Pilar Carral

The diversity of methoxyphenols released by pyrolysis-gas chromatography as predictor of soil carbon storage

The variable extent to which environmental factors are involved in soil carbon storage is currently a subject of controversy. In fact, justifying why some soils accumulate more organic matter than others is not trivial. Some abiotic factors such as organo-mineral associations have classically been invoked as the main drivers for soil C stabilization. However, in this research indirect evidences based on correlations between soil C storage and compositional descriptors of the soil organic matter are presented. It is assumed that the intrinsic structure of soil organic matter should have a bearing in the soil carbon storage. This is examined here by focusing on the methoxyphenols released by direct pyrolysis from a wide variety of topsoil samples from continental Mediterranean ecosystems from Spain with different properties and carbon content. Methoxyphenols are typical signature compounds presumptively informing on the occurrence and degree of alteration of lignin in soils. The methoxyphenol assemblages (12 major guaiacyl- and syringyl-type compounds) were analyzed by pyrolysis-gas chromatography-mass spectrometry. The Shannon-Wiener diversity index was chosen to describe the complexity of this phenolic signature. A series of exploratory statistical analyses (simple regression, partial least squares regression, multidimensional scaling) were applied to analyze the relationships existing between chemical and spectroscopic characteristics and the carbon content in the soils. These treatments coincided in pointing out that significant correlations exist between the progressive molecular diversity of the methoxyphenol assemblages and the concentration of organic carbon stored in the corresponding soils. This potential of the diversity in the phenolic signature as a surrogate index of the carbon storage in soils is tentatively interpreted as the accumulation of plant macromolecules altered into microbially reworked structures not readily recognized by soil enzymes. From a quantitative viewpoint, the partial least squares regression models exclusively based on total abundances of the 12 major methoxyphenols were especially successful in forecasting soil carbon storage.

Assessment of Soil Organic Matter Molecular Characteristics Related to Hydrophysical Properties in Semiarid Soils (Central Spain)

The objective of this study is to analyze soil hydrophysical properties in addition to the molecular composition of the organic matter in semiarid Mediterranean cereal agroecosystems. Aggregate water-stability, total porosity, water holding capacity, and b and c parameters of the Kostiakovs equation for soil infiltration were determined using a double-ring infiltrometer. The soil humic acid (HA) was isolated and characterized mainly by analytical pyrolysis-gas chromatography mass spectrometry. Multivariate data treatments suggested favourable hydrophysical parameters associated to HAs with a substantial aliphatic domain. Soils with HAs consisting of condensed aromatic structures, often considered to correspond to advanced stages of humification and high resilience, do not coincide with those displaying favourable hydrophysical properties. The results discussed in this article provide support for moderate aliphatic hydrophobicity to be considered as a soil organic matter quality parameter positively influencing hydrophysical properties of semiarid soils.

Application of dendrochronological analysis of Retama sphaerocarpa L. (Boiss) for dating agricultural abandonment

Abandonment of agricultural land leads to changes in soil characteristics that may result in better or worse soil conditions. These changes are slow therefore the use of indicators for dating the time of abandonment is particularly useful. This study was carried out in Madrid, Spain with the aim to establish for the first time the use of Retama sphaerocarpa L. (Boiss) as a dendrochronological tool for dating land abandonment. This offers the possibility to take into consideration a period of time long enough for changes in soil to be determined. Such changes can be indicated by fluctuations in soil organic carbon content (SOC), porosity or water availability. Three different situations resulted from the dendrochronological analysis: soil currently tilled; soil recently abandoned (less than 5 years), and prolonged abandonment (in average 10 years). In addition the influence of Retama sphaerocarpa L. (Boiss) on soils was checked for these periods of abandonment. The rate of SOC gain can be considered fast. Tilled soils accounted for 0.48% SOC, and reached 1% in less than 5 years, although with wide standard deviations. Due to prolonged abandonment SOC reached 1.41%, (P = 0.09). Total soil porosity under tillage was 49%, and decreased to 38% after 4-5 years, but recovered to 41% under prolonged abandonment. Water availability (volumetric soil moisture between field capacity and permanent wilting point) remained the same, ranging from 7.7 to 8.5% along the whole period of time. The presence of R. sphaerocarpa L. (Boiss) accelerates soil changes as SOC in prolonged abandonment increased to 2.65%, porosity was 41% and water availability 10.3%.