The hypothesis that the biological availability of soil organic matter (SOM) pools is inversely proportional to their thermal stability was tested using th
<p>Aggregation of mineral and organic particles is a key process of soil development, which promotes carbon (C) stabilization by hindering decomposition of plant and microbial residues. All microbial utilization and C stabilization processes lead to <sup>13</sup>C fractionation and consequently to various δ<sup>13</sup>C values of organic matter in aggregate size classes, sand, silt, and clay-sized particles, as well as density fractions. Differences in δ<sup>13</sup>C within the aggregates and density fractions may have two reasons: 1) preferential stabilization of organic compounds with light or heavy δ<sup>13</sup>C and/or 2) stabilization of organic materials after passing one or more microbial utilization cycles, leading to respiring of <sup>13</sup>C depleted CO<sub>2</sub> and heavier δ<sup>13</sup>C in remaining C. Assuming these two reasons, the new approach based on the natural differences in stable C isotopic composition between SOM fractions was proposed and tested on soils developed solely under C3 vegetation (arable, coniferous and deciduous forests) in boreal climate (Gunina and Kuzyakov, 2014). This approach assumes that: 1) <sup>13</sup>C enrichment between the SOM fractions corresponds to successive steps of SOM formation; 2) <sup>13</sup>C fractionation (but not the δ<sup>13</sup>C signature) depends mainly on the transformation steps and not on the C precursors. Consequently, <sup>13</sup>C enrichment of SOM fractions allows reconstructing the SOM formation pathways. To prove these initial results we reviewed  δ<sup>13</sup>C values of soils globally and focused on the i) estimation of the validity of this approach for soils developed under various climatic conditions and parent materials, and depending on fertilization, and ii) C flows not only between aggregate size classes and density fractions but also between various particle size classes of the soils (i.e. sand, silt, and clay) and iii) on revealing the intensities of natural <sup>13</sup>C fractionation during the stabilization of litter C in aggregates, particle size classes, and density fractions. Results showed that density fractions were <sup>13</sup>C enriched in the order: free particulate organic matter (POM) < light occluded POM < heavy occluded POM < mineral fraction, with the strongest increase between the light occluded and heavy occluded POM. The maximum <sup>13</sup>C fractionation during stabilization of litter C in density fractions and aggregate size classes was < 2‰. Δ<sup>13</sup>C 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. Thus, despite some limitations, δ<sup>13</sup>C natural abundance approach based on <sup>13</sup>C fractionation within individual steps of SOM formation is very useful and probably the sole approach to estimate C flows under steady-state without labeling.</p>
Obwohl Sorption an Mineraloberflachen als dominierender Prozess zur Erklarung des langsamen Umsatzes mineralassoziierter organischer Bodensubstanz (OBS) dient, widerspricht diese Idee einer zunehmenden Zahl an Inkubationsstudien, die zeigen, dass fur niedermolekulare Substanzen nicht nur die mikrobielle Aufnahme kompetitivier als die Sorption ist, sondern auch sorbierte Substanzen in hohem Mase desorbiert und mikrobiell verwertet werden konnen. Dabei wurde gezeigt, dass sich die Verstoffwechselung desorbierter Substanzen zugunsten eines erhohten Recyclings verschiebt. Dies wirft die Frage auf, ob Recycling von intakten Metaboliten, d.h. unter Erhalt des Kohlenstoffgerustes, generell ein bisher stark unterschatzter Prozess ist, der die relativ hohen 14C Alter der OBS teilweise erklaren kann. Nach Applikation hoher Toxindosen konnte nachgewiesen werden, dass die nachfolgende Reetablierung der mikrobiellen Gemeinschaft zu grosem Anteil auf Recycling der Nekromasskomponenten der Vorgeneration basiert. Intaktes Metabolitrecycling ist jedoch unter steady-state Bedingungen nur auserst schwierig von der direkten Stabiliserung zu unterscheiden. Um diesen Prozess im Fliesgleichgewicht nachzuweisen muss 1) ein Biomolekul untersucht werden, dessen Biosyntheseweg so aufwandig ist, dass Recycling einen deutlichen Vorteil fur die Zelle im Vergleich zur Neusynthese darstellt, 2) dieses Biomolkul in lebenden Zellen in einer anderen Form gebunden sein, als die zu recycelnde Einheit in der Bodenlosung, so dass beide Zustandsformen unterschieden werden konnen und 3) dieses Biomolekul positionsspezifisch isotopenmarkiert zugegeben werden, so dass uber einen identischen Einbau der Positionen die Intaktheit des Kohlenstoffgerustes nachgewiesen werden kann. Am Beispiel der Alkylketten von Fettsauren, die in mikrobiellen Zellen primar als Phospholipide in den Membranen gebunden sind, soll dieses Prinzip veranschaulicht werden. Eine erste Abschatzung des intakten Recyclings von Alkylketten durch Mikroorganismen in Boden zeigt, dass von den 0.03% der basierend auf Alkyl-Kohlenstoff neugebildeten PLFA mehr als 75% aus intaktem Recycling dieser Ketten hervor gingen. Obwohl der Beitrag des Recyclings intakter Metabolite zur Umsatzzeit der gesamten OBS aufgrund der geringen Anzahl bisher untersuchter Metabolite noch nicht final quantifiziert werden kann, untermauern die hier vorgestellten Ergebnisse jedoch die hohe Relevanz dieses Prozesses fur die Dynamik der OBS.
Most subsoil carbon (C) turnover occurs in biopore hotspots such as root channels and earthworm burrows. Biopores allocate large C amounts into the subsoil, where a vast capacity for long-term C sequestration is predicted. We hypothesise that organic matter (OM) cycling in biopores depends on their origin. Earthworm and root biopores were induced under field conditions and were sampled from the subsoil (45–75 and 75–105 cm) after two years of biopore formation. The effects of biopore formation on OM decomposition were studied by biomarkers: neutral sugars, cutin and suberin-derived lipids, lignin-derived phenols and free lipids. The degradation stage of OM was biopore type-specific but was only governed by the soil depth in root biopores. Degradation of OM increased from earthworm biopores to root biopores and bulk soil. Hemicelluloses (GM/AX ratio) were more strongly degraded than lignin side-chains (relative change from initial values). Two years of microbial processing during biopore formation increased the GM/AX ratio in earthworm biopores from 0.65 to 1.05 and in root biopores from 0.15 to 1.35 (both relative to source biomasses). Root biopores and bulk soil had the highest GM/AX ratios (1.2–1.3), hinting to rapid processing of plant residues and accumulation of microbial residues. The regular, frequent OM inputs by earthworms stimulated microbial growth and processing of mostly bioavailable OM and, thus, relatively enriched more persistent OM (e.g. lignin). Syringyl subunits of lignin underwent low (ratio changed from 0.35 to 0.55 relative to initial input) and vanillyl subunits underwent almost no processing in earthworm biopores indicating the preferential microbial utilisation of the easily available compounds frequently replenished by earthworm activity. After two years of decomposition of the root detritus, mainly structural plant material was enriched in root biopores. Short periods (6 months) of earthworm activity effectively recharged the highly processed OM in root biopores with fresh OM. In total, deep-rooting catch crops and short-term earthworm activities promote C accumulation in the subsoil followed by biopore-specific microbial processing predominantly governed by the C input frequency. As root biopores are up to 40 times more common than earthworm biopores, they dominate the OM input into subsoils. Such C inputs create several years lasting hotspots for preferential root growth and nutrient mobilisation in the subsoil. We conclude that root- and earthworm-derived biopores are vertical pathways for plant C from the soil surface into the subsoil and for intensive processing of litter C and sequestration of microbial necromass.
Silicon (Si) is the second-most abundant element in the earth’s crust. In the pedosphere, however, huge spans of Si contents occur mainly caused by Si redistribution in soil pro-files and landscapes. Here, we summarize the current knowl-edge on the different pools and fluxes of Si in soils and terres-trial biogeosystems. Weathering and subsequent release of soluble Si may lead to (1) secondarily bound Si in newly formed Al silicates, (2) amorphous silica precipitation on sur-faces of other minerals, (3) plant uptake, formation of phyto-genic Si, and subsequent retranslocation to soils, (4) translo-cation within soil profiles and formation of new horizons, or (5) translocation out of soils (desilication). The research car-ried out hitherto focused on the participation of Si in weather-ing processes, especially in clay neoformation, buffering mechanisms for acids in soils or chemical denudation of land-
Different inputs of mineral and organic fertilizers are mainly responsible for increasing yield of crop coupled with an increase in the amount of crop residues. Transformation of the crop residues into soil organic C is an important factor improving soil fertility and aggregation. Apart from long-term organic matter influence, aggregation is also promoted by soil bacterial and fungal biomass. These effects on soil aggregation in relation to soil organic C content have not yet been well investigated. We studied the long-term effects of 20-years application of mineral and organic fertilizers in an intensive horticultural crop rotation. Treatments: without fertilization or manuring (control soil), nitrogen applied by mineral fertilizer (N), and manure with low or high application rates (M and 2M). The short-term effect on soil aggregation was simulated by addition of K-polyacrylate to the soil and incubation for 2 weeks. Long-term fertilization led to increase of soil organic C content by (42-73) %. This corresponded to the highest portion of small macroaggregates (1-0.25 mm) instead of large macroaggregates (1-2 mm) in fertilized soil compared to control soil. On contrary, K-polyacrylate induced short-term aggregation, appreciably (14-18%) improved large macroaggregates proportion of soils independent on fertilization. Long-term fertilization accelerated the rate of soil organic C (SOC) decomposition in the initial days of laboratory incubation. The CO2 emission rate became almost static for all soils after 21 days of incubation. Cumulative CO2 efflux was high (4.2-5.2% of SOC after 80 days of incubation). Short-term aggregation by K-polyacrylate slowed down the rate of mineralization of labile pools of SOC and allowed greater protection of SOC against decomposition.
Abstract Investigating the response of the rhizosphere and bulk soil to fertilization, along with the mechanisms governing their dynamic and stability, is essential to understand root and microbial functions in ecosystems. Here we synthesized 7606 pairwise observations based on 3803 paired samples from rhizosphere and bulk soil to assess how chemical and microbiological properties respond to application of mineral and organic fertilizers across croplands, grasslands, and forests. Fertilization positively altered the overall chemical properties of rhizosphere and bulk soil by 21% and 18%, respectively, while microbiological properties remained largely unchanged except for microbial biomass and the activities of certain enzymes. Fertilization decreased the response variability of soil properties, resulting in reduced heterogeneity, particularly in bulk soil. The response slopes of rhizosphere and bulk soil properties to fertilization were below 1.0, indicating that the rhizosphere had greater resistance, especially under organo-mineral fertilizer application. This high resistance stems from i) large input or available compounds by roots maintaining the stability of microbial communities in rhizosphere, ii) stronger microbial control over chemical properties, and iii) a larger gap in response variability between rhizosphere and bulk soil. In conclusion, while rhizosphere and bulk soil properties respond similarly to fertilization, the rhizosphere demonstrates greater stability due to its higher resistance.
Purpose . This study is aimed to develop a model of priming effect (accelerated mineralisation of soil organic matter (SOM)) induced by root exudate input into nitrogen (N) limited rhizosphere soil as a typical case for most terrestrial ecosystems. This ecologically important process in the functioning of the “plant-soil” system was parameterized for temperate and boreal forests. Methods. A model of priming effect has been developed based on the concept of N mining to making up for the N scarcity in exudates by accelerating SOM mineralisation. Lacking N for microbial growth is mined from the SOM mineralisation considering C:N ratio of soil. The model has a built-in food web module, which calculates soil fauna feeding on microorganisms, the release of by-products of faunal metabolism and mineral N used for root uptake. Results . The model verification demonstrated the similar order of the priming effect as in the published experiments. Testing at the pedon level revealed a high sensitivity of the model to N content in root exudates. Testing of the model at the ecosystem level revealed that CO 2 emission from the priming can reach 25–30% of CO 2 emission from the whole Ah horizon of forest soil. The same intensities were simulated for the fauna-derived N released within the rhizosphere. Conclusion. The new model reflects important ecological consequences of the main target function of priming effects within the “plant – soil – microorganisms – fauna” system – the microbial acceleration of C and N cycling in the rhizosphere and detritusphere to mobilise mineral N for plants.
<p>Among the factors controlling root exudation, root hair proliferation and warming strongly influence exudate release, microbial substrate utilization and enzyme activities. The interactions of these two factors are important but poorly known in the rhizosphere. To clarify these interactions, two maize varieties – a wild type with root hairs and a hairless mutant – were grown at 20 and 30 °C for 2 weeks. We applied a unique combination of zymography to localize hotspots of β-glucosidase with microcalorimetry and substrate-induced respiration from soil sampled in hotspots. This approach enabled monitoring exudate effects on microbial growth strategy, enzyme kinetics (V<sub>max</sub> and K<sub>m</sub>), heat release and CO<sub>2</sub> production in the hotspots in response to warming.</p><p>Root hair effects on enzyme activity and efficiency were pronounced only at the elevated temperature: i) β-glucosidase activity of the wild type at 30 °C was higher than that of the hairless maize; ii) temperature shifted the microbial growth strategy, whereas root hairs (i.e. C input) promoted the fraction of growing microbial biomass; iii) K<sub>m</sub> and the activation energy for β-glucosidase under the hairless mutant was lower than that under wild maize. These results suggest that microorganisms inhabiting hotspots of the wild type synthesized more enzymes to fulfill their higher energy and nutrient demands than those of the hairless mutant. In contrast, at higher temperature the hairless maize produced an enzyme pool with higher efficiencies rather than higher enzyme production, enabling metabolic needs to be met at lower cost. These changes in enzyme kinetics and metabolic shifts confirmed evolutionary theory on tradeoffs of enzyme structure–function and thermal–substrate under warming at the soil hotspot level. We conclude that, if microbial and enzymatic activities are stimulated by more substrate input under warming, then this shift in the microbial community and in enzyme systems to a lower efficiency could offset C losses.</p>
Despite its fundamental role for carbon (C) and nutrient cycling, rhizodeposition remains ‘the hidden half of the hidden half’: it is highly dynamic and rhizodeposits are rapidly incorporated into microorganisms, soil organic matter, and decomposed to CO2. Therefore, rhizodeposition is rarely quantified and remains the most uncertain part of the soil C cycle and of C fluxes in terrestrial ecosystems. This review synthesizes and generalizes the literature on C inputs by rhizodeposition under crops and grasslands (281 data sets). The allocation dynamics of assimilated C (after 13C-CO2 or 14C-CO2 labeling of plants) were quantified within shoots, shoot respiration, roots, net rhizodeposition (i.e., C remaining in soil for longer periods), root-derived CO2, and microorganisms. Partitioning of C pools and fluxes were used to extrapolate belowground C inputs via rhizodeposition to ecosystem level. Allocation from shoots to roots reaches a maximum within the first day after C assimilation. Annual crops retained more C (45% of assimilated 13C or 14C) in shoots than grasses (34%), mainly perennials, and allocated 1.5 times less C belowground. For crops, belowground C allocation was maximal during the first 1-2 months of growth and decreased very fast thereafter. For grasses, it peaked after 2-4 months and remained very high within the second year causing much longer allocation periods. Despite higher belowground C allocation by grasses (33%) than crops (21%), its distribution between various belowground pools remain very similar. Hence, the total C allocated belowground depends on the plant species, but its further fate is species independent. This review demonstrates that C partitioning can be used in various approaches, e.g. root sampling, CO2 flux measurements, to assess rhizodeposits’ pools and fluxes at pot, plot, field and ecosystem scale and so, to close the most uncertain gap of the terrestrial C cycle. This article is protected by copyright. All rights reserved.
Plant photosynthetic products are microbially and chemically transformed and retained as soil organic matter (SOM), which is essential for ecosystem functioning, particularly in the detritusphere. The energy quantity (EQ) of an organic substance, defined as the net energy release per gram of the sample, depends on its OM content and the energy density (ED, the net energy release per gram of OM). During the transformation of plant leaves to SOM, both the energy quantity retention ratio (EQSoil/Leaf, the ratio of soil’s EQ to leaves’ EQ) and the OM retention ratio (OMSoil/Leaf, the ratio of SOM to leaves’ OM) are consistently less than one, reflecting the loss of OM as CO2 and coupled heat dissipation. However, how the energy density conversion ratio (EDSOM/Leaf-OM) changes and its influence on EQSoil/Leaf ratio remain unknown. Here, we 1) examined the EDSOM/Leaf-OM ratio along an elevation gradient (481–3035 m), where environmental conditions shift from warm-dry to cold-wet, and 2) explored how the EDSOM/Leaf-OM ratio was influenced by chemical composition (e.g., the H/C and C/N ratios of plant leaves, the ratio of stable-to-labile OM thermal pool (Exo2/Exo1)), climate, and edaphic factors. The EDSOM/Leaf-OM ratio was < 1 (0.7 ± 0.04) in warm-dry areas, indicating the decrease in OM’s energy density from plant leaves to SOM, whereas it remained 1 (1.0 ± 0.1) in cold-wet areas. In warm-dry areas, the EDSOM/Leaf-OM ratio decreased with the ratio of themal pools Exo2/Exo1 of SOM due to the protection of low-energy–density OM by Fe/Al oxides. Conversely, in cold-wet areas, EDSOM/Leaf-OM declined with increasing Exo2/Exo1 ratio of leaf, which was featured by low H/C and high C/N ratio. The EDSOM/Leaf-OM ratio modulates energy retention as OMSoil/Leaf in cold-wet areas, underlining the dynamics of SOM energy beyond mass loss. This study highlights that energy retention during SOM formation is environment-sensitive, mediated by two mechanisms: mineral protection in warm-dry areas versus biochemical transformation in cold-wet areas. We recommend integrating energy and mass flux assessments to better understand OM decomposition, stabilization, and ecosystem carbon storage.
