We related carbon allocation by roots to distribution of acid and alkaline phosphatase activity in the rhizosphere of Lupinus albus L. To do so, we further developed soil zymography - an in situ method for the analysis of the two-dimensional distribution of enzyme activity in soil - integrating fluorescent substrates. Soil zymography was combined with.sup.14C imaging, a technique that gives insights into the distribution of photosynthates after labeling plants with.sup.14C. Both acid and alkaline phosphatase activity were up to 5.4-times larger in the rhizosphere than in the bulk soil. While acid phosphatase activity (produced by roots and microorganisms) was closely associated with roots, alkaline phosphatase activity (produced only by microorganisms) was more widely distributed, leading to a 2.5-times larger area of activity of alkaline than of acid phosphatase. These results indicate a spatial differentiation of different ecophysiological groups of organic P mineralizing organisms. The spatial differentiation could be either between microorganisms and L. albus or between microorganisms that produce exclusively alkaline phosphatases on the one hand, and L. albus and root associated microorganisms that produce acid phosphatases on the other hand. The spatial separation of different organic P mineralizing organisms might alleviate a potential competition between them. While alkaline phosphatase activity strongly decreased with P fertilization, acid phosphatase activity was not affected by fertilization, suggesting that alkaline phosphatase-producing microorganisms react more strongly to it than other organic P mineralizing organisms. Alkaline phosphatase activity was high in parts of the rhizosphere where relatively little recent photosynthates were allocated, indicating that rhizodeposition and the activity of alkaline phosphatase-producing microorganisms are not directly related. Our study indicates, first, a spatial differentiation of organic P mineralization by various ecophysiological groups that react differently to inorganic P fertilization and second, that rhizodeposition and alkaline phosphatase-producing microorganisms were not directly related. Finally, we conclude that soil zymography with fluorescent substrates is a very promising approach for studying the distribution of a broad range of extracellular enzymes at microscales. Author Affiliation: (a) Department of Soil Science of Temperate Ecosystems, Georg-August-University Gottingen, Germany (b) Department of Agricultural Soil Science, Georg-August-University Gottingen, Germany Article History: Received 3 April 2013; Revised 11 August 2013; Accepted 12 August 2013
    Keywords: Photosynthesis ; Enzymes ; Soil Microbiology ; Phosphatases ; Ecosystems
    ISSN: 0038-0717
    Source: Cengage Learning, Inc.
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  • 7
    Language: English
    In: Soil Biology and Biochemistry, June, 2013, Vol.61, p.69(7)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.soilbio.2013.02.013 Byline: Marie Spohn, Yakov Kuzyakov Abstract: Despite the importance of phosphorus (P) mineralization to maintain soil fertility, little is known about the mechanisms that regulate microbial P mineralization. We tested the hypothesis that microbial P mineralization can be driven by microbial need for carbon (C). For this purpose, net microbial uptake kinetics of.sup.14C and.sup.33P from glucose-6-phosphate were studied in a Leptosol depending on availability of C, nitrogen (N), and P. After 60 h of incubation, 16.4% of the.sup.14C from glucose-6-phosphate was recovered in the microbial biomass, while.sup.33P incorporation into the microbial biomass was a third less. The higher net uptake of.sup.14C than of.sup.33P from the glucose-6-phosphate indicates that soil microorganisms use the organic moiety of phosphorylated organic compounds as a C source, but only use a small proportion of the P. Hence, they mineralize P without incorporating it. Our finding that the net uptake of.sup.14C and.sup.33P in the soils amended with inorganic P did not differ from the control treatment indicates that P mineralization was not driven by microbial need for P but rather for C. In a second experiment with three temperate forest soils we found that the activity of.sup.14C from glucose-6-phosphate in soil solution decreased faster than the activity of.sup.33P from glucose-6-phosphate. This might suggest that higher net uptake of C than of P from glucose-6-phosphate can also be observed in other temperate forest soils differing in C, N, and P contents from the Leptosol of the main experiment. In conclusion, the experiments show that microbial P mineralization can be a side-effect of microbial C acquisition from which plants potentially can benefit. Author Affiliation: Department of Soil Science of Temperate Ecosystems, Georg-August-University, Gottingen, Germany Article History: Received 29 September 2012; Revised 12 February 2013; Accepted 13 February 2013
    Keywords: Glucose Metabolism ; Soil Microbiology ; Phosphates ; Glucose ; Forest Soils
    ISSN: 0038-0717
    Source: Cengage Learning, Inc.
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  • 8
    Language: English
    In: Soil Biology and Biochemistry, Dec, 2012, Vol.55, p.40(8)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.soilbio.2012.06.007 Byline: Johanna Pausch (a)(b), Yakov Kuzyakov (a) Abstract: The production of CO.sub.2 in soil strongly depends on the availability of organic carbon (C) for microorganisms. It is obvious, that C that entered the soil recently is more easily available for microorganisms in comparison to older C. However, only very few approaches allow for a quantitative estimation of the availability of C in relation to the time it is entering the soil. We hypothesized that [delta].sup.13C values of CO.sub.2 and of soil organic matter (SOM) after a C.sub.3 to C.sub.4 vegetation change will enable to calculate the relative availability of younger (C.sub.4-derived) and older C (C.sub.3-derived) sources for microorganisms. Soil CO.sub.2 was sampled over one vegetation period at depths of 10, 40-50 and 60-70 cm at three treatments: a C.sub.3 reference (wheat), a C.sub.4/fallow (fallow after one year of maize cropping), and a C.sub.4/C.sub.4 (two years of maize cropping). Based on the [delta].sup.13C of CO.sub.2 purified from the admixture of atmospheric CO.sub.2 by the Miller/Tans model and on the [delta].sup.13C values of SOM, the contributions of younger and older C sources to CO.sub.2 and SOM were assessed. Depending on the soil depth and the presence of living roots, the contribution of younger C to soil CO.sub.2 ranged from 16 to 50%, but that to SOM was less than 5%. By comparing the contributions of older and younger C to CO.sub.2 and SOM, we found that the relative availability of organics recently introduced into the soil (C.sub.4-derived) was about 7 times higher than the availability of C stabilized in soil for longer than one year (C.sub.3-derived). We concluded that simultaneous analysis of the [delta].sup.13C values of both SOM and of CO.sub.2 allows not only for the quantification of the CO.sub.2 sources, but also for the estimation of the availability of soil C pools of different age for microorganisms. Author Affiliation: (a) Department of Soil Science of Temperate Ecosystems, University of Gottingen, Germany (b) Department of Agroecosystem Research, BayCEER, University of Bayreuth, Universitatsstr. 30, 95440 Bayreuth, Germany Article History: Received 16 January 2012; Revised 4 June 2012; Accepted 6 June 2012
    Keywords: Agroecosystems -- Analysis ; Soil Microbiology -- Analysis ; Wheat -- Analysis ; Soil Carbon -- Analysis
    ISSN: 0038-0717
    Source: Cengage Learning, Inc.
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  • 9
    Language: English
    In: Plant and Soil, 2012, Vol.351(1), pp.23-30
    Description: Byline: Olga Gavrichkova (1), Yakov Kuzyakov (2) Keywords: Canopy-rhizosphere coupling; Labeling approach; Phloem transport; Soil respiration; Pressure concentration waves; Regression analysis; CO.sub.2 partitioning Abstract: Background Two recent reviews raised a fundamental question: what is the time lag between photosynthetic C uptake and CO.sub.2 efflux from soil. Both reviews, however, in describing the linkages between CO.sub.2 assimilation and CO.sub.2 efflux from soil, were unable to evaluate the significance of two mechanisms i) the direct transport of assimilates to the roots and rhizosphere and ii) phloem pressure concentration waves. This uncertainty led to a further discussion about the suitability of labeling when evaluating time lags. Scope Here, we estimated the importance of the direct transport of assimilates to the rhizosphere and the importance of phloem pressure concentration waves by various approaches, and reveal further differences between both reviews. Conclusions We show that the pressure concentration waves only briefly play a role for CO.sub.2 efflux. In contrast, the direct transport influence lasts longer and so, it is more important for rhizosphere processes and for CO.sub.2 efflux. To evaluate the significance of these two mechanisms, we suggested a new approach based on regressions between the time lag and tree height, which confirms significance of pressure concentration waves only for trees, smaller than 2.5 m. Author Affiliation: (1) CNR, Institute of Agro-Environmental and Forest Biology (IBAF), 38050, Cinte Tesino, Italy (2) Department of Soil Science of Temperate Ecosystems, University of Gottingen, 37077, Gottingen, Germany Article History: Registration Date: 08/07/2011 Received Date: 18/02/2011 Accepted Date: 08/07/2011 Online Date: 20/08/2011 Article note: Responsible Editor: Katja Klumpp.
    Keywords: Canopy-rhizosphere coupling ; Labeling approach ; Phloem transport ; Soil respiration ; Pressure concentration waves ; Regression analysis ; CO partitioning
    ISSN: 0032-079X
    E-ISSN: 1573-5036
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  • 10
    Language: English
    In: Soil Biology and Biochemistry, April, 2014, Vol.71, p.95(10)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.soilbio.2014.01.011 Byline: Anna Gunina, Yakov Kuzyakov Abstract: Aggregate formation is a key process of soil development, which promotes carbon (C) stabilization by hindering decomposition of particulate organic matter (POM) and its interactions with mineral particles. C stabilization processes lead to.sup.13C fractionation and consequently to various [delta].sup.13C values of soil organic matter (SOM) fractions. Differences in [delta].sup.13C within the aggregates and fractions may have two reasons: 1) preferential stabilization of organic compounds with light or heavy [delta].sup.13C and/or 2) stabilization of organic materials after passing one or more microbial utilization cycles, leading to heavier [delta].sup.13C in remaining C. We hypothesized that: 1).sup.13C enrichment between the SOM fractions corresponds to successive steps of SOM formation; 2).sup.13C fractionation (but not the [delta].sup.13C signature) depends mainly on the transformation steps and not on the C precursors. Consequently, minimal differences between [DELTA].sup.13C of SOM fractions between various ecosystems correspond to maximal probability of the SOM formation pathways. We tested these hypotheses on three soils formed from cover loam during 45 years of growth of coniferous or deciduous forests or arable crops. Organic C pools in large macroaggregates, small macroaggregates, and microaggregates were fractionated sequentially for four density fractions to obtain free POM with I 〈 1.6 g cm.sup.-3, occluded POM with two densities (I 〈 1.6 and 1.6-2.0 g cm.sup.-3), and mineral fraction (I 〉 2.0 g cm.sup.-3). The density fractions were.sup.13C enriched in the order: free POM 〈 light occluded POM 〈 heavy occluded POM 〈 mineral fraction. This, as well as their C/N ratios confirmed that free POM was close to initial plant material, whereas the mineral fraction was the most microbially processed. To evaluate the successive steps of SOM formation, the [DELTA].sup.13C values between [delta].sup.13C of SOM fractions and [delta].sup.13C of bulk SOM were calculated. The [DELTA].sup.13C indicated that, parallel with biochemical transformations, the physical disintegration strongly contributed to the formation of free and occluded light POM. In contrast, biochemical transformations were more important than physical disintegration for formation of heavy occluded POM from light occluded POM. This was confirmed by review of 70 [DELTA].sup.13C values from other studies analyzed [DELTA].sup.13C depending on the density of SOM fractions. Accordingly, the successive steps of SOM formation were: fLF.sub.〈1.6 = oLF.sub.〈1.6 [right arrow] oDF.sub.1.6-2.0 = MF.sub.〉2.0. The obtained steps of C stabilization were independent on the initial precursors (litter of coniferous forest, deciduous forest or grasses). To test the second hypothesis, we proposed an extended scheme of C flows between the 3 aggregate size classes and 4 SOM fractions. [DELTA].sup.13C enrichment of the SOM fractions showed that the main direction of C flows within the aggregates and SOM fractions was from the macroaggregate-free POM to the mineral microaggregate fraction. This confirmed the earlier concept of SOM turnover in aggregates, but for the first time quantified the C flows within the aggregates and SOM density fractions based on [delta].sup.13C values. So, the new.sup.13C natural abundance approach is suitable for analysis of C pathways by SOM formation under steady state without.sup.13C or.sup.14C labeling. Author Affiliation: (a) Department of Agricultural Soil Science, Georg-August-University of Gottingen, Germany (b) Max Planck Institute for Biogeochemistry, Jena, Germany (c) Faculty of Soil Science, Moscow Lomonosov State University, Russia (d) Department of Soil Science of Temperate Ecosystems, Georg-August-University of Gottingen, Germany Article History: Received 10 November 2013; Revised 3 January 2014; Accepted 11 January 2014
    Keywords: Coniferous Forests -- Analysis ; Deciduous Forests -- Analysis
    ISSN: 0038-0717
    Source: Cengage Learning, Inc.
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