Assessing air-drying and rewetting pre-treatment effect on some soil enzyme activities under Mediterranean conditions

Soil enzyme activities are useful indicators of soil quality as they are very sensitive to disturbance. Sample storage or pre-treatments could affect the results in these assays, which are normally determined in fresh samples, kept cold or frozen. The objectives of this study were to (i) evaluate the effect of air-drying or air-drying and rewetting on β-glucosidase, acid phosphatase and urease activities in soils from different locations, degradation status and sampling seasons, and (ii) assess if air-drying or air-drying and rewetting is an accurate sample storage and pre-treatment procedure for enzyme activities in soil quality evaluations under semiarid Mediterranean conditions. Our results showed that urease, phosphatase and β-glucosidase activities were hardly affected by air-drying of degraded and non-degraded soils from the two locations studied in all seasons. Short incubations (4, 8 and 12 d at 23 °C) of rewetted air-dried soil at 55% of water-holding capacity showed different patterns depending on the enzyme studied. Urease and β-glucosidase activities were relatively stable during incubation, with several significant (P<0.05) shifts up and down in some soils and samplings. However, acid phosphatase showed an increase in activity with incubation, of between 5% and 50% relative to air-dried samples. These increases followed no pattern and were unrelated to soil characteristics or sampling date. Hence, urease, phosphatase and β-glucosidase activities determined in air-dried soil samples seem to be representative of those obtained under field-moist conditions. In contrast, short incubations of rewetted soil samples can produce fluctuations in these enzyme activities, mainly of acid phosphatase, and estimations in these conditions are not so representative of field-moist soil values.     

干土效应对土壤生物组成及矿化与硝化作用的影响

将经过风干、过筛后的2种旱地红壤加水培养,并和新鲜土培养条件相比较,研究干土效应对土壤生物组成及矿化与硝化作用的影响.试验共4个处理(1)农田旱地风干土加水培养(RU);(2)农田旱地新鲜土培养(FU);(3)苗圃旱地风干土加水培养(RN);(4)苗圃旱地新鲜土培养(FN).结果表明红壤风干土加水预培养5 d后,细菌、放线菌、真菌数量比新鲜土显著增加(p＜0.01),细菌数量增加最为明显,农田旱地和苗圃旱地风干土处理分别是新鲜土的6.26倍和6.84倍,红壤风干土加水培养处理的微生物量碳、氮也随之增加.培养28 d后土壤中微生物数量趋于稳定,与预培养5 d时的数量相当或稍有下降,但风干后加水培养处理的微生物数量仍保持大于新鲜土的趋势(农田旱地的放线菌除外),微生物量碳、氮也存在同样的趋势.风干土加水培养后微生物数量的迅速增加,使得氮素矿化速度加快,由此导致NH+4-N量显著增加(p＜0.01),培养28 d后,NH+4-N量较预培养5 d时有所增加,且明显高于新鲜土培养处理;NO-3-N含量也增加,但新鲜土处理显著高于风干土处理.土壤风干处理对土壤自由生活线虫的影响比较大,农田旱地和苗圃旱地风干土加水培养28 d后,其自由生活线虫数量仅为新鲜土的16.0%和30.1%,显示风干土加水培养难以恢复土壤微型动物的数量.28 d的矿化和硝化培养试验结果显示,风干土加水培养处理的净矿化量和矿化率均高于新鲜土处理,苗圃旱地风干土处理的增量达到了显著水平(p＜0.05),但是硝化作用却刚好相反,农田旱地和苗圃旱地的净硝化量及硝化率均是新鲜土处理显著高于风干土处理(p＜0.05),其原因是对硝化作用起重要作用的硝化菌(氨氧化细菌和亚硝酸氧化细菌)数量在经历了风干过程后很难恢复到新鲜土水平.     

Relationships of soil respiration to microbial biomass, substrate availability and clay content

The roles of microbial biomass (MBC) and substrate supply as well as their interaction with clay content in determining soil respiration rate were studied using a range of soils with contrasting properties. Total organic C (TOC), water-soluble organic carbon, 0.5 M K₂SO₄-extractable organic C and 33.3 mM KMnO₄-oxidisable organic carbon were determined as C availability indices. For air-dried soils, these indices showed close relationship with flush of CO₂ production following rewetting of the soils. In comparison, MBC determined with the chloroform fumigation-extraction technique had relatively weaker correlation with soil respiration rate. After 7 d pre-incubation, soil respiration was still closely correlated with the C availability indices in the pre-incubated soils, but poorly correlated with MBC determined with three different techniques—chloroform fumigation extraction, substrate-induced respiration, and chloroform fumigation-incubation methods. Results of multiple regression analyses, together with the above observations, suggested that soil respiration under favourable temperature and moisture conditions was principally determined by substrate supply rather than by the pool size of MBC. The specific respiratory activity of microorganisms (CO₂-C/MBC) following rewetting of air-dried soils or after 7 d pre-incubation was positively correlated with substrate availability, but negatively correlated with microbial pool size. Clay content had no significant effect on CO₂ production rate, relative C mineralization rate (CO₂-C/TOC) and specific respiratory activity of MBC during the first week incubation of rewetted dry soils. However, significant protective effect of clay on C mineralization was shown for the pre-incubated soils. These results suggested that the protective effect of clay on soil organic matter decomposition became significant as the substrate supply and microbial demand approached to an equilibrium state. Thereafter, soil respiration would be dependent on the replenishment of the labile substrate from the bulk organic C pool.     

风干土保存时间和湿土培育时间对黄淮海平原潮土酶活性的影响

以经历18年不同施肥管理的土壤为研究对象,阐明它们经过4个不同时间保存或处理后的土壤脲酶、转化酶、脱氢酶、及FDA酶活性的动态变化。处理包括:风干保存30天或鲜土状态、风干保存210天、风干土湿润至田间持水量(25℃)条件下分别培育15天和51天;同时评估这些酶活性的变化程度与土壤本身有机碳含量之间的关系。结果表明,风干土保存时间和风干土湿润后短期培育均对脲酶活性影响很小,但风干土湿润培养51天后其活性则显著降低;随风干土保存时间延长,转化酶活性显著降低;与鲜土相比,风干土湿润培养15天后,脱氢酶活性显著提高,但继续湿润培养至51天后,其活性又降至与鲜土相当,因此风干土湿润培育一定时间后测定的脱氢酶活性可用来代表其田间自然湿度时的状态;FDA酶活性的变异程度最大,与其从鲜土状态至风干状态的活性急剧下降有关。土壤本身有机碳含量与脲酶和脱氢酶的活性变化程度成显著负相关关系,说明土壤有机碳含量是决定它们随环境条件改变而变化的主要因素之一。另外,土壤NH4+-N、NO3--N和可溶性有机碳含量对上述4种处理的响应程度也存在差异。其中风干状态土壤经湿润培育处理后,NH4+-N含量呈先降后升趋势,正好与脲酶活性变化趋势相反;而NO3--N含量整体上呈上升趋势,可溶性有机碳含量则正好相反。     

Determination of ATP in soils: Effect of soil treatments

The adenosine triphosphate (ATP) contents of seven soil samples were determined after air-drying, freeze-drying, storage, incubation with glucose or water. The amount of ATP extracted was rapidly reduced after air-drying of the field moist soils, but a short period of wetting of the air-dried soils increased their ATP contents significantly. Addition of an ATP-uncoupler to the air-dried soils indicated that the additional amount of ATP extracted after wetting may not be due to synthesis during the wetting, but from some other processes. Freeze-drying of moist soils reduced the amounts of ATP extracted from soils to a lesser extent than air-drying. Storage of the freeze-dried soils at 25° and — 15°C led to substantial losses of ATP.Incubation of soils with and without glucose rapidly increased the ATP contents of soils, particularly those which had been air-dried previously.Biomass C: ATP ratios in two soils declined during the first few days of incubation and then became relatively constant as incubation proceeded, although significantly different for a loam and a clay soil.     

Microbial biomass and its activities in salt-affected coastal soils

Seasonal fluctuations in salinity are typical in coastal soils due to the intrusion of seawater in the groundwater. We studied the effect of salinity on the microbial and biochemical parameters of the salt-affected soils of the coastal region of Bay of Bengal, Sundarbans, India. The average pH values and average organic C (OC) contents of soils from nine different sites cultivated with rice (Oryza sativa) ranged from 4.8 to 7.8 and from 5.2 to 14.1 g kg⁻¹, respectively. The average electrical conductivity of the saturation extract (EC_e) during the summer season was about five times higher than that during the monsoon season. Within the nine sites, three soils (S₃, S₄, and S₅) were the most saline. The average microbial biomass C (MBC), average basal soil respiration (BSR), and average fluorescein diacetate hydrolyzing activity (FDHA) were lowest during the summer season, indicating a negative influence of soil salinity. About 59%, 50%, and 20% variation in MBC/OC, FDHA/OC, and BSR/MBC (metabolic quotient, qCO₂), respectively, which are indicators of environmental stress, could be explained by the variation in EC_e. The decrease in MBC and microbial activities with a rise in salinity is probably one of the reasons for the poor crop growth in salt-affected coastal soils.     

Microbial biomass dynamics in recently air-dried and rewetted soils compared to others stored air-dry for up to 103 years

Our aim was to compare the soil microbial biomass concentration and its activity (measured as CO₂-C evolved) following the rewetting and aerobic incubation of soils which have previously been stored air-dry for different periods. Some of the soils have been stored in the Rothamsted sample archive for 103 years, others were comparable freshly sampled soils following air-drying and rewetting and other soils were stored air-dry for 2 years then rewetted for the work described here. Following air-drying, soil ATP concentrations were variable in recently air-dried soil, comprising about 10-35% of the initial ATP concentrations in fresh soil. Following rewetting, the percentage recovery of ATP increased in all soils by 7 days, then declined to between 73% and 87% of the original ATP concentration in the air-dried soils by day 12. Storage of air-dried soils decreased the ability of the microbial biomass to restore its ATP concentrations. For example, the ATP concentration in a soil sampled from stubbed (i.e. tree seedling, saplings and bushes cut frequently to ground level) grassland of the Broadbalk continuous wheat experiment at Rothamsted then air-dried for 2 years was only about 14% of that in the fresh soil at 2 days after rewetting. In other soils from the Hoosfield Barley Experiment, also at Rothamsted, previously given NPK or FYM since 1852, and sampled then stored air-dry for between 13 and 83 years, from 52% to 57% of the ATP in the comparable fresh soils was measured at two days after rewetting. The soil ATP concentration then changed little more up to 12 days. One of the most interesting findings was that while the microbial biomass ATP concentration in the above NPK soils only ranged from about 2 to 4 μmol ATP g⁻¹ biomass C, in the FYM soil the microbial biomass ATP concentrations (range 11.5-13.6 μmol ATP g⁻¹ biomass C) were the same as we repeatedly measure in fresh moist aerobic soil. We do not yet know the reasons for this. More than twice as much CO₂-C was evolved from the long-term stored soils than from freshly sampled ones. However, the specific respiration of the microbial biomass did not change much after the first 12 years of storage, indicating that loss of viability mainly occurred in the earlier years.     

土样制备和保存方法对亚热带土壤反硝化的影响

比较了两种土样制备和保存方法对厌氧培养1周内土壤反硝化及矿化的动态影响。试验结果表明,强烈风干后并经长期存放过的土样显著促进了NO3--N浓度降低速率和N2O排放速率的提高,其反硝化速率和矿化速率分别较稍微风干后无存放时间(即立即开始培养试验)的土样提高了47.3%和31.0%。强烈风干土有机C矿化作用的增强以及易矿化有效态C含量的提高是促进反硝化作用增强的主要原因。风干程度和存放时间对反硝化的促进程度取决于其对有机质矿化影响的相对大小,对有机质矿化的影响越大,反硝化强度增加的幅度也越大。由试验结果可推测,利用风干土的实验室培养方法测定得到的土壤反硝化势可能会过高估计田间原位测定的反硝化势。     

Enhancing Soil Quality through Residue Management in a Rice-Wheat System in Fukuoka,Japan

A long-term experiment (LTE) on a rice-wheat system was initiated in 1963 at the Kyushu National Agricultural Experiment Station, in Fukuoka, Japan, to determine the effects of continuous application of rye grass/wheat straw, rice straw and rice straw compost, alone or in combination with inorganic N on crop yields. Increase in rice yields and enhancement of total soil C and N contents with the application of organic residues in this LTE have been reported earlier. However, evaluation of the changes in the soil microbiological properties and the decomposable C fraction of soil organic matter that is needed for soil quality assessment is still lacking. Soil samples were collected after rice harvest in 2003 from the organic residue treatments and unfertilized control, air-dried and incubated for 1 month under aerobic [50% water-filled pore space (WFPS)] and flooded conditions prior to the analysis of the amount of microbial biomass C (MBC), soil respiration and the amount of potential mineralizable N (PMN). The contents of total C (TC), total N (TN), organic C (OC), hot water-extractable C (HWEC) and permanganate-oxidizable C (POC) were determined from air-dried soils. Organic residue incorporation brought about significant increases in the contents of TC, TN, OC, POC, HWEC and PMN. The largest accumulation of total C (23%) and N (72%) in the soil was from rice straw compost, compared with that from rice straw (C, 7% and N, 33%) and rye grass/wheat straw (C, 9% and N, 29%). Incorporation of rice straw compost also increased the amount of MBC under both aerobic and flooded conditions and basal soil respiration under aerobic conditions only. An efficient utilization of C by microorganisms was indicated by a significantly lower metabolic quotient (qCO₂) in the composted and uncomposted rice straw treatments compared with the control in the “-” N treatment under aerobic conditions. Similarly, the flush of CO₂ after rewetting of dry soil per unit of HWEC was lower in the organic matter treatments, indicating a more efficient C utilization and lower C losses per unit of available C. The content of HWEC was significantly correlated with the basal soil respiration (at 50% WFPS), the amounts of MBC, PMN and with the increase in the content of soil organic C in the residuetreated soils. In the treatments without inorganic N fertilizer, grain yield was significantly correlated with the amounts of total organic C, HWEC, MBC (at 50% WFPS), basal soil respiration (at 50% WFPS) and the amount of PMN.     

Interactions between soil properties and moisture content in crust formation, runoff and interrill erosion from tilled loess soils

Soil-surface seals and crusts resulting from aggregate breakdown reduce the soil infiltration rate and may induce erosion by increasing runoff. The cultivated loess areas of northwestern Europe are particularly prone to these processes.Surface samples of ten tilled silty loamy loess soils, ranging in clay content from 120 to 350 g kg⁻¹ and in organic carbon from 10 to 20 g kg⁻¹, were packed into 0.5 m² plots with 5% slopes and subjected to simulated rainfall applied at 30 mm h⁻¹. The 120 minutes rainfall events were applied to initially field-moist soil, air-dried soil and rewetted soil to investigate the effect of soil moisture content prior to rainfall. Runoff and eroded sediments were collected at 5 minutes intervals. Aggregate stability of the soils was assessed by measuring particle-size distribution after different treatments.All soils formed seals. Runoff rates were between 70 and 90% by the end of the rainfall event for field-moist plots. There were large differences between soil runoff rates for the air-dried and rewetted plots. Interrill erosion was associated with runoff, and sediment concentration in runoff readily reached a steady-state value. Measurements of aggregate stability for various treatments were in good agreement with sealing, runoff and erosion responses to rainfall. Runoff and erosion were lower for air-dried plots than for field-moist plots, and were either intermediate or lowest for rewetted plots, depending on soil characteristics. Soils with a high clay content had the lowest erosion rate when they were rewetted, whereas the soil with a high organic-carbon content had the lowest erosion rate in air-dry conditions. The results indicate the complexity of the effect of initial moisture content, and the interactions between soil properties and climate.     

Air-drying changes the distribution of Hedley phosphorus pools in forest soils

Hedley labile phosphorus(P)pools in soil tend to be several times larger than annual forest requirements,even in highly weathered soils characterized by P limitation.The discrepancy between plant and soil P status could be partly attributable to the frequently adopted air-drying pretreatment that tends to increase soil P solubility.In this study,the effects of air-drying on the distribution of Hedley P fractions were examined using soils collected under 4 forest types at Gongga Mountain,southwestern China.The results showed that the microbial biomass P(Pmic)in the organic horizon decreased markedly after air-drying.The concentrations of Hedley labile P in the air-dried samples were 31%–73%more than those in the field-moist samples.Consequently,the air-drying-induced increments of Hedley labile P pools in the surface soil horizons were 0.8–3.8 times the annual plant P requirements.The organic horizon was more susceptible to the air-drying-induced increases in Hedley labile P than the mineral horizon,probably because of the stronger release of Pmicand disruption of soil organic matter.The quality of P,indexed by the ratio of Hedley labile P to slowly cycling P,shifted in favor of the Hedley labile fractions after air-drying,further revealing that air-drying changed the distribution of Hedley P pools in forest soils.These indicated that the effects of air-drying could not be ignored when interpreting the discrepancy between the P status of plants and the Hedley labile P pools in forest soils.To more efficiently evaluate the P status in forest soils via the Hedley fractionation procedure,the use of field-moist soils is recommended.     

Responses of carbon,nitrogen and phosphorus to two consecutive drying–rewetting cycles in soils

Drying and rewetting cycles are known to be important for the dynamics of carbon (C), phosphorus (P), and nitrogen (N) in soils. This study reports the short‐term responses of these nutrients to consecutive drying and rewetting cycles and how varying soil moisture content affects microbial biomass C and P (MBC and MBP), as well as associated carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions. The soil was incubated for 14 d during which two successive drying–rewetting episodes were imposed on the soils. Soils subjected to drying (DRW) were rewetted on the seventh day of each drying period to return them to 60% water holding capacity, whilst continually moist samples (M), with soil maintained at 60% water holding capacity, were used as control samples. During the first seven days, the DRW samples showed significant increases in extractable ammonium, total oxidized nitrogen, and bicarbonate extractable P concentrations. Rewetting after the first drying event produced significant increases only in CO₂ flux (55.4 µg C g^?1 d^?1). The MBC and MBP concentrations fluctuated throughout the incubation in both treatments and only the second drying–rewetting event resulted in a significantly MBC decrease (416.2 and 366.8 mg kg^?1 in M and DRW soils, respectively). The two drying–rewetting events impacted the microbial biomass, but distinguishing the different impacts of microbial versus physical impacts of the perturbation is difficult. However, this study, having a combined approach (C, N, and P), indicates the importance of understanding how soils will react to changing patterns of drying–rewetting under future climate change.     

冀北辽河源油松天然林土壤微生物碳代谢特征研究

本文以冀北辽河源地区不同林龄油松天然林为研究对象,研究其土壤微生物生物量碳、微生物呼吸及微生物代谢熵随油松林龄的变化趋势。结果表明:随着油松天然林林龄的增加,土壤微生物生物量碳逐渐增加;而土壤微生物呼吸则呈现出先减小后增加的趋势;微生物代谢熵表现为随油松林林龄的增加而降低。相关性分析表明,土壤微生物生物量碳、微生物呼吸分别与微生物代谢熵之间呈现高度的极显著线性负相关。微生物生物量碳与微生物呼吸呈极显著正相关,但线性相关程度较弱。土壤微生物生物量碳和微生物呼吸与土壤温度和含水量均呈极显著正相关,而土壤微生物代谢熵则与土壤温度、土壤含水量呈极显著负相关。上述结果表明,在冀北辽河源地区,土壤微生物生物量碳、微生物呼吸、微生物代谢熵与油松天然林林龄密切相关。随着油松天然林林龄的增加,其土壤微生物活性增强,碳代谢效率增加,土壤质量及可持续利用潜力更高,土壤生态体系更加成熟。     

Effect of sample pretreatment on aggregate stability measured by wet sieving or turbidimetry on soils of different cropping history

The effects of cropping history (pasture or arable) and sample pretreatment (field-moist, air-dried or air-dried and then tension or vacuum rewetted) on aggregate stability as measured by wet sieving or turbidimetry were compared. When field-moist samples were used there was a tendency for aggregate stability, as measured by wet sieving, to decline with increasing time under arable cropping (i.e. decreasing soil organic matter content). Air-drying samples caused a pronounced decline in stability of soils from under arable management and as a consequence there was a marked decline in stability with increasing time under arable. Use of tension or vacuum rewetted samples resulted in high values of stability which were unaffected by cropping history. For turbidimetry, there was a marked decline in measured stability with increasing time under arable cropping when field-moist samples were used. Air-drying caused an increase in measured stability that was relatively greater for the less stable samples. In comparison with air-dried samples, tension and vacuum rewetting caused a decrease in stability values for relatively unstable soils. It is suggested that, upon air-drying (and contraction of aggregates), additional intermolecular associations were formed between soil constituents thus conferring greater stability on aggregates. This resulted in reduced dispersion (and the release of particles <0.04 mm in diameter) from the surfaces of aggregates and slaked aggregate fragments following rapid rewetting. As a consequence stability as measured by turbidimetry was increased by drying. For aggregates from a predominantly arable history, this stabilization was not great enough to prevent slaking occurring following rapid rewetting, with the formation of a large proportion of stabilized fragments <0.5 mm in diameter. The stability of these aggregates as measured by wet sieving was therefore decreased by drying.     

Evaluation of the Effects of Soil Depth on Microbial Activity in Three Agroecosystems in Venezuela

The objective of this study was to determine the total organic carbon (TOC), basal respiration (BR), microbial biomass carbon (MBC), the metabolic quotient (qCO₂), and ratio of microbial biomass carbon to total organic carbon (MBC/TOC) at different soil depths in three agroecosystems in Fundo Zamorano Independencia, Cojedes State, Venezuela. The types of agroecosystems studied were short-cycle corn and bean cropland, pastureland, and gallery forest, which was used as the control soil. The BR was determined by measuring the carbon dioxide released in a closed system, and the MBC was analyzed by applying the substrate-induced respiration method. The BR and the MBC varied depending on depth and transect position, decreasing between 18% and 38% at a depth of >10 cm with respect to the measurements taken between 0 and 10 cm. Significantly greater BR and MBC values were found at the middle transect position, where the soils with the greatest moisture content are located. No significant differences for BR, MBC, and qCO₂ values were found among the agroecosystems. This was due to the high spatial variability of the physical and chemical properties of the soils in the study site. However, with the application of the Kruskal–Wallis test, significant differences for the TOC and MBC/TOC values were found among the agroecosystems. These results highlight the importance of the type of plant cover over soil in different agroecosystems, mainly on pastureland, which maintains the potential of the inocular mycorrhizal of the soil.     

Interactions between soil organic matter status, cropping history, method of quantification and sample pretreatment and their effects on measured aggregate stability

 The effects of sample pretreatment (field-moist, air-dried or tension rewetted) on aggregate stability measured by wet sieving or turbidimetry were compared for a group of soil samples ranging in organic C content from 20 to 40 g C kg^–1. Concentrations of total N, total and hot-water-extractable carbohydrate and microbial biomass C were linearly related to those of organic C. Aggregate stability measured by wet sieving using air-dried or field-moist samples and that measured by turbidimetry, regardless of sample pretreatment, increased curvilinearly with increasing soil organic C content. However, when tension-rewetted samples were used for wet sieving, aggregate stability was essentially unaffected by soil organic C content. Measurements of aggregate stability (apart from wet sieving using rewetted soils) were closely correlated with one another and with organic C, total and extractable carbohydrate and microbial biomass C content of the soils. The short-term effects of aggregate stability were also studied. Soils from under long-term arable management and those under long-term arable followed by 1 or 3 years under pasture had similar organic C contents, but aggregate stability measured by turbidimetry and by wet sieving using air-dried or field-moist samples increased with increasing years under pasture. Light fraction C, microbial biomass and hot-water-extractable carbohydrate concentrations also increased. It was concluded that both total and labile soil organic C content are important in relation to water-stable aggregation and that the use of tension-rewetted samples to measure stability by wet sieving is unsatisfactory since little separation of values is achieved. Received: 6 January 1999     

Effect of rewetting air-dried soils on pH and accumulation of mineral nitrogen

The effect of rewetting a number of air-dried soils on pH and on accumulation of mineral-N was examined in a laboratory incubation study. When rewetted-soils were incubated at 25°C three patterns of change in soil _pH and in accumulation of mineral-N were observed. Ammonification and nitrification proceeded together in soils with pH values greater than 6.0; soil pH decreased whilst concentrations of nitrate rose and those of ammonium remained low. By contrast, in soils with pH values less than 5.0, although ammonification proceeded there was no appreciable nitrification; soil pH increased whilst concentrations of ammonium rose and those of nitrate remained very low. In a third group of soils with pH values between 5.0 and 5.5, there was a delay in nitrification, but ammonification was not retarded; soil _pH initially rose as concentrations of ammonium increased, but when nitrification subsequently commenced the _pH decreased, concentrations of nitrate rose and those of ammonium declined. When microbial activity in rewetted soils was inhibited by incubation at 3°C, or in a chloroform atmosphere at 25°C, there was little change in concentrations of ammonium and nitrate, and soil pH remained relatively constant.
Such changes in soil _pH, induced by ammonification and nitrification, are likely to have important consequences to soil chemical studies where _pH-dependent reactions are being studied using rewetted soils. Changes in pH can be minimized by using field moist rather than air-dried soils.     

Land use changes the soil carbon stocks,microbial biomass and fatty acid methyl ester (FAME) in Brazilian semiarid area

Land use affect soil C and microbial structure, especially in tropical dry areas. The objective of this study was to evaluate the effects of the land use on physical, chemical, and microbiological attributes of soils from Brazilian semiarid. We analyzed soil physical, chemical, total carbon stocks (TCS), water-soluble carbon (WSC), microbial biomass carbon (MBC) and microbial structure of soil from forest, no irrigation maize, succession areas (Anadenanthera falcate and Tabebuia alba) and secondary shrubby vegetation. The use of soil influences C stock. The forest soil showed higher TCS and MBC. The conversion in T. alba reduced in 9% soil total bacteria. The multivariate analysis showed that TCS, MBC and FAMEs contributed to separation of natural forest and other areas in the superficial layer. This study indicates that the conversion of forest into successional areas can decrease by up to 44% TCS and 68% MBC. The present study provided alarming data concerning the impact of land use on quality of soil in a tropical dry region in Northeastern Brazil. Our results provide an alternative tool for the management of deforested dry areas that could serve as guideline for management plan to sustainability for agricultural impacted dry areas.     

The relationships among microbial parameters and the rate of organic matter mineralization in forest soils,as influenced by forest type

Vegetation type influences the rate of accumulation and mineralization of organic matter in forest soil, mainly through its effect on soil microorganisms. We investigated the relationships among forest types and microbial biomass C (MBC), basal respiration (R_B), substrate-induced respiration (R_S), N mineralization (N_min), specific growth rate μ, microbial eco-physiology and activities of seven hydrolytic enzymes, in samples taken from 25 stands on acidic soils and one stand on limestone, covering typical types of coniferous and deciduous forests in Central Europe. Soils under deciduous trees were less acidic than soils of coniferous forests, which led to increased mineralizing activities R_B and N_min, and a higher proportion of active microbial biomass (R_S/MBC) in the Of horizon. This resulted in more extractable organic C (0.5 M K₂SO₄) in soils of deciduous forests and a higher accumulation of soil organic matter (SOM) in coniferous forest soil. No effect of forest type on the microbial properties was detected in the Oh horizon and in the 0–10 cm layer. The microbial quotient (MBC/C_org), reflecting the quality of organic matter used for microbial growth, was higher in deciduous forests in all three layers. The metabolic quotient qCO₂ (R_B/MBC) and the specific growth rate μ, estimated using respiration growth curves, did not differ in soils of both forest types. Our results showed that the quality of SOM in coniferous forests supported microorganisms with higher activities of β-glucosidase, cellobiosidase and β-xylosidase, which suggested the key importance of fungi in these soils. Processes mediated by bacteria were probably more important in deciduous forest soils with higher activities of arylsulphatase and urease. The results from the stand on limestone showed that pH had a positive effect on microbial biomass and SOM mineralization.     

Reduced tillage with residue retention improves soil labile carbon pools and carbon lability and management indices in a seven-year trial with wheat-mung bean-rice rotation

Soil total organic carbon (TOC) is a composite indicator of soil quality with implications for crop production and the regulation of soil ecosystem services. Research reports on the dynamics of TOC as a consequence of soil management practices in subtropical climatic conditions, where microbial carbon(C) loss is high, are very limited. The objective of our study was to evaluate the impact of seven years of continuous tillage and residue management on soil TOC dynamics (quantitative and qualitati...