Comparison of factors limiting bacterial growth in different soils

Lack of carbon has been assumed to be the most common limiting factor for bacterial growth in soil, although there are reports of limitation by other nutrients, e.g. nitrogen and phosphorus. We have studied which nutrient(s) limited instantaneous growth rates of bacteria in 28 Swedish soils using the thymidine or leucine incorporation technique to measure increased growth rate after adding different combinations of organic carbon (glucose), nitrogen and phosphorus. The soils ranged in pH between 3.1 and 8.9, in organic matter content between 1% and 91% and in soil C/N ratio between 10 and 28. We also tested the effect of adding different amounts of carbon on the bacterial change in growth rate for two soils with different organic matter content. We found that bacterial growth in most of the 28 soils was limited by a lack of carbon, indicated by an increased bacterial growth rate 48 h after adding glucose. In some soils, adding carbon together with nitrogen increased the bacterial growth rates even further. In three soils no effects were seen upon adding nutrients separately, but adding carbon and nitrogen together increased bacterial growth rates. Nitrogen addition tended to decrease bacterial growth rates, while phosphorus addition had little effect in most soils. No correlations were found between the soil C/N ratio, ammonium or nitrate content in soil and bacterial growth limitation, indicating that even soils with a C/N ratio of 28 could be carbon limited. Although the interpretation of the effects of a single limiting nutrient was in most cases straightforward, an interaction between the amount of carbon added and the organic matter content of the soil confounded the interpretation of the extent of a second limiting nutrient.     

Nutrition of drip irrigated corn by phosphorus under sandy calcareous soils

Phosphorus (P) is an essential nutrient for the higher plants and has low mobility and availability in soils. Good fertilization management for this nutrient may be lead to increasing its availability. A field experiment in a split plot design was conducted out during the 2003 and 2004 summer growing seasons to study the effect of frequency of P fertigation and different forms of P fertilizers on uptake of P by corn plant as well as yield production. The high frequency of P fertigation increased P uptake significantly (p < 0.05) by 12 and 19% in the first and second season respectively. Phosphoric acids (PA) followed by urea phosphate (UP) gave the highest significant biological yield (25.67 and 24.5 ton/ ha). From the results of this study, it may be recommended that in sandy calcareous soils P must be added every three days using acid forms.     

Reconciling the dichotomy between single species and ecosystem conservation: black kites (Milvus migrans) and eutrophication in pre-Alpine lakes

In recent times there has been a growing dichotomy between preservation of single species and broader, ecosystem-based approaches to conservation. Freshwater habitats are among the most highly human-impacted ecosystems. We used a long-term data set on black kites (Milvus migrans), a threatened raptor dependent on aquatic habitats, to explore ways to reconcile single species and biodiversity-driven approaches to conservation. In the pre-Alpine lakes of northern Italy, black kite populations showed medium-low density, extremely low breeding success and widespread declines. Spatio-temporal variations showed density and breeding success to be positively related to ecosystem productivity (as estimated by phosphorus concentrations in lakes), availability of aquatic habitats and grassland, and to be negatively related to extent of farmland and fish harvest by professional fishermen. Fish species richness, used as a surrogate of biodiversity, was highest in one oligotrophic lake, but on average increased with increasing ecosystem productivity (i.e. lake eutrophication). Given expected future declines in ecosystem productivity, kite conservation will be helped by enhancing populations of alternative prey in terrestrial habitats (e.g. through incentives for grassland), and higher regulation of fish harvest. On the other hand, ecosystem management may profit from the use of black kites or other aerial piscivores as indicators of biodiversity and of diffuse ecosystem stress, such as sustainability of fishing practices.     

鄱阳湖洲滩湿地土壤-水-植物系统中磷的静态迁移研究

通过采集鄱阳湖典型洲滩湿地土壤和植物来开展磷的静态迁移模拟实验,研究了鄱阳湖洲滩湿地土壤-水-植物系统对磷的吸收、释放及转移规律,并对静态实验前后,不同形态磷素在系统中的迁移量进行了监测。在此基础上,利用Freundlich模型和Langmuir模型对鄱阳湖洲滩湿地土壤基质对磷的吸附解吸特性进行了一元线性回归分析。结果表明:磷素不断从土壤和水体中释放出来,被植物大量的吸收。在培育之前,植物体内磷含量为1.0 mg g-1,水样磷浓度为2.3 mg L-1;而试验后植物体内磷含量为2.1 mg g-1,水样磷浓度为0.062 mg L-1。随着植物根系的生长,对磷素需求不断变大。直至试验进行到28d时,植物体内的磷素含量趋于恒定。同时,由土壤磷分级试验可看出,试验前后Al-P含量变化较小,Ca-P含量略有增加,Fe-P含量增幅较大。相对于Freundlich模型而言,采用Langmuir模型对湿地土壤基质对磷的吸附过程进行模拟相关性更好。该研究成果旨在为防治湖泊水体污染、开展湿地污染物的环境效应研究提供科学的理论依据。     

Effects of phosphate fertilizer on biomass production and N2(C2H2) fixation by pot-grown Ulex gallii Planchon in a forest soil

Summary N₂(C₂H₂) fixation by Ulex gallii Planchon (dwarf or autumn flowering gorse/furze) seedlings was determined following 8 months of growth (December-August) in the glasshouse in a very acid, N- and P-deficient forest soil. Application of Na₂HPO₄·12H₂O or North African ground rock phosphate fertilizer was essential for growth, nodulation and C₂H₂ reduction activity. Overall, both the sodium phosphate and the rock phosphate were equally effective P sources and the maximum acetylene reduction by intact roots was measured as 4.09 and 4.69 mol C₂H₄g^-1 fresh weight nodule h^-1, respectively. Applied NH₄Cl severely inhibited nodulation and restricted acetylene reduction activity but not seedling growth. The results are discussed in relation to the spread of U. gallii in the south of Ireland and its potential as a leguminous nurse crop for Sitka spruce on the very impoverished forest soils of the region.     

Effects of long-term phosphorus fertilization and winter cover cropping on soil phosphorus transformations in less weathered soil

Information concerning sources and sinks of available P in soil is needed to improve soil P management and protect water quality. This study, conducted from 1989 to 1998 on a Sultan silt loam soil (Aquantic Xerochrept), determined the annual P removal rate by corn (Zea mays L.) and P transformation as affected by P rate and winter cover cropping. Treatments included two P rates (0 and 44 kg P ha^–1) applied to corn at planting each year. All cover crops received 19.6 kg P ha^–1 at seeding each fall. Also included was a control without any cover crop and with no P addition. Corn yield and P uptake were affected by P fertilizer additions, but not by cover crops. A fairly constant amount of P was supplied from indigenous soil P when no external P was added. When the amount of P added exceeded that removed by corn, the excess P was converted mainly to NaOH-extractable inorganic P (NaOH-P_i). When the amount of P applied was below that removed by corn, indigenous soil NaOH-P_i acted as a source of available P for the plant. With no reduction of organic P (P_o) extractable by NaOH or NaHCO₃, the contribution from P_o to the available P pool appeared limited. The role of NaOH-P_i in P availability in the soil was substantiated by its significant correlation with labile NH₄Cl-extractable P (NH₄Cl-P; r² =0.60, P <0.001) or NaHCO₃-P_i (r² =0.81, P <0.001) pools. The NaOH-P_i for the soil reflected the changes in soil P resulting from past fertilizer P input and P removal by the crops.Scientific Paper Number 0005-34     

Influence of soluble phosphorus fertilizer on the interaction between a vesicular-arbuscular mycorrhizal fungus and Azospirillum brasilense in barley (Herdeum vulgare L.)

Summary Pot-culture studies were carried out to examine the response of barley (Hordeum vulgare L.) to inoculation with Azospirillum brasilense and Glomus versiforme, singly and/or in combination, under varying levels of nitrogenous [(¹⁵NH₄)₂SO₄] and soluble phosphatic (single superphosphate) fertilizers. The interaction between both the endophytes led to increased growth and nutrition of the barley plants. Roots from plants inoculated with Azospirillum brasilense and Glomus versiforme exhibited very low acetylene reduction activity. N₂ fixation in the plants increased with the increase in plant growth but the mycorrhiza alone gave a low level of N₂ fixation in the plants compared to combined inoculation with both the endophytes.     

Differences in mycorrhizal infection and P uptake of sweet potato cultivars (Ipomoea batatas L.) during their early growth in three soils

Summary Mycorrhizal infection in the roots of 10 sweet potato cultivars was assessed 7 weeks after planting in three soils collected from Ibadan, Fashola and Onne in southern Nigeria, three soils which contained 21.0, 7.8 and 54.8 mg P kg^–1, respectively. Mycorrhizal infection averaged 17% in the soil from Ibadan, 24% in the soil from Fashola and 7% in the acid soil from Onne. The plants grown in the Fashola soil contained the same percentage of P as plants grown in the Onne soil. Although the percentage of P in sweet potato was lowest in the Ibadan soil, shoot dry weights were 35% higher in this soil than in the other two soils. There was no correlation between the level of mycorrhizal infection and plant dry weight in the partially sterilized soil from Ibadan. Sweet potato inoculated in this soil with infected roots of Leucaena leucocephala showed a higher level of mycorrhizal infection than uninoculated plants. Dry-matter production was, however, the same for all treatments. The sweet potato cultivars differed in their level of mycorrhizal infection and in their response to applied P. Cultivars TIS 2498 and TIS 70357 consistently showed the lowest percentage of infection; and TIb 4, TIS 8441 and TIS 8524 showed infection levels above 20% in the Fashola and Ibadan soils. When the low-yielding cultivar, TIb 4, and an improved clone, TIS 9265, were grown in the presence of 50 and 100 mg single superphosphate per kg soil, TIb 4 produced more dry matter in the presence of P fertilizer than it did without the fertilizer. Growth and mycorrhizal infection of TIS 9265 were not affected by the fertilizer.     

Response of soil microbial biomass dynamics to quality of plant materials with emphasis on P availability

Soil microbial biomass is a major sink and source of plant available P and transformer of soil organic P. The dynamics of microbial biomass P (B_P) and C (B_C) and resin extractable P (P_ex) in a sandy loam soil (Kandic Rhodustalf) after amending with plant materials of diverse quality were examined in a laboratory incubation study. Quality variables of the plant materials were described by the proportion of dry matter as total P, total N, lignin (LIG), and soluble polyphenolics (Pp). The materials were added to the soil at a rate equivalent to 10 t ha⁻¹ (dry weight) and kept at 50% field moisture capacity and 25 °C. Included was an unamended control soil. During incubation of up to 56 d, soil samples were periodically analyzed for P_ex, B_P and B_C. Quality variables affecting P_ex, B_P, B_C and B_P-to-B_C ratio were total P, total N, N-to-P and C-to-P ratios. Total P appeared to be the most important quality factor affecting P_ex, B_P, B_C and B_P-to-B_C ratio. There was no consistent significant correlation between P_ex and B_P or between B_P and B_C. This was attributed to the complexity of the dynamics of P_ex and B_P pools and the absence of a significant microbial turnover due to the short incubation periods assessed in the study. By virtue of the high P content and resulting in a low B_P-to-B_C ratio, leaves of the shrubby non-legume Tithonia diversifolia (Hemsley) A. Gray, had a distinct advantage of replenishing P_ex in the short-term and yet maintain a high potential to release P through microbial turnover. The tree legume Sesbania sesban, on the other hand, resulted in a low B_P-to-B_C ratio but also had a low P content and may be useful for the build up of soil organic matter (SOM) and P release in the long term. Exploiting plant materials with quality variables exhibited by T. diversifolia might create the potentials for fast replenishment of solution P as well as improving SOM.     

有机肥和无机肥对马铃薯生长发育及块茎产量的影响

摘要：本试验在田间条件下，以‘尤金’和‘克新23号’为供试品种，研究了单施无机肥或有机肥以及有机肥和无机肥配施对马铃薯生长发育及块茎产量的影响，对马铃薯生产中有机肥替代无机肥的可行性及替代效果进行验证和阐述。结果表明，施用有机肥可显著提高两品种的株高、茎粗、叶面积指数及叶片SPAD值（p<0.05），延长了‘尤金’地上茎叶的功能期，提高‘尤金’的平均薯重（p<0.05）；单施无机肥可促进两个品种地上茎叶及根系的生长，提高单株结薯数和块茎干物质积累量。两个马铃薯品种的最高产量均为单施无机肥处理，其中‘尤金’施用90 kg P2O5 /hm2时达到最高产量41.50 t/hm2，‘克新23号’施用135 kg P2O5 /hm2时达到最高产量41.26 t/hm2。在本试验条件下，两个品种有机替代的产量均为中等水平。