Content In Bulk Soil Research Articles

Organic matter decomposition and carbon content in soil fractions as affected by a gradient of labile carbon input to a temperate forest soil

Labile carbon (C) input to soils is expected to affect soil organic matter (SOM) decomposition and soil organic C (SOC) stocks in temperate coniferous forests. We hypothesized that the SOM decomposition rate, C content in soil fractions, and microbial and faunal abundance and activity were increased along the gradient in labile C input around wood ant nests. Three distances from the nest that differed in annual labile C input to soil were selected: 4 m with 6379 mg C m−2, 30 m with 9060 mg C m−2, and 70 m with 9215 mg C m−2. Soil from the organic horizon (Oe+Oa), surface mineral horizon (A), and subsoil mineral horizon (B) was analyzed for C content in soil fractions and for activity and abundance of soil microorganisms and fauna. In addition, a 1-year litter-bag and soil-bag decomposition experiment was conducted. Although the rate of soil decomposition did not differ along the labile C input gradient, the rate of litter decomposition in the B horizon increased as labile C input increased with distance from the nest. Correspondingly, the C content in bulk soil and in the labile and less-protected soil fractions in the B horizon decreased as labile C input increased. We infer that, because the O and A horizons are less C-limited than the B horizon, the changes in the labile C input along the gradient affected the B horizon more than the surface O and A horizons. However, soil microbial and faunal activity and abundances were not consistently affected by the gradient. Apparently, C stocks in soil fractions are more important for microbial and faunal communities than labile C inputs. Although the results indicate that SOC content changes very slowly in the coniferous forest soil of the current study, increases in the input of natural labile C leads to decreases in the SOC stock in the B horizon. By increasing the labile C input to temperate forest soils, future increases in atmospheric CO2 concentration may therefore lead to a significant loss of SOC in deep soil layers.

Variation in soil lignin protection mechanisms in five successional gradients of mixed broadleaf–pine forests

AbstractOver 200 yr of ecosystem succession in northeastern China, conifers have replaced broadleaf trees. Vegetation changes during forest succession may affect soil organic C (SOC) stability, which is associated with altered protection mechanisms. Lignin phenol composition, a predictor of soil organic matter variation, was assessed for macroaggregates, microaggregates, and silt–clay (SC) fractions from successional gradients in five forests aged 19 to 239 yr in the Changbai Mountains nature reserve. The large macroaggregates (4.00–8.00 and 2.00–4.00 mm) comprised 45.17 to 59.87% of bulk dry weight and 40.22 to 60.89% of SOC in the 19‐, 32‐, and 48‐yr‐old pioneer forests. However, we detected increased mass proportions and SOC contents in small macroaggregates (1.00–2.00 and 0.25–1.00 mm) in the mixed broadleaf–Korean pine (Pinus koraiensis Siebold & Zucc.) forest after 122 yr. Lignin in bulk soil and aggregates in the 122‐yr‐old stand had lower acid/aldehyde ratios for the vanillyl and syringyl types than other stands, indicating less lignin decomposition. The highest C (198.20 g kg−1 soil) and lignin concentrations (6.14 mg 100 mg−1 C) were detected in the bulk soil from the 239‐yr‐old stand, where SC fraction occupied 56.18% of the C and 84.17% of the lignin content in bulk soil. Forest succession from broadleaf to a broadleaf–pine mixture shifted SOC sequestration and lignin protection from aggregates to SC fractions, along with the development of plant litter composition, fine‐root biomass and turnover, and microorganism biomass, which prompted effective long‐term SOC accumulation in successional forest communities.

Under temperate climate, the conversion of grassland to arable land affects soil nutrient stocks and bacteria in a short term

Projected population growth and climate change will make it inevitable to convert neglected and marginal land into productive arable land. We investigate the influence of agricultural management practices on nutrient stocks and soil functions during the conversion of former extensively used grassland to arable land. Effects of grassland removal, tillage, intercropping with faba bean (Vicia faba) and its later incorporation were studied with respect to soil properties and bacterial community structure. Therefore, composite samples were collected with a core sampler from the topsoil (0–20 cm) in (a) the initial grassland, (b) the transitional phase during the vegetation period of V. faba, (c) after ploughing the legume in, and (d) untreated controls. In all samples, nitrate-N, ammonium-N, dissolved organic carbon (DOC) and total nitrogen bound (TNb) were analyzed and comparisons of the bacterial community structure after 16S-amplicon sequencing were performed to assess soil functions. Mineralization after grassland conversion followed by the biological nitrogen fixation of broad beans enhanced the nitrate-N content in bulk soil from 4 to almost 50 μg N g−1dw. Bacterial community structure on phylum level in bulk soil was dominated by Proteobacteria, Actinobacteria, Acidobacteria, Chloroflexi, and Bacteroidetes and remained almost stable. However, alpha and beta-diversity analysis revealed a change of the bacterial composition at the final state of the conversion. This change was primarily driven by increasing abundances of the genera Massilia and Lysobacter, both members of the Proteobacteria, after the decay of the leguminous plant residues. Furthermore, increasing abundances of the family Gaiellaceae and its genus Gaiella fostered this change and were related to the decreasing carbon to nitrogen ratio. In short, gentle management strategies could replace the input of mineral fertilizer with the aim to contribute to future sustainable and intensified production even on converted grassland.

Particulate organic carbon is more vulnerable to nitrogen addition than mineral-associated organic carbon in soil of an alpine meadow

Long-term nitrogen (N) addition can affect soil organic carbon (SOC) pool within different soil fractions with different turnover rates. However, the mechanisms of these effects, particularly in alpine grassland ecosystems, are not clear. We studied the responses of SOC content in different soil fractions to N addition based on a six-year N addition field experiment in an alpine meadow ecosystem on the Tibetan Plateau. We measured soil chemical and microbial properties, and SOC content in bulk soil, particular organic matter (POM) and mineral-associated organic matter (MAOM) fractions in response to N addition. N addition increased soil N availability, decreased soil pH and microbial biomass, but had minimal effect on plant biomass, soil enzyme activity, and SOC content in bulk soil. With increasing levels of N addition, SOC in the POM fraction (POC) showed a significant negative trend, while SOC in the MAOM fraction (MAOC) did not change significantly. As plant biomass input and soil enzyme activity were not significantly altered with N addition, the decline in POC was likely caused by changes in microbial physiology (carbon use efficiency), while the insignificant change in MAOC may be determined by the balance between input (from microbial necromass) and output (from microbial decomposition). Taken together, our study showed that the less-protected POC fraction is more vulnerable to N addition than the more-protected MAOC fraction in the alpine grassland. This finding may improve the prediction of soil C dynamics in response to N deposition in alpine grassland ecosystems on the Tibetan Plateau.

Soil organic carbon associated with aggregate-size and density fractions in a Mollisol amended with charred and uncharred maize straw

Straw return has been strongly recommended in China, whereas applying biochar into soil is considered to provide more benefits for agriculture as well as the environment. In this study, a five-year (2011–2015) field experiment was conducted to evaluate the effects of uncharred maize straw amendment (MS) and charred maize straw amendment (charred MS) on organic carbon (C) contents in bulk soil and in various soil aggregate-size and density fractions. Compared to no amendment (CK), the bulk soil organic C content significantly improved by 9.30% for MS and by 23.4% for charred MS. Uncharred and charred maize straw applied annually at a consistent equal-C dosage resulted in 19.7 and 58.2% organic C sequestration efficiency in soil, respectively, after the five years of the field experiment. The percentages of macroaggregates (>0.25 mm) and occluded microaggregates (0.25–0.053 mm) obviously increased by 7.73 and 18.1% for MS and by 10.7 and 19.6% for charred MS, respectively. Moreover, significant incremental increases of 19.4 and 35.0% in macroaggregate-associated organic C occurred in MS and charred MS, respectively. The occluded microaggregates associated organic C significantly increased by 21.7% for MS and 25.1% for charred MS. Mineral-associated organic C (<0.053 mm) inside the macroaggregates and the occluded microaggregates obviously improved by 24.7 and 33.3% for MS and by 18.4 and 44.9% for charred MS. Organic C associated with coarse particulate organic matter (POM) within the macroaggregates markedly increased by 65.1 and 41.2% for MS and charred MS, respectively. Charred MS resulted in a noteworthy increment of 50.4% for organic C associated with heavy POM inside the occluded microaggregates, whereas charred MS and MS observably improved organic C associated with heavy POM inside the free microaggregates by 36.3 and 20.0%, respectively. These results demonstrate that uncharred and charred maize straw amendments improve C sequestration by physically protecting more organic C in the macroaggregates and the occluded microaggregates. Compared to the feedstock straw amendment, charred maize straw amendment is more advantageous to C sequestration.

Effects of combined organic manure and mineral fertilization on soil aggregation and aggregate-associated organic carbon in two agricultural soils

The effect of organic manure and inorganic fertilizer on soil aggregate size distribution and stability, and associated carbon (C) within aggregates varies greatly in previous studies because of the differences in soil conditions, cropping systems, and management practices. This study was conducted as two field fertilization experiments, with different cropping systems, under a subtropical climate in China. The two field experiment sites were located in Jinhua (established in April 2011) in the Jinqu basin in Zhejiang province and Jintan (established in October 2010) in the low-middle Yangtze River plain in Jiangsu province. Both experiments consisted of four treatments, including unfertilized (CK), mineral fertilizer nitrogen (N)–phosphorus (P)–potassium (K) (NPK), NPK plus straw (NPK + SR), and NPK plus cattle manure (NPK + FYM) or half NPK plus cattle manure (1/2NPK + FYM). Water stable aggregate size classes (>5, 2–5, 1–2, 0.5–1, 0.25–0.5, and <0.25 mm) and associated soil organic C (SOC) at 0–15 cm depth were measured. The mean weight diameter (MWD), geometric mean diameter (GMD), and water stable aggregates (WSA) > 0.25 mm were also determined. The results showed that aggregate-size distribution varied with soil types. Combined application of NPK and organic matter (straw residue or cattle manure), unlike the CK and NPK treatments, significantly increased the WSA >0.25 mm, MWD, and GMD, while obviously reducing the proportion of <0.25 mm aggregates. However, no differences in WSA >0.25 mm, MWD, GMD, and associated C were observed between CK and NPK at both sites. The addition of FYM to the NPK treatment yielded the highest SOC contents in bulk soil, and showed significantly higher associations of C within all size aggregates at both sites. In contrast, NPK + SR significantly increased SOC within aggregate classes (2–5 mm, 0.5–1 mm, 0.25–0.5 mm, and <0.25 mm) at Jinhua and (>5 mm and 1–2 mm) at Jintan compared to the CK and NPK treatments. Overall, the combined application of FYM and mineral NPK was the best sustainable management practice for the improvement of aggregate stability and SOC sequestration.

Historical charcoal additions alter water extractable, particulate and bulk soil C composition and stabilization

AbstractThe objective of this work was to investigate the chemical composition and the quantitative changes in soil organic matter (SOM) fractions in response to multiple historical inputs of charcoal that ceased &gt; 60 years ago. The topsoil (0–5 cm) and subsoil (5–20 cm) samples of charcoal enriched soils and the unamended reference soils were assessed for C and N contents in bulk soil, particulate organic matter (POM) fractions and water extractable organic matter (WEOM). The SOM molecular characteristics were investigated in the solid phase by nuclear magnetic resonance (NMR) and in the WEOM by Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS). Formerly added charcoal additions reduced the extracted amount of WEOM and altered POM pattern: an increased proportion of C and N stored in coarse, intermediate, and fine POM relative to corresponding total C and N was found in subsoils. In contrast, C and N stored in the residual fraction (&lt; 20 µm) decreased. NMR results revealed a higher aromaticity of SOM in charcoal enriched soils, while the FT‐ICR‐MS results indicated an increased presence of lignin‐ and tannin‐like compounds in the WEOM of these soils. Former charcoal additions enhanced soils capacity to retain and stabilize C and N. Particularly, the presence of charcoal particles elevated C and N stored in large POM fractions &gt; 20 µm, which presumably increases soil porosity and thus the soils' capacity to retain water.

Soil aggregate-associated organic carbon dynamics subjected to different types of land use: Evidence from 13C natural abundance

The technique of 13C natural abundance provides new information that can be applied to study carbon (C) incorporation into soil aggregates and to improve our understanding of the aggregate hierarchy theory. The organic matter (OM) in soil undergoes microbial decomposition, which preferentially removes the lighter 12C isotope, enriching the remaining soil in the heavier 13C isotope. We hypothesized that the soil aggregate turnover would gradually lead to C isotope fractionation, and changes in the 13C natural abundance are closely linked to C turnover that, in turn, is influenced by aggregates turnover. We examined how land use affects soil organic carbon (SOC) dynamics in soil aggregate with the following objectives: (1) evaluate the influence of land uses (woodland, orchard, paddy and upland) on aggregate-associated C isotope compositions and the distribution of new and old SOC in the Danjiangkou Reservoir area of central China, and (2) propose an extended scheme of C transfers between the aggregate size classes. The results showed that SOC content in aggregates generally increased with increasing aggregate size but the δ13C values of aggregates decreased. The SOC contents in bulk soil decreased in the order of paddy > woodland > upland > orchard. The most negative δ13C values in bulk soil and aggregates were observed in the paddy and woodland soils, whereas maximum δ13C values were obtained in the orchard and upland soils. The stable C isotope results suggest that SOC sequestration of fresh OM generally starts in macro-aggregates (>0.25 mm), and, after disaggregation processes and microbiological consumption, the resulting degraded OM is sequestered in micro-aggregates (<0.25 mm). This general trend is consistent with the concept of SOM stabilization by association with aggregate hierarchy theory. We conclude that different land uses and management practices significantly affect C incorporation in the aggregate system and C transfer in soil aggregates was considerably greater in the orchard and upland soils than in the woodland and paddy soils. These findings help improve the theory of soil aggregates.

Long-term effects of nitrogen fertilization on aggregation and localization of carbon, nitrogen and microbial activities in soil

Long-term nitrogen (N) fertilization affects soil aggregation and localizations of soil organic carbon (SOC), N and microbial parameters within aggregates. The mechanisms of these N effects are poorly understood. We studied these processes in a loamy soil from a 23-year repeated N addition field experiment under a rice–barley rotation. Nitrogen fertilization increased plant productivity and the portion of large macroaggregates (>2mm). However, SOC contents in macro- and micro-aggregates remained constant despite an N-induced increase of 27% in root C input into soil. Therefore, N fertilization accelerated SOC turnover. Nitrogen addition increased total N (TN) content in bulk soil and two macroaggregates (>2, and 1–2mm), but not in microaggregates (<0.25mm). Also, N fertilization increased the phospholipid fatty acids (PLFAs) contents of fungi in the large macroaggregates, but not in the microaggregates. In contrast, the effect of N addition on contents of bacterial and total microbial PLFAs was not apparent. Nitrogen fertilization increased N-acetyl-β-D-glucosaminidase (NAG) activities in the two larger macroaggregate size classes (>2, and 1–2mm), but not in the aggregates (<1mm). In both control and N fertilization, the large macroaggregates localized more TN, microbial PLFAs, and NAG activities than the microaggregates. In conclusion, long-term N fertilization not only directly promotes soil N resource but also indirectly improves soil structure by forming large macroaggregates, accelerates SOC turnover, and shiftes localization of microorganisms to the macroaggregates.

Effect of land degradation on carbon and nitrogen pools in two soil types of a semi-arid landscape in West Africa

To determine the resilience of soil organic C and N pools during land degradation processes in a semi-arid landscape of West Africa, we compared the magnitude of soil organic C and N differences in bulk soil and aggregate fractions between contrasting types of land cover (degraded land and native land cover) and soil (Luvisols and Cambisols). We analyzed the following soil key indicators: CEC, soil respiration, C and N contents, and δ13C and δ15N signatures of soil organic C.The average CO2 respired from native land cover was at least 82% higher than its value from degraded land cover and was significantly higher in Luvisols than in Cambisols. Likewise, the soil organic C and N contents in bulk soil were significantly affected by land cover and soil contrasts. The average C loss in bulk soil from degraded land cover was equivalent to 49% in Cambisols and 54% in Luvisols. In both soil types, all aggregate fractions were sensitive to land degradation processes and the C loss decreased from macroaggregates to the clay+silt fraction. Compared to the native land cover, organic C loss from the macroaggregates in degraded land cover was 92% and 84%, respectively, in Cambisols and Luvisols. The soil type affected significantly the C content only in the clay+silt fraction. The C/N ratio of finer fractions (microaggregates and clay+silt) was significantly higher in degraded land cover than in native land cover, indicating greater losses of N than C during land degradation processes. The differences of δ13C signatures throughout C pools between the two types of land cover suggest a relative dominance of C3 derived C in macroaggregates and C4 derived C in the clay+silt fraction in the degraded lands. The reduction of soil respiration and the rapid N loss in degraded land cover slowed down the humification processes of C3 plant derived materials which were effectively dominant in macroaggregates.

Effect of Organic Inputs on Strength and Stability of Soil Aggregates Under Rice-Wheat Rotation

Abstract The study aims to elucidate the impact of organic inputs on strength and structural stability of aggregates in a sandy loam soil. Tensile strength, friability and water stability of aggregates, and the carbon contents in bulk soil and in large macro (&gt;2 mm), small macro (0.25-2 mm), micro (0.053-0.25 mm) and silt+clay size (&lt;0.053) aggregates were evaluated in soils from a long-term experiment with rice-wheat rotation at Modipuram, India, with different sources and amounts of organic C inputs as partial substitution of N fertilizer. Addition of organic substrates significantly improved soil organic C contents, but the type and source of inputs had different impacts. Tensile strength of aggregates decreased and friability increased through organic inputs, with a maximum effect under green gram residue (rice)-farmyard manure (wheat) substitution. Higher macroaggregates in the crop residue- and farmyard manure-treated soils resulted in a higher aggregate mean weight diameter, which also had higher soil organic C contents. The bulk soil organic C had a strong relation with the mean weight diameter of aggregates, but the soil organic C content in all aggregate fractions was not necessarily effective for aggregate stability. The soil organic C content in large macroaggregates (2-8 mm) had a significant positive effect on aggregate stability, although a reverse effect was observed for aggregates &lt;0.25 mm. Partial substitution of nitrogen by organic substrates improved aggregate properties and the soil organic C content in bulk soil and aggregate fractions, although the relative effect varied with the source and amount of the organic inputs.

Diffusion‐controlled mobilization of water‐dispersible colloids from three German silt loam topsoils: effect of temperature

SummaryThe effects of time and temperature on the release kinetics of water‐dispersible colloids (WDCs) from three German silt loam topsoils in deionized water were investigated in batch experiments under low‐energy rotating shaking conditions. The measured critical coagulation concentrations of Ca2+ and Na+ for extracted WDC were much larger than the experimental ionic conditions. This indicates a fast dispersion rate in the first detachment step of WDC mobilization from soil aggregates. The cumulative released WDC fraction F(t) (released WDC/clay content in bulk soil) was satisfactorily fitted to the square root of shaking time by a linear function in three soils with a similar clay content. This implies diffusion‐controlled release kinetics in the second step of the WDC mobilization process. The mobilization kinetics were modelled by considering a diffusion‐controlled transport through an immobile water layer in the macropores of soil aggregates formed by silt and sand particles. The effects of temperature on the mobilization kinetics and sedimentation volumes of saturated soils were compared at 7, 23 and 35°C. A linear correlation was found between immobile water layer thickness in soil macropores (lt) and the water volume (Vwater) in soil sediment, which indicates a strong dependence of lt on the soil texture. Temperature‐sensitive lt and Vwater influenced the effect of temperature on WDC release, which counteracts the estimated effect of temperature on particle diffusion according to the Stokes‐Einstein relation. A larger decrease in F(t) was found in grassland and forest soils than in an arable soil and can be related to greater stagnant water contents (larger lt and Vwater) in soil macropores, where particulate organic matter and polyvalent cations in their oxide forms at acidic pH will thus contribute to water immobilization.

Metals in residential soils and cumulative risk assessment in Yaqui and Mayo agricultural valleys, northern Mexico

This investigation examines the extent of soil metal pollution associated with the Green Revolution, relative to agricultural activities and associated risks to health in the most important agricultural region of Mexico. Metal contents in bulk soil samples are commonly used to assess contamination, and metal accumulations in soils are usually assumed to increase with decreasing particle size. This study profiled the spatial distribution of metals (Ni, Cr, Pb, Cu, Fe, Cd, V, Hg, Co, P, Se, and Mn) in bulk soil and fine-grained fractions (soil-derived dust) from 22 towns and cities. The contamination of soil was assessed through the use of a geoaccumulation index (Igeo) and pollution index (PI). The results of this study indicated that a number of towns and cities are moderately to highly polluted by soil containing Be, Co, Hg, P, S, V, Zn, Se, Cr, and Pb in both size fractions (coarse and fine). Hazard index in fine fraction (HIchildren=2.1) shows that risk assessment based on Co, Mn, V, and Ni spatially related to power plants, have the potential to pose health risks to local residents, especially children. This study shows that risk assessment based on metal content in bulk soil could be overestimated when compared to fine-grained fraction. Our results provide important information that could be valuable in establishing risk assessment associated with residential soils within agricultural areas, where children can ingest and inhale dust.

Soil profile distribution of total C content and natural abundance of 13C in two volcanic soils subjected to crop residue burning versus crop residue retention

Soil organic carbon (SOC) plays a key role in crop productivity and soil quality. Conservation agriculture has a positive effect on SOC accumulation in the surface soil horizons, but little information is available regarding the effect of the removal of crop residues by burning. This study aimed to assess the impact of different types of crop residue management practices on the total C distribution and natural abundance of 13C (‰, δ13C). Two volcanic soils, located in the Mediterranean temperate zone of Southern Chile, were studied: an Ultisol (Collipulli Series, CPL) and an Andisol (Santa Bárbara Series, SBA). Both soils had been cultivated under direct-drilling and a typical annual crop rotation system for a long period of time. Two different types of crop residue management practices were imposed in both soils: (i) crop residue burning (CPL-B; SBA-B) and (ii) crop residue retention over the soil (CPL-R; SBA-R), corresponding to treatments B and R, respectively. Soil profile distribution of the C content and natural abundance of 13C were analysed for bulk soils (down to 100 cm depth) and three particle-size fractions of the soils (down to 20 cm of soil depth): (a) ≤ 53 µm, (b) 53-212 µm and (c) ≥ 212 µm. It was found that the effect of crop residue management can be observed in the variations of C content and δ13C in the soil profile in both volcanic soils. Crop residue burning (B treatment) increased the C content in bulk soil and the particle-size fractions. On the other hand, soil organic matter of crop residue retention (R treatment) showed higher natural abundance of 13C (δ13C) compared with residue burning (B treatment) in the two volcanic soils. R treatment enriched the particle-size fractions (except ≥ 212 µm fraction of CPL soil) with 13C. Factors that could account for these findings are also discussed here.

Paired-site approach for studying soil organic carbon dynamics in a Mediterranean semiarid environment

Abstract This work investigated the effects of land cover and land-use change (LUC) on the ability of a soil to store carbon (C) and reduce carbon dioxide (CO2) emissions, in a Mediterranean area. Using a paired-site approach, we estimated the effect of land-cover change on the C stock from 1972 to 2008 in a natural reserve (Grotta di Santa Ninfa) in western Sicily. We selected 15 paired sites representative of five LUCs. We studied the effect of land use on soil organic C (SOC) content in bulk soil and in different particle-size fractions (2000–1000 μm, 1000–500 μm, 500–250 μm, 250–63 μm, 63–25 μm, and

Preservation of fire-derived carbon compounds and sorptive stabilisation promote the accumulation of organic matter in black soils of the Southern Alps

Cryptopodzols are black soils that occur under forests dominated by chestnut trees ( Castanea sativa) in Southern Switzerland. Their soil organic carbon (SOC) stocks reach an average of 150 t C ha − 1 and are thus among the highest of European forest soils. We investigated the processes leading to the accumulation and stabilisation of SOC in these soils by analysing three Cryptopodzols and one Cambisol for charred organic matter content (macrocharcoal and BPCA), the amounts of Fe and Al, and the colour and SOC content in bulk soil and density fractions. The results showed that charred organic matter produced by frequent fires in the area for more than 10,000 years is highly abundant in Cryptopodzols: the stocks of macrocharcoal and BPCA-C amount to up to 31 t ha − 1 and 17 t ha − 1, respectively. These high amounts of charred organic matter are responsible for the dark soil colour and high SOC concentrations that are, however, also closely related to Fe p and Al p concentrations. We concluded that the occurrence of charcoal across the whole profiles of Cryptopodzols seems to be the dominating factor, although both the formation of organo-metallic or organo-mineral complexes in the subsoil and the high abundance and stability of charred organic matter are responsible for the high SOC stocks in Cryptopodzols.

Fractionation analysis of trace metals in humic substances of soils irrigated with wastewater in Central Mexico by particle induced X-ray emission

The Mezquital Valley in Central Mexico has received wastewater from Mexico City for nearly 100 years. Wastewater brings in organic matter and nutrients but also trace metals. Humic substances, the main components of organic soil matter, are responsible for retaining and regulating the mobility of trace metals in soils. In this study, humic substances were extracted from the soil and separated into distinct fractions (humic acids, fulvic acids and humins). The particle induced X-ray emission (PIXE) technique was applied to determine the metal content in bulk soil as well as in humic acids and fulvic acids not soluble in H 3PO 4. In order to assess whether the long-term input of organic matter and metals modifies the metal association with these humic substances, parcels irrigated for three time periods (5, 47 and 89 years) were selected for this study. It was observed that metals such as Zn and Cu are mainly associated with the humic acids. Fulvic acids retain mainly Cr while Pb is distributed among humic and fulvic acids. It was also observed that in general, metal retention by humic substances increases with irrigation time. Depth also affects metal association with the humic substances.

Preferential erosion of black carbon on steep slopes with slash and burn agriculture

In this study we investigated the quantitative and qualitative aspects of soil organic matter (SOM) losses caused by water erosion within a small catchment in Northern Laos, under steep slopes and slash and burn agriculture. The soils in the region have a high contribution of black carbon to soil organic matter and high erosion rates. The aim of the study was to quantify the erosion of black carbon and to identify the processes involved. The conceptual approach included the measurement of contents of SOM, black carbon and mineral bound SOM in bulk soils, sediments eroded from 1 m 2 plots and in sediments at the outlet of the 0.6 ha catchment. Additionally, the enrichment factors of bulk SOM, BC and mineral bound SOM were calculated for eroded sediments. Carbon content of eroded sediments was higher than carbon content in bulk soil. The C enrichment factors in the sediments eroded from 1 m 2 plots ranged between 1.7 and 2.7, which confirms that SOM rich material such as clay or silt along with particulate organic matter such as black carbon and plant debris is preferentially eroded. At the watershed level a mean carbon enrichment factor of 1.5 showed that part of the eroded carbon is lost from the watershed. Analysis of the carbon types showed that mineral bound SOM is representing on average 30% and black carbon 15% of the carbon of the bulk soil. Mineral bound organic matter is not eroded preferentially from the soil. Enrichment factors from 1.1 to 1.8 were recorded for black carbon in sediments eroded from 1 m 2 plots. At the watershed level, black carbon represents 30% of carbon in eroded sediments. The average enrichment factor of black carbon in sediments eroded from the watershed is 2.3 compared to bulk soil C content. These results show a continuous enrichment of sediments in black carbon during the whole process of soil erosion. The preferential erosion of black carbon compared to other SOM types can be explained by its light nature, absence of mineral interactions right after its formation and its resistance to biodegradation during transport. Carbon erosion in these tropical soils may thus lead to a decrease of their ability to act as a sink for CO 2 due to the preferential exportation of a very stable SOM form. These results further suggest that erosion of black carbon could be a crucial process determining its fate in terrestrial ecosystems. Moreover, black carbon erosion could be conceptually simpler than soil erosion and could therefore be tackled by specific models.

Long‐term effects of cropped vs. fallow and fertilizer amendments on soil organic matter I. Organic carbon

AbstractA long‐term field experiment, conducted since 1962 in Gumpenstein (Austria) on a Dystric Cambisol, was used for the present investigation. We combined a physical fractionation procedure with the determination of natural abundance of 13C and FT‐IR spectroscopy to study the influence of fertilizer amendments (organic manure and mineral fertilizers) and management practices (fallow vs. cropped) on changes in organic carbon (OC) associated with different particle‐size fractions. The OC content in bulk soil decreased or was not affected by slurry+straw, PK, and NPK treatments in both fallow and cropped plots after 28 and 38 yr of treatment. However, OC in plots receiving organic manures increased depending on the quality of the organic manures applied. The ranking among the different treatments under both fallow and cropped plots was: animal manure (liquid) &gt; animal manure (solid) &gt; cattle slurry = slurry+straw = PK = NPK. Results showed that the two types of management practices, fallow (non‐tilled) vs. cropped (tilled) had effects on OC concentrations. Comparing the OC contribution of particle‐size fractions to the total OC amount revealed the following ranking: silt &gt; clay &gt; fine sand &gt; coarse sand except in the plots receiving solid or liquid animal manure. Size fractions within treatments showed larger variations of 13C abundances than bulk samples between treatments. The natural abundances of 13C increased especially in cropped (and tilled) plots. It was shown by cluster analysis that FT‐IR spectra differentiated between the different treatments originating from different land management practices. The present study revealed that below‐ground C deposition by agricultural plants can hardly compensate the C losses due to tillage.

Soil organic carbon and fractions of a Rhodic Ferralsol under the influence of tillage and crop rotation systems in southern Brazil

Soil organic matter (SOM) and its different pools have key importance in optimizing crop production, minimizing negative environmental impacts, and thus improving soil quality. The objective of this study was to evaluate the soil C and N contents in bulk soil and in different SOM pools (light and heavy fractions) of a clayey Rhodic Ferralsol after 13 years of different tillage and crop rotations in Passo Fundo, State of Rio Grande do Sul, Brazil. Soil samples were collected from no-tillage (no soil disturbance except for sowing; NT) and conventional tillage (disc plough followed by light disc harrowings; CT) applied to wheat/soybean (W/S) and wheat/soybean–vetch/maize (W/S–V/M) rotations. As reference, soil was sampled from a non-cultivated area adjacent to the field experiment. The greatest soil C and N contents were found in non-cultivated soils in the 0–5 cm depth (45 g C kg −1 soil and 3.6 g N kg −1 soil). Crop cultivation led to a decrease in SOM content which was higher for CT soils (approx. 60% decrease in C and N contents) than NT soils (approx. 43% decrease in C and N contents) at 0–5 cm. Tillage had the greatest impact on soil C and N storage. Soils under NT did not contain higher C and N storage than CT soils below 5 cm depth. Significantly, higher amounts of organic carbon of FLF in CT (0.5–0.7 g C kg −1 soil) than in NT soils (0.2 g C kg −1 soil) at 10–20 cm depth were also observed and the differences in C and N storage between CT and NT soils in the 0–30 cm layer were not significant. Silt and clay fractions contained the largest amount of organic carbon (60–95% of total organic carbon), and free light fraction was the most sensitive pool of organic carbon to detect changes in SOM due to soil tillage and crop rotations.