Active and passive organic matter fractions in Mediterranean forest soils

Soil organic carbon (C) is a complex set of pools, and to understand its dynamics it is necessary to know which of these pools are active at a given moment, and which act as passive, due to either physical protection or biochemical recalcitrance, or both. This matter has been studied mainly in agricultural soils. For forest soils, especially in Mediterranean areas, there is a data gap that needs to be filled. Therefore, we studied three profiles in Catalonia (NE Spain) over marl and under Pinus halepensis stands. Soil horizons were incubated under optimal conditions for 45 days. The respiration rate on day 45 was taken as basal respiration rate (BR_R). The following fractions were quantified: (1) soluble C, (2) microbial C, both corrected (MC_C) and uncorrected (MC_UC) (i.e., applying or not a correction factor to account for the non-extractable microbial carbon), (3) C in size fractions, isolated by ultrasonic dispersion and sieving plus sedimentation, and (4) labile and recalcitrant C, quantified by acid hydrolysis, applied to both the whole soil horizons and the size fractions. The basal respiration rate (BR_R) correlated best with the sum soluble + MC_UC, which altogether seem the best estimator of the active C pool. The correlation between BR_R and MC_C was worse, thus suggesting that not all microbial C should be included in the active pool. The correlation of BR_R with the C associated to coarse fractions (>50 μm) was positive, whereas that\with C associated to fine fractions (<20 μm) was negative. The correlations were lower than those obtained with the soluble + MC_UC, thus suggesting that the coarse organic fractions are probably the main source of active C, but not active C itself. Alone, the pools obtained by acid hydrolysis (labile and recalcitrant) correlated poorly with BR_R, but the combination of size fractionation with acid hydrolysis resulted in some of the best predictors of microbial activity. Hydrolyzable polyphenolic compounds inhibited microbial activity. Unhydrolyzable C associated to fine fractions (<20 μm) seemed the most stable of all the C pools studied. By contrast the unhydrolyzable part was apparently as unstable as the hydrolyzable part in the coarse organic debris. Overall, our results point to a hierarchy of constraints: both the physical protection and the biochemical quality affect microbial activity, but the physical protection goes first. In the profiles studied, C did not appear to be more stable in deep horizons than in surface horizons.     

地中海山地土壤中团聚体的形成和有机质的储存

Soil aggregation and organic matter of soils from the pre-Pyrenean range in Catalonia (NE Spain) were studied, in order to assess their quality as carbon sinks and also to select the best soil management practices to preserve their quality. Aggregate stability, organic carbon and micromorphology were investigated. The highest amount of organic carbon was found in alluvial, deep soils (228 Mg C ha^-1), and the lowest was in a shallow, stony soil with a low plant cover (78 Mg C ha^-1). Subsurface horizons of degraded soils under pastures were the ones with smaller and less-stable aggregates. Fresh residues of organic matter (OM) were found mostly in interaggregate spaces. Within the aggregates there were some organic remains that were beginning to decompose, and also impregnative nodules of amorphous OM. Although OM was evenly distributed among the aggregate fractions, the larger blocky peds had more specific surface, contained less decomposed OM and had a lower organic/mineral interphase than smaller crumb aggregates, which were also more stable. Soil carbon storage was affected primarily by the OM inputs in the surface horizons. In order to store organic carbon over the mid-and long-term periods, the mechanisms favouring structuration through biological activity and creating small aggregates with intrapedal stable microporosities seemed to be the most effective.     

Identification of labile and stable pools of organic matter in an agrogray soil

The intensity of decomposition of the organic matter in the particle-size fractions from a agrogray soil sampled in a 5-year-long field experiment on the decomposition of corn residues was determined in the course of incubation for a year. The corn residues were placed into the soil in amounts equivalent to the amounts of plant litter in the agrocenosis and in the meadow ecosystem. A combination of three methods—the particle-size fractionation, the method of ¹³C natural abundance by C3–C4 transition, and the method of incubation—made it possible to subdivide the soil organic matter into the labile and stable pools. The labile pool reached 32% in the soil of the agrocenosis and 42% in the meadow soil. Owing to the negative priming effect, the addition of C4 (young) carbon favored the stabilization of the C3 (old) carbon in the soil. When the young carbon was absent, destabilization or intense decomposition of the old organic matter was observed. This process was found even in the most stable fine silt and clay fractions.     

The influence of organic matter on aggregate stability in some British soils

The stability of aggregates from 26 soils selected from agricultural areas was measured by wet-sieving and the results correlated with sand, silt, clay, nitrogen, organic matter and iron contents and with cation exchange capacity. Highly significant correlations were obtained for the relationships between aggregate stability and organic matter and some properties associated with it. No other soil constituent investigated had a significant relationship with aggregate stability, indicating that organic matter is mainly responsible for the stabilization of aggregates in these soils. The relationships between aggregate stability, and organic matter content plus some of its component fractions were examined in more detail using 120 soils. Total organic matter, total carbohydrate and humic material extracted by various reagents each gave highly or very highly significant correlations with aggregate stability. However, whilst it was not possible to distinguish whether any one organic component was more important than another, the results indicate that soil organic matter levels can be used diagnostically to identify soils which may show problems of structural instability.     

The influence of slope aspect on soil weathering processes in the Springerville volcanic field,Arizona

A comparison was made between soils on north- and south-facing slopes of six cinder cones in the Springerville volcanic field (SVF), Arizona, in order to determine the influence of slope aspect on soil weathering processes. Twenty-four soil pedons were sampled on different aspects of six cinder cones. To control for the influence of slope on pedogenesis, all sample sites possessed slopes of 17±2°. Soil weathering processes were characterized by solum depth, texture, and Ca:Zr chemical weathering indices. Quartz and mica were used to identify eolian additions to the volcanic soils. Accelerated rates of weathering and soil development were found to occur in soils on south-facing slopes while no trend with aspect was found for eolian additions. Accelerated rates of weathering and soil development may influence cinder cone degradation and cone morphology.     

Effect of temperature on the decomposition rate of labile and stable organic matter in an agrochernozem

An hypothesis about the different temperature dependences of the decomposition of the labile and stable organic carbon pools has been tested using an agrochernozem sampled from an experimental plot of 42-year-old continuous corn in Voronezh oblast. The partitioning of the CO₂ loss during the decomposition of the labile and stable soil organic matter (SOM) at 2, 12, and 22°C in a long-term incubation experiment was performed using the method of ¹³C natural abundance by C3–C4 transition. On the basis of the determined decomposition constants, the SOM pools have been arranged in an order according to their increasing stability: plant residues < new (C4) SOM < old (C3) SOM. The tested hypothesis has been found valid only for a limited temperature interval. The temperature coefficient Q ₁₀ increases in the stability order from 1.2 to 4.3 in the interval of 12–22°C. At low temperatures (2–12°C), the values of Q ₁₀ insignificantly vary among the SOM pools and lie in the range of 2.2–2.8. Along with the decomposition constants of the SOM, the new-to-old carbon ratio in the CO₂ efflux from the soil and the magnitude of the negative priming effect for the old SOM caused by the input of new organic matter depend on the temperature. In the soil under continuous corn fertilized with NPK, the increased decomposition of C3 SOM is observed compared to the unfertilized control; the temperature dependences of the SOM decomposition are similar in both agrochernozem treatments.     

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.     

Management-induced change in labile soil organic matter under continuous corn in eastern Canadian soils

Soil samples taken from four experimental sites that had been cropped to continuous corn for 3–11 years in Ontario and Quebec were analyzed to evaluate changes in quantity and quality of labile soil organic carbon under different nitrogen (N) fertility and tillage treatments. Addition of fertilizer N above soil test recommendations tended to decrease amounts of water-soluble organic carbon (WSOC) and microbial biomass carbon (MBC). The quality of the WSOC was characterized by ¹³C nuclear magnetic resonance and infrared spectrophotometry and the results indicated that carbohydrates, long-chain aliphatics and proteins were the major components of all extracts. Similar types of C were present in all of the soils, but an influence of management was evident. The quantity of soil MBC was positively related to the quantities of WSOC, carbohydrate C, and organic C, and negatively related to quantities of long-chain aliphatic C in the soil. The quantity of WSOC was positively related to the quantities of protein C, carbohydrate C, and negatively related to the quantity of carboxylic C. The quantity of soil MBC was not only related to quantities of soil WSOC but also to the quality of soil WSOC. Received: 2 April 1997     

The influence of bioturbation on the vertical distribution of soil organic matter in volcanic ash soils: a case study in northern Ecuador

Soil faunal bioturbation (‘bioturbation’) is often cited as a major process influencing the vertical distribution of soil organic matter (SOM). The influence of bioturbation on vertical SOM transport is complex because it is the result of interaction between different groups of soil faunal species that redistribute SOM through the soil profile in distinct ways. We performed a semi‐quantitative micromorphological analysis of soil faunal pedofeatures and related their occurrence to the vertical distribution of SOM and high‐resolution radiocarbon dating in volcanic ash soils under montane forest and grassland (páramo) vegetation in the northern Ecuadorian Andes. The páramo soil data suggest that bioturbation was largely responsible for the vertical distribution of SOM, while illuviation and root input were of minor importance. Bioturbation was caused by endogeic species, which typically mix the soil only over short vertical distances. Short vertical distance mixing was apparently enhanced by the upward shifting of bioturbation as a result of soil thickening due to SOM accumulation. A change from páramo to forest vegetation was accompanied by a change from endogeic to epigeic species. As these latter species do not redistribute material vertically, this eventually resulted in the formation of thick ectorganic horizons in the forest.     

The influence of heating on organic matter of forest litters and soils under experimental conditions

The specific features of changes in the content and mobility of organic matter in litters and cryogenic soils under heating were revealed. The thermal stability of the organic matter and litters is different. In the soils, the maximal loss of matter was recorded at a temperature of 300°C. In the litters, the maximal losses were found at 300, 400 and 550°C and depended inversely on the carbon content in them. The heating to 200°C caused insignificant changes in the mass of the litters and soils but increased the content of the water-soluble fraction of organic matter and the concentration of the water-soluble mineral nitrogen forms.     

Hydrophobicity of organic matter in arable soils: influence of management

The affinity of soil organic matter for water influences resistance to microbial degradation, the rate of wetting and adsorption processes. Such properties play key roles in organic matter and microbial biomass dynamics, aggregate stability, water infiltration, leaching of organic and inorganic pollutants, chemical composition and the dynamics of dissolved organic matter (DOM). The hydrophobicity of the organic matter as a function of management have been studied in two soils with contrasting textures using diffuse reflectance infrared fourier transform spectroscopy (DRIFT). The results show that agricultural management clearly influences the amount of aliphatic C-H units and implicitly the hydrophobicity of the soil organic matter. A decrease of organic C due to management is accompanied by a decrease of hydrophobicity as well as of soil microbial activity and aggregate stability. The hydrophobicity index is a sensitive quantity to characterize the‘quality’ of soil organic matter. DRIFT spectroscopy proves to be a rapid and reliable technique to determine quantitatively the hydrophobicity of soil organic matter.     

土壤活性有机质测定方法的比较

用 16种化学方法对红壤、黑土、垆土、褐土、灰漠土五个典型土壤活性有机质进行测定 ,结果表明 ,333mmol/LKMnO4常温振荡 1h的方法测定结果变异系数小 ,重现性好 ,且操作简便快速 ,可作为土壤活性有机质测定的基本方法。虽然不同方法测定的活性有机质数量不同 ,但它们之间表现出较好的相关性 ,说明它们在一定程度上都可反映土壤有机质的活性     

Modelling changes in soil organic matter after planting fast-growing Pinus radiata on Mediterranean agricultural soils

The Kyoto Protocol explicitly allows the storage of carbon (C) in ecosystems resulting from afforestation to be offset against a nation's carbon emissions and paves the way for carbon storage in soils to be eligible as carbon offsets in the future. More information is required about how afforestation affects carbon storage, especially in the soil. We report a study in which soil carbon storage in first‐rotation Mediterranean Pinus radiata plantations, established on former cereal fields and vineyards, was measured and modelled. Measurements were made on plantations of several ages, as well as repeat measurements at the same site after 5 years. We tested the ability of two widely used soil organic matter models (RothC and Century) to predict carbon sequestration in Mediterranean forest soils. Increases in the top 5 cm of soil of about 10 g C m^?2 year^?1 were observed after afforestation of former vineyards, but nitrogen (N) either remained the same or decreased slightly. During afforestation, most organic matter accumulated in the ectorganic layers at a rate of 19 g C m^?2 year^?1 in former vineyards and 41 g C m^?2 year^?1 in former cereal fields. The RothC and Century models were sensitive to previous land use and estimated a carbon sequestration potential over 20 years of 950 and 700 g C m^?2, respectively. The accurate simulation of the dynamics of soil organic matter by RothC, together with measured above‐ground inputs, allowed us to calculate below‐ground inputs during afforestation. The Century model simulated total C and N, including the ectorganic horizons, well. Simulations showed a depletion of N in the below‐ground fractions during afforestation, with N limitation in the former vineyard but not on former cereal land. The approach demonstrates the potential of models to enhance our understanding of the processes leading to carbon sequestration in soils.     

Biogeochemical factors related with organic matter degradation and C storage in agricultural volcanic ash soils

Laboratory incubation experiments in addition to physicochemical analyses of volcanic ash soils were carried out in order to identify biogeochemical factors related with soil organic C (SOC) stabilization in the long term and with the potential for C sequestration of agroecosystems. Up to 24 vineyard plots under similar subtropical conditions in Tenerife Island (Spain) were sampled. Soil samples were incubated for 30 days in laboratory conditions (27 °C and 66% water holding capacity) and the CO₂ released was periodically measured to plot C mineralization curves. Soil organic matter (SOM) with special emphasis paid on the humic acid (HA) was characterized by elemental composition, spectroscopic techniques: visible, infrared (IR) and ¹³C nuclear magnetic resonance (¹³C NMR) and analytical pyrolysis–gas chromatography/mass spectrometry (GC/MS). The dependent variables examined were either the total mineralization coefficient (TMC, g C · kg C soil^?1 day^?1) in laboratory incubations, or the SOC. A very significant negative correlation was found between SOC and TMC, i.e., in our soils, the higher the biodegradation rates under laboratory conditions, the lower the soil C sequestered in the corresponding plots. In it was also observed that the concentration of amorphous minerals (Al_o + ½ Fe_o index) and the water holding capacity at 0.033 MPa were associated with lower CO₂ release; the latter could suggest microanaerobic conditions hampering biodegradation in these thixotropic soils. Conversely, no correlation was found between SOC or TMC and typical soil physical and chemical factors, such as granulometric fractions or exchangeable calcium. The molecular characteristics of the HAs showed also predictive potential as regards SOC resilience, reflecting the comparatively fast biodegradation of SOM composed mainly of biomass constituents (prominent lignin signature and O-alkyl ¹³C NMR region). The poor correlation between total aromaticity of the HAs and SOM resistance against biodegradation could be explained by a dual origin of aromatic structures in HAs, either consisting of methoxyl-containing non-decomposed lignin structures or condensed black carbon-like polyaromatic structures. The results suggested the possibility of predicting the vulnerability of SOC to biodegradation from laboratory incubation experiments, which results of interest for modeling global change scenarios.     

平菇栽培废料等有机肥对土壤活性有机质和土壤酶活性的影响

通过大田试验研究了不施有机肥(CK)、施用平菇栽培废料(T1)、施用干腐熟牛粪(T2)和烘干鸡粪(T3)在种植黄瓜01～50.d内土壤中活性有机质和4种土壤酶活性的变化。结果表明:施入不同有机肥对土壤总有机质含量的影响为烘干鸡粪平菇栽培废料干腐熟牛粪对照;对活性有机质含量的影响为平菇栽培废料烘干鸡粪干腐熟牛粪对照;施用平菇栽培废料的土壤中脲酶、转化酶和脱氢酶活性最高,施用干腐熟牛粪的土壤中过氧化氢酶活性最高。相关性分析显示,脲酶、转化酶和脱氢酶活性与土壤活性有机质显著相关。用平菇栽培废料做有机肥能有效提高土壤活性有机质含量和土壤酶活性。     

Aggregate and organic matter dynamics in reclaimed soils as indicated by stable carbon isotopes

Recovery of belowground ecosystem processes, such as soil aggregation and organic matter (OM) accumulation, in reconstructed soils is crucial to successful reclamation of disturbed lands. Objectives of this study were to track soil aggregate recovery in combination with aggregate associated OM on a chronosequence of reclaimed surface mine sites and a native, undisturbed reference site. Macroaggregate and micro-within-macroaggregate proportions increased with reclamation age, while microaggregate proportions decreased. Organic carbon (C) and total nitrogen (N) concentrations increased with reclamation age for each aggregate fraction and were higher in the OM fraction observed within soil aggregates than in the free OM fraction found between soil aggregates. Naturally occurring isotopic signatures of ¹³C decreased rapidly with reclamation age, indicating over 50% of total aggregate C to be new C from predominately C₃ plant community inputs after 26 years of reclamation. Soil aggregate size distribution trends of increasing macroaggregation and micro-within-macroaggregates along with rapid rates of OM accumulation with time indicated that reclaimed soils had recovered structurally towards a native soil condition after a period of 10-15 years.     

Mineralization rate of C-labelled dissolved organic matter from leaf litter in soils of a weathering chronosequence

During the processes of primary succession and soil development, large stocks of organic C with very long residence times accumulate in many soils. Soluble organic C adsorbed by soils may contribute to the stock of organic C accumulating during soil development. We determined whether the mineralization rate of water-soluble organic C and the insoluble residue from ¹⁴C-labelled leaf litter added to soils from a weathering chronosequence decrease as soil age and adsorption capacity increase. The soils were formed on mudflows of andesitic material deposited about 75, 255, 616 y ago, and another older but undetermined time before this study. The percentage of the DOC adsorbed by the soils increased with age. After 1 year of incubation there were no significant differences in the mineralization rates of DOC added to soils of different ages. The DOC appeared to be comprised of two fractions, one that comprises about 32% of the total that mineralized with a half decay time of 0.02 y (7 d) and a second fraction comprising 68% with a half decay time of about 1.6 y. Consequently, the slowly mineralized fraction of the soluble C contributed to the accumulation of slowly mineralized C in the soil. Both the slowly and rapidly mineralized fractions of the insoluble residue decomposed more slowly than the corresponding fractions in DOC. We found no support for the idea that increased adsorption capacity due to weathering resulted in protection of soluble organic C from microbial mineralization.     

Grade of weathering and fertility of volcanic ash soils of Aso volcano

Texture is one of the major criterions in soil classification, probably because it has a decisive influence on soil properties. This is particularly true for volcanic ash soils. Most ashes are largely composed of sand and silt particles with little clay (11, 15). The ash weathers very rapidly (1), and clay site particles less than 2 microns in diameter occur even within a few months, as shown by Ishii at the authors' laboratory. Those clay size particles produced in the early stage of weathering are slightly weathered ones (2), and are still subject to rapid weathering, losing bases and silica under humid and well drained conditions. In consequence the clay fraction of volcanic ash soils is composed of particles which vary in degree of weathering from slightly altered glass and feldspar to true clay mineraloids and minerals. The clay fraction of younger soils as a whole is less and that of older ones is more weathered. Weathering brings a remarkable change in the properties of volcanic ash soils; for example, an inerease in soil acidity, lowering of base saturation and bulk density, or accumlation of organic matter. These changes must exert a great influence on soil fertility directly or indirectly.     

The distribution of nitrogen in some highly organic tropical volcanic soils

The qualitative and quantitative distribution of N-compounds in 10 tropical soils, and in a number of humic materials extracted from representative samples thereof, was determined after 6 N HCl hydrolysis.Eighty to 98% of the total N in the soils and humic materials was hydrolysable by 6n HCl. Slightly less than one half the hydrolysable N in the soils and humic fractions consisted of amino acids. Well-drained soils and fulvic acids extracted from them contained unusually high concentrations of the acidic amino acids, aspartic and glutamic acids. Between 80 and 95% of the amino acids in the soils was accounted for in the humic materials + NaOH-insoluble organic residues. NH⁺₄-N released by acid hydrolysis was generally higher for the soil samples than for the humic materials. Amino sugar-N constituted relatively small proportions of the total N in the soils and humic fractions.Our data suggest that large quantities of amorphous allophanic materials coupled with relatively high enzymic activity are responsible for the observed accumulation of acidic amino acids in the well-drained tropical volcanic soils.     

The influence of the moisture content,texture and organic matter on the aggregation of sandy and loamy soils

It is generally accepted that water is one of the major factors involved in the aggregation process. This is proved by the change in bulk density of a soil wetted to different water contents and submitted to a certain number of standardized knocks.The water content corresponding to a minimal bulk density, resulting in an optimal aggregation, is called optimal or critical moisture content. The corresponding suction or critical capillary depression is inversely proportional to the particle size and thus will increase as the latter decreases. The pF-value at the critical moisture content ranges between 2.30 for sand and 3.32 for sandy loam.Organic matter also plays an important role in the aggregation process, but has no influence on the critical capillary depression. From a quantitative point of view the organic matter is a better aggregation agent than clay and silt.