Transformation of organic rhizodepositions by rhizosphere bacteria and its influence on the availability of tertiary calcium phosphate

This study assesses the influence of saccharides in the rhizodeposition on the phosphate solubilizing ability of rhizosphere bacteria. Water‐soluble rhizodeposits were analyzed of ¹⁴C‐labeled pea plants (Pisum sativum, cv. ‘Grapis’) which were grown at two different levels of P‐nutrition. The sugars produced were fed in vitro either as single compounds or as synthetic mixtures to three bacterial strains and the ability of the bacteria to mobilize Ca₃(PO₄)₂ was measured. The relative glucose proportion of pea exudates decreased under P deficiency while the content of galactose, ribose, xylose and fucose increased. In vitro feeding of single sugars and sugar mixtures showed that the ability of Pseudomonas fluorescens (PsIA12) to dissolve tertiary calcium phosphate was lower with pentoses and the mixed sugars of the P‐deficient plants than with glucose. On the other hand, the shifted sugar pattern observed under P deficiency increased the phosphate mobilization ability of Pantoea agglomerans (D5/23) and Azospirillum sp. (CC 322) considerably. This observation can only partly be explained by the acidification of the nutrient medium. Bacteria also produced different carboxylic anions depending on sugar supply. In addition to low‐molecular mono‐, di‐, and tricarboxylic acids which are known as P‐solubilizing substances, sugar acids also played an important role in cultures D 5/23 and CC 322.     

Phosphorus availability,root exudates,and microbial activity in the rhizosphere

Field and pot experiments showed that the P demand of wheat is highest in early stages of growth (up to 1.67 μg P per cm² root surface and day). The needed orthophosphate ions H₂PO₄? and HPO₄²-move from soil to the root by diffusion. This process is controlled by the concentration gradient of the diffusible phosphate and the effective diffusion coefficient according to Pick's first law. Root excretions (rhizodeposition) are able to affect both characteristics. The water soluble portion of rhizodeposition contains more than 50% of up to 8 different sugars, 10–40% carboxylic acids and 10–15 amino acids and amides. The composition varies in dependence on the age of the root parts and on nutrition (Zea mays L., Brassica napus L., Pisum sativum L.). Diffusion experiments using small soil blocks showed that 50–75% of the root exudates were decomposed by respiration within 3 days. The rest was largely chemically converted. Originally present sugars disappeared. Due to the biosynthesis of different organic acids from the individual sugars the mobilisation of Ca₃(PO₄)₂ by Pantoea agglomerans increased when the sugar mixture was derived from the rhizodeposition of P deficient plants with more pentoses instead of glucose and fructose (mainly effect of anions). In the rhizosphere therefore a mixture of rhizodeposition and its conversion products exists which affects the binding of phosphorus in soil and the P transport to the root. This should be considered both for the development of new soil extractants and for modelling the P supply to plants.     

长期施肥对黑土磷素积累、形态转化及其有效性影响的研究

1980年开始,在小麦大豆玉米轮作制中,研究长期定位施用常量的氮、磷、钾(小麦、玉米施肥量为N150、P2O575、K2O75kg/hm2;大豆为N75、P2O5150、K2O75kg/hm2)和有机肥(马粪,折N75kg/hm2,只在玉米后茬上施用),以及二倍和四倍量对土壤磷素积累、形态变化及磷肥后效的影响。23年研究结果表明,长期不施肥,黑土土壤全磷下降37.4%、速效磷下降了60%;施用磷肥土壤全磷增加53.9%～65.7%、速效磷增加6～15倍。积累的磷素大部分以有效性较高的Ca2-P、Ca8-P、Al-P形态积累在土壤中,施用磷肥可使Ca2-P增加4～15倍,Ca8-P增加4～16倍,Al-P增加1.6～11.8倍,Fe-P增加1.4～4.4倍,O-P增加0.6～1.7倍,Ca10-P增加0.3～0.7倍。所积累在土壤中的磷素具有生物有效性。     

The quantitative effect of chemical phosphate mobilization by carboxylate anions on P uptake by a single root. II. The importance of soil and plant parameters for uptake of mobilized P

Model calculations were made in order to quantify the effect of carboxylate excretion on phosphate (P) uptake by a single root. The uptake of chemically mobilized P increased exponentially with increasing concentration of adsorbed citrate or oxalate in soil because of the exponential relationship between adsorbed carboxylate and the solubilizing effect of carboxylate on P. The effect of local citrate excretion compared with uniform citrate excretion along the whole root was also calculated. Local exudation increased the uptake of chemically mobilized P because the higher concentration of citrate increases the solubilization of P. Additionally the effect of citrate excretion by root clusters e.g. proteoid roots was evaluated. Uptake of chemically mobilized P by root clusters was much higher than that of single roots, especially if the ratio of P buffering to citrate buffering was high. This is often the case in P fixing soils where by definition P buffering is high and citrate buffering is low because of the short time of reaction between root excreted citrate and rhizosphere soil. The reason for the superiority of cluster roots lies in the fact that most of the mobilized P is transported away from a single root to be absorbed by neighbouring roots in the clusters. This phenomenon demonstrates the strong ecological significance of cluster roots in relation to nutrient mobilization. The calculations on the effect of oxalate excretion by sugar beet roots on the uptake of mobilized P show that under P fixing conditions the influx of mobilized P will exceed that of P transported by diffusion to the root surface by a factor of 1.5—6.0.     

长期不同施肥对红壤性水稻土磷素及水稻磷营养的影响

【目的】合理的土壤磷素管理对作物生产和环境保护具有重要意义。南方双季稻田土壤磷素特征及磷素吸收信息相对缺乏,本文利用江西省稻田土壤质量演变定位监测试验为平台,系统分析长期不同施肥措施下土壤全磷、磷活化系数及水稻磷素吸收量的变化特征和全磷与磷盈亏的响应关系等,为指导磷肥合理施用提供重要科学依据。【方法】从1984年开始在江西省南昌市进行长期定位试验,设置8个处理,分别为不施肥对照(CK),PK、NP、NK、NPK、70%化肥氮+30%有机肥氮(70F+30M)、50%化肥氮+50%有机肥氮(50F+50M)、30%化肥氮+70%有机肥氮(30F+70M)。早稻施用纯N、P2O5和K2O量分别为150、60和150 kg/hm^2,晚稻分别为180、60和150 kg/hm^2。早、晚稻施用的氮、磷、钾化肥均分别为尿素、过磷酸钙和氯化钾,有机肥分别为紫云英(N、P2O5、K2O含量分别为0.30%、0.08%、0.23%)和腐熟猪粪(N、P2O5、K2O含量分别为0.45%、0.19%、0.60%)。除30F+70M处理,其余处理均为等氮磷钾设计。于1984-2012年每年早、晚稻收获期采集秸秆和稻谷计产,并于晚稻收获后,测定土壤全磷和有效磷含量。分析土壤全磷、磷活化系数(PAC)及早、晚稻磷素吸收量随种植年限的变化规律,研究土壤全磷含量与磷累积盈亏的响应关系。【结果】经29年连续试验,NK处理土壤全磷含量以每年4.6 mg/kg的速度下降,而含磷化肥处理土壤全磷含量升高速率为3.3~19.4 mg/(kg·a)。有机无机配施处理(70F+30M、50F+50M和30F+70M)升高速率平均为16.1 mg/(kg·a),是施NPK肥处理的4.89倍。施磷土壤全磷含量平均增至1.07 g/kg (2010-2012平均值),较初始值提高了1.18倍。不施磷肥处理土壤磷活化系数(PCA)由试验初始的4.24%下降至2.5%左右,施磷肥处理则均显著升高,其中有机无机配施处理平均升高至8.51%,平均年升高速率是施NPK处理的2.89倍。早、晚稻磷素吸收量,施磷肥(PK、NP和NPK)和化肥配施有机肥处理(70F+30M、50F+50M和30F+70M)均显著高于CK,提高幅度分别为29.9%~124%和28.6%~103%,均衡施肥(NPK、70F+30M、50F+50M和30F+70M)磷素吸收量显著高于不均衡施肥(PK和NP)处理,前者平均分别较后两者提高了38.7%和32.9%。早、晚稻产量与磷素吸收量呈极显著线性正相关关系,每吸收磷(P) 1 kg,早稻和晚稻产量分别可提高115和106 kg/hm^2。不施肥(CK)条件下,土壤全磷变化与累积磷盈亏间无显著相关关系,施NK肥处理土壤中每亏缺磷100 kg/hm^2,土壤全磷含量降低6.0 mg/kg,施化学磷肥的3个处理,土壤中每盈余磷100 kg/hm^2,平均提高9.3 mg/kg,而3个有机–无机配施处理,土壤中每盈余磷100 kg/hm^2,平均增加63.3 mg/kg,是无机磷肥的6.78倍。【结论】无论是单施化学磷肥,还是有机无机配施均有效提高土壤全磷含量及磷活化系数,且在等磷量投入条件下,有机无机配施较单施化肥的效果更优。建议减少中国南部红壤性稻田土壤的总磷输入量和提高有机肥施用比例,以改善粮食生产和保护环境。     

土壤磷有效性及其与土壤性质关系的研究

选取我国14个不同地点土样,测定理化性质、全磷、速效磷、水溶性磷含量,采用通径分析研究土壤磷有效性与土壤性质的关系。结果表明,土壤磷有效性是各个土壤性质综合作用的结果。土壤CEC、有机碳、粘粒、砂粒、碳酸钙含量对土壤速效磷比例影响显著,土壤CEC、粘粒、砂粒含量对土壤速效磷比例贡献为正值,而土壤有机碳对土壤速效磷比例贡献为负值;土壤pH和CEC含量对土壤水溶性磷含量影响显著,土壤pH对水溶性磷比例贡献为正值,土壤CEC对土壤水溶性磷比例贡献为负值。本文所选土样基本符合土壤磷有效性与土壤性质之间通径分析的结果。     

Effects of aerobic conditions in the rhizosphere of rice on the dynamics and availability of phosphorus in a flooded soil – A model experiment

The secretion of O₂ by rice roots results in aerobic conditions in the rhizoshere compared to the bulk flooded soil. The effect of this phenomenon on the adsorption/desorption behavior and on the availability of phosphorus (P) in a flooded soil was investigated in a model experiment. An experimental set‐up was developed that imitates both O₂ release and P uptake by the rice root. The results showed that O₂ secretion significantly reduced P adsorption/retention and increased P desorption/release in the “rhizosphere” soil, compared to the anaerobic bulk soil. The P uptake by an anion exchange resin from both unfertilized and P‐amended soil was significantly increased. The results confirm that the O₂ secretion is an important mechanism to enhance P availability and P uptake of rice under flooded conditions, where the “physico‐chemical” availability of P in the anaerobic bulk soil is strongly reduced. The decrease of P availability in the P‐amended flooded bulk soil was mainly associated with the almost complete transformation of the precedingly enriched Al‐P fraction into Fe‐bound P with extremely low desorption/release characteristics during the subsequent flooding.     

Mineral dust around the Sahara—from source to sink. A review with emphasis on contributions of the German soil science community in the last twenty years

Natural mineral dust has manifold environmental effects reaching from fertilizing aquatic and terrestrial ecosystems to affecting the earth's radiation balance and thus impacting on climate. The Sahara is considered the largest source of natural mineral dust on the globe, so much research attention has been paid to source identification, dust mobilization, transport, and effects in the sink areas. This paper gives a review of the research results concerning these topics emphasizing soil‐science‐related aspects of the last 20 y and the identification of knowledge gaps.     

Nitrogen and phosphorus fertilization leads to soil arbuscular mycorrhizal fungal diversity changes and rainfed crop yield increase on the Loess Plateau of China: A 37-year study

More than 80% of plants form mutualistic symbiotic relationships with arbuscular mycorrhizal fungi (AMF), and the application of fertilizers, such as nitrogen (N) and phosphorus (P) fertilizers, is a common agricultural management practice to improve crop yield and quality. However, the potential effects of long-term N and P fertilization on the AMF community in the rainfed agricultural system of the Loess Plateau of China are still not well understood. In this study, a long-term field experiment was conducted based on orthogonal design, with three N levels (0, 90, and 180 kg ha^-1 year^-1) and three P levels (0, 90, and 180 kg ha^-1 year^-1) for wheat fertilization. Changes in AMF community and correlations between AMF community composition, soil environmental factors, and wheat yield component traits were analyzed using traditional biochemical methods and high-throughput sequencing technology. The results showed that long-term N and P addition had a significant effect on the AMF community structure and composition. Nitrogen application alone significantly reduced the richness and diversity of AMF community, whereas the combined application of N and P significantly increased the richness and diversity of AMF community. The AMF community was driven mainly by soil available P, total P, and pH. There was a significant positive correlation between Glomus abundance and wheat yield and a significant negative correlation between Paraglomus abundance and wheat yield. Long-term N and P addition directly increased crop yield and affected yield indirectly by influencing soil chemical properties and the AMF community. Combined application of N and P both at 90 kg ha^-1 year^-1 could improve the ecological and physiological functions of the AMF community and benefit the sustainable development of rainfed agriculture.