Quantifying microbial biomass phosphorus in acid soils

 This study aimed to validate the fumigation-extraction method for measuring microbial biomass P in acid soils. Extractions with the Olsen (0.5 M NaHCO₃, pH 8.5) and Bray-1 (0.03 M NH₄F–0.025 M HCl) extractants at two soil:solution ratios (1 : 20 and 1 : 4, w/v) were compared using eight acid soils (pH 3.6–5.9). The data indicated that the flushes (increases following CHCl₃-fumigation) of total P (P_t) and inorganic P (P_i) determined by Olsen extraction provided little useful information for estimating the amount of microbial biomass P in the soils. Using the Bray-1 extractant at a soil:solution ratio of 1 : 4, and analysing P_i instead of P_t, improves the reproducibility (statistical significance and CV) of the P flush in these soils. In all the approaches studied, the P_i flush determined using the Bray-1 extractant at 1 : 4 provided the best estimate of soil microbial biomass P. Furthermore, the recovery of cultured bacterial and fungal biomass P added to the soils and extracted using the Bray-1 extractant at 1 : 4 was relatively constant (24.1–36.7% and 15.7–25.7%, respectively) with only one exception, and showed no relationship with soil pH, indicating that it behaved differently from added P_i (recovery decreased from 86% at pH 4.6 to 13% at pH 3.6). Thus, correcting for the incomplete recovery of biomass P using added P_i is inappropriate for acid soils. Although microbial biomass P in soil is generally estimated using the P_i flush and a conversion factor (k _P) of 0.4, more reliable estimates require that k _P values are best determined independently for each soil. Received: 3 February 2000     

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.     

土壤矿物钾的释放及其在植物营养中的意义

本文对土壤钾的形态、矿物钾释放的影响因素及钾的固定、土壤含钾矿物与土壤供钾能力间关系、矿物钾的植物有效性进行了综述 ,表明土壤钾素从有效性的角度进行划分时 ,不能仅把速效性钾和1mol/L热硝酸提取的缓效性钾列为作物吸钾的主要来源 ,而忽视矿物钾的作用 .     

Implication of relationships among suspended sediment size, water discharge and suspended sediment concentration: the Yellow River basin, China

According to the yearly maximum suspended sediment concentrations (SSC) in the Yellow River and its tributaries, the rivers are divided into three types of more than 300, 20–300, and less than 20 kg/m³. The middle Yellow River basin is located in the transitional zone from subhumid to semiarid climates, and covered by a thick loess mantle. Neighboring on the desert areas to the northwest, the surface material of the Loess Plateau exhibits some marked areal differentiation in grain size and forms three zones covered by sandy loess, (typical) loess and clayey loess from northwest to southeast. Controlled by these physico-geographical conditions, the grain size of river-transported sediment shows some particular characteristics; at small water discharge or SSC, the grain size of suspended sediment abruptly decreases to a minimum with increasing water discharge or SSC. At water discharge of more than ca. 40 m³/s or at SSC of more than ca. 40 kg/m³, the grain size increases with water discharge or SSC.During the low-stage season, the relatively clear baseflow may scour the coarse bed materials, so the suspended sediment is relatively coarse. In the rainy season, rainstorm runoff washes out fine loess materials to the river, making suspended sediment fine. During relatively strong rainstorms, there often occur hyperconcentrated flows at SSC of more than 300 kg/m³. The relatively coarse grains could then remain suspended in the mixture of water and fine suspended sediment.     

Influence of water potential on nitrification and structure of nitrifying bacterial communities in semiarid soils

The objective of this study was to examine the relationship between soil water potential, nitrifier community structure and nitrification activity in semiarid soils. Soils were collected after a 5-month dry period (end of summer) and subsequently rewetted to specific water potentials and incubated for 7 days prior to analysis of nitrification activity and nitrifier community structure. The approach used in this study targeted a 491bp segment of the amoA gene which encodes the active site of the ammonia monooxygenase enzyme, which is the key enzyme for all aerobic ammonia oxidisers. amoA serves as a useful target for environmental studies since it is both specific and universal for all ammonia oxidisers and reflects the phylogeny of the ammonia oxidisers. Our results suggest that in semiarid soils water potential plays a key role in determining the structure of ammonia oxidising bacteria (AOB), and that additionally AOB community structure is correlated to potential nitrification rate in these soils.     

不同利用年限红壤水稻土有机碳和养分含量的粒级分布变化

通过田间采样结合沉降法分级提取,研究了不同利用年限红壤水稻土有机碳和养分含量的粒级分布变化特征。结果表明,红壤水稻土有机碳和养分含量随土壤颗粒粒径的增大而下降,但在各粒级中的分布比例存在显著差异。<0.002mm、0.002～0.02mm、0.02～0.05mm、>0.05mm粒级的有机碳占全土有机碳的比例分别是29.2%、30.7%、11.9%、15.4%,氮的相应数值为36.7%、31.9%、10.2%、14.0%,磷为49.2%、26.5%、11.1%、12.4%,钾为36.9%、33.4%、12.9%、20.0%。总体来说,黏粒和粉粒中有机碳和养分的分布比例较高。红壤水稻土有机碳和养分含量及分布比例还随利用年限而有明显变化。开垦利用不到10a的水田土壤,有机碳和养分含量较低且主要集中在<0.002mm粒级中;而利用超过10a的水稻土,有机碳和养分在粉粒中(0.002～0.05mm)的比例大于50%。各利用年限的红壤水稻土多以0.02～0.05mm粒级的C/N为最高,并随利用年限延长而下降。红壤水稻土各粒级有机碳和养分含量及分布状况随利用年限的变化反映了土壤肥力熟化和养分有效性的提高过程。     

Nitrogen fertilizer–induced mineralization of soil organic C and N in six contrasting soils of Bangladesh

A 90‐day laboratory incubation study was carried out using six contrasting subtropical soils (calcareous, peat, saline, noncalcareous, terrace, and acid sulfate) from Bangladesh. A control treatment without nitrogen (N) application was compared with treatments where urea, ammonium sulfate (AS), and ammonium nitrate (AN) were applied at a rate of 100 mg N (kg soil)^–1. To study the effect of N fertilizers on soil carbon (C) turnover, the CO₂‐C flux was determined at nine sampling dates during the incubation, and the total loss of soil carbon (TC) was calculated. Nitrogen turnover was characterized by measuring net nitrogen mineralization (NNM) and net nitrification (NN). Simple and stepwise multiple regressions were calculated between CO₂‐C flux, TC, NNM, and NN on the one hand and selected soil properties (organic C, total N, C : N ratio, CEC, pH, clay and sand content) on the other hand. In general, CO₂‐C fluxes were clearly higher during the first 2 weeks of the incubation compared to the later phases. Soils with high pH and/or indigenous C displayed the highest CO₂‐C flux. However, soils having low C levels (i.e., calcareous and terrace soils) displayed a large relative TC loss (up to 22.3%) and the added N–induced TC loss from these soils reached a maximum of 10.6%. Loss of TC differed depending on the N treatments (urea > AS > AN >> control). Significantly higher NNM was found in the acidic soils (terrace and acid sulfate). On average, NNM after urea application was higher than for AS and AN (80.3 vs. 71.9 and 70.9 N (kg soil)^–1, respectively). However, specific interactions between N‐fertilizer form and soil type have to be taken into consideration. High pH soils displayed larger NN (75.9–98.1 mg N (kg soil)^–1) than low pH soils. Averaged over the six soils, NN after application of urea and AS (83.3 and 82.2 mg N (kg soil)^–1, respectively) was significantly higher than after application of AN (60.6 mg N (kg soil)^–1). Significant relationships were found between total CO₂ flux and certain soil properties (organic C, total N, CEC, clay and sand content). The most important soil property for NNM as well as NN was soil pH, showing a correlation coefficient of –0.33** and 0.45***, respectively. The results indicate that application of urea to acidic soils and AS to high‐pH soils could be an effective measure to improve the availability of added N for crop uptake.     

不同浓度硫酸盐对水稻土中异化铁还原过程的影响

研究不同电子受体之间的竞争关系对揭示厌氧水稻土中微生物作用导致的氧化还原过程变化机理具有重要的理论意义。本研究采用土壤泥浆厌氧培养、人工合成氧化铁体系接种土壤浸提液厌氧培养及接种铁还原菌纯培养等试验方法,通过向培养体系中添加SO24-,探讨了硫酸盐作为竞争电子受体对不同铁还原体系中Fe（Ⅲ）还原的影响。结果表明,在2种水稻土的泥浆培养过程中,Fe（Ⅲ）还原速率均随着SO24-浓度增加而降低,但Fe（Ⅱ）的最终累积量却较对照处理有明显的增加。添加硫酸盐对Fe（Ⅲ）还原速率（k）的影响表现为：石灰性水稻土〉酸性水稻土;而最终Fe（Ⅱ）累积增加率则为：酸性水稻土〉石灰性水稻土。由接种不同水稻土浸提液的培养试验看出,添加SO24-后Fe（Ⅲ）还原受到显著的抑制,但随着培养时间延长Fe（Ⅲ）还原反应依然可以进行,并且Fe（Ⅱ）累积量最终达到与CK相同的水平。在接种铁还原菌的纯培养试验中,添加SO24-对供试的4株铁还原菌的Fe（Ⅲ）还原过程并未产生抑制效应,表明铁还原菌本身并不受硫酸盐的影响。     

西昌市土壤Fe、Mn、Cu、Zn有效性评价及其影响因素分析

在分析西昌市土壤微量元素Fe、Mn、Cu、Zn有效含量状况及其分布特征的基础上,从成土母质、土地利用方式、有机质和pH等4个方面探讨其对土壤微量元素有效含量的影响。结果表明:西昌市土壤有效Fe含量丰富,有效Cu和Zn略低且分布不均,有效锰有32.18%的土壤缺乏。成土母质是重要的影响因素,第四纪紫色洪冲积物发育的土壤Fe、Mn、Zn有效含量较高,二迭纪岩浆岩洪冲积物发育的土壤有效Cu含量较高,坡残积物发育的土壤4种微量元素有效态含量均较低;水田土壤各元素有效含量均大于旱地;有机质与有效Fe、Cu间显著正相关;pH值在中性偏碱时,有效Cu和Zn含量较低。     

拖拉机液压悬挂控制系统动态特性分析

本文分重分析间控泵单作用油缸系统运动规律，建立拖拉机阀控泵单作用油缸液压悬挂闭环控制系统动态特性数学模型，并加以分析，指出提高稳定性的措施，提供稳定性系统设计的理论依据．