Publications

Assessment of the Screening Procedure for Measuring Nitrate in the Soils of Baden-Württemberg (Germany)

Labile carbon retention compensates for CO₂released by priming in forest soils

Increase of belowground C allocation by plants under global warming or elevated CO2 may promote decomposition of soil organic carbon (SOC) by priming and strongly affects SOC dynamics. The specific effects by priming of SOC depend on the amount and frequency of C inputs. Most previous priming studies have investigated single C additions, but they are not very representative for litterfall and root exudation in many terrestrial ecosystems. We evaluated effects of (13)C-labeled glucose added to soil in three temporal patterns: single, repeated, and continuous on dynamics of CO2 and priming of SOC decomposition over 6 months. Total and (13)C labeled CO2 were monitored to analyze priming dynamics and net C balance between SOC loss caused by priming and the retention of added glucose-C. Cumulative priming ranged from 1.3 to 5.5 mg C g(-1) SOC in the subtropical, and from -0.6 to 5.5 mg C g(-1) SOC in the tropical soils. Single addition induced more priming than repeated and continuous inputs. Therefore, single additions of high substrate amounts may overestimate priming effects over the short term. The amount of added glucose C remaining in soil after 6 months (subtropical: 8.1-11.2 mg C g(-1) SOC or 41-56% of added glucose; tropical: 8.7-15.0 mg C g(-1) SOC or 43-75% of glucose) was substantially higher than the net C loss due to SOC decomposition including priming effect. This overcompensation of C losses was highest with continuous inputs and lowest with single inputs. Therefore, raised labile organic C input to soils by higher plant productivity will increase SOC content even though priming accelerates decomposition of native SOC. Consequently, higher continuous input of C belowground by plants under warming or elevated CO2 can increase C stocks in soil despite accelerated C cycling by priming in soils.

Organic matter chemistry and bacterial community structure regulate decomposition processes in post-fire forest soils

Wildfires decrease forest aboveground biomass and have long-term legacy effects on carbon (C) stocks in soil via alterations of microbial communities and functions. However, the interactions between soil organic C (SOC) chemodiversity and bacterial communities that drive C decomposition remain unclear. Soils from two boreal forest sites, 3 months (S1) and 15 years (S2) after fire events, were incubated for 53 days to quantify the mineralization of sucrose (mimicking rhizodeposits, δ13C = −11.97‰) and SOC priming. To reveal SOC-bacterial interactions that regulate SOC decomposition, the isotopic abundance, SOC chemical composition (13C NMR), and associated bacterial community structure (16S rRNA gene sequencing) were analyzed. The best multivariate model (DISTLM) analysis indicated that aromatic C (phenolic-C and aryl-C) in S1 and di-O-alkyl C in S2 were the largest contributors to bacterial community structure. The co-occurrence network confirmed SOC-bacteria interactions, and revealed the highly co-occurrent groups, i.e. Paenibacillus in S1 and Bacillus in S2, both of which belong to the Firmicutes, correlated with recalcitrant C and labile C, respectively, and are potentially linked to decomposition. For example, Firmicutes (as well as Proteobacteria and Actinobacteria) were correlated with aryl-C and phenolic-C in S1 and highly correlated with SOC priming intensity. The limited C resources (enriched refractory components, i.e. phenolic substances) in S1 favored oligotrophs to outcompete other bacterial groups, which likely aided decomposition of more recalcitrant SOC via co-metabolisms. The slow decomposition of sucrose and large soil priming effects observed in S1 suggested a faster SOC turnover via bidirectional processes of additional sucrose-C gain and native soil-C loss. Collectively, changes in SOC chemistry were coupled with an altered bacterial community, and their interactions might further correlate to decomposition, with implications for C sequestration in the post-fire boreal forest soils.

Kinetics of [2-14C]glycine decomposition and its incorporation into humus fractions of sierozem

No abstract is provided for this article.

Distribution, Storage, and Factors Influencing Particulate and Mineral‐Associated Organic Matter in Paddy Soils

Abstract Soil organic matter (SOM) reserves in paddies are approximately two times larger than those in upland soils, and therefore, rice paddies have a strong impact on terrestrial carbon (C) sequestration. Functional partitioning of SOM into particulate organic matter (POM) and mineral‐associated organic matter (MAOM) facilitates our understanding of C sequestration capacity in paddy soils. We analyzed POM and MAOM contents in 104 samples of topsoil and 81 samples of subsoil collected from paddies, and investigated how climate, nitrogen (N) fertilization, and soil depth regulate POM and MAOM storage. MAOM was the predominant fraction (45.3%–63.7%) of SOM in all paddy soils. As the SOC content increased, POM increased linearly, while the increase rate of MAOM slowed down, indicating a tendency for MAOM to reach saturation. The influence of mineral types on POM and MAOM protection exhibited depth‐dependent patterns: clay minerals showed stronger associations in topsoil, whereas amorphous iron oxides displayed increasing importance in subsoil. Climatic factors, particularly mean annual temperature (MAT), had contrasting effects on POM and MAOM storage: increasing MAT reduced MAOM content and stability while having a minor impact on POM. Increasing the N application rate had minimal impact on POM and MAOM storage due to crop harvest and the balance between microbial activity and mineral protection mediated by soil acidification. These findings are valuable for facilitating the sequestration and increasing the stability of SOM in paddies, providing information for global soil carbon storage strategies.

How to link soil C pools with CO<sub>2</sub> fluxes?

. Despite the importance of carbon (C) pools and CO2 fluxes in terrestrial ecosystems and especially in soils, as well as many attempts to assign fluxes to specific pools, this challenge remains unsolved. Interestingly, scientists investigating pools are not closely linked with scientists studying fluxes. This review therefore focused on experimental approaches enabling soil C pools to be linked with CO2 flux from the soil. The background, advantages and shortcomings of uncoupled approaches (measuring only pools or fluxes) and of coupled approaches (measuring both pools and fluxes) were evaluated and their prerequisites – steady state of pools and isotopic steady state – described. The uncoupled approaches include: (i) monitoring the decrease of C pools in long-term fallow bare soil lacking C input over decades, (ii) analyzing components of CO2 efflux dynamics by incubating soil without new C input over months or years, and (iii) analyzing turnover rates of C pools based on their 13C and 14C isotopic signature. The uncoupled approaches are applicable for non-steady state conditions only and have limited explanatory power. The more advantageous coupled approaches partition simultaneously pools and fluxes based on one of three types of changes in the isotopic signature of input C compared to soil C: (i) abrupt permanent, (ii) gradual permanent, and (iii) abrupt temporary impacts. I show how the maximal sensitivity of the approaches depends on the differences in the isotopic signature of pools with fast and slow turnover rates. The promising coupled approaches include: (a) δ13C of C pools and CO2 efflux from soil after C3/C4 vegetation changes or in FACE experiments (both corresponding to continuous labeling), (b) addition of 13C or 14C labeled organics (corresponding to pulse labeling), and (c) bomb-14C. I show that physical separation of soil C pools is not a prerequisite to estimate pool size or to link pools with fluxes. Based on simple simulation of C aging in soil after the input, the discordance of MRT of C in pools and of C released in CO2 was demonstrated. This discordance of MRT between pools and fluxes shows that the use of MRT of pools alone underestimates the fluxes at least for two times. The future challenges include combining two or more promising approaches to elucidate more than two C sources for CO2 fluxes, and linking scientific communities investigating the pools with those investigating the fluxes.

Interactions between biochar and litter priming: A three-source 14C and δ13C partitioning study

Although it has been separately reported that biochar primes the decomposition of soil organic matter (SOM) or fresh organic matter, little is known about the simultaneous effects of biochar on SOM versus plant litter mineralization. We applied dual 13C/14C isotopic labels to partition soil CO2 efflux and C pools into three sources: SOM, litter and biochar. Biochar made by slow pyrolysis (400 °C) of 14C labeled residues of rice (Oryza sativa, C3) and maize (Zea mays, C4) litter were added separately or in combination to a silty Fluvisol with a C3 isotopic signature and incubated at 25 °C over a period of 6 months. Biochar decomposition was very slow, with a mean rate of 0.017% d−1. Approximately 63% of biochar-derived CO2 was produced in the first month. Mixing with litter reduced biochar mineralization by 14%. Addition of biochar alone to soil induced a cumulative positive priming effect (0.24 mg C g−1 soil) on SOM decomposition over 183 days, a much smaller effect than litter-induced priming (1.05 mg CO2-C g−1 soil). Compared to soils with only litter amended, biochar and litter added in combination decreased SOM mineralization by 19% while increasing litter mineralization by 6.9%, with no net changes in total CO2 release. Increased litter- but not SOM-derived C in microbial biomass in the presence of biochar suggested that biochar caused preferential microbial utilization of litter over SOM. Given that immobilization of mineral N in the soil-litter mixture was markedly enhanced following the addition of biochar, we proposed that the biochar-induced preferential microbial utilization of litter over SOM was due primarily to alterations in N cycling. In conclusion, the priming effects of litter on SOM are changed by the presence of biochar.

Reviews and syntheses: Agropedogenesis – humankind as the sixth soil-forming factor and attractors of agricultural soil degradation

. Agricultural land covers 5.1×109 ha (ca. 50 % of potentially suitable land area), and agriculture has immense effects on soil formation and degradation. Although we have an advanced mechanistic understanding of individual degradation processes of soils under agricultural use, general concepts of agropedogenesis are absent. A unifying theory of soil development under agricultural practices, of agropedogenesis, is urgently needed. We introduce a theory of anthropedogenesis – soil development under the main factor “humankind” – the sixth factor of soil formation, and deepen it to encompass agropedogenesis as the most important direction of anthropedogenesis. The developed theory of agropedogenesis consists of (1) broadening the classical concept of factors→processes→properties→functions along with their feedbacks to the processes, (2) a new concept of attractors of soil degradation, (3) selection and analysis of master soil properties, (4) analysis of phase diagrams of master soil properties to identify thresholds and stages of soil degradation, and, finally, (5) a definition of the multidimensional attractor space of agropedogenesis. The main feature of anthropedogenesis is the narrowing of soil development to only one function (e.g. crop production for agropedogenesis), and this function is becoming the main soil-forming factor. The focus on only one function and the disregard of other functions inevitably lead to soil degradation. We show that the factor humankind dominates over the effects of the five natural soil-forming factors and that agropedogenesis is therefore much faster than natural soil formation. The direction of agropedogenesis is largely opposite to that of natural soil development and is thus usually associated with soil degradation. In contrast to natural pedogenesis leading to divergence of soil properties, agropedogenesis leads to their convergence because of the efforts to optimize conditions for crop production. Agricultural practices lead soil development toward a quasi-steady state with a predefined range of measured properties – attractors (an attractor is a minimal or maximal value of a soil property toward which the property will develop via long-term intensive agricultural use from any natural state). Based on phase diagrams and expert knowledge, we define a set of “master properties” (bulk density and macroaggregates, soil organic matter content, C:N ratio, pH and electrical conductivity – EC, microbial biomass and basal respiration) as well as soil depth (A and B horizons). These master properties are especially sensitive to land use and determine the other properties during agropedogenesis. Phase diagrams of master soil properties help identify thresholds and stages of soil degradation, each of which is characterized by one dominating process. Combining individual attractors in a multidimensional attractor space enables predicting the trajectory and the final state of agrogenic soil development and developing measures to combat soil degradation. In conclusion, the suggested new theory of anthro- and agropedogenesis is a prerequisite for merging various degradation processes into a general view and for understanding the functions of humankind not only as the sixth soil-forming factor but also as an ecosystem engineer optimizing its environment to fulfil a few desired functions.

Carbon pools and fluxes measured during a field campaign conducted in 2010 on the Tibetan Plateau at Kema

Functional Soil Organic Matter Fractions, Microbial Community, and Enzyme Activities in a Mollisol Under 35 Years Manure and Mineral Fertilization

Fertilization is a worldwide practice to maintain and increase crop productivity and improve soil quality in agricultural ecosystems. The interactive mecha

Shower thoughts: why scientists should spend more time in the rain

Stormwater is a vital resource and dynamic driver of terrestrial ecosystem processes. However, processes controlling interactions during and shortly after storms are often poorly seen and poorly sensed when direct observations are substituted with technological ones. We discuss how human observations complement technological ones and the benefits of scientists spending more time in the storm. Human observation can reveal ephemeral storm-related phenomena such as biogeochemical hot moments, organismal responses, and sedimentary processes that can then be explored in greater resolution using sensors and virtual experiments. Storm-related phenomena trigger lasting, oversized impacts on hydrologic and biogeochemical processes, organismal traits or functions, and ecosystem services at all scales. We provide examples of phenomena in forests, across disciplines and scales, that have been overlooked in past research to inspire mindful, holistic observation of ecosystems during storms. We conclude that technological observations alone are insufficient to trace the process complexity and unpredictability of fleeting biogeochemical or ecological events without the shower thoughts produced by scientists’ human sensory and cognitive systems during storms.

Using natural ¹³C abundances to differentiate between three CO₂ sources during incubation of a grassland soil amended with slurry and sugar

This study describes a novel approach to separate three soil carbon (C) sources by one tracer method (here 13 C natural abundance). The approach uses the temporal dynamics of the CO 2 efflux from a C 3 grassland soil amended with added C 3 or C 4 slurry and/or C 3 or C 4 sugar to estimate contributions of three separate C sources (native soil, slurry, and sugar) to CO 2 efflux. Soil with slurry and/or sugar was incubated under controlled conditions, and concentration and δ 13 C values of evolved CO 2 were measured over a 2‐week period. The main assumption needed for separation of three C sources in CO 2 efflux, i.e. , identical decomposition of applied C 3 and C 4 sugars in soil, was investigated and proven. The relative contribution to the CO 2 efflux was higher, but shorter with an increased (microbial) availability of the C source, i.e. , sugar > slurry > SOM. The shortcomings and limitations as well as possible future applications of the suggested method are discussed.

Changes in Mineralization Activity of Microbial Communities Depending on Physico-Chemical Properties of Soils and Cadmium Contamination

No abstract is provided for this article.

Combined biochar and nitrogen application stimulates enzyme activity and root plasticity

Biochar (BC) and nitrogen (N) fertilizers are frequently applied to improve soil properties and increase crop productivity. Nonetheless, our mechanistic understanding of plant-soil interactions under single or combined application of BC and N remains incomplete. For the first time, we applied a split-root system to evaluate how BC or N contributes to the changes in soil enzyme activities, N and phosphorus (P) cycling as well as root plasticity. Left and right parts of rhizoboxes were filled with silty-clay loamy soil amended with BC (15 g kg−1 soil, from wheat straw, 300 °C), N (0.05 g KNO3-N kg−1 soil) or a control (no amendments), resulting in the following combinations: BC/Control, N/Control, BC/N. Soil enzyme activities, available N and P, root morphology and plant biomass were analyzed after plant harvest. Plant biomass (shoot + root) ranged from 0.56 g pot−1 (BC/Control) to 0.91 g pot−1(BC/N). The decreased soil bulk density and increased P availability in the BC compartment (BC/Control and BC/N) stimulated root length by 1.4–1.8 times – an effect that was independent of N availability in the same rhizobox. Biochar stimulated activities of β-glucosidase and leucine aminopeptidase (by 33–39%) compared to N due to the coupling of C, N and P cycles in BC/N treated soil. Nitrogen fertilization also increased β-glucosidase activity compared to the unfertilized control, whereas root elongation remained unaffected. Thus, the combined application of BC/N had more efficient benefits for plant growth than BC or N alone. This is linked with i) the stimulation of enzyme activities at the BC locations to reduce N limitation for both microorganisms and plants, and ii) an increase of fine root production to improve N uptake efficiency. Thus, combined BC/N application is potentially especially sustainable to overcome nutrient limitation as well as to maintain crop productivity because it accelerates root-microbial interactions.

Plant and soil elemental C:N:P ratios are linked to soil microbial diversity during grassland restoration on the Loess Plateau, China

Plant and soil elemental ratios of carbon (C), nitrogen (N) and phosphorus (P) play a central role in shaping the composition and structure of microbial communities. However, the relationships between plant and soil elemental C:N:P ratios and microbial diversity are still poorly understood. Here, we evaluated the effects of C:N:P ratios in plant-soil systems on microbial diversity in a chronosequence of restored grasslands (1, 5, 10, 15, 25, and 30 years since restoration) on the Loess Plateau. We found that C and N concentrations, C:N and C:P ratios in leaf, root, soil and microbial biomass, bacterial and fungal diversity (Shannon-Wiener index) gradually increased with year since grassland restoration. Microbial C:N:P ratios ranged from 17.8:4.5:1 to 24.3:6.6:1, and C:P ratio increased from 17.8:1 at the 1-year site to 24.3:1 at the 30-year site, indicating the increasing P limitation for soil microorganisms during grassland development. Soil microbial diversity increased with root, soil, and microbial C and N concentrations, and decreased with P concentration (p < 0.05). Structural equation modeling indicated that soil and microbial C:N and N:P ratios had the greatest influences on soil bacterial and fungal diversity, and elemental C:N:P ratios had a greater effect on soil fungal than bacterial diversity. Our findings emphasize the importance of elemental C:N:P ratios on soil microbial diversity, which is critical for formulating policies for sustainable biodiversity conservation in terrestrial ecosystems.

Composition and metabolism of microbial communities in soil pores

Delineation of microbial habitats within the soil matrix and characterization of their environments and metabolic processes are crucial to understand soil functioning, yet their experimental identification remains persistently limited. We combined single- and triple-energy X-ray computed microtomography with pore specific allocation of 13 C labeled glucose and subsequent stable isotope probing to demonstrate how long-term disparities in vegetation history modify spatial distribution patterns of soil pore and particulate organic matter drivers of microbial habitats, and to probe bacterial communities populating such habitats. Here we show striking differences between large (30-150 µm Ø) and small (4-10 µm Ø) soil pores in (i) microbial diversity, composition, and life-strategies, (ii) responses to added substrate, (iii) metabolic pathways, and (iv) the processing and fate of labile C. We propose a microbial habitat classification concept based on biogeochemical mechanisms and localization of soil processes and also suggests interventions to mitigate the environmental consequences of agricultural management.

Contributors

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Effect of land use types on decomposition of 14C-labelled maize residue (Zea mays L.)

The effect of three land use types on decomposition of 14C-labelled maize (Zea mays L.) residues and soil organic matter were investigated under laboratory conditions. Samples of three Dystric Cambisols under plow tillage (PT), reduced tillage (RT) and grassland (GL) collected from the upper 5cm of the soil profile were incubated for 159days at 20°C with or without 14C-labelled maize residue. After 7days cumulative CO2 production was highest in GL and lowest in PT, reflecting differences in soil organic C (SOC) concentration among the three land use types and indicating that mineralized C is a sensitive indicator of the effects of land use regime on SOC. 14CO2 efflux from maize residue decomposition was higher in GL than in PT, possibly due to higher SOC and microbial biomass C (MBC) in GL than in PT. 14CO2 efflux dynamics from RT soil were different from those of PT and GL. RT had the lowest 14CO2 efflux from days 2 to 14 and the highest from days 28 to 159. The lowest MBC in RT explained the delayed decomposition of residues at the beginning. A double exponential model gave a good fit to the mineralization of SOC and residue-14C (R 2 >0.99) and allowed estimation of decomposition rates as dependent on land use. Land use affected the decomposition of labile fractions of SOC and of maize residue, but had no effect on the decomposition of recalcitrant fractions. We conclude that land use affected the decomposition dynamics within the first 1.5months mainly because of differences in soil microbial biomass but had low effect on cumulative decomposition of maize residues within 5months.