Andic soils contain a large amount of stabilised soil organic matter (SOM). The present study aims to review and integrate the determining factors and mechanisms of SOM stabilisation in andic compared with other (non-andic) soil types. We have reviewed recent literature regarding the nature of SOM and its stabilisation processes in the top- and sub-soil to address and discuss the interaction between the SOM and the mineral phase. The carbon (C) storage capacity by the metal-humus-complex formation of volcanic soils is also evaluated. The most important stabilisation processes are related to the incorporation and decomposition of microbial-derived C along with the changing C storage capacity with increasing soil development. The priming and destabilisation of adsorbed SOM are crucial mechanisms influencing the soil C sequestration in subsoils. The C storage capacity of andic soils was closely related to the Na-pyrophosphate extractable Al and Fe. The upper boundary for SOM saturation with Al and Fe was a molar metal:C ratio of 0.18. The influence of climate, mineralogy and soil disturbances on the SOM storage capacity of andic soils also require further attention.
<p>Electron acceptors (NO<sub>3</sub><sup>–</sup>, SO<sub>4</sub><sup>2–</sup>, Fe<sup>3+</sup>, Mn<sup>4+</sup>) play a crucial function in the oxidation of soil recalcitrant organic compounds. Soils that present large amount of total Fe (8-57 g kg<sup>-1</sup>soil) and organic (C) (10-110 g kg<sup>-1</sup>soil), iron-reducing bacteria (IRB)  may play a importan role. In the present study we hypothesized that IRB which reduce Fe(III)(oxyhydr)oxide of low solubility to soluble Fe(II), can contribute substantially to the degradation of lignin from soil organic matter (SOM). The aim of this study was to isolate IRB and evaluate their importance in lignin degradation. IRB were obtained from topsoils of different climates (humid temperate, cold temperate, subpolar), vegetation type (steppe, rainforest) and parent materials (granitic, volcanic, fluvio-glacial, basaltic-Antartic and metamorphic). The potential of IRB to reduce Fe(III) was assessed with lactate substrate as source of carbon (C) and anthraquinone-2,6-disulfonate (AQDS) as electron acceptor. The contribution of IRB to lignin degradation was assessed in an anaerobic microcosms experiment for 36 h. The CO<sub>2</sub> efflux from sterilized and reinoculated soil with IRB was compared with sterilized (abiotic), non-sterilized (biotic) and induced Fenton reaction. Lignin degradation by IRB was examined by: 1) bacterial growth containing alkali lignin and alkali lignin disappearance during incubation, 2) Lignin peroxidase and manganese peroxidase activities originated from IRB, 3) cells abundance estimated from ATP synthase from bacteria growing in alkali lignin and 4) lignin degradation monitored by fluorescence disappearance intensity. The major microbial group for Fe(III) reduction, as essayed by PLFA and nested-PCR and sequencing different species were Geobactericeae-strains (G. metallireducens and G. lovleyi) in all studied. The CO<sub>2</sub> respiration in reinoculated soils was 140% higher than the CO<sub>2 </sub>release by abiotic and Fenton reaction and, 40% lower than biotic treated soil. The Fe(II) extractable in HCl in soil derived from basaltic-Antarctic parent material showed 362 % more Fe(II) solubilisation than that of biotic treatment. Fluorescence intensity decreased during lignin degradation and it was closely correlated with CO<sub>2</sub> release in the same sample. We conclude that IRB community such as Geobacter spp. Uses intensively Fe(III) as an electron acceptor to oxidize lignin compounds, and this process is especially active in Fe rich soils.</p>
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Over the last decades, Chile has experienced a long-term drought with significant consequences for water availability, forest productivity, and soil degradation, ultimately dramatically increasing the surface of burned area. Here, we quantify the Palmer Drought Severity Index (PDSI) to ascertain the extent of “moisture deficiency” across the central-southern region of Chile from 2000 to 2023 to assess the drought’s relationship with the frequency of wildfires focusing on the impact of native forests. Our methodology quantifies the PDSI from the burned area data using MODIS MCD64A1 satellite imagery, validated by in situ wildfire occurrence records. The findings indicate that 85.2% of fires occurred under moderate to severe drought conditions. We identified 407,561 ha showing varying degrees of degradation due to wildfires, highlighting the critical areas for targeted conservation efforts. A significant increase in both the frequency of wildfires and the extent of the affected area in native forests was observed with the intensification of drought conditions in the 21st century within mesic to humid Mediterranean climatic zones where drought explains up to 41% of the variability in the burned area (r2 = 0.41; p < 0.05). This study highlights the relationship between drought conditions and wildfire frequency, showing the paramount need to adopt comprehensive wildfire mitigation management in native forests.
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The effects of cultivation in hillside (> 30% slope) on soil C and N stocks after forest clearance and subsequent cultivation in new re-growth forest sites not always induce rapid soil organic matter (SOM) loss in subtropical areas. In the present study we evaluated the sensitivity to SOM changes of material floatable in water (Fw), coarse sand (> 250 µm), fine sand (50-250 µm), silt (2-50 µm) and clay (0-2 µm) fractions by the impact of cultivation of Mexican Ferrasols. This research also determined the relative degree of C and N saturation in the silt and clay size particles. The soil physical fractions were obtained after ultrasonic vibration and sedimentation. Soil organic matter stock (0-20 cm depth) in cultivated sites ranged from 54 to 146 Mg C ha-1 and from 3.9 to 7.1 Mg N ha-1 and in the forest soil 46-94 Mg C ha-1 and 3.7-5.1 Mg N ha-1. The relative distribution of C and N was lower in the coarse sand and highest in the silt fraction; the latter paralleled the changes of whole soil C and N. Coarse sand was the most sensitive fraction responding to cultivation, thus representing a diagnostic pool to assess the SOM shifts under cultivation. Half of our studied soil showed a low degree of C and N saturation in the silt and clay fraction indicating potential for accumulation of C and N, while the other half were well above the saturation limit.
The silt and clay particles play a key role as stabilizing agents of soil organic carbon (SOC). Several lines of evidence indicate a theoretical maximum or C saturation in individual particles. In the present study, we hypothesized that a C fraction displaying linear accumulation relative to the SOC is not influenced by C saturation, while a fraction displaying an asymptotic relationship is regarded as saturated (Stewart et al., 2008). The aim of the present study was to compare the amount of C in the silt and clay sized fractions in temperate and subtropical cropping soils across a range of textures with different mineralogy. Twenty-one and 18 soil samples containing 1:1 and 2:1 clay of temperate soil from Chile under monoculture of maize (Zea maiz L.) for at least 30 years and 9 subtropical soils from Mexico under maize and bean (Phaseolus vulgaris L.) cropping for 9 years having mixed clay were collected at 0-0.1 m. The SOC of 2:1 soils was significantly higher (14±0.5 g kg-1 dry soil) than 1:1 soils (10±0.7 g kg-1). However, subtropical soils showed the highest values (59±0.5 g kg-1). A positive (P < 0.01) relationship was observed between the SOC and the C in the silt fraction (R2 0.80-0.97, P < 0.01). In contrast, the clay fraction remained constant or showed asymptotic behavior. We conclude that the silt fraction, unlike clay, showed no evidence of C saturation, while clay accumulates C to a maximum. On average, the 2:1 clay was saturated at 1-2 g C kg-1 and 1:1 at 1 g C kg-1, and subtropical soils at 14 g C kg-1
