Labile, recalcitrant, and inert organic matter in Mediterranean forest soils

The biochemical quality of soil organic matter (SOM) was studied in various profiles under Quercus rotundifolia Lam. stands on calcareous parent material. Special attention was paid to the question of how biochemical quality is affected by position within the soil profile (upper versus lower horizons). The following global SOM characteristics were investigated: (a) overall recalcitrance, using hydrolysis with either hydrochloric or sulphuric acid; (b) hydrolyzable carbohydrates and polyphenolics; (c) extractability by hot water and quality of the extract; and (d) abundance of inert forms of SOM: charcoal and soot-graphite. The recalcitrance of soil organic carbon (OC) decreases with depth, following the order: H horizons>A horizons>B horizons. In contrast, the recalcitrance of nitrogen is roughly maintained with depth. The ratio carbohydrate C to total OC increases from H to B horizons, due to the increasing importance of cellulosic polysaccharides in B horizons, whereas other carbohydrates are maintained throughout the soil profile at a relatively constant level, 12-15% of the total OC in the horizon. Whereas the quality of the hydrolyzable carbon (measured by the carbohydrate to polyphenolic C ratio) decreases with depth from H to B horizons, the quality of the hot-water extractable organic matter is much higher in B horizons than in A or H horizons. The relative importance of both charcoal and soot-graphitic C and N tends to increase with depth. The ratio black/total is usually higher for N than for C, a result that suggests that inert SOM may represent a relevant compartment in the nitrogen cycle. Overall, our data suggest that in Mediterranean forest soils the organic matter in B horizons could be less stable than often thought.     

Characterization of ester sulphate in a gypsum-amended podzol using an immobilized sulphatase reactor

An immobilized sulphatase reactor column was successfully used to determine the biochemical stability of ester sulphate in soil organic matter extracted from a podzol amended with gypsum. The sulphatase from Helix pomatia was covalently attached to controlled-porosity glass beads, and the immobilized enzyme was packed into a small glass column. The optimum pH, the time required to reach equilibrium, and the percentage of substrate consumed for the enzymatic hydrolysis of soil ester sulphate (pH 7.7, 90 min, 23–59%) were substantially different from those of p-nitrophenyl sulphate at similar concentrations of substrate (pH 7.0, 40 min, 99%). The striking difference in the biochemical stability and kinetic behaviour between soil ester sulphate and the simple synthetic substrate reflected their different chemical nature and structural features. The amounts of enzymatically hydrolysable (labile) ester sulphate in soil organic matter extracted from the podzol amended with gypsum at rates of 0, 50, and 200 kg S ha^-1 were significantly different (P=0.0004), being 0.5, 1.1, and 1.4 g S ml^-1 soil extract (or 5, 11, and 14 g S g^-1 soil), respectively. The labile ester sulphate was not correlated with the total hydriodic acid-reducible organic sulphate with the soil organic matter extracts but with the hydriodic acid-reducible organic sulphate: organic C ratio, which increased as a result of gypsum amendment. This study revealed that input of inorganic sulphate as gypsum substantially increased the accumulation of labile ester sulphate in a podzol.     

Incorporation of nitrogen from crop residues into light-fraction organic matter in soils with contrasting management histories

The proportion of N from crop residues entering the light-fraction organic matter (LFOM) pool was investigated in soils with contrasting soil organic matter and microbial characteristics arising from different management histories. A laboratory experiment was conducted in which ¹⁵N-labelled sugar beet, Brussels sprout or ryegrass shoots, which possessed a range of C/N contents, and hence different biochemical qualities, were incorporated into a sandy–loam soil collected from within a field (FC) or from the field margin (FM). Amounts of C and N incorporated into LFOM were determined after 112 days. The FC and FM soils had organic C contents of 0.9% and 2.5%, respectively. Addition of crop residues increased total LFOM N content and reduced its C/N in FC soil but had no effect on total LFOM N or its C/N in FM soil. Ryegrass incorporation into FC was the only treatment in which there was a net increase in LFOM C. Isotopic analysis indicated that more crop-residue-derived N became incorporated into the LFOM N pool in FM relative to FC soil, with per cent crop residue N incorporated ranging from 25.9% to 35.3% in FC and between 38.9 and 68.5 in FM. Incorporation of crop residues had a positive priming effect on pre-existing LFOM N in FM but not FC soil. We conclude that the characteristics of plant material, together with differences in soil organic matter and microbiology resulting from contrasting management, determined the amount of crop residue C and N incorporated into both HFOM and LFOM.     

Dynamic of organic matter in the heavy fraction after abandonment of cultivated wetlands

The abandonment of cultivated wetland soil increased the contents of light fraction organic matter (LFOM), heavy fraction organic matter (HFOM) and soil organic matter (SOM). The LFOM and HFOM content increased to 13.3 g kg⁻¹ and 62.4 g kg⁻¹ after 5 years whereas they were 8.4 and 47.9 g kg⁻¹ after 9 years of cropping, respectively. Fourteen years after abandonment, HFOM content increased to 104.3 g kg⁻¹. LFOM was positively correlated with HFOM (p < 0.001). A Langmuir equation was used to calculate the highest HFOM value. The value for the natural wetland soil was closed to this theoretical value (140.8 g kg⁻¹). After 14 years of abandonment, the HFOM maximum (HFOM_Max) value was lower than the equilibrium value suggesting that a further increase in HFOM can occur after abandonment. Assuming a linear accumulation (3.87 Mg C ha⁻¹yr⁻¹), it would take approximately 24 years after the abandonment to reach the HFOM_Max value.     

Testing a practical indicator for changing soil organic matter

Long-term changes in soil organic carbon (SOC) resulting from management change are documented for many experimental situations, and corresponding trends in the field have been observed by national survey. Since these changes are relevant to atmospheric carbon balance a practical measure to confirm the impact of recent management decisions at any location, without resorting to repeated sampling, is highly attractive but none has previously been tested. This study assessed intra-aggregate C to fulfil the role, based on a temporary deviation from its predictable contribution to total SOC under stable management. A total of 166 surface soil samples (0–15 cm) were analyzed for intra-aggregate C using an established physical fractionation protocol or compatible scaled-up procedure. Soils were arable (or ley-arable) managed by conventional or minimum-tillage, or permanent grassland, and assigned ‘stable’ or ‘changing’ status on the basis of a verbal account of management history. Log-normal populations of intra-aggregate C were compared for soils of stable and changing status using F-tests. Intra-aggregate C shows promise as an indicator of changing SOC in arable soils up to 30% clay content, particularly soils <20% clay. A larger dataset is required to establish its utility in grassland soils. It is not certain that intra-aggregate C is capable of confirming direction of change or trajectory (endpoint), and functions to indicate change, rather than confirm stable status. Supplementary information on the history of soil use and management is therefore essential in the interpretation of such measurements.     

Biochemical properties and biodegradation of dissolved organic matter from soils

Various biologically mediated processes are involved in the turnover of dissolved organic matter (DOM) in soil; however, relatively little is known about the dynamics of either the microbial community or the individual classes of organic molecules during the decomposition of DOM. We examined the net loss of DOC, the mineralisation of C to CO₂ and the degradation of DOC from six different soils by soil microorganisms. We also quantified the changes in the concentrations of protein, carbohydrate and amino acid C during microbial biodegradation. Over a 70-day incubation period at 20°C, the mineralisation of DOC to CO₂ was described by a double exponential model with a labile pool (half-life, 3–8 days) and a stable pool (half-life, 0.4–6 years). However, in nearly all cases, the mass loss of DOC exceeded the C released as CO₂ with significant deviations from the double exponential model. Comparison of mass DOC loss, CO₂ production and microbial cell counts, determined by epifluorescence microscopy, showed that a proportion of the lost DOC mass could be accounted for by microbial assimilation. Carbohydrate and protein C concentrations fluctuated throughout the incubation with a net change of between 3 to 13 and −30 to 22.4% initial DOC, respectively. No amino acid C was detected during the incubation period (level of detection, 0.01 mg C l⁻¹).     

Properties of dissolved organic matter related to soil organic matter quality and nitrogen additions in Norway spruce forest floors

The quality of dissolved organic matter (DOM) is highly variable and little information is available on the relation of DOM quality to the structure and composition of its parent soil organic matter (SOM). The effect of increasing N inputs to forest soils on the structure and composition of both SOM and DOM also remains largely unclear. Here we studied the release of DOM, its specific UV absorption and two humification indices (HIX) derived from fluorescence spectra from Oa material of 15 North- and Central-European Norway spruce (Picea abies (L.) Karst.) stands. The Oa material was incubated aerobically at 15 °C and water holding capacity over a period of 10 months and extracted monthly with an artificial throughfall solution. Soil respiration was determined weekly. The influence of mineral N inputs on composition of DOM and on respiration rates was investigated on periodically NH₄NO₃-treated Oa samples of eight selected sites. Release of dissolved organic carbon (DOC) from untreated Oa material samples ranged from 0.0 to 58.6 μg C day⁻¹ g C⁻¹ and increased with increasing C-to-N ratio. One HIX and UV absorption of DOM were negatively correlated to the degree of oxidation of lignin-derived compounds and positively to the C-to-N ratio and – HIX only – to the aromatic C content of SOM. Mineral N addition had no distinct effect on respiration rates. In six of eight samples the N-treatment caused an increase in specific UV absorption or one HIX of DOM. However, these effects were not statistically significant. Addition of mineral N did not affect the rates of DOM release. Our results show that properties of SOM largely determine the amount and quality of DOM in forest floors. Changes of DOM quality due to mineral N additions are likely, but we cannot confirm significant changes of DOM release.     

Chemical characteristics of glomalin-related soil protein (GRSP) extracted from soils of varying organic matter content

Glomalin is described in the literature as a N-linked glycoprotein and the putative gene product of arbuscular mycorrhizal fungi (AMF). Since the link between glomalin and various protein fractions in soil is not yet clearly defined, glomalin-related soil protein (GRSP) more appropriately describes glomalin's existence in natural organic matter (NOM). The objective of this study was to examine the chemical characteristics of GRSP present in several mineral and organic soils of varying organic carbon content. GRSP was isolated using high temperature sodium citrate extraction followed by either trichloroacetic acid (TCA) or hydrochloric acid (HCl) precipitation. GRSP was characterized by quantitative solid-state ¹³C DPMAS NMR, infrared (IR) spectroscopy, elemental analysis, and the Bradford assay for protein content. GRSP accounted for 25% and 52% of total C in the mineral soils and organic soil, respectively. Molar C/N and H/C ratios reveal that GRSP has less nitrogen than bovine serum albumin (BSA), and that GRSP extracted from the Pahokee peat soil possessed a more unsaturated, and thus aromatic character relative to the mineral soil GRSP, respectively. GRSP's high aromatic (42-49%) and carboxyl (24-30%) carbon contents and low aliphatic (4-11%) and carbohydrate-type carbon contents (4-16%) suggests that GRSP does not resemble a typical glycoprotein. In fact, the NMR spectra of GRSP closely resemble that of humic acid. GRSP extracted from mineral and organic soils possessed the same NMR fingerprint regardless of the precipitation method used (i.e., either TCA or HCl). It is likely that the current GRSP extraction methods, because of their similarity to the method used to extract humic acid, are coextracting both materials.     

Effect of drying and rewetting on phosphorus transformations in red brown soils with different soil organic matter content

In this study, the effect of drying and rewetting on native P transformations in two red brown soils with different management history was investigated. Three treatments, T1 (constantly moist), T2 (dried for 4 days and then kept dry), T3 (rewetted after 4 days drying) were used. Drying and rewetting caused a rapid increase in microbial P (Pm) and labile organic P (labile Po) within 1 day and a gradual increase in available inorganic P (Colwell). These increases were only temporarily, as Pm and labile Po decreased with time and were at the same level as in the constantly moist soil by the end of the incubation period of 21 days. The effect of drying and rewetting on P transformations strongly depended on soil organic matter content, being more pronounced in the soil with high organic matter content, compared to the soil with low soil organic matter content.     

Influence of soil compaction on carbon and nitrogen mineralization of soil organic matter and crop residues

 We studied the influence of soil compaction in a loamy sand soil on C and N mineralization and nitrification of soil organic matter and added crop residues. Samples of unamended soil, and soil amended with leek residues, at six bulk densities ranging from 1.2 to 1.6 Mg m^–3 and 75% field capacity, were incubated. In the unamended soil, bulk density within the range studied did not influence any measure of microbial activity significantly. A small (but insignificant) decrease in nitrification rate at the highest bulk density was the only evidence for possible effects of compaction on microbial activity. In the amended soil the amounts of mineralized N at the end of the incubation were equal at all bulk densities, but first-order N mineralization rates tended to increase with increasing compaction, although the increase was not significant. Nitrification in the amended soils was more affected by compaction, and NO₃ ^–-N contents after 3 weeks of incubation at bulk densities of 1.5 and 1.6 Mg m^–3 were significantly lower (by about 8% and 16% of total added N, respectively), than those of the less compacted treatments. The C mineralization rate was strongly depressed at a bulk density of 1.6 Mg m^–3, compared with the other treatments. The depression of C mineralization in compacted soils can lead to higher organic matter accumulation. Since N mineralization was not affected by compaction (within the range used here) the accumulated organic matter would have had higher C : N ratios than in the uncompacted soils, and hence would have been of a lower quality. In general, increasing soil compaction in this soil, starting at a bulk density of 1.5 Mg m^–3, will affect some microbially driven processes. Received: 10 June 1999     

Microbial respiration activities related to sequentially separated, particulate and water-soluble organic matter fractions from arable and forest topsoils

This study aimed to reveal differences in the relevance of particulate as well as water-soluble organic matter (OM) fractions from topsoils to the easily biodegradable soil organic matter (SOM). We selected eight paired sites with quite different soil types and soil properties. For each of these sites, we took samples from adjacent arable and forest topsoils. Physically uncomplexed, macro-, and micro-aggregate-occluded organic particle, as well as water-soluble OM fractions were sequentially separated by a combination of electrostatic attraction, ultrasonic treatment, density separation, sieving, and water extraction. The easily biodegradable SOM of the topsoil samples was determined by measuring microbial respiration during a short-term incubation experiment (OC_R). The organic carbon (OC) contents separated by i) the physically uncomplexed water-soluble OM, ii) the macro-, and iii) the micro-aggregate-occluded organic particle as well as water-soluble OM fractions were significantly correlated with OC_R. The correlation coefficients vary between 0.54 and 0.65 suggesting differences in the relevance of these OM fractions to the easily biodegradable SOM. The strongest correlation to OC_R was detected for the OC content separated by the physically uncomplexed water-soluble OM indicating the most distinct relation to the easily biodegradable SOM. This was found to be independent from land use or soil properties.     

Thermal destruction of soil water repellency and associated changes to soil organic matter as observed by FTIR spectroscopy

The heat generated during wildfires often leads to increases in soil water repellency. Above a critical heating threshold, however, its destruction occurs. Although the temperature thresholds for repellency destruction are relatively well established, little is known about the specific changes in the soil organic matter that are responsible for repellency destruction. Here we report on the analysis of initially water repellent surface soil samples (Dystric Cambisol, 0–5 cm depth) by transmission Fourier Transform Infrared (FTIR) spectroscopy analysis before and after destruction of its water repellency by heating to 225 °C in order to investigate heating-induced changes in soil organic matter (SOM) composition. Although assignment of absorption bands is made difficult by overlapping of some bands, it was possible to distinguish bands relevant for hydrophobicity of SOM in the soil before heat treatment. The most significant decrease in absorbance following water repellency destruction took place in the frequency area corresponding to stretching vibrations of aliphatic structures within SOM. The results suggest that besides a general decrease of SOM content during heating, the loss of soil water repellence is primarily caused by the selective degradation of aliphatic structures.     

Labile soil organic matter as a potential nitrogen source in golf greens

The loss of fertilizer N from golf greens can be high depending upon management (irrigation schedule, N source, rate and timing of fertilizer application) as well as soil conditions. Although soil organic matter (SOM) is acknowledged as a major source of N and other nutrients, its potential as an N source seems to be neglected in the management of golf greens. The susceptibility of SOM to degradation is one indication of how active a role SOM plays as a nutrient source. An extraction method developed by Olk et al. [Geoderma 65 (1995) 195] distinguishes humic acid fractions by their binding to dominant stabilizing soil cations and separates them into calcium-bound (CaHA) and non calcium-bound or mobile (MHA) fractions. Mobile humic acid is a relatively young, N-rich HA fraction that does not appear to form stable complexes with Ca. The MHA could therefore play a greater role in nutrient availability than CaHA. We determined C and N distributions within SOM extracted from these two HA fractions in 11 golf greens ranging in age from 4 to 28 yr. Because SOM in golf greens is recently formed, and MHA is an N-rich fraction representing an early stage of SOM evolution, we hypothesized that the MHA fraction would account for a larger proportion of soil organic N than CaHA. The amounts of both HA-C and HA-N increased significantly with green age. MHA accounted for a larger proportion (20-27%) of total soil C than CaHA-C (8-14%). MHA was also enriched in N compared to CaHA with consistently smaller C-to-N ratios. Thus, the greater abundance of MHA and its higher N concentration accounted for a larger proportion of soil organic N (24-45%). The equivalence of MHA-N ranged between 250 kg N ha⁻¹ for a 4 yr-old green and 775 kg N ha⁻¹ for a 21 yr-old green. Thus, soils of established greens contain significant quantities of labile SOM rich in N that could through mineralization supply part of the fertilizer N requirement of turf grass. A greater understanding of the dynamics of this resource is needed if we are to manage golf greens for optimal use without negative consequences to the environment.     

Effects of cropping systems on soil organic matter in a pair of conventional and biodynamic mixed cropping farms in Canterbury, New Zealand

Effects of cropping systems on soil organic matter (SOM) in a pair of conventional and biodynamic mixed cropping farms were investigated. Soil samples (0–75 and 75–150-mm depths) were analysed for total carbon (TC), total nitrogen (TN), microbial biomass C (BC) and microbial biomass N (BN), and sequentially extracted for labile and stable SOM using cold water, hot water, acid mixtures and alkalis. In the biodynamic farm, TC and TN decreased with increasing period of cropping but the reverse occurred under pastures. These were not shown in soils from the conventional farm, probably due to N fertilizer additions. Under pastures, increases in SOM were attributed to greater biological N₂ fixation and the return of plant residues and excreta from grazing animals. Overall, sensitive SOM quality indicators found for labile SOM were BN, BN:TN and HC:TC, and for stable SOM were HCl/HFC, HCl/HFC:TC, humin C, humin N, humin C:TC and humin N:TN. The BN and BN:TN were better indicators than BC and BC:TC. The humin fraction was strongly related to both labile and stable SOM fractions suggesting that humin contained non-extractable strongly complexed SOM components with mineral matter and also non-extractable plant and microbial residual components. Received: 10 October 1996     

Changes in mineral nitrogen, soil organic matter fractions and microbial community level physiological profiles after application of digested pig slurry and compost from municipal organic wastes to burned soils

In this study, mineralization of digested pig slurry and compost from municipal organic wastes in burned soils was followed for 60 days. The effects of amendments on organic matter fractions and microbial community level physiological profiles (CLPP) were also investigated at the end of the incubation period. Soil from a forest 10 days after a fire had a greater basal respiration, and more organic matter that a nearby soil that was not affected by fire, presumably as a consequence of black ash addition following the wildfire. Nitrification was inhibited in soils treated at 105 and 250 °C in the laboratory, but amendment application allowed nitrification to take place in the latter soil, and led to significant flushes of mineralization. Slurry amendment resulted in greater increases in mineral N compared with compost. Soil treated at 250 °C had the greatest content of water-extractable compounds (WE) at the expense of acid-extractable compounds (AE), but during the incubation the variations in these two fractions had an opposite trend, i.e. soil gained AE and lost WE fractions. The variation in N-acetyl-glucosamine-induced respiration was different between compost- and slurry-amended soils, with the greater values in the former. The effect of amendments could be further differentiated by principal component (PCA) and cluster analyses based on the variations in organic matter fractions and CLPP between the beginning and the end of the incubation period. Amendment application led to shifts on the PCA maps that depended both on the amendment and soil treatment. In fresh soil and in that treated at 250 °C, the unamended, compost- and slurry-amended treatments remained relatively close on the PCA maps and had linkage distances <1.0. In contrast, amendment application to other soils led to large shifts on the PCA maps and to linkage distances >1.0. Pig slurry led to the greatest changes in burned soil, while compost induced the greatest shifts in soil treated at 105 °C.This study suggests that an application of organic amendments after a severe fire event may contribute to a faster recovery of soil functions.     

Soil organic matter status of Chernozem soil in North Kazakhstan: effects of summer fallow

In North Kazakhstan there is concern about the degradation of Chernozem soil and agricultural sustainability by the inclusion and frequency of summer fallows in crop rotations in terms of their influence on the changes of soil organic matter (SOM) quality and quantity. We examined five fallow-wheat (Triticum aestivum L.) cropping systems with different frequencies of the fallow phase in Chernozem soil of North Kazakhstan; continuous wheat (CW), 6-y rotation (6R), 4-y rotation (4R), 2-y rotation (2R) and continuous fallow (CF). Soil samples were collected from the two phases of each rotation, pre- and post-fallow, and analyzed for potentially mineralizable C (PMC) and N (PMN), ‘light fraction’ organic matter (LF-OM), C (LF-C) and N (LF-N). Potentially mineralizable C was inversely proportional to the frequency of fallow and was highest in CW. Mineral N (min-N) and PMN were more responsive to rotation phase than other indices of SOM. Mineral N was higher in the post-fallow phase while PMN was higher in the pre-fallow phase. Light fraction organic matter was negatively correlated to the frequency of fallow and was higher in the pre-fallow than in the post-fallow phase in a rotation. The results suggested that the yearly input of plant residue in a less frequently fallowed system built up more PMC, whereas PMN was closely correlated to recent inputs of substrate added with plant residues. We conclude that a frequent fallow system may deplete SOM via accelerated mineralization. Also that LF-OM, PMC and PMN are more sensitive to detect subtle changes in SOM quality than total SOM. Our results may provide prediction of SOM response to fallow frequency in wheat-based rotation systems in Chernozem soils of semi-arid regions.     

Relation of soil organic matter concentration to climate and altitude in zonal soils of China

With a total of 886 data sets distributed in different regions of China, the relation of soil organic matter (SOM) concentration to climate and altitude was investigated. These data sets were obtained from the 2nd National Soil Survey of China that was completed in early 1980s. According to climate gradient and vegetation community succession, six geographical regions, including eastern, southern, northern, northeastern, northwestern and southwestern China, were divided to identify the key factors regulating surface SOM concentration in different geographical regions. Correlation analysis indicates that surface SOM concentration is in general negatively correlated with annual mean temperature (T) and positively correlated with annual mean precipitation (P) and altitude (H). A further investigation suggested that multiple regression models with different combination of T, P and H could explain 41.5%–56.2% of the variability in surface SOM concentration for different geographical regions, while the driving variables are different. Variables of T and P determined surface SOM concentration in northern, northeastern and northwestern China. In eastern and southern China, variables of P and H are key factors regulating surface SOM concentration. Surface SOM concentration in southwestern China is determined by a linear combination of T, P and H.     

Biological, chemical and thermal indices of soil organic matter stability in four grassland soils

The various ecosystem functions of soil organic matter (SOM) depend on both its quantity and stability. Numerous fractionation techniques have been developed to characterize SOM stability, and thermal analysis techniques have shown promising results to describe the complete continuum of SOM in whole soil samples. However, the potential link between SOM thermal stability and biological or chemical stability has not yet been adequately explored. The objective of this study was to compare conventional chemical and biological methods used to characterize SOM stability with results obtained by thermal analysis techniques. Surface soil samples were collected from four North American grassland sites along a continental mean annual temperature gradient, each with a native and cultivated land use. Soil organic C concentrations ranged from 6.8 to 33 g C kg⁻¹ soil. Soils were incubated for 588 days at 35 °C, and C mineralization rates were determined periodically throughout the incubation by measuring CO₂ concentration using an infrared gas analyzer (IRGA) to calculate biological indices of SOM stability. Hot-water extractable organic C (HWEOC) contents were determined before and after incubation as chemical indices. Finally, samples from before and after incubation were analyzed by simultaneous thermal analysis (i.e., thermogravimetry (TG) and differential scanning calorimetry (DSC)) to determine thermal indices of SOM stability. Long-term incubation resulted in the mineralization of up to 33% of initial soil C. The number of days required to respire 5% of initial soil organic carbon (SOC), ranged from 27 to 115 days, and is proposed as a standardized biological index of SOM stability. The number of days was greater for cultivated soils compared to soils under native vegetation, and generally decreased with increasing site mean annual temperature. HWEOC (as % of initial SOC) did not show consistent responses to land use, but was significantly lower after long-term incubation. Energy density (J mg⁻¹ OM) was greater for soils under native vegetation compared to cultivated soils, and long-term incubation also decreased energy density. The temperatures at which half of the mass loss or energy release occurred typically showed larger responses to land use change than to incubation. Strong correlations demonstrated a link between the thermal and biogeochemical stability of SOM, but the interpretation of the thermal behavior of SOM in bulk soil samples remains equivocal because of the role the mineral component and organo-mineral interactions.     

Assessment of biogeochemical trends in soil organic matter sequestration in Mediterranean calcimorphic mountain soils (Almería, Southern Spain)

Total soil organic matter levels and humic acid formation processes in mountain calcimorphic soils from Sierra María-Los Vélez Natural Park (Almería, Southern Spain) were found to differ depending on soil use (pine and oak forests, and cleared areas either cultivated or affected by bush encroachment). Biogeochemical indicators such as the concentration of exchangeable cations, or the concentration of the different types of humic substances were neither influenced by the type of vegetation nor soil use. In fact, multidimensional scaling and multiple correlations suggest that soil carbon sequestration processes are controlled by small-scale topographical features and their impact on water holding capacity. From a qualitative viewpoint, there were two more or less defined sets of soils: one set consisted of soils with humic acids with marked aliphatic character, displayed intense 2920 cm⁻¹ infrared band, and had low optical density. The resolution-enhanced infrared spectra suggested typical lignin patterns and well-defined amide bands, which point to a selective preservation of comparatively young organic matter. This situation contrasts with that in other set of soils with low C levels (<20 g kg⁻¹) where humic acids with featureless infrared spectra showed high aromaticity and were associated with perylenequinonic chromophors of fungal origin: this is considered the consequence of overlapping biogeochemical mechanisms involving both microbial synthesis and condensation processes. The results from visible and infrared derivative spectroscopies suggest that the reliability of statistically assessing the biogeochemical performance of the different uses on the site studied in terms of the intensity of the prevailing humic acid formation mechanisms, i.e., accumulation of inherited macromolecular substances in the former set, vs. microbial synthesis including the condensation of precursors of low molecular weight substances in the latter.