Humification Of Soil Organic Matter Research Articles

Carbon Turnover and Carbon‐13 Natural Abundance in Organo‐Mineral Fractions of a Tropical Dry Forest Soil under Cultivation

Soil organic matter (SOM) turnover is more rapid in tropical than in temperate soils. One possible reason is a limited ability of tropical soils to stabilize SOM. To test this, we measured C turnover resulting from 12 years' cultivation of a forest soil with sorghum [Sorghum bicolor (L.) Moench]. Carbon‐13 natural abundance (δ13C) signatures of forest‐ and sorghum‐derived C were used to quantify C losses and gains in organo‐mineral fractions separated by particle size, and further by density (for sands and silts) and magnetic susceptibility (for clays). Nearly 50% of original C was in the silt‐sized fraction, mostly in microaggregates of intermediate density; 30% was held by clays, particularly those of intermediate magnetic susceptibility; and 20% was of sand‐size, low‐density, often recognizable plant residues. The δ13C values in the forest soil showed the more humified SOM to be associated with finer, denser, and less magnetic fractions. After cultivation, total C content was 28% lower, with 59% of this reduction in the silts, 28% in the sand, and 19% in the clays. Loss of forest‐derived C amounted to 45%. The sand fraction lost 54% of its forest C, the silts 45% (mostly from intermediate density fractions), and the clays 23% (mostly from intermediate magnetic fractions). Gains in sorghum‐derived C amounted to 32% of C in the sand fraction, 12% in the silts (relatively evenly distributed among densities) and 13% in the clays (mostly in the nonmagnetic fraction). Thus, losses of forest C and gains of sorghum C occurred in different organo‐mineral fractions, indicating that there were no unique active fractions corresponding with the concept of C pools with defined turnover characteristics used in models of organic matter turnover.

Transformation and mineralization of synthetic 14C-labeled humic model compounds by soil-feeding termites

The majority of termite species are considered to be humivorous, but the exact nature of their carbon and energy source, the mechanisms involved in digestion and the impact of this feeding habit on the humification of soil organic matter are largely obscure. We performed feeding trials with soil-feeding termites (Termitidae: Termitinae), using 14C-labeled humic model compounds as substrates. In the case of Cubitermes orthognathus, the components of chemically identical synthetic humic acids (HA), labeled either in their proteinaceous or aromatic building blocks, and synthetic fulvic acids (FA), labeled in the aromatic building blocks, all had similarly low mineralization rates (2.5, 2.2 and 2.7%, respectively) when incubated in soil for 10 days in the absence of termites. When termites were present, the mineralization rate of the aromatic component of HA and FA increased only slightly but significantly (2.4 and 3.1%, respectively; P < 0.05), whereas that of the proteinaceous component increased more than 10-fold (30.8%). Similar results were obtained when the protein was peptonized prior to polymerization (9.7 vs. 27.4% mineralization). Mineralization of HA was accompanied by a transformation of the residual peptide label to FA, whereas the aromatic label of HA and FA was partly transformed to humin during gut passage. High-performance gel permeation chromatography showed a strong shift in the size-class distribution of peptide label towards low-molecular-weight products, especially of the material ingested by the termites; the smallest molecules were recovered from the termite bodies. Similar results were obtained in feeding trials with Cubitermes umbratus and Thoracotermes macrothorax. Together with previous findings, the current results provide strong evidence that during gut passage, the combined action of extreme alkalinity in the anterior hindgut, autoxidative processes, and probably also proteolytic activities, renders a large reservoir of potential substrates accessible. While peptidic components of humic substances are selectively digested, aromatic components are apparently not an important food source for soil-feeding termites. These findings have important implications for the mobilization of organic nitrogen in tropical soils.

Correlation of spectroscopic indicators of humification with mean annual rainfall along a temperate grassland climosequence

The stable free radical concentration, as measured by electron spin resonance spectroscopy, and the E 4/ E 6 absorbance ratio were determined for a suite of humic acids extracted from uncultivated surface soils collected along a 620 km climosequence in the Argentine Pampa (mean annual temperature, 14–15°C; mean annual rainfall, 300–1000 mm). These two spectroscopic properties were interpreted as indicators of the degree of humification of soil organic matter, with free radical concentration expected to increase and the E 4/ E 6 ratio to decrease with greater humification. Both the free radical concentration and the E 4/ E 6 ratio were found to be significantly correlated with increasing mean annual rainfall, which was accompanied by an increase in the stable free radical concentration and a decrease in the E 4/ E 6 ratio. Our results indicate that, in the sequence of temperate grassland soils investigated, organic matter humification increased with increasing rainfall.

A comparison of plant available nutrients on decomposed granite cut slopes and adjacent natural soils

Decomposed granite (DG) slopes are often difficult to revegetate after existing topsoil and vegetative cover have been removed. In this study, naturally vegetated soils and denuded DG cut slopes were evaluated to compare edaphic (soil-related) conditions that may differ between the vegetated and non-vegetated states. Potential plant nutrient deficiencies were evaluated by soil nutrient extraction, plant tissue analysis and by greenhouse bioassay methods. The DG cut slopes have approximately one-quarter to one-half the clay content, soil organic matter, water-holding capacity, and total and available nitrogen (N) of the adjacent natural soils. Soil extraction tests for phosphorus (P) availability gave variable results, but a growth response to P amendment was observed in a greenhouse bioassay. The native topsoils and the DG cut slope materials had non-limiting levels of pH, exchangeable Ca, Mg and K, and total P. Cation exchange capacity and sulfate–sulfur (S) were low but similar between the two soil groups. Principle nutrient limitations are interpreted to be N, P, and possibly S and boron (B), in order of decreasing severity. Based on these results, we conclude that the removal of humified soil organic matter on the cut slopes reduces plant growth directly by lowering several of these essential nutrients, and indirectly by decreasing the soil's water-retention and infiltration capacity. Soil organic matter is viewed as a critical factor and major contrast between barren and revegetated DG materials. © 1998 John Wiley & Sons, Ltd.

Influence of humified and non-humified natural organic compounds on mineral dissolution

The most important solution chemical controls on weathering rates are the pH and concentration of chelating ligands. Here, data from controlled laboratory experiments are used to highlight pertinent differences between non-humified (root exudates, simple ligands) soil organic compounds and humic substances. Particular attention is paid to the role that mycorrhizae-forming fungi play in the production of chemical weathering agents. Using aluminum oxide as a model mineral, it is shown that exudates from non-mycorrhizal roots and humic substances do not significantly affect weathering rates, while exudates from ectomycorrhizal roots effectively enhance dissolution rates. It is proposed that weathering rates in the rhizosphere may be fast in comparison with rates. in the bulk soil, due to exudation of simple (non-humified) organic ligands by mycorrhizal roots. On the other hand, the influence of humified soil organic matter on mineral dissolution may be smaller than generally expected.

Interactions between tree seedling roots and humus forms in the control of soil C and N cycling

The hypothesis that roots enhance soil-N turnover in humified soil organic matter (SOM) (mull) but not in lignified SOM (mor) was tested in a study involving the growth of eight species of tree seedlings on the two contrasting humus forms. After 12 and 22 weeks of seedling growth, soil-CO2 efflux was measured with (1) growing seedlings, and after 22 weeks, with (2) roots only, shoots excised, and (3) with roots removed and soils amended with different rates of glucose. Indices of C-flux and of soil available-C were derived and compared to plant-N uptake, extractable soil mineral-N, anaerobically mineralized soil-N, N bioavailability to Agrostis grass following harvest of seedlings, and to seedling fine root C-chemistry. Significant soil x species interactions were found for total soil-CO2 efflux, root-dependent CO2, soil available-C and microbial biomass. In all cases, roots were important contributors to C-cycling in the mull soil but not in the mor soil. C was more limiting in the mor than in the mull microbial community. Plant-N uptake and the mineral-N pool was greater in the mor soil, reflecting that soil's higher specific N-supplying capacity (N-mineralized:CO2). Seedlings decreased the mineral-N pool in both soils, but the presence of roots increased N-mineralization in the mull soil and decreased N-mineralization in the mor soil. Significant positive relationships were observed in the mull soil only between soil respiration and plant N uptake at mid-season, and between soil respiration and N-mineralization at late-season. Birch root activity in the mull soil was greater than that of all other seedlings and this observation is discussed with respect to the autecology of birch. Soil respiration correlated with the non-polar extract content but not the lignin:N ratio of fine roots. Results suggest that root-released C in mull SOM is sufficient to relieve energy limitation to soil microbes and allow them to access appreciable amounts of soil-N, whereas ligninolytic activity, which may ultimately control soil-N turnover in mor SOM, is not increased by rhizodeposition.

Relationship between neutral sugars and the degree of humification in Andisols

Abstract We investigated the relationship between the amounts of neutral sugar carbon (NSC) and total carbon content (TC) or the degree of humification of soil organic matter (SOM) using 14 Andisols in Japan. The amounts of 7 kinds of NSC, TC content and the degree of humification of humic acids expressed as RF values were determined. Positive linear correlations were observed between the amount of every kind of NSC and TC Especially the amount of mannose carbon which was likely to be adsorbed on the mineral components in soil and is considered to be mainly of microbial origin was highly correlated with TC (r = 0.912**). However the correlation of cellulose-type glucose-C which was assumed to be mainly of plant origin was less significant (r = 0.603*) compared with other kinds of NSC. The proportion of total NSC (TNSC) in TC was negatively correlated with RF values (r=− 0.623*). Furthermore the proportion of TNSC in humic acid carbon was also negatively correlated with RF values (r= −0.891**). In a conclu...

Patterns of mono- and difunctional carboxylic acids in hydrothermal leachates of humified soil organic matter and peat: geological and environmental implications

Maturation dependent generation of C 1-C 5 mono- and dicarboxylic acids is investigated by hydrous pyrolysis of soil and peat humates. Evolution and extinction patterns of acid populations released during progressive thermal maturing of these humates are examined. Abundant carboxylic acids yields were obtained and two types of carboxylic acid assemblages were identified: an immature acid assemblage, characterised by an abundant C 2-C 4 diacid component and a mature acid assemblage which is nearly devoid of these acids, except for the C 4 diacid. C 1-C 5 monoacids are abundant in both types of acid assenblages, however acid populations characterised by a n-C 4/ i-C 4 monoacid ratio greater than 3.0 and enriched in C 5 monoacids may also indicate a mature acid population. The trans and cis forms of the C 4 unsaturated diacid may also prove useful in defining these assemblages. Geologically these different assemblages imply that the composition of sediment expelled carboxylic acids in the subsurface environment may depend on the timing of expulsion and thermal maturity of the source rock. The abundant production of these acids from humates also implies that where industrial activities involve heating of soil or rock organic matter, waste carboxylic acid effluents may cause industrial and environmental problems.

Humification and nitrogen mineralization of crop residues in semi-arid Argentina

Crop residues in the soil play a fundamental role in organic matter dynamics and have effects on numerous beneficial soil properties. The humification of residues depends on soil nutrient status and physical and meteorological conditions. The prediction of the humification rate and of the size and decomposition of fresh and humified soil organic matter has been attempted with equations and models which require several constants. To simplify the procedure. Janssen developed an equation which utilizes only one constant, permits calculation of the young (fresh) and old (humified) soil organic matter pools and predicts the production of available or mineralized nitrogen. The validity of Janssen's equation was tested in three production systems (a crop rotation and two tillage procedures) in a sandy-loam textured, thermic, entic Haplustoll soil in the pampean semi-arid region of Argentina. The model produced results compatible with actual field data, but information on soil available water was essential for fitting and interpreting the organic matter dynamics.

Independence of Savanna Grasses from Soil Organic Matter for Their Nitrogen Supply

In Lamto savanna (Cote d'Ivoire), grass δ15N (≈‐1.3%) is much lower (has a smaller absolute value) than soil organic matter δ15N (≈‐1.3%). In order to understand such a discrepancy, we have analyzed 15N natural concentrations in the four major sources of mineral nitrogen that can meet the annual requirements of plants: bulk precipitation, mineralization of humified soil organic matter, atmospheric dinitrogren fixation, and decomposition of plant litter. The first source (negative δ15N) only contributes ≈7% of nitrogen requirements, as does the second (δ15N≈+7%) due to a very low humus mineralization rate. The third source (δ15N = 0) contributes up to 17%, due to nonsymbiotic N2 fixation by microorganisms associated with grasses, legumes being almost absent from the savanna. All these processes cannot account for the low δ15N of grasses, suggesting that most of the assimilated nitrogen originates from the decay of root material (δ15N≈‐1.1%).

Form and behavior of radium, uranium, and thorium in central Pennsylvania soils derived from dolomite

In a pair of well‐ and poorly‐drained soils developed on Ordovician carbonate rocks in Central Pennsylvania, the distribution with depth and chemical form of 238U, 232Th, and 226Ra have been compared to soil properties. Concentrations range from 3.3 to 8 ppm U and 6.3 to 11 ppm Th in the soils. Although radium is in equilibrium with U at depth in both soils, in surficial A‐horizons Ra is well in excess of U (165–180%). Ra and U are enriched 10X‐12X over the bedrock. Th is enriched, along with Al and Fe, by 20X.Progressive selective extractions for U, Th, and Ra in soil show half the radium occurring in pedogenic phases, chiefly in the organic fractions (24% of total Ra). By contrast, &lt;1% of the U and Th occurs in any of the organic fractions; up to 80% occurs in the silt and clay fractions with a significant occurrence in the Fe‐oxides, up to 50% for Th.Ra and chemically similar Ba are cycled by vegetation but Ra is retained in soil, bound in humified soil organic matter while Ba is not. Radon emanation of these soils (8–27%) can be almost entirely attributed to highly emanative Ra bound in organic matter. In these soils the action of vegetation leads to high soil gas radon by maintaining high Ra concentration and enhancing its emanating power.

Effects of macrofauna on soil properties in tropical ecosystems

Earthworms, termites and ants influence soil physical, chemical and biological properties. Earthworms are most active in humid and subhumid tropics, termites and ants are more active in semi-arid and arid regions. Soil fauna influence rates of soil turnover, mineralization and humification of soil organic matter, soil texture and consistency, total and macroporosity, infiltration rate and soil-water retention characteristics. Biologically, worked soil often has more organic matter and more plant-available nutrients and water reserves than adjacent soil. Population, species diversity and activity of soil fauna are influenced by agricultural practices. The practices that adversely affect faunal population include mechanized land clearing, plowing, monoculture and indiscriminate use of agrochemicals. Soil- and crop-management techniques that favor and enhance the activity of soil fauna include mulch farming, no-tillage, cover crops, agroforestry and other ecologically compatible farming systems. Soil degradation in the tropics is related to drastic reductions in activity and diversity of soil fauna.

An IR spectroscopic study of soil humin and its relationship to other soil humic substances and fungal pigments

Humin, with a relatively low ash content, was isolated from a sandy soil by exhaustively dissolving humic and fulvic fractions in cold 0.5 M NaOH, and the mineral components in 20% HF. The infrared spectrum of the insoluble humin was similar to that of humic acid derived from the same soil, and resembled those of the humic-like pigments obtained from the soil fungi Coniothyrium minitans and Rhizoctonia solani. It was less like the spectrum of the pigment from Aspergillus niger which closely resembled that of the melanin pigment from Sclerotinia sclerotiorum. The brown debris remaining after extracting some of the fungi with NaOH, produced, on acid hydrolysis, fractions having IR spectra similar to acid hydrolysed humin. It is concluded from the similarity of humin, humic acid and fungal pigments, that well humified soil organic matter is to a large extent microbial in origin. When 14C-labelled humin from a sandy loam was incubated with fresh soil, about 20% of the radioactivity was lost after 3 months and 25% after 6 months, but subsequently there was virtually no further loss. A similar pattern of stability was obtained using 14C-labelled humic acid and this result, in conjunction with those from infrared spectroscopy, indicates a close relationship between humin and humic acid in terms of their structure, and their turnover in the soil. It is concluded that the composition of a soil humin is likely to be determined by the soil's microbial population.

CHANGES IN MOLECULAR WEIGHT DISTRIBUTION OF SOIL ORGANIC MATTER DURING SOIL DEVELOPMENT

SummaryTen different topsoils representing members of three soil chronosequences were pre‐treated with 0.1 M HC1 and extracted with neutral 0.1 M Na4P2O7 followed by 0.5 M NaOH. Extracts were purified and fractionated into five different nominal molecular weight fractions using gel filtration. With increasing soil development, the proportion of large molecules (&gt;200 000) in the organic matter decreased with an increase in the proportion of intermediate size molecules (200 000 – 10 000). This effect was more evident in the mild Na4P2O7 extracts than in the subsequent NaOH extracts, and in soils of the Manawatu chronosequence than in those of the other two chronosequences and are attributed to changes in the nature and humification of soil organic matter during soil development.

Note on the action of hydrogen peroxide on farmyard manure in different stages of decomposition

G. W. Robinson and J. O. Jones (1) have shown that humified can be distinguished from non-humified organic matter by the use of 6 per cent, hydrogen peroxide. Humified organic matter is apparently oxidizedor rendered soluble by this reagent, whilst structural organic matter is unattacked. It seems reasonable to suppose that a similar distinction might be made in the case of farmyard manure between the amorphous decomposed material and the unaltered fibre of the faeces and litter. In other words, the degree of decomposition of farmyard manure might be determined by a method similar to that suggested for the degree of humification of soil organic matter. It is recognised that farmyard manure differs somewhat from soil organic matter in that the former includes the naturally soluble constituents of the litter, faeces and urine, which are either oxidized completely or rendered soluble in the peroxide treatment. However, they may be regarded as analogous in that both have undergone putrefactive decomposition. In the present paper, humification is used as a convenient term for the processes whereby organic matter is changed to structureless colloidal material and not as implying their exact correspondence with humification in the soil.

A method for determining the degree of humification of soil organic matter

(1) Hydrogen peroxide oxidises or renders soluble a portion of the organic matter of soils. It appears to be without action on fibrous organic substances such as cellulose and lignins. Humified material seems to be oxidised completely or brought into a water soluble state.(2) Treatment with hydrogen peroxide is proposed as the basis of a method for estimating the approximate degree of humification of soil organic matter.(3) Among the soils examined, the highest degree of humification is found in peats and uncultivated soils. It is possible that the degree of humification is correlated with the prevalence of anaerobic conditions.