Retention and release of dissolved organic matter in Podzol B horizons

The main objectives were to study the effects of pH on the retention and release of organic matter in acid soil, and to determine the main differences in results obtained from batch experiments and experiments in columns. We took soil material from the B horizons of a Podzol at Skånes Värsjö (southern Sweden). In batch experiments, soil was equilibrated with solutions varying in pH and concentration of dissolved organic C. In Bh samples, the release of dissolved C gradually increased with increase in pH. In the Bs1 material there was a minimum at pH 4.1, and in the Bs2 soil the minimum occurred at pH 4.6. The ability to retain added dissolved C increased in the order Bh < Bs1 < Bs2. The column experiment was run for 160 days under unsaturated flow conditions. Columns were packed with Bh, Bh + Bs1 or Bh + Bs1 + Bs2 samples to calculate mass balances for each horizon. Solutions either without any dissolved organic C or ones containing 49 mg C dm^?3 with pH of 4.0 or 3.6 were used to leach columns. The pH of input solutions only little affected the concentration of dissolved C in the effluent. Relative proportions of hydrophobic substances decreased with increasing column length and decreasing pH. For input solutions containing dissolved C, near steady state was achieved for both the Bs1 and Bs2 horizons with approximately 25% dissolved organic matter retention. Thus, no maximum sorption capacity for dissolved C could be defined for these horizons. This behaviour could not have been predicted by batch data, showing that column experiments provide useful additional information on interactions between organic compounds and solid soil material.     

Long-term effects of biochar amount on the content and composition of organic matter in soil aggregates under field conditions

Purpose

Soil aggregates play an important role in promoting soil fertility, as well as increasing the sink capacity and stability of soil carbon. In this study, we consider the following research questions:1. Under field conditions, do different dosages of biochar increase the soil aggregation after 3 years of application?2. How does the application of biochar affect the concentration and distribution of soil total organic carbon (TOC) and total nitrogen (TN) in different sizes of aggregates?3. Can the application of biochar alter the composition of organic carbon in soil aggregates?

Materials and methods

Different amounts of biochar (up to 90 t ha^?1) were applied to a calcareous soil in a field experiment in 2009 along with the application of chemical fertilizer annually and the returning of winter wheat and summer maize straws. After 3 years, 0–20-cm soil samples were taken to measure the size distribution of soil water-stable aggregates by wet sieving, the concentrations of TOC and TN in whole aggregates and light or heavy fractions by elemental analysis equipment, and composition of TOC by Fourier transform infrared (FTIR) and pyrolysis-gas chromatography/mass spectrometer (Py–GC/MS).

Results and discussion

(1) The 3 years of biochar application had no significant effects on degree of soil aggregation but reduced the breakage of large soil aggregates (>1000 μm); (2) biochar significantly increased the contents of TOC and TN in soil macro-aggregates (>250 μm), as well as their ratios to total soil amount. Biochar also significantly increased the contents of TOC and TN in light fractions as well as the C/N ratio, which made the soil organic matter more active. The biochar dosage showed a significant positive correlation with organic carbon, total nitrogen, and C/N ratio in light fraction components of aggregates (>250 μm). Biochar mainly affected the organic matter in the heavy fraction components of macro-aggregates; (3) from the Py–GC/MS results, biochar increased the CO₂ content originated from active organic carbon.

Conclusions

Long-term application of biochar improved the stability of soil aggregates, increased the contents of TOC and TN as well as organic carbon and total nitrogen in macro-aggregates, and usually increased the contents of CO₂ originated from active organic carbon in light fractions. The findings were helpful in evaluating the effects of biochar on soil aggregation and organic matter stability.

     

Long-term effects of mineral and organic fertilization on soil organic matter fractions and sorghum yield under Sudano-Sahelian conditions

Abstract. Knowledge of changes in soil organic matter (SOM) fractions resulting from agricultural practice is important for decision‐making at farm level because of the contrasting effects of different SOM fractions on soils. A long‐term trial sited under Sudano‐Sahelian conditions was used to assess the effect of organic and inorganic fertilization on SOM fractions and sorghum performance. Sorghum straw and kraal manure were applied annually at 10 t ha^?1, with and without urea at 60 kg N ha^?1. The other treatments included fallowing, a control (no fertilization), and inorganic fertilization only (urea, 60 kg N ha^?1). Fallowing gave significantly larger soil organic carbon and nitrogen (N) levels than any other treatment. Total soil SOM and N concentrations increased in the following order: urea only < straw < control < straw+urea < manure with or without urea < fallow. Farming had an adverse effect on SOM and N status; however, this mostly affected the fraction of SOM >0.053 mm (particulate organic matter, POM). The POM concentrations in the control, straw and urea‐only treatments were about one‐half of the POM concentrations in the fallow treatment. POM concentrations increased in the following order: urea only < control < straw with or without urea < manure with or without urea < fallow. The fraction of SOM <0.053 mm (fine organic matter, FOM) was greater than POM in all plots except in fallow and manure+urea plots. Total N concentration followed the same trend as SOM, but cultivation led to a decline in both POM‐N and FOM‐N. Crop yield was greatest in the manure plots and lowest in the straw, control and urea‐only plots. Results indicate that under Sudano‐Sahelian conditions, SOM, POM and FOM fractions and crop performance were better maintained using organic materials with a low C/N ratio (manure) than with organic material with a high C/N ratio (straw). Urea improved the effect of straw on crop yield and SOM concentration.     

Chlorophenol binding to dissolved and particulate soil organic matter determined in controlled equilibrium systems

We determined the sorption of 2,4‐dichlorophenol (DCP), 2,4,5‐trichlorophenol (TCP) and pentachlorophenol (PCP) to dissolved (DOM) and particulate soil organic matter (POM) from the same soil in controlled equilibrium systems, using ¹⁴C‐labelled chlorophenols in combination with reversed‐phase high‐performance liquid chromatography (RP‐HPLC) and liquid scintillation. Associations of DCP, TCP and PCP to DOM and POM were satisfactorily described by linear adsorption isotherms. Together with the absence of substantial competition between DCP and TCP for binding sites, this indicates a hydrophobic partitioning mechanism. The organic carbon normalized partitioning coefficient (K_OC) for the binding of DCP was similar in magnitude for POM (K_POC) and for DOM (K_DOC), whereas K_POC for the more hydrophobic compounds TCP and PCP were approximately one order of magnitude greater than K_DOC. On the basis of the relationships between log K_OC and the organic carbon normalized partitioning coefficient (log K_OW), the extent of association to POM increases more with the hydrophobicity of the chlorophenol than the extent of association to DOM. This holds for our data obtained for DOM and POM of similar origin, as well as for various sources of POM and DOM reported in the literature. Differences in the magnitude of K_POC and K_DOC in our study could not be accounted for by differences in gross carbon chemistry of POM and DOM, as determined by nuclear magnetic resonance (¹³C‐NMR) and X‐ray photoelectron spectroscopy (XPS). Thus, other factors such as the average size and capacity of hydrophobic moieties could explain differences in chlorophenol association between POM and DOM. We conclude that K_POC and K_DOC need to be determined explicitly, when the transport and retention of chlorophenols is modelled, and not calculated from relationships between log K_OC and log K_OW.     

Effect of temperature and moisture on rates of carbon mineralization in a Mediterranean oak forest soil under controlled and field conditions

We examined the relationship between carbon mineralization (C_min), moisture and temperature in a Mediterranean forest soil under controlled and field conditions. We studied the following. 1 The temperature sensitivity at three soil depths: soil samples were incubated at 4, 10, 20 and 30°C at optimal water content of 60% of water holding capacity (WHC). Values of C_min of the top layer were more than 12 times faster than those measured in deeper layers. We found a temperature sensitivity factor (Q₁₀) of 3.3, 2.7 and 2.2 for the 0–5 cm, 5–10 cm and 10–20 cm layers, respectively. 2 The relationship between C_min, moisture and temperature (top layer). The sensitivity of C_min to fluctuating moisture depended on temperature. However, the Q₁₀ was not significantly affected by soil moisture. We fitted a multiple polynomial model that predicted C_min as a multiplicative function of temperature and moisture (R² > 0.99). 3 The response of C_min of soil to rewetting after 1 and 24 hours. In all cases, the response was rapid. The soil incubated at 60% WHC or less responded positively to a sudden increase in water content, with the largest increase in the 20% WHC treatment. The model predicted C_min in the field well when rewetting effects were taken into account (R² > 0.81). These results indicate that sudden changes in soil moisture can lead to increased carbon mineralization during the dry summer. It is necessary to include such responses in models as they may represent a substantial loss of carbon in the overall carbon balance of Mediterranean ecosystems.     

Modeling salinity effects on soil reflectance under various moisture conditions and its inverse application: A laboratory experiment

Soil salinization is an important desertification process that threatens the stability of ecosystems, especially in arid lands. Quantifying and mapping soil salinity to monitor soil salinization is difficult because of its large spatial and temporal variability. There has been a growing interest in the use of hyperspectral reflectance as a rapid and inexpensive tool for soil salinity characterization in the recent past. However, as soil moisture often jointly affects soil reflectance, a moisture-insensitive reflectance model is needed to provide the base for soil salinity monitoring from soil reflectance. In this paper, we developed an exponent reflectance model to estimate soil salt contents inversely under various soil moisture conditions, based on a control laboratory experiment on the two factors (soil salinity and soil moisture) to soil reflectance. Main soil salt types (Na₂SO₄, NaCl, Na₂CO₃) with wide soil salinity (0% to 20%) and soil moisture (1.75% to 20%) levels (in weight base) from Western China were examined for their effects on soil reflectance through a model based approach. Moisture resistant but salt sensitive bands of reflected spectra have been identified for the model before being applied to inversely estimate soil salt content. Sensitive bands for Na₂SO₄ type of salt affected soils were identified as from 1920 to 2230 nm, and 1970 to 2450 nm for NaCl, 350 to 400 nm for Na₂CO₃ type of salt affected soils, respectively. The sensitive bands focused on ranged from 1950 to 2450 nm when all data were considered when ignoring salt types. The model was then applied to inversely estimate soil salt contents. High R² of 0.87, 0.79, and 0.66, and low mean relative error (MRE) of 16.42%, 21.17%, and 27.16%; have been obtained for NaCl, Na₂SO₄ and Na₂CO₃, respectively. Performance of the inverse model dropped but remained significant when ignoring salt types with an R² of 0.56 and a MRE of 33.25%. The approach proposed in this study should thus provide a new direction for estimating salinity from reflectance under various soil moisture conditions and should have wide applications in future monitoring of soil salinization.     

微集水种植技术的农田水分调控效果模拟研究

农田微集水种植技术是提高旱区农田作物生产力的一项重要的技术选择，带型(沟垄的宽度比例和数值)的优化设计是其研究和开发所面临的关键问题之一。该研究利用模拟降水等方法，通过降水在沟垄间的分配比例、在沟土中水平分布的不均匀程度，在沟内的最大下渗深度及在沟内的垂直分布特征值4个描述降水在沟垄间分布特征参数对同一沟垄宽度比例的不同宽度值处理对农田水分调控的效果进行了比较分析。结果表明：各种降水处理下，对同一沟垄宽度比值，随着带型的窄化(宽度变窄)，降水向垄中的侧渗作用增强，在沟内水平分布的不均匀性降低，在沟内的垂直下渗略有减弱；在蓄水保墒效果上，窄带型要优于同一沟垄比值下的宽带型。     

Carbohydrate and amino acid composition of dissolved organic matter leached from soil

Low molecular weight organic substances (LMWOS) in soil and soil solution include mainly amino acids, carboxylic acids, and carbohydrates. Due to their high bioavailability they play a crucial role in the cycles of C and nutrients in soils. The variety of soil processes that involve LMWOS requires identifying their composition to elucidate reactions and transformations. In most studies, LMWOS are extracted under artificial conditions, e.g. batch experiments, which may overestimate the actual concentrations. This study measures the composition of carbohydrates and amino acids in solution of a Haplic Luvisol leached in a column experiment. A combined system for simultaneous leaching and blowout of CO₂ was used to estimate LMWOS decomposition. ¹⁴C-labeled glucose was added as a highly sensitive tracer to control the efficiency of the LMWOS extraction by leaching and to estimate LMWOS decomposition during leaching. High performance liquid chromatography (HPLC), optimized for soil extracts, was used to analyze LMWOS composition. For HPLC optimization, different preparations of leached solutions (filtration vs. centrifugation, and drying vs. no-drying) were compared. For sugar determination, drying had no influence on the solution concentrations. In contrast, amino acid concentrations significantly decreased by drying LMWOS eluted substances. Combining the HPLC identification of eluted substances with ¹⁴C tracer application revealed that about 5% of the glucose could be leached unchanged within 786 min (13.1 h), whereas about 84% remained in the soil, 9% were decomposed to CO₂, and 2% were transformed to other LMWOS and recovered in the soil solution. The total amino acid concentration (TAC) in soil solution was about 8.2 μmol l⁻¹, dominated by alanine (14.4% of TAC), glycine (13.4%), glutamic acid (9.9%), serine (9.4%), and leucine (9.3%). The total carbohydrate concentration was about 2.4 μM, dominated by glucose (29.9%), glucuronic acid (26.8%), and galacturonic acid (17.3%). Ratios of hexoses to pentoses, amino sugars glucosamine to galactosamine, and neutral sugars to uronic acids were determined. All three parameters pointed to the dominant influence of plants as the source of LMWOS in the leached soil solution. Within the small contribution of microorganisms, bacteria dominated over fungi. These used biomarker ratios as well as LMWOS concentrations differed widely from the ones obtained with conventional batch extraction. More research is necessary to evaluate the application of these biomarkers to soil solutions.     

Influence of inorganic fertilizers and organic amendments on soil organic matter and soil microbial properties under tropical conditions

 Soil organic matter level, mineralizable C and N, microbial biomass C and dehydrogenase, urease and alkaline phosphatase activities were studied in soils from a field experiment under a pearl millet-wheat cropping sequence receiving inorganic fertilizers and a combination of inorganic fertilizers and organic amendments for the last 11 years. The amounts of soil organic matter and mineralizable C and N increased with the application of inorganic fertilizers. However, there were greater increases of these parameters when farmyard manure, wheat straw or Sesbania bispinosa green manure was applied along with inorganic fertilizers. Microbial biomass C increased from 147 mg kg^–1 soil in unfertilized soil to 423 mg kg^–1 soil in soil amended with wheat straw and inorganic fertilizers. The urease and alkaline phosphatase activities of soils increased significantly with a combination of inorganic fertilizers and organic amendments. The results indicate that soil organic matter level and soil microbial activities, vital for the nutrient turnover and long-term productivity of the soil, are enhanced by use of organic amendments along with inorganic fertilizers. Received: 6 May 1998     

Emission of nitric oxide and nitrous oxide from soil under field and laboratory conditions

A detailed short-term (12 d) laboratory study was carried out to investigate the effects of applying animal urine, fertilizer (ammonium nitrate) and fertilizer+urine on emission of NO and N₂O from soil. A complementary 24 d field study measured the effect of fertilizer or fertilizer+sheep grazing on NO and N₂O emissions from pasture. The data generated were used to interpret the transformations responsible for the release of these gases. Application of urine to the soil (at a rate equivalent to 930 kg N ha⁻¹) increased the amount of mineral and microbial N in the soil. This was followed by increases in emissions of NO (from 0.02 to 1.76 mg NO-N m⁻² d⁻¹) and N₂O (from 15 to 330 mg N₂O-N m⁻² d⁻¹). Molar ratios of NO-N-to-N₂O-N were very low (<0.001 to 0.011) indicating that denitrification was the main process during the first 12 d after application. In the laboratory, nitrification was inhibited during the first 7 d due to an inhibitory effect of the urine, but even though nitrification was clearly underway 7–12 d after application, denitrification was still the dominant process. The fertilizer was applied at a lower rate (120 kg N ha⁻¹) than the urine. Consequently, the effect on soil mineral N was smaller. Nevertheless the fertilizer still increased NO and N₂O emission with denitrification the dominant process. The effects of fertilizer and grazing on NO and N₂O emissions was less obvious in the field compared with the laboratory and fluxes returned to background rates within 4 d. This was attributed to the rapid decline in soil mineral N in the field trial due to plant uptake and leaching, processes that did not occur in the laboratory.     

Stock, turnover time and accumulation of organic matter in bulk and density fractions of a Podzol soil

Temperate forest soils store large amounts of organic matter and are considered as net sinks for atmospheric carbon dioxide. Information about the sink strength and the turnover time of soil organic carbon (SOC) is required to assess the potential response of soils to climate change. Here we report on stocks, turnover times (TT) and accumulation of SOC in bulk soil and density fractions from genetic horizons of a Podzol in the Fichtelgebirge, Germany. Stocks of SOC, total nitrogen and exchangeable cations determined in nine quantitative soil pits strongly varied with stone content and thickness of horizons in both the organic layer and the mineral soil. On the basis of radiocarbon signatures, mean turnover times of 4, 9 and 133 years, respectively, were calculated for Oi, Oe and Oa horizons from three soil pits, using a non-steady-state model. The Oa horizons accumulated 4–8 g C m⁻² year⁻¹ whereas the Oi and Oe horizons were close to steady-state during the past decade. Free particulate organic matter (FPOM) was the most abundant fraction in the Oa and EA horizons with TT of 70–480 years. In the B horizons, mineral associated organic matter (MAOM) dominated with over 40% of total SOC and had TT of 390–2170 years. In contrast to other horizons, MAOM in the Bsh and Bs horizon had generally faster TT than occluded particulate organic matter (OPOM), possibly because of sorption of dissolved organic carbon by iron and aluminium oxides/hydroxides. Our results suggest that organic horizons with relatively short turnover times could be particularly vulnerable to changes in climate or other disturbances.     

Effect of CaCl2 on the kinetics of dissolved organic matter release from a sandy soil�

The dissolution of organic matter in soil is of fundamental relevance for the fate of organic contaminants associated with organic matter and for the microbial availability of organic matter. In this study, the kinetics of soil organic matter (SOM) dissolution from a sandy forest soil was investigated under different electrolyte conditions, using a continuous extraction method. The mathematical analysis of the concentration signal obtained from extractions with constant flow rates and after sudden flow rate changes showed that the dissolution of SOM is diffusion limited. The dissolution rate was lower during extraction with 0.01 M CaCl₂. The reaction on sudden flow rate changes was slower when extracting with 0.01 M CaCl₂ as compared to water, and the mechanism was different. These observations were explained by a gel phase developing in the swelling SOM. The lower dissolution rates found for extractions with 0.01 M CaCl₂ could indicate a more stable gel structure in the presence Ca²⁺. The development of the gel phase may be influenced by mechanical strain due to increased flow rates.     

Electron transfer capacity of soil dissolved organic matter and its potential impact on soil respiration

Purpose

Soil dissolved organic matter (DOM) as the labile fraction of soil organic carbon (SOC) is able to facilitate biogeochemical redox reactions effecting soil respiration and carbon sequestration. In this study, we took soil samples from 20 sites differing in land use (forest and agriculture) to investigate the electron transfer capacity of soil DOM and its potential relationship with soil respiration.

Materials and methods

DOM was extracted from 20 soil samples representing different land uses: forest (nos. 1–12) and agriculture (nos. 13–20) in Guangdong Province, China. Chronoamperometry was employed to quantify the electron transfer capacity (ETC) of the DOM, including electron acceptor capacity (EAC) and electron donor capacity (EDC), by applying fixed positive or negative potentials to a working electrode in a conventional three-electrode cell. The reversibility of electron accepting from or donating to DOM was measured by applying switchable potentials to the working electrode in the electrochemical system with the multiple-step potential technique. Carbon dioxide produced by soil respiration was measured with a gas chromatograph.

Results and discussion

Forest soil DOM samples showed higher ETC and electron reversible rate (ERR) than agricultural soil DOM samples, which may be indicative of higher humification rate and microbial activity in forest soils. The average soil respiration of forest soil (nos. 1–12) and agricultural soil (nos. 13–10) was 26.34 and 18.58 mg C g^?1 SOC, respectively. Both EDC and EAC of soil DOM had close relationship with soil respiration (p?<?0.01). The results implied that soil respiration might be accelerated by the electroactive moieties contained in soil DOM, which serve as electron shuttles and facilitate electron transfer reactions in soil respiration and SOC mineralization.

Conclusions

DOM of forest soils showed higher ETC and ERR than DOM of agricultural soils. As soil represents one of the largest reservoirs of organic carbon, soil respiration affects C cycle and subsequently CO₂ concentration in the atmosphere. As one of the important characteristics of soil DOM related to soil respiration, ETC has a significant impact on greenhouse gas emission and soil carbon sequestration but has not been paid attention to.     

Straw management and tillage effects on soil water storage under field conditions

Abstract. A two year field study was conducted to evaluate the effects of straw management and tillage on the soil profile (1.5m) water storage, nature of the moisture profile, infiltration and sorptivity as influenced by rainfall, evaporativity (E₀) and soil texture. The straw mulch treatment stored more moisture under low E₀ rainy conditions in three coarse to medium textured soils. Straw incorporation treatment was better under low E₀ rain free conditions, as well as under high E₀ rainy conditions in the two coarser textured soils. In the coarsest textured soil, tillage and straw mulching were not effective in maintaining greater soil water storage under high E₀ because of the very open nature of the soil. The soil moisture profiles showed a sharper increase in water content below the tilled layer in the tillage and straw- incorporation treatments than the untreated and straw mulch treatments. Tillage and straw incorporation treatments increased the sorptivity of the soil compared with untreated and straw mulch treatments respectively. The results of this study suggest that when selecting a suitable soil water conservation practice to increase water storage in the soil profile, information on soil texture and weather (rain and evaporativity) must be considered.     

Bioaugmentation and biostimulation effects on PAH dissipation and soil ecotoxicity under controlled conditions

The degradation of spiked anthracene (ANT), pyrene (PYR) and benzo[a]pyrene (B[a]P) in soil (3000 mg ∑ 3 PAHs kg⁻¹ dry soil) was studied in aerobically incubated microcosms for 120 d. The applied treatments aimed at enhancing PAH removal from the heavily contaminated soils are: (i) bioaugmentation by adding aged PAH-contaminated soil (ACS) containing activated indigenous degraders; and (ii) combined bioaugmentation/biostimulation by incorporating sewage sludge compost (SSC) and decaying rice straw (DRS). The adopted treatments produced higher PAH dissipation rates than those observed in unamended PAH-spiked soils, especially for ANT and PYR in the presence of DRS or ACS (>96%). However, B[a]P was the most recalcitrant hydrocarbon to biodegradation. Extracellular enzyme investigation revealed the existence of ligninolytic activities in all soil treatments, including control but no relationship could be found with PAH dissipation. The ecotoxicological assessment indicated that regardless of applied treatment, PAH-spiked soils were chronically lethal to ostracod Heterocypris incongruens and confirmed the sensitivity of the microcrustacean to the concomitant presence of these three hydrocarbons. Lettuce root elongation inhibition was correlated with PAH level but the presence of SSC conferred a strong phytotoxic capacity to PAH-spiked soils. DRS amendment may constitute a cost-effective alternative for hydrocarbon bioremediation as it has impacted positively on soil microbial activity and enhanced PAH removal with no apparent changes in soil physico-chemical properties.     

Quantifying soil water effects on nitrogen mineralization from soil organic matter and from fresh crop residues

A loamy sand was incubated with and without addition of carrot leaves at six different water contents ranging from 6% to 20% (g 100 g^-1 dry soil) and N mineralization was monitored during 98 days. We calculated zero- and first-order rates for mineralization in the unamended soil and first-order rates for N mineralization in the residue-amended soil. Although N mineralization was strongly affected by soil moisture, rates were still important at 6% water content (corresponding to permanent wilting point), particularly in the residue-amended soil. Soil water content was recalculated as soil water tension and as percent water-filled pore space (%WFPS) and a parabolic, a logistic and a Gaussian-type function were fitted to the relation between N mineralization rates and water content, %WFPS or pF. Water potential was a less suitable parameter than either %WFPS or water content to describe the soil water influence on N mineralization, because N mineralization rates were extremely sensitive to changes in the water potential in the range of pF values between 1.5 and 2.5. In the residue-amended soil the Gaussian model yielded an optimum %WFPS of 56% for N mineralization, which is slightly lower than optimum values cited in literature. N mineralization in the unamended soil was more influenced by soil water than N mineralization from fresh crop residues. This could be explained by less water limitation of the microbial population decomposing the residues, due to the water content of the residues. The effect of the water contained in the residues was most pronounced in the lowest water content treatments. The water retention curves of both undisturbed and repacked soil were determined and suggested that extrapolation of results obtained during laboratory incubations, using disturbed soil, to field conditions will be difficult unless soil bulk density effects are accounted for, as is the case with the use of %WFPS.     

The quality of exogenous organic matter: short-term effects on soil physical properties and soil organic matter fractions

We examined the short-term effect of five organic amendments and compared them to plots fertilized with inorganic fertilizer and unfertilized plots on aggregate stability and hydraulic conductivity, and on the OC and ON distribution in physically separated SOM fractions. After less than 1 year, the addition of organic amendments significantly increased ( P  <   0.01) the aggregate stability and hydraulic conductivity. The stability index ranged between 0.97 and 1.76 and the hydraulic conductivity between 1.23 and 2.80 × 10⁻³ m/s for the plots receiving organic amendments, compared with 0.34–0.43, and 0.42–0.64 × 10⁻³ m/s, respectively, for the unamended plots. There were significant differences between the organic amendments (P <  0.01), although these results were not unequivocal for both soil physical parameters. The total OC and ON content were significantly increased ( P  <   0.05) by only two applications of organic fertilizers: between 1.10 and 1.51% OC for the amended plots versus 0.98–1.08% for the unamended and between 0.092 and 0.131% ON versus 0.092–0.098% respectively. The amount of OC and ON in the free particulate organic matter fraction was also significantly increased ( P  <   0.05), but there were no significant differences ( P  <   0.05) in the OC and ON content in the POM occluded in micro-aggregates and in the silt + clay-sized organic matter fraction. The results showed that even in less than 1 year pronounced effects on soil physical properties and on the distribution of OC and ON in the SOM fractions occurred.     

控制条件下水肥耦合对黄泥田还田秸秆腐解及土壤碳转化的影响

【目的】研究秸秆还田后不同水温和肥剂管理措施下土壤碳素转化特征。【方法】以华中双季稻区低产水稻土黄泥田为供试材料,模拟早稻和晚稻秸秆还田的田间环境,在实验室控制条件下,开展了两种温度环境中(15℃、35℃)不同水分(40%和100%最大田间持水量,即40%WHC、100%WHC)、配施氮肥类型(尿素、猪粪即U、M)、以及促腐菌剂添加对秸秆腐解效果及其过程中土壤碳素转化影响的研究。对水稻秸秆腐解过程中土壤CO2释放量、以及土壤可溶性有机碳(DOC)和总有机碳(TOC)含量在105天培养周期内变化特征进行动态监测分析。【结果】两种温度环境中整个培养周期内,各处理的CO2释放速率和释放总量通常表现为100%WHC-M100%WHC-U40%WHC-M40%WHC-U,即猪粪优于尿素的规律,而不论配施何种氮肥都存在100%WHC40%WHC(P0.01)的现象,同时40%WHC条件下辅施菌剂可显著提升CO2释放量;与此相反,两种温度环境下DOC含量都表现为40%WHC-M40%WHC-U100%WHC-M100%WHC-U(后两者差异小),即40%WHC条件下DOC含量显著高于100%WHC(P0.05),且配施猪粪处理优于配施尿素处理,但这两种氮肥处理间差异随培养时间延长而减小;以CO2-C释放量计算0 7 d、0 28 d、0 105 d内物料分解率,结果表明,35℃时100%WHC-U的处理中物料分解最快,15℃时40%WHC-M的处理中物料分解最慢。与之对应,105 d内TOC含量和净增量则在35℃时100%WHC-U的处理中最小(P0.01),而在15℃时40%WHC-M的处理中最大(P0.01);TOC的净增量和净损失量在相同温度条件下,尤其试验前期不同水分(P0.01)、氮素(P0.05)间均存在显著差异,且促腐菌剂添加普遍减小TOC含量;培养周期内所有处理的CO2释放速率与DOC含量间存在显著相关(P0.05)。【结论】水分状况对碳素的转化存在极大影响,其次是氮肥类型,且氮肥的影响作用随秸秆还田时间的延长而减弱;高湿条件更利于促进秸秆腐解,但导致土壤DOC含量较低,TOC的固持量也较少,而配施猪粪则可促进土壤DOC含量的提升及TOC的固持;促腐菌剂添加可促进秸秆腐解,但由于40%WHC条件下显著激发了CO2的释放而不利于土壤固碳。因此在华中低产黄泥田双季轮作稻区,早稻还田时由于气温高周期短,建议保持100%WHC、辅施适量尿素、并配合添加秸秆腐解菌剂,侧重秸秆快腐;而晚稻还田时气温低周期长,建议保持40%WHC并辅施缓效猪粪,侧重土壤固碳。