Nitrogen,phosphorus and organic carbon pools in natural and transplanted marsh soils

Total nitrogen, phosphorus and organic carbon were compared in natural and transplanted estuarine marsh soils (top 30 cm) to assess nutrient storage in transplanted marshes. Soils were sampled in five transplanted marshes ranging in age from 1 to 15 yr and in five nearby natural marshes along the North Carolina coast. Dry weight of macroorganic matter (MOM), soil bulk density, pH, humic matter, and extractable P also were measured. Nutrient pools increased with increasing marsh age and hydroperiod. Nitrogen, phosphorus and organic carbon pools were largest in soils of irregularly flooded natural marshes. The contribution of MOM to marsh nutrient reservoirs was 6–45%, 2–22%, and 1–7% of the carbon, nitrogen and phosphorus, respectively. Rates of nutrient accumulation in transplanted marshes ranged from 2.6–10.0, 0.03–1.10, and 84–218 kmol ha^?1yr^?1 of nitrogen, phosphorus and organic carbon, respectively. Accumulation rates were greater in the irregularly flooded marshes compared to the regularly flooded marshes. Approximately 11 to 12% and 20% of the net primary production of emergent vegetation was buried in sediments of the regularly flooded and irregularly flooded transplanted marshes, respectively. Macroorganic matter nutrient pools develop rapidly in transplanted marshes and may approximate natural marshes within 15 to 30 yr. However, development of soil carbon, nitrogen and phosphorus reservoirs takes considerably longer.     

Bulk organic matter characteristics in the Pichavaram mangrove – estuarine complex,south-eastern India

The Pichavaram mangrove ecosystem is located between the Vellar and Coleroon Estuaries in south-eastern India. To document the spatial-depth-based variabilities in organic matter (OM) input and cycling, five sediment cores were collected. A comparative study was carried out of grain-size composition, pore water salinity, dissolved organic C (DOC), loss-on-ignition (LOI), elemental ratios (C/N and H/C), pigments (Chl a, Chl b, and total carotenoids), and humification indices. Sand is the major fraction in these cores ranging from 60% to 99% followed by silt and clay; cores from the estuarine margin have high sand content. In mangrove forests, pore-water DOC concentrations are high (32 ± 14 mg L⁻¹), whereas salinity levels are low (50 ± 5.5‰). Likewise, LOI, organic C and N, and pigment concentrations are high in mangroves. OM is mainly derived from upstream terrestrial matter and/or mangrove litter, and marine OM. The humification indices do not vary significantly with depth because of rapid OM turnover. The bulk parameters indicate that the Vellar and Coleroon Estuaries are more affected by anthropogenic processes than mangrove forests. Finally, greater variability and sometimes lack of specific trends in bulk parameters implies that the 2004 tsunami caused extensive mixing in sediments.     

Variability of tetraether lipids in Yellow River-dominated continental margin during the past eight decades: Implications for organic matter sources and river channel shifts

Glycerol dialkyl glycerol tetraethers (GDGTs) and bulk organic geochemical parameters were examined for a short core from the Bohai Sea, a Yellow River-dominated continental margin. A three end member mixing model using branched/isoprenoid tetraethers (BIT) index, δ¹³C and C/N shows that the average fractions of soil, marine and plant organic matter (OM) during the period of 1933–2011 are 67.7% (38–92%), 26.1% (0–58%) and 6.2% (0–23%), respectively. Abrupt changes of sedimentary OM compositions around 1953, 1976 and 1996 are synchronous with the Yellow River mouth relocations. The BIT index values (0.33–0.80) present a stronger correlation with crenarchaeol abundance (R² = 0.88) than branched GDGTs abundance (R² = 0.27), suggesting that variations of marine Thaumarchaeota abundance rather than soil OM inputs is the first order factor controlling the BIT index values, although this proxy has been widely used for soil OM. The comparison between the BIT index, nutrient status and historical Yellow River sediment load indicates that the high sensitivity of the BIT index to the Yellow River channel shifts cannot be explained by a nutrient stimulation mechanism, but instead is likely caused by the restriction of Thaumarchaeota growth in highly turbid water due to the enormous sediment inputs from Yellow River. Our study demonstrates that local conditions should be considered when applying the BIT index as an environmental proxy.     

Assessment of Cd concentration of crude oil polluted arable soils

Thirty soil samples were analyzed for their properties and cadmium concentration in polluted and unpolluted sites of Isikwuato, Abia State, Nigeria. Polluted soils were more acidic (pH = 4.38) than unpolluted ones (pH = 5.22). Bulk density increased in polluted soils (1.51 g/cm³). Higher average value of organic matter was recorded in polluted soils (mean value = 1.42 %) unlike 0.98 % found in unpolluted soils. Cadmium concentration was higher in polluted soils (0.76 mg/kg) contrasting with 0.02 mg/kg obtained in their unpolluted counterparts. Good relationship existed between exchangeable acidity and cadmium status in polluted soils (R = 0.83, R² = 0.77, N = 30) as opposed to values in unpolluted soils (R = 0.58, R² = 0.49, N = 30).     

Effect of N content and soil texture on the decomposition of organic matter in forest soils as revealed by solid-state CPMAS NMR spectroscopy

N has a controlling effect on litter biodegradation in the forest floor, while stabilization of organic matter in the mineral soil may be influenced by physical parameters related to soil texture. In this study, in order to understand the processes involved in soil organic matter (SOM) formation, the chemical composition of SOM was followed and evaluated with regards to N contents and soil texture. Samples were taken on sites covered with Norway spruce and displaying contrasting values of C/N ratios in the forest floor. The chemical structure of OM was characterized using solid-state CPMAS ¹³C and ¹⁵N nuclear magnetic resonance (NMR) spectroscopy, along with Proton Spin Relaxation Editing (PSRE) sequences. Four groups of sampling sites were defined based on the NMR spectra of Oh and A horizons. In each group displaying similar NMR characteristics, N content and soil texture could be highly different among sites. Some Oh horizons with similar NMR spectra had very different N contents. Highly humified OM in Oh horizons were observed mainly on sites with low N contents. Some A horizons with different soil texture displayed similar OM chemical structure. High contents of O-alkyl C in some A horizons could originate from higher fresh root material input.     

Wetland soil organic matter composition in a Mediterranean semiarid wetland (Las Tablas de Daimiel, Central Spain): Insight into different carbon sequestration pathways

Wetland soils from a Mediterranean semiarid wetland (Las Tablas de Daimiel, Central Spain) were studied to characterize the organic matter (OM) and determine its origin and transformation. Cross polarization magic angle spinning (CPMAS) ¹³C nuclear magnetic resonance (NMR) spectroscopy and mathematical molecular mixing allowed analysis of the organic fraction in terms of six generic components (carbohydrate, protein, lignin, lipid, char and “carbonyl”). Las Tablas is an active carbon sink, with total organic carbon (TOC) content independent of soil OM quality; the TOC content of the upper sediment is 10.0 ± 7.8%. The inorganic carbon content is also high (5.4 ± 3.3%) and is associated mainly with OM of aliphatic character. The OM composition is variable; samples predominantly aliphatic (carbohydrate, lipid and protein) are characteristic of the northern sector, whereas predominantly aromatic samples are typical of the southern Tablas. A strong negative relationship between protein content and lignin content was found, interpreted as a consequence of different proportions of vascular vs. non-vascular (mostly charophyte) litter input. The effect of perturbation is apparent in the extended presence of char, particularly abundant in fire-prone areas. OM quantity and quality do not seem to depend on hydrology (although seasonal flooding is associated with lower TOC wetland soils) or soil characteristics. Dominant vegetation and fire are the main drivers of OM content and composition. Structural carbohydrate, protein and lipid (>60% of total organic fraction) dominate. Widespread anaerobic conditions and the recent character of the sediments could explain the preservation of different fractions of the original detritus composition (due to different vegetation and presence of microbes).     

Separation of total nitrogen from sediments into organic and inorganic forms for isotopic analysis

Nitrogen isotopic (δ¹⁵N) analysis is widely applied in palaeoceanography, palaeolimnology and soil science. Most measurements are performed on untreated bulk samples containing both organic and inorganic nitrogen (ON and IN), which could sometimes confound isotopic signals. Here we propose a sample pretreatment method to physically and quantitatively separate the total nitrogen into ON and IN for subsequent concentration and isotopic measurements. In the method, the sediment is first demineralized with HCl and HCl-HF, adsorbed and bound IN being liberated into the acid solutions during the demineralization. Then, organic matter (OM) dissolved in the acid solutions is recovered by a sorbent of styrene divinyl benzene polymer (Bond Elut PPL, Varian) and is then combined with the acid-resistant OM to obtain the total OM. Finally, IN in the acid solutions passing through the PPL sorbent is recovered by way of steam distillation. Total nitrogen recovery with the method is 92.4% and isotopic mass balance calculations show that the δ¹⁵N value of total nitrogen can be quantitatively partitioned into the organic and inorganic forms. The δ¹³C values of the separated OM show a small positive excursion of 0.2‰ relative to those of bulk OM, ruling out significant loss of ON during the pretreatment. By comparison, the KOBr method, widely used to remove ON from bulk samples for isotope measurements, gives lower IN concentration and higher δ¹⁵N_IN values, attributed to the partial loss of IN and escape of some oxidation-resistant ON.     

苏北潮滩湿地不同生态带碳、氮、磷分布特征*

通过对比苏北潮滩湿地不同生态带的表层以及柱状沉积物中总有机碳、总氮、总磷和有机磷含量,并结合不同植被不同植株部位中碳、氮、磷的含量变化,分析了潮滩沉积物中碳、氮、磷的垂向和水平分布特征与规律,探讨了不同生态带以及潮滩植被对碳、氮、磷等生源要素的富集作用。对比分析结果表明:苏北潮滩湿地各生态带对不同的测量指标有着不同的富集作用,互花米草滩的总有机碳、总氮和有机磷含量要远大于其他几个生态带,光滩沉积物中总磷的含量最高; 粒度效应是控制互花米草前缘地带以及互花米草滩沉积物中有机碳和氮分布的一个重要因素,盐蒿和芦苇滩中有机碳和氮的分布更多的是受粒度之外的其他因素影响; 不同生态带表层沉积物中的C/N比值分布,大致可反映其有机物来源的差异,而不同生态带中柱状沉积物中的C/N比值相对接近,很难根据C/N比值大小来对不同生态带中的有机物来源进行判断,这可能是埋藏在柱状沉积物中的有机物更多的受到了早期成岩作用造成的。植被对潮滩湿地中碳、氮、磷的分布有着重要影响,3种物质在互花米草、盐蒿和芦苇中的含量差别不是很大,因此潮滩植被对沉积物中上述3种物质的贡献差别主要是由不同植被的生物量和其所处环境的沉积动力差异造成的。     

Organic Geochemistry of the Cenomanian–Turonian Bahloul Formation Petroleum Source Rock,Central and Northern Tunisia

Total organic carbon (TOC) determination, Rock‐Eval pyrolysis, extractable organic matter content (EOM) fractionation, gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS) analyses, were carried out on 79 samples from eleven outcrop cross sections of the Bahloul Formation in central and northern Tunisia. The TOC content varied between 0.23 to 35.6%, the highest average values (18.73%, 8.46% and 4.02%) being at the east of the study area (at Ain Zakkar, Oued Bahloul and Dyr Ouled Yahia localities, respectively). The Rock‐Eval maximum pyrolysis temperature (Tmax) values in the 424–453°C range delineated a general east‐west trend increase in the organic matter (OM) maturity. The disparity in hydrogen index (HI) values, in the range 114–824 mg hydrocarbons (HC) g^?1 TOC, is relevant for the discrepancy in the level of OM preservation and maturity among localities and samples. The n‐alkane distributions, maximizing in the C17 to C20 range, are typical for a marine planktonic origin, whereas pristine/phytane (Pr/Ph) average values in the 1–2 range indicate an oxic to suboxic depositional environment. Pr/n‐C17 and Ph/n‐C18 ratios in the 0.38–6.2 and 0.68–3.25 range, respectively, are consistent with other maturity indicators and the contribution of specific bacteria to phytol as a precursor of isoprenoids. The thermal maturity varies between late diagenesis to main‐stage of petroleum generation based on the optic and the cis‐trans isomerisation of the C29 sterane [20S/(20S+20R) and 14β(H),17β(H)/(14β(H),17β(H)+14α(H),17α(H)), respectively] and the terpane [18α(H)22,29,30‐Trisnorneohopane/(18α(H)22,29,30‐Trisnorneohopane+17α(H)22,29,30‐Trisnorhopane): Ts/(Ts+Tm)] ratios. The Bahloul OM is represented by an open marine to estuarine algal facies with a specific bacterial contribution as revealed by the relative abundance of the ααα‐20R C27 (33–44%), C28 (22–28%) and C29 (34–41%) steranes and by the total terpanes/total steranes ratio (1.2–5.33). These results attested that the Bahloul OM richness was controlled both by an oxygen minimum zone induced by high productivity and restricted circulation in narrow half graben structures and around diapirs of the Triassic salt.     

Transfer of litter-derived N to soil mineral–organic associations: Evidence from decadal 15N tracer experiments

Mineral–organic associations act as mediators of litter-derived N flow to the mineral soil, but the time scales and pathways involved are not well known. To close that gap, we took advantage of decade old ¹⁵N litter labeling experiments conducted in two European forests. We fractionated surface soils by density with limited disaggregating treatment and investigated organic matter (OM) characteristics using δ¹³C, δ¹⁵N and the C/N ratio. Mineral properties were studied by X-ray diffraction and selective dissolution of pedogenic oxides.Three types of associations were isolated: plant debris with few trapped minerals (<1.65 g/cm³), aggregates dominated by phyllosilicates (1.65–2.4 g/cm³), and single mineral grains and pedogenic oxides with little OM (>2.4 g/cm³). A small proportion of ¹⁵N tracer was rapidly attached to single mineral grains, while most of it moved from plant debris to aggregates of low density and progressively to aggregates of higher density that contain a more microbially processed OM. After a decade, 60% of the ¹⁵N tracer found in the investigated horizon was retained in aggregates, while plant debris still contained 40% of the tracer.We present a conceptual model of OM and N flow through soil mineral–organic associations, which accounts for changes in density, dynamics and chemistry of the isolated structures. It suggests that microbial reworking of OM entrapped within aggregates (1.65–2.4 g/cm³) causes the gradient of aggregate packing and, further on, controls the flow of litter-derived N through aggregates. For associations with denser material (>2.4 g/cm³), mineralogy determines the density of the association, the type of patchy OM attached to mineral surfaces and controls the extent of litter-derived N incorporation.     

Organic matter sources and transport in an agriculturally dominated temperate watershed

The quality, quantity, and origin of suspended organic matter were studied in the highly agricultural Upper Scioto River in Central Ohio. Late summer baseflow conditions were compared to late autumn high flow conditions. Variables examined in the suspended matter were the total suspended solids concentration, semi-quantitative concentrations of lignin, carbohydrate concentrations, total organic C, total and organic P, and δ-¹³C. Also examined were ratios of C to N, organic C to organic P ratios and fluxes of total organic C. The primary hypothesis of this research was that the quality (or biodegradability) and quantity of organic matter in the Upper Scioto River would increase during autumn stormflow conditions due to inputs of fresh terrestrial organic matter. The autumn suspended matter was also expected to reflect C4 plant contributions from corn organic matter. Results show that the quality and quantity of organic matter were greater during summer, as reflected in low molar ratios (178:1) of organic C to organic P, and higher organic C content of the suspended matter in summer. Summer suspended matter was 3.6% organic C and autumn suspended matter was 2.3% organic C. Carbon to N molar ratios in both seasons were very close to the Redfield ratio (6.6:1 in summer and 6.7:1 in autumn). Total suspended matter and total organic C concentrations were lower in autumn (8.7 mg/l⁻¹ TOC and 17.7 mg/l⁻¹ TSS) than in summer (17.5 mg/l⁻¹ TOC and 39.0 mg/l⁻¹ TSS), but the fluxes were greater in autumn due to greater stream flow. Stable isotope analyses suggested a phytoplankton or C3 plant source (most likely corn) for summer organic C (mean δ¹³C of −24.8‰) and a phytoplankton or C4 plant source for autumn organic matter (δ¹³C=−21.5‰).     

Marsh-water column interactions in two Louisiana estuaries. II. Nutrient dynamics

The exchange of dissolved nutrients between marshes and the inundating water column was measured using throughflow marsh flumes built, in two microtidal Louisiana estuaries: the Barataria Basin estuary and Fourleague Bay. The flumes were sampled between September 1986 and April 1988, coincident with an extended period of low sea level on the Louisiana coast. The Barataria Basin estuary is in the later, deteriorating stage of the deltaic cycle, characterized by low freshwater inputs and subsiding marshes. Both brackish and saline marshes supplied dissolved organic nitrogen (DON), inorganic nitrogen (ammonium + nitrate + nitrite = DIN), dissolved organic carbon (DOC), and total nitrogen (as total Kjeldahl nitrogen = TKN) to the water column. The export of DIN is probably related to the N accumulated in earlier stages of deltaic development and released as these marshes deteriorate. Coastal brackish marshes of Fourleague, Bay, part of an accreting marsh system in an early, developmental stage of the deltaic cycle, exported TKN to the open water estuary in all samplings. This marsh apparently acted as a short-term buffer of DIN by taking up NH₄ ⁺ in spring, when baywide concentrations were high, and supplying DIN to the estuary in summer and fall, when concentrations, in the bay were lower. Differences in phosphorus (P), DOC, and DON fluxes between these two estuaries were also observed. The Fourleague Bay site exported soluble reactive phosphorus (SRP) and total phosphorus (TP) and imported DOC. This P export may be related to remobilization of sediment-bound riverine P by the reducing, soils of the marshes. Fluxes of SRP at the Barataria Basin sites were variable and low while DOC was imported. Most imports of dissolved nutrients were correlated with higher upstream [source] concentrations, and flux rates were fairly consistent throughout the tide. Dissolved nutrient exports, did not correlate with upstream concentrations, though, and in many cases the flux was dominated by early, flood tide nutrient release. This pulsed behavior may be caused by rapid diffusion from the sediments early in the tidal cycle, when the sediment-water concentration gradient is largest. Interestuary differences were also seen in particulate organic matter fluxes, as the Fourleague Bay marsh exported POC and PON during all samplings while Barataria Basin imported these nutrients. In general, the magnitude and direction of nutrient exchanges in Louisiana marshes, seem to reflect the deltaic successional stage of the estuary.     

Radiocarbon Dating of Soil Organic Matter

Radiocarbon ages of soil organic matter are evaluated with a model which incorporates the dynamics of the¹⁴C content of soil organic matter. Measured¹⁴C ages of soil organic matter or its fractions are always younger than the true ages of soils due to continuous input of organic matter into soils. Differences in soil C dynamics due to climate or soil depth will result in significantly different¹⁴C signatures of soil organic matter for soils of the same age. As a result, the deviation of the measured¹⁴C age from the true age of soil formation could differ significantly among different soils or soil horizons. Our model calculations also suggest that¹⁴C ages of soil organic matter will eventually reach a steady state provided that no climatic or ecological perturbations occur. Once a soil or a soil horizon has reached a steady state,¹⁴C dating of soil organic matter will provide no useful information regarding the age of the soil. However, for soils in which steady state has not been reached, it is possible to estimate the age of soil formation by modeling the measured¹⁴C contents of soil organic matter. Radiocarbon dating of buried soils could, in general, overestimate the true age of the burial by as much as the steady-state age of the soil or soil horizon.     

Quantification of carbon flow from stable isotope fractionation in rice field soils with different organic matter content

Rice fields are an important source for the greenhouse gas methane produced by acetoclastic and hydrogenotrophic methanogenesis. Fractionation of ¹³C/¹²C can in principle be used to quantify the relative contribution of these pathways, but our knowledge of isotopic fractionation during reduction of CO₂ and turnover of acetate in different methanogenic environments is still scarce. We therefore measured δ¹³C signatures in two types of anoxic Italian rice field soils, one with high and one with low degradable organic matter (OM) content. Both soils were incubated in the presence and absence of methyl fluoride, a specific inhibitor of acetoclastic methanogenesis. Optimization of methyl fluoride concentration resulted in complete inhibition of acetoclastic methanogenesis. CH₄ was then exclusively produced by hydrogenotrophic methanogenesis, allowing determination of the isotopic signatures and fractionation factors specific for this methanogenic pathway. Acetate, which was then no longer consumed, accumulated and was used for determination of the isotopic signature of the fermentatively produced acetate (both total acetate and methyl carbon of acetate). Hence, all isotopic signatures, including fractionation factors were determined for the methanogenic soil. These data, were then used for computation of the relative contribution of the two methanogenic pathways. In the high OM soil, the contribution of acetoclastic methanogenesis to total CH₄ production increased simultaneously with decreasing acetate concentration. In the low OM soil, methanogenesis from H₂/CO₂ was clearly greater than theoretically expected. Furthermore, isotope fractionation of hydrogenotrophic methanogenesis indicated that the in situ energy status of methanogens strongly depended on the availability of organic carbon in the rice field soil system. Collectively, our data show that the study of isotopic fractionation in methanogenic environments allows a deeper insight into the ongoing processes, which may be quite different in the same ecosystem with different content of degradable OM.     

Assessment of degradation of agricultural soils arising from brick burning in selected soil profiles

The study was conducted with the selected soil profile of burnt (soil around brick kilns) and unburnt (agricultural land) soils in the Dinajpur, Rangpur, Rajshahi, Khulna and Patuakhali districts at the western part of Bangladesh to evaluate the effects of brick kilns on soil degradation and environmental pollution. The pH values of the unburnt soils increased as a function of the soil depth for Rangpur, Khulna and Patuakhali, while decreased for the soil profiles in Dinajpur. Burning of soils significantly (p<0.05) decreased the average pH values of soils by 0.4 pH units (7 % increased over average content = IOAC), but strikingly increased the average EC values from 0.26 to 1.77 mS/cm (592 % IOAC) and the effect was pronounced with the depth function. The average sand content of the soil profiles increased by 330%, while the silt and clay contents decreased by 49 and 40 %, respectively. The average losses arising from the burning of agricultural soils were amounted to 63% for organic matter, 56 to 86 % and 23 to 88 % for available and total N, P, K and S, respectively. This huge loss through the burning of 1 m deep soil profile, i.e. almost 3/4^th of the deterioration of soil fertility is not only reducing the crop production but also polluting the associated environment and atmosphere. The burning of enormous C, N and S not only degrade the agricultural soils but also contributing to the changes in the global climate.     

Analytical problems in determining peptide nitrogen in the humic fractions of three Italian soils

Peptides were released from organic matter fractions of three Italian soils (humin, humic and fulvic acids), when the samples were hydrolyzed in Ba(ON)₂-saturated solution at 105°C for 2 hr. The peptides obtained were separated using electrophoresis and paper chromatography. The presence of polypeptides in the soil organic matter was indicated by: (1) their hydrolysis by pronase; (2) the amino acids released by 6 N HCl hydrolysis; (3) The comparison of i.r. spectra of humic fractions before and after hydrolysis with 6 N HCl.Attempts at isolating the native proteinaceous compounds using electrophoresis in polyacrylamide gel failed; additionally, our attempts to hydrolyze proteinaceous components enzymatically in unfractionated soil organic matter, as well as in its fractions, before and after methylation, with pepsin, papain and pronase, were unsuccessful. Pronase demonstrated a weak proteolytic activity only at very low substrate-enzyme ratios (20 : 1) in humic and fulvic fractions and in whole phyrophosphate extract. Deproteinated substrates treated with pronase also released free amino acids, suggesting autodigestion.In humin, humic and fulvic fractions we found a total amino acid content of 40–45%, 12–24% or 1–85, respectively. Amino acid recovery from single fractions was about 70–80% of the total content in the unfractionated soil.     

Deforestation modifying terrestrial organic transport in the Rio Tapajós, Brazilian Amazon

The concentration and biomarker compositions of sedimentary organic matter (OM) as well as fine and coarse suspended particles were analysed to identify the impact of deforestation on the transport of terrigenous organic matter (OM) in the Rio Tapajós, a major tributary to the Amazon. Substantial shifts in the concentration and composition of recently deposited sedimentary OM suggest that intensive deforestation over the last few decades has considerably modified the natural inputs of sedimentary materials to the aquatic ecosystems by disrupting the terrigenous fluxes of humus and soil materials from the drainage basin. The observed compositional changes of bulk OM and land derived biomarkers (e.g. lignin) in recent sediments illustrate a sedimentary enrichment in OM from soil horizons that, under normal forest cover, tend to be retained in the drainage basin. On average, the recently accumulated OM is nitrogen-rich ((C/N)a=12–15) and more highly degraded ((Ac/Al)v=0.4–0.6 and DHBA/V=0.15–0.20) than deep materials ((C/N)a=20–30, (Ac/Al)v=0.25–0.4, and DHBA/V=0.05–0.10), showing that this recently accumulated material is more humified than original inputs to the aquatic system, and consistent with increased exportation of fine eroded mineral and organic particles from surface soils along river banks. The present study illustrates the relevance of using OM oxidation products in sediment profiles to evaluate deforestation impacts on aquatic ecosystems and to characterize the nature of eroded soil materials, complementing studies on mineral/metal cycling.     

Simulation of the availability index of soil copper content using general regression neural network

Excessive soil copper (Cu) availability leads to plant growth retardation and leaf chlorosis, and the contamination of Cu in the food chain would be detrimental to human and animal health. The most important path for Cu accumulation in plants is uptake from soils. It is therefore important to understand the availability of soil Cu and its controlling factors to modify Cu availability and prevent excessive Cu from entering the food chain. The present study proposed a general regression neural network (GRNN) to simulate the availability index of soil Cu (available heavy mental concentrations/total heavy metal concentrations), based on the influencing factors of total Cu concentration, pH, organic matter (OM), available phosphorus (AP), and readily available potassium (RAK). Results showed that total Cu concentration, combined with OM and AP, achieved the lowest RMSE value (0.0524) for the modeled value of the availability index of soil Cu. The simulated results by GRNN and the ground truth values had better agreement (R ² = 0.7760) than that by a linear model (R ² = 0.6464) for 23 test samples. Moreover, GRNN obtained lower averaged relative errors than linear model. This demonstrated that GRNN could be used to simulate the availability index of soil heavy metals and gained better results than linear model.     

Carbon isotope and C/N ratios of suspended matter in rivers: an indicator of seasonal change in C4/C3 vegetation

A combination of δ ¹³C values with C/N ratios in suspended matter has been used to examine the seasonal relationship between C₄ and C₃ vegetation along the Loess Plateau, NW China. The C isotopic composition of suspended organic matter in rivers, together with C/N ratios can differentiate between soil and plant material, and can be used to estimate the relative contributions of soil organic C and plant litter to the suspended matter. The relationship between C isotopic composition and C/N ratios indicates that the samples are a mixture of two end members: (1) modern soils with relatively constant δ ¹³C values, low C content and low C/N ratios; (2) plant litter with varying δ ¹³C values, high C content and high C/N ratios. The results reflect the seasonal distribution of C₄/C₃ vegetation within the area studied, as part of the Loess Plateau. The abundance of C₄ grasses is about 20% for the current summer vegetation ecosystem in the eastern part of the Loess Plateau. Hence, the use of δ ¹³C values and C/N ratios of suspended matter in rivers and modern soil may be useful for reflecting seasonal distribution of C₄/C₃ vegetation in catchments. This could be a useful tool for distinguishing between catchments for GIS studies, and long term planning for ecological management of catchment areas.