Short-term temporal changes of soil carbon losses after tillage described by a first-order decay model

Tillage stimulates soil carbon (C) losses by increasing aeration, changing temperature and moisture conditions, and thus favoring microbial decomposition. In addition, soil aggregate disruption by tillage exposes once protected organic matter to decomposition. We propose a model to explain carbon dioxide (CO₂) emission after tillage as a function of the no-till emission plus a correction due to the tillage disturbance. The model assumes that C in the readily decomposable organic matter follows a first-order reaction kinetics equation as: dC_sail(t)/dt = −kC_soil(t) and that soil C-CO₂ emission is proportional to the C decay rate in soil, where C_soil(t) is the available labile soil C (g m⁻²) at any time (t). Emissions are modeled in terms soil C available to decomposition in the tilled and non-tilled plots, and a relationship is derived between no-till (F_NT) and tilled (F_T) fluxes, which is: F_T=a₁F_NT e^−a₂t, where t is time after tillage. Predicted and observed fluxes showed good agreement based on determination coefficient (R²), index of agreement and model efficiency, with R² as high as 0.97. The two parameters included in the model are related to the difference between the decay constant (k factor) of tilled and no-till plots (a₂) and also to the amount of labile carbon added to the readily decomposable soil organic matter due to tillage (a₁). These two parameters were estimated in the model ranging from 1.27 and 2.60 (a₁) and −1.52 × 10⁻² and 2.2 × 10⁻² day⁻¹ (a₂). The advantage is that temporal variability of tillage-induced emissions can be described by only one analytical function that includes the no-till emission plus an exponential term modulated by tillage and environmentally dependent parameters.     

Relationship of host recurrence in fungi to rates of tropical leaf decomposition

Here we explore the significance of fungal diversity on ecosystem processes by testing whether microfungal ‘preferences’ for (i.e., host recurrence) different tropical leaf species increases the rate of decomposition. We used pairwise combinations of γ-irradiated litter of five tree species with cultures of two dominant microfungi derived from each plant in a microcosm experiment. The experiment was designed to test whether early leaf decomposition rates differed depending on relationships between the leaf litter from which the fungi were derived (i.e., the source plant) and the leaf substrata decomposed by these fungi in microcosms. Relationships tested were phylogenetic relatedness between the source and substratum leaves, and similarity in litter quality (lignin, N and P) between the source and substratum. We found a significant interaction between microfungi and leaf species (P<0.0001), and differences among the four classes of source–substratum relationships were highly significant (P=0.0004). Combinations in which fungal source leaves were of the same species or family as the substratum, or the fungal source resembled the substratum in quality had marginally faster decomposition than when the fungal source and substratum leaves were mismatched (i.e., unrelated and of dissimilar quality). In some microcosms, a basidiomycete contaminant had a strong additive effect on decomposition of Croton poecilanthus leaves resulting in faster decomposition than with microfungi alone (P<0.0001). Comparisons among leaf–microfungal combinations were made after the effect of the basidiomycete covariate was adjusted to zero. The data on microfungi suggest differential abundance in particular hosts, which contributes to species diversity of decomposer fungi in tropical forests, affects rates of decomposition.     

Comparison of concentrated-flow detachment equations for low shear stress

Several equations exist to describe the relationship between concentrated-flow detachment and shear stress (τ). However, the advantages and disadvantages of these equations for specific circumstances remain unclear. This study examines the performance of linear and power equations with and without a critical shear stress (τ_c) term for concentrated-flow detachment at low shear stress. Equations were fit to data collected from experiments on five midwestern US soils using flume experimental data at low shear stress levels. Field experimental data were also available for these soils. The linear equation was simple to use and parameter values were easily estimated with linear regression. However, significant lack of fit was found when the linear equation was applied to data collected from low to medium shear stress levels. The value of soil erodibility (K) for a soil varied by a factor of 3 and critical shear stress (τ_c) varied by a factor of 2.5. The linear equation prediction underestimated detachment (D) by 25% at high shear stress and overestimated detachment by 30% at low shear stress. In contrast, the power equations gave more stable erodibility parameters because these equations reduced the systematic nature of the observation residuals found with the linear equation. Correlation between rill detachment D and τ was generally lower with the linear compared to the power equations for conditions tested. For higher shear stresses and longer slopes, the linear equation may be acceptable where field experiments show a linear trend. It is suggested that τ_c only be used when it has a value significantly different from zero.     

A multi-step inflow method for estimating hydraulic conductivity at low pressure under the wetting process

The accurate determination of the wetting soil hydraulic properties for a wide range of water contents is essential for studying and predicting infiltration processes. We present a laboratory infiltration method for determining hydraulic conductivity function, K(θ), in the low-to-medium water content range. An initially air-dry soil core is subjected to infiltration from the bottom where the pressure head, ψ_bot, is controlled through a membrane. As soon as the wetting front arrives at the soil surface, the top 0.5-cm layer is sliced for measurement of the water content. The ψ_bot is stepwise increased as the hydraulic equilibrium is nearly attained at each step. The wetting water retention function, ψ(θ), is determined by curve-fitting the equilibrium inflow data and from the independently measured data obtained from vapor equilibrium. The parameter in the K(θ) is estimated inversely using the cumulative inflow and water content of the sliced layer. This method is verified through comparisons with K(θ) obtained by the Boltzmann transform method. Although requiring an additional operation, the slicing procedure is found to be valuable in enhancing the reliability of the optimized parameter. A sensitivity analysis shows that water vapor movement would be negligible under our experimental conditions.     

Study of ephemeral gully erosion in a small upland catchment on the Inner-Mongolian Plateau

Ephemeral gully erosion is an important soil erosion process on the Inner-Mongolia Plateau in North China, and although its damage is very intense, little research on the area has been published. In this paper, a global positioning system (GPS) is used to measure the morphology of ephemeral gullies in a small catchment, the Inner-Mongolia Autonomous Region. First, this paper presents the characteristics of ephemeral gullies and soil loss due to ephemeral gully erosion. The network of ephemeral gullies takes on the shapes of tree branches, and there are 16 hole-ephemeral gullies in the middle of the ephemeral gullies. An average gully length of about 19.6 m ha⁻¹ and an average soil loss of 8.8 m³ ha⁻¹ due to ephemeral gully erosion were measured. Second, soil erosion influences crop production in cropland and combinations of vegetation in fallow. The difference between vegetation in the middle of ephemeral gullies and in other places is very obvious. Third, this paper discusses hole-ephemeral gullies that are holes locating in the middle of ephemeral gullies whose widths and depths are more than 0.5 m (Fig. 6) for the first time. The relationship between local hill slope gradient S (m m⁻¹) and upslope contributing area A (ha) can be expressed as S = 0.064A^−0.375 and may be a key indicator for determining the position of existing hole-ephemeral gully heads and for predicting where hole-ephemeral gullies could form in the small watershed on the Inner-Mongolian Plateau.     

硫酸根硫和单质硫对青菜硫素营养的影响

为探究施用硫酸根硫(SO42--S)和单质硫(S⁰-S)对土壤有效硫含量及青菜硫素营养的影响,以矮脚黄青菜品种为材料,采用小区试验,分析不同类型硫肥(硫酸钾和硫磺粉)处理对供试土壤pH值、土壤有效硫含量、青菜产量及硫素营养含量的影响。结果表明,施用硫酸钾(SO42--S)和硫磺粉(S⁰-S)均降低了弱碱性土壤pH值,增加了土壤有效硫含量,且施用SO42--S的影响较S⁰-S更明显。施用SO42--S和S⁰-S均可有效提高青菜株高、可食部分干重和含硫量,均在106 g·m^-2处理水平下达到最大值,且施用SO42--S的提升作用优于S⁰-S。53 g·m^-2SO42--S处理水平下,与对照相比,青菜可食部分硝酸盐含量下降了38.6%,可溶性蛋白、可溶性糖和维生素C含量分别增加了115.1%、120.5%和21.7%。综上,适度施用SO42--S肥料有利于改善弱碱性缺硫土壤物理化学性质,提高青菜产量和营养品质。本研究结果为缺硫地区青菜优质优产种植提供了科学依据。     

Soil carbon and nitrogen dynamics using stable isotopes in 19- and 10-year-old tropical agroforestry systems

Conversion of forests to agricultural land in the American tropics, through traditional agricultural practices such as shifting cultivation, has not been able to maintain stocks of soil organic carbon (SOC), and increasing population pressure has led to shortened fallow periods, causing further losses of soil fertility. However, land management practices such as agroforestry can provide a sustainable alternative to single cropping because of its ability to maintain or increase the SOC pool. This study quantified SOC and nitrogen (N) pools, gross SOC turnover, residue stabilization efficiency (RSE_AC) in the alley crop, soil δ¹³C partitioning, C₃-C abundance and δ¹⁵N dynamics in 19- and 10-year Gliricidia sepium and Erythrina poeppigiana alley cropping system. Each system was studied at two fertilizer levels (tree prunings only [−N or −A], and tree prunings plus chicken manure [+N], or Arachis pintoi as a groundcover [+A]), and was compared to a sole crop system. The SOC and N pools were significantly higher (p < 0.05) in the 19-year-old alley crop compared to the sole crop, but not significantly different (p < 0.05) in the 10-year-old system. Soil C and N (%) showed a similar trend as that of the SOC and N pools in both 19- and 10-year-old systems. Gross SOC turnover, to a 20 cm depth, ranged from 12 to 21 years in the 19-year-old alley crop compared to 50 years in the sole crop, and from 20 to 32 years in the 10-year-old alley crop compared to 106 years in the sole crop. The RSE_AC ranged from 10% to 58% in the 19-year-old system, and from 3% to 43% in the 10-year-old system. The δ¹³C signature of the soil shifted significantly (p < 0.05) towards that of C₃ vegetation in the alley crop due to the greater input of organic residues from tree prunings compared to the sole crop. The proportion of input from tree prunings only in the 19-year-old alley crop ranged from 14% to 20%, and from 9% to 11% in the 10-year-old system to a soil depth of 20 cm. The δ¹⁵N signature of the soil showed two patterns: that of the 19-year-old system being enriched in δ¹⁵N, and that of the 10-year-old system being depleted in δ¹⁵N compared to the sole crop. The addition of manure in the 19-year-old system has enriched the soil δ¹⁵N and in the 10-year-old system the soil was depleted due to the N₂-fixing groundcover A. pintoi.     

Interaction between gas exchange rates, physical and microbiological properties in soils recently subjected to agriculture

The impact of conventional tillage (CT) or no-till (NT) management on soil microbial respiration as well as microbial abundance was studied in soils from the El Salado basin river (Buenos Aires, Argentina) recently subjected to agriculture under a corn-pasture rotation since 1996. Both management systems were monitored for several soil (micro)biological, physical and chemical properties during the second (1997) to fourth (1999) years from the beginning of the experiment. O₂ and CO₂ composition of the soil atmosphere and the rate at which soil consumes O₂ (qO₂) or produces CO₂ (qCO₂), under conditions that approximate the soil environment in the field, were quantitated following an experimental method and a mathematical model developed by ourselves [Soil Sci. 166 (2001) 68] to interpret the data. qO₂ and qCO₂ expressed in terms of kg O₂ or CO₂-C per ha per day or per kg C of microbial biomass (microbial respiration), increased from the lowest values measured at 10–30% water-filled pore space (WFPS) up to 60% WFPS, decreasing thereafter. Low respiratory quotients, RQ (qCO₂/qO₂<1.0), were detected, with gas exchanges being slightly higher in NT than in CT. Correspondingly, higher bacterial and fungal biomass were measured in NT than in CT. Apparently, bacteria were more sensitive to high WFPS than fungi. When aerobic bacteria or fungi counts were compared at low or high WFPS, they differed significantly only in the upper soil profile whereas microaerophilic bacteria and fungi were significatively different in both depths tested (D1=5–10 cm; D2=15–20 cm). The results are discussed in terms of microbial metabolism behavior and abundance as a function of management and soil air/water balance in soils recently subjected to agriculture.     

壳寡糖锌配合物对氧化衰老模型小鼠的抗氧化作用

为探讨壳寡糖-锌配合物(COS-Zn²⁺)对氧化衰老模型小鼠的抗氧化作用,在体外条件下测定COS-Zn²⁺对O2·-、DPPH自由基的清除能力和Fe³⁺的还原能力,并通过D-半乳糖注射法制备氧化衰老小鼠模型,进而评价COS、ZnSO₄、COS+ZnSO₄及COS-Zn²⁺配合物对模型小鼠血清及各组织中丙二醛(MDA)含量、总抗氧化能力(T-AOC)、超氧化物歧化酶(SOD)、谷胱甘肽过氧化物酶(GSH-Px)和过氧化氢酶(CAT)活性的影响。结果表明,COS-Zn²⁺对O2·-清除作用显著优于COS、ZnSO₄及二者混合物组(P<0.05);当COS-Zn²⁺浓度为10 mg·mL^-1时,其对DPPH自由基清除能力显著优于其他试验组(P<0.05);COS-Zn²⁺分子中由于-NH₂、-OH等活性基团结合了Zn²⁺,故表现出相对较弱的Fe³⁺还原能力。动物模型试验结果显示,与正常组相比,模型组小鼠体重及脏器指数均发生了显著性变化(P<0.05),COS-Zn²⁺给药对小鼠体重增加和脏器指数恢复具有一定的帮助作用,且有效降低了模型小鼠体内MDA含量,显著提高了其机体血清、肾脏和肝脏中的SOD、GSH-Px、CAT活性和T-AOC能力,尤其对血清中相关保护酶活性的改善作用显著优于COS、ZnSO₄及COS+ZnSO₄组(P<0.05)。COS-Zn²⁺表现出较好的体外抗氧化活性,同时能显著增强小鼠对氧化损伤的拮抗能力。综上,COS-Zn²⁺配合物具有较好的抗衰老及增强机体免疫功能作用。本研究结果为海洋壳寡糖及其金属修饰物的活性开发及功能应用提供了一定的理论基础与技术参考。     

A new approach to calculate the particle density of soils considering properties of the soil organic matter and the mineral matrix

The particle density of soil (ρ_S) represents one of the soil's basic physical properties and it depends on the composition of both the mineral and the organic soil components. It therefore varies for different soils, e.g. within the group of mineral soils, and ranges from 2.4–2.9 g cm⁻³. Hence, awareness of this variability is important for properties estimated by a calculation involving particle density. Because ρ_S depends on both the soil's solid mineral particles and soil organic matter composition, we derived a function based on the mixture ratio of these two soil components. This approach represents a further development of earlier investigations dealing with the influence of organic carbon (C_org) on ρ_S. To parameterise this function, two data sets were used: (1) data from soils with C_org contents between 0% and 54.88% and corresponding values of ρ_S between 1.49 and 2.72 g cm⁻³; and (2) data from soils of 17 German long-term experiments contrasting in soil texture and in soil mineral inventory. Data set 1 was used to quantify the influence of soil organic matter on ρ_S, and data set 2 was used to calculate the influence of mineral matrix on ρ_S. The soil organic matter has two major influences on ρ_S: (1) via a mass effect (expressed as a mixture ratio between organic and mineral soil components); and (2) via a quality effect (expressed as calculated changes in particle density of organic soil components). Here, we calculated that with increasing content of soil organic matter (0–100%), the particle density of organic soil components rose from about 1.10 to 1.50 g cm⁻³, and present possible reasons for this phenomenon. Additionally, we demonstrate that the mineral matrix of the soil affects ρ_S especially via variations in the mineral inventory, but conclude that differences in particle size distribution of soils were to a lesser extent suitable for describing the influence of the mineral matrix on ρ_S. Overall, using our approach should generate more realistic values of ρ_S, and consequently of all calculated parameters which are sensitive to ρ_S.     

烟嘧磺隆胁迫对甜玉米幼苗活性氧积累、抗氧化系统及相关基因表达的影响

为减轻烟嘧磺隆对甜玉米(Zea mays L. seccharata Sturt)的药害作用,探究甜玉米幼苗抗氧化系统对烟嘧磺隆胁迫的响应机制,本研究以一对甜玉米姊妹系(对烟嘧磺隆表现耐药性的甜玉米自交系HK301和对烟嘧磺隆表现敏感的甜玉米自交系HK320)为试材,研究烟嘧磺隆胁迫对不同耐药性甜玉米品种氧化压力、抗氧化酶、抗氧化系统非酶类物质和抗氧化酶关键基因表达量的影响。结果表明,烟嘧磺隆胁迫后,与对照(HK301-CK)相比,HK301的超氧阴离子自由基( {\mathrm{O}}_2^{\frac{-}{·}} )产生速率、过氧化氢(H₂O₂)含量和丙二醛(MDA)含量先增加后减少,超氧化物歧化酶(SOD)、过氧化氢酶(CAT)、抗坏血酸过氧化物酶(APX)、单脱氢抗坏血酸还原酶(MDHAR)、脱氢抗坏血酸还原酶(DHAR)、谷胱甘肽还原酶(GR)活性均呈先升高后降低的趋势,抗坏血酸(AsA)含量、脱氢抗坏血酸(DHA)、还原型谷胱甘肽(GSH)含量先增加后减少,氧化型谷胱甘肽(GSSG)含量持续增加,抗坏血酸氧化还原状态(AsA/DHA比值)显著降低,谷胱甘肽氧化还原状态(GSH/GSSG比值)无显著差异。与对照(HK320-CK)相比,HK320的 {\mathrm{O}}_2^{\frac{-}{·}} 产生速率和H₂O₂含量显著增加, CAT、APX、DHAR、GR活性先升高后下降,SOD、MDHAR活性和GSH含量、GSH/GSSG 比值、AsA/DHA 比值呈下降趋势,DHA含量持续增加,GSSG含量无显著差异。烟嘧磺隆胁迫后,相较于HK320,HK301显著上调了SOD、CAT、APX、MDHAR、DHAR和GR基因的相对表达量。综合分析显示,不同耐药性甜玉米幼苗通过调节体内抗氧化物质含量和酶活性来清除活性氧,从而提高幼苗对除草剂的耐药性。本研究为揭示烟嘧磺隆胁迫下甜玉米幼苗抗氧化系统的代谢机制提供了理论依据。     

大豆鼓粒期叶片荧光参数与叶绿素含量的关系

叶绿素含量是研究大豆光合作用的重要生理指标,对大豆产量和品质具有重要影响,因此,进行叶绿素含量和光合功能的同步分析具有重要意义。本研究以鼓粒期大豆叶片为研究对象,对63个样本的叶绿素含量和荧光参数进行相关分析,建立回归模型,并用28个样本验证集进行验证及评价。结果表明,可变荧光与初始荧光比(F_v/F_o)、最大光化学效率(φ_PO)、初始时间点活性反应中心捕获的单个激子驱动除QA外的电子传递的效率(ψ_EO)、初始时间点用于电子传递的量子产额(φ_EO)、t=t_FM时单位面积内电子传递的量子产额(ET_O/CS_M)、性能指数(PI_ABS)、J点相对可变荧光(V_J)7个荧光参数与叶绿素含量相关性较好,相关系数分别为0.78、0.76、0.75、0.80、0.82、0.77、-0.75。回归模型方程为y=-0.138x₁+ 2.154x₂+0.002x₃+0.077x₄+0.076(R²=0.694)(x₁、x₂、x₃、x₄和y分别为F_v/F_o、φ_PO、ET_O/CS_M、 PI_ABS和叶绿素含量),验证模型决定系数(R²)=0.805 8,均方根误差(RMSE)=0.293 4,预测残差(RPD)=1.773 8,该模型具有较好的预测效果,可以丰富非生物逆境胁迫下无损监测大豆叶绿素估算方法。回归和通径分析发现,ET_O/CS_M对叶绿素含量直接作用最大,φ_PO次之,F_v/F_o对叶绿素含量起直接负作用,PI_ABS直接作用最小,直接通径系数分别为0.706、0.382、-0.303、0.078。本研究实现了大豆叶绿素含量与光合功能的同步分析,明确了荧光参数对叶绿素含量的影响效应,可为调节栽培措施,实现大豆高产提供理论基础。     

Rice roots and CH4 oxidation: the activity of bacteria, their distribution and the microenvironment

Irrigated rice fields account for 10–30% of global methane emissions. Rice plants ventilate the soil and enlarge the oxic–anoxic interface by their root system, thus supplying the necessary O₂ to aerobic CH₄ oxidizing bacteria (MOB). Rice plants (Oryza sativa type japonica var. Roma) were grown in microcosms in a greenhouse. The roots were sandwiched between two blocks of flooded rice field soil separated by a nylon gauze bag. A root mat developed which mimicked the dense root texture in the upper layer of a natural rice field. Flux measurements under oxic and anoxic conditions showed that CH₄ was oxidized with a constant rate of 19% of the anoxically emitted CH₄, suggesting that CH₄ oxidation in the rhizosphere was at least sometimes limited by CH₄ availability. Washed rice roots could both produce and oxidize CH₄, depending upon incubation conditions. CH₄ production by washed rice roots accounted for at most 10% of the CH₄ emitted under anoxic conditions. Initial CH₄ oxidation rates of washed roots equaled oxidation rates calculated from the difference between oxic and anoxic fluxes in situ. Oxidation rates became twice as high after an induction period of 20 h, indicating a limitation by O₂ or CH₄ in situ. The micro-environmental conditions near to the root mat were measured using microelectrodes for O₂, redox potential and NH₄⁺ and diffusion probes for CH₄. Up to 42 μM O₂ was detected in the root mat and concentrations were >2.5 μM in 45% of all measurements. In the bulk soil, no O₂ was detected below 2 mm depth, but the root mat significantly increased the redox potential. Plant roots and associated bacteria decreased porewater CH₄ and NH₄⁺ concentrations. In the root mat, concentrations of dissolved CH₄ were below the detection limit of our probes (<5 μM). Cell numbers of MOB increased with time in the rhizosphere and in the rhizoplane. MOB and aerobic heterotrophic bacteria (AHB) each numbered from 10⁶ to 10⁸ cells g⁻¹ dry weight of soil or root biomass). Active MOB occurred near to a root mat similar to the dense root texture in the upper layer of rice fields. We speculate about O₂ or CH₄ limitation of MOB.     

Soil quality indicators under continuous cropping systems in the Argentinean Pampas

Over the last two decades, soil cultivation practices in the southern Argentinean Pampas have been changing from a 7 year cash-crop production system alternated with 2–3 years under pasture, to a continuous cropping system. A better understanding of the impact of the period of time a field has been under continuous cropping on a broad spectrum of soil properties related to soil quality is needed to target for sustainable cropping systems. The objectives of this study were to: (i) assess the relationship between physical and chemical soil parameters related to soil quality and (ii) identify soil quality indicators sensitive to soil changes under continuous cropping systems in the Argentinean Pampas.

Correlation analysis of the 29 soil attributes representing soil physical and chemical properties (independent variables) and years of continuous cropping (dependent variable) resulted in a significant correlation (p < 0.05) in 78 of the 420 soil attribute pairs. We detected a clear relationship between hydraulic conductivity at tension h (K_h) and structural porosity (ρ_e); ρ_e being a simple tool for monitoring soil hydraulic conditions.

Soil tillage practice (till or no-till) affected most of the soil parameters measured in our study. It was not possible to find only one indicator related to the years under continuous cropping regardless of the cultivation practice. We observed a significant relationship between years under continuous cropping and K_h under no-till (NT) and wheat fallow (p < 0.001, R² = 0.70). Under these conditions, K₋₄₀ diminished as the number of years under continuous cropping increased.

The change in mean weight diameter (CMWD) was the only physical parameter related to the number of years under continuous cropping, explaining 36% of the variability in the number of years under continuous cropping (p < 0.001) The combination of three soil quality indicators (CMWD, partial R² = 0.38; slope of the soil water retention curve at its inflexion point (S), partial R² = 0.14 and cation exchange capacity (CEC), partial R² = 0.13) was able to explain, in part, the years under continuous cropping (R² = 0.65; p value > 0.001), a measure related to soil quality.     

Soil macropore dynamics affected by tillage and irrigation for a silty loam alluvial soil in southern Portugal

Soil tillage can have a significant effect on soil porosity and water infiltration. This study reports field measurements of near saturated hydraulic conductivity in an undisturbed soil under two tillage treatments, conventional tillage (CT) and minimum tillage (MT). The objective was to determine effective macro and mesoporosities, porosity dynamics during the irrigation season, and their contribution to water flow. Field observations were performed during the 1998 maize (Zea mays L.) cropping season in an Eutric Fluvisol with a silty loam texture, located in the Sorraia River Watershed in the south of Portugal. Infiltration measurements were done with a tension infiltrometer. At each location an infiltration sequence was performed corresponding to water tensions (φ) of 0, 3, 6 and 15 cm. Five sets of infiltration measurements were taken in both treatments in the top soil layer between May and September. One set of measurements was done at the depth of 30 cm at the bottom of the plowed layer in the CT plot. After 5 years of continuous tillage treatments the results show that regardless of the tillage treatment, saturated conductivity values K(φ₀) were several times larger than near saturation conductivity K(φ₃). This indicates that subsurface networks of water conducting soil pores can exist in both CT and MT maize production systems. In CT, the moldboard plow created macro and mesoporosity in the top soil layer while breaking pore continuity at 30 cm depth. This porosity was partially disrupted by the first irrigation, resulting in a significant decrease of 45% in the macropore contribution to flow. Later in the season, the irrigation effect was overlaid by the root development effect creating new channels or continuity between existing pores. In MT macroporosity contribution to flow did not show significant differences in time, representing 85% of the total flow. In both the treatments, macropores were the main contributing pores to the total flow, in spite of the very low macroporosity volumes.     

Long-term effects of lantana (Lantana spp. L.) residue additions on soil physical properties under rice–wheat cropping: I. Soil consistency, surface cracking and clod formation

Poor soil tilth is a major constraint in realizing optimum yield potential of wheat (Triticum aestivum L.) in rice (Oryza sativa L.)–wheat cropping system. The effect of long-term additions of lantana (Lantana spp. L.) biomass, a wild sage, on physical properties of a silty clay loam soil under rice–wheat cropping was studied in north-west India. Lantana was added to soil 10–15 d before puddling at 10, 20 and 30 Mg ha⁻¹ yr⁻¹ (fresh weight). At the end of 10th rice crop, liquid limit, plastic limit, shrinkage limit and plasticity index of soil increased significantly with lantana additions. The friability range of lantana-treated soil decreased from 8.9 to 7.8–8.2% gravimetric-moisture content, but soil became friable at relatively higher moisture content. Soil cracking changed from wide, deep cracks in hexagonal pattern to a close-spaced network of fine cracks. The cracks of sizes <5 mm increased, 10–20 mm and wider decreased, while 5–10 mm remained almost unchanged with lantana additions. The volume density of cracks decreased by 36–76% and surface area density by 19–37% compared with control. The clods of sizes <2 cm diameter increased, while 2–4 cm and 4–6 cm diameter decreased with lantana additions. The MWD of clods varied between 2.15 and 2.34 cm in lantana-treated soil as against 2.83 cm in the control. The bulk density and breaking strength of soil clods were lower in lantana-treated soil by 4–9% and 29–42% than in the control. About 23–47% less energy was required to prepare seed-bed in lantana-treated than in the control soil.     

A simple equation to determine the breakdown of individual aggregate size fractions in the wet-sieving method

Wet-sieving method using nested sieves is one common method to measure aggregate stability. However, this method cannot be used to measure the stability of individual aggregate size fractions, only of whole soils. Thus, this study was to develop an equation to estimate the aggregate breakdown of individual aggregate size fractions in this particular method. The key to develop the equation was to assume that aggregate breakdown happens sequentially and consistently, and that the aggregate breakdown between any two aggregates in the same aggregate size fractions is equal in percentage. Applying these two assumptions, this equation was developed: x_i=(W_ai×D_i)/(W_ai+D_i−1), where x_i is the weight of aggregate breakdown in aggregate size fraction i, W_ai is the weight of the aggregates in aggregate size fraction i before wet-sieving, and D_i and D_i−1 are the weight of aggregates that have passed through sieve i and i−1, respectively. This equation was tested with five soil series. The soils were separated into six aggregate size fractions: 4.76–8.0, 2.83–4.76, 2.0–2.83, 1.0–2.0, 0.5–1.0 and 0.3–0.5 mm. For every soil, each of their aggregate size fraction was separately wet-sieved to determine the actual aggregate breakdown. The separate wet-sievings results were then combined in such a way to simulate the usual wet-sieving method; that is, to construct the data that would have been produced if each of the aggregate size fractions was wet-sieved together in the same nested sieves. Paired sample t-test showed that the differences between the actual and estimated aggregate breakdown values were significant at 5%. However, there was very close correlation between the actual and estimated values (r=0.974; p<0.001); thus, the equation was calibrated by simple linear regression. The calibrated equation was: _i=100 sin²_i, where _i is the calibrated breakdown estimate for aggregate size fraction i, and _i is 0.0166x_i+0.1 in unit radians. This calibrated equation was highly significant at 1% (F=766.039; p<0.001), with the values fitting very tightly along the regression line (R²=0.961), and with very small standard error (std. error=0.023). The calibrated equation was validated with three additional soils. Paired sample t-test showed there was insignificant differences between the actual and calibrated breakdown estimate values. Moreover, using fewer aggregate size fractions did not affect the accuracy of the calibrated equation, as this equation still predicted the actual values with very small errors.     

Dynamics of soil hydraulic properties during fallow as affected by tillage

There is limited information on the effects of tillage practices on soil hydraulic properties, especially changes with time. The objective of this study was to evaluate on a long-term field experiment the influence of conventional tillage (CT), reduced tillage (RT) and no-tillage (NT) on the dynamics of soil hydraulic properties over 3 consecutive 16–18 month fallow periods. Surface measurements of soil dry bulk density (ρ_b), soil hydraulic conductivity (K(ψ)) at −14, −4, −1 and 0 cm pressure heads using a tension disc infiltrometer, and derived hydraulic parameters (pore size, number of pores per unit of area and water-transmission porosity) calculated using the Poiseuille's Law were taken on four different dates over the fallow period, namely, before and immediately after primary tillage, after post-tillage rains and at the end of fallow. Under consolidated structured soil conditions, NT plots presented the most compacted topsoil layer when compared with CT and RT. Soil hydraulic conductivity under NT was, for the entire range of pressure head applied, significantly lower (P < 0.05) than that measured for CT and RT. However, NT showed the largest mean macropore size (0.99, 0.95 and 2.08 mm for CT, RT and NT, respectively; P < 0.05) but the significantly lowest number of water-conducting pores per unit area (74.1, 118.5 and 1.4 macropores per m² for CT, RT and NT, respectively; P < 0.05). Overall, water flow was mainly regulated by macropores even though they represented a small fraction of total soil porosity. No significant differences in hydraulic properties were found between CT and RT. In the short term, tillage operations significantly increased K (P < 0.05) for the entire range of pressure head applied, which was likely a result of an increase in water-conducting mesopores despite a decrease in estimated mesopore diameter. Soil reconsolidation following post-tillage rains reduced K at a rate that increased with the intensity of the rainfall events.     

Seasonal variations in soil erosion resistance during concentrated flow for a loess-derived soil under two contrasting tillage practices

Soil erodibilty during concentrated flow (K_c) and critical flow shear stress (τ_cr), both reflecting the soil's resistance to erosion by concentrated runoff, are important input parameters in many physically-based soil erosion models. Field data on the spatial and temporal variability of these parameters is limited but crucial for accurate prediction of soil loss by rill or gully erosion. In this study, the temporal variations in K_c and τ_cr for a winter wheat field on a silt loam soil under three different tillage practices (conventional ploughing, CP; shallow non-inversion tillage, ST; deep non-inversion tillage, DT) in the Belgian Loess Belt were monitored during one growing season. Undisturbed topsoil samples (0.003 m³) were taken every three weeks and subjected to five different flow shear stresses (τ = 4–45 Pa) in a laboratory flume to simulate soil detachment by concentrated flow. To explain the observed variation, relevant soil and environmental parameters were measured at the time of sampling. Results indicated that after two years of conservation tillage, K_c(CP) > K_c(DT) > K_c(ST). K_c values can be up to 10 times smaller for ST compared to CP but differences strongly vary over time, with an increasing difference with decreasing soil moisture content. The beneficial effects of no-tillage are not reflected in τ_cr. K_c values vary from 0.006 to 0.05 sm⁻¹ for CP and from 0.0008 to 0.01 sm⁻¹ for ST over time. Temporal variations in K_c can be mainly explained by variations in soil moisture content but consolidation effects, root growth, residue decomposition and the presence of microbiotic soil crusts as well play a role. τ_cr values increase with increasing soil shear strength but K_c seems more appropriate to represent the temporal variability in soil erosion resistance during concentrated flow. The large intra-seasonal variations in K_c, which are shown to be at least equally important as differences between different soil types reported in literature, demonstrate the importance of incorporating temporal variability in soil erosion resistance when modelling soil erosion by concentrated flow.     

Changes in δC composition of soil carbonates driven by organic matter decomposition in a Mediterranean climate: A field incubation experiment

The current view on the relationship between the δ¹³C of pedogenic carbonates and soil organic matter is based on static studies, in which soil profiles are analysed at a given moment of their development. A dynamic approach to this question should also be possible by studying under field conditions how the δ¹³C of carbonates changes as organic matter decomposes. No such study has been undertaken owing to the slowness of the changes in the δ¹³C of carbonates, since it has been calculated that a detectable change will occur only after millenia. Nevertheless, this may not be true where soil CO₂ efflux is intense, as expected in soil zones with high microbial activity. In this paper we test the latter assumption by incubating mixtures of plant material and carbonate-rich red earth in the field at depths of 5, 20 and 40 cm. Four types of plant material were tested: Medicago sativa, Eucalyptus globulus, Quercus ilex and Pinus halepensis. Because the isotopic composition of these plant materials is known, we can determine the isotopic composition of the respired C and study how it relates to the (expected) changes in the δ¹³C. After two years of field incubation, the changes in δ¹³C of carbonates were high enough to be reliably detected and quantified, thus showing that the isotopic composition of soil carbonates can change quite rapidly in biologically active soil horizons. The observed changes are possible only if we assume that the increase in δ¹³C in the overall path respired C → pedogenic carbonate is much higher than the usually applied standard factors (about 15‰). These enrichments can be explained by assuming, as does the currently accepted paradigm, that the precipitation of new carbonates occurs in an open system in which the penetration of free-air CO₂ plays a major role. On the other hand, these enrichments can also be explained by an alternative interpretation, which assumes that the dissolution–precipitation carbonate cycles occur in systems that can be at least temporarily closed. Thus, we suggest that both possibilities (carbonate dissolution and precipitation in either an open or closed system) can coexist in a given soil, even though one or the other will dominate in any given time period.