SOM genesis: microbial biomass as a significant source

Proper management of soil organic matter (SOM) is needed for maintaining soil fertility and for mitigation of the global increase in atmospheric CO₂ concentrations and should be informed by knowledge about the sources, spatial organisation and stabilisation processes of SOM. Recently, microbial biomass residues (i.e. necromass) have been identified as a significant source of SOM. Here, we propose that cell wall envelopes of bacteria and fungi are stabilised in soil and contribute significantly to small-particulate SOM formation. This hypothesis is based on the mass balance of a soil incubation experiment with ¹³C-labelled bacterial cells and on the visualisation of the microbial residues by means of scanning electron microscopy (SEM). At the end of a 224-day incubation, 50% of the biomass-derived C remained in the soil, mainly in the non-living part of SOM (40% of the added biomass C). SEM micrographs only rarely showed intact cells. Instead, organic patchy fragments of 200–500 nm size were abundant and these fragments were associated with all stages of cell envelope decay and fragmentation. Similar fragments, developed on initially clean and sterile in situ microcosms during exposure to groundwater, provide clear evidence for their formation during microbial growth and surface colonisation. Microbial cell envelope fragments thus contribute significantly to SOM formation. This origin and the related macromolecular architecture of SOM are consistent with most observations on SOM, including the abundance of microbial-derived biomarkers, the low C/N ratio, the water repellency and the stabilisation of biomolecules, which in theory should be easily degradable.     

Bacterial,Archaeal and Fungal Succession in the Forefield of a Receding Glacier

Glacier forefield chronosequences, initially composed of barren substrate after glacier retreat, are ideal locations to study primary microbial colonization and succession in a natural environment. We characterized the structure and composition of bacterial, archaeal and fungal communities in exposed rock substrates along the Damma glacier forefield in central Switzerland. Soil samples were taken along the forefield from sites ranging from fine granite sand devoid of vegetation near the glacier terminus to well-developed soils covered with vegetation. The microbial communities were studied with genetic profiling (T-RFLP) and sequencing of clone libraries. According to the T-RFLP profiles, bacteria showed a high Shannon diversity index (H) (ranging from 2.3 to 3.4) with no trend along the forefield. The major bacterial lineages were Proteobacteria, Actinobacteria, Acidobacteria, Firmicutes and Cyanobacteria. An interesting finding was that Euryarchaeota were predominantly colonizing young soils and Crenarchaeota mainly mature soils. Fungi shifted from an Ascomycota-dominated community in young soils to a more Basidiomycota-dominated community in old soils. Redundancy analysis indicated that base saturation, pH, soil C and N contents and plant coverage, all related to soil age, correlated with the microbial succession along the forefield.     

Microbial dynamics in soils of the Damma glacier forefield show succession in the functional genetic potential

Glacier retreat is a visible consequence of climate change worldwide. Although taxonomic change of the soil microbiomes in glacier forefields have been widely documented, how microbial genetic potential changes along succession is little known. Here, we used shotgun metagenomics to analyse whether the soil microbial genetic potential differed between four stages of soil development (SSD) sampled along three transects in the Damma glacier forefield (Switzerland). The SSDs were characterized by an increasing vegetation cover, from barren soil, to biological soil crust, to sparsely vegetated soil and finally to vegetated soil. Results suggested that SSD significantly influenced microbial genetic potential, with the lowest functional diversity surprisingly occurring in the vegetated soils. Overall, carbohydrate metabolism and secondary metabolite biosynthesis genes overrepresented in vegetated soils, which could be partly attributed to plant–soil feedbacks. For C degradation, glycoside hydrolase genes enriched in vegetated soils, while auxiliary activity and carbohydrate esterases genes overrepresented in barren soils, suggested high labile C degradation potential in vegetated, and high recalcitrant C degradation potential in barren soils. For N-cycling, organic N degradation and synthesis genes dominated along succession, and gene families involved in nitrification were overrepresented in barren soils. Our study provides new insights into how the microbial genetic potential changes during soil formation along the Damma glacier forefield.     

冰川前缘土壤微生物原生演替的生态特征——以乌鲁木齐河源1号冰川为例

为了解冰川前缘土壤微生物在原生演替过程中的生态特征及其影响因素,用空间距离代替时间序列,以乌鲁木齐河源1号冰川终碛堤为起点,沿6个不同演替时期(0,4,15,31,43a和对照)的样带采集土样,以冰川附近发育良好的土壤为对照,测定土壤酶活性、微生物氮矿化与脱氨作用以及微生物生物量。结果表明,土壤脲酶、蛋白酶、酸性磷酸酶、芳基硫酸酯酶、蔗糖酶活性、微生物氮矿化及脱氨作用随演替时间而增加,微生物生物量碳和氮变化呈波动状,趋势不明显。相关分析表明,土壤有机质与酶活、微生物生物量存在极显著正相关(P0.01)。1号冰川前缘微生物多样性指数随着演替时间持续增加,但目前仍未达稳定状态。     

Microbial Diversity and Activity along the Forefields of Two Receding Glaciers

Forefields of two receding glaciers were sampled along either a 150 or 200 m long transect at identical spatial intervals for assessment of soil microbial activity and community diversity trends. The forefields belonged to the Dammaglacier (forefield area is 157 ha, 2000 m above sea level) and Rotfirnglacier (100 ha, 2200 m) and at the time of sampling were receding at an estimated rate of 8 and 10 m yr(-1) over the past 5 years, respectively. Direct counting of bacteria (DAPI staining), assessment of dehydrogenase activity (DH), and fluorescein diacetate hydrolysis activity (FDA) were performed to estimate bacteria number and soil microbial activity. Along the Dammaglacier forefield (from youngest to oldest soil), bacteria number (8.21 x 10(7) to 1.49 x 10(9) cells g(-1) soil), DH activity (0 to 61 mg TTC reduced g(-1) soil h(-1)), and FDA activity (0 to 100 mg fluorescein produced g-1 soil h-1) increased, suggesting the development of microbial populations increasing in number and activity. The Rotfirn forefield exhibited similar trends per gram of soil in bacteria number (1.13 x 10(8) to 5.93 x 10(9) cells), DH activity (0 to 36 mg TTC reduced), and FDA activity (2 to 70 mg fluorescein produced), but with more variability among samples than the Damma forefield samples. Molecular assessment of bacterial diversity included denaturing gradient gel electrophoresis (DGGE) and ribosomal intergenic spacer analysis (RISA) of soil DNA. DGGE and RISA revealed that the composition and succession of bacterial populations were different in both forefields. Comparison of Shannon diversity index values indicated that all populations sampled from the Damma forefield were significantly different (p < 0.05). Conversely, similar populations existed in the Rotfirn forefield succession. Overall, the results indicate that diverse bacterial assemblages increasing in number and activity characterize these glacier forefield soils with both forefield successions exhibiting differing modes of bacterial community establishment.     

Microbial dynamics in glacier forefield soils show succession is not just skin deep

All over the world, glaciers are receding. One key consequence of glacier area loss is the creation of new terrestrial habitats. This presents an experimental opportunity to study both community formation and the implications of glacier loss for terrestrial ecosystems. In this issue of Molecular Ecology, Rime et al. ( 2015 ) describe how microbial communities are structured according to soil depth and development in the forefield of Damma glacier in Switzerland. The study provides insights into the contrasting structures of microbial communities at different stages of soil development. An important strength of the study is the integration of soil depth into the paradigm of primary succession, a feature which has rarely been considered by other studies. These findings underscore the importance of studying the interactions between microbial communities and glaciers at a time when Earth's glacial systems are experiencing profound change.     

Rapid soil formation after glacial retreat shaped by spatial patterns of organic matter accrual in microaggregates

Global change contributes to the retreat of glaciers at unprecedented rates. The deglaciation facilitates biogeochemical processes on glacial deposits with initiating soil formation as an important driver of evolving ecosystems. The underlying mechanisms of soil formation and the association of soil organic matter (SOM) with mineral particles remain unclear, although further insights are critical to understand carbon sequestration in soils. We investigated the microspatial arrangement of SOM coatings at intact soil microaggregate structures during various stages of ecosystem development from 15 to >700 years after deglaciation in the proglacial environment of the Damma glacier (Switzerland). The functionally important clay‐sized fraction (<2 μm) was separated into two density fractions with different amounts of organo‐mineral associations: light (1.6–2.2 g/cm³) and heavy (>2.2 g/cm³). To quantify how SOM extends across the surface of mineral particles (coverage) and whether SOM coatings are distributed in fragmented or connected patterns (connectivity), we developed an image analysis protocol based on nanoscale secondary ion mass spectrometry (NanoSIMS). We classified SOM and mineral areas depending on the ¹⁶O^?, ¹²C^?, and ¹²C¹⁴N^? distributions. With increasing time after glacial retreat, the microspatial coverage and connectivity of SOM increased rapidly. The rapid soil formation led to a succession of patchy distributed to more connected SOM coatings on soil microaggregates. The maximum coverage of 55% at >700 years suggests direct evidence for SOM sequestration being decoupled from the mineral surface, as it was not completely masked by SOM and retained its functionality as an ion exchange site. The chemical composition of SOM coatings showed a rapid change toward a higher CN:C ratio already at 75 years after glacial retreat, which was associated with microbial succession patterns reflecting high N assimilation. Our results demonstrate that rapid SOM sequestration drives the microspatial succession of SOM coatings in soils, a process that can stabilize SOM for the long term.     

Climate‐sensitive ecosystem carbon dynamics along the soil chronosequence of the Damma glacier forefield,Switzerland

We performed a detailed study on the carbon build‐up over the 140‐year‐long chronosequence of the Damma glacier forefield, Switzerland, to gain insights into the organic carbon dynamics during the initial stage of soil formation and ecosystem development. We determined soil carbon and nitrogen contents and their stable isotopic compositions, as well as molecular‐level composition of the bulk soils, and recalcitrance parameters of carbon in different fractions. The chronosequence was divided into three age groups, separated by small end moraines that resulted from two glacier re‐advances. The net ecosystem carbon balance (NECB) showed an exponential increase over the last decades, with mean annual values that range from 100 g C m^?2 yr^?1 in the youngest part to over 300 g C m^?2 yr^?1 in a 60–80 years old part. However, over the entire 140‐year chronosequence, the NECB is only 20 g C m^?2 yr^?1, similar to results of other glacier forefield studies. The difference between the short‐ and long‐term NECB appears to be caused by reductions in ecosystem carbon (EC) accumulation during periods with a colder climate. We propose that two complementary mechanisms have been responsible: 1) Reductions in net primary productivity down to 50% below the long‐term mean, which we estimated using reconstructed effective temperature sums. 2) Disturbance of sites near the terminus of the re‐advanced glacier front. Stabilization of soil organic matter appeared to play only a minor role in the coarse‐grained forefield. We conclude that the forefield ecosystem, especially primary productivity, reacts rapidly to climate changes. The EC gained at warm periods is easily lost again in a cooling climate. Our conclusions may also be valid for other high mountain ecosystems and possibly arctic ecosystems.     

Variations in soil culturable bacteria communities and biochemical characteristics in the Dongkemadi glacier forefield along a chronosequence

The variations in the soil culturable bacterial communities and biochemical parameters of early successional soils from a receding glacier in the Tanggula Mountain were investigated. We examined low organic carbon (C) and nitrogen (N) contents and enzymatic activity, correlated with fewer bacterial groups and numbers in the glacier forefield soils. The soil pH values decreased, but the soil water content, organic C and total N significantly increased, along the chronosequence. The soil C/N ratio decreased in the early development soils and increased in the late development soils and it did not correlate with the soil age since deglaciation. The activities of soil urease, sucrase, protease, polyphenol oxidase, catalase, and dehydrogenase increased along the chronosequence. The numbers of culturable bacteria in the soils increased as cultured at 25°C while decreased at 4°C from younger soils to older soils. Total numbers of culturable bacteria in the soils cultured at 25°C were significantly positively correlated to the soil total N, organic C, and soil water content, as well as the activities of soil urease, sucrase, dehydrogenase, catalase, and polyphenol oxidase. We have obtained 224 isolates from the glacier forefield soils. The isolates were clustered into 28 groups by amplified ribosomal DNA restriction analysis (ARDRA). Among them, 27 groups and 25 groups were obtained from the soils at 25°C and at 4°C incubation temperatures, respectively. These groups are affiliated with 18 genera that belong to six taxa, viz, Actinobacteria, Gammaproteobacteria, Bacteroidetes, Firmicutes, Alphaproteobacteria, and Betaproteobacteria. The dominant taxa were Actinobacteria, Gammaproteobacteria, and Bacteroidetes in all the samples. The abundance and the diversity of the genera isolated at 25°C incubation temperature were greater than that at 4°C.     

土壤活性有机质及其与土壤质量的关系

活性有机质是土壤的重要组成部分 ,主要包括溶解性有机碳、微生物生物量、轻组有机质。它在土壤中具有重要作用 :(1)可以表征土壤物质循环特征、评价土壤质量 ,可以作为土壤潜在生产力以及由土壤管理措施引起土壤有机质变化的早期指标 ;(2 )在养分周转中起重要作用 ,是植物的养分库 ,可以提供植物所需要的养分如氮、磷、硫等 ;(3)能稳定土壤结构 ,对维持团粒结构稳定性有重要作用。从土壤养分、土壤物理、化学性质方面讨论了活性有机质与土壤质量的关系。土壤中的溶解性有机碳、微生物生物量碳氮含量与土壤有机碳、全氮和碱解氮等物质的含量呈正相关。活性有机质受土壤质地、含水量、温度等因素影响 ,与土壤酸碱度、阳离子交换量等也有关。土壤微生物生物量碳和微生物量 C/有机碳比与土壤粘粒、粉粒含量呈正相关、与砂粒含量呈负相关     

典型矿冶区周边土壤微生物功能特征及影响因素

土壤微生物对重金属污染胁迫敏感,但在实际野外环境中,土壤微生物群落生态效应通常是污染胁迫和环境因素综合作用的结果。为探究重金属污染土壤中微生物群落生态效应发生变化的主控因素,本研究以湖南省某典型矿冶区周边不同土地利用类型土壤为研究对象,以土壤碳氮循环过程主要的微生物功能指标土壤微生物生物量碳(MBC)、基础呼吸(BR)、诱导呼吸(SIR)和硝化潜势(PNR)为生态效应终点,进行采样调查分析。结果表明: 土地利用类型对MBC、BR和SIR影响均不显著;研究区土壤微生物功能的主要影响因子包括CaCl₂提取态Pb(CaCl₂-Pb)含量与土壤有机质(SOM)含量。多元回归分析结果表明,在CaCl₂-Pb含量为0.004～13.14 mg·kg^-1及SOM含量为0.24%～4.34%的条件下,土壤CaCl₂-Pb和SOM含量可以共同解释土壤中BR、SIR和PNR总变异的39.8%～58.3%;中等含量下(SOM在1.70%～2.36%,CaCl₂-Pb在0.004～12.98 mg·kg^-1),土壤CaCl₂-Pb和SOM含量与BR、SIR和PNR的变化能够建立显著的暴露-效应关系,可以作为测定终点定量评价重金属污染对微生物群落功能的生态效应。     

High Diversity of Diazotrophs in the Forefield of a Receding Alpine Glacier

Forefields of receding glaciers are unique and sensitive environments representing natural chronosequences. In such habitats, microbial nitrogen fixation is of particular interest since the low concentration of bioavailable nitrogen is one of the key limitations for growth of plants and soil microorganisms. Asymbiotic nitrogen fixation in the Damma glacier (Swiss Central Alps) forefield soils was assessed using the acetylene reduction assay. Free-living diazotrophic diversity and population structure were resolved by assembling four NifH sequence libraries for bulk and rhizosphere soils at two soil age classes (8- and 70-year ice-free forefield). A total of 318 NifH sequences were analyzed and grouped into 45 unique phylotypes. Phylogenetic analyses revealed a higher diversity as well as a broader distribution of NifH sequences among phylogenetic clusters than formerly observed in other environments. This illustrates the importance of free-living diazotrophs and their potential contribution to the global nitrogen input in this nutrient-poor environment. NifH diversity in bulk soils was higher than in rhizosphere soils. Moreover, the four libraries displayed low similarity values. This indicated that both soil age and the presence of pioneer plants influence diversification and population structure of free-living diazotrophs.     

Soil microbial diversity affects soil organic matter decomposition in a silty grassland soil

Soil microorganisms play a pivotal role in soil organic matter (SOM) turn-over and their diversity is discussed as a key to the function of soil ecosystems. However, the extent to which SOM dynamics may be linked to changes in soil microbial diversity remains largely unknown. We characterized SOM degradation along a microbial diversity gradient in a two month incubation experiment under controlled laboratory conditions. A microbial diversity gradient was created by diluting soil suspension of a silty grassland soil. Microcosms containing the same sterilized soil were re-inoculated with one of the created microbial diversities, and were amended with ¹³C labeled wheat in order to assess whether SOM decomposition is linked to soil microbial diversity or not. Structural composition of wheat was assessed by solid-state ¹³C nuclear magnetic resonance, sugar and lignin content was quantified and labeled wheat contribution was determined by ¹³C compound specific analyses. Results showed decreased wheat O-alkyl-C with increasing microbial diversity. Total non-cellulosic sugar-C derived from wheat was not significantly influenced by microbial diversity. Carbon from wheat sugars (arabinose-C and xylose-C), however, was highest when microbial diversity was low, indicating reduced wheat sugar decomposition at low microbial diversity. Xylose-C was significantly correlated with the Shannon diversity index of the bacterial community. Soil lignin-C decreased irrespective of microbial diversity. At low microbial diversity the oxidation state of vanillyl–lignin units was significantly reduced. We conclude that microbial diversity alters bulk chemical structure, the decomposition of plant litter sugars and influences the microbial oxidation of total vanillyl–lignins, thus changing SOM composition.     

Preparation techniques for the electron microscopy of granular sludge from an anaerobic digester

Several fixation and dehydration techniques for scanning and transmission electron microscopy of glycocalyx and microbial populations within granules from an upflow anaerobic sludge blanket digester purifying a brewery wastewater were compared. Sputter-cryo and freeze-drying techniques prior to scanning electron microscopy (SEM) allowed viewing of the glycocalyx. In contrast standard fixation and dehydration techniques were suitable for examination of underlying microbial populations by both SEM and transmission electron microscopy.     

Effects of Soil Organic Matter Properties and Microbial Community Composition on Enzyme Activities in Cryoturbated Arctic Soils

Enzyme-mediated decomposition of soil organic matter (SOM) is controlled, amongst other factors, by organic matter properties and by the microbial decomposer community present. Since microbial community composition and SOM properties are often interrelated and both change with soil depth, the drivers of enzymatic decomposition are hard to dissect. We investigated soils from three regions in the Siberian Arctic, where carbon rich topsoil material has been incorporated into the subsoil (cryoturbation). We took advantage of this subduction to test if SOM properties shape microbial community composition, and to identify controls of both on enzyme activities. We found that microbial community composition (estimated by phospholipid fatty acid analysis), was similar in cryoturbated material and in surrounding subsoil, although carbon and nitrogen contents were similar in cryoturbated material and topsoils. This suggests that the microbial community in cryoturbated material was not well adapted to SOM properties. We also measured three potential enzyme activities (cellobiohydrolase, leucine-amino-peptidase and phenoloxidase) and used structural equation models (SEMs) to identify direct and indirect drivers of the three enzyme activities. The models included microbial community composition, carbon and nitrogen contents, clay content, water content, and pH. Models for regular horizons, excluding cryoturbated material, showed that all enzyme activities were mainly controlled by carbon or nitrogen. Microbial community composition had no effect. In contrast, models for cryoturbated material showed that enzyme activities were also related to microbial community composition. The additional control of microbial community composition could have restrained enzyme activities and furthermore decomposition in general. The functional decoupling of SOM properties and microbial community composition might thus be one of the reasons for low decomposition rates and the persistence of 400 Gt carbon stored in cryoturbated material.     

How interactions between microbial resource demands,soil organic matter stoichiometry,and substrate reactivity determine the direction and magnitude of soil respiratory responses to warming

Recent empirical and theoretical advances inform us about multiple drivers of soil organic matter (SOM) decomposition and microbial responses to warming. Absent from our conceptual framework of how soil respiration will respond to warming are adequate links between microbial resource demands, kinetic theory, and substrate stoichiometry. Here, we describe two important concepts either insufficiently explored in current investigations of SOM responses to temperature, or not yet addressed. First, we describe the complete range of responses for how warming may change microbial resource demands, physiology, community structure, and total biomass. Second, we describe how any relationship between SOM activation energy of decay and carbon (C) and nitrogen (N) stoichiometry can alter the relative availability of C and N as temperature changes. Changing availabilities of C and N liberated from their organic precursors can feedback to microbial resource demands, which in turn influence the aggregated respiratory response to temperature we observe. An unsuspecting biogeochemist focused primarily on temperature sensitivity of substrate decay thus cannot make accurate projections of heterotrophic CO₂ losses from diverse organic matter reservoirs in a warming world. We establish the linkages between enzyme kinetics, SOM characteristics, and potential for microbial adaptation critical for making such projections. By examining how changing microbial needs interact with inherent SOM structure and composition, and thus reactivity, we demonstrate the means by which increasing temperature could result in increasing, unchanging, or even decreasing respiration rates observed in soils. We use this exercise to highlight ideas for future research that will develop our abilities to predict SOM feedbacks to climate.     

Abundance of Microbes Involved in Nitrogen Transformation in the Rhizosphere of Leucanthemopsis alpina (L.) Heywood Grown in Soils from Different Sites of the Damma Glacier Forefield

Glacier forefields are an ideal playground to investigate the role of development stages of soils on the formation of plant–microbe interactions as within the last decades, many alpine glaciers retreated, whereby releasing and exposing parent material for soil development. Especially the status of macronutrients like nitrogen differs between soils of different development stages in these environments and may influence plant growth significantly. Thus, in this study, we reconstructed major parts of the nitrogen cycle in the rhizosphere soil/root system of Leucanthemopsis alpina (L.) Heywood as well as the corresponding bulk soil by quantifying functional genes of nitrogen fixation (nifH), nitrogen mineralisation (chiA, aprA), nitrification (amoA AOB, amoA AOA) and denitrification (nirS, nirK and nosZ) in a 10-year and a 120-year ice-free soil of the Damma glacier forefield. We linked the results to the ammonium and nitrate concentrations of the soils as well as to the nitrogen and carbon status of the plants. The experiment was performed in a greenhouse simulating the climatic conditions of the glacier forefield. Samples were taken after 7 and 13 weeks of plant growth. Highest nifH gene abundance in connection with lowest nitrogen content of L. alpina was observed in the 10-year soil after 7 weeks of plant growth, demonstrating the important role of associative nitrogen fixation for plant development in this soil. In contrast, in the 120-year soil copy numbers of genes involved in denitrification, mainly nosZ were increased after 13 weeks of plant growth, indicating an overall increased microbial activity status as well as higher concentrations of nitrate in this soil.     

Decomposition and stabilization of root litter in top- and subsoil horizons: what is the difference?

Mechanisms leading to high mean residence times of organic matter in subsoil horizons are poorly understood. In lower parts of the soil profile root material contributes greatly to soil organic matter (SOM). The objective of this study was to elucidate the decomposition dynamics of root-derived C and N in different soil depths during a 3 year field experiment and to examine the importance of different protection mechanisms as well as abiotic factors for the decomposition dynamics. Additionally, we assessed the effect of root litter addition on native SOM. Our conceptual approach included the exposure of litterbags with ¹³C and ¹⁵N labeled wheat root material mixed to loamy agricultural soil at three different soil depths (30, 60 and 90 cm). During the incubation period, we monitored soil temperature, humidity and the incorporation of root derived C and N into the soil microbial biomass and physical SOM fractions. Our results showed that abiotic decay conditions were better in subsurface horizons compared to the topsoil. Root litter addition significantly increased the size of microbial biomass in all three soil horizons. In the topsoil, root-derived C decomposition was significantly higher in the first 6 months of incubation compared to subsoil horizons. In 60 and 90 cm depths, a lag phase with development of soil microbial biomass seemed to be prevailing before decomposition was activated. For root-derived N, similar decomposition kinetics could be observed in top- and subsoil horizons. Despite of higher SOM contents, better soil structure and higher microbial activity in the topsoil horizon compared to subsoil horizons, the amounts of root-derived C and N remaining after 3 years were similar for all three depths. Most of the root-derived C and N was present as organo-mineral complexes or occluded in soil aggregates (oPOM), illustrating similar importance of these two protection mechanisms in all three soil depths. Addition of fresh root litter caused small losses of native soil C whereas native N was retained. We conclude that despite of similar SOM protection mechanisms, there are distinct differences in decomposition dynamics of root litter between top- and subsoil horizons. In the long run, the better abiotic decay conditions prevailing in subsoil horizons may compensate for their poorer physico-chemical characteristics.     

Biochar amendment to soils with contrasting organic matter level: effects on N mineralization and biological soil properties

Four biochar types, produced by slow pyrolysis of poultry litter (PL) and pine chips (P) at 400 or 500 °C, were added to two adjacent soils with contrasting soil organic matter (SOM) content (8.9 vs. 16.1 g C kg^?1). The N mineralization rate was determined during 14‐week incubations and assessments were made of the microbial biomass C, dehydrogenase activity, and the microbial community structure (PLFA‐extraction). The addition of PL biochars increased the net N mineralization (i.e., compared to the control treatment) in both soils, while for treatments with P biochars net N immobilization was observed in both soils. Increasing the pyrolysis temperature of both feedstock types led to a decrease in net N mineralization. The ratio of Bacterial to Fungal PLFA biomarkers also increased with addition of biochars, and particularly in the case of the 500 °C biochars. Next to feedstock type and pyrolysis temperature, SOM content clearly affected the assessed soil biological parameters, viz. net N mineralization or immobilization, MBC and dehydrogenase activity were all greater in the H soil. This might be explained by an increased chance of physical contact between the microbial community activated by SOM mineralization upon incubation and discrete biochar particles. However, when considering the H soil's double C and N content, these responses were disproportionally small, which may be partly due to the L soil's, somewhat more labile SOM. Nonetheless, increasing SOM content and microbial biomass and activity generally appears to result in greater mineralization of biochar. Additionally, higher N mineralization after PL addition to the H soil with lower pH than the L soil can be due to the liming effect of the PL biochars.