水稻优生区农户资本禀赋对其耕地保护决策行为的影响——基于双栏模型的实证研究

水稻优生区农户对耕地实施保护性耕作的决策行为对于水稻产量及品质具有重要影响。本文基于农户资本禀赋视角,将农户资本禀赋分为人力资本禀赋、经济资本禀赋、自然资本禀赋和社会资本禀赋,借助水稻优生区宁夏青铜峡394户农户的微观数据,运用双栏模型将农户参与耕地保护决策行为划分为参与意愿和参与程度两个决策行为阶段,实证分析了水稻优生区农户资本禀赋对其参与耕地保护决策行为的影响,以期为科学制定水稻优生区耕地保护相关政策、保障农业绿色经济发展提供参考和借鉴。研究表明：1）绝大多数农户愿意参加耕地保护行为,但其从事耕地保护的支付意愿相对较低,愿意参加耕地保护的农户占73.86%,而平均支付意愿仅为3 100.65元·hm^-2;2）人力资本禀赋（文化程度和农业培训）、经济资本禀赋（家庭收入和农机投入）以及社会资本禀赋（社会关系和社会参与）对农户耕地保护参与意愿和参与程度均为正向影响,自然资本禀赋（耕地禀赋和破碎程度）对农户耕地保护参与意愿有正向影响,对参与程度有负向影响;3）耕地保护重要性认知仅对农户耕地保护参与意愿有促进作用,而耕地保护政策了解程度对农户耕地保护参与意愿和参与程度均有促进作用。最后,提出建立健全耕地保护经济补偿政策、积极组建各类农业合作社和耕地保护组织以及培养新型农民、加大稻作区耕地保护宣传力度等政策建议。     

土地用途管制框架下农户土地利用行为特征及影响因素——四川省成都市和湖北省武汉市2个典型区域的比较

[目的]探讨农户土地利用行为的区域差异及其受土地用途管制的影响,为更好引导农户合理利用土地,提高土地利用效率及完善土地用途管制制度提供借鉴。[方法]以率先试点耕地保护基金的四川省成都市和尚未实行耕地保护经济补偿政策的湖北省武汉市2个典型城市的43个村庄433名受访农民为例,利用非参数检验及回归模型分别从土地利用意愿、土地利用结构、土地利用强度和土地保护程度4方面探讨农户土地利用行为的基本特征及影响因素。[结果](1)农户土地利用行为呈现出农地利用意愿较低,倾向兼种粮食作物和经济作物,单位农地投资强度较低,农地利用效率呈两极分化且忽视农田环境保护的特点。(2)农户土地利用行为具有显著的区域差异,成都受访农户农地流转参与率较武汉农户农地流转参与率高12.61%。(3)农户个体和家庭特征、土地用途管制程度及区域因素均对农户土地利用行为有显著影响。[结论]农户土地利用行为呈现两极分化,且其受农户个体及地域差异显著影响。     

粮食生产核心区不同类型农户耕地保护的认知与行为

[目的]剖析粮食生产核心区内不同类型农户参与耕地保护的认知与行为差异,为制定相关差异化政策,引导农户加强对耕地的监督和保护提供支持。[方法]选择粮食生产核心区河南省周口市为样本点,采用分层随机抽样与乡村参与性评估法(PRA)对各类农户参与耕地保护的认知与行为进行调查,运用Logistic回归模型进行影响因素分析。[结果](1)随着受教育程度的提高和兼业程度的加深,各类农户对耕地重要性和多功能性的认识、时间投入偏好、对耕地保护前景的信心、对农业污染的防范都呈现出:缺失型(Ⅰ)基本型(Ⅱ)自然资产型(Ⅲ)人力资产型(Ⅳ),得到补偿后参与意愿、追加补偿到耕地的意愿、定期施用有机肥或采用过测土配方施肥、制止他人破坏耕地的比例都呈现出缺失型(Ⅰ)基本型(Ⅱ)自然资产型(Ⅲ)人力资产型(Ⅳ)。(2)影响各类农户耕地保护行为的因素既存在相同之处也存在各自差异。相同因素包括户主文化程度、投入偏好、补偿意愿。户均年龄高,家庭劳动力少的缺失型农户影响因素还包括户主年龄、务农人数、补偿方式;专门从事农业生产的基本型农户影响因素还包括家庭毛收入、经营土地面积、耕地破碎度;从事短期兼业且非农收入较高的自然资产型农户影响因素还包括非农收入比、粮食种植经济效益、农户兼业;长期居住城镇,较少参与农业生产的人力资产型农户的影响因素还包括耕地经济区位、当期粮食价、政策性补偿方式。[结论]农户作为耕地保护的直接参与者,其认知与行为很大程度决定着耕地保护的成效。     

耕地土壤质量保护中农户认知行为影响因素研究——基于结构方程模型的估计

探讨耕地土壤质量保护中农户认知行为的影响因素,可以在深化已经研究基础上,为政府制定引导农户合理利用和保护耕地土壤质量的政策提供依据。本研究采用结构方程模型,利用辽宁省沈阳市沈北新区233户农户调查数据进行实证分析。研究结果表明:(1)社会经济因素特征、农户自身条件特征和耕地自然条件特征这3类外源潜变量对农户耕地质量土壤质量保护认知均有显著的正向影响,农户自身条件特征是影响耕地土壤质量保护中农户认知行为的主要因素,其特征每提高一个单位,农户耕地土壤质量保护认知程度会提高0.877个单位;(2)可观测变量对3类外源潜变量的解释能力均较强,且显著水平、影响程度和作用方向存在明显的差异,其中农产品价格、户主受教育程度和典型地块面积分别对这3类外源潜变量的解释能力相对较好。基于以上的研究结果,本研究认为提高农业生产的比较效益、稳定的农村土地经营权和将土地向以农业收入为主的农户流转实现规模经营,并针对这种类型的农户进行农业生产和耕地土壤质量保护方面的技术服务和培训,是未来政策制定和实施的重要方向。     

基于CVM的耕地保护外部性估算与检验

为了科学评估耕地保护的外部性,在耕地利用效益体系重构和耕地保护外部性界定的基础上,基于条件价值评估法(CVM)构建了耕地保护外部性测度、分析和检验一体化的方法体系,并以河南省焦作市为研究区域进行了实证应用。研究结果表明:1耕地生态社会效益属于一种纯粹的、具有正外部性和私人(主要为农户)联合提供的混合类跨区域性公共物品,可以运用CVM等非市场价值评估法进行估算。2受访者的支付意愿较大程度地受社会经济发展水平的制约和影响,农户和城镇居民对耕地生态社会效益认知度的高低是造成城镇居民与农户支付率差异的重要影响因素。3基于CVM测算出的焦作市耕地整体外部性为1.91亿元·a-1,耕地保护外部性为993.7元·hm-2·a-1,该测算结果可作为耕地保护外部性补偿的最低标准。4CVM的有效性与可靠性检验是CVM理论、方法和实践内容的重要组成部分,可通过问卷设计优化、预调查、问卷内容修正、合理的询价方式、不同估算方法的对照分析、同一方法重复试验等手段进行有效性与可靠性检验;检验结果表明,CVM进行耕地保护外部性估算的可靠性和有效性较高。     

重金属污染区耕地轮作休耕治理农户参与意愿实证研究

  目的  研究我国农村重金属污染区耕地轮作休耕治理农户参与意愿,分析影响农户参与重金属污染区耕地轮作休耕治理影响因素,为我国重金属污染耕地长期治理的有效实施提供参考依据。  方法  基于343户的调查数据,构建累积logistic回归模型,探究我国重金属污染区耕地轮作休耕治理农户参与意愿及其影响因素。  结果  相比于男性,女性更愿意参与重金属污染轮作休耕治理（?0.1341）;农户参与意愿与受教育程度（?0.098）、家庭收入（0.2591）正相关,但与年龄（0.2036）负相关;农户参与意愿与轮作休耕治理政策了解程度（0.3049）、治理过程评价（0.0985）正相关,参与意愿受亲朋好友（?0.1110）影响较大,由于信任度原因现阶段农村基层组织政策宣传对农户参与意愿影响较小;农户参与意愿与轮作休耕后家庭收入变化（0.4851）正相关;农户参与意愿与自有耕地污染程度（?0.3995）负相关,与农户对环境关注度（?0.1953）呈负相关。  结论  农户个体特征及基本情况、参与后的家庭收入变化、耕地污染特征及对环境感知、政策宣讲及实施过程对农户参与意愿具备实质性影响。因此,要提高基层村组在政策宣讲和政策执行过程在农户心中的信任度;提高农业从业人员的受教育水平,降低农业从业人员平均年龄;增加农户家庭收入,拓宽农户收入来源渠道;从环境保护和收入层面加强治理的预期效应宣传,逐步消除农户污染治理与自身收入平衡的矛盾心理。     

基于CVM的耕地保护外部性估算与检验

为了科学评估耕地保护的外部性, 在耕地利用效益体系重构和耕地保护外部性界定的基础上, 基于条件价值评估法(CVM)构建了耕地保护外部性测度、分析和检验一体化的方法体系, 并以河南省焦作市为研究区域进行了实证应用。研究结果表明: ①耕地生态社会效益属于一种纯粹的、具有正外部性和私人(主要为农户)联合提供的混合类跨区域性公共物品, 可以运用CVM等非市场价值评估法进行估算。②受访者的支付意愿较大程度地受社会经济发展水平的制约和影响, 农户和城镇居民对耕地生态社会效益认知度的高低是造成城镇居民与农户支付率差异的重要影响因素。③基于CVM测算出的焦作市耕地整体外部性为1.91亿元·a^-1, 耕地保护外部性为993.7元·hm^-2·a^-1, 该测算结果可作为耕地保护外部性补偿的最低标准。④CVM的有效性与可靠性检验是CVM理论、方法和实践内容的重要组成部分, 可通过问卷设计优化、预调查、问卷内容修正、合理的询价方式、不同估算方法的对照分析、同一方法重复试验等手段进行有效性与可靠性检验; 检验结果表明, CVM进行耕地保护外部性估算的可靠性和有效性较高。     

基于农户调查的江汉平原地区农村土地流转意愿研究及影响因素分析

农户是参与农地流转的主体之一,实现土地合理流转必须以尊重农民意愿为前提。以江汉平原为研究区,运用Logistic二元回归模型对研究区278户农户土地流转意愿的影响因素进行分析,得出以下结论:(1)研究区农户的土地流转意愿较低,流出土地的农户比例为17.99%,流入土地的农户比例为25.17%;(2)影响农户土地流出意愿的主要因素是农地收入功能、外出务工劳动比例和耕地面积,农地收入功能越高,外出务工人数越少,耕地面积越多,农户的土地流出意愿越强;(3)影响农户土地流入意愿的主要因素是户主文化水平、农地政策了解程度、是否具备耕种能力、农地收入功能、耕地质量和村集体经济状况,户主文化水平越高,对农地政策越了解,耕种能力越强,农地收入越高,农户的农地流入意愿越强,而耕地质量越高,村集体经济状况越好,农户的农地流入意愿越弱。     

城镇化进程中农户土地退出意愿影响因素分析

随着城镇化进程的不断加快,鼓励农户退出土地承包经营权以提高土地资源的利用效率成为解决土地资源短缺的关键途径。为了分析农户土地承包经营权退出意愿的影响因素,利用陕西省关中地区579户农户调研数据,运用结构方程模型,对农户土地退出意愿的影响因素及影响方向进行了研究。研究结果表明:农户家庭特征中的人均收入、离最近县城的距离、家庭总收入、户主文化程度,农户生产特征中的农业收入、耕地经营面积、劳动力数量,农户意识特征中的农户产权意识、农户行为意识、农户福利意识、农户技能意识,对农户的退地意愿产生显著影响。研究结果表明农户土地承包经营权退出意愿是受到多因素综合影响的,并且各影响因素之间也存在一定的共变关系。对农户土地退出意愿的影响因素进行分析可以为制定合理的土地退出政策和机制提供意见参考。     

农户耕地质量提升行为的逻辑路径与驱动力:研究进展与展望

新时代我国耕地保护已转型为数量、质量、生态"三位一体"格局,提升耕地质量是确保国家粮食安全和重要农产品有效供给的资源基础,是实现农业高质量发展的条件保障。农户是耕地的直接利用者与经营者,也是耕地质量保护与提升的直接主体,促进农户的耕地质量提升行为是实现耕地保护战略目标的关键。本文通过文献统计与归纳总结,对国内外相关研究进行梳理和分析发现:农户耕地质量提升行为的逻辑一般沿"认知→意愿→行为"路径,也存在"认知→行为"的可能;农户采取耕地质量提升行为的响应程度不高,通过调节关键驱动因素可提高农户的积极性。从行为逻辑与响应过程来看,农户耕地质量提升行为的关键驱动力为认知水平、经营规模、土地经营权稳定性和政策支持。此外,现有文献的研究方法主要为定性与简单定量研究,缺乏对农户耕地质量提升行为的障碍因素、契约关系稳定性、不同经营主体行为差异性等方面的定量研究。因此,从多元主体等视角,采用定量模型等方法,开展农户耕地质量提升行为的关键因素识别、障碍因素分析和政策支持探索成为未来研究的重点方向。     

硫酸根自由基高级氧化技术对污染场地多环芳烃的修复效果研究

硫酸根自由基高级氧化技术(sulfate radical(SO_4~(·–))based advanced oxidation processes,SR-AOPs)是一种被广泛应用于降解土壤有机污染物的原位氧化修复技术。然而,关于SR-AOPs降解土壤多环芳烃(polycyclic aromatic hydrocarbons,PAHs)的报道相对较少。本研究以南京某炼钢厂附近土壤作为试验样本,通过设置不同比例混合体系的过硫酸钠(Na_2S_2O_8)和亚铁离子(Fe~(2+))以及反应不同时间,探究SR-AOPs对土壤中16种PAHs的修复效果以及最佳技术方案。结果表明:Na_2S_2O_8和Fe~(2+)的配比会显著影响土壤PAHs的降解效果,当两者比例达到10︰1时,即Na_2S_2O_8用量为5 mmol/g,Fe~(2+)用量为0.5 mmol/g,反应时间为24 h时,PAHs总降解率最高,可达到29.32%;不同环数的PAHs决定了SR-AOPs的降解效果,其中SR-AOPs对四环PAHs降解效率最高,总降解率达到37.32%;此外,降解效率随反应时间增加而增加,在24 h达到效果最佳。因此,本研究结果可为SR-AOPs修复土壤PAHs提供理论依据。     

A method for estimating coefficients of soil organic matter dynamics based on long-term experiments

The one-compartment C model C_t=C₀e^−k₂t+k₁A/k₂(1−e^−k₂t) is being long used to simulate soil organic C (SOC) stocks. C_t is the SOC stock at the time t; C₀, the initial SOC stock; k₂, the annual rate of SOC loss (mainly mineralization and erosion); k₁, the annual rate to which the added C is incorporated into SOC; and A, the annual C addition. The component C₀e^−k₂t expresses the decay of C₀ and, for a time t, corresponds to the remains of C₀ (C_0 remains). The component k₁A/k₂(1−e^−k₂t) refers, at time t, to the stock of SOC derived from C crops (C_crop). We herein propose a simple method to estimate k₁ and k₂ coefficients for tillage systems conducted in long-term experiments under several cropping systems with a wide range of annual C additions (A) and SOC stocks. We estimated k₁ and k₂ for conventional tillage (CT) and no-till (NT), which has been conducted under three cropping systems (oat/maize −O/M, vetch/maize −V/M and oat + vetch/maize + cowpea −OV/MC) and two N-urea rates (0 kg N ha⁻¹ −0 N and 180 kg N ha⁻¹ −180 N) in a long-term experiment established in a subtropical Acrisol with C₀ = 32.55 Mg C ha⁻¹ in the 0–17.5 cm layer. A linear equation (C_t = a + bA) between the SOC stocks measured at the 13th year (0–17.5 cm) and the mean annual C additions was fitted for CT and NT. This equation is equivalent to the equation of the model C_t=C₀e^−k₂t+k₁A/k₂(1−e^−k₂t), so that a=C₀e^−k₂t and bA=k₁A/k₂(1−e^−k₂t). Such equivalences thus allow the calculation of k₁ and k₂. NT soil had a lower rate of C loss (k₂ = 0.019 year⁻¹) than CT soil (k₂ = 0.040 year⁻¹), while k₁ was not affected by tillage (0.148 year⁻¹ under CT and 0.146 year⁻¹ under NT). Despite that only three treatments had lack of fit (LOFIT) value lower than the critical 5% F value, all treatments showed root mean square error (RMSE) lower than RMSE 95% indicating that simulated values fall within 95% confidence interval of the measurements. The estimated SOC stocks at steady state (C_e) in the 0–17.5 cm layer ranged from 15.65 Mg ha⁻¹ in CT O/M 0 N to 60.17 Mg ha⁻¹ in NT OV/MC 180 N. The SOC half-life (t_1/2 = ln 2/k₂) was 36 years in NT and 17 years in CT, reflecting the slower C turnover in NT. The effects of NT on the SOC stocks relates to the maintenance of the initial C stocks (higher C_0 remais), while increments in C_crop are imparted mainly by crop additions.     

Interpreting the dependence of soil respiration on soil temperature and water content in a boreal aspen stand

Continuous half-hourly measurements of soil CO₂ efflux made between January and December 2001 in a mature trembling aspen stand located at the southern edge of the boreal forest in Canada were used to investigate the seasonal and diurnal dependence of soil respiration (R_s) on soil temperature (T_s) and water content (θ). Daily mean R_s varied from a minimum of 0.1 μmol m⁻² s⁻¹ in February to a maximum of 9.2 μmol m⁻² s⁻¹ in mid-July. Daily mean T_s at the 2-cm depth was the primary variable accounting for the temporal variation of R_s and no differences between Arrhenius and Q₁₀ response functions were found to describe the seasonal relationship. R_s at 10 °C (R_s10) and the temperature sensitivity of R_s (Q_10Rs) calculated at the seasonal time scale were 3.8 μmol m⁻² s⁻¹ and 3.8, respectively. Temperature normalization of daily mean R_s (R_sN) revealed that θ in the 0–15 cm soil layer was the secondary variable accounting for the temporal variation of R_s during the growing season. Daily R_sN showed two distinctive phases with respect to soil water field capacity in the 0–15 cm layer (θ_fc, 0.30 m³ m⁻³): (1) R_sN was strongly reduced when θ decreased below θ_fc, which reflected a reduction in microbial decomposition, and (2) R_sN slightly decreased when θ increased above θ_fc, which reflected a restriction of CO₂ or O₂ transport in the soil profile.Diurnal variations of half-hourly R_s were usually out of phase with T_s at the 2-cm depth, which resulted in strong diurnal hysteresis between the two variables. Daily nighttime R_s10 and Q_10Rs parameters calculated from half-hourly nighttime measurements of R_s and T_s at the 2-cm depth (when there was steady cooling of the soil) varied greatly during the growing season and ranged from 6.8 to 1.6 μmol m⁻² s⁻¹ and 5.5 to 1.3, respectively. On average, daily nighttime R_s10 (4.5 μmol m⁻² s⁻¹) and Q_10Rs (2.8) were higher and lower, respectively, than the values obtained from the seasonal relationship. Seasonal variations of these daily parameters were highly correlated with variations of θ in the 0–15 cm soil layer, with a tendency of low R_s10 and Q_10Rs values at low θ. Overall, the use of seasonal R_s10 and Q_10Rs parameters led to an overestimation of daily ranges of half-hourly R_s (ΔR_s) during drought conditions, which supported findings that the short-term temperature sensitivity of R_s was lower during periods of low θ. The use of daily nighttime R_s10 and Q_10Rs parameters greatly helped at simulating ΔR_s during these periods but did not improve the estimation of half-hourly R_s throughout the year as it could not account for the diurnal hysteresis effect.     

Genetic diversity and differentiation of cultivated ginseng (<Emphasis Type="Italic">Panax ginseng</Emphasis> C. A. Meyer) populations in North-east China revealed by inter-simple sequence repeat (ISSR) markers

Panax ginseng C. A. Meyer is a medicinally important herb with a long history of cultivation, and includes three cultivated types, viz. garden ginseng (GGS), forest ginseng (FGS) and transplanted wild ginseng (TGS). In the present study, inter-simple sequence repeat (ISSR) markers were employed to investigate the genetic variability in 282 individuals, which corresponded to 16 cultivated ginseng populations. Genetic diversity was high at the species level (h = 0.2886; I = 0.4382; PPB = 98.96%), but relatively lower at the cultivated-type level (GGS: h = 0.2294, I = 0.3590, PPB = 85.42%; FGS: h = 0.1702, I = 0.2559, PPB = 57.29%; TGS: h = 0.2021, I = 0.3125, PPB = 76.04%). The hierarchical analysis of molecular variance (AMOVA) revealed pronounced genetic differentiation among populations (Φ _ST = 53.94%), which was confirmed by the gene differentiation coefficient (G _ST = 0.4910) and low gene flow (N _m = 0.5184). Both Principal Coordinates Analysis (PCoA) and UPGMA cluster analysis supported the clustering of all 16 populations into three groups, corresponding to the three cultivated types, among which there occurred remarkable genetic differentiation (Φ _ST = 37.43%). Pronounced genetic differentiation was also detected among populations within the three cultivated types (GGS: Φ _ST = 40.83%, G _ST = 0.3187, N _m = 1.0691; FGS: Φ _ST = 22.85%, G _ST = 0.2328, N _m = 1.6480; TGS: Φ _ST = 30.68%, G _ST = 0.2540, N _m = 1.4686). Mantel test indicated no significant correlation between geographic and genetic distances at both species and cultivated-type levels (P > 0.05). These findings have profound implications for the sustainable utilization of this precious medicinal herb.     

Perchlorate Depositional History as Recorded in North American Ice Cores from the Eclipse Icefield, Canada, and the Upper Fremont Glacier, USA

Temporal depositional rates are important in order to understand the production and occurrence of perchlorate (ClO₄⁻) as limited information exists regarding the impact of anthropogenic production or atmospheric pollution on ClO₄⁻ deposition. Perchlorate concentrations in discrete ice core samples from the Eclipse Icefield (Yukon Territory, Canada) and Upper Fremont Glacier (Wyoming, USA) were analyzed using ion chromatography tandem mass spectrometry to evaluate temporal changes in the deposition of ClO₄ ⁻ in North America. The ice core samples cover a time period from 1726 to 1993 and 1970 to 2002 for the Upper Fremont Glacier (UFG) and Eclipse ice cores, respectively. The average ClO₄ ⁻ concentration in the Eclipse ice core for the time period from 1970 to 1973 was 0.6 ± 0.3 ng L⁻¹, with higher values of 2.3 ± 1.7 and 2.2 ± 2.0 ng L⁻¹ for the periods 1982–1986 and 1999–2002, respectively. All pre-1980 ice core samples from the UFG had ClO₄ ⁻ concentrations <0.2 ng L⁻¹, and the post-1980 samples ranged from <0.2 ng L⁻¹ to a maximum of 2.6 ng L⁻¹ for the year 1992. A significant positive correlation (R = 0.75, N = 15, p < 0.001) of ClO₄⁻ with SO₄²⁻ was found for the annual UFG ice core layers and of ClO₄ ⁻ with SO₄²⁻ and NO₃⁻ in sub-annual Eclipse ice samples (R > 0.3, N = 121, p < 0.002). The estimated yearly ClO₄⁻ depositional flux for the Eclipse ice core ranged from 0.6 (1970) to 4.7 μg m⁻² year⁻¹ (1982) and the UFG from <0.1 (pre-1980) to 1.4 μg m⁻² year⁻¹ (1992). There was no consistent seasonal variation in the ClO₄⁻ depositional flux for the Eclipse ice core, in contrast to a previous study on the Arctic region. The presence of ClO₄⁻ in these ice cores might correspond to an intermittent source such as volcanic eruptions and/or any anthropogenic forcing that may directly or indirectly aid in atmospheric ClO₄⁻ formation.     

Estimating the component of soil respiration not dependent on living plant roots: Comparison of the indirect y-intercept regression approach and direct bare plot approach

Distinguishing between root and non-root derived CO₂ efflux is important when determining rates of soil organic matter turnover, however, in practice they remain difficult to separate. Our aim was to evaluate two methods for determining the component of below-ground respiration not dependent on plant roots (i.e., basal soil respiration; R_b). The first approach estimated R_b indirectly from the y-intercept of linear regressions between below-ground respiration (BGR) and root biomass. The second approach involved direct measurements of soil respiration from bare plots. To compare the contrasting approaches, BGR and crop biomass measurements were collected throughout the year in a range of agricultural systems. We found that both methods were very closely correlated with each other. Values of R_b determined by the intercept approach, however, were slightly higher than those determined by measurement of bare plots. Both approaches showed a seasonal trend with estimates of R_b lowest in winter months at 0.02 t C ha⁻¹ month⁻¹ for the y-intercept approach and 0.11 t C ha⁻¹ month⁻¹ for the bare plots approach, even after the data had been corrected for the influence of soil temperature. Highest rates of R_b occurred from the height to the end of the crop growing season (0.8-1.5 t C ha⁻¹ month⁻¹). The annual CO₂ efflux due to R_b was estimated to be 8.1 t C ha⁻¹ y⁻¹ from the y-intercept approach and 6.8 t C ha⁻¹ y⁻¹ from bare plots. Annual BGR was 12.1 t C ha⁻¹ y⁻¹. We conclude that both methods provide similar estimates of R_b, however, logistically the bare plots approach is much easier to undertake than the y-intercept approach.     

Methane fluxes from three ecosystems in tropical peatland of Sarawak, Malaysia

Methane fluxes were measured monthly over a year from tropical peatland of Sarawak, Malaysia using a closed-chamber technique. The CH₄ fluxes in forest ecosystem ranged from −4.53 to 8.40 μg C m⁻² h⁻¹, in the oil palm ecosystem from −32.78 to 4.17 μg C m⁻² h⁻¹ and in the sago ecosystem from −7.44 to 102.06 μg C m⁻² h⁻¹. A regression tree approach showed that CH₄ fluxes in each ecosystem were related to different underlying environmental factors. They were relative humidity for forest and water table for both sago and oil palm ecosystems. On an annual basis, both forest and sago were CH₄ source with an emission of 18.34 mg C m⁻² yr⁻¹ for forest and 180 mg C m⁻² yr⁻¹ for sago. Only oil palm ecosystem was a CH₄ sink with an uptake rate of −15.14 mg C m⁻² yr⁻¹. These results suggest that different dominant underlying environmental factors among the studied ecosystems affected the exchange of CH₄ between tropical peatland and the atmosphere.     

Effects of nitrite toxicity on soil bacteria under aerobic and anaerobic conditions

Isolates of a soil Pseudoimonas, as well as other soil bacteria, showed a different sensitivity towards NO^?₂ when grown under aerobic or anaerobic conditions. The tolerance to NO^?₂ was increased in the presence of O₂: for instance, a concentration of $\text{200parts}\text{10}{}^6$ of NO^?₂-N proved to be toxic to a Pseudomonas sp. under anaerobic conditions, whereas over $\text{400 parts}\text{10}{}^6$ were needed aerobically to suppress its growth completely. The addition of NO^?₃ as an electron acceptor for anaerobic respiration did not overcome the inhibitive effect of NO^?₃. The pH range, at which NO^?₂ was utilized anaerobically, was narrowed with increasing NO^?₂ concentration (pH 6.8–8.8 at $\text{70 parts}\text{10}{}^6$ of NO^?₂-N and 7.4–8.5 and $\text{140 parts}\text{10}{}^6$ of NO^?₂-N).Tolerance to nitrite varied considerably among the bacteria tested. Each species was able to overcome the inhibitory effect of NO^?₂ up to a certain concentration, while the length of the lag phase was related to NO^?₂ concentration.