Peter Buurman

Amount and composition of clay-associated soil organic matter in a range of kaolinitic and smectitic soils

Abstract In the global carbon cycle, soil organic matter (SOM) is a major source/sink of atmospheric carbon. Clay minerals stabilize part of the SOM through mineral–organic matter binding. Stabilization of organic matter is essential for tropical soils. Since the climatic conditions of the tropics favor decomposition of organic matter, tropical soils would be very poor in organic matter without this stabilization process. This research aims at determining the effect of clay mineralogy on the amount and composition of organic matter that is bound to the mineral surface. We focused on organic matter that is associated with kaolinite and smectite. We characterized kaolinite- and smectite-associated SOM in soils from seven countries, employing 13C NMR spectroscopy and Py-GC/MS. The content of carbon in the total clay-size fraction showed no significant difference between kaolinitic and smectitic soils. This suggests that the total amount of organic carbon in the clay-size fraction is independent of the clay mineralogy. We first extracted the clay fraction with NaOH and thereafter with Na4P2O7. About half of the kaolinite-associated SOM was extractable by NaOH. In the smectitic soils, pyrophosphate extracted more organic carbon than did NaOH. The Py-GC/MS and NMR results indicate that kaolinite-associated SOM is enriched in polysaccharide products, while smectite-associated organic matter contains many aromatic compounds. We suggest that different clay minerals use different binding mechanisms to complex SOM. As a result, the composition of clay-associated organic matter would be influenced by the type of clay that is dominantly present in the soil.

Composition of plant tissues and soil organic matter in the first stages of a vegetation succession

Differences in soil organic matter (SOM) composition in consecutive soil profiles (under algae, grass, moss, heather and pine) from a primary vegetation succession (central Netherlands) were investigated by Cross Polarization-Magic Angle Spinning (CPMAS) 13C NMR, chemical degradation (acid hydrolysis of polysaccharides) and pyrolysis-GC/MS (Py-GC/MS) in relation to the vegetation. All mineral soil horizons had lower contents of polysaccharides than the ectorganic layer and/or the fresh plant material as suggested by the diminishing O-alkyl peaks by NMR. The monosaccharide distribution pointed to an increase of microbial sugars in the soil as suggested by the raising C6/C5 ratios. Soil polysaccharides may be accompanied by a significant contribution of carbohydrates from mosses in the first stages, because the undecomposed moss tissues have high C6/C5 ratios, which falsify the usual interpretation that such high ratios may be due to microbial sugars. Fresh litter and ectorganic layers under heather and pine had similar contents of aromatics, but the contribution of lignin (guaiacols and syringols) was lower in the ectorganic horizons. The degree of oxidation, as determined by the acid/aldehyde ratio, increased with progressive humification. All Ah horizons contained less aromatics, more oxidized lignin remnants and higher amounts of aliphatic compounds than the litter. One or two macromolecules, which produced n-alkenes/n-alkanes and branched alkenes/alkanes upon pyrolysis and were derived from mosses, were mainly responsible for the accumulation of aliphatics in soil, although other aliphatic polymers, such as suberin, might also play an important role. Preservation of components from earlier vegetation might significantly influence SOM composition.

Soil organic matter dynamics in density and particle size fractions as revealed by the 13C/12C isotopic ratio in a Cerrado's oxisol

Abstract In order to better understand the dynamics of soil organic matter (SOM) in Oxisols and the impact of converting native cerrado (savannah) into pasture, we studied the dynamics of different physically separated SOM pools at different depths in a cerrado oxisol (Typic Haplustox), under natural conditions and after 23 years of cultivated pasture ( Brachiaria spp . ) via the replacement of the native C (C 3 -derived) by pasture C (C 4 -derived). Organic C stocks of the original cerrado (15±3 kg m −2 ) and pasture (17±3 kg m −2 ) were not significantly different, which was attributed to the high biomass production of the tropical grasses and the protective effect of the high clay content (>800 g kg −2 ). We observed that 89–91% of the total organic C accumulated in the clay+silt fraction. The replacement of cerrado-derived C by pasture-derived C was in average 36%, 34%, and 19% for A p , AB 1 , and B w2 horizons, respectively, suggesting a fast turnover rate of organic C regardless of the high clay content. The replacement decreased in the order: free low-density organic matter (LDOM)>heavy fractions (sand, silt, clay)>occluded-LDOM. The lower replacement of the occluded-LDOM compared to the heavy fractions was attributed to protection inside aggregates and to a possible accumulation of C 3 -derived charcoal (black carbon). After 23 years of pasture, about 50% of the total organic C in the free-LDOM in the topsoil was still from cerrado, indicating that a significant part of this fraction was relatively recalcitrant. Charcoal fragments observed in the fraction suggested that the recalcitrance was probably due to charred material.

Laser-diffraction and pipette-method grain sizing of Dutch sediments: correlations for fine fractions of marine, fluvial, and loess samples

To evaluate correlations between silt and clay fractions determined by pipette method and laser diffraction, samples from Dutch fine marine, fluvial, and loess deposits were analysed by both methods. For fluvial deposits, correlations for fractions 50 ∝m were excellent (R 2 > 0.95), those for 2-4, 4-8, 16-32 and 32-50 ∝m were satisfactory (R 2 = 0.80 - 0.95), while that for the fraction 8-16 ∝m had an R 2 of only 0.68. For marine deposits, correlations for 50 ∝m were in the same range, but those of all other fractions except 8-16 ∝m were lower. In the loess samples, correlations for all but the 8-16 ∝m fraction were unsatisfactory. Laser diffraction gave 42% of pipette clay in marine samples, and 62% in fluvial and loess sam- ples if regressions are forced through 0. Sand fractions detected by laser diffraction were 107% of the sieve fraction in marine samples, and 99% in the fluvial samples. Correlations for fractions smaller than reference size are generally better than those for individual size fractions. Both the 2 ∝m and the 50 ∝m boundary cause problems in the comparison. The first because of platy shape of clay minerals, and the second due to both a change in method in the pipette/sieving procedure, and to non- sphericity of particles. Apparently, correlations for clay- and silt-size fractions obtained by pipette method and laser diffrac- tion will be different for each type of sediment.

Effects of fire on soil organic matter in a "cerrado sensu-stricto" from Southeast Brazil as revealed by changes in δ13C.

We studied the effect of increasing fire incidence on litter and soil organic matter in a Dark Red Latosol (Typic Haplustox) under a “cerrado sensu-stricto” vegetation in Southeast Brazil. After 21 years, C4-grass population significantly increased in the plot under high fire incidence (“cerrado 3”) compared to the plots of low fire incidence (“cerrado 1” and “cerrado 2”). In general, the variability of organic carbon (OC) content, organic nitrogen (ON) content and δ13C, both in litter and in the topsoil, was higher in “cerrado 3” than in the other plots. The higher fire incidence in “cerrado 3” decreased the litter stock of carbon and nitrogen by 3703±930 and 104±26 kg ha−1, respectively, in relation to the other plots. However, no difference in carbon (195.0±8.2 Mg ha−1) and nitrogen (13.4±1.0 Mg ha−1) stock in the first meter of soil was observed between the plots. The values of δ13C increased between 2.06‰ and 3.39‰ under C3 plants, and decreased 8.31‰ under C4 plants from litter to topsoil. For all plots, the δ13C profile showed an increase in δ13C with depth. In “cerrado 1” and “cerrado 2”, the δ13C values ranged from −25.80‰ to −23.19‰, typical for a C3-dominated vegetation profile. In “cerrado 3”, the replacement of C3- by C4-derived C was in average 34.6% throughout the profile. We conclude that: (i) the studied undisturbed “cerrado sensu-stricto” is a C3-dominated vegetation; (ii) biannual fire incidence for 21 years reduced the C3 dominance and decreased the stock of C and N in litter, but not in the soil; (iii) in the same period, at least about one-third of the C stock in the soil, or 67.5 Mg ha−1 (to 1-m depth), was replaced by C4-derived below ground litter; and (iv) in choosing a δ13C reference profile for soils under cerrado, it is necessary to take fire incidence into account.

Soils of volcanic regions in Europe

Soils of volcanic areas often exhibit unique properties differentiating them from other soils on Earth. This book gives comprehensive coverage of soils in volcanic regions within Europe, dealing with most aspects of modern day soil science. It covers a range of issues such as mineralogy, chemistry, physics, soil genesis, classification, and more.

Distribution of some selected major and trace elements in four Italian soils developed from the deposits of the Gauro and Vico volcanoes

The vertical distribution of 21 elements (K, Ca, Ti, Fe, Mn, Cr, Ni, Cu, Zn, Ga, As, Se, Br, Rb, Sr, Y, Zr, Pb, Th, U, Hg) in four Italian soils derived from volcanic materials has been investigated. Most of the elements, and in particular lithogenic ones, show little enrichment/depletion indicating a low degree of weathering and a great similarity to the parent material. In relative terms, the degree of soil evolution (horizon expression and weathering) and the depletion of the more mobile elements (K, Ca and Sr) seem to increase with increasing rainfall and soil age. Discontinuities in the soil profile due to the accretion of new volcanic material are detectable from the geochemical composition of the horizons. Some metallic elements such as Hg, Pb, Cu, Mn and Zn show enrichment in the organic-rich horizons (O and A), but the intensity of enrichment suggests that, besides biocycling and weathering processes, human activity (soil management, fertilizing and atmospheric pollution) may be significant sources of trace elements.

Chemical trends in a perhumid soil catena on the Turrialba volcano (Costa Rica)

Abstract The variation in chemical composition of soil samples (XRFS data) from a soil catena of 14 soil profiles on the northern slope and foot slope of the andesitic Turrialba volcano (3300 m) has been analysed to test the two hypotheses, which underlie our interpretation that this catena is a weathering sequence. The first hypothesis is that all parent material is chemically homogeneous and the second hypothesis is that development of chemical variation is the net result of progression of weathering and of diversity of weathering, which both depend on altitude via gradients in temperature and precipitation. The aim of this purely chemical approach is to find out if trace elements are significant “tracers” for different weathering paths leading to different amorphous or crystalline products, for example imogolite, allophane, halloysite, gibbsite, and ferrihydrite. The results show that volcanic ash is sufficiently homogeneous to detect the well-known dominance of a general weathering trend with relative depletion of mobile elements (Mg, K, Si, etc.) and relative enrichment of less-mobile elements (Ti, Fe, Al, Ce, Cr, V, Zr, etc.). The least-mobile elements were Ti, V, and Zr. From their contents, a Weathering Index for all samples has been derived that describes 95% of their total variance (hypothesis 1). In addition to this general trend, Ti, V, and Zr showed systematic deviations that could be defined as a chemical differentiation factor. In combination with the general weathering trend, this explained 99% of the total variation of Ti, V, and Zr (hypothesis 2). The contents of other (major, minor, and trace) elements used can only partially be described by the Ti–V–Zr-derived Weathering Index and Chemical Differentiation. The level of explanation decreases roughly with increasing mobility of the chemical element in the system. Triangular plots of element ratios suggest that contents of mobile elements are influenced by too many weathering processes. Trace elements did not show a more specific Chemical Differentiation than major and minor oxides. The leaching pattern of Mg indicates the presence of three different processes: (1) short-term preferential leaching of basic cations at high altitude, (2) subsequent allophane formation between 660 and 1220 m altitude, and (3) long-term loss of basic oxides (and not of silica) upon ageing of fluvio-volcanic sediments in the foot plain below 240 m. Fe shows anomalous depletion (redistribution) due to oxidation and reduction, and Al shows extra depletion in two of the allophanic profiles.

Mineralogical and (sub) microscopic aspects of iron oxides in polygenetic Oxisols from Minas Gerais, Brazil

Iron oxides are known to be useful paleo-environmental indicators, and their properties may give information about past and present circumstances of weathering and soil formation. Iron oxides from polygenetic Oxisols developed on saprolites of metamorphic rocks (soil-saprolite sequences) and on sediments (soil-sediment sequences) from Minas Gerais, Brazil, were studied by X-ray diffraction, Mossbauer spectroscopy, micromorphology, and electron microscopy to establish the connection between their properties and circumstances of formation. Hematite and goethite are the only iron oxides found in the clay and silt fractions. Micromorphology and mineralogy indicated the presence of three genetically different forms of hematite and four forms of goethite. XRD and Mossbauer spectroscopy indicate that these forms have differences in mean coherence length (MCL) and Al substitution. In soil-saprolite sequences, hematite is mainly formed during saprolite formation, while goethite formed mainly during soil formation. Microscopic hematite formed in the saprolite has low Al substitution and uniform MCL. In response to soil formation, it diminishes to submicroscopic size and goethite is formed towards toposoils. Goethite formed in yellow surface soils reflects recent soil formation and has high Al substitution. Hematites and goethites related to gley phenomena have low Al substitution and large MCLs. Different coexisting forms of either goethite or hematite point to varying pedo-environments, either in time, corroborating the polygenetic nature of the studied soils, or at micron-scale.

Carbon Sequestration in Pastures, Silvo-Pastoral Systems and Forests in Four Regions of the Latin American Tropics

SUMMARY Tropical America (TA) holds 8% of the worlds population, 11% of the worlds continental area, 23% and 22%, respectively, of the worlds forest and water resources, and 13% of the worlds pasture and agro-pastoral land, this representing 77% of TAs agricultural land. Recent interest in carbon sequestration and preliminary research suggest that well-managed pasture systems in TA could provide a good combination of economic production, poverty reduction, recovery of degraded areas and delivery of environmental services, particularly, carbon sequestration. This paper presents 3-year research results generated by the “Carbon Sequestration Project, The Netherlands Cooperation CO-01002” on soil carbon stocks (SCS) for a range of pasture and silvo-pastoral systems prevalent in agro-ecosystems of TA compared to native forest and degraded land. In the tropical Andean hillsides, Colombia (1350-1900 m.a.s.L, 1800 mm rainfall/yr, 14-18°C mean annual temperature, medium to high slopes, medium fertility soils), SCS from Brachiaria decumbenspastures were statistically lower than those from native forest, but higher than those from natural regeneration of a degraded pasture (fallow land), degraded pasture and mixed-forage bank. In contrast, in the humid tropical forest of the Atlantic Coast, Costa Rica (200 ma.s.L, 28-35°C, 3500 mm/year, poor acid soils), pasture or silvo-pastoral systems with native or planted pasture species such as Ischaemum ciliare, Brachiaria brizantha + Arachis pintoiand Acacia mangium + Arachis pintoishowed statistically higher SCS than native forest. Similar rankings were found in the humid tropical forest of Amazonia, Colombia (800 ma.s.L, 30-42°C, 4200 mm/yr, flat, very poor acid soils) where improved Brachiariapastures (monoculture and legume-associated) showed statistically higher SCS than native forest. In the sub-humid tropical forest of the Pacific Coast, Costa Rica (200 ma.s.L, 6-monfh dry season, 2200 mm/year, poor acid soils) no statistical differences in SCS were found between land-use systems. In tropical ecosystems, improved pasture and silvo-pastoral systems show comparable or even higher SCS than those from native forests, depending on climatic and environmental conditions (altitude, temperature, precipitation, topography and soil), and represent attractive alternatives as C-improved systems.