Thermal Stability Of Soil Organic Matter Research Articles

Nutrient addition affects stability of soil organic matter and aggregate by altering chemical composition and exchangeable cations in desert steppe in northern China

AbstractNitrogen and phosphorus can influence the stability of soil organic matter (SOM) and aggregate, which are important for ecosystem functions. However, understanding remains limited regarding the impact of nutrient addition on soil aggregate stability, aggregate‐relevant organic matter and nutrients in desert steppe. Here, we studied the variation of soil aggregate stability, nutrients, and the exchangeable cations distribution characteristics, and the chemical composition and thermal stability of SOM were analyzed using thermogravimetric (TG) and Fourier transformed infrared spectroscopy. Samples of soil were collected under four conditions, phosphorus addition (P), nitrogen addition (N), combined phosphorus and nitrogen addition (NP), and no addition (CK), from 0 to 10 cm depth over 4‐year experiments in desert steppe in northern China. The percentage of micro‐aggregates (0.25–0.053 mm) and large macroaggregates (>2 mm) dramatically increased by 34.2% and 18.1%, respectively, under the N treatment condition compared to the CK treatment, and micro‐aggregate content significantly increased under the P treatment, respectively. Nitrogen addition significantly increased the soil aggregate stability index, but the P addition reduced their value. The both macro‐aggregates (>0.25 mm) and silt‐clay (<0.053 mm) were preferentially enriched with soil nutrients and the exchangeable cations. The N and P addition was beneficial to nutrients accumulating in macro‐aggregates, and enhanced the aliphatic‐C and aromatic‐C abundances respectively. The TG‐T50 value (i.e., temperature when 50% of SOM is lost) increased after nutrient addition, indicating that the SOM has higher thermal stability, especially in the micro‐aggregates. The addition of N may have potentially a greater influence on aggregate stability through their influence exchangeable polyvalent cations (Ca2+ and Mg2+), and stability of SOM was influenced by chemical composition and exchangeable K+, Na+, Ca2+ contents in desert steppe. Overall, nutrient addition effects stability of SOM and aggregate by altering chemical composition and exchangeable cations in desert steppe in northern China.

Effects of fire on soil organic matter in northern Amazonian forest fragments

ABSTRACT Forest fires incorporate pyrogenic organic matter into the soil, affecting the characteristics of soil organic matter (SOM) due to its high aromaticity, increasing its renewal time. However, the factors that control the concentration of pyrogenic organic matter and its chemical composition and structure are still little known. Forest fragments dispersed in a savanna matrix of the northern Brazilian Amazon are frequently impacted by fires, which can affect the composition of SOM within the fragments. The aim of this study was to analyse the chemical composition of SOM in the border and interior of six forest fragments in the Roraima savanna, in the northern Brazilian Amazon. Soil samples were collected at 10-cm intervals up to 1 m in depth at the border and in the interior of each fragment. Soil organic material concentration was determined with 10% HF solution and its elemental composition, thermogravimetric index, and 13C CPMAS NMR spectroscopic analysis were determined. There was no significant difference in the aromaticity index between border and interior. The concentration of alkyl C structures between border (22 to 25%) and interior (19 to 29%) indicated the occurrence of medium-intensity fires in the study area. The thermogravimetric analysis showed no significant difference in the thermal stability of SOM between border and interior. Our results showed no pronounced difference in SOM quality up to 1 m depth between the border and the interior of the evaluated forest fragments, indicating that this compartment is stable throughout the fragments.

Evaluating biodegradability of soil organic matter by its thermal stability and chemical composition

The stability of soil organic matter (SOM) as it relates to resistance to microbial degradation has important implications for nutrient cycling, emission of greenhouse gases, and C sequestration. Hence, there is interest in developing new ways to quantify and characterise the labile and stable forms of SOM. Our objective in this study was to evaluate SOM under widely contrasting management regimes to determine whether the variation in chemical composition and resistance to pyrolysis observed for various constituent C fractions could be related to their resistance to decomposition. Samples from the same soil under permanent pasture, an arable cropping rotation, and chemical fallow were physically fractionated (sand: 2000-50 μm; silt: 50-5 μm, and clay: <5 μm). Biodegradability of the SOM in size fractions and whole soils was assessed in a laboratory mineralization study. Thermal stability was determined by analytical pyrolysis using a Rock-Eval pyrolyser, and chemical composition was characterized by X-ray absorption near-edge structure (XANES) spectroscopy at the C and N K-edges. Relative to the pasture soil, SOM in the arable and fallow soils declined by 30% and 40%, respectively. The mineralization bioassay showed that SOM in whole soil and soil fractions under fallow was less susceptible to biodegradation than that in other management practices. The SOM in the sand fraction was significantly more biodegradable than that in the silt or clay fractions. Analysis by XANES showed a proportional increase in carboxylates and a reduction in amides (protein) and aromatics in the fallow whole soil compared to the pasture and arable soils. Moreover, protein depletion was greatest in the sand fraction of the fallow soil. Sand fractions in fallow and arable soils were, however, relatively enriched in plant-derived phenols, aromatics, and carboxylates compared to the sand fraction of pasture soils. Analytical pyrolysis showed distinct differences in the thermal stability of SOM among the whole soil and their size fractions; it also showed that the loss of SOM generally involved preferential degradation of H-rich compounds. The temperature at which half of the C was pyrolyzed was strongly correlated with mineralizable C, providing good evidence for a link between the biological and thermal stability of SOM.

Thermal stability of soil organic matter was affected by 23-yr maize and soybean continuous cultivation in northeast of China

Long-term continuous cropping affects the biochemical quality of soil organic matter (SOM), but whether the effects are relevant with their thermal stability is less clear. In northeast China, long-term continuous cropping occurred frequently owing to higher yield and economic interest requirement. To verify the thermal stability properties of SOM affected by the long-term continuous cropping, the study focused on 23-yr continuous cultivated maize and soybean plots, where the effect of cropping is likely to be detected. Bulk soils sampled in 1991 and 2014 were studied by thermogravimetry and differential scanning calorimetry (DSC). The results showed typical bimodal peaks in DSC curve in bulk Mollisols. A labile fraction peak was observed at 354–366 °C low-temperature zone and recalcitrant fraction one at high temperature of 430–438 °C. Energy density (J mg−1 OM) was greater in soybean plots compared to maize plots; in contrast, long-term continuous maize cultivation also increased energy density, in reverse in soybean plots after 23-yr cultivation. The DSC-T50, temperature at which half of energy release occurred, typically showed larger responses to long-term cultivation than crop species. Results obtained support the hypothesis of a potential link between long-term continuous cropping and the thermal stability of SOM, and a correlation with crop species.

Effects of atmospheric CO2 enrichment on δ 13C, δ 15N values and turnover times of soil organic matter pools isolated by thermal techniques

CO2 applied for Free-Air CO2 Enrichment (FACE) experiments is strongly depleted in 13 C and thus provides an opportunity to study C turnover in soil organic matter (SOM) based on its δ 13 Cv alue. Simultaneous use of 15 N labeled fertilizers allows N turnover to be studied. Various SOM fractionation approaches (fractionation by density, particle size, chemical extractability etc.) have been applied to estimate C and N turnover rates in SOM pools. The thermal stability of SOM coupled with C and N isotopic analyses has never been studied in experiments with FACE. We tested the hypothesis that the mean residence time (MRT) of SOM pools is inversely proportional to its thermal stability. Soil samples from FACE plots under ambient (380 ppm) and elevated CO2 (540 ppm; for 3 years) treatments were analyzed by thermogravim- etry coupled with differential scanning calorimetry (TG- DSC). Based on differential weight losses (TG) and energy release or consumption (DSC), five SOM pools were distinguished. Soil samples were heated up to the respective temperature and the remaining soil was analyzed for δ 13 Ca ndδ 15 Nb y IRMS. Energy con- sumption and mass losses in the temperature range 20- 200°C were mainly connected with water volatilization. The maximum weight losses occurred from 200-310°C. This pool contained the largest amount of carbon: 61% of the total soil organic carbon in soil under ambient treatment and 63% in soil under elevated CO2, respectively. δ 13 C values of SOM pools under elevated CO2 treatment showed an increase from −34.3‰ of the pool decomposed between 20-200°C to −18.1‰ above 480°C. The incorporation of new C and N into SOM pools was not inversely proportional to its thermal stability. SOM pools that decomposed between 20-200 and 200-310°C contained 2 and 3% of the new C, with a MRT of 149 and 92 years, respectively. The pool decomposed between 310-400°C contained the largest proportion of new C (22%), with a MRT of 12 years. The amount of fertilizer-derived N after 2 years of application in ambient and elevated CO2 treatments was not significantly different in SOM pools decomposed up

Effect of C3–C4 Vegetation Change on δ13C and δ15N Values of Soil Organic Matter Fractions Separated by Thermal Stability

Carbon isotopic composition of soils subjected to C3–C4 vegetation change can be used to estimate C turnover in bulk soil and in soil organic matter (SOM) pools with fast and intermediate turnover rates. We hypothesized that the biological availability of SOM pools is inversely proportional to their thermal stability, so that thermogravimetry can be used to separate SOM pools with contrasting turnover rates. Soil samples from a field plot cultivated for 10.5 years with the perennial C4 plant Miscanthus×gigantheus were analyzed by thermogravimetry coupled with differential scanning calorimetry (DSC). Three SOM fractions were distinguished according to the differential weight losses and exothermic or endothermic reactions measured by DSC. The δ13C and δ15N values of these three fractions obtained by gradual soil heating were measured by IRMS. The weight losses up to 190 °C mainly reflected water evaporation because no significant C and N losses were detected and δ13C and δ15N values of the residual SOM remained unchanged. The δ13C values (−16.4‰) of SOM fraction decomposed between 190 and 390 °C (containing 79% of total soil C) were slightly closer to that of the Miscanthus plant tissues (δ13C = −11.8‰) compared to the δ13C values (−16.8‰) of SOM fraction decomposed above 390 °C containing the residual 21% of SOM. Thus, the C turnover in the thermally labile fraction was faster than that in thermally stable fractions, but the differences were not very strong. Therefore, in this first study combining TG-DSC with isotopic analysis, we conclude that the thermal stability of SOM was not very strongly related to biological availability of SOM fractions. In contrast to δ13C, the δ15N values strongly differed between SOM fractions, suggesting that N turnover in the soil was different from C turnover. More detailed fractionation of SOM by thermal analysis with subsequent isotopic analysis may improve the resolution for δ13C.

Studies on the composition and formation of soil organic matter in terrestrial soils by wet-chemical methods and pyrolysis—field ionization mass spectrometry

Soil organic matter (SOM) in soils from the Northwest Lower Plain in Germany was investigated to characterize its composition and formation. Using the combination of wet-chemical methods and pyrolysis—field ionization mass spectrometry (Py—FIMS) two problems of soil science are discussed. Firstly, the composition of SOM of an Orthic Luvisol and selected Bh-horizons of Haplic Podzols was analysed to examine the suitability of a wet-chemical distinction between plant constituents (litter) and humic compounds for the explanation of soil formation. The distribution of litter and humic compounds determined by wet-chemical methods was compared with the assignment of Py—FIMS signals to compound classes of SOM. The observed chemical structural changes of plant materials during formation of SOM were less distinct than formerly supposed on the basis of previously published investigations. Py—FIMS showed that differences between litter and humic compounds are reflected to a lesser extent by changes of chemical units, but can be clearly recognized by differences in the thermal stability of SOM, i.e. by intra/intermolecular bonds of humic compounds and organo-mineral complexes. Secondly, a preliminary comparison of wet-chemical results and Py—FIMS intensities of compound classes of SOM of a Podzolic Cambisol is presented. Depth functions of compound classes obtained by both analytical methods showed corresponding trends. Moreover, for seven soil horizons, results of the wet-chemical determination of litter and humic compounds correlated significantly with intensities of Py—FIMS signals typical for selected soil constituents. Thus, data of thermally evolved marker molecules obtained by the direct Py—FIMS of whole soils are suitable for qualitative descriptions and quantitative comparisons of whole soils and soil horizons.