Contribution Of Soil Organic Matter Research Articles

Chernozem Land Use Differentiation by Temperature-Dependent IR Spectra

Granulometric aggregate fractions (20 µm–2 mm) of chernozem soils with different agriculture-use histories (native steppe, permanent bare fallow, arable land, and shelterbelt) were investigated in mid-IR and far IR regions (4000–100 cm−1) by heating in the air from 25 to 215 °C, using ATR FTIR and linear discriminant analysis to differentiate the land-use samples without chemical perturbation. The temperature dependences of the band maxima significantly differed for bands of silicate matrix and bands with the contribution of soil organic matter and were more stable to experimental conditions compared to the absolute positions. The thermal behavior of the integral intensities of the IR bands at 790–750 cm−1 and 440–420 cm−1 that was different compared to pure quartz, may be attributed to –C–H bending of alkanes and (poly)aromatic structures and skeletal bending, and could be used to distinguish intact soils from agriculturally used samples. The different temperature behaviors of the bands for fractions of soils with different land use are shown, with the maximum difference in fractions below 20–50 µm and medium fractions (50–200 µm). Changes in the band-maximum frequencies and the integral intensities of the bands were reversible for a heating–cooling cycle. The linear discriminant analysis of the spectra obtained for granulometric fractions of chernozem soils made it possible to separate the samples of native steppe, arable land, bare fallow, and shelterbelt with a high probability based on the type of vegetation and agrogenic load, mainly on the basis of the spectral ranges associated with biogenic forms of quartz and phytoliths.

Analysis of mechanisms of soil potassium‐holding capacity using different soils of China

AbstractSoil is the most important source of K for plants; many studies have focused on soil K status and phytoavailability. However, there is still a lack of quantitative explanation for how soils hold plant‐available K. Based on the previous study, the ability to hold plant‐available K for soil was defined as soil potassium‐holding capacity (SKHC). Therefore, this study aimed to investigate the mechanisms underlying SKHC. Fourteen soils from different sites in China were collected and the contribution of soil organic matter (SOM) and clay minerals to SKHC were analyzed based on waterlogging simulation experiments, chemical determination, and data‐fitting analysis. Results showed that SKHC mainly depended on the absorption and holding effects of exchangeable sites deriving from both SOM and clay minerals (presented as cation exchange capacity [CEC]) and nonexchangeable sites deriving from clay minerals, which contributed 14.7 and 82.5%, respectively, to SKHC, at similar soil water‐soluble K levels. After removing the SOM from the soil, the average SOM and CEC decreased from 26.0 to 7.73 g kg–1 and from 13.7 to 11.8 cmol kg–1, respectively. The SOM contributed an average of 24.5% to CEC, with the remaining 75.5% provided by clay minerals. Further analysis showed when SOM increased by 1%, CEC increased by 1.14 cmol kg–1, a relative proportion of 8.32% of the initial soil. Therefore, SKHC is mainly determined by the nonexchangeable sites of soil clay minerals, and increasing CEC by improving SOM content to enhance SKHC should be an effective but slow approach.

Rice plants reduce methane emissions in high-emitting paddies.

Background: Rice is understood to enhance methane emissions from paddy fields in IPCC guidelines. However, rice actually has two opposite functions related to methane: i) emission enhancement, such as by providing emission pathways (aerenchyma) and methanogenetic substrates; and ii) emission suppression by providing oxygen pathways, which suppress methanogenesis or enhance methane oxidation. The overall role of rice is thus determined by the balance between its enhancing and suppressing functions. Although previous studies have suggested that rice enhances total methane emissions, we aimed to demonstrate in high-emitting paddy fields that the overall methane emission is decreased by rice plants. Methods:We compared methane emissions with and without rice plants in triple cropping rice paddy fields in the Mekong Delta, Vietnam. The gas samples are collected using chamber method and ware analyzed by gas chromatography. Results:We found that rice, in fact, suppressed overall methane emissions in high-emitting paddies. The emission reductions increased with the growth of rice to the maximum tillering stage, then decreased after the heading stage, and finally recovered. Discussion: Our result indicates that the overall methane emission is larger than that of rice planted area. In addition, although many studies in standard-emitting paddies have found that the contribution of soil organic matter to methanogenesis is small, prior studies in high-emitting paddies suggest that methanogenesis depended mainly on soil organic matter accumulated from past crops. The higher the methane emission level, the lower the contribution of the rice-derived substrate; conversely, the higher the contribution of the rice providing oxygen. Finally, rice plants reduce methane emissions in high-emitting paddies. Conclusion: The present study demonstrates that during the growing season, rice is suppressing methane emissions in high-emitting paddies. This means the significance of using the rice variety which has high suppressing performance in high-emitting paddies.

Rice plants reduce methane emissions in high-emitting paddies

Background: Rice is understood to enhance methane emissions from paddy fields in IPCC guidelines. However, rice actually has two separate functions related to methane: i) emission enhancement, such as by providing emission pathways (aerenchyma) and methanogenetic substrates; and ii) emission suppression by providing oxygen pathways, which suppress methanogenesis or enhance methane oxidation. The overall role of rice is thus determined by the balance between its enhancing and suppressing functions. Although previous studies have suggested that rice enhances total methane emissions, we aimed to demonstrate in high-emitting paddy fields that the overall methane emission is decreased by rice plants. Methods: We compared methane emissions of with and without rice plants in triple cropping rice paddies in the Mekong Delta, Vietnam. The gas samples are collected using chamber method and ware analyzed by gas chromatography. Results: We found that rice, in fact, suppressed overall methane emissions in high-emitting paddies. The emission reductions increased with the growth of rice to the maximum tillering stage, then decreased after the heading stage, and finally recovered. Discussion: Our result indicates that the overall methane emission by ebullition is larger than the overall emission of rice planted area. In addition, although many studies in standard-emitting paddies have found that the contribution of soil organic matter to methanogenesis is small, our results in high-emitting paddies suggest that methanogenesis depended mainly on soil organic matter accumulated from past crops. The higher the methane emission level, the lower the contribution of the rice-derived substrate; therefore, the role of rice in high-emitting paddies is the opposite to in that of standard-emitting paddies. Conclusion: The present study demonstrates that during the growing season, rice is suppressing methane emissions in high-emitting paddies. This means the significance of using the rice variety which has high suppressing performance in high-emitting paddies.

Rice plants reduce methane emissions in high-emitting paddies

Background: Rice is understood to enhance methane emissions from paddy fields in IPCC guidelines. However, rice actually has two opposite functions related to methane: i) emission enhancement, such as by providing emission pathways (aerenchyma) and methanogenetic substrates; and ii) emission suppression by providing oxygen pathways, which suppress methanogenesis or enhance methane oxidation. The overall role of rice is thus determined by the balance between its enhancing and suppressing functions. Although previous studies have suggested that rice enhances total methane emissions, we aimed to demonstrate in high-emitting paddy fields that the overall methane emission is decreased by rice plants. Methods: We compared methane emissions with and without rice plants in triple cropping rice paddy fields in the Mekong Delta, Vietnam. The gas samples are collected using chamber method and ware analyzed by gas chromatography. Results: We found that rice, in fact, suppressed overall methane emissions in high-emitting paddies. The emission reductions increased with the growth of rice to the maximum tillering stage, then decreased after the heading stage, and finally recovered. Discussion: Our result indicates that the overall methane emission is larger than that of rice planted area. In addition, although many studies in standard-emitting paddies have found that the contribution of soil organic matter to methanogenesis is small, prior studies in high-emitting paddies suggest that methanogenesis depended mainly on soil organic matter accumulated from past crops. The higher the methane emission level, the lower the contribution of the rice-derived substrate; conversely, the higher the contribution of the rice providing oxygen. Finally, rice plants reduce methane emissions in high-emitting paddies. Conclusion: The present study demonstrates that during the growing season, rice is suppressing methane emissions in high-emitting paddies. This means the significance of using the rice variety which has high suppressing performance in high-emitting paddies.

High-resolution reconstruction of sedimentary organic matter variability during the Holocene in the mud area of the Yellow Sea using multiple organic geochemical proxies

As a result of terrestrial input and local circulation, an area containing continuously deposited mud with abundant organic matter (OM) has formed in the Yellow Sea (YS) off the Shandong Peninsula, China, during the Holocene. The understanding of temporal variations in sedimentary organic matter (SOM) from this mud area helps to evaluate the influences of Yellow River discharge and marine productivity on the burial of organic matter in the YS. In this study, multiple organic geochemical proxies were used to reconstruct Holocene changes in SOM in a sediment core (A02C) recovered from the mud area of the YS. Higher total organic carbon (TOC) and biomarker contents during the early Holocene relative to the middle and late Holocene are attributed to sea-level fluctuations. The δ13C values of TOC (δ13CTOC) range from −21.8‰ to −23.5‰, becoming more positive from the bottom to the top of the core. TMBR' (terrestrial and marine biomarker ratio) values vary from 0.43 to 0.80, with a decreasing trend during the Holocene. The decrease in TOM contribution was related to elevated sea-level, shifting of the mouth of the Yellow River, and weakening of the East Asian Winter Monsoon (EAWM) during the Holocene. A mixing model with three end-members, based on δ13CTOC and TMBR', reveals that the contribution of soil OM (average 42%) is higher than that of plant OM (average 17%). Simultaneous decreases in the plant OM fraction and the marine OM fraction occurred around 10.3, 8.2, 5.9, 4.2, and 2.8 kyr BP, corresponding to drift-ice events in the North Atlantic. Spectral analyses also reveal that all three OM fractions exhibit 1015-year periodicity and that the soil and plant OM fractions display 1420-year periodicity, consistent with the 1000-year cycle of solar activity and the 1500-year cycle of the Kuroshio Current (KC), respectively.

Rice cultivation reduces methane emissions in high-emitting paddies

Background: Rice is typically understood to enhance methane emissions from paddy fields. However, rice actually has two separate functions related to methane: i) emission enhancement, such as by providing emission pathways (aerenchyma) and methanogenetic substrates; and ii) emission suppression by providing oxygen pathways, which suppress methanogenesis or enhance methane oxidation. The overall role of rice is thus determined by the balance between its enhancing and suppressing functions. Although existing studies have suggested that rice enhances total methane emissions, we aimed to demonstrate that the balance between rice’s emitting and suppressing functions changes according to overall methane emission levels, which have quite a large range (16‍–500 kg methane ha−1 crop−1). Methods: Using PVC chambers, we compared methane emissions emitted by rice paddy fields with and without rice plants in rice fields in the Mekong Delta, Vietnam. Samples were analyzed by gas chromatograph. Results: We found high overall methane emission levels and our results indicated that rice in fact suppressed methane emissions under these conditions. Emission reductions increased with the growth of rice, up to 60% of emission rate at the maximum tillering stage, then decreased to 20% after the heading stage, and finally recovering back to 60%. Discussion: It is known that methane is emitted by ebullition when the emission level is high, and methane emission reductions in rice-planted fields are thought to be due to oxidation and methanogenesis suppression. However, although many studies have found that the contribution of soil organic matter to methanogenesis is small, our results suggested that methanogenesis depended mainly on soil organic matter accumulated from past crops. The higher the methane emission level, the lower the contribution of rice-providing substrate. Conclusion: As a result, during the growing season, rice enhanced methane emissions in low-emission paddy fields but suppressed methane emissions in high-emission paddy fields.

Effects of soil components and solution inorganic cations on interactions of imidazolium-based ionic liquid with soils

Effects of alkyl chain length of ionic liquid (IL), soil components and solution inorganic cations on a selected IL (1-methyl-3-octylimidazolium chloride, [OMIM]Cl) interaction with Chinese soils were investigated using batch sorption experiments. The results indicated that sorption energy was mainly controlled by chain length of [OMIM]Cl and contents of soil organic matter (SOM). [OMIM]Cl sorption on soils was mainly controlled by cation exchange process. Contributions of SOM and clay minerals (CMs) to [OMIM]Cl sorption were 7.3%–53.8% and 46.2%–92.7%, respectively. SOM possessed higher energy cation-exchange binding sites than CMs. To predict the sorption of [OMIM]Cl on soils, a model for the relationship between sorption coefficient (Kd) and cation exchange capacity (CEC) from soil components (SOM and CMs, i.e., CECSOM and CECCMs) as well as solution concentration (Ce) was established: LogKd = Log(1.67*CECSOM + 3.22*CECCMs) - 0.58LogCe. This model could provide a good prediction for sorption coefficients and the prediction errors were within 0.48 log unit. Competitive effects caused by inorganic cations followed the order of Ca2+ = Mg2+ > K+ > Na+. Concentrations and valence of coexisting ions both affect their competitive capability on [OMIM]Cl sorption. The finding of this study provided valuable information for evaluating the fate of [OMIM]Cl in soils.

System of Rice Intensification for Increased Productivity and Ecological Security: A Report

The present technology of paddy production leaves behind ecological foot prints caused by overuse of fossil fuels and synthetic fertilizers that pollute almost a quarter to one third of world’s annual fresh water supply. Overuse of water contributes to emission of greenhouse gases causing more global warming. Besides, input costs are high and the additional inputs, particularly synthetic fertilizers to increase production are leading to diminishing or constant returns over the years. The method is ill suited for rain-fed areas. Despite this, India has relied on such input management practices attempting to improve yield for its food security. Such highly intensive agriculture dependent on fossil fuels, damaging to soil and fresh water, and crop diversity is becoming questionable today. The real challenge therefore is to develop/adopt strategies based on sound ecological principles and integrate traditional organic farming practices and biodiversity with scientific knowledge. System of Rice Intensification (SRI) is a promising resource conserving method of growing rice both under irrigated and rain fed conditions as the method is based on alternate wetting and drying interval. Application of controlled irrigation reduces nitrogen export to the environment in comparison with flooding irrigation management and reverses climate change. Around 40 countries of the world today are reaping the benefits of SRI. Rice is grown on 42.5 m ha in India, which is the largest area among rice growing counties and provides 29% of the calorie requirement in India with a production of 85.59 million tons and average yield of 2.2 t per ha. Area under rice is expected to be reduced to about 40 million ha in the country in the next 15 to 20 years due to water shortage and rapid industrialization and urbanization. The Problems confronted in rice production particularly by resource deficient small and marginal farmers and rain-fed areas are well known. G. B. Pant Social Science Institute, University of Allahabad, Allahabad organized a two-day National Consultation session on the subject on 6th and 7th, April, 2015. The Consultation explored the alternatives ways of resource efficient SRI method for paddy cultivation in comparison to the existing resource intensive post Green Revolution conventional method. It focused on examination of the SRI method, considering particularly the contributions of soil organic matter, indigenous rice varieties, responses and roles of soil microbes, resourceuse efficiency, and labor use efficiency and productivity. It considered SRI and its relationships to biodiversity and ecological security, linking agricultural practices to the larger natural environment in which we live and produce. The Consultation also envisioned on a more desirable future, looking at SRI expansion across the country, at the roles and empowerment of farmers, and at the institutions, policies and knowledge-deepening that will make this all possible.

Testing the alkenone D/H ratio as a paleo indicator of sea surface salinity in a coastal ocean margin (Mozambique Channel)

Reconstructing past ocean salinity is important for assessing paleoceanographic change and therefore past climatic dynamics. Commonly, sea water salinity reconstruction is based on planktonic foraminifera oxygen isotope values combined with sea surface temperature reconstruction. However, the approach relies on multiple proxies, resulting in rather large uncertainty and, consequently, relatively low accuracy of salinity estimates. An alternative tool for past ocean salinity reconstruction is the hydrogen isotope composition of long chain (C37) alkenones (δDalkenone). Here, we have applied δDalkenone to a 39ka sedimentary record from the Eastern South African continental shelf in the Mozambique Channel, close to the Zambezi River mouth. Despite changes in global seawater δD related to glacial – interglacial ice volume effects, no clear changes were observed in the δDalkenone record throughout the entire 39ka. The BIT index record from the same core, which provides information on relative contributions of soil organic matter (OM) vs. marine input, indicates high soil OM input during the glacial and low input during the Holocene. This suggests a more pronounced freshwater influence at the core location during the glacial, resulting in alkenones depleted in D during that time, thereby explaining the lack of a clear glacial-interglacial alkenone δD shift. The correlation between the BIT index and δDalkenone during the glacial period suggests that increased continental runoff potentially changed the growth conditions of the alkenone-producing haptophytes, promoting coastal haptophyte species with generally more enriched δDalkenone values. We therefore suggest that the application of δDalkenone for reconstructing past salinity in coastal settings may be complicated by changes in the alkenone-producing haptophyte community.

Soil respiration in an old‐growth subtropical forest: Patterns, components, and controls

AbstractThe patterns, components, and controls of soil respiration in an old‐growth subtropical forest were investigated using an automatic chamber system. We measured soil respiration in three treatments (control, trenching, litter removal) over 15 months. The annual total soil respiration (1248 gC m–2 yr–1) showed considerable spatial variation (coefficient of variation = 27.8%) within the forest. Thirty samples were required to obtain results within 10% of the mean value at a 95% confidential level. A distinctive cosine‐like diel pattern of soil respiration was observed; the time lag between gross primary production and soil respiration at this scale was calculated to be 4–5 h. Seasonality of soil respiration was strong (~1 µmol m–2 s–1 near the end of winter; ~6 µmol m–2 s–1 in midsummer). No time lag was discerned between gross primary production and soil respiration at the seasonal scale. Soil temperature at 5 cm below surface can explain most (>91%) of the observed annual variation in soil respiration. The apparent respiration temperature sensitivity index (Q10) was 3.05. The lowest Q10 value was observed in winter, when soil moisture was low. Soil respiration was overestimated by a Q10 function during both dry and wet periods. The relative contributions of soil organic matter (RSOM), litterfall decomposition (RL), and root respiration (RR) to total soil respiration are 65.25%, 18.73%, and 16.01%, respectively; the temperature sensitivity of these components differ: RL (Q10 = 7.22) > RSOM (2.73) > RR (1.65). This relationship between Q10 values for litter respiration, soil organic matter decomposition, and root respiration still holds after minimizing the confounding effect of moisture. A relatively constant substrate supply and/or thermal acclimation could account for the observed low‐temperature sensitivity in root respiration. Given the high carbon stocks and fluxes, the old‐growth subtropical forests of China seem important in the global carbon budget and climate change.

Climate conditions in the westernmost Mediterranean over the last two millennia: An integrated biomarker approach

Climate conditions in the westernmost Mediterranean (Alboran Sea basin) over the last two millennia have been reconstructed through integration of molecular proxies applied for the first time in this region at such high resolution. Two temperature proxies, one based on isoprenoid membrane lipids of marine Thaumarchaeota (TEX86H-tetraether index of compounds consisting of 86 carbons) and the other on alkenones produced by haptophytes (U37K′ ratio) were applied to reconstruct sea surface temperature (SST). Both records reveal a progressive long term decline in SST over the last two millennia and an increased rate of warming during the second half of the 20th century. This is in accord with previous temperature reconstructions for the Northern Hemisphere. TEX86H temperature values are higher than those inferred from U37K′, probably due to differences in the bloom season of haptophytes and Thaumarchaeota, and reflect summer SST. The branched vs. isoprenoid tetraether index (BIT index) suggests a low contribution of soil organic matter (OM) to the sedimentary OM. The stable carbon isotopic composition of long chain n-alkanes indicates a predominant C3 plant contribution, with no major change in vegetation over the last 2000yr. The distribution of long chain 1,14-diols (most likely sourced by Proboscia spp. in this setting) provides insight into variation in upwelling conditions during the last 2000yr and depicts a correlation with the North Atlantic Oscillation (NAO) index, providing evidence of enhanced wind induced upwelling during periods of a persistent positive mode of the NAO.

Partitioning of CH4 and CO2 Production Originating from Rice Straw, Soil and Root Organic Carbon in Rice Microcosms

Flooded rice fields are an important source of the greenhouse gas CH4. Possible carbon sources for CH4 and CO2 production in rice fields are soil organic matter (SOM), root organic carbon (ROC) and rice straw (RS), but partitioning of the flux between the different carbon sources is difficult. We conducted greenhouse experiments using soil microcosms planted with rice. The soil was amended with and without 13C-labeled RS, using two 13C-labeled RS treatments with equal RS (5 g kg−1 soil) but different δ13C of RS. This procedure allowed to determine the carbon flux from each of the three sources (SOM, ROC, RS) by determining the δ13C of CH4 and CO2 in the different incubations and from the δ13C of RS. Partitioning of carbon flux indicated that the contribution of ROC to CH4 production was 41% at tillering stage, increased with rice growth and was about 60% from the booting stage onwards. The contribution of ROC to CO2 was 43% at tillering stage, increased to around 70% at booting stage and stayed relatively constant afterwards. The contribution of RS was determined to be in a range of 12–24% for CH4 production and 11–31% for CO2 production; while the contribution of SOM was calculated to be 23–35% for CH4 production and 13–26% for CO2 production. The results indicate that ROC was the major source of CH4 though RS application greatly enhanced production and emission of CH4 in rice field soil. Our results also suggest that data of CH4 dissolved in rice field could be used as a proxy for the produced CH4 after tillering stage.

Carbon and nitrogen isotopes analysis and sources of organic matter in the upper reaches of the Chaobai River near Beijing, China

The carbon and nitrogen isotopes in the surface sediments, plants, and soil in the upper reaches of the Chaobai River have been researched. The results showed −27.75‰ — −21.58‰ and 1.32‰-6.74‰ for carbon and nitrogen isotopic ratios in the surface sediments, respectively. The sources of sedimentary organic matter in this area are soil organic matter, aquatic vascular plants, and riverine plankton, respectively, and a significant contributor to sediment in the Chaohe River, the Baihe River, and the Miyun Reservoir areas is soil organic matter. Furthermore, part of sedimentary organic matter in the Miyun Reservoir is attributed to the input from the Chaohe River and the Baihe River, the other is from C4 vegetation growing around individual point stations at the Miyun Reservoir. Compared with the situation in Hebei Province, the contribution of soil organic matter decreased significantly and river plankton and aquatic vascular plants increased significantly in Beijing municipal areas. This study reveals that the source of organic matter has a close relationship with the soil erosion.

Radiocarbon based assessment of soil organic matter contribution to soil respiration in a pine stand of the Campine region, Belgium

The contribution of decomposing soil organic carbon (SOC) to total annual soil respiration (SR) was evaluated by radiocarbon measurements at a Scots pine stand growing on a plaggen soil in the Belgian Campine region. Two approaches were used to estimate the contribution of different C pools to SR. In the first approach, the variations in 14C content of soil CO2 efflux were monitored during one year (2003) and compared to the atmospheric and SOC 14C signatures to determine the contribution of “fast” (root respiration and fast decomposing SOC) and “slow” cycling C pools to total SR. In the second approach an estimate of the total heterotrophic soil respiration (Rh), comprising the slow cycling C and the heterotrophic part of the fast-cycling C pools, was derived applying a box model based on the amount of the bulk SOC pool and its 14C-derived mean residence time (MRT). The quantification of the Rh and the decomposition rate of the slow-cycling SOC allows to indirectly determining the contribution of the heterotrophic C that decompose within a year. Measurements of total SR performed in the field allowed assessing the contribution of the different C pools to total soil C efflux. On an annual basis, the fast-cycling C was the main contributor to SR, about 85%, while the contribution of the slow-cycling C (with MRT >1 yr) to total SR was 15%. Total annual Rh was 36% of total SR, which is in the lower range reported for temperate coniferous forests. The comparison of Rh with other estimates for the same site (47–50% of total SR) suggest a possible underestimation of the C flux from the mineral soil. In fact, the “very old” C contained in the plaggen horizon strongly affects the signature of the mostly young C leaving the soil. In conclusion, our results indicate that the contribution of SOC decomposition to total soil CO2 flux in this forest is less than 40%, and at least half of it comes from organic compounds less than 1 year old.

Acid and Alkali Buffer Capacity of Typical Fluvor-Aquic Soil in Huang-Huai-Hai Plain

Soil acid and alkali buffer capacity, as a major indicator for evaluating its vulnerability and resistibility to acidification and alkalization, is an important factor affecting the sustainable agriculture, through knowledge on which soil acidification process can be predicted and modified. In this study, titration curve method was adopted to investigate the pH buffer capacity (pHBC) of fluvor-aquic soil, and separate titration curves were established by adding incremental amounts of either standardized hydrochloric acid (HCl) (0.12 mol L −1) or sodium hydroxide (NaOH) (0.10 mol L −1) to soil suspended in deionized water (soil:solution = 1:5). Soil pH was measured after 7 d resuspension and isothermal equilibrium (T = 25°C). Linear regressions were fitted to the linear portion of each titration curve and the slopes of these lines were derived as the soil pHBC. The results showed that significant correlations between the amounts of adding acid or alkali and each pH change were presented, and titration curve method was feasible for measurement of pHBC on typical fluvor-aquic soil in Huang-Huai-Hai Plain, and the coefficients of determination were higher than the similar researches on acid soil ( R 2 = 0.96). The slope-derived pHBC of acid and alkali were 158.71 and 25.02 mmol kg −1, respectively. According to the classification of soil buffer systems, the soil tested belongs to the calcium carbonate buffer system, carbonates contribute the most to pHBC, and the contribution of soil organic matter relatively less than it.

Soil pH controls the environmental availability of phosphorus: Experimental and mechanistic modelling approaches

Abstract Inorganic P is the least mobile major nutrient in most soils and is frequently the prime limiting factor for plant growth in terrestrial ecosystems. In this study, the extraction of soil inorganic P with CaCl 2 (P–CaCl 2 ) and geochemical modelling were combined in order to unravel the processes controlling the environmentally available P (EAP) of a soil over a range of pH values (pH ∼ 4–10). Mechanistic descriptions of the adsorption of cations and anions by the soil constituents were used (1-pK Triple Plane, ion-exchange and NICA-Donnan models). These models are implemented into the geochemical code Visual MINTEQ. An additive approach was used for their application to the surface horizon of a Cambisol. The geochemical code accurately reproduced the concentration of extracted P at the different soil pH values ( R 2 = 0.9, RMSE = 0.03 mg kg −1 ). Model parameters were either directly found in the literature or estimated by fitting published experimental results in single mineral systems. The strong agreement between measurements and modelling results demonstrated that adsorption processes exerted a major control on the EAP of the soil over a large range of pH values. An influence of the precipitation of P-containing mineral is discounted based on thermodynamic calculations. Modelling results indicated that the variations in P–CaCl 2 with soil pH were controlled by the deprotonation/protonation of the surface hydroxyl groups, the distribution of P surface complexes, and the adsorption of Ca and Cl from the electrolyte background. Iron-oxides and gibbsite were found to be the major P-adsorbing soil constituents at acidic and alkaline pHs, whereas P was mainly adsorbed by clay minerals at intermediate pH values. This study demonstrates the efficacy of geochemical modelling to understand soil processes, and the applicability of mechanistic adsorption models to a ‘real’ soil, with its mineralogical complexity and the additional contribution of soil organic matter.

Effects of long term oxic degradation on the [formula omitted], TEX 86 and BIT organic proxies

One of the primary prerequisites for the application of organic proxies is that they should not be substantially affected by diagenesis. However, studies have shown that oxic degradation of biomarker lipids can affect their relative distribution. We tested the diagenetic stability of the U 37 K ′ and TEX 86 palaeothermometers upon long term oxygen exposure. For this purpose, we studied the distributions of alkenones and glycerol dialkyl glycerol tetraethers (GDGTs) in different sections of turbidites at the Madeira Abyssal Plain (MAP) that experienced different degrees of oxygen exposure. Sediments were deposited anoxically on the shelf and then transported by turbidity currents to the MAP, which has oxic bottom water. This resulted in partial degradation of the turbidite organic matter as a result of long term exposure to oxic bottom water. Concentrations of GDGTs and alkenones were reduced by one to two orders of magnitude in the oxidized parts of the turbidites compared to the unoxidized parts, indicating substantial degradation. High-resolution analysis of the Pleistocene F-turbidite showed that the U 37 K ′ index of long chain alkenones increased only slightly (0.01, corresponding to <0.5 °C) in the oxidized part of the turbidite, suggesting minor preferential degradation of the C 37:3 alkenone, in agreement with previous studies. TEX 86 values showed a small increase (0.02, corresponding to ∼2 °C) in the F-turbidite, like U 37 K ′ , while for other Pliocene/Miocene turbidites it either remained unchanged or decreased substantially (up to 0.06, corresponding to ∼6 °C). Previous observations showed that the BIT index, a proxy for the contribution of soil organic matter to total organic carbon, was always substantially higher in the oxidized part in all the turbidites, as a result of preferential degradation of marine-derived GDGTs. This relative increase in soil-derived GDGTs affects TEX 86, as the isoprenoid GDGT distribution on the continent can be quite different from that in the marine environment. Our results indicate that the organic proxies are affected by long term oxic degradation to different extents; this should be taken into account when applying these proxies in palaeoceanographic studies of sediments which have been exposed to prolonged oxic degradation.

Conversion of cerrado into agricultural land in the south-western Amazon: carbon stocks and soil fertility

Land use change and land management practices can modify soil carbon (C) dynamics and soil fertility. This study evaluated the effect of tillage systems (no-tillage - NT and conventional tillage - CT) on soil C and nutrient stocks in an Oxisol from an Amazonian cerrado following land use change. The study also identified relationships between these stocks and other soil attributes. Carbon, P, K, Ca and Mg stocks, adjusted to the equivalent soil mass in the cerrado (CE), were higher under NT. After adoption of all but one of the NT treatments, C stocks were higher than they were in the other areas we considered. Correlations between C and nutrient stocks showed positive correlations with Ca and Mg under NT due to continuous liming, higher crop residue inputs and lack of soil disturbance, associated with positive correlations with cation exchange capacity (CEC), base saturation and pH. The positive correlation (r = 0.91, p &lt; 0.05) between C stocks and CEC in the CE indicates the important contribution of soil organic matter (SOM) to CEC in tropical soils, although the exchange sites are - under natural conditions - mainly occupied by H and Al. Phosphorus and K stocks showed positive correlations (0.81 and 0.82, respectively) with C stocks in the CE, indicating the direct relationship of P and K with SOM in natural ecosystems. The high spatial variability of P and K fertilizer application may be obscuring these soil nutrient stocks. In this study, the main source of P and K was fertilizer rather than SOM.

Agricultural soil erosion and global carbon cycle: controversy over?

AbstractRecent research on the contribution of soil erosion on agricultural land to atmospheric carbon dioxide (CO2) emphasizes either the contribution of soil organic matter (SOM) mineralization during transport as source for atmospheric CO2, or the deep burial of SOM‐rich sediment in agricultural landscapes as a sink. The contribution of either process is subject to a controversial debate. In this letter, we present preliminary results on our research on interrill carbon (C) erosion, SOM transport by rill erosion and the stationarity of C erosion during the Holocene. None of those issues has been incorporated comprehensively and with global coverage in the debate on the role of C erosion in the global C cycle. Therefore, we argue that only an eco‐geomorphologic perspective on organic C movement through landscapes can reconcile the two positions. Copyright © 2009 John Wiley &amp; Sons, Ltd.