接种耐酸根瘤菌和施钙对酸性土上紫花苜蓿生长的影响

采用裂区设计法研究了重庆缙云山酸性土上施钙与接种耐酸根瘤菌对紫花苜蓿生长和品质的影响。结果发现:接种耐酸苜蓿根瘤菌对苜蓿植株瘤重、根鲜重、株高、植株上部鲜重、全氮含量和酸性土壤中根瘤菌数量的提高影响显著;施Ca2+ 5 mmol/L与10 mmol/L处理与不施Ca2+处理相比,能显著增加植株根鲜重、株高和地上部鲜重,也能显著增加瘤重;瘤重与植株地上部鲜重、根鲜重和株高呈极显著相关关系,与植株全氮含量呈显著相关关系,说明良好的结瘤性能能够提高酸性土上紫花苜蓿的产量和品质。     

酸性土壤中接种耐酸根瘤菌对豆科植物根际微生态的影响

【目的】接种耐酸豆科根瘤菌可以提高豆科植物耐酸能力。应用PLFA等方法研究接种耐酸根瘤菌对根际土壤微生态的综合影响,从土壤角度阐明接种耐酸根瘤菌缓解土壤酸度对豆科植物胁迫的机制。【方法】盆栽条件下分别向种植于pH 4.8酸性土壤中的葛藤和苜蓿植株相应接种耐酸葛藤根瘤菌068、389、390与耐酸苜蓿根瘤菌91512、91522、91532,于接种后30 d、60 d、90 d、120 d、150 d取样,分析耐酸根瘤菌对土壤养分、可培养微生物、微生物群落结构多样性的影响。【结果】1) 接种耐酸葛藤和苜蓿根瘤菌处理后,根际土壤pH由4.8显著提高至6.0左右,有效磷、速效钾、铵态氮、硝态氮及有机质含量的增加效果显著 (P < 0.05),至120 d 达到最高。120 d样品的有效磷、铵态氮、硝态氮及有机质含量分别为23.16～48.68 mg/kg、61.21～81.96 mg/kg、65.05～86.38 mg/kg和11.85～12.87 g/kg,分别比未接种对照提高24.8%～162.4%、16.6%～56.2%、145.4%～225.8%、1.4%～10.1%;2) 接种苜蓿和葛藤耐酸根瘤菌后土壤中可培养微生物数量显著提高 (P < 0.05),土壤中可培养细菌、真菌和放线菌数量在120 d达到峰值,比同期未接种对照分别提高了61.5%～348.4%、3.4%～441.7%和18.9%～255.2%,分别达到48.00 × 102～133.3 × 107、20.11 × 104～155.9 × 104和3.21 × 104～9.59 × 104 cfu/g鲜土;3) PLFA分析表明,虽然接种耐酸葛藤、苜蓿根瘤菌处理的根际土壤的特征脂肪酸种类数与未接种处理相比并无明显差异,但接种葛藤或苜蓿根瘤菌均明显降低了根际土壤中异构PLFA/反异构PLFA值,显示土壤根际微生态的稳定性得到提高。【结论】在种植豆科植物的酸性土壤中接种相应耐酸根瘤菌能显著提高根际土壤pH,提高根际土壤有机质含量和速效氮磷养分含量,缓解土壤酸性对根际微生物的胁迫。     

生物质炭和Ca(OH)2缓解土壤酸化过程中植物铝毒性的模拟对比研究

随着外源酸输入,酸性土壤改良剂的石灰效应逐渐消退,土壤再次酸化形成铝毒害。作为一种新型酸性土壤改良剂,生物质炭施用后土壤的复酸化过程尚不清楚。本研究通过循环酸浸洗耦合根伸长试验,对比研究了施用生物质炭和熟石灰（Ca(OH)2）后土壤的复酸化过程及其对植物的铝毒性。结果表明,循环酸浸洗有效模拟了土壤的复酸化过程。随着模拟酸化年限增加,生物质炭和Ca(OH)2处理土壤中玉米根系伸长均逐渐受到了抑制。生物质炭相较于Ca(OH)2有效缓解了酸化过程对植物根系的抑制作用。在模拟12年酸输入时,生物质炭处理中玉米根相对伸长率较Ca(OH)2处理高18.6%,生物质炭相较于Ca(OH)2处理展现出更为长效的酸性土壤改良潜力。这一方面是由于生物质炭通过表面阴离子官能团质子化作用减缓了酸化过程中土壤pH的降低,抑制了土壤铝的活化。在模拟12年酸输入时,生物质炭处理土壤溶液Al3+浓度较Ca(OH)2处理低33%。另一方面,酸化过程中生物质炭持续释放Mg2+,在模拟12年酸输入时,生物质炭处理土壤溶液Mg2+浓度和植物Mg2+吸收量均较Ca(OH)2处理高2倍以上。较高的Mg2+浓度可通过调控植物对Al3+的生理响应,缓解植物铝毒害症状。该研究结果可为土壤酸化长效阻控提供理论依据和技术支撑。     

苜蓿内生根瘤菌分布部位与数量变化动态

为探明根瘤菌在苜蓿植株及其种子内的分布规律,对苜蓿植株及种子各部位在不同生育期内生根瘤菌的分布和数量变化进行了研究.结果表明:苜蓿植株内根瘤菌的分布和数量在空间和时间上都具有很大异质性.空间分布上,不同部位组织的内生根瘤菌数量随植株光合产物源-库的运输方向呈逐渐增大的趋势,绝大部分内生根瘤菌分布于植株的根系,并主要分布于毛根,在主根内主要存在于表皮和皮层,中柱分布较少;在植株地上部分,内生根瘤菌主要在营养期末分布于花芽[8.6～9.6×103cfu·g-1(FW)],在蕾期和花期分布于雌蕊子房的子房壁,在结英期分布干荚果果皮(1.07×103cfu·pod-1),在种子成熟期则主要存在于新生的种子中;茎内的根瘤菌数量在营养期和结荚期不足2×102 cfu·g-1(FW),而在蕾期和花期完全绝迹;叶片内则始终不存在根瘤菌.在时间上,结荚期根、荚果皮内的根瘤菌数量明显高于其他时期;花内各器官(不包括花梗)在授粉后根瘤菌数量迅速增加;由子房向荚果发育的过程中,子房壁和胚珠内的根瘤菌数量随时间呈对数增长;胚珠在受精后即存在有内生根瘤菌.并且幼嫩种子内生根瘤菌数量远高于受精胚珠,证明内生根瘤菌能被转运并定殖在发育早期的种子中.两个苜蓿品种成熟种子在收获120 d后,根瘤菌数量比刚收获时分别增加131.46倍("陇东")和11.76倍("游客"),说明根瘤菌进入种子后,仍然有一个继续繁殖增长的过程.     

复合型PGPR和苜蓿对新垦地土壤培肥效果研究

研究了利用根际有益微生物和豆科植物相结合培肥新垦地土壤的效果。试验采用裂区设计法研究了重庆北碚新垦坡耕地中性土壤上种植紫花苜蓿并接种根瘤菌和其他根际有益微生物(PGPR)(如联合固氮菌、解磷菌和解钾菌等)对土壤养分的影响。结果显示:接种根瘤菌+其他PGPR的处理对土壤有机质、全氮、全磷、全钾、有效磷和速效钾的提高均达到显著水平,较只接种根瘤菌的处理分别提高33.5%、22.7%、3.8%、11.5%、11.4%和22.3%,较不接种根瘤菌和PGPR的处理分别高42.2%、58.8%、8%、12.6%、37.2%和40.2%,接种根瘤菌+其他PGPR的效果优于只接种根瘤菌和不接种的。同时,上述处理对豆科植物苜蓿植株瘤重、株高、根鲜重、地上部鲜重以及植株全氮含量的提高均达到显著水平,比只接种根瘤菌的处理分别高44.5%、33.2%、77.3%、76.7%和17.7%。将苜蓿和相应的PGPR两者联合使用有更好的土壤改良效果,加速了新垦地贫瘠土壤的培肥过程。     

铝胁迫下大豆根系分泌物对根际土壤微生态的影响

通过对19个不同基因型大豆品种的耐铝性筛选实验,选择耐铝型的浙春2号和敏感型的浙春3号作为实验材料;设置5个铝处理浓度(0,0.2,0.4,0.6,0.8 g kg-1,即土壤总Al3+浓度分别为0.293,0.493,0.693,0.893,1.093 g kg-1)土壤,大棚种植大豆30d后,取大豆根际、非根际土壤和外源根系分泌物作用下的土壤样品,对各类微生物生理群进行分析,同时测定土壤呼吸速率、纤维分解作用、氨化作用、硝化作用,以及酸性磷酸酶、过氧化氢酶和蔗糖酶等土壤重要酶类的活性。结果显示,土壤铝含量较低条件下(0.2,0.4g kg-1),大豆根系分泌物的应激分泌促使土壤微生物数量增多、微生物物质转化能力增强和土壤酶活性增大;土壤铝含量较高(0.8 g kg-1)时,根系分泌物的分泌相对受抑制,土壤微生物活性和土壤酶活性相应地受到抑制。实验结果还表明,外源根系分泌物也能影响土壤微生态,引起土壤微生物数量和土壤酶活性的变化,以及各种土壤生化作用(如硝化作用等)的改变,显示出一定的缓解铝毒能力,从而减少铝毒对植物的伤害。     

长期施用氮磷钾肥和石灰对红壤性水稻土酸性特征的影响

利用34年的长期定位施肥试验,研究不施肥(CK)、施氮磷钾肥(NPK)和氮磷钾化肥配施石灰(NPK+Ca O)对红壤性水稻土不同形态酸、土壤盐基离子及水稻植株阳离子吸收量的影响,探讨土壤交换性H+和Al3+占交换性酸的比例、土壤盐基离子、植株带出阳离子数量与土壤酸度的关系。结果表明,长期NPK处理早、晚稻土壤p H较CK处理分别降低0.2和0.3个单位,交换性酸提高2.3倍和4.2倍,水解性酸提高35.4%和40.0%;NPK+Ca O处理早、晚稻土壤p H较NPK处理分别提高0.5和0.7个单位,较CK处理分别提高0.3和0.4个单位,交换性酸、水解性酸均显著低于NPK和CK处理(p0.05)。土壤交换性H+、Al3+含量高低顺序均为NPK+Ca OCKNPK。土壤交换性盐基离子以交换性Ca2+所占比例最大(81.8%~89.3%),NPK+Ca O处理交换性Ca2+较CK和NPK处理分别提高40.1%和62.9%。交换性Ca2+、交换性盐基离子、盐基饱和度与土壤p H正相关,与交换性酸、水解性酸负相关,交换性Mg2+与交换性酸、水解性酸负相关,交换性Na+与水解性酸负相关。植株移出带走的钙、镁、钾、钠离子量及其总量对土壤p H、交换性酸和水解性酸有一定影响,但其相关性均不显著。研究表明长期施用化肥条件下通过配施石灰可有效缓解稻田土壤的酸化,促进酸性稻田土壤的生态修复与改良。     

南方酸性旱坡地桔园有机无机肥料配合施用效应研究

通过2年定位试验研究了有机无机肥料配施对南方旱坡地果园柑桔(沙糖桔)产量和品质的影响,并比较了土壤可培养微生物数量、土壤微生物碳、氮、柑桔根际土壤活性铝含量及柑桔根系铝含量的变化。结果表明,与100%化肥处理(CK)相比,20%有机肥+80%化肥(T1)、40%有机肥+60%化肥(T2)、100%有机肥(T3)处理的细菌、放线菌、真菌数量均显著提高,平均分别是对照的1.30、1.40、1.46倍;土壤微生物量碳、氮显著增加,其中T3处理的土壤微生物量碳、氮分别增加45.6%6、3.3%。施用有机肥后,活跃了土壤中的微生物种群,改善了土壤的生物学质量。施用有机肥可缓解旱坡地柑桔园根际土壤铝毒的危害,显著减少柑桔根系对土壤铝的吸收累积。尤其是T3处理,根际土壤活性铝含量较CK有大幅度的降低,降幅达到77.4%。施用有机肥明显地促进了柑桔根系的生长发育,提高果实产量和品质,沙糖桔平均增产率18.5%,增产效果显著。     

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。     

城市污水再生水灌溉对黑麦草生长及土壤磷素转化的影响

为了进一步明确城市污水再生水的农业利用价值,在温室条件下采用盆栽试验方法种植黑麦草,以自来水(clean water,CW)灌溉为对照,分别进行全再生水(reclaimed municipal wastewater,RW)和混合再生水(自来水+再生水,CW+RW,1∶1)灌溉处理,研究了再生水灌溉对黑麦草生长和土壤磷素的转化特征。结果表明,城市污水再生水灌溉显著增加了黑麦草地上部和根系的生物量,CW+RW处理黑麦草地上部和根系生物量在播种55 d后分别较对照(CW)增加18.92%和6.42%,RW处理分别增加26.79%和10.55%;黑麦草地上部磷含量分别显著增加8.48%和10.93%。再生水灌溉土壤全磷含量变化不大并有减少趋势,但土壤速效磷含量CW+RW和RW处理分别较对照(CW)增加29.15%和43.80%;CW+RW和RW处理显著增加了土壤有机磷组分中的活性有机磷和中活性有机磷,与对照CW相比,其中活性有机磷增幅分别为50.30%和81.57%,中活性有机磷增幅分别为7.66%和13.68%;也显著增加了无机磷组分中的Ca2-P和Ca8-P,CW+RW和RW处理Ca2-P含量由对照的12.90 mg·kg-1分别增加到16.42 mg·kg-1和15.49 mg·kg-1,与对照相比,增幅分别为27.29%和19.38%,Ca8-P增幅分别为19.94%和16.03%。再生水灌溉显著降低了土壤pH并显著增加了土壤有机质含量,这可能是增加土壤磷活性的原因之一。再生水灌溉对提高土壤磷素利用率有促进作用。     

Calcium and humic acid affect seed germination,growth, and nutrient content of tomato (Lycopersicon esculentum L.) seedlings under saline soil conditions

The effects of calcium and humic acid on seed germination, growth and macro- and micro-nutrient contents of tomato (Lycopersicon esculentum L.) seedlings in saline soil conditions were evaluated. Different levels of humic acid (0, 500, 1000 and 2000 mg kg^?1) and calcium (0, 100, 200 and 400 mg kg^?1) were applied to growth media treated with 50 mg NaCl kg^?1 before sowing seeds. Seed germination, hypocotyl length, cotyledon width and length, root size, shoot length, leaf number, shoot and root fresh weights, and shoot and root dry weights of the plant seedlings were determined. Macro- and micro-nutrient (N, P, K, Ca, Mg, S, Cu, Fe, Mn and Zn) contents of shoot and root of seedlings were also measured. Humic acid applied to the plant growth medium at 1000 mg kg^?1 concentration increased seedling growth and nutrient contents of plants. Humic acid not only increased macro-nutrient contents, but also enhanced micro-nutrient contents of plant organs. However, high levels of humic acid arrested plant growth or decreased nutrient contents. Levels of 100 and 200 mg kg^?1 Ca²⁺ application significantly increased N, Ca and S contents of shoot, and N and K contents of root.     

A microcosm study on the influence of pH and the host-plant on the soil persistence of two alfalfa-nodulating rhizobia with different saprophytic and symbiotic characteristics

The acid tolerance of Sinorhizobium meliloti in culture media and in soils is considered a useful criteria to select for strains with improved survival in agricultural acidic soils. Using a glass tube system with gamma-irradiated soil at different pH values, we analysed the survival of two different alfalfa-nodulating rhizobia: S. meliloti (pH_limit for growth 5.6–6.0) and the acid-tolerant Rhizobium sp. LPU83, closely related to the strain Rhizobium sp. Or191 (pH_limit for growth below 5.0). Although the acid-tolerant rhizobia showed a slightly better survival during the first months in acid soil (pH=5.6), none of the strains could be detected 2 months after inoculation (bacterial counts were below 10³ colony-forming units (cfu)/30 g of soil). The inclusion of two alfalfa plants/glass tube with soil, however, supported the persistence of both types of rhizobia at pH 5.6 for over 2 months with counts higher than 9×10⁶ cfu/30 g of soil. Remarkably, in the presence of alfalfa the cell densities reached by S. meliloti were higher than those reached by strain LPU83, which started to decline 1 week after inoculation. Although more acid-sensitive in the culture medium than the Or191-like rhizobia, in the presence of the host plant the S. meliloti strains showed to be better adapted to the free-living condition, irrespective of the pH of the soil.     

Tolerance of forage legumes to lime‐stabilized sludge

Abstract

Lime‐stabilized sludge (LSS) from dairy processing waste‐water treatment plants is a desirable product for land application. The material contains lime, which neutralizes soil acidity, and P, which is useful as a plant nutrient. The fineness of the lime and the solubility of P make LSS especially desirable in establishing forage legumes. This greenhouse study had two objectives: to determine a reasonable quantity of LSS for establishing forage legumes such as alfalfa (Medicago sativa L.) and red clover (Trifolium pratense L.) and to prevent adverse effects on seedlings. Sludge was applied at 0, 2.5, 5.0, 7.5 g kg^‐1 to an acid, low P soil in pots, and alfalfa and red clover seeds were sown. All treatments received 123 μg g^‐1 potassium as KCl. A completely randomized design with four replications was used. Each species was handled as a separate study. Dry matter production was measured at one‐tenth bloom stage. Plant samples were analyzed for P, K, Ca, and Mg content. Soil samples taken at the end of the study were analyzed for pH, organic matter, Bray P, K, Ca, Mg, exchangeable Al, EC, and CEC. The higher quantities of LSS (7.5 g kg^‐1 for alfalfa and 5.0 g kg^‐1 for red clover) had negative effects on seedling germination and establishment. Lime‐stabilized sludge resulted in an increase in total nutrient uptake of Ca, Mg, K, and P up to 5.0 and 2.5 g kg^‐1 in alfalfa and red clover, respectively. In both species significant dry matter yield increases were obtained with LSS up to 5.0 g kg^‐1; however, 7.5 g kg^‐1 caused a reduction in dry matter yield. Based on these results, applications of LSS at 5.0 for alfalfa and 2.5 g kg^‐1 for red clover had positive effects in seedling establishment, nutrient uptake, and dry matter production. Lime‐stabilized sludge application resulted in significant increases in soil pH, available P, Ca, Mg, EC, and CEC; decreases were seen in neutralizable acidity and exchangeable Al levels in soil. This study indicates that LSS is appropriate for the acidic, low P soils of Southern Missouri for alfalfa and red clover establishment and production, if applied in appropriate quantities.     

Soil Depletion of Ca, Mg and K Due to Vicinal Intensive Hog Farming Operation Located in East Mediterranean

One of the main environmental impacts of concentrated animal feeding operations is soil degradation in the vicinity of the livestock breeding facilities due to substances such as ammonia emitted from the various stages of the process. In this research, the soil degradation effects of an intensive hog farming operation (IHFO) located at a Mediterranean limestone soil coastal area have been investigated. Soil samples of the upper mineral soil were taken in various distances and directions from the IHFO boundaries. Thirteen experimental cycles were carried out in the duration of 1.5?years starting in March 2009 until October 2010. The soil samples were analysed on total, exchangeable and water-soluble Ca, Mg and K as well as water-soluble ammonium concentrations. Significantly lower concentrations of the exchangeable and water-soluble base cations were observed on soil samples at increasing proximity downwind from the farm (south). Southern soil average concentrations of exchangeable base cations ranged between 78.6 and 128.52?mmol Ca²⁺?kg^?1 soil, 8.42?C21.39?mmol?Mg²⁺?kg^?1 soil and 4.25?C8.1?mmol?K⁺?kg^?1 soil, respectively. Southern soil average concentrations of water-soluble base cations ranged between 0.57 and 2.17?mmol Ca²⁺?kg^?1 soil, 0.16?C0.89?mmol?Mg²⁺?kg^?1 soil and 0.48?C0.95?mmol?K⁺?kg^?1 soil, respectively.     

Phosphate sorption by calcareous Vertisols and Inceptisols as evaluated from extended P-sorption curves

The plant availability of phosphate applied to calcareous soils is affected by precipitation and adsorption reactions, the relative significance of which is not well known. We used extended P-sorption curves obtained at phosphate addition rates up to 340 mmol P kg^?1 soil to examine the relative contribution of precipitation and adsorption by 24 calcareous Spanish Vertisols and Inceptisols. Adsorption was dominant at 1 day and at small rates of addition (10–35 mmol P kg^?1). With increasing clay and Fe and Al oxides contents of the soil, more phosphate was sorbed before the sorption curve bent upwards, as a result of Ca phosphate precipitation. Sorption curves showed a nearly vertical intermediate region, the length of which increased with time, suggesting that a Ca phosphate buffered the concentration of P in solution. The buffering concentration decreased with time, suggesting a progressive transformation of more to less soluble forms of Ca phosphate. A phase less soluble than octacalcium phosphate seemed to control the concentration of P in solution at 180 days in most soils. The apparent solubility of this phase decreased with increasing carbonate content in the soil. Precipitation of poorly soluble Ca phosphates apparently predominated up to a P addition dose ranging from about 30 mmol P kg^?1 in some soils to more than 340 mmol P kg^?1 in others. At larger doses, the way additional P was bound to the solid phase was different; phosphate was probably adsorbed, at least in part, to low-affinity sites on silicate clays and oxides. The proportion of sorbed phosphate that was isotopically exchangeable decreased with time, soil carbonate content and P addition dose for doses <100 mmol P kg^?1. This is consistent with the idea that P in Ca phosphates is less isotopically exchangeable than P adsorbed on mineral surfaces. At larger additions of P, isotopic exchangeability was unrelated to the soil properties measured, probably because there was a variety of sorbed P forms influenced in turn by different soil components.     

Cadmium and lead affect the status of mineral nutrients in alfalfa grown on a calcareous soil

ABSTRACT

Cadmium (Cd) and lead (Pb) are toxic trace elements which are not essential for plants but can be easily taken up by roots and accumulated in various organs, and cause irreversible damages to plants. A pot experiment was carried out to investigate the individual and combined effects of Cd (0, 10, 20 mg kg^?1) and Pb (0, 500, 1000 mg kg^?1) level in a calcareous soil on the status of mineral nutrients, including K, P, Ca, Mg, S, Fe, Mn, Cu, and Zn, in alfalfa (Medicago sativa L.) plants. Soil Pb level considerably (P ≤ 0.05) affected the concentrations of more elements in plants than soil Cd level did, and there were combined effects of soil Cd level and Pb level on the concentrations of some nutrients (Ca, Mg, and Cu) in plants. The effects of soil Cd level and Pb level on plant nutrient concentrations varied among plant parts. Cd and Pb contamination did not considerably affect the exudation of carboxylates in the rhizosphere. An increase in rhizosphere pH and exudation of significant amounts of carboxylates (especially oxalate) in the rhizosphere might contribute to the exclusion and detoxification of Cd and Pb. Neither shoot dry mass nor root dry mass was significantly influenced by soil Cd level, but both of them were considerably reduced (by up to 25% and 45% on average for shoot dry mass and root dry mass, respectively) by increasing soil Pb level. The interaction between soil Cd level and Pb level was significant for root dry mass, but not significant for shoot dry mass. The results indicate that alfalfa is tolerant to Cd and Pb stress, and it is promising to grow alfalfa for phytostabilization of Cd and Pb on calcareous soils contaminated with Cd and Pb.     

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.     

Proton release by N2‐fixing plant roots: A possible contribution to phytoremediation of calcareous sodic soils

With a world‐wide occurrence on about 560 million hectares, sodic soils are characterized by the occurrence of excess sodium (Na⁺) to levels that can adversely affect crop growth and yield. Amelioration of such soils needs a source of calcium (Ca²⁺) to replace excess Na⁺ from the cation exchange sites. In addition, adequate levels of Ca²⁺ in ameliorated soils play a vital role in improving the structural and functional integrity of plant cell walls and membranes. As a low‐cost and environmentally feasible strategy, phytoremediation of sodic soils — a plant‐based amelioration — has gained increasing interest among scientists and farmers in recent years. Enhanced CO₂ partial pressure (P_CO2) in the root zone is considered as the principal mechanism contributing to phytoremediation of sodic soils. Aqueous CO₂ produces protons (H⁺) and bicarbonate (HCO₃^‐). In a subsequent reaction, H⁺ reacts with native soil calcite (CaCO₃) to provide Ca²⁺ for Na⁺ Ca²⁺ exchange at the cation exchange sites. Another source of H⁺ may occur in such soils if cropped with N₂‐fixing plant species because plants capable of fixing N₂ release H⁺ in the root zone. In a lysimeter experiment on a calcareous sodic soil (pH_s = 7.4, electrical conductivity of soil saturated paste extract (EC_e) = 3.1 dS m^‐1, sodium adsorption ratio (SAR) = 28.4, exchangeable sodium percentage (ESP) = 27.6, CaCO₃ = 50 g kg^‐1), we investigated the phytoremediation ability of alfalfa (Medicago sativa L.). There were two cropped treatments: Alfalfa relying on N₂ fixation and alfalfa receiving NH₄NO₃ as mineral N source, respectively. Other treatments were non‐cropped, including a control (without an amendment or crop), and soil application of gypsum or sulfuric acid. After two months of cropping, all lysimeters were leached by maintaining a water content at 130% waterholding capacity of the soil after every 24±1 h. The treatment efficiency for Na⁺ removal in drainage water was in the order: sulfuric acid > gypsum = N₂‐fixing alfalfa > NH4NO3‐fed alfalfa > control. Both the alfalfa treatments produced statistically similar root and shoot biomass. We attribute better Na+ removal by the N₂‐fixing alfalfa treatment to an additional source of H⁺ in the rhizosphere, which helped to dissolve additional CaCO₃ and soil sodicity amelioration.     

Rhizosphere dynamics under nitrogen‐induced root modification: The interaction of phosphorus and calcium

Optimizing root phosphorus (P) acquisition to reduce intensive fertilizer use is a crucial pathway for sustainable agriculture, particularly as P is an important plant macronutrient, often limiting in a majority of soils worldwide. Although many studies have assessed plant growth and P acquisition, few studies have investigated the interactive effects of nitrogen (N)‐induced root modification on soil P processes or the understudied effects of soil calcium (Ca) dynamics on soil P bioavailability. In this study, we investigate soil P and Ca response in the rhizosphere of durum wheat (Triticum turgidum L. spp. durum). Wheat grown under controlled conditions preloaded for 20 d with two N treatments [preloaded low N (1 mmol KNO₃ plant^?1) and preloaded high N (2 mmol KNO₃ plant^?1)] were transferred to rhizoboxes for 12 d [days after transfer (DAT)]. Shoot and root biomass, P and Ca concentration, and plant‐available P and extractable Ca were determined every three days (0, 3, 6, 9, 12 DAT). Significantly higher root mass (P = 0.7%), root length (P = 1.8%) and total biomass (P = 2.2%) were found at the end of the experiment but exclusively for high N preloaded wheat. This greater root biomass was associated with lower root P concentration, suggesting a dilution response, while little difference was observed in shoot P concentration over the 12 d. However, Ca accumulated in both roots and shoots under both preloading N levels. Concurrently, soil‐extractable Ca declined, and plant‐available P increased (r = –0.62; P = 0.03%), presumably due to a promoting effect of Ca uptake on soil P availability; lower soil Ca in turn increased the repulsive forces between P ions and the negatively charged soil surface, resulting in an increased P availability in the soil solution. This study contributes to the understanding of the complex interplay between multi‐nutrient dynamics within the rhizosphere.