Activation energy (Ea) of (bio)chemical reactions – the fundamental parameter, defining the reaction rates – has never been critically evaluated and generalized for processes of organic matter transformations in soil. Based on the database of Ea for a broad range of i) oxidative and hydrolytic exoenzyme activities, ii) CO2 production and iii) heat release during soil incubation, as well as iv) thermal decomposition of soil organic matter (SOM), we assessed the Ea for processes of SOM transformation. After a short description of the four approaches to assess Ea of SOM transformation – all based on the Arrhenius equation – we present the Ea of chemical (79 kJ mol-1) and microbial (67 kJ mol-1) mineralization, microbial decomposition (40 kJ mol-1), and exoenzyme-catalyzed depolymerization (33 kJ mol-1) of SOM. The catalyzing effects of exoenzymes reduce the energy barrier of SOM decomposition by more than twice that of its chemical oxidation (from 79 to 33 kJ mol-1). The Ea of exoenzymatic hydrolysis of N-, P-, and S-containing organic compounds is about 9 kJ mol-1 lower (40-fold faster reactions) than that of other (N-, P-, and S-free) organic substances. Under real soil conditions (not in suspension as in enzyme activity analysis), where organic substrates are physically protected and exoenzymes are partly deactivated, microbial mineralization of SOM is 140-fold faster compared to its chemical oxidation. The Ea of microbial mineralization of SOM increases from biochemically labile to stable pools. This is one of the reasons for the decrease in the CO2 efflux from soil during long-term incubations.Since processes with higher Ea are more sensitive to temperature increase, global warming will stimulate faster decomposition of stable organic compounds and accelerate the C cycle much stronger than the cycling of the nutrients N, P, and S. The consequence will be a shift in the stoichiometric ratios of microbially utilized substrates. Overall, Ea is an easily measurable crucial parameter of (bio)chemical transformations of organic matter in soil, enabling the assessment of process rates and the inherent stability of SOM pools, as well as their responses to global warming.
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.
. 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.
The soil-water interfaces (SWI) in soil pores are hotspots for organic matter (OM) transformation. However, due to the heterogeneous and opaque nature of soil microenvironment, direct and continuous tracing of interfacial reactions, such as OM transformations and formation of organo-mineral associations, are rare. To investigate these processes, a new soil microarray technology (SoilChips) was developed and used. Homogeneous 800-μm-diameter SoilChips were constructed by depositing a dispersed Oxisol A horizon suspension on a patterned glass. Dissolved organic matter from the original soil was added on the SoilChips to mimic SWI processes. The effects of ammonium fertilization (90 mg N kg-1 soil) on chemical composition of SWIs were evaluated via X-ray photoelectron spectroscopy. Over 21 days, ammonium addition increased OM coatings at SWIs and modified the OM chemical structure with more alcoholic- and carboxylic-C compared to the unfertilized control. Molecular modeling of OM composition at SWIs showed that N fertilization mainly facilitated the microbial production of glucans. We demonstrated that N availability modifies the specific OM molecular processing and its immobilization on SWIs, thereby providing a direct insight into biogeochemical transformation of OM at micro-scale.
The anaerobic oxidation of methane (AOM) in marine ecosystems is ubiquitous and largely coupled to sulfate reduction. In contrast, the role of AOM in terrestrial environments and the dominant electron acceptors driving terrestrial AOM needs deeper understanding. Submerged rice paddies with intensive CH4 production have a high potential for AOM, which can be important for greenhouse gas mitigation strategies. Here, we used 13CH4 to quantify the AOM rates in paddy soils under organic (Pig manure, Biochar) and mineral (NPK) fertilization. Alternative-to-oxygen electron acceptors for CH4 oxidation, including Fe3+, NO3 −, SO4 2−, and humic acids, were examined and their potential for CH4 mitigation from rice paddies was assessed by 13CH4 oxidation to 13CO2 under anoxic conditions. During 84 days of anaerobic incubation, the cumulative AOM (13CH4-derived CO2) reached 0.15–1.3 μg C g-1 dry soil depending on fertilization. NO3- was the most effective electron acceptor, yielding an AOM rate of 0.80 ng C g-1 dry soil h-1 under Pig manure. The role of Fe3+ in AOM remained unclear, whereas SO42- inhibited AOM but strongly stimulated the production of unlabeled CO2, indicating intensive sulfate-induced decomposition of organic matter. Humic acids were the second most effective electron acceptor for AOM, but increased methanogenesis by 5–6 times in all fertilization treatments. We demonstrated for the first time that organic electron acceptors (humic acids) are among the key AOM drivers and are crucial in paddy soils. The most pronounced AOM in paddy soils occurred under Pig manure, followed by Control and NPK, while AOM was the lowest under Biochar. We estimate that nitrate (nitrite)-dependent AOM in paddy fields globally consumes ~3.9 Tg C–CH4 yr-1, thereby offsetting the global CH4 emissions by ~10–20%. Thus, from a broader agroecological perspective, the organic and mineral fertilizers control an important CH4 sink under anaerobic conditions in submerged ecosystems. Appropriate adjustments of soil fertilization management strategies would therefore help to decrease the net CH4 flux to the atmosphere and hence the global warming.
No abstract is provided for this article.
Distribution and diffusion of root exudates of Zea mays were studied in a loamy Haplic Luvisol by means of 14CO2 pulse labelling of shoots in two-compartment pots, in which the roots were separated from sterile or nonsterile soil by a screen. Root hairs but not roots could penetrate the screen into the soil. Root-free soil in the bottom pot compartment of one treatment was sterilized with cycloheximide and streptomycin to inhibit microbial decomposition of exudates. The soil from the bottom part of the pots was frozen and sliced by a microtome into 15 layers, each 1 mm thick. Four zones of exudate concentrations were found according to the distribution of the 14C activity in the rhizosphere profile: 1) 1…2 (3) mm: maximal concentration of exudates around root hairs; 2) 3…5 mm: presence of exudates is caused by their diffusion from the root hairs; 3) 6… 10 mm: insignificant amounts of exudates diffused from the previous zones; 4) > 10 mm: complete lack of exudates. The amount of 14C exudates was higher in the first 1-mm layer from the roots of nonsterile soil compared to the sterile soil due to stimulation of exudation by microorganisms. The coefficient of vertical exudate diffusion in the soil was 1.9 × 10•7 cm2/Vs.
Psychrotolerant microbes are crucial for carbon cycling and biotechnological applications. Nonetheless, the mechanisms enabling their survival and function
Carbon isotopic composition of soils subjected to C3–C4 vegetation change can be used to estimate C turnover in bulk soil and in soil organic matter
Understanding the pathways of nitrogen (N) retention in pristine forest soils is essential for effective ecosystem management and nutrient conservation. The incorporation of nitrate (NO<sub>3</sub><sup>-</sup>) and nitrite (NO<sub>2</sub><sup>-</sup>) into organic N in soils without microbiological contribution remains a very intriguing question. This study explores the abiotic incorporation of nitrate (NO<sub>3</sub><sup>-</sup>) and nitrite (NO<sub>2</sub><sup>-</sup>) into organic N of volcanic soil under sterilized and anoxic conditions, providing insights into mineral N losses occurring as dissolved organic N (DON) rather than the commonly accepted nitrate leaching. We evaluated the hypothesis that nitrate (NO<sub>3</sub><sup>-</sup>) can be reduced to nitrite (NO<sub>2</sub><sup>-</sup>), which subsequently reacts with organic matter through nitration and nitrosation, leading to the formation of organic nitrogen. This mechanism, which is of great ecological significance, supports the Ferrous Wheel Hypothesis (FWH). The FWH proposes that ferrous iron, Fe(II), reduces NO<sub>3</sub><sup>-</sup> to NO<sub>2</sub><sup>-</sup> within anaerobic microsites, and that Fe(II) is then re-oxidised to ferric iron, Fe(III), contributing to the formation of dissolved organic N (DON). Both NO<sub>3</sub><sup>-</sup> and NO<sub>2</sub><sup>-</sup> declined rapidly by 51 and 94 %, while labelled organic N increased by 20-38 % for NO<sub>3</sub><sup>-</sup> and 42-44 % for NO<sub>2</sub><sup>-</sup> within seconds. The incorporation of <sup>15</sup>N into organic forms was confirmed using ATR-FTIR and benzene:isopropanol extraction, with the lowest and highest accumulation observed at 5 and 15 mg NO<sub>₃</sub>⁻ kg⁻¹, respectively. These results demonstrate that NO<sub>3</sub><sup>-</sup> incorporation into organic N can occur primarily through abiotic processes, supporting the FWH, as both DON and solid-phase organic N were measured. These findings highlight the natural resilience of volcanic soils in unpolluted old-growth temperate rainforests to N loss and provide new insights into long-term ecosystem stability and nutrient cycling. Further research should investigate the interplay between abiotic and biotic N transformations under field conditions and across diverse forest ecosystems.
Priming effect is the change of soil organic matter (SOM) decomposition due to the addition of labile carbon (C) sources. Earthworms incorporate organic matter into their burrow-linings thereby creating preferred habitats for microorganisms, but the roles of such burrows in priming effect initiation is unknown. Here we study the mechanisms driving SOM decomposition in top- and subsoil biopores and additionally in the rhizosphere. Given the topsoil was newly formed after ploughing 10 months prior to sampling, we hypothesized that (1) SOM accessibility, enzyme activities and efficiency of enzymatic reaction (K a ) are main drivers of different priming effect in biopores vs. bulk soil and rhizosphere, subsoil vs. topsoil and (2) the production of microbial enzymes in biopores depends on microbial community composition. To test these hypotheses, biopores formed by Lumbricus terrestris L. and bulk soil were sampled from topsoil (0–30 cm) and two subsoil depths (45–75 and 75–105 cm). Additionally, rhizosphere samples were taken from the topsoil. Total organic C (Corg), total N (TN), total P (TP) and enzyme activities involved in C-, N-, and P-cycling (cellobiohydrolase, β-glucosidase, xylanase, chitinase, leucine aminopeptidase and phosphatase) were measured. Priming effects were calculated as the difference in SOM-derived CO2 from soil with or without 14C-labeled glucose addition. Enzyme activities (V max ) and the catalytic efficiency (K a ) were higher in biopores compared to bulk soil and the rhizosphere, indicating that the most active microbial community occurred at this site. Negative correlations between some enzymes and C:N ratio in bulk soil are explained by higher content of fresh organic C in the topsoil, and the corresponding C and nutrient limitations in the subsoil. The positive correlation between enzyme activities and Corg or TN in biopores, however, was associated with the decrease of C and TN with pore age in the subsoil. In the subsoil, priming effect in biopores was 2.5 times higher than bulk soil, resulting from the favorable conditions for microorganisms in biopores and the stimulation of microbial activities by earthworm mucus. We conclude that earthworm burrows provide not only the linkage between top- and subsoil for C and nutrients, but strongly increase microbial activities and accelerate SOM turnover in subsoil, contributing to nutrient mobilization for roots.
Although most organic matter (OM) in soil is mineralized by microorganisms, the nonmicrobial processes, e.g., Fenton reactions and photo-degradation, strongly contribute to OM decomposition and CO2 emission and are frequently the chemical background of biotic transformations. Fenton oxidation is a catalytic reaction chain of hydrogen peroxide (H2O2) with ferrous iron (Fe(II)) and Fe (oxyhydr)oxides that generates highly reactive hydroxyl radicals (HO•) oxidizing OM to CO2. Globally, reactive Fe (oxyhydr)oxides store at least one quarter (~600 Gt) of organic C in soil, which may be subjected to Fenton reactions, in which nano-sized Fe (oxyhydr)oxides act as nanocatalysts. The Fenton mechanisms depend on the sources and pathways of reactive oxygen species (ROS): O2 •−, H2O2 and HO•. Given that microorganisms continuously produce ROS, biotic Fenton chemistry is ubiquitous in all soils (including subsoil), especially in those with strong UV radiation, fluctuating O2 concentrations and redox conditions, microbial hotspots such as rhizosphere and detritusphere, and high contents of amorphous or short-range ordered Fe (oxyhydr)oxides. Charcoal and biochar mediate heterogeneous catalysis and ROS formation in soil directly – as an electron shuttle – or indirectly by electron transfer from the valence band to the conduction band in the minerals under UV irradiation. Despite the extremely short lifetime (from nanoseconds to a few days), ROS are continuously produced and sustain the ubiquity of chelators and Fe(III) reduction. For the first time, we calculated the fundamental Eh-pH diagrams for ROS species and showed their relevance for Fenton reactions under specific soil conditions. Based on its extremely high reactivity (Eo = 2.8 V), HO• is one of the most powerful oxidants and may provide the most efficient energy release from Fenton reactions in soil. Even though the direct contribution of Fenton reactions to OM oxidation and CO2 emission is less than 0.5% on the global level, in some soils and ecosystems (e.g., hot deserts, red soils in the tropics and wet subtropics) it can reach 30% and even exceed 50% of total CO2 emissions. Fenton reactions are omnipresent and play a dual role for soil C cycling: they stimulate OM mineralization (including the most stable C pools such as charred C) and facilitate long-term C stabilization due to the increased recalcitrance of remaining OM and organo-mineral complex formation. Agricultural management positively affects Fenton reactions, accelerating C cycling and nutrient acquisition by plants. Accordingly, Fenton reactions and their effects on OM decomposition and formation are an emerging research field that explains the chemical background of many oxidative enzymatic processes. This may crucially change our views on C, energy and nutrient cycling in soils, especially in a changing world.
