Publications

Volcanic ash soils display distinctive morphological, physical and chemical properties and they contain several times more organic matter than non‐volcanic soils. So far, there are few studies of soil organic matter (SOM) distribution in different chemically and physically protected carbon pools of soil horizons of volcanic soils. The aim of this study was to determine the SOM distribution (and its δ 13 C and δ 15 N composition) in different chemical and physical fractions at various depth horizons of two Andisols under pasture or rain forest in southern Chile. We used the amount of humus‐complexes (C p ) extracted with Na pyrophosphate as a measure of C stabilized by aluminum (Al p ) and iron (Fe p ) in combination with density fractionation to separate particulate organic matter as free (fPOM), occluded (oPOM) and organic matter associated with the mineral fraction (MF). The results showed that soil SOM stock (0–40 cm) in the pasture soil was 166 Mg C ha −1 (11.7 Mg N ha −1 ) and in the forest soil 100 Mg C ha −1 (4.1 Mg N ha −1 ). The SOM variation was explained largely by the differences in C p , Al p and Fe p . About 34% of total soil C was found as C p in both oPOM and MF in the topsoil, whereas 33–53% was found in the subsoil horizons. The oPOM fraction was more important in the forest soil and generally decreased in the subsoil where these fractions were enriched with δ 13 C and δ 15 N. Our results emphasize the importance of the humus complex and oPOM formation as the SOM stabilization mechanism in the forest Andisol, whereas under pasture organo‐mineral interaction, including the formation of humic‐metal complexes, is the most important stabilization mechanism. A conceptual model is lacking to demonstrate the major areas of uncertainty within known mechanisms and factors that explain the distribution of SOM through soil profiles in Andisols.

No abstract is provided for this article.

In previous studies, Al extracted by acid ammonium acetate (Ala) or Na-pyrophosphate (Alp), rather than silt or clay content and climate conditions, was the most important factor that controls organic matter (OM) levels in volcanic soils. Here, the hypothesis was tested that Ala is a comparable method (as much as CuCl2) to quantify the proportion of Al bound to OM in allophanic soils. As far as we know, there are no previous antecedents in which selective dissolution method has been compared with this extractant. Secondly, we examine the effects of (a) Al, (b) silt plus clay content (particles size 0–53 µm) and (c) clay mineralogy on the control of organic carbon (OC) level in Chilean volcanic soils. This was achieved by sampling 16 soils series (11 Andisols, one Alfisol and four Ultisols, USDA classification) including 48 soil pedons up to 0.4 m depth. Soils were analyzed for Ala, Alp, oxalate (Alo, Sio and Feo), cold NaOH (Aln) and un-buffered salts, CuCl2 (AlCu), LaCl3 (AlLa) and KCl (Alk). We also measured the Al-humus as soluble C fraction after pyrophosphate extraction and the C associated to the silt plus clay fraction after sonication and gravity decantation. The statistical package (S)MATR was used to examine bivariate linear regressions among soil properties by computing the standardized major axis (SMA). Our results indicate that Ala had a good correspondence with Alp (R 2 =0.76) in the top soil with Ala/Alp ratio of 0.19 and both extractans presented significant and positively relationship with soil OC (R2 >0.62). Acid ammonium acetate was as effective as AlCu to determine the Al–OM in allophanic soils. It is cheaper than AlCu and Alp and 0.5 h shaking is required compared to 2 h of AlCu and 16 h of Alp. The efficiency of the extraction was: Aln ≥Alo >Alp >AlCu ≥Ala >AlLa >Alk. We also found that allophane content (estimated by Al/Si ratio) was strongly correlated (R 2 =0.82) with the OC in the fine silt plus clay and that Al-humus together with C in the finest particles explained (R 2 >0.60) the largest proportion of variation of soil OC across studied soils.

Understanding the pathways of nitrogen (N) retention in pristine forest soils is essential for effective ecosystem management and nutrient conservation. The incorporation of nitrate (NO₃^-) and nitrite (NO₂^-) into organic N in soils without microbiological contribution remains a very intriguing question. This study explores the abiotic incorporation of nitrate (NO₃^-) and nitrite (NO₂^-) 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 (NO₃^-) can be reduced to nitrite (NO₂^-), 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 NO₃^- to NO₂^- 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 NO₃^- and NO₂^- declined rapidly by 51 and 94 %, while labelled organic N increased by 20-38 % for NO₃^- and 42-44 % for NO₂^- within seconds. The incorporation of ¹⁵N 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_₃⁻ kg⁻¹, respectively. These results demonstrate that NO₃^- 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.

ISHS VII International Symposium on Pear Growing EFFECT OF PARTICLE SIZE AND AGE OF PRUNING RESIDUES OF PYRUS COMMUNIS, CV. BEURRE BOSC AND PYRUS PYRIFOLIA, CV. HOSUI ON C- AND N- MINERALIZATION

Aim We sought to clarify the role of juvenile tree growth rates in species sorting along fine‐scale fertility gradients in the coast range forests of southern Chile. Location In montane forests in the coast range, the conifer Fitzroya cupressoides occupies the poorest soils, whereas dominance shifts to the evergreen angiosperms Nothofagus nitida and/or Weinmannia trichosperma on moderately infertile sites. On the most fertile sites, N. nitida is usually the sole overstorey dominant. Methods We examined the effects of soil depth and nitrogen availability on height growth and leaf nitrogen levels of the three common overstorey dominants. Sapling height growth rates and foliage nitrogen concentration were measured in 20 large gaps (>0.1 ha), on sites of varied soil depth and potential N‐mineralization rates. Results Height growth rates of Weinmannia and Nothofagus were strongly positively correlated with both soil depth and potential N‐mineralization. Height growth of Fitzroya was strongly correlated with soil depth, but not with N‐mineralization. Nothofagus and Weinmannia enjoyed large growth‐rate advantages over Fitzroya on the more fertile soils of the study area, and this advantage was reduced or nullified (but not reversed) on poor soils. This pattern was associated with species differences in nitrogen use: on fertile soils, Nothofagus and especially Weinmannia , achieved more height growth per unit of leaf N than Fitzroya , whereas differences were minimal on poor soils. Main conclusions Results suggest that comparative height growth rates of juvenile trees, associated with nitrogen productivity differences, play a key role in determining eventual dominance on productive sites, but that traits other than growth (possibly related to nutrient retention) are more important on low fertility sites. Disturbance regimes appear to vary in relation to site fertility, and differential species responses to this superimposed disturbance gradient are also probably influential in determining overstorey composition.

Soil biological activity has important implications for soil carbon (C) sequestration.However, very little is known about the environmental factors, particularly the effect of soil mineralogy on availability of C for soil microorganisms.In this study, we have investigated the influences of soil type (clay mineralogy) on C mineralization and its effects on biological activity at different levels of soil organic matter stability.Two soils an allophanic, derived from recent volcanic ash and a kaolinitic, resulting from metamorphic parent materials were physically fractioned in to light (LF, coarse sand 250-2000 µm), intermediate (IF, fine sand53-250 µm) and mineral (MF, silt and clay < 53 µm) fractions.Several biological and biochemical analyses at Ah horizons of mineral soil and physical fractions were conducted: soil respiration, enzymatic activities, carbohydrates and microbial biomass, amongst others soil variables.The results indicated that the bulk soil and physical fractions had a significant impact on cumulative C mineralized after 30 days of incubation and soil enzyme activities.More than 76% of total C-CO 2 variation was explained by stepwise multiple regression analysis including factors such as soil enzymes (β-glucosidase, dehydrogenase and phosphatase) and inorganic P. Soil ATP extraction was a good indicator of microbial activity, because of a positive and significant correlation among ATP and i) C-CO 2 and ii) metabolic quotient (soil respiration rate divided by microbial biomass).We also found an inverse and significant relationship between Al pyrophosphate (Al bound to SOM) and the C-CO 2 in volcanic soil, whereas the same correlation did not occur in kaolinitic soil.Our results confirmed a greater stabilization capacityof MF in allophanic than in kaolinitic soils due to the amorphous minerals clay materials.

We evaluated the abiotic formation of dissolved organic nitrogen (DON) by the fast reaction of iron (Fe) with nitrate (NO3 −) in the dissolved organic matter (DOM) of volcanic soils in a temperate rainforest (>5000 mm precipitation per year). During five days, the educts and products of abiotic reactions under anoxic conditions were measured in a microcosm experiment depending on the Fe and NO3 − concentrations. A control zero-Fe was not used because there was no chemical reaction with nitrate addition. Using a novel technique of automated sample preparation for inorganic N (SPIN) attached to a membrane inlet quadrupole mass spectrometry (MIMS), the 15N abundances and inorganic N concentrations were determined directly in aqueous solutions. The results were explained in the context of the Ferrous Wheel Hypothesis which states that Fe(II) is utilized to reduce NO3 − to nitrite (NO2 −) that is incorporated into DOM. Fe(II) is regenerated from Fe(III) in anaerobic soil microsites. Here we tested one part of this hypothesis, the processes occurring in DOM (instead of soil organic matter). Using the SPIN-MIMS technique, we could overcome Ferrous Wheel Hypothesis criticism regarding possible Fe interference during NO3 − analysis. The total recovery of 15N added as NO3 − fluctuated between 63 and 101%, and the remaining 15N was measured as gaseous N2O. The 15N-labelled NO3 − added decreased immediately after 15 min of incubation. After five days of incubation, approximately 25% of the labelled NO3 − (e− acceptors) added was transformed to DON in the presence of a high amount of Fe(II) (e− donors). Small amounts of N2O and CO2 provided further evidence of NO3 − reduction and DOM oxidation, respectively. From these results, we propose a new theoretical model that includes the Ferrous Wheel Hypothesis, where only the transformation of NO3 − to DON was proven. The present results explain the high retention of NO3 − in DOM from volcanic soils in ecosystems with high precipitation.

The major priority of research in the present day is to conserve the environment by reducing GHG emissions. A proposed solution by an expert panel from 195 countries meeting at COP 21 was to increase global SOC stocks by 0.4% year-1 to compensate for GHG emissions, the '4 per 1000' agreement. In this context, the application of biocrusts is a promising framework with which to increase SOC and other soil functions in the soil-plant continuum. Despite the importance of biocrusts, their application to agriculture is limited due to: (1) competition with native microbiota, (2) difficulties in applying them on a large scale, (3) a lack of studies based on carbon (C) balance and suitable for model parameterization, and (4) a lack of studies evaluating the contribution of biocrust weathering to increase C sequestration. Considering these four challenges, we propose three perspectives for biocrust application: (1) natural microbiome engineering by a host plant, using biocrusts; (2) quantifying the contribution of biocrusts to C sequestration in soils; and (3) enhanced biocrust weathering to improve C sequestration. Thus, we focus this opinion article on new challenges by using the specialized microbiome of biocrusts to be applied in a new environment to counteract the negative effects of climate change.

CCCada suelo posee una limitada capacidad de proteger fisico - quimicamente la materia organica contra la biodegradacion en sus particulas de arcilla y limo. Las tasas de mineralizacion de carbono (C) y nitrogeno (N) de los suelos estarian relacionadas al grado de saturacion de C en estas particulas. En el presente estudio se investigo la hipotesis de que la disminucion del C en la arcilla y limo a medida que el contenido de estas fracciones aumenta en los suelos se debe a una disminucion en la superficie especifica en las particulas de arcilla y limo. Tambien se investigo la relacion entre las tasas de mineralizacion de C y N con el grado de saturacion con C en las particulas de arcilla y limo. Seis suelos cultivados (cereales) y tres suelos de praderas (Medicago sativa L.) fueron muestreados a 0-20 cm de profundidad. Todos los suelos fueron dispersados con ultrasonido y gravimetricamente separados en fracciones > 50 µm (arena) y < 50 µm (arcilla y limo). A todas las muestras y al suelo entero se les determino el C - organico y la superficie especifica con etilenglicol. Por otra parte, se probo que existe una estrecha relacion entre el grado de saturacion con C en las particulas de arcilla y limo con las tasas de mineralizacion. Sin embargo, tambien se encontro una buena relacion entre el C-organico total del suelo y las tasas de mineralizacion. Esto ultimo coincidio con el hecho de haber encontrado una buena correlacion entre el C-organico del suelo y el grado de saturacion. Nuestros resultados explican parcialmente la hipotesis de que las tasas de mineralizacion estan mas relacionadas al grado de saturacion que a la textura (limo + arcilla) y al contenido de materia organica del suelo.

Efecto del tamaño de partícula y calidad de la madera de residuos de poda de peras asiáticas