Root exudate effects on the bacterial communities, CO2 evolution, nitrogen transformations and ATP content of rhizosphere and bulk soils

The ATP content, soil respiration, bacterial community composition, and gross N mineralization and immobilization rates were monitored under laboratory condition at 25 °C for 28 d in a model system where low molecular weight root exudates (glucose and oxalic acid) were released by a filter placed on the surface of a forest soil also treated with ¹⁵N, so as to simulate rhizosphere conditions. Periodically, the soil was sampled from two layers, 0-2 and 6-14 mm below the filter's surface, which were indicated as rhizosphere and bulk soils, respectively. The isotope dilution technique was used to determine the effect of these low molecular weight organic compounds (LMWOCs) on gross N mineralization and immobilization rates. From 0 to 3 d both glucose and oxalic acid amended soils showed a rapid evolution of CO₂, more pronunced in the latter treatment together with a decrease in the amount of mineral N of the rhizosphere soil, probably due to N immobilization. Nevertheless, these changes were accompanied by a very small increase in the net ATP content probably because the low C application rate stimulated microbial activity but microbial growth only slightly. A positive ‘priming effect’ probably developed in the oxalic acid amended soil but not in the glucose amended soil. Gross N mineralization and immobilization rates were only observed in the rhizosphere soil, probably due to the greater C and N concentrations and microbial activity, and were a little higher in both amended soils than in the control soil, only between 1 and 7 d. Both glucose and oxalic acid influenced the bacterial communities of the rhizosphere soil, as new bands in the DGGE profiles appeared at 3 and 7 d. Glucose induced lower changes in the bacterial community than oxalic acid, presumably because the former stimulated a larger proportion of soil microorganisms whereas the latter was decomposed by specialized microorganisms. Peaks of net daily soil respiration and net ATP content and the appearence of new dominant bacterial populations were shifted in time, probably because there was less ATP synthesis and DGGE patterns changed after complete substrate mineralization.     

单宁酸对不同pH茶园土壤中活性铝形态分布的影响

采集云南省普洱市和江西省南昌县两地典型的茶园土壤,通过添加HCl和Ca(OH)2调节土壤pH,研究不同pH(3.0、3.5、4.0、4.5)茶园土壤添加0.4 mmol·kg 1、2.0 mmol·kg 1、4.0 mmol·kg 1、8.0 mmol·kg 1、12.0 mmol·kg 1单宁酸后,活性铝形态交换态铝(Al3+)、单聚体羟基铝[Al(OH)2+、Al(OH)+2]、酸溶无机铝[Al(OH)03]和腐殖酸铝[Al-HA]的分布特征。结果表明:单宁酸添加量为0~0.4 mmol·kg 1和0~2.0 mmol·kg 1时,江西南昌和云南普洱茶园土壤中交换态铝随土壤pH的增加呈明显下降趋势,而羟基态铝、酸溶无机铝和腐殖酸铝呈逐渐上升趋势;当单宁酸浓度增至2.0 mmol·kg 1以上时,随土壤pH的增加,单宁酸对活性铝释放的抑制作用增强,各形态活性铝含量都较低,且不同pH处理土壤间的差异不显著。0~20 cm土层土壤与20~40 cm土层土壤变化规律大致相似,总体上看,下层土壤活性铝总量高于上层。云南普洱茶园土壤活性铝总量明显高于江西南昌的茶园土壤。相关分析表明,0~20 cm土层土壤中,pH与羟基态铝、腐殖酸铝、土壤酸碱缓冲容量(pHBC)呈正相关(r=0.796,P0.01;r=0.960,P0.01;r=0.852,P0.01);pHBC与交换态铝、羟基态铝呈负相关(r=0.904,P0.01;r=0.645,P0.05),而与腐殖酸铝呈正相关(r=0.795,P0.01)。同时,单宁酸加入浓度为0~0.4 mmol·kg 1时,土壤pH明显上升,之后随着单宁酸加入浓度的增加土壤pH持续下降,土壤pH(YpH)与单宁浓度(CDN)在此阶段基本符合方程:YpH=0.04CDN+3.82(R2=0.95,P0.01)的线性变化趋势,在单宁酸浓度达到8.0~12.0 mmol·kg 1时,土壤pH基本不再变化。     

种植苹果树对渭北果园土壤胶结物质分布的影响

本研究通过系统研究种植果树对土壤胶结性物质的演化规律及其与土壤团聚体稳定性之间关系的影响,探索影响果园土壤团聚体状态的因素,以期为果园科学管理提供理论依据。在渭北旱塬苹果主产区分别选取10 a、20 a的苹果园和农田(冬小麦-夏玉米轮作,对照)各4个,在果树冠层投影范围内距树干2/3处逐层采集0~100 cm土层土壤样品和0~50 cm土层原状土壤样品,研究不同植果年限果园及农田土壤剖面黏粒、有机质、CaCO_3等团聚体胶结物质的分布及其与团聚体稳定性之间的关系。结果发现:在0~100 cm土层范围内,各果园土壤黏粒含量基本随土层深度的增加而递增,且在0~40 cm土层表现为农田10 a果园20 a果园,40 cm以下土层则呈现相反的态势;种植果树相比农田可显著增加0~100 cm土层土壤有机质总储量,但随着种植果树年限的增加,土壤有机质总储量呈递减趋势;在0~100 cm土层土壤CaCO_3总储量表现为10 a果园农田20a果园,但在0~40 cm土层CaCO_3含量及储量表现为10 a果园农田20 a果园,而40~100 cm土层则为20 a果园10 a农田。皮尔森相关分析发现(29)0.25 mm土壤团聚体的数量和平均重量直径(MWD)与土壤黏粒、有机质和CaCO_3含量密切相关,其中机械稳定性团聚体的数量和稳定性主要受土壤中CaCO_3、有机质含量的影响,水稳性团聚体的数量和稳定性主要受土壤中黏粒和CaCO_3的影响。总之,植果显著改变了土壤中黏粒、有机质、CaCO_3的演化过程和趋势,随植果年限增加,果园土壤黏粒和CaCO_3在土壤较深土层淋溶淀积明显;各果园土壤有机质总储量虽然高于农田,但随植果年限增加,有逐渐减少的趋势。可见植果明显加速了渭北黄土塬地土壤的残积黏化和钙化过程,影响着表层土壤团聚作用和底层土壤的紧实化和坚硬化程度。     

有机酸对几种土壤胶体吸附解吸镉离子的影响

用平衡法研究了有机酸对土壤胶体吸附 解吸Cd2 的影响。结果表明 ,黄棕壤、红壤、砖红壤胶体Cd2 最大吸附容量 (Qm)分别为 4 3 7、16 8、1 5 8mmolkg-1。在加入Cd2 浓度相同的条件下 ,土壤胶体Cd2 吸附量随有机酸浓度的升高呈峰形曲线变化。当有机酸与Cd2 共存时 (竞争吸附 ) ,低浓度的草酸 (小于0 5～ 2mmolL-1)或柠檬酸 (小于 0 0 2 5～ 0 2mmolL-1)提高Cd2 吸附量 ,高浓度的草酸或柠檬酸能降低Cd2 吸附量。吸附有机酸后的土壤胶体 (次级吸附 )对Cd2 次级吸附量的影响与竞争吸附一致 ,但两者的Cd2 吸附量变化幅度不一样。这是由于两种吸附体系液相中有机酸残留浓度不同所致。土壤胶体吸附态Cd2 的解吸结果表明 ,草酸浓度不仅影响Cd2 的总解吸量、总解吸率 ,还影响土壤胶体表面KNO3 解吸态与DTPA解吸态Cd2 的分配比例     

Degradation of [14C]phosphinothricin (glufosinate) in soil under laboratory conditions: Effects of concentration and soil amendments on 14CO2 production

Summary Degradation of the herbicide phosphinothricin (L-homoalanine-4-yl-(methyl)-phosphinic acid) in a phaeozem was investigated by monitoring the ¹⁴CO₂ release from [1-¹⁴C] and [3,4-¹⁴C]phosphinothricin. The degradation was largely due to microbial activity, since the rate decreased by more than 95% when the soil was sterilized by -radiation. Data obtained with both labels suggested that decarboxylation of phosphinothricin preceded oxidation of its C-atoms 3 and 4, since a metabolite, probably 3-methylphosphinico-propanoic acid, was only labelled when [3,4-¹⁴C]phosphinothricin was used as the substrate. Maximum rates of ¹⁴CO₂ production from both the 1- and 3,4-label positions occurred without a lag phase during the breakdown of phosphinothricin as monitored for a total of 30 days at 5-day intervals. This result indicated that a phosphinothricin-degrading microbial community was already present in the soil. With low concentrations of [1-¹⁴C]phosphinothricin (10.7 mg kg^-1 soil), complete decarboxylation at 25°C was observed within 30 days of incubation, compared to 15.9% ¹⁴CO₂ release from [3,4-¹⁴C]phosphinothricin. Increasing the quantity of the herbicide in the soil (10.7–1372 mg kg^-1) resulted in increased degradation rates, irrespective of whether the herbicide was labelled in the positions 1 or 3 and 4. Addition of glucose and other carbohydrates stimulated ¹⁴CO₂ release while addition of a yeast extract had a negative effect. Glucose stimulation was reversed by ammonium nitrate, suggesting that the microorganisms were using the herbicide as a source of N.     

周原全新世复合古土壤和成壤环境的微形态学研究

通过对陕西周原黄土剖面的微形态研究 ,结合微量元素、粒度及磁化率分析 ,探讨了黄土风化成壤的特征 ;揭示出该剖面的全新世古土壤S0 是温暖湿润的环境下形成的具有强烈粘化特征的复合土壤 ,是 60 0 0～ 5 0 0 0aB .P .出现的区域性干旱气候事件造成风尘加速堆积的结果。关中盆地在 85 0 0～60 0 0aB .P .风化成壤最为强烈 ,气候最为暖湿。在 5 0 0 0～ 3 1 0 0aB .P .又出现了一个比较强烈的成壤时期。而最近 3 1 0 0年以来是一个风尘堆积较强的相对干旱期。     

Spatial variability of 2,4-dichlorophenoxyacetic acid (2,4-D) mineralisation potential at a millimetre scale in soil

We analysed the ability of soil units of millimetre size to mineralise a herbicide, 2,4-D, using incubations of individual aggregates (2-7 mm diameter) and 6×6×6 mm³ cubes dissected from soil cores, under standard conditions. Mineralisation of ¹⁴C-ring labelled 2,4-D was measured using a barite paper trap and a Phosphorimager to record the evolved ¹⁴C-CO₂ from these very small soil samples. We found a large variability of 2,4-D mineralisation potential between aggregate size classes, between individual aggregates of the same size and between the different dissected cubes from a given core. We explained this variability by an uneven distribution of the degrading microorganisms at this scale, and to a lesser extent, an uneven distribution of C, necessary for co-metabolism. Furthermore, we found that in a soil core, the dissected cubes with a large mineralisation potential were not randomly distributed, but rather organised into centimetre sized hot spots.     

玉米秸秆全量深翻还田对高产田土壤结构的影响

为达到玉米生产耕层最适深度(22 cm)和耕层最适土壤容重(1.1~1.3 g×cm~(-3)),解决内蒙古平原灌区耕层浅、犁底层坚硬且厚的农田土壤结构问题,分别选用连续1、2、3、4年秸秆深翻还田定位试验地,秋收后玉米秸秆全量粉碎深翻还田,秸秆年均还田量为20 034.97 kg×hm-2,形成秸秆深翻还田1~4年的4个试验处理(SF1-SF4),以不深翻秸秆还田的处理为对照(CK),研究土壤容重、土壤坚实度、土壤团聚体及其稳定性、土壤肥力及p H随不同年限秸秆深翻还田的变化规律。结果表明:1)SF1-SF4处理0~40 cm土层,土壤容重和土壤坚实度比CK显著减小。2)0~20 cm土层,SF4处理0.25 mm团聚体比例(R0.25)、几何平均直径(GWD)和平均重量直径(MWD)均比CK显著减小;SF1处理土壤团聚体破坏率(PAD)比CK显著降低9.56%,不稳定指数(SWA)随深翻年限增加而显著降低;团聚体分形维数SF4比CK显著增大7.30%。3)20~40 cm土层,SF1和SF2处理R0.25比CK分别显著增加13.69%和17.83%;SF2处理的MWD和GWD分别比CK显著增加23.92%和53.38%;SF1-SF4处理的PAD比CK显著降低,且SF2显著高于SF1和SF3;而SF1-SF4的SWA比CK显著增加,且随秸秆深翻年限的增加呈逐渐升高趋势;团聚体分形维数SF2比CK显著降低7.39%。4)土壤有机质含量SF1-SF4比CK显著增加,且SF2-SF4处理显著大于SF1;速效氮、速效磷和速效钾SF1-SF4比CK显著增加,土壤p H SF3、SF4比CK显著降低。总之,深翻秸秆还田1~4年对0~40 cm土层土壤影响显著;深翻秸秆还田2年适合土壤犁底层结构的改良,深翻秸秆还田3年和4年适合土壤耕层结构的改良。玉米秸秆全量深翻还田既能达到耕作土壤的目的,同时也增加了土壤有机质,降低土壤团聚体破坏率和土壤水稳性团聚体的不稳定系数,利于培肥耕层土壤。     

Soil CO2 flux in six monospecific forest plantations in Southern Rwanda

Forest soils contain the largest carbon stock of all terrestrial biomes and are probably the most important source of carbon dioxide (CO₂) to atmosphere. Soil CO₂ fluxes from 54 to 72-year-old monospecific stands in Rwanda were quantified from March 2006 to December 2007. The influences of soil temperature, soil water content, soil carbon (C) and nitrogen (N) stocks, soil pH, and stand characteristics on soil CO₂ flux were investigated. The mean annual soil CO₂ flux was highest under Eucalyptus saligna (3.92 μmol m⁻² s⁻¹) and lowest under Entandrophragma excelsum (3.13 μmol m⁻² s⁻¹). The seasonal variation in soil CO₂ flux from all stands followed the same trend and was highest in rainy seasons and lowest in dry seasons. Soil CO₂ flux was mainly correlated to soil water content (R² = 0.36-0.77), stand age (R² = 0.45), soil C stock (R² = 0.33), basal area (R² = 0.21), and soil temperature (R² = 0.06-0.17). The results contribute to the understanding of factors that influence soil CO₂ flux in monocultural plantations grown under the same microclimatic and soil conditions. The results can be used to construct models that predict soil CO₂ emissions in the tropics.     

地膜覆盖的玉米地的土壤空气状况

The effects of plastic mulching on soil aeration at the soil depth of 0-100 cm were studied in a corn field. The results indicated that the CO₂ concentration of unmulched soil in the 0-100 cm layer ranged from 0.001 to 0.016 m³/m^3, and that of mulched soil 0.002 to 0.018m³/m³, about 32.39% higher than the former on the average. Such a CO₂ concentration in the soil air is still suitable for crop growth. The O₂ concentration was inversely correlated with CO₂ concentration in the soil air (unmulching r=-0.92^**, mulching r=-0.79^*). O₂ concentration ranged from 0.11 to 0.17 m³/m³ in the mulched soil and 0.13 to 0.18 m³/m³ in the unmulched soil. By contrast, N₂ concentration in soil air remained relatively steady, with no difference between the two treatments. The relationship between the soil respiratory intensity and the depth of a soil layer appeared to be a power function. At the layer of 0-20 cm, the soil respiration intensity in the mulched soil was obviously higher than that in the unmulched. Plastic mulching could also affect soil structure. In comparison with the unmulched soil, the content of >0.25 mm aggregate and 0.05-0.001 mm microaggregate in the mulched soil was reduced by 82.1% and 35.8%, respectively; the soil total porosity, gaseous phase rate and aeration porosity in the depth of 10-20 cm were reduced by 2.85%, 19.89% and 26.54% respectively, but contrary at the depth of 0-10 cm.     

不同灌水量对黄河三角洲盐碱地改良效果研究

通过室内土柱模拟试验,研究不同灌水量对盐渍土的改良效果。试验共设计3个不同灌水量,分别为S1 (200 mm)、S2 (300 mm)、S3 (400 mm)。结果表明:(1)灌溉淋洗对0—40 cm土壤盐分淋失影响较大,其中0—20 cm脱盐率最高,表现为S3S2S1,不同处理在剖面上均出现积盐,S1、S2在40—60 cm出现积盐,说明低灌水量对土壤表层盐分具有淋洗作用,但会造成底层土壤盐分累积;(2)K~+、Na~+、Ca~(2+)、Mg~(2+)、Cl~-、SO_4~(2-)、HCO_3~-含量在灌水后有较大幅度的下降,且整体溶脱率随灌溉水量的增加而增加。各离子在0—20 cm随灌水量增加表现为不同的变化规律,Ca~(2+)先溶脱后积累,HCO_3~-变化规律与Ca~(2+)相反,表现为先积累后溶脱,其他离子均随灌水量的增加而减少;(3)灌水后,S1、S2的pH在剖面上的分布与灌水前相似,0—40 cm土壤总碱度随灌水量增加呈先增后减的趋势,表现为先碱化再脱碱,与土壤pH变化一致。研究成果可为盐碱地改良和节水灌溉提供参考。     

Microbial activity and hydrolase activities during decomposition of root exudates released by an artificial root surface in Cd-contaminated soils

The aim of this study was to assess the stimulatory effects of different low molecular weight organic compounds commonly present in root exudates on microbial activity and hydrolase activities, and the effects of high Cd concentrations in sandy soils collected from contaminated field plots on the stimulatory effects. Glucose, glutamic acid, citric acid, oxalic acid, or a mixture of all compounds were released by an artificial root surface in a simplified rhizosphere system. The effects were measured at <2 mm (rhizosphere soil layer) and >4 mm (bulk soil layer) distance from the root surface, 7 d after the root exudates release. Results showed that different root exudates were mineralized at different extent and had different stimulatory effects on microbial growth estimated by dsDNA content of soil, and on hydrolase activities, mostly localized in the rhizosphere soil layer. Mineralization of root exudates, microbial growth and stimulation of most of the measured hydrolase activities were drastically reduced by high Cd concentrations in soil.     

Decomposition of C-labeled roots in a pasture soil exposed to 10 years of elevated CO2

The net flux of soil C is determined by the balance between soil C input and microbial decomposition, both of which might be altered under prolonged elevated atmospheric CO₂. In this study, we determined the effect of elevated CO₂ on decomposition of grass root material (Lolium perenne L.). ¹⁴C-labeled root material, produced under ambient (35 Pa pCO₂) or elevated CO₂ (70 Pa pCO₂) was incubated in soil for 64 days. The soils were taken from a pasture ecosystem which had been exposed to ambient (35 Pa pCO₂) or elevated CO₂ (60 Pa pCO₂) under FACE-conditions for 10 years and two fertilizer N rates: 140 and 560 kg N ha⁻¹ year⁻¹. In soil exposed to elevated CO₂, decomposition rates of root material grown at either ambient or elevated CO₂ were always lower than in the control soil exposed to ambient CO₂, demonstrating a change in microbial activity. In the soil that received the high rate of N fertilizer, decomposition of root material grown at elevated CO₂ decreased by approximately 17% after incubation for 64 days compared to root material grown at ambient CO₂. The amount of ¹⁴CO₂ respired per amount of ¹⁴C incorporated in the microbial biomass (q¹⁴CO₂) was significantly lower when roots were grown under high CO₂ compared to roots grown under low CO₂. We hypothesize that this decrease is the result of a shift in the microbial community, causing an increase in metabolic efficiency. Soils exposed to elevated CO₂ tended to respire more native SOC, both with and without the addition of the root material, probably resulting from a higher C supply to the soil during the 10 years of treatment with elevated CO₂. The results show the importance of using soils adapted to elevated CO₂ in studies of decomposition of roots grown under elevated CO₂. Our results further suggest that negative priming effects may obscure CO₂ data in incubation experiments with unlabeled substrates. From the results obtained, we conclude that a slower turnover of root material grown in an ‘elevated-CO₂ world’ may result in a limited net increase in C storage in ryegrass swards.     

旱地小麦休闲期覆盖施磷对土壤水库的调控作用

为探明休闲期覆盖配施磷肥对土壤水分运行规律、小麦产量和水分利用效率的影响,在山西省闻喜县进行了休闲期覆盖与不覆盖条件下75 kg(P2O5)·hm-2、112.5 kg(P2O5)·hm-2、150 kg(P2O5)·hm-23个施磷量的田间试验。结果表明:与不覆盖相比,休闲期覆盖后,播种—孕穗期0~100 cm土壤蓄水量显著提高,小麦播种期提高38~41 mm;增加施磷量,越冬—孕穗期土壤蓄水量提高,尤其拔节期40~100 cm土层。覆盖后,播种—拔节期土壤贮水减少量及其占整个生育期比例显著提高,拔节—开花期土壤贮水减少量增加;增加施磷量,拔节—开花期土壤贮水减少量及其比例显著提高,开花—成熟期80~100 cm土层贮水减少量显著提高。覆盖后增加施磷量,产量和水分利用效率显著提高,产量提高1 452 kg·hm-2,水分利用效率提高16%。覆盖配施磷肥条件下,拔节—开花期60~100 cm、开花—成熟期80~100 cm土层贮水减少量与产量呈极显著相关。因此认为,旱地小麦休闲期覆盖有利于蓄积休闲期降雨,提高底墒,可实现伏雨春夏用;覆盖促进小麦生育前期和中期吸收土壤水分;增施磷肥有利于提高土壤水分,促进小麦生育后期深层吸水;旱地小麦休闲期覆盖配施磷肥150 kg·hm-2有利于蓄水保墒,达到增产、高效的目的。     

Seasonal changes in multi-scale spatial structure of soil pH and related parameters along a tropical dry evergreen forest slope

Seasonal changes in multi-scale spatial variation in soil chemical properties, which may be controlled simultaneously by biotic and abiotic factors, have not been studied in tropical dry forests. We evaluated the spatial variation of physico-chemical soil properties, plant litter and terrain attributes at multiple scales in a tropical dry evergreen forest using multivariate geostatistics. Soil samples were collected at different depths using nested interval sampling during 1- and 10-m intervals in both the wet and dry seasons. We measured pH, exchangeable cations (Ex-K⁺ and Ex-Ca²⁺), acidity (Ex-H⁺ and Ex-Al³⁺), particle size (clay and sand contents), and forest floor mass (O_i and O_a). Pronounced spatial variation in pH was observed in surface soil (0-5 cm) but not in deeper soil (5-55 cm). Multi-scale spatial structures with short (20 m) and long (86 m) ranges were observed in the auto- and cross-variograms of soil, litter and slope gradient. Pronounced multi-scale structures were observed simultaneously in pH and Ex-Ca²⁺ both in the wet and dry seasons. Only a short-range structure was observed in Ex-K⁺ and O_a, whereas a long-range structure was pronounced in sand contents and slope gradients. Although the variograms had similar shapes between wet and dry seasons for almost all variables, the short-range structure of the cross-variogram between O_a with pH and base cations was more pronouncedly developed in the wet season than in the dry season. Scale-dependent correlation coefficients suggest that a small-scale spatial variation in pH was connected to heterogeneous litter accumulation via base-cation input, whereas long-range spatial variation was simultaneously linked to particle size and slope gradient. This multivariate geostatistical approach applied within a stand detected biotic and abiotic factors controlling spatial variation in soil properties at both short and long distances.     

草酸/草酸盐对森林暗棕壤的磷释放效应

模拟森林凋落物淋洗液中的草酸/草酸盐浓度范围,设计了不同浓度草酸/草酸盐溶液一次性浸提和多次连续浸提系列实验,其中的草酸(阴离子)载荷量为0~200 mmol kg-1。结果表明,草酸能显著促进暗棕壤A1层(腐殖质层)磷的释放,土壤磷溶出量随草酸溶液浓度升高而线性增加;但对B层土壤磷的释放效应相对较弱,草酸浓度低于5mmol L-1时B层磷的释放不明显。pH 5.16草酸钠溶液比相同浓度的草酸溶液具有更高的解磷效率,在设置二者浓度为0.5~20.0 mmol L-1时,前者的解磷量是后者的1.51~2.98倍,推断草酸盐溶液或凋落物淋洗液中草酸(盐)类物质促进暗棕壤磷释放的主要机理在于草酸阴离子(C2O42-)配位反应。草酸盐对暗棕壤磷的释放效应具有一定累加性,土壤磷释放量主要由草酸阴离子累积载荷量决定,而与其加入方式(多次或一次性)关系不大;当以pH5.16草酸钠溶液加入时,土壤磷释放量Y(mgkg-1)与草酸阴离子累积载荷量X(mmol kg-1)间的回归方程为Y=-0.000 4X2 0.176 6X 0.425 3,R2=0.990 2。仅以凋落物层溶出的草酸(阴离子)量进行估计,由此增加的A1层土壤磷释放量达2.40 kg hm-2a-1,大约相当于中龄林年吸收磷量的1/3~1/5,因此其实际作用是不可忽视的。     

不同Zn浓度对黑麦草根分泌物、根际Zn形态及植物Zn蓄集的影响

A glasshouse experiment was conducted using a root-bag technique to study the root exudates, rhizosphere Zn fractions, and Zn concentrations and accumulations of two ryegrass cultivars (Lolium perenne L. cvs. Airs and Tede) at different soil Zn levels (0, 2, 4, 8, and 16 mmol kg-1 soil). Results indicated that plant growth of the two cultivars was not adversely affected at soil Zn level≤8 mmol kg-1. Plants accumulated more Zn as soil Zn levels increased, and Zn concentrations of shoots were about 540 /μg g-1 in Aris and 583.9μg g-1 in Tede in response to 16 mmol Zn kg-1 soil. Zn ratios of shoots to roots across the soil Zn levels were higher in Tede than in Airs, corresponding with higher rhizosphere available Zn fractions (exchangeable, bound to manganese oxides, and bound to organic matter) in Airs than in Tede. Low-molecular-weight (LMW) organic acids (oxalic, tartaric, malic, and succinic acids) and amino acids (proline, threonine, glutamic acid, and aspartic acid, etc.) were detected in root exudates, and the concentrations of LMW organic acids and amino acids increased with addition of 4 mmol Zn kg-1 soil compared with zero Zn addition. Higher rhizosphere concentrations of oxalic acid, glutamic acid, alanine, phenylalanine, leucine, and proline in Tede than in Airs likely resulted in increased Zn uptake from the soil by Tede than by Airs. The results suggested that genotypic differences in Zn accumulations were mainly because of different root exudates and rhizosphere Zn fractions.     

格氏栲天然林与人工林土壤异养呼吸特性及动态

通过用静态碱吸收法对中亚热带福建三明格氏栲自然保护区内的格氏栲天然林和33年生的格氏栲人工林及杉木人工林的土壤异养呼吸进行为期2年的定位研究。结果表明，三种森林枯枝落叶层呼吸和无根土壤呼吸速率季节变化均呈单峰曲线，最大峰值出现在5月至6月，最小值出现在12月至1月。格氏栲天然林、格氏栲人工林和杉木人工林枯枝落叶层呼吸速率平均值分别为CO2 79．88、44．37和21．02mgm^-2h^-1，无根土壤呼吸速率平均值分别为CO2 217．4、85．85和94．04mg m^-2h^-1。2002年枯枝落叶层呼吸速率和无根土壤呼吸速率主要受土壤温度影响，但在极端干旱的2003年则主要受土壤湿度的影响。双因素关系模型（R=ae^bTW^c）拟合结果优于仅考虑土壤温度或土壤湿度的单因素关系模型，土壤温度和土壤湿度共同解释不同年份枯枝落叶层呼吸和无根土壤呼吸速率季节变化的82％～85％和85％～92％。不同森林枯枝落叶层呼吸对土壤温度和湿度的敏感性均高于无根土壤呼吸的。格氏栲天然林、格氏栲人工林和杉木人工林枯枝落叶层呼吸年通量分别为C3．76、2．63和1．23t hm^-2a^-1，无根土壤呼吸年通量则分别为C3．44、2．79和1．49t hm^-2a^-1。不同森林土壤异养呼吸通量的差异主要与枯落物数量和质量、土壤有机质数量和质量的差异有关。杉木林枯枝落叶层呼吸对干旱敏感性高于格氏栲（天然林和人工林）的，而人工林（杉木和格氏栲）的土壤有机C对干旱敏感性则要高于格氏栲天然林。     

No tillage and crop rotation effects on soil aggregation and organic carbon in a Rhodic Ferralsol from southern Brazil

In Brazil, no tillage (NT) is a soil conservation practice now widely adopted by farmers, including smallholders. The effect of NT and conventional tillage (disc ploughing followed by two light disc harrowings, CT) was investigated on the aggregation properties of a clayey Rhodic Ferralsol from southern Brazil under different crop rotations. The same soil type under secondary forest was used as reference. Macro- and microaggregate classes were separated by wet sieving using a series of eight sieves (8, 4, 2, 1, 0.5, 0.25, 0.125, 0.053 mm) at four sampling layers (0–5, 5–10, 10–20, 20–30 cm). The soil in general had high structural stability. At 0–5 cm, meanweight diameter (MWD, 11.1 mm) and total organic C in macroaggregates (TOC, 39 g kg⁻¹ soil) were highest for the forest soil. Soil under NT had a more similar distribution of aggregate size classes and TOC to the forest soil than CT. The most pronounced difference between tillage systems was observed in the surface soil layer (0–5 cm). In this layer, NT had higher aggregate stability (AS_NT: 96%; AS_CT: 89%), had higher values of aggregate size distribution (MWD_NT: 7.9 mm, MWD_CT: 4.3 mm), and had on average 28% greater TOC in all aggregate size classes than CT. Soil under NT had greater TOC in macroaggregates (NT: 22 g kg⁻¹; CT: 13 g kg⁻¹). Crop rotation did not have a significant effect on soil aggregate distribution and TOC. By increasing macroaggregation NT increased organic carbon accumulation in soil.     

C and N mineralization of undisrupted and disrupted soil from different structural zones of conventional tillage and no-tillage systems in northern France

Differences in soil structure created by tillage systems are often believed to have large impacts on C and N mineralization, in turn influencing total soil C and N stocks, CO₂ emissions and soil mineral N supply. The objectives of our work were therefore (i) to study C and N mineralization in undisrupted fresh soils from long-term conventional till (CT) and no-till (NT) systems in northern France and (ii) to evaluate at which scale soil structure plays a significant role in protecting organic matter against C and N mineralization. The in situ heterogeneity of soil structure was taken into account during sampling. Two megastructure zones induced by tillage and compaction were identified in the ploughed layer of CT: zones with loose structure (CT_Loose) and clods with dense structure (CT_Dense). The soil samples in NT were taken from layers that differed in both structure and organic matter content (NT_0-5 and NT_5-20). Soil from the two zones of different megastructure in CT showed similar levels of protection and similar C and N mineralization. Undisrupted soil from NT_0-5 showed greater absolute and specific C and N mineralization than CT_Loose, CT_Dense and NT_5-20. Limited soil structure destruction (sieving through 2 mm) had no effect on C and N mineralization. Increased disturbance (sieving down to 250 μm) only induced a significant increase of both C and N mineralization in the 5-20 cm layer of NT. Further disruption of soil structures (sieving through 50 μm) resulted in greater C and N mineralization for all treatments except C mineralization in the upper layer of NT. Protection in the four structural zones in CT and NT was, in general, greatest in the NT deeper layer and least in the NT upper layer. Our results therefore suggest that physical protection in the 5-20 cm soil layer can partly account for larger C and N stocks in NT, but that the large C and N concentrations in the 0-5 cm soil layer are determined by mechanisms other than physical OM protection.