岷江上游山地森林/干旱河谷交错带不同植被恢复模式对根际土壤微生物量碳氮及固氮菌群落结构的影响

以岷江上游山地森林/干旱河谷交错带5种植被恢复模式的根际与非根际土壤为研究对象,对土壤微生物生物量碳、氮和土壤固氮菌群落结构的影响进行了初步研究。结果表明,5种不同植被根际与非根际土壤的微生物生物量存在显著差异且根际土壤含量大于非根际土壤,依次为:青冈次生林>灌木林>刺槐林>人工幼林>针阔混交林;DGGE图谱显示,5种植被根际与非根际土壤固氮类群有一条明显的共有条带,其中刺槐林和灌木林的固氮菌多样性最高;多样性指数、群落丰富度分析表明,灌木根际土壤固氮菌的群落种群结构最丰富,其次为刺槐;聚类分析结果显示,不同植被恢复的10种供试土壤样品分为4大类群。总之,自然恢复(青冈林和灌木林)效果好于人工林(刺槐林、人工幼林、针阔林);3种人工林相比较,刺槐林比其它两种人工林更有利于提高微生物生物量及土壤固氮菌群落结构多样性;在人工幼林地采取的封禁措施也有利于植被恢复。

不同南丰蜜橘园挂果期土壤微生物的多样性

通过对江西省抚州市南丰县市山镇郊橘园土壤、莱溪水田边橘园土壤、军峰山石桥橘园下层土壤、旴江边橘园上层土壤的微生物进行培养,再通过菌落观察、测量、计数及细胞形态、大小的观测,计算出各土壤培养组中固氮菌群落的Margalef丰富度指数(dma)、Simpson优势度指数(D)、Shannom-Winener多样性指数(H')、Pielou均匀度指数(Jsw)、每皿平均种类数和平均个体数,再进行种群间的相似性分析,以期能体现南丰蜜橘的独特性和南丰蜜橘橘园土壤微生物的多样性状况。试验结果表明,南丰蜜橘不同耕作方式对土壤有一定影响,合理的种植措施可以改善土壤微生物多样性,从而使南丰蜜橘高产、优质。初步得出结论:除保证品系、苗木质量、合理施肥外,保护产地的生态条件是目前情形下保护南丰蜜橘优良品质最有效的措施。

Effects of N,P and K Fertilizers on Growth of Clover Nitrogen-fixing Rhizobia and Soil Fertility after Plantation

In order to develop organic rice and increase paddy soil fertility by cloverorganic rice rotation, the effects of N, P and K fertilizers on growth of clover nitrogen-fixing rhizobia and soil fertility after plantation were investigated, thereby providing certain reference for reasonable cultivation of clover and improvement of soil fertility. A two-year experiment was conducted from 2012 to 2013. The clover was cultivated after rice every year, and different levels of N, P and K fertilizers were applied before winter. In the treatment group, no fertilizer was applied. The effects of different fertilizers and different application amounts on clover yield, nitrogen-fixing rhizobia quantity, nitrogen-fixing rhizobia weight and soil fertility after plantation were analyzed. The results showed that the application of N, P and K fertilizers in the early stage had significantly effect on the growth of clover. When the application amount of N fertilizer was 75 kg/hm^2（N 46%）, the clover yield, nitrogen-fixing rhizobia quantity and nitrogen-fixing rhizobia weight were highest. The soil nitrogen content after plantation increased with the increase of the application amount of N fertilizer, while the P and K content decreased and then increased with the increased application amounts. Soil available P content increased with the increased application amounts of N and P fertilizers, but it did not change significantly with the increased application amount of K fertilizer. Soil available K content increased first and then decreased with the increased application amounts of N and P fertilizers.When the application amounts of N and P fertilizers were 150（N 46%） and 300（P_2O_5 12%） kg/hm^2, soil available K content reached the maximum. Soil organic matter content increased with the increased application amounts of N, P and K fertilizers. Therefore, in the cultivation of clover, appropriate application of N, P and K fertilizers in the early stage can improve clover yield and soil fertility.

Efficiency of Biological Nitrogen Fixation of Bradyrhizobium under Different Soil Fertility Levels as Measured by 15N Dilution Analysis

The objective of this study was to determine the efficiency of biological nitrogen fixation （BNF） of local Bradyrhizobium isolates in soil of various fertility levels using 15N dilution technique. Local isolates were obtained from cowpea rhizosphere in fields of different Iraqi provinces. Six isolates were selected in this study, which was conducted as a pot experiment under greenhouse conditions. Effects of the following fertility levels were evaluated： at F1, 0 mg N, P and K was added; at F2, 25 mg N/kg soil, 10 mg P/kg soil and 25 mg K/kg soil were added, respectively; the other two levels were F3 at which 50, 20 and 50 mg/kg soil and at F4 75, 30 and 75 mg/kg soil for N, P and K, respectively, were added. Urea, labeled with 15N 10% access atom （aa）, was used as a source of N. The highest BNF was observed under the lowest fertility level, i.e., F1. BNF across all isolates was markedly decreased with the increase of nutrient application to soil, being totally eliminated at the highest fertility level F4. Numbers of nodules per plant root of all isolates were the least under the zero nutrients application and the highest nodules number were found under the highest levels of N, P and K application. Number of nodules does not necessarily reflect the best BNF efficiency of all isolates. However, fertility levels were of significant effect on average nodule number of all isolates. The lowest plant dry weight was under the first fertility level F1 irrelevant of Rhizobium isolates. In general, the highest plant dry weight was under the second soil fertility level F2.

Complete genome sequence of a novel bacteriophage infecting Bradyrhizobium diazoefficiens USDA110

Dear Editor,The nitrogen-fixing symbiotic bacteria, rhizobia are the most important beneficial bacteria in soil, as they form nodules with host legume plants to fix nitrogen to maintain soil fertility and facilitate plant growth. Although these bacteria are critically important to agriculture, bacteriophages of rhizobia (rhizobiophages) are commonly present in soils, rhizosphere,and nodules, which have major influences on the composition and population of rhizobia in soils and affect nodulation and nitrogen fixation. Despite the importance of rhizobiophages,only 23 phage full genomes have been sequenced.