Water-soluble organic materials in paddy soil ecosystem

Abstract

A column experiment was conducted to analyse the composition of organic materials in the leachate from the plow layer and their fate in the subsoil. Water-soluble organic materials in the leachate were fractionated by insoluble polyvinylpyrrolidone (PVP) and ion exchange resins. The content of total organic carbon (TOC) in the leachate increased by the addition of rice straw (RS) to the plow layer soil sample. The leachate contained a constant amount of PVP-adsorbed Fraction, while that of the PVP-non-adsorbed Fraction changed during the 45 day incubation period. In the fractionation using ion exchange resins, the fraction adsorbed onto the anion exchange resin was the major one.

By the connection of a subsoil column to the plow layer soil column with RS, the TOC content in the leachate decreased by percolation into the subsoil sample. In the Anjo soil sample (Yellow Soil), the decrease occurred throughout the incubation period, and about 90% of the PVP-adsorbed Fraction in the leachate decreased by percolation into the subsoil sample. In the Fukushima soil sample (Gray Lowland Soil), the TOC content decreased in the early and middle periods of incubation, while in the late period the decrease was negligible. This decrease of the TOC content by percolation into the subsoil sample was mainly due to retention in the subsoil sample of the Anjo soil, while in the Fukushima soil sample it was due to decomposition and retention. It was considered that easily decomposable organic materials like organic acids were decomposed in the early to middle periods of incubation, while in the late period the contents of such substances in the leachate from the plow layer soil sample with RS were small and the decrease of TOC was negligible.     

Bacterial numbers and biomass in leachate and in subsoil of a submerged paddy field

By a leaching experiment with glass columns packed with submerged paddy soils, the relationships among numbers of total bacteria, total Gram-positive bacteria, culturare aerobic bacteria and a amount of bacterial biomass both in the leachate, and in the subsoil after leaching incubation were studied. The leachate from soil columns was collected every 3 d during the 30-d incubation period. The soil columns were packed with plow layer soil samples with and without rice straw (RS) amendment, and the subsoil column was connected to the plow layer soil column without RS. Numbers of total bacteria, culturable aerobic bacteria, and a amount of bacterial biomass in the leachate decreased with the incubation time. There was no correlation between the number of total bacteria in the leachate and the concentration of total organic carbon in the leachate. Bacteria less than 0.1 µsm³ in size predominated in the leachate, especially in that from the plow layer soil column with RS. Percentages of the number of Gram-positive bacteria in the leachate were very low (less than 7% of the total bacterial number), while the percentage in the subsoil after the leaching experiment was in the range of 21–82%. The sum of the number of bacteria percolated from the plow layer soil column with RS during the 30-d period of incubation and the sum of the amount of biomass C were 39 and 77% less than the corresponding values for the bacteria percolated from the plow layer soil column without RS. Percentages of culturable aerobic bacteria among the total bacteria ranged between 2.8 and 37% in the leachate, while less than 0.6% in the subsoil after the leaching experiment.     

Origin and properties of humus in the subsoil of irrigated rice paddies

Stability of humus in the plow layer soil is considered to affect the quantity and quality of dissolved organic matter leached from the plow layer soil. Therefore, a model experiment was conducted to analyze the effect of soil reduction under submerged conditions on the stability of humus in the plow layer soil. The changes in the stability of humus in the plow layer soil during submerged incubations with and without rice straw application were evaluated based on the changes in the binding type of humus. Binding type of humus in the plow layer soil was analyzed by successive extractions of organic matter with water, 0.25 M Na₂SO₄, 0.1 M Na₄P₂O₇ (pH 7.0), 0.1 M Na₄P₂O₇ (pH 10.5), and 0.1 M Na₄P₂O₇ (pH 10.5) with NaBH₄. Amounts of Fe, Mn, and Mg in each fraction were also determined to estimate the relationships between humus and metals.

The successive extraction of humus indicated that the amount of organic carbon which was extractable with the (NaBH₄ +0.1 M Na₄P₂O₇) solution decreased while that of the 0.1 M Na₄P₂O₇ (pH 7.0}-extractable organic carbon increased during submerged incubation with rice straw application. The origin of the increase in the amount of organic carbon in the Na₄P₂O₇ (pH 7.0)-extractable fraction during submerged incubation was investigated further by another incubation experiment using ¹³C-glucose as a reducing agent. Atom- 13C% analysis showed that the contribution of organic carbon derived from compounds other than glucose to the increase in the contents of humic acids and fulvic acids in the Na₄P₂O₇ (pH 7.0)-extractable fractions was ca. 80%. Therefore, it was concluded that the binding type of humus changed from (NaBH₄ + Na₄P₂O₇)-extractable to Na₄P₂O₇ (pH 7.0)-extractable humus under reducing conditions. Since the amounts of organic carbon and Fe increased in the Na₄P₂O₇ (pH 7.0)-extractable fraction and decreased in the (NaBH₄ +0.1 M Na₄P₂O₇)-extractable fraction simultaneously, iron reduction was presumably associated with the change in the binding type of humus in submerged paddy soil.     

Methane production and its fate in paddy fields

Abstract

Oxidation of methane and total water soluble organic carbon (TOC) in the subsoil, which percolated from the plow layer, was investigated in a column experiment. The amounts of both methane and TOC in the leachate decreased by percolation in the subsoil.

Fe²⁺ percolated from the plow layer was nearly completely retained in the subsoil. The decomposition of methane and TOC in the subsoil was considered to result in the coupling with the formation of Fe²⁺. Methane was estimated to contribute ca. 19–21% to the total amount of Fe²⁺ formed in the subsoil by the organic materials in the leachate.     

Nutrient leaching from the plow layer by water percolation and accumulation in the subsoil in an irrigated paddy field

To estimate the impact of water percolation on the nutrient status in paddy fields, the seasonal variations of the concentrations of cations, anions, inorganic carbon (IC), and of dissolved organic carbon (DOC) in percolating water that was collected from just below the plow layer (PW-13) and from drainage pipes at the 40 em depth (PW-40), as well as in irrigation water were measured in an irrigated paddy field. Total amounts of Ca, Mg, K, Fe, and Mn leached from PW-13 during the period of rice cultivation were estimated to range from about 390 to 770, 65 to 130, 33 to 66, 340 to 680, and 44 to 87 kg ha^-1, respectively. Amounts of losses that were estimated from the differences between the input by irrigation water and the output by percolation water from the plow layer corresponded to 11 to 26, 22 to 47,5.9 to 12, and 13 to 26% of exchangeable Ca and Mg, amorphous Fe, and easily reducible Mn in the plow layer, respectively. The concentrations of Ca, Mg, K, Fe, and Mn in PW13 were higher than those in PW-40. The amounts of these nutrients that were retained in the subsoil between the 13 em and 40 em soil depth corresponded to 83, 86, 61, 99, and 89% of the amounts that percolated from the plow layer, respectively. Total amounts of IC and DOC that percolated from the plow layer ranged from 750 to 1,500 and 85 to 170 kg-C ha^-1, which corresponded to 5.0 to 10.0% and 0.6 to 1.1% of the total carbon content in the plow layer, respectively. Eighty eight % of IC in the percolating water from the plow layer was also retained in the subsoil.     

Carbon cycling in rice field ecosystems in the context of input, decomposition and translocation of organic materials and the fates of their end products (CO2 and CH4)

Rice fields are intensively managed, unique agroecosystems, where soil flooding is general performance for rice cultivation. Flooding the field results in reductive soil conditions, under which decomposition of organic materials proceeds during the period of rice cultivation. A large variety of organic materials are incorporated into rice soils according to field management. In this review, the kind and abundance of organic materials entering carbon cycling in the rice field ecosystem are evaluated first. Then, decomposition of plant residues and soil organic matter in rice fields is reviewed quantitatively. Decomposition of plant residues is shown to be the active process in carbon cycling in rice fields. Rice releases photosynthates into the rhizosphere (rhizodeposition), and they follow a different avenue of decomposition in soil from that of plant residues. Incorporation of rhizodeposition into microbial biomass and soil organic matter during the period of rice cultivation, and their fates after harvesting are evaluated quantitatively from ¹³C pulse labeled experiments. Percolating water transports inorganic and organic carbon from the plow layer to the subsoil layer. The amounts of their transport and accumulation in the subsoil layer are evaluated in relation to the amounts of soil organic C in the plow layer. Not only CO₂ but also CH₄ are produced in the decomposition process of organic materials in flooded rice fields. CH₄ evolution from rice fields is of global concern from the viewpoint of global warming. Origins of CH₄ evolved from rice fields are estimated first, followed by the fates of CH₄ in rice field ecosystems. Rhizodeposition is shown to be the main origin of CH₄ evolved from rice fields. Evolution to the atmosphere is not the sole pathway of CH₄ produced in rice fields. The amounts of CH₄ retained in soil, percolated to the subsoil layer and decomposed in soil are evaluated in the context of the amounts of CH₄ efflux. Thus, this review focuses on carbon cycling in the rice field ecosystem from the viewpoints of input, decomposition, and translocation of organic materials and the fates of their end products (CO₂ and CH₄).     

Water-soluble organic materials in paddy soil ecosystem

Abstract

Column experiments were conducted to analyze the effect of the temperature on the amounts of organic materials in the leachate, especially organic acids and methane, from samples of the plow layer soil amended with rice straw. Total amount of inorganic carbon in the leachate during the 30-d period of incubation in relation to the temperature was 18°C < 25°C ≤ 30°C > 37°C > 45°C. Total amount of organic carbon in the leachate was signiicantly larger under 45°C incubation than that at other temperatures.

Acetic acid was the dominant organic acid in the leachate regardless of the temperature. Butylic and propionic acids were also present in large amounts in the early and the late period of incubation of temperatures ranging between 18 and 37°C, while only acetic acid was the dominant organic acid during the 30-d period of incubation at 45°C.

The total amount of methane in leachate during the 30-d period of incubation was very small at 18°C, while very large at 25, 30, and 37°C. It decreased nearly to one half at 45°C compared with that at 30°C. Based on the values of δ¹³CH₄ in the leachate, 3 different stages were recognized in the predominant processes of methane production in the submerged paddy soil amended with rice straw: the stage when methane production from CO₂-B₂ was predominant followed by the stages of methane production from acetic acid and from CO₂-H₂ in this order. The second stage coincided with the time of decrease of the organic acid contents in the leachate. Under 45°C incubation, methane production from CO₂-H₂ was predominant throughout the 30-d period of incubation.     

Characterization of Compost Leachate Fractions Using NMR Spectroscopy

A distilled water leachate was prepared from a hardwood-leaf compost. The ¹³C NMR (nuclear magnetic resonance) spectra of organic acids isolates from this leachate provided support for the hypothesis that the composition of the dissolved organic carbon (DOC) in the leachate is very similar to that in natural soil pore waters. The leachate DOC was adsorbed by a model mineral surface, and the organic coating on the surface was characterized by solid-state ¹³C NMR spectroscopy.     

Sites and processes of nutrient accumulation in the subsoil through water percolation from the plow layer in a submerged paddy soil microcosm

The leaching of nutrients from the plow layer by water percolation and their accumulation in the subsoil observed in a Japanese paddy field (Katoh et al. 2004: Soil Sci. Plant Nutr., 50, 721-729) were determined semi-quantitatively in a soil column experiment. Ca²⁺, Mg²⁺, K⁺, Mn²⁺, Fe²⁺, and phosphate in percolating water from the plow layer soil column were retained in the subsoil columns that were connected to the plow layer soil column. Fe²⁺, K⁺, and phosphate accumulated in the uppermost part of the subsoil. Accumulation of Fe²⁺ in the uppermost part of the subsoil was presmnably due to the cation exchange process with concomitant desorption of Ca²⁺. In contrast, Ca²⁺ and Mg²⁺ in percolating water from the plow layer soil colmnn accumulated once in the subsoil, and translocated downwards slowly with successive water percolation. Considerable amounts of inorganic carbon (IC) and dissolved organic carbon (DOC) in percolating water from the plow layer soil column were also retained in the subsoil columns. IC did not accumulate a gaseous form.     

Chemical characteristics and potential source of fulvic acids leached from the plow layer of paddy soil

To understand better the chemical characteristics and source of dissolved organic matter (DOM) leached from the plow layer of rice paddies, a lysimeter study was conducted, which simulated submerged paddy topsoil during rice growth. The fulvic acid (FA) fraction in the percolation water from the lysimeter was collected by adsorption onto insoluble polyvinylpyrrolidone (PVP), and the temporal variations in its elemental composition, molecular size distribution, IR spectra, ¹³C CPMAS NMR spectra, and δ¹³C values were investigated. The proportion of the FA fraction to bulk DOM varied greatly, but the chemical characteristics of the FA fraction did not change appreciably during the experimental period. Thus, it is considered that the changes in the DOM composition in percolation water were mainly due to the differing contributions of the FA fraction. Further, to investigate the source of the FA fraction in the leachate, the chemical characteristics of the FA fraction in the leachate were compared with those extracted from the plow layer soil. A sequential extraction of the FA fraction was conducted using a sequence of water, 0.25 M Na₂SO₄, 0.1 M Na₄P₂O₇ (pH 7.0), 0.1 M Na₄P₂O₇ (pH 10.5), and NaBH₄+0.1 M Na₄P₂O₇ (pH 10.5). It was found that the water- and 0.25 M Na₂SO₄-extractable fractions, which were most mobile, were not the only source of the FA fraction in the leachate. The small molecular size sub-fraction of the NaBH₄+0.1 M Na₄P₂O₇ (pH 10.5)-extractable FAs, most of which are probably bound mainly to iron oxides, are considered to be another source of the FA fraction leached from the plow layer of paddy fields.     

Loss of labile organic carbon from subsoil due to land-use changes in subtropical China

Topsoil carbon (C) stocks are known to decrease as a consequence of the conversion of natural ecosystems to plantations or croplands; however, the effect of land use change on subsoil C remains unknown. Here, we hypothesized that the effect of land use change on labile subsoil organic C may be even stronger than for topsoil due to upward concentration of plantations and crops root systems. We evaluated soil labile organic C fractions, including particulate organic carbon (POC) and its components [coarse POC and fine POC], light fraction organic carbon (LFOC), readily oxidizable organic carbon, dissolved organic carbon (DOC) and microbial biomass down to 100 cm soil depth from four typical land use systems in subtropical China. Decrease in fine root biomass was more pronounced below 20 cm than in the overlying topsoil (70% vs. 56% for plantation and 62% vs. 37% for orchard. respectively) driving a reduction in subsoil labile organic C stocks. Land use changes from natural forest to Chinese fir plantation, Chinese chestnut orchard, or sloping tillage reduced soil organic C stocks and that of its labile fractions both in top and subsoil (20–100 cm). POC reduction was mainly driven by a decrease in fine POC in topsoil, while DOC was mainly reduced in subsoil. Fine POC, LFOC and microbial biomass can be useful early indicators of changes in topsoil organic C. In contrast, LFOC and DOC are useful indicators for subsoil. Reduced proportions of fine POC, LFOC, DOC and microbial biomass to soil organic C reflected the decline in soil organic C quality caused by land use changes. We conclude that land use changes decrease C sequestration both in topsoil and subsoil, which is initially indicated by the labile soil organic C fractions.     

Effect of nitrate leaching in oxisol columns on 15N abundance and nitrate breakthrough curves

Abstract

Nitrate (20 mg N0₃‐ N l^?1) was leached through 180 cm columns of oxisol subsoil until the leachate attained the initial nitrate concentration. Leaching was continued with water until no nitrate was detectable in the leachate.

The Δ¹⁵N for the first aliquot of leachate containing nitrate was 2.2 units lower than that of the added solution indicating that ¹⁵Nwas preferentially adsorbed to ¹⁴N. The breakthrough curve for nitrate indicated that nitrate adsorption decreased after six pore volumes. The implications for modelling nitrate leaching are discussed.     

Incorporation of nitrogen from urea fertilizer into soil organic matter in rice paddy and cassava upland fields in Indonesia

Incorporation of newly-immobilized N into major soil organic matter fractions during a cropping period under paddy and upland cropping systems in the tropics was investigated in Jawa paddy fields with and without fish cultivation and a Sumatra cassava field in Indonesia. ¹⁵N-labelled urea (¹⁵N urea) was applied as basal fertilizer, and the soil samples were collected after harvest. The percentage of distribution of the residual N in soil from ¹⁵N urea into the humic acids, fulvic acid fraction, and humin were 13.1–13.9, 19.0–20.5, and 53.4–54.3%, respectively, for the Jawa paddy soils, and 14.9, 27.4, and 52.4%, respectively, for the Sumatra cassava soil. These values were comparable to the reported ones for other climatic zones. The percentage of distribution of ¹⁵N urea-derived N into humic acids was larger than that of total N into the same fraction in all the soils. The distribution into the fulvic acid fraction was also larger for ¹⁵N urea-derived N than for total N in the Jawa soils. Humic and non-humic substances in the fulvic acid fraction were separated using insoluble polyvinylpyrrolidone (PVP) into the adsorbed and non-adsorbed fractions, respectively. Less than 5% of the ¹⁵N urea-derived N in fulvic acid fraction was detected in the PVP-adsorbed fraction (generic fulvic acids). The proportion of non-hydrolyzable N remained after boiling with 6 M HCl in the ¹⁵N urea-derived N was 9.4–13.5%, 17.3–26.7%, and 8.4–16.6% for the humic acids, generic fulvic acids, and humin, respectively. The significantly low resistance to acid hydrolysis suggested that the ¹⁵N urea-derived N was less stable than the total N in soil regardless of the fractions of humus.     

Denitrification Rate and Controlling Factors for Accumulated Nitrate in the Deep Subsoil of Intensive Farmlands: A Case Study in the North China Plain

Denitrification in subsoil(to a depth of 12 m) is an important mechanism to reduce nitrate(NO_3~-) leaching into groundwater.However, regulating mechanisms of subsoil denitrification, especially those in the deep subsoil beneath the crop root zone, have not been well documented. In this study, soil columns of 0–12 m depth were collected from intensively farmed fields in the North China Plain. The fields had received long-term nitrogen(N) fertilizer inputs at 0(N0), 200(N200) and 600(N600) kg N ha~(-1) year~(-1). Main soil properties related to denitrification, i.e., soil water content, NO_3~-, dissolved organic carbon(DOC), soil organic carbon(SOC),pH, denitrifying enzyme activity(DEA), and anaerobic denitrification rate(ADR), were determined. Statistical comparisons among the treatments were performed. The results showed that NO_3~- was more heavily accumulated in the entire soil profile of the N600 treatment, compared to the N0 and N200 treatments. The SOC, DOC, and ADR decreased with increasing soil depth in all treatments,whereas considerable DEA was observed throughout the subsoil. The long-term fertilizer rates affected ADR only in the upper 4 m soil layers. The ADRs in the N200 and N600 treatments were significantly correlated with DOC. Multiple regression analysis indicated that DOC rather than DEA was the key factor regulating denitrification beneath the root zone. Additional research is required to determine if carbon addition into subsoil can be a promising approach to enhance NO_3~- denitrification in the subsoil and consequently to mitigate groundwater NO_3~- contamination in the intensive farmlands.     

Solute Release from Weathering of Spent Mushroom Substrate under Controlled Conditions

Weathering of piled material in the field is a popular method to treat spent mushroom substrate (SMS) before reuse. During the weathering process, rainfall and snowmelt pass through SMS piles and a large amount of solutes is released in the leachate. To investigate solute release patterns, the field weathering process was simulated under controlled conditions in the laboratory. Fresh SMS was packed in an acrylic column (20 cm i.d.) to 150 cm height and leached intermittently with a cumulative total of 230 cm of deionized water over 180 days. Leachate was collected and analyzed for dissolved organic carbon (DOC), dissolved organic nitrogen (DON), electrical conductivity (EC), and inorganic salts. Solute release patterns were described using first order models, and total released solutes were calculated. The SMS leachate had DOC, DON and EC values ranging from 450 to 15,500 mg L^?1, 50 to 1,700 mg L^?1, and 3 to 50 dS m^?1, respectively. The major inorganic cations were K⁺, Na⁺, Ca²⁺, Mg²⁺ and NH₄⁺, and anions were Cl^? and SO₄^2?. Release of DOC, DON, and bivalent cations Ca²⁺ and Mg²⁺ were described by a first order Exponential Rise to Maximum model, while releases of monovalent ions Cl^?, K⁺, Na⁺ and NH₄⁺ were described as a first order Sigmoidal Logistic process, and SO₄^2? release was best modeled by a Sigmoidal Chapman equation. Following six months and 230 cm applied water, 3.1 kg of DOC, 0.58 kg of dissolved N, and 8.6 kg of inorganic salts were leached per cubic meter of bulk SMS (220 kg oven dry mass). Weathering of SMS involves a significant removal of nutrients from the composted material, which can contribute to pollution of soil and groundwater.     

Effects of arsenic on nutrient dynamics of grassland microcosms and field plots

Grassland field plots were compared to two sizes of excised microcosms (15 and 30 cm diameter) and 30-cm diameter homogenized microcosms to assess the effects of As on soil-plant nutrient dynamics. The goal of this experiment was to determine the feasibility of using microcosms for screening chemicals which are potentially toxic to terrestrial ecosystems. All experimental units were treated with As, at rates of 0, 0.5, and 5.0 mg cm^?2, by superficial application of dissolved Na₂AsO₄. Calcium, PO₄-P, NO₃-N, dissolved organic carbon (DOC), and NH₄-N were determined in soil leachate and soil extracts. Greater loss of PO₄-P, NO₃-N, NH₄-N, and DOC occurred via soil leachate from microcosms treated with the greatest level of As relative to untreated microcosms. Field plots were unaffected by As treatment. Excised microcosms were found to be more representative of field plots in reflecting microbial dynamics than were homogenized microcosms. Thus excised microcosms were recommended for testing potentially hazardous chemicals.     

Depth Distribution of Soil Organic Carbon Fractions in Relation to Tillage and Cropping Sequences in Some Dry Lands of Punjab,Pakistan

Depth distribution of soil organic carbon (SOC) fractions depends on the efficiency of agro‐technical managements. Information on depth distribution of SOC fractions mostly confined to the plow layer and scant in dry lands of Punjab, Pakistan. Therefore, a field experiment was laid out with moldboard plow (MP) (control), tine cultivator (TC), and minimum tillage (MT) as main plots, and cropping sequences fallow wheat (Triticum aestivum L.), (FW, control), mungbean (Vigna radiata L.) wheat (MW), sorghum (Sorghum bicolor L.) wheat (SW), green manure wheat (GW), and mungbean‐chickpea (MC) (Cicer arietinum L.) as sub‐plots. Treatment effects were assessed for microbial biomass carbon (MBC), potentially mineralizeable carbon (PMC), particulate organic carbon (POC), dissolved organic carbon (DOC), HCl insoluble carbon (HIC), and stratification ratio (SR) in Rawal series: Udic Haplustalf. Alfisols. The MBC concentration was the highest in MT system, at 15 to 30‐cm depth under MW and PMC concentration was highest under SW with MT at 45–60 cm. MP had higher POC in FW sequence. The highest DOC was at 0 to 15‐cm depth under MC with TC and stock of HIC was more under TC with FW sequence. The highest SR of PMC was under MT with FW at 0–15:15–30 and POC was under TC and MP with FW at depths of 0–15:45–60 cm. The highest SR for DOC was under MP with GW at 0–15:45–60 cm and HCl insoluble C was under MT with SW at 0–15:45–60. In broad‐spectrum, labile organic fractions revealed differential sensitivity, and POC stocks are also a sensitive indicator to detect the short‐management effects. Copyright © 2015 John Wiley & Sons, Ltd.     

The contribution of fresh litter to dissolved organic carbon leached from a coniferous forest floor

The relative contributions of litter and humified organic matter as the source of dissolved organic carbon (DOC) leached from organic layers of forest soils are poorly understood. In the present investigation, ¹³C labelled spruce litter was used to study the role of recent litter in the leaching of DOC from a coniferous forest floor in southern Sweden, while litterbags were used to quantify the total loss of C from the labelled litter. The labelled litter applied on bare lysimeters released considerable amounts of DOC during the first weeks, but the concentration of DOC originating from labelled litter decreased gradually from 176 mg litre⁻¹ during the first sampling period in May to 5 mg litre⁻¹ in the last sampling period in October. Only a moderate flush of DOC from the labelled litter occurred under the Oe and Oa horizons, with concentrations of 20 and 6 mg litre⁻¹ from labelled litter, equal to 19 and 9% of the total DOC flux, respectively, during the first sampling period. Total flux of DOC from labelled litter from May to September was 16 g m⁻², whereas only 2.2 and 0.9 g m⁻² were captured under the Oe and Oa horizons, respectively. The almost complete loss of new DOC implies that DOC leached from the Oe and Oa horizons consists not of recent litter‐derived carbon, but of DOC produced in these two horizons themselves. Water‐extractable organic carbon from labelled litter left in litterbags in the field for 4 months consisted of about one‐third native carbon from external sources at the experimental site and two‐thirds of the labelled litter. In contrast, the ¹³C content of the bulk litter from the litterbags was not changed by the incubation in the field. We suggest that the soluble native carbon in water extracts originated from throughfall DOC that had been assimilated by microorganisms in the litterbags.     

UV-TiO2 Photocatalytic Degradation of Landfill Leachate

Mature landfill leachate contains some macromolecular organic substances that are resistant to biodegradation. The photocatalytic process helps to enhance biodegradability of landfill leachate. Batch experiments were employed to determine the optimum conditions for removal of organic matter by UV-TiO₂ photocatalysis. Under optimum conditions, the removal of chemical oxygen demand (COD), dissolved organic carbon (DOC), biological oxygen demand (BOD), and color was determined. Moreover, gas chromatography coupled with mass spectrometry (GC/MS) was used to analyze the organic matter in the treated leachate before and after treatment by the photocatalysis. The experimental results indicated that the removal of COD, DOC, and color by UV-TiO₂ photocatalysis could reach above 60%, 70% and 97%, respectively. Under optimal conditions, the ratio of biological oxygen demand (BOD)/chemical oxygen demand (COD) was elevated from 0.09 to 0.39, representing substantial improvement in biodegradability. GC/MS analysis revealed that 37 out of 72 kinds of organic pollutants in the leachate remained after 72 h treatment. Esters were produced during photocatalytic process and ketones, hydrocarbons, aromatic hydrocarbons, hydroxybenzenes, and acids were easier to be degraded during photocatalytic oxidation processes. The UV-TiO₂ photocatalysis systems proposed may be a cost-effective approach for pre-treatment of landfill leachate.     

Bioavailability of DOC in leachates, soil matrix solutions and soil water extracts from beech forest floors

The biodegradability of dissolved organic carbon (DOC) in different fractions from the forest floor was studied. Soil leachate (SL, the soil solution in macropores which is freely drained from forest floor after rainfall), the soil matrix solution (SMS, the soil solution in meso-/micropores of the soil matrix), and soil water extracts (SWE) from two different beech forest floors were compared. Zero-tension and tension lysimeters were used to collect SL and SMS, respectively. Loss of DOC (during 21 days) and respiration of CO₂-C (during 7 days) were used as conventional measures of the availability of DOC. Bacterial production, measured using the leucine incorporation technique, and bacterial growth efficiency were also estimated. All methods were used to study differences in biodegradability between plots with and without ground flora (Deschampsia flexuosa or Anemone nemorosa) and different type of forest floor (with an organic (O) horizon or a mull (A) horizon). There were no differences in bioavailability of DOC from soil solutions extracted from plots with and without ground flora. The bioavailability of DOC in the different collected soil solutions varied, however. DOC in SWE was the most available, with a mean of 39% of DOC-loss in 21 days, and 18% of DOC being respired in 7 days. DOC in soil matrix solution was the least available of the soil solutions (7% respired), significantly less than DOC in soil leachate (11% respired). The methods measuring biodegradation of DOC, DOC-loss and CO₂-C respiration gave similar results and were comparable to bacterial production and bacterial growth efficiency, with the exception of SWE from the O-horizon at the D. flexuosa site, which had low bacterial production and bacterial growth efficiency, indicating a limitation of the bacterial growth. This study is one of the first to use bacterial production and bacterial growth efficiency for measuring bioavailability in terrestrial environments, giving an extra dimension for the process of biodegradation of DOC.