The diversity of orchid mycorrhizal fungi (OMF) and other beneficial root-associated fungi in tem... more The diversity of orchid mycorrhizal fungi (OMF) and other beneficial root-associated fungi in temperate forests has scarcely been examined. This study aimed to analyze the diversity of mycorrhizal and rhizosphere-associated fungal communities in the terrestrial orchids Gavilea lutea and Chloraea collicensis growing in high-orchid-population-density areas in the piedmont of the Andes Cordillera with native forest (Nothofagus-Araucaria) and Coastal Cordillera with an exotic plantation (Pinus-Eucalyptus) in south-central Chile. We focused on rhizosphere-inhabiting and peloton-associated OMF in a native forest (Andes Cordillera) and a mixed forest (Coastal Cordillera). The native terrestrial orchids G. lutea and C. collicensis were localized, mycorrhizal root segments were taken to isolate peloton-associated OMF, and rhizosphere soil was taken to perform the metabarcoding approach. The results revealed that Basidiomycota and Ascomycota were the main rhizosphere-inhabiting fungal phyla, ...
A computer model is described that is able to trace the fate of nitrogen in crop residues added t... more A computer model is described that is able to trace the fate of nitrogen in crop residues added to field soils. Clover and wheat residues labelled with 15N were added to a clay and a sand soil and the fate of the label traced over a period of almost 16 months under field conditions. Using a simple function to retard the decomposition of crop residues according to how much fibrous tissue they contain, the model was able to estimate the organic N remaining in soil, and the mineral N and microbial biomass N derived from the crop residues. It proved necessary, however, to postulate the existence of a pool of organic matter derived from crop residues that was more labile than native humus in soil.
Plants and microorganisms, besides the climate, drive nitrogen (N) cycling in ecosystems. Our obj... more Plants and microorganisms, besides the climate, drive nitrogen (N) cycling in ecosystems. Our objective was to investigate N losses and N acquisition strategies along a unique ecosystem-sequence (ecosequence) ranging from arid shrubland through Mediterranean woodland to temperate rainforest. These ecosystems differ in mean annual precipitation, mean annual temperate, and vegetation cover, but developed on similar granitoid soil parent material, were addressed using a combination of molecular biology and soil biogeochemical tools. Soil N and carbon (C) contents, δ15N signatures, activities of N acquiring extracellular enzymes as well as the abundance of soil bacteria and fungi, and diazotrophs in bulk topsoil and rhizosphere were determined. Relative fungal abundance in the rhizosphere was higher under woodland and forest than under shrubland. This indicates toward plants' higher C investment into fungi in the Mediterranean and temperate rainforest sites than in the arid site. Fu...
In der chilenischen Kustenkordillera wurden entlang eines klimatischen Gradienten von 1500 km, vo... more In der chilenischen Kustenkordillera wurden entlang eines klimatischen Gradienten von 1500 km, von arid bis masig humid, naturliche Okosysteme ausgewahlt, um den Abbau der organischen Bodensubstanz (OBS) sowie die Nahrstofffreisetzung zu untersuchen. Mikroorganismen konnen mithilfe extrazellularer Enzyme organische Verbindungen aufspalten und Nahrstoffe fur Pflanzen bereitstellen. Es stellt sich die Frage, welchen Einfluss die Bodenfeuchte und der Kohlenstoffeintrag uber das Wurzelsystem auf den mikrobiellen Abbau haben. Es wurde die Hypothese gepruft, dass feuchte Bodenbedingungen und Wurzelnahe den enzymatischen OBS-Abbau und die Nahrstofffreisetzung fordern. In zwei Klimaregionen, einem humid gemasigtem und einem semiariden Waldgebiet, wurden entlang vertikaler (Bodentiefe) und horizontaler (Wurzelabstand) Gradienten folgende Parameter bestimmt: Bodenfeuchte, C- und N-Gehalte, d13C- und d15N-Werte sowie die Aktivitaten von sechs extrazellularen Enzymen, beteiligt in den C-, N- un...
Abstract Low-molecular-weight organic acids (LMWOAs) are crucial for the mobilization and acquisi... more Abstract Low-molecular-weight organic acids (LMWOAs) are crucial for the mobilization and acquisition of mineral phosphorus by plants. However, the role of LMWOAs in mobilizing organic phosphorus, which is the predominant phosphorus form in at least half of the world's ecosystems, especially in humid climates, is unclear. The mechanisms of phosphorus mobilization by LMWOAs depend on climate, mainly precipitation, and shape the phosphorus nutrition strategies of plants. We disentangled the impact of roots and associated microorganisms on mechanisms of phosphorus cycling mediated by LMWOAs by studying soils along an ecosystem-sequence (ecosequence) from arid shrubland (~70 mm yr−1), and Mediterranean woodland (~370 mm yr−1) to humid-temperate forest (~1470 mm yr−1). Phosphorus speciation in soil was examined by X-ray absorption near edge structure analysis (XANES). LMWOAs were quantified as biological rock-weathering and organic phosphorus mobilization agents and compared to kinetics of acid phosphatase as a proxy for organic phosphorus mineralization. Calcium-bound phosphorus in topsoils decreased from 126 mg kg−1 in the arid shrubland, to 19 mg kg−1 in the Mediterranean woodland and was undetectable in the humid-temperate forest. In contrast, organic phosphorus in topsoils in close root proximity (0–2 mm distance to roots) was absent in the arid shrubland but raised to 220 mg kg−1 in the Mediterranean woodland and to 291 mg kg- 1 in the humid-temperate forest. The organic phosphorus content in topsoils was 1.6 to 2.4 times higher in close root proximity (0–2 mm distance to roots) compared to bulk soil (4–6 mm distance to roots) in the Mediterranean woodland and humid-temperate forest, showing intensive phosphorus bioaccumulation in the rhizosphere. Redundancy analysis (RDA) revealed that LMWOAs were explained by the content of hydroxyapatite and variscite phosphorus-species in the arid shrubland, indicating that LMWOAs contribute to mineral weathering in this soil. LMWOA contents, phosphatase activity, and microbial biomass carbon correlated strongly with organic phosphorus in the humid-temperate forest soil, which implies a high relevance of LMWOAs for organic phosphorus recycling. In the Mediterranean woodland soil, however, oxalic acid correlated with organic phosphorus in the topsoil (suggesting phosphorus recycling), whereas in the subsoil malic and citric acid were correlated with primary and secondary phosphorus minerals (implying mineral weathering). We conclude that phosphorus acquisition and cycling depend strongly on climate and that the functions of LMWOAs in the rhizosphere change fundamentally along the precipitation gradient. In the arid shrubland LMWOAs facilitate biochemical weathering (rock eating), while in the humid-temperate forest their functions change towards supporting organic phosphorus recycling (vegetarian).
The effect of mineral properties on soil organic carbon (SOC) destabilization processes is poorly... more The effect of mineral properties on soil organic carbon (SOC) destabilization processes is poorly known. Andisols developed under temperate old-growth rain forest store important amounts of SOC due to strong mineral adsorption capacity. We evaluated the effect of mineral soil properties on microbial activity and native SOC mineralization in an incubation experiment with and without the addition of labile organic carbon (C) and nutrients. Top- and subsoil horizons of two Andisols with contrasting age from temperate old-growth rain forest were sampled. We used the selective dissolution methods to determine the metals complexed with SOC and allophane content. Radiocarbon analysis activity was performed to determine the 14 C activity and 14 C ages of all soils as a proxy for SOC persistence. Additionally, we carried out a laboratory incubation under controlled conditions with and without 13 C labeled cellulose and nutrient solution (NH 4 NO 3 , KH 2 PO 4 ) addition. A lower microbial activity in the subsoils of both Andisols compared to topsoils in accordance with the lower 14 C activity at depth was recorded. After cellulose and nutrient additions, both subsoils horizons showed native SOC mineralization after 80 days of incubation. In contrast, these amendments did not induce native SOC mineralization in topsoils showing differences on C turnover rates between top- and subsoil horizons. In both soils, the native SOC mineralization after labile C and nutrient additions was negatively related to Na-pyrophosphate extractable C and may therefore be controlled by the formation of organo-mineral complexes with increasing soil C age. We conclude that the physicochemical soil characteristics evolving during pedogenesis control the SOC stabilization and also destabilization processes in Andisols developed under temperate old-growth rain forests.
&... more &am…
&... more <p>Temperate rain forest soils (>8000 mm yr -1 ) of south of Chile in the East Andes range are<br>intensively affected by increasing freezing and thawing cycles (FTC) due to increasing<br>climate variability in the last 20 years. Most of these volcanic forests soils are unpolluted<br>(pristine) and receive seasonal snow-cover. In spite of pollutant free precipitations, the<br>snow cover in these ecosystems contains aerosols, nutrients and microorganisms from<br>circumpolar south west winds. These inputs and FTC generate specific conditions at the<br>shallow layer at the soil surface for soil microbiology and biochemistry. The objectives of<br>the study were to compare (micro)biological and chemical properties of topsoil and snow<br>cover in an pristine forest and after clear-cut. The organic matter mineralization was<br>monitored in a microcosm experiment to explore the effects of FTC and snow melting on<br>redox potential and other topsoil parameters. FTC for soil+snow released more CO 2…
<p&amp... more <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…
<p&amp... more <p>Temperate forests in Chile have experienced increasing temperatures and extreme climatic events, such as severe drought and short winters in unique Araucaria araucana forest in Nahuelbuta National Park. Therefore, it is relevant to understand the impact of drying and rewetting (D/R) or freezing and thawing (F/T) on SOM turnover in these ecosystems. Particularly important is the destabilization of soil organic matter (SOM) by microbial activity, which is highly heterogeneous and influenced by soil properties and water cycles. Drying and rewetting or F/T cycles accelerate particulate organic matter (POM) decomposition by aggregate disruption, thereby, decreasing carbon (C) availability for soil microorganism. We hypothesized that frequent D/R and F/T cycles release labile organic C locked away in the aggregates for microbial consumption. We assumed that a repeated number of D/R and F/T cycles enhance the preferential C utilization of fresh organic substrate. In the present study an incubation experiment was conducted for 27 days to assess the effect of F/T (-18 ºC to room temperature) and D/R (-500 kPa to 33 kPa, field capacity) cycles on labelled <sup>14</sup>C glucose and <sup>13</sup>C lignocellulose decomposition, soil aggregates size and POM fractions distributions. CO<sub>2</sub> efluxes and priming effect (PE), i.e. the turnover acceleration or retardation of native C mineralization, C use efficiency (CUE) and C allocation in soil aggregate classes as POM-light, POM-occluded and heavy fractions were also determined. Labelled glucose was mainly allocated in macro (> 250 mm) and microaggregates (< 250 mm) as part of the POM-light fraction. In contrast, labelled lignocellulose was allocated in microaggregate in the POM-occluded and heavy fraction. CUE was similar amongst all treatments. The PE was negative in soil with and without cycles and it was much more pronounced (-125 mg C kg<sup>-1</sup> soil) for F/T cycles than D/R (-50 mg C kg<sup>-1</sup> soil) at the end of incubation. The C:N ratio of soil following mining theory is further discussed. We conclude that D/R cycles clearly retarded the native C mineralization by preferential use of labelled <sup>13</sup>C-lignocellulose, while F/T cycles led to preferential use of <sup>14</sup>C-glucose.</p>
The diversity of orchid mycorrhizal fungi (OMF) and other beneficial root-associated fungi in tem... more The diversity of orchid mycorrhizal fungi (OMF) and other beneficial root-associated fungi in temperate forests has scarcely been examined. This study aimed to analyze the diversity of mycorrhizal and rhizosphere-associated fungal communities in the terrestrial orchids Gavilea lutea and Chloraea collicensis growing in high-orchid-population-density areas in the piedmont of the Andes Cordillera with native forest (Nothofagus-Araucaria) and Coastal Cordillera with an exotic plantation (Pinus-Eucalyptus) in south-central Chile. We focused on rhizosphere-inhabiting and peloton-associated OMF in a native forest (Andes Cordillera) and a mixed forest (Coastal Cordillera). The native terrestrial orchids G. lutea and C. collicensis were localized, mycorrhizal root segments were taken to isolate peloton-associated OMF, and rhizosphere soil was taken to perform the metabarcoding approach. The results revealed that Basidiomycota and Ascomycota were the main rhizosphere-inhabiting fungal phyla, ...
A computer model is described that is able to trace the fate of nitrogen in crop residues added t... more A computer model is described that is able to trace the fate of nitrogen in crop residues added to field soils. Clover and wheat residues labelled with 15N were added to a clay and a sand soil and the fate of the label traced over a period of almost 16 months under field conditions. Using a simple function to retard the decomposition of crop residues according to how much fibrous tissue they contain, the model was able to estimate the organic N remaining in soil, and the mineral N and microbial biomass N derived from the crop residues. It proved necessary, however, to postulate the existence of a pool of organic matter derived from crop residues that was more labile than native humus in soil.
Plants and microorganisms, besides the climate, drive nitrogen (N) cycling in ecosystems. Our obj... more Plants and microorganisms, besides the climate, drive nitrogen (N) cycling in ecosystems. Our objective was to investigate N losses and N acquisition strategies along a unique ecosystem-sequence (ecosequence) ranging from arid shrubland through Mediterranean woodland to temperate rainforest. These ecosystems differ in mean annual precipitation, mean annual temperate, and vegetation cover, but developed on similar granitoid soil parent material, were addressed using a combination of molecular biology and soil biogeochemical tools. Soil N and carbon (C) contents, δ15N signatures, activities of N acquiring extracellular enzymes as well as the abundance of soil bacteria and fungi, and diazotrophs in bulk topsoil and rhizosphere were determined. Relative fungal abundance in the rhizosphere was higher under woodland and forest than under shrubland. This indicates toward plants' higher C investment into fungi in the Mediterranean and temperate rainforest sites than in the arid site. Fu...
In der chilenischen Kustenkordillera wurden entlang eines klimatischen Gradienten von 1500 km, vo... more In der chilenischen Kustenkordillera wurden entlang eines klimatischen Gradienten von 1500 km, von arid bis masig humid, naturliche Okosysteme ausgewahlt, um den Abbau der organischen Bodensubstanz (OBS) sowie die Nahrstofffreisetzung zu untersuchen. Mikroorganismen konnen mithilfe extrazellularer Enzyme organische Verbindungen aufspalten und Nahrstoffe fur Pflanzen bereitstellen. Es stellt sich die Frage, welchen Einfluss die Bodenfeuchte und der Kohlenstoffeintrag uber das Wurzelsystem auf den mikrobiellen Abbau haben. Es wurde die Hypothese gepruft, dass feuchte Bodenbedingungen und Wurzelnahe den enzymatischen OBS-Abbau und die Nahrstofffreisetzung fordern. In zwei Klimaregionen, einem humid gemasigtem und einem semiariden Waldgebiet, wurden entlang vertikaler (Bodentiefe) und horizontaler (Wurzelabstand) Gradienten folgende Parameter bestimmt: Bodenfeuchte, C- und N-Gehalte, d13C- und d15N-Werte sowie die Aktivitaten von sechs extrazellularen Enzymen, beteiligt in den C-, N- un...
Abstract Low-molecular-weight organic acids (LMWOAs) are crucial for the mobilization and acquisi... more Abstract Low-molecular-weight organic acids (LMWOAs) are crucial for the mobilization and acquisition of mineral phosphorus by plants. However, the role of LMWOAs in mobilizing organic phosphorus, which is the predominant phosphorus form in at least half of the world's ecosystems, especially in humid climates, is unclear. The mechanisms of phosphorus mobilization by LMWOAs depend on climate, mainly precipitation, and shape the phosphorus nutrition strategies of plants. We disentangled the impact of roots and associated microorganisms on mechanisms of phosphorus cycling mediated by LMWOAs by studying soils along an ecosystem-sequence (ecosequence) from arid shrubland (~70 mm yr−1), and Mediterranean woodland (~370 mm yr−1) to humid-temperate forest (~1470 mm yr−1). Phosphorus speciation in soil was examined by X-ray absorption near edge structure analysis (XANES). LMWOAs were quantified as biological rock-weathering and organic phosphorus mobilization agents and compared to kinetics of acid phosphatase as a proxy for organic phosphorus mineralization. Calcium-bound phosphorus in topsoils decreased from 126 mg kg−1 in the arid shrubland, to 19 mg kg−1 in the Mediterranean woodland and was undetectable in the humid-temperate forest. In contrast, organic phosphorus in topsoils in close root proximity (0–2 mm distance to roots) was absent in the arid shrubland but raised to 220 mg kg−1 in the Mediterranean woodland and to 291 mg kg- 1 in the humid-temperate forest. The organic phosphorus content in topsoils was 1.6 to 2.4 times higher in close root proximity (0–2 mm distance to roots) compared to bulk soil (4–6 mm distance to roots) in the Mediterranean woodland and humid-temperate forest, showing intensive phosphorus bioaccumulation in the rhizosphere. Redundancy analysis (RDA) revealed that LMWOAs were explained by the content of hydroxyapatite and variscite phosphorus-species in the arid shrubland, indicating that LMWOAs contribute to mineral weathering in this soil. LMWOA contents, phosphatase activity, and microbial biomass carbon correlated strongly with organic phosphorus in the humid-temperate forest soil, which implies a high relevance of LMWOAs for organic phosphorus recycling. In the Mediterranean woodland soil, however, oxalic acid correlated with organic phosphorus in the topsoil (suggesting phosphorus recycling), whereas in the subsoil malic and citric acid were correlated with primary and secondary phosphorus minerals (implying mineral weathering). We conclude that phosphorus acquisition and cycling depend strongly on climate and that the functions of LMWOAs in the rhizosphere change fundamentally along the precipitation gradient. In the arid shrubland LMWOAs facilitate biochemical weathering (rock eating), while in the humid-temperate forest their functions change towards supporting organic phosphorus recycling (vegetarian).
The effect of mineral properties on soil organic carbon (SOC) destabilization processes is poorly... more The effect of mineral properties on soil organic carbon (SOC) destabilization processes is poorly known. Andisols developed under temperate old-growth rain forest store important amounts of SOC due to strong mineral adsorption capacity. We evaluated the effect of mineral soil properties on microbial activity and native SOC mineralization in an incubation experiment with and without the addition of labile organic carbon (C) and nutrients. Top- and subsoil horizons of two Andisols with contrasting age from temperate old-growth rain forest were sampled. We used the selective dissolution methods to determine the metals complexed with SOC and allophane content. Radiocarbon analysis activity was performed to determine the 14 C activity and 14 C ages of all soils as a proxy for SOC persistence. Additionally, we carried out a laboratory incubation under controlled conditions with and without 13 C labeled cellulose and nutrient solution (NH 4 NO 3 , KH 2 PO 4 ) addition. A lower microbial activity in the subsoils of both Andisols compared to topsoils in accordance with the lower 14 C activity at depth was recorded. After cellulose and nutrient additions, both subsoils horizons showed native SOC mineralization after 80 days of incubation. In contrast, these amendments did not induce native SOC mineralization in topsoils showing differences on C turnover rates between top- and subsoil horizons. In both soils, the native SOC mineralization after labile C and nutrient additions was negatively related to Na-pyrophosphate extractable C and may therefore be controlled by the formation of organo-mineral complexes with increasing soil C age. We conclude that the physicochemical soil characteristics evolving during pedogenesis control the SOC stabilization and also destabilization processes in Andisols developed under temperate old-growth rain forests.
&... more &am…
&... more <p>Temperate rain forest soils (>8000 mm yr -1 ) of south of Chile in the East Andes range are<br>intensively affected by increasing freezing and thawing cycles (FTC) due to increasing<br>climate variability in the last 20 years. Most of these volcanic forests soils are unpolluted<br>(pristine) and receive seasonal snow-cover. In spite of pollutant free precipitations, the<br>snow cover in these ecosystems contains aerosols, nutrients and microorganisms from<br>circumpolar south west winds. These inputs and FTC generate specific conditions at the<br>shallow layer at the soil surface for soil microbiology and biochemistry. The objectives of<br>the study were to compare (micro)biological and chemical properties of topsoil and snow<br>cover in an pristine forest and after clear-cut. The organic matter mineralization was<br>monitored in a microcosm experiment to explore the effects of FTC and snow melting on<br>redox potential and other topsoil parameters. FTC for soil+snow released more CO 2…
<p&amp... more <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…
<p&amp... more <p>Temperate forests in Chile have experienced increasing temperatures and extreme climatic events, such as severe drought and short winters in unique Araucaria araucana forest in Nahuelbuta National Park. Therefore, it is relevant to understand the impact of drying and rewetting (D/R) or freezing and thawing (F/T) on SOM turnover in these ecosystems. Particularly important is the destabilization of soil organic matter (SOM) by microbial activity, which is highly heterogeneous and influenced by soil properties and water cycles. Drying and rewetting or F/T cycles accelerate particulate organic matter (POM) decomposition by aggregate disruption, thereby, decreasing carbon (C) availability for soil microorganism. We hypothesized that frequent D/R and F/T cycles release labile organic C locked away in the aggregates for microbial consumption. We assumed that a repeated number of D/R and F/T cycles enhance the preferential C utilization of fresh organic substrate. In the present study an incubation experiment was conducted for 27 days to assess the effect of F/T (-18 ºC to room temperature) and D/R (-500 kPa to 33 kPa, field capacity) cycles on labelled <sup>14</sup>C glucose and <sup>13</sup>C lignocellulose decomposition, soil aggregates size and POM fractions distributions. CO<sub>2</sub> efluxes and priming effect (PE), i.e. the turnover acceleration or retardation of native C mineralization, C use efficiency (CUE) and C allocation in soil aggregate classes as POM-light, POM-occluded and heavy fractions were also determined. Labelled glucose was mainly allocated in macro (> 250 mm) and microaggregates (< 250 mm) as part of the POM-light fraction. In contrast, labelled lignocellulose was allocated in microaggregate in the POM-occluded and heavy fraction. CUE was similar amongst all treatments. The PE was negative in soil with and without cycles and it was much more pronounced (-125 mg C kg<sup>-1</sup> soil) for F/T cycles than D/R (-50 mg C kg<sup>-1</sup> soil) at the end of incubation. The C:N ratio of soil following mining theory is further discussed. We conclude that D/R cycles clearly retarded the native C mineralization by preferential use of labelled <sup>13</sup>C-lignocellulose, while F/T cycles led to preferential use of <sup>14</sup>C-glucose.</p>
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