Iron hydroxides serve as an efficient ‘rusty sink’ promoting the stabilization of rhizodeposits into soil organic carbon (SOC). Our work aimed to understand the physicochemical and microbial mechanisms promoting rhizodeposit (rhizo-C) stabilization as influenced by goethite (α-FeOOH) or nitrogen (N), using 13C natural abundance methodologies and DNA sequencing, in the rhizosphere of maize (Zea mays L.). The addition of N fertilizer to soil increased the mineralization of both rhizo-C and SOC, while amendment with α-FeOOH decreased rhizo-C derived CO2 and lowered the rhizosphere priming effect by 0.57 and 0.74-fold, respectively, compared to the control soil. This decrease resulted from the co-precipitation of rhizo-C at the reactive α-FeOOH surfaces as Fe-organic matter complexes (FeOM), which was 10-times greater than the co-precipitation on short-range ordered minerals. The highest portion of rhizo-C (67% of the total accumulated in soil) was protected within macroaggregates (>2 mm). Carbon overlapped with α-FeOOH mainly in >2 mm aggregates, as shown by HRTEM-EDS imaging, suggesting that α-FeOOH associated rhizo-C stimulated aggregate formation. Random forest analysis confirmed that the stabilization of rhizo-C was controlled mainly by physiochemical binding within FeOM complexes and macroaggregates. Rhizo-C mineralization was regulated by the keystone microbiome: Paucimonas (β-Proteobacteria) being an r-strategist with rapid growth under soil without nutrient limitation (N treated) and Steroidobacter (Actinobacteria) with branched filaments that can access C and nutrients under oligotrophic conditions (goethite enriched soil). Two-way orthogonal partial least squares analysis revealed that the rhizosphere priming effect was facilitated mainly by the same genera, most likely due to co-metabolism. The genera belonging to Acidimicrobiaceae (Actinobacteria), Cryptococcus and Cystofilobasidium (Basidiomycota) were positively correlated with the accumulation of rhizo-C in the >2 mm aggregate size, which might due to their high affinity towards α-FeOOH and contribution to the development of aggregation via filamentary structures that interact with microaggregates. We suggest that rhizodeposit stabilization in soil was balanced by microbial mineralization and abiotic associations with the “rusty sink” and organisms with branched filaments contributing to the development of aggregation.
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. 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.
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.
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.
