Chemodestructive fractionation of soil organic matter

The method of chemodestructive fractionation is suggested to assess the composition of soil organic matter. This method is based on determination of the resilience of soil organic matter components and/or different parts of organic compounds to the impact of oxidizing agents. For this purpose, a series of solutions with similar concentration of the oxidant (K₂Cr₂O₇), but with linearly increasing oxidative capacity was prepared. Chemodestructive fractionation showed that the portion of easily oxidizable (labile) organic matter in humus horizons of different soil types depends on the conditions of soil formation. It was maximal in hydromorphic soils of the taiga zone and minimal in automorphic soils of the dry steppe zone. The portion of easily oxidizable organic matter in arable soils increased with an increase in the rate of organic fertilizers application. The long-lasting agricultural use of soils and burying of the humus horizons within the upper one-meter layer resulted in the decreasing content of easily oxidizable organic matter. It was found that the portion of easily oxidizable organic matter decreases by the mid-summer or fall in comparison with the spring or early summer period.     

Recovery of soil organic matter,organic matter turnover and nitrogen cycling in a post-mining forest rehabilitation chronosequence

Recovery of soil organic matter, organic matter turnover and mineral nutrient cycling is critical to the success of rehabilitation schemes following major ecosystem disturbance. We investigated successional changes in soil nutrient contents, microbial biomass and activity, C utilisation efficiency and N cycling dynamics in a chronosequence of seven ages (between 0 and 26 years old) of jarrah (Eucalyptus marginata) forest rehabilitation that had been previously mined for bauxite. Recovery was assessed by comparison of rehabilitation soils to non-mined jarrah forest references sites. Mining operations resulted in significant losses of soil total C and N, microbial biomass C and microbial quotients. Organic matter quantity recovered within the rehabilitation chronosequence soils to a level comparable to that of non-mined forest soil. Recovery of soil N was faster than soil C and recovery of microbial and soluble organic C and N fractions was faster than total soil C and N. The recovery of soil organic matter and changes to soil pH displayed distinct spatial heterogeneity due to the surface micro-topography (mounds and furrows) created by contour ripping of rehabilitation sites. Decreases in the metabolic quotient with rehabilitation age conformed to conceptual models of ecosystem energetics during succession but may have been more indicative of decreasing C availability than increased metabolic efficiency. Net ammonification and nitrification rates suggested that the low organic C environment in mound soils may favour autotrophic nitrifier populations, but the production of nitrate (NO₃^?) was limited by the low gross N ammonification rates (≤1 μg N g^?1 d^?1). Gross N transformation rates in furrow soils suggested that the capacity to immobilise N was closely coupled to the capacity to mineralise N, suggesting NO₃^? accumulation in situ is unlikely. The C:N ratio of the older rehabilitation soils was significantly lower than that of the non-mined forest soils. However, variation in ammonification rates was best explained by C and N quantity rather than C:N ratios of whole soil or soluble organic matter fractions. We conclude that the rehabilitated ecosystems are developing a conservative N cycle as displayed by non-mined jarrah forests. However, further investigation into the control of nitrification dynamics, particularly in the event of further ecosystem disturbance, is warranted.     

土地利用方式影响黄棕壤有机质周转和分配

土地利用方式影响土壤有机质平衡和周转。选取南京信息工程大学农业气象实验站长期定位试验的3种利用方式即水稻小麦(稻麦)轮作、大豆小麦(豆麦)轮作和自然植被(休闲)土壤作为研究对象。将土壤筛分团聚体和进行密度分组,测定全土有机质含量和13C丰度、各组分有机质含量,计算全土有机质周转率和半衰期,以揭示土地利用方式对土壤有机质含量及稳定性的影响。结果表明:与休闲处理相比,稻麦轮作对全土有机质含量无明显影响,而豆麦轮作降低了全土有机质含量。稻麦轮作促进了微团聚体黏结形成大团聚体,增加了土壤团聚体的平均重量直径。密度组分的结果表明,稻麦轮作中土壤游离态轻组和闭蓄态轻组有机质含量均未见明显变化,这是保持全土有机质含量无明显增加的主控因素。稻麦轮作处理土壤有机质的高周转率和较短半衰期有利于土壤有机质的更新和养分的释放。豆麦轮作破坏了大团聚体,且有机质源输入较少,土壤有机质的周转率相对较低,不利于有机质的周转和保持其含量稳定。相关分析的结果证实,粒径大于1 mm团聚体、游离态轻组和闭蓄态轻组中有机质含量都与全土有机质含量显著正相关。上述结果表明,在黄棕壤区,稻麦轮作不仅可以保持土壤有机质的含量,还会促进土壤有机质的更新周转,从而提高土壤肥力水平,是未来农业可持续发展需要的科学有效耕作方式。     

Effects of organic matter and calcium on soil structural stability

The cationic bridging effect of the calcium ion (Ca²⁺) and the flocculating ability of clay and organic matter are crucial in the formation and stability of soil aggregates. They are therefore likely to influence the soil's saturated hydraulic conductivity ( K _s). We tested the individual effects of these factors on aggregate stability and related hydraulic properties, and studied the influence of clay mineralogy also. Samples from the surface (0–10 cm) of three contrasting soils in Trinidad were used. The soils were treated with three levels of Ca²⁺ and three levels of organic matter in a 3 × 3 × 3 factorial design and incubated for 14 days. Both aggregate stability and saturated hydraulic conductivity were influenced by all factor combinations. Interactions between soil type and Ca²⁺ revealed the importance of polyvalent cations in aggregate stability of soils with low activity minerals. The influence of organic matter varied with quantity; the more there was, the more stable the soil became, particularly in the soil containing little clay. Clay dispersion and slaking of expanding minerals occurred even with large additions of Ca²⁺ and organic matter, emphasizing the overall influence of mineralogy in determining the response of soils to stability treatments.     

Estimation of turnover and equilibrium of soil organic matter using a mathematical approach

The methods based on N uptake of aerial-plants, soil organic matter （SOM） dynamics, Jenny＇s equation, and actual measurement of long-term field experiments in Jiaxing, Quzhou, Huangyan and Hangzhou of Zhejiang Province, China were used to determine the organic mineralization rate being helpful in estimating the organic requirement for SOM equilibrium. The results showed that the estimated mineralization ratios of SOM for Jiaxing and Quzhou were, respectively, 0.0404 and 0.0508 based on N uptake of aerial-plants in non-fertilized plots; 0.0405 and 0.012 using SOM dynamics in non-fertilized plots; and 0.0413 and 0.0513 using the actual investigated data and Jenny＇s equation. With Jenny＇s equation, soil organic C balance in manure ＋ N-P-K plots was estimated at nearly 28.8 g kg^-1 for Jiaxing and 32.4 g kg^-1 for Quzhou with predicted SOM linearly related to the actual investigated values （r^2 = 0.9640 for Jiaxing and 0.8541 for Quzhou）. To maintain the SOM balance in the non-fertilized plots the recommended rate of organic materials was 3 000-6 600 kg ha^-1, and the relevant rates of farm yard manure （FYM） in the manure and N-P-K plots were estimated at 3 375 （dry） and 17670 kg ha^-1 （wet） for Jiaxing, 1845 （dry） and 6090 kg ha^-1 （wet） for Quzhou.     

Movement and turnover of soil organic matter as indicated by carbon isotope measurements

The distribution in a New Zealand pasture soil of total organic carbon, of ¹³C/¹²C and ¹⁴C/¹²C ratios, up to a depth of l m and over a period of 15 yr, has been analysed to provide models for radiocarbon enrichment and organic matter turnover. For the enrichment model, the parameters of C input, rates of decomposition and diffusion down the soil profile, and turnover rates have been determined. Input rates are compatible with herbage productivity measurements; the turnover period is estimated as 63 yr, and the downward diffusivity of C is estimated at 13cm² yr^?1. The steady state model indicates that a small fraction of the soil organic C, about 16%, is relatively very old and uniformly distributed throughout the soil profile. Most of the remaining fraction of modern C, other than “bomb” C, is less than 100 yr old and decreases exponentially with increasing depth. The models provide a rational integration of empirical measurements of C input, rates of organic matter decomposition, diffusivity and turnover, enabling an unknown to be calculated from measured parameters. They also provide a rational and precise method for determining the age of soil organic matter from radiocarbon measurements and have been used to examine alternative hypotheses for the distribution of UC with depth.     

Quantifying water-stable soil aggregate turnover and its implication for soil organic matter dynamics in a model study

Recent studies have concluded that the dynamics of soil structure are central to the understanding of soil organic matter (SOM) cycling and the ensuing soil‐water–nutrient relationships. Aggregate turnover directly controls the stabilization and physical protection of SOM. Therefore, quantifying aggregate dynamics will improve our ability to predict SOM behaviour as affected by ecosystem management and global change. We present an approach to directly quantify aggregate dynamics using rare‐earth oxides as tracers. A 6‐week laboratory incubation was set up to measure aggregate dynamics at different times. We made samples in which each different aggregate size‐fraction contained a different tracer. By following the redistribution of these tracers into the other aggregate size‐fractions, we could quantify all soil mass transfers between aggregate size‐fractions. A comparison with a control soil showed that the tracer did not affect soil respiration or the aggregation process itself. Tracer mixing homogeneity, recovery and immobility were tested and validated. While initially macroaggregate formation occurred rapidly, microaggregate formation occurred more slowly during the experiment. Subsequent aggregate stabilization was more pronounced for the newly formed microaggregates than for the newly formed macroaggregates. Calculated turnover times were smaller for macroaggregates than for microaggregates (i.e. 30 vs. 88 days). Further research is needed to investigate to what extent these results can be extrapolated to the field. Our results confirmed existing qualitative views and concepts on aggregate dynamics in a quantitative way and will be valuable in directly linking aggregate turnover to the stabilization and protection of SOM.     

Distribution of transformed organic matter in structural units of loamy sandy soddy-podzolic soil

The effect of land use types and fertilizing systems on the structural and aggregate composition of loamy sandy soddy-podzolic soil and the quantitative parameters of soil organic matter has been studied. The contribution of soil aggregates 2–1 mm in size to the total C_org reserve in the humus horizon is higher than the contributions of other aggregates by 1.3–4.2 times. Reliable correlations have been revealed between the contents of total (C_org), labile (C_lab), and active (C₀) organic matter in the soil. The proportion of C₀ is 44–70% of C_lab extractable by neutral sodium pyrophosphate solution. The contributions of each of the 2–1, 0.5–0.25, and <0.25 mm fractions to the total C₀ reserve are 14–21%; the contributions of each of the other fractions are 4–12%. The chemically labile and biologically active components of humic substances reflect the quality changes of soil organic matter under agrogenic impacts. A conceptual scheme has been proposed for the subdivision of soil organic matter into the active, slow (intermediate), and passive pools. In the humus horizon of loamy sandy soddy-podzolic soil, the active, slow, and passive pools contain 6–11, 34–65, and 26–94% of the total C_org, respectively.     

Physical protection and biochemical quality of organic matter in mediterranean calcareous forest soils: a density fractionation approach

Physical protection is one of the most important ways for stabilization of organic carbon (OC) in soils, and in order to properly manage soils as a sink for carbon, it is necessary to know how much OC a given soil could protect. To this end, we studied individual horizons taken from 16 soil profiles under Quercus rotundifolia stands, all over calcareous parent materials. Horizons were subjected to a sequential extraction using solutions of sodium polytungstate (NaPT) of increasing density: (i) NaPT d=1.6, using slight hand agitation, to obtain the free light fraction (FL); (ii) NaPT d=1.6 and ultrasonic dispersion, to obtain the Occluded Fraction I (Ocl I); (iii) NaPT d=1.8, to obtain the Occluded Fraction II (Ocl II); and (iv) NaPT d=2.0, to obtain the Occluded Fraction III (Ocl III). The fraction of density>2.0 are taken as dense fraction (DF). The free organic matter was further divided into FL>50 (retained by a 50 μm mesh: coarse organic fragments) and FL<50 (non-retained: fine organic fragments). The fractions FL>50 and FL<50 were taken together as free organic matter. The rest of the fractions are taken together as protected organic matter. The obtained fractions were analyzed for total OC, total N, and carbohydrate content. The percentage of non-hydrolyzable OC and N in each fraction was taken as an indicator of OC and N recalcitrance, respectively.For both OC and N, the fractions FL>50 and DF are dominant; the rest of the fractions are of much lower quantitative importance. In H horizons and in most A horizons, most of the OC and N are free, whereas in B horizons both OC and N are mostly protected. Overall, the percentages of free OC and N are very high and are currently amongst the highest ever recorded.Organic matter recalcitrance is lowest in the two most protected fractions (Ocl III and especially DF), and highest in the first occluded fractions (Ocl II and especially Ocl I). The free organic matter (FL>50 fraction) has an intermediate quality: it includes recognizable plant fragments, but the indicators tested (recalcitrance, carbohydrate content, cellulose to total carbohydrates ratio) suggest that it is not always the most fresh and non-decomposed fraction.There are clear maxima for both protected OC and N, which can be approached by curve fitting. By exponential fit, the obtained maxima are 84.1 g of OC and 7.7 g of N kg⁻¹ of mineral particles <20 μm. These maxima are much higher than the upper limits obtained by other authors. Differences in the sampling approach are suggested as the reason for such discrepancies.     

Photodissolution of soil organic matter

Sunlight has been shown to enhance loss of organic matter from aquatic sediments and terrestrial plant litter, so we tested for similar reactions in mineral soil horizons. Losses of up to a third of particulate organic carbon occurred after continuous exposure to full-strength sunlight for dozens of hours, with similar amounts appearing as photodissolved organic carbon. Nitrogen dissolved similarly, appearing partly as ammonium. Modified experiments with interruption of irradiation to include extended dark incubation periods increased loss of total organic carbon, implying remineralization by some combination of light and microbes. These photodissolution reactions respond strongly to water content, with reaction extent under air-dry to fully wet conditions increasing by a factor of 3–4 fold. Light limitation was explored using lamp intensity and soil depth experiments. Reaction extent varied linearly with lamp intensity. Depth experiments indicate that attenuation of reaction occurs within the top tens to hundreds of micrometers of soil depth. Our data allow only order-of-magnitude extrapolations to field conditions, but suggest that this type of reaction could induce loss of 10–20% of soil organic carbon in the top 10 cm horizon over a century. It may therefore have contributed to historical losses of soil carbon via agriculture, and should be considered in soil management on similar time scales.     

Stock, turnover time and accumulation of organic matter in bulk and density fractions of a Podzol soil

Temperate forest soils store large amounts of organic matter and are considered as net sinks for atmospheric carbon dioxide. Information about the sink strength and the turnover time of soil organic carbon (SOC) is required to assess the potential response of soils to climate change. Here we report on stocks, turnover times (TT) and accumulation of SOC in bulk soil and density fractions from genetic horizons of a Podzol in the Fichtelgebirge, Germany. Stocks of SOC, total nitrogen and exchangeable cations determined in nine quantitative soil pits strongly varied with stone content and thickness of horizons in both the organic layer and the mineral soil. On the basis of radiocarbon signatures, mean turnover times of 4, 9 and 133 years, respectively, were calculated for Oi, Oe and Oa horizons from three soil pits, using a non-steady-state model. The Oa horizons accumulated 4–8 g C m⁻² year⁻¹ whereas the Oi and Oe horizons were close to steady-state during the past decade. Free particulate organic matter (FPOM) was the most abundant fraction in the Oa and EA horizons with TT of 70–480 years. In the B horizons, mineral associated organic matter (MAOM) dominated with over 40% of total SOC and had TT of 390–2170 years. In contrast to other horizons, MAOM in the Bsh and Bs horizon had generally faster TT than occluded particulate organic matter (OPOM), possibly because of sorption of dissolved organic carbon by iron and aluminium oxides/hydroxides. Our results suggest that organic horizons with relatively short turnover times could be particularly vulnerable to changes in climate or other disturbances.     

Mineral surfaces and soil organic matter

The organic carbon content of soil is positively related to the specific surface area (SSA), but large amounts of organic matter in soil result in reduced SSA as determined by applying the Brunauer–Emmett–Teller (BET) equation to the adsorption of N₂. To elucidate some of the controlling mechanisms of this relation, we determined the SSA and the enthalpy of N₂ adsorption of separates with a density > 1.6 g cm^?3 from 196 mineral horizons of forest soils before and after removal of organic matter with NaOCl. Likewise, we investigated these characteristics before and after sorption of increasing amounts of organic matter to four mineral soil samples, oxides (amorphous Al(OH)₃, gibbsite, ferrihydrite, goethite, haematite), and phyllosilicates (kaolinite, illite). Sorption of organic matter reduced the SSA, depending on the amount sorbed and the type of mineral. The reduction in SSA decreased at larger organic matter loadings. The SSA of the mineral soils was positively related to the content of Fe oxyhydroxides and negatively related to the content of organic C. The strong reduction in SSA at small loadings was due primarily to the decrease in the micropores to which N₂ was accessible. This suggests preferential sorption of organic matter at reactive sites in or at the mouths of micropores during the initial sorption and attachment to less reactive sites at increasing loadings. The exponential decrease of the heat of gas adsorption with the surface loading points also to a filling or clogging of micropores at early stages of organic matter accumulation. Desorption induced a small recovery of the total SSA but not of the micropore surface area. Destruction of organic matter increased the SSA of all soil samples. The SSA of the uncovered mineral matrix related strongly to the amounts of Fe oxyhydroxides and the clay. Normalized to C removed, the increase in SSA was small in topsoils and illuvial horizons of Podzols rich in C and large for the subsoils containing little C. This suggests that micropores preferentially associate with organic matter, especially at small loadings. The coverage of the surface of the soil mineral matrix as calculated from the SSA before and after destruction of organic matter was correlated only with depth, and the relation appeared to be linear. We conclude that mineralogy is the primary control of the relation between surface area and sorption of organic matter within same soil compartments (i.e. horizons). But at the scale of complete profiles, the surface accumulation and stabilization of organic matter is additionally determined by its input.     

Microbial biomass and organic matter turnover in wetland rice soils

A decline in rice yields has been associated with intensification of rice production. In continuously irrigated systems this has been attributed to a decline in soil N supply. Nutrient mineralisation and immobilisation is constrained by the quantity and nature of the organic substrates and the physico-chemical environment of the soil system itself. A flooded soil is very different from an aerobic one; electron acceptors other than oxygen have to be used. The transition to continuously anaerobic conditions associated with the intensification of wetland rice systems affects their organic matter turnover and may adversely affect their productivity.     

Effects of iron and organic matter on the porosity and structural stability of soil aggregates

Knowledge of the soil components controlling aggregate formation and stability is fundamental to the conservation of soil structure. In this work, the effects of Fe and organic matter (OM) on the porosity and structural stability of aggregates <4 mm of two cropped soils from Galicia (NW Spain) were determined. Porosity was estimated directly, by mercury intrusion porosimetry, and indirectly, from moisture characteristic and shrinkage curves. The three porosities obtained were similar and indicated that Soil 1, with the highest Fe and OM contents, had lower total porosity and a wider pore-size distribution than Soil 2. As regards structural stability, Hénin and Monnier's test, simulated rainfall and dispersion experiments, and determination of textural tensile strength all indicated Soil 1 to be the more stable soil. Oxidation of OM and selective extraction of Fe, which were most concentrated in the clay and silt fractions, indicated both these components to be important soil aggregants. It is suggested that the higher content of Soil 1 in Fe and OM is responsible for its greater stability.     

Modelling refractory soil organic matter

Most models for the turnover of soil organic matter (SOM) include a compartment that is either considered inert, or has a very slow turnover time (refractory SOM; RSOM). The RSOM content of soils varies markedly between sites, and knowledge of its size and variability are essential for determining whether soils behave as sources or sinks of atmospheric CO₂. It has also been suggested that the accurate specification of RSOM pools is essential to modelling studies, and that uncertainty in estimates of the size of RSOM pool could be a major source of error in modelling soil organic C. In this paper, current SOM models are reviewed, and approaches to modelling RSOM and its significance are discussed. Simulations of SOM turnover for the Rothamsted Broadbalk winter wheat experiment using the Rothamsted C model and CENTURY are presented as examples. Received: 13 July 1999     

Are the links between soil aggregate size class, soil organic matter and respiration rate artefacts of the fractionation procedure?

Our aim was to see how variations in aggregate fractionation procedures influence the chemical and biological properties of different sized soil aggregates. Soil was fractionated using two different physical procedures: (1) slaking to simulate a major wetting stress in the field or (2) shaking to simulate mechanical disruption by tillage followed by wet sieving. In the slaked treatment, macro-aggregates (<250 μm dia) contained about 17% more soil organic C and had about 30% faster rates of respiration. This was in contrast to the shaken treatment where micro-aggregates (<250 μm dia) contained about 12% more soil organic C and had about 14% faster rates of respiration. The biological and chemical properties of different sized aggregates were used to describe two different models. These were the aggregate heirarchy model and one based on maximum biological activity at soil surfaces. Our results suggest that the chemical and biological properties of aggregates depend on the fractionation procedure. On this basis we suggest that the observed relationships between aggregate size and other properties, for example biological activity, must be interpreted in terms of the disruptive mechanisms used to fractionate aggregated soil. Our results suggest that the aggregate hypothesis has serious weaknesses: the aggregates measured being largely an artefact of the chosen method of separation. We therefore suggest that future work should also consider biological activities at soil pore surfaces. It is at the surface of these channels that parameters such as oxygen supply, plant roots, root exudates and fresh organic matter inputs first interact with the soil. Biological processes in this region are therefore likely to be more important than those occurring in the bulk soil.     

Stabilization of organic matter in soil lysimeters

Using stationary soil lysimeters it was demonstrated that the type of phytocenose determines the processes of stabilization of organic matter and the type of litter reflects the transformation of organic matter. The maximum quantity of large aggregates at the lowest concentrations of carbon is detected in the upper horizons of soils under fallow conditions as compared to those under other communities. The maximum possible loss of carbon, as estimated by the intensity of respiration, is observed in the soils under mixed plantings, while the least was observed for agricultural lands.