Stabilization mechanisms of organic matter in four temperate soils: Development and application of a conceptual model

Based on recent findings in the literature, we developed a process‐oriented conceptual model that integrates all three process groups of organic matter (OM) stabilization in soils namely (1) selective preservation of recalcitrant compounds, (2) spatial inaccessibility to decomposer organisms, and (3) interactions of OM with minerals and metal ions. The model concept relates the diverse stabilization mechanisms to active, intermediate, and passive pools. The formation of the passive pool is regarded as hierarchical structured co‐action of various processes that are active under specific pedogenetic conditions. To evaluate the model, we used data of pool sizes and turnover times of soil OM fractions from horizons of two acid forest and two agricultural soils. Selective preservation of recalcitrant compounds is relevant in the active pool and particularly in soil horizons with high C contents. Biogenic aggregation preserves OM in the intermediate pool and is limited to topsoil horizons. Spatial inaccessibility due to the occlusion of OM in clay microstructures and due to the formation of hydrophobic surfaces stabilizes OM in the passive pool. If present, charcoal contributes to the passive pool mainly in topsoil horizons. The importance of organo‐mineral interactions for OM stabilization in the passive pool is well‐known and increases with soil depth. Hydrophobicity is particularly relevant in acid soils and in soils with considerable inputs of charcoal. We conclude that the stabilization potentials of soils are site‐ and horizon‐specific. Furthermore, management affects key stabilization mechanisms. Tillage increases the importance of organo‐mineral interactions for OM stabilization, and in Ap horizons with high microbial activity and C turnover, organo‐mineral interactions can contribute to OM stabilization in the intermediate pool. The application of our model showed that we need a better understanding of processes causing spatial inaccessibility of OM to decomposers in the passive pool.     

Indications for soil organic matter quality in soils under different management

Changes in management practice are reflected by soil carbon and nitrogen status, in particular by the proportion of soil organic matter (SOM) being easily transformed (active SOM). We describe SOM quality for three management practices, Organic Farming system (OF), Integrated Crop Production (ICP) and pasture sites (G), which intend to achieve sustainable management practice. The experimental sites were conventionally farmed until 1992. SOM quality was examined by describing active SOM pools, such as the decomposed ‘young soil organic matter’ (YSOM), ratio of microbial biomass carbon (C_mic) to organic carbon (C_org), ecophysiological status of the microbial biomass (qCO₂), and the ratio of light particulate organic matter (POM-LF) to C_org. Ratios of soil microbial biomass (C_mic/C_org) and POM-LF (POM-LF/C_org) and the amount of decomposed YSOM were relatively similar to each other, despite differences in management practice and soil texture. Soil microbial parameters (C_mic, C_mic/C_org and qCO₂) were significantly (p<0.05) affected by the amount of decomposed YSOM and the silt content in the OF. In the ICP, soil microbial parameters depended only on the amount of decomposed YSOM, which was considered to be a consequence of the more heterogeneous texture at the OF-sites. Management effects were detectable for no-tillage in the ICP leading to an accumulation of active SOM in the surface soil (0–10 cm). The ratio POM-LF/C_org showed no difference between G and OF despite markedly higher C_org-contents at the G-sites. Conclusively, all methods used indicate comparable SOM qualities for the three management systems, despite differences in soil texture and soil management during 7 years. Management practices seem to be well adapted to the site conditions.     

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.     

Characteristics of soil organic matter in temperate soils by Curie point pyrolysis-mass spectrometry.

This series of papers examines some significant differences in the constitution of organic matter found amongst major soil groups and their genetic and morphological horizons, as determined by the dynamic pyrolysis-mass spectrometry of whole soil samples. Multivariate methods of data analysis identify groups of co-variant mass ions which successfully partition samples in terms of the above soil categories, and correlate with other major properties such as humification and hydromorphism. This first study covers the A horizons from a group of 22 Scottish soils chosen from the major temperate soil groups including brown forest soils, podzols, and noncalcareous, peaty and humic gleys (phaeozems, cambisols, podzols and gleysols). The principal components analysis of 52 mass ion intensities was found to partition the soils successfully in terms of two main factors which represented 62% of the total statistical variance. The first factor was found to correspond closely to the extent of hydromorphism, being high for highly gleyed A horizons, and depended chiefly on the presence of aromatic hydrocarbon ions. The second factor corresponded to the extent of mull humus formation, being high for brown forest soils (phaeozems, cambisols) and low for podzols, and depended chiefly on the abundance of polypeptide products relative to polysaccharide products. This second factor was evidently a more comprehensive expression of a humification factor found in previous studies. The relationships of these factors with C and N content and with base status properties were examined.     

Evolution of organic matter added to soils under cultivation conditions

The development of an organic matter (OM) based on mixed sheep manure and peat, when it was incorporated into soils as fertilizer, was studied. The experiment was carried out in soils under almond tree culture, with drip irrigation and non irrigation regimes. Two doses, 10 and 4.5 kg tree^–1, were assayed. Changes in the humic acid fraction one year after incorporation into soils showed oxidation and enrichment in condensed structures, as observed by an increase of the O*:H* ratio and a decrease of the H*:C* ratio, and also by FTIR spectra. The oxidative process was more significant in the coarser textured and also in the non‐irrigated soil. The evolution of the ratios C_ext:C_ox and C_HA:C_FA throughout the culture cycle was followed by sampling and chemical analysis of different forms of organic carbon. Evolution of C_ext:C_ox showed a uniform humification state in the irrigated soil, and a significant decrease in the non‐irrigated soil, at the beginning of the experiment. Curves of C_HA:C_FA evolution showed changes attributed to mineralization or drainage of the fulvic acids fraction, giving a maximum in spring in both soils and a final increase at the end of the cycle by drainage only in the irrigated soil.     

Long‐term irrigation effects on soil organic matter under temperate grazed pasture

Irrigation of grazed pasture significantly increases plant and animal production, which may in turn increase soil organic carbon (SOC), depending on the balance between primary production and below‐ground allocation of C on the one hand, and the decomposition and export of C from the soil on the other. To evaluate the effect of irrigation on SOC we sampled a grazed pasture field experiment maintained under different irrigation treatments for 62 years. The dry‐land treatment in this experiment only received rainfall at an average of 740 mm year^?1. The 10 and 20% irrigation treatments involved application of 100 mm of irrigation when the soil reached 10 and 20% gravimetric moisture content, respectively. The 10 and 20% irrigation treatments received average total annual irrigation inputs of 260 and 770 mm year^?1, respectively. The 10 and 20% irrigation treatments increased pasture production by 44 and 74%, respectively, compared with that from the dry‐land. Analysis of soils taken to 1‐m depth revealed that amounts of SOC were not significantly different between the dry‐land (125.5 Mg ha^?1) and 10% irrigation (117.8 Mg ha^?1) treatments, but these were significantly greater than the 20% irrigation treatment (93.0 Mg ha^?1). At 50–100 cm, SOC was also less (34%) for the 20% irrigation treatment than for the 10% irrigation treatment. The relative quantities of carbon (C) and nitrogen (N) in the light fraction (LF) at all soil depths decreased successively from dry‐land to the 20% irrigation treatment, suggesting that wetter soil conditions accelerated decomposition of the LF fraction, a comparatively labile SOC fraction. The C‐to‐N ratio of the bulk soil was also less for the 20% irrigation treatment, indicating more decomposed SOM in the irrigated than in the dry‐land treatment. There were no significant differences in the microbial biomass between the three different irrigation treatments, but the respiration rate (CO₂ production) of soil organisms in the 20% irrigation treatment was consistently greater than in the other two treatments. It was concluded that large increases in plant productivity as a result of irrigation had either no effect or significantly reduced SOC stocks under grazed pasture. The reduced SOC content observed in the 20% irrigation treatment was attributed to a combination of increased C losses in animal products and drainage associated with greater stocking, together with accelerated decomposition of organic C resulting from elevated soil moisture maintained throughout the growing season.     

Biological degradation of pyrogenic organic matter in temperate forest soils

Pyrogenic organic matter (PyOM), derived from the incomplete combustion of plant biomass and fossil fuels, has been considered one of the most stable pools of soil organic matter (SOM) and a potentially important terrestrial sink for atmospheric CO₂. Recent evidence suggests that PyOM may degrade faster in soil than previously thought, and can affect native SOM turnover rates. We conducted a six-month laboratory incubation study to better understand the processes controlling the degradation of PyOM in soils using dual-enriched (¹³C/¹⁵N) PyOM and its precursor wood (Pinus ponderosa). We examined the effects of soil type and inorganic N addition on PyOM and wood C and N mineralization rates, microbial C utilization patterns, and native SOM turnover rates. PyOM charred at 450 °C or its precursor pine wood was incubated in two temperate forest subsoils with contrasting short range order (SRO) clay mineralogy (granite versus andesite parent material). Duplicates of experimental treatments with and without PyOM added were sterilized and abiotic C mineralization was quantified. In a second incubation, PyOM or wood was incubated in granitic soil with and without added NH₄NO₃ (20 kg N ha⁻¹). The fate of ¹³C/¹⁵N-enriched PyOM and wood was followed as soil-respired ¹³CO₂ and total extractable inorganic ¹⁵N. The uptake of ¹³C from PyOM and wood by soil microbial community groups was quantified using ¹³C-phospholipids fatty acids (PLFA). We found that (1) The mean residence time (MRT) of PyOM-C was on a centennial time scale (390–600 yr) in both soil types; (2) PyOM-C mineralization was mainly biologically mediated; (3) Fungi more actively utilized wood-C than PyOM-C, which was utilized by all bacteria groups, especially gram (+) bacteria in the andesite (AN) soil; (4) PyOM-N mineralization was 2 times greater in granite (GR) than in AN soils; (5) PyOM additions did not affect native soil C or N mineralization rates, microbial biomass, or PLFA-defined microbial community composition in either soil; (6) The addition of N to GR soil had no effect on the MRT of C from PyOM, wood, or native SOM. The centennial scale MRT for PyOM-C was 32 times slower than that for the precursor pine wood-C or native soil C, which is faster than the MRT used in ecosystem models. Our results show that PyOM-C is readily utilized by all heterotrophic microbial groups, and PyOM-C and -N may be more dynamic in soils than previously thought.     

Repeated freeze–thaw cycles changed organic matter quality in a temperate forest soil

Under temperate climate, the frequency of extreme weather events such as intensive freezing or frequent thawing periods during winter might increase in the future. It was shown that frost and subsequent thawing may affect the fluxes of C and N in soils. In a laboratory study, we investigated the effect of frost intensity and repeated freeze–thaw cycles on the quality and quantity of soil organic matter (SOM) in a Haplic Podzol from a Norway spruce forest. Undisturbed soil columns comprising O layer and top mineral soil were treated as followed: control (+5°C), frost at –3°C, –8°C, and –13°C. After a 2‐week freezing period, frozen soils were thawed at +5°C and irrigated with 80 mm water at a rate of 4 mm d^–1. Lignin contents were not significantly affected by repeated freeze–thaw cycles. Phospholipid fatty acid (PLFA) contents decreased in the mineral soil, and PLFA patterns indicate that fungi are more susceptible to soil frost than bacteria. Amounts of both plant and microbial sugars generally decreased with increasing frost intensity. These changes cannot be explained by increased mineralization of sugars or by leaching with DOM nor by a decreased microbial activity and, thus, sugar production with increasing frost intensity. Also physical stabilization of sugars due to frost‐induced changes in soil structure can be ruled out as sugar extraction was carried out on ground bulk soil. Therefore, the only possible explanation for the disappearance of plant and microbial sugars upon soil freezing are chemical alterations of sugar molecules leading to SOM stabilization.     

含水率对土壤有机质含量高光谱估算的影响

土壤含水率对有机质(soil organic matter,SOM)含量高光谱估算精度有很大的影响。为了探讨SOM高光谱估算中土壤含水率的影响,该文对烘干土、风干土和质量含水率为5%~40%(按5%递增)的土壤样本进行了室内高光谱测量,对光谱数据进行了反射率、反射率一阶导数和反射率倒数对数3种光谱数据变换,运用偏最小二乘回归法(partial least squares regression,PLSR)建立了相应的SOM估算模型。结果表明,风干土的SOM高光谱估算精度较好;当含水率水平小于25%时,SOM估算模型精度受含水率的影响较大,光谱数据进行反射率倒数对数变换后的模型精度最高;当含水率水平大于等于25%时,水分对土壤光谱反射率的影响要大于SOM,不适宜利用土壤光谱数据进行SOM含量高光谱估算。该研究可为大田环境不同含水率情况下光谱估算SOM提供参考。     

Laser-based spectroscopic methods to evaluate the humification degree of soil organic matter in whole soils: a review

Purpose

The objective of this review is to survey critically the results obtained by the application of laser-induced fluorescence spectroscopy (LIFS) and laser-induced breakdown spectroscopy (LIBS) to the evaluation of the humification degree (HD) of soil organic matter (SOM) directly in untreated, intact whole soils.

Materials and methods

A large number of soils of various origin and nature, either native or under various cultivations, land use, and management, at various depths, have been studied to evaluate the HD of their SOM directly in intact whole samples. The LIFS spectra were obtained by either a bench or a portable argon laser apparatus that emits UV-VIS light of high power, whereas the LIBS spectra were obtained using a Q-switched Nd:YAG laser at 1064 nm.

Results and discussion

The close correlations found by comparing H_LIF values of whole soil samples with values of earlier proposed humification indexes confirmed the applicability of LIFS to assess the HD of SOM in whole soils. The high correlation found between HD_LIBS values and H_LIF values showed the promising potential of LIBS for the evaluation HD of SOM.

Conclusions

The LIFS technique shows to be a valuable alternative to evaluate the HD of SOM by probing directly the whole solid soil sample, thus avoiding the use of any previous chemical and/or physical treatments or separation procedures of SOM from the mineral soil matrix. The emerging application of LIBS to evaluate the HD of SOM in whole soils appears promising and appealing due to its sensitivity, selectivity, accuracy, and precision.

     

Biochar application to temperate soils: Effects on soil fertility and crop growth under greenhouse conditions

Biochar (BC) application as a soil amendment has aroused much interest and was found to considerably improve soil nutrient status and crop yields on poor, tropical soils. However, information on the effect of BC on temperate soils is still insufficient, with effects expected to differ from tropical soils. We investigated the effects of BC on soil nutrient dynamics, crop yield, and quality in a greenhouse pot experiment. We compared three agricultural soils (Planosol, Cambisol, Chernozem), and BCs of three different feedstocks (wheat straw [WS], mixed woodchips [WC], vineyard pruning [VP]) slowly pyrolyzed at 525°C, of which the latter was also pyrolyzed at 400°C. The BCs were applied at two rates (1% and 3%, which would correspond to 30 and 90 t ha^–1 in the field). Three crops, namely mustard (Sinapis alba L.), barley (Hordeum vulgare L.), and red clover (Trifolium pretense L.) were grown successively within one year. The investigated soil properties included pH, electrical conductivity (EC), cation‐exchange capacity (CEC), calcium‐acetate‐lactate (CAL)–extractable P (P_CAL) and K (K_CAL), C, N, and nitrogen‐supplying potential (NSP). The results show a pH increase in all soils. The CEC increased only on the Planosol. The C : N ratio increased at 3% application rate. Despite improving the soil nutrient status partly, yields of the first crop (mustard) and to a lesser extent of the second crop (barley) were significantly depressed through BC application (by up to 68%); the yield of clover as third crop was not affected. Only the BC from WS maintained yields in the range of the control and even increased barley yield by 6%. The initial yield reduction was accompanied by notable decreases (Cu, Fe, Mn, Zn) and increases (Mo) in micronutrient concentrations of plant tissues while nitrogen concentrations were hardly affected. The results of the pot experiment show that despite additional mineral fertilization, short‐term growth inhibition may occur when applying BC without further treatment to temperate soils.     

Exploring soil microbial communities and soil organic matter: Variability and interactions in arable soils under minimum tillage practice

This study describes an integrated approach (1) to monitor the quantity and quality of water extractable organic matter (WEOM) and size, structure and function of microbial communities in space (depth) and time, and (2) to explore the relationships among the measured properties. The study site was an arable field in Southern Germany under integrated farming management including reduced tillage. Samples of this Eutric Cambisol soil were taken in July 2001, October 2001, April 2002 and July 2002 and separated into three depths according to the soil profile (0–10 cm, 10–28 cm and 28–40 cm). For each sample, the quantity and quality (humification index, HIX) of water extractable organic matter (WEOM) were measured concomitantly with soil enzyme activities (alkaline phosphatase, β-glucosidase, protease) and microbial community size (C_mic). Furthermore, microbial community structure was characterised based on the fingerprints of nucleic acids (DNA) as well as phospholipid fatty acids (PLFA). We observed strong influences of sampling date and depth on the measured parameters, with depth accounting for more of the observed variability than date. Increasing depth resulted in decreases in all parameters, while seasonal effects differed among variants. Principal component (PC) analysis revealed that both DNA and PLFA fingerprints differentiated among microbial communities from different depths, and to a smaller extent, sampling dates. The majority of the 10 PLFAs contributing most to PC 1 were specific for anaerobes. Enzyme activities were strongly related to C_mic, which was depending on water extractable organic carbon and nitrogen (WEOC and WEON) but not to HIX. HIX and WEOM interact with the microbial community, illustrated by (1) the correlation with the number of PLFA peaks (community richness), and (2) the correlations with community PC analysis scores.     

Dynamics of organic matter in soils of temperate regions: Its action on pedogenesis

The author studies the reciprocal action of the organic and inorganic components during the process of humification in temperate climates. In an Atlantic climate, three main types of organo-mineral complexes occur: humus—CaCO₃ in rendzina soils, humus-allophane in andosols and humus-iron in brown acid soils. These complexes differ in their properties, both biologically (ability to withstand biodegradation) and chemically (ability to withstand extractants).The humus-swelling clay complex, occurring in fluctuating pedoclimates, shows a high degree of polymerization of organic matter (maturation).The temperate humus forms are classified with regard to the kind and degree of transformation of the fresh organic matter: (1) little-transformed humus (e.g., calcic mull and moder); (2) strongly transformed humified humus (e.g., acid mull, andosol); (3) very much transformed, mature humus type (e.g., Vertisol, chernozem).     

Microbial transformation of organic matter in soils of montane grasslands under different management

The study compared the effects of mowing, mulching, and no-treatment, applied to a mountain grassland over five years, on microbial transformation of soil organic matter (SOM). Microbial biomass, microbial respiration, cellulose decomposition and mineralization were measured in the laboratory eight times during the three-year experiment. In addition, soil phosphatase activity and factors limiting microbial growth were assessed once to complete the results. Mowing increased soil microbial biomass and carbon use efficiency, which supported carbon sequestration in soil. In contrast, mulching led to a decrease in microbial biomass and microbial metabolic efficiency due to the limitation of easily decomposable carbon. This was a consequence of changes in temperature and light conditions under the mulch layer, which suppressed plant growth and rhizodeposition. Processes causing organic matter transformation in the mulched grassland were similar to those of the untreated grassland. Annual mowing appears to be most suitable for maintenance of SOM content and sustainability of montane grasslands.     

Influence of inorganic fertilizers and organic amendments on soil organic matter and soil microbial properties under tropical conditions

 Soil organic matter level, mineralizable C and N, microbial biomass C and dehydrogenase, urease and alkaline phosphatase activities were studied in soils from a field experiment under a pearl millet-wheat cropping sequence receiving inorganic fertilizers and a combination of inorganic fertilizers and organic amendments for the last 11 years. The amounts of soil organic matter and mineralizable C and N increased with the application of inorganic fertilizers. However, there were greater increases of these parameters when farmyard manure, wheat straw or Sesbania bispinosa green manure was applied along with inorganic fertilizers. Microbial biomass C increased from 147 mg kg^–1 soil in unfertilized soil to 423 mg kg^–1 soil in soil amended with wheat straw and inorganic fertilizers. The urease and alkaline phosphatase activities of soils increased significantly with a combination of inorganic fertilizers and organic amendments. The results indicate that soil organic matter level and soil microbial activities, vital for the nutrient turnover and long-term productivity of the soil, are enhanced by use of organic amendments along with inorganic fertilizers. Received: 6 May 1998     

Characteristics of soil organic matter in temperate soils by Curie-point pyrolysis-mass spectrometry, III. Transformations occurring in surface organic horizons

In previous parts of this study, transformations of organic matter in mineral A and B horizons from a set of soils of temperate regions were characterised by Curie-point pyrolysis-mass spectrometry (Py/MS) and elucidated in terms of changes in the biopolymer composition. These changes were associated with the processes of eutrophic humification, illuviation and hydromorphism. Within the set (23 profiles from Scotland, chosen as representative of soil groups of the temperate zone) some 13 podzols and humic gleysols displayed raw humus surface horizons above their mineral A horizons.

The L, F and H layers of these organic horizons, where they were morphologically distinct, were successfully differentiated in terms of their biopolymer composition by Py/MS. These differences were of degree rather than kind. The principal components analysis of 50 ion intensities from the mass spectra demonstrated a single dominant factor of composition. The corresponding reconstructed factor spectrum showed, in the sense L→F→H, the loss of lignin and polysaccharide products derived from raw plant material and the increase of residual or humified structures characterized by homologous alkene and higher benzene pyrolysis products. Peat was found to be similar to the L and F material. The successful observation of these chemical differences by a rapidly applied method is contrasted with alternative indices of organic composition, C%, N%, C/N, and also with pH, none of which showed any significant differences between the L, F and H horizons.

The retarded process of humification which occurs in these horizons compares closely in terms of Py/MS with that occurring in anaerobic mineral soils and differs from the more rapid eutrophic humification in aerobic soils. The products resemble those from geochemical sediments.     

Organo‐mineral associations in temperate soils: Integrating biology,mineralogy, and organic matter chemistry

We summarize progress with respect to (1) different approaches to isolate, extract, and quantify organo‐mineral compounds from soils, (2) types of mineral surfaces and associated interactions, (3) the distribution and function of soil biota at organo‐mineral surfaces, (4) the distribution and content of organo‐mineral associations, and (5) the factors controlling the turnover of organic matter (OM) in organo‐mineral associations from temperate soils. Physical fractionation achieves a rough separation between plant residues and mineral‐associated OM, which makes density or particle‐size fractionation a useful pretreatment for further differentiation of functional fractions. A part of the OM in organo‐mineral associations resists different chemical treatments, but the data obtained cannot readily be compared among each other, and more research is necessary on the processes underlying resistance to treatments for certain OM components. Studies using physical‐fractionation procedures followed by soil‐microbiological analyses revealed that organo‐mineral associations spatially isolate C sources from soil biota, making quantity and quality of OM in microhabitats an important factor controlling community composition. The distribution and activity of soil microorganisms at organo‐mineral surfaces can additionally be modified by faunal activities. Composition of OM in organo‐mineral associations is highly variable, with loamy soils having generally a higher contribution of polysaccharides, whereas mineral‐associated OM in sandy soils is often more aliphatic. Though highly reactive towards Fe oxide surfaces, lignin and phenolic components are usually depleted in organo‐mineral associations. Charred OM associated with the mineral surface contributes to a higher aromaticity in heavy fractions. The relative proportion of OC bound in organo‐mineral fractions increases with soil depth. Likewise does the strength of the bonding. Organic molecules sorbed to the mineral surfaces or precipitated by Al are effectively stabilized, indicated by reduced susceptibility towards oxidative attack, higher thermal stability, and lower bioavailability. At higher surface loading, organic C is much better bioavailable, also indicated by little ¹⁴C age. In the subsurface horizons of the soils investigated in this study, Fe oxides seem to be the most important sorbents, whereas phyllosilicate surfaces may be comparatively more important in topsoils. Specific surface area of soil minerals is not always a good predictor for C‐stabilization potentials because surface coverage is discontinuous. Recalcitrance and accessibility/aggregation seem to determine the turnover dynamics in fast and intermediate cycling OM pools, but for long‐term OC preservation the interactions with mineral surfaces, and especially with Fe oxide surfaces, are a major control in all soils investigated here.     

Determinants of soil organic matter chemistry in maritime temperate forest ecosystems

While the influence of climate, vegetation, management and abiotic site factors on total carbon budgets and turn-over is intensively assessed, the influences of these ecosystem properties on the chemical complexity of soil organic matter (SOM) remains poorly understood. This study addresses the chemical composition of NaOH-extracted SOM from maritime temperate forest sites in Flanders (Belgium) by pyrolysis-GC/MS. The studied forests were chosen based on dominant tree species (Pinus sylvestris, Fagus sylvatica, Quercus robur and Populus spp.), soil texture and soil-moisture conditions. Differences in extractable-SOM pyrolysis products were correlated to site variables including dominant tree species, management of the woody biomass, site history, soil properties, total carbon stocks and indicators for microbial activity. Despite of a typical high intercorrelation between these site variables, the influence of the dominant tree species is prominent. The extractable-SOM composition is strongly correlated to litter quality and available nutrients. In nutrient-poor forests with low litter quality, the decomposition of relatively recalcitrant compounds (i.e. short and mid-chain alkanes/alkenes and aromatic compounds) appears hampered, causing a relative accumulation of these compounds in the soil. However, if substrate quality is favorable, no accumulations of recalcitrant compounds were observed, not even under high soil-moisture conditions. Former heathland vegetation still had a profound influence on extractable-SOM chemistry of young pine forests after a minimum of 60 years.     

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