Depletion of non‐exchangeable NH$ _4^+ $ at the root–soil and hyphae–soil interface of VA mycorrhizal maize (Zea mays) and parsley (Petroselinum sativum)

Mobilization of non‐exchangeable ammonium (NH ) by hyphae of the vesicular‐arbuscular mycorrhizal (VAM) fungus Glumus mosseae was studied under controlled experimental conditions. Maize (Zea mays) and parsley (Petroselinum sativum) were grown either alone or in symbiosis with Glomus mosseae in containers with separated compartments for roots and hyphal growth. In one experiment, ¹⁵NH was added to the soil to differentiate between the native non‐exchangeable NH and the non‐exchangeable NH derived from N fertilization. Non‐exchangeable NH was mobilized by plant growth. Plant dry weight and N uptake, however, were not significantly influenced by mycorrhizal colonization of the roots. The influence of root infection with mycorrhizal fungus on the mobilization of non‐exchangeable NH was negligible. In the hyphal compartment, hyphal uptake of N resulted in a decrease of NH in the soil solution and of exchangeable NH . However, the NH concentration was still too high to permit the release of non‐exchangeable NH . The results demonstrate that, in contrast to roots, hyphae of VAM fungi are not able to form a non‐exchangeable‐NH depletion zone in the adjacent soil. However, under conditions of a more substantial depletion of the exchangeable NH in the mycorrhizal sphere (e.g., with longer growth), an effect of mycorrhiza on the non‐exchangeable NH might be found.     

Comparison of urease inhibitor N‐(n‐butyl) thiophosphoric triamide and oxidized charcoal for conserving urea‐N in soil

Charcoal‐based amendments have a potential use in controlling NH₃ volatilization from urea fertilization, owing to a high cation‐exchange capacity (CEC) that enhances the retention of NH . An incubation study was conducted to evaluate the potential of oxidized charcoal (OCh) for controlling soil transformations of urea‐N, in comparison to urease inhibition by N‐(n‐butyl) thiophosphoric triamide (NBPT). Four soils, ranging widely in texture and CEC, were incubated aerobically for 0, 1, 3, 7, and 14 d after application of ¹⁵N‐labeled urea with or without OCh (150 g kg^?1 fertilizer) or NBPT (0.5 g kg^?1 fertilizer), and analyses were performed to determine residual urea and ¹⁵N recovery as volatilized NH₃, mineral N (as exchangeable NH , NO , and NO ), and immobilized organic N. The OCh amendment reduced NH₃ volatilization by up to 12% but had no effect on urea hydrolysis, NH and NO concentrations, NO accumulation, or immobilization. In contrast, the use of NBPT to inhibit urea hydrolysis was markedly effective for moderating the accumulation of NH , which reduced immobilization and also controlled NH₃ toxicity to nitrifying microorganisms that otherwise caused the accumulation of NO instead of NO . Oxidized charcoal is not a viable alternative to NBPT for increasing the efficiency of urea fertilization.     

Nitrate stability in loess soils under anaerobic conditions—laboratory studies

The objective of this laboratory study with six loess soils (three Eutric CambisoIs and three Haplic Phaeozems) incubated under flooded conditions was to examine the effect of a wide range of NO doses under anaerobic conditions on soil redox potential and N₂O emission or absorption. Due to the fact that loess soils are usually well‐drained and are expected to be absorbers during prevailing part of the season, the study aimed at determination of the conditions decisive for the transition from emission to absorption process. On the basis of the response to soil nitrate level, the two groups of soils were distinguished with high and low denitrification capacity. The soil denitrification activity showed Michaelis‐Menten kinetics with respect to soil nitrate content with K_M in the range 50–100 mg NO ‐N kg^–1. Percentage of nitrates converted to N₂O increased linearly with nitrate concentration in the range from 25 to 100 mg NO ‐N kg^–1 up to 43% and decreased linearly at higher concentrations reaching practically zero at concentrations about 600 mg NO ‐N kg^–1. No denitrification was observed below 25 mg NO ‐N kg^–1. Nitrous oxide absorption in soil occurred only at nitrate concentrations to 100 mg NO ‐N kg^–1 and in this concentration range was proportional to the denitrification rate. Nitrous oxide was formed at redox potentials below +200 mV and started to disappear at negative E_h values.     

Plant availability and leaching of (heavy) metals from ammonium‐, calcium‐, carbohydrate‐, and citric acid‐treated uranium‐mine‐dump soil

Remediation of an uranium‐mine soil from Settendorf (East Germany) includes phytoextraction under conditions which make its heavy metals more plant‐available but less leachable. A second way is active inhibition of heavy metal uptake by the plant. In a pot trial with Chinese cabbage (Brassica chinensis L.), planted and unplanted soil samples were daily irrigated with deionized water or aqueous solutions with a total of (g (kg soil)^–1) CaCl₂ (0.26 Ca), NH₄Cl (1.39), casein, sucrose, citric acid (13), and an extract of rape (B. napus L.) shoots (13 DW) in a phytotron for 26 d. Water‐irrigated plants were also treated with a 50 mM citric acid solution (10.5 g (kg soil)^–1) 6 and 7 d prior to harvesting. Total elements in plant tissue and soluble elements in aqueous extracts from control and postharvest soils were determined by ICP‐AES. Supplements of NH , and the NH ‐generating casein and rape extract reduced soil pH during nitrification, and increased plant uptake of Cd, Cu, Ni, and Zn. Citric acid at 50 mM adjusted soil to pH 4.5–6.0 and enhanced uptake of all elements. Long‐term application of sucrose and citric acid increased pH and inhibited uptake of Cd, Cr, Cu, Ni, and Zn. Contemporarily, leaching of heavy metals and humic substances was lowest with Ca and NH and highest with sucrose and citric acid amendments. It is concluded that Chinese cabbage grown for chelate‐assisted phytoextraction should be supplied with Ca and NH to obtain a high plant biomass on soil with a low hazard of leaching. Metal uptake should be stimulated by application of chelator 7 d prior to harvesting. Undesired uptake of heavy metals by Chinese cabbage determined as food should be inhibited with carbohydrate amendments. Long‐term application of NH or chelator, which reduces the solubility of certain elements but increases their uptake moderately, is recommended as a tool for continuous phytoextraction technologies.     

The role of biochar in retaining nutrients in amended tropical soils

This study investigated the effect of biochar amendments on the retention and availability of plant nutrients and Al in seven acidic tropical soils from Zambia and Indonesia. The experiments carried out investigated whether the adsorption capacity of NH$ _4^+ $ in the soils increased upon the addition of biochar and which effect biochar had on available concentrations of NO$ _3^- $ , K⁺, Mn²⁺, Mg²⁺ , PO$ _4^{3‐} $ , and Al³⁺. These nutrients were selected as they represent those important to plant growth and soil quality. No significant increases or decreases in aqueous NH$ _4^+ $ ‐N concentration with additions of biochar were detected. The Gaines–Thomas model was used in order to calculate selectivity coefficients for NH$ _4^+ $ exchange (K_gt values). Following the addition of biochar to soil, K_gt values decreased showing a reduction in the selective binding of NH$ _4^+ $ in the biochar amended soil compared to the control. The concentration of NO$ _3^- $ increased following the addition of biochar to the soils. The addition of 5 and 10% biochar to the Indonesian soil did not significantly alter (t‐test confidence level 0.05) the sorption of PO$ _4^{3‐} $ to the soil–biochar mixtures as compared to the soil alone. However, the addition of biochar to the soil from Zambia increased the sorption of PO$ _4^{3‐} $ compared to the soil alone. The concentrations of K⁺ and Mg²⁺ were significantly increased for almost all soils (t‐test at the 0.05 confidence level) following the addition of biochar. Addition of biochar to all but two soils significantly decreased (t‐test confidence level 0.05) Mn²⁺ concentrations. The concentration of Al³⁺ in the soils decreased exponentially significantly (t‐test confidence level 0.05) following the amendment of biochar in accordance with the increase in pH observed when biochar was added to the soil. These results show that biochar has the ability to release essential plant growth nutrients as well as alleviate Al toxicity in these soils.     

Effects of rainfall pattern on carbon and nitrogen dynamics in soil amended with biogas slurry and composted cattle manure

Soil moisture affects the degradation of organic fertilizers in soils considerably, but less is known about the importance of rainfall pattern on the turnover of C and N. The objective of this study was to determine the effects of different rainfall patterns on C and N dynamics in soil amended with either biogas slurry (BS) or composted cattle manure (CM). Undisturbed soil cores without (control) or with BS or CM, which were incorporated at a rate of 100 kg N ha^–1, were incubated for 140 d at 13.5°C. Irrigation treatments were (1) continuous irrigation (cont_irr; 3 mm d^–1); (2) partial drying and stronger irrigation (part_dry; no irrigation for 3 weeks, 1 week with 13.5 mm d^–1), and (3) periodic heavy rainfall (hvy_rain; 24 mm d^–1 every 3 weeks for 1 d and 2 mm d^–1 for the other days). The average irrigation was 3 mm d^–1 in each treatment. Cumulative emissions of CO₂ and N₂O from soils amended with BS were 92.8 g CO₂‐C m^–2 and 162.4 mg N₂O‐N m^–2, respectively, whereas emissions from soils amended with CM were 87.8 g CO₂‐C m^–2 and only 38.9 mg N₂O‐N m^–2. While both organic fertilizers significantly increased CO₂ production compared to the control, N₂O emissions were only significantly increased in the BS‐amended soil. Under the conditions of the experiment, the rainfall pattern affected the temporal production of CO₂ and N₂O, but not the cumulative emissions. Cumulative NO leaching was highest in the BS‐amended soils (9.2 g NO ‐N m^–2) followed by the CM‐amended soil (6.1 g NO ‐N m^–2) and lowest in the control (4.7 g NO ‐N m^–2). Nitrate leaching was also independent of the rainfall pattern. Our study shows that rainfall pattern may not affect CO₂ and N₂O emissions and NO leaching markedly provided that the soil does not completely dry out.     

Verteilung von Stickstoff auf Sproß und Wurzel bei jungen Bohnenpflanzen nach der Aufnahme von NO und NH

Translocation of nitrogen to the shoot of young bean plants after uptake of NO and NH by the root Phaseolus vulgaris plants (var. nana, cv. Saxa) at the primary leaf stage (without nodules) were fed during 6 hours with ¹⁵NO and ¹⁵NH, respectively. 24 hours after the absorption period more ₁₅N from the absorbed NO was translocated from the root to the shoot. The presence of NH in the nutrient solution enhanced the translocation of ¹⁵NON, probably by an inhibition of nitrate reductase. NH_4-⁺¹⁵N is mainly retained in the root by a high incorporation into the root protein. It can be concluded that nitrogen from newly absorbed NO is not retained and used for protein synthesis in the root according to the root's potential to synthesize protein. Nitrate reduction in the root is considered to be the limiting factor. This is supported by the fact that withdrawal of NO in the nutrient solution prior to the ¹⁵N-experiment increased NOtranslocation to the shoot as a consequence of a lowered level of nitrate reductase. In an experiment with ¹⁴NOsupply to the roots and ¹⁵NOapplication to the primary leaves (infiltration method) a considerable amount of ¹⁵N was translocated from the leaves to the roots. This indicates that an insufficient NOreduction in the root can be substituted by a retranslocation of reduced N-compounds from leaves to the roots. The proportion of NO reduced in the root influences also the pattern of primary distribution of nitrogen in the shoot of plants at the 4 leaf stage. At a concentration of 0,2 meq/l NO in the nutrient solution as compared to 20 meq/l NO during 10 hours a relative higher amount of ₁₅N was transported from the root to the younger, growing leaves i.e. via the phloem to metabolic sinks.     

Effects of nitric oxide and nitrogen on seedling emergence,ion accumulation,and seedling growth under salinity in the euhalophyte Suaeda salsa

Recently nitric oxide (NO) has emerged as a key signal molecule in plants. However, little is known about the role of NO in the salt tolerance of halophytes. Effects of the NO donors sodium nitroprusside (SNP) and nitrate (NO ) on growth and ion accumulation in the euhalophyte Suaeda salsa under salinity were investigated in the present study. The results showed that higher SNP supply increased seedling emergence, but SNP had no effect on shoot growth and the concentrations of Na⁺, K⁺, Cl^–, and NO . Higher NO had no effect on seedling emergence of the species. Shoot Cl^– decreased, but NO₃^– increased markedly, with a higher NO supply. The decrease in the estimated contribution of Cl^– to the osmotic potential was compensated for by an increase in that of NO . It appears that NO plays an important osmotic role in S. salsa under high salinity with a higher NO supply, and this trait may increase salt tolerance of the species under high salinity.     

Effect of nitrogen forms and levels on β‐glucan accumulation in grains of oat (Avena sativa L.) plants

Oat consumption has been rapidly increased worldwide because its high β‐glucan concentration may lower the level of blood cholesterol. The rates of β‐glucan accumulation in two husk and two nude oats were determined with an interval of 5 d after anthesis in a pot experiment. The results showed that a higher nitrogen level increased oat grain yield per plant, thousand‐grain weight, and concentrations of protein and β‐glucan. β‐glucan concentration generally increased with development of grain after anthesis, whereas the accumulation rate of β‐glucan was greater at the early stage of grain development and reached a maximum around 25 d after anthesis. We also found a larger β‐glucan concentration when plants were grown with NO ‐N as compared to NH ‐N. This study may suggest that the agronomic approaches such as nutrient management by optimizing the time and the level of nitrogen application and a higher ratio of NO ‐N to NH ‐N can play an important role in enhancing β‐glucan concentration in oat grains.     

Effect of ammonium or nitrate nutrition on photosynthesis,growth, and nitrogen assimilation in tomato plants

Nitrogen (N) is taken up by most plant species in the form of nitrate (NO ) or ammonium (NH ). Plant response to continuous NH nutrition is species‐dependent. In this study, we compare the responses of tomato (Solanum lycopersicum L. cv. Rio Grande) plants to N source (NO or NH ). To this end, early plant growth, photosynthesis, chlorophyll, carbohydrate, and N‐compound concentrations as well as the activities of main enzymes involved in N metabolism (nitrate reductase, nitrite reductase, glutamine synthetase, and glutamate dehydrogenase) were analyzed. Early plant growth was remarkably ameliorated under NH ‐ in comparison to NO ‐based nutrition. Concomitantly, photosynthetic activity, total chlorophyll, and carbohydrate concentrations were significantly increased. With increasing external NH concentration, NH accumulated mainly in roots. In addition, root protein concentration was significantly increased, reflecting high NH incorporation into organic nitrogen. Root glutamine synthetase (GS) activity was enhanced by NH for concentrations below 5 mM, whereas root glutamate dehydrogenase (GDH) activity increased in parallel to NH availability. Together with the positive effect of NH on tomato plant cv. Rio Grande growth, these results reveal that GDH could have, in addition to GS, a possible role in NH detoxification and tolerance of NH ‐based nutrition.     

Copper and temperature modify microbial communities,ammonium and sulfate release in soil

Recent studies suggest an important role of thermophilic bacterial communities of the Phylum Firmicutes on soil C, N and S cycling, and a positive effect on crop productivity through the production of sulfate (SO $ _4^{2 - } $ ) and ammonium (NH $ _4^+ $ ), essential plant nutrients. Copper (Cu) is commonly supplemented to soils as a fungicide in phytosanitary treatments although its consequences to the bacterial communities is frequently overlooked. Herein, we report on the influence of temperature and Cu on the microbial communities, namely those of the Phylum Firmicutes, from a soil collected at an olive orchard in S Portugal. Community fingerprints and band identification through sequencing was combined with measurement of SO $ _4^{2 - } $ and NH $ _4^+ $ production at different supplemented amounts of Cu and at moderate and high temperatures (30°C and 50°C, respectively). Both temperature and Cu induced changes in these communities, selecting for specific bacteria. Temperature induced the dominance of Brevibacillus, and Cu addition to soil caused a reduction of SO $ _4^{2 - } $ release by soil bacteria. Ammonium production during bacterial growth at moderate and high temperatures was not affected by Cu addition. A Cu‐tolerant thermophilic isolate, belonging to the Bacillus genus, showed significant inhibition by high Cu concentrations and a reduction of NH $ _4^+ $ release during growth; genera Brevibacillus and Bacillus have been previously reported as high NH $ _4^+ $ and SO $ _4^{2 - } $ producers of the Firmicutes phylum. Results indicate that Cu treatments select specific tolerant bacterial strains which could influence natural soil fertilization in Cu‐treated orchards.     

Effect of HCO $ _3^ - $ on rice growth and iron uptake under flood irrigation and drip irrigation with plastic film mulch

There has been a partial shift away from conventional flood irrigation (FI) practices for rice (Oryza stativa L.) production in water‐scarce northern China. Drip irrigation with plastic film mulch (DI‐PFM) can maintain high rice yields with significant water savings. However, rice seedlings often develop chlorosis when grown with DI‐PFM on calcareous soil. Bicarbonate is a concern with regard to chlorosis in calcareous soil. The objective of this simulation experiment was to determine the effect of irrigation method and irrigation water HCO $ _3^ - $ concentration on (1) soil pH and DTPA‐Fe concentration, (2) chlorophyll, total Fe, and active Fe concentrations of rice leaves, and (3) rice root and shoot biomass. The experiment consisted of four treatments: FI with water containing either 2 or 10 mM HCO $ _3^ - $ (referred to as FI‐2 and FI‐10, respectively) and DI‐PFM with water containing 2 or 10 mM HCO $ _3^ - $ (referred to as DI‐2 and DI‐10, respectively). The results show that the HCO $ _3^ - $ concentrations of the soil solution were greater under FI than under DI‐PFM, because more irrigation water was applied in the FI system. Soil pH increased as the HCO $ _3^ - $ concentration of the irrigation water increased. The increase in soil pH was greater in DI‐PFM than in FI. Soil DTPA‐Fe concentration, leaf SPAD values, leaf total Fe concentration, leaf active Fe concentration, shoot biomass, and root biomass decreased as the HCO $ _3^ - $ concentration of the irrigation water increased. The decreases were less under DI‐PFM than under FI. Overall, the results indicate that rice plants are more sensitive to the HCO $ _3^ - $ concentration of irrigation water under FI than under DI‐PFM.     

Small amounts of ammonium (NH$ _4^+ $) can increase growth of maize (Zea mays)

Nitrate (NO$ _3^ - $ ) and ammonium (NH$ _4^+ $ ) are the predominant forms of nitrogen (N) available to plants in agricultural soils. Nitrate concentrations are generally ten times higher than those of NH$ _4^+ $ and this ratio is consistent across a wide range of soil types. The possible contribution of these small concentrations of NH$ _4^+ $ to the overall N budget of crop plants is often overlooked. In this study the importance of this for the growth and nitrogen budget of maize (Zea mays L.) was investigated, using agriculturally relevant concentrations of NH$ _4^+ $ . Maize inbred line B73 was grown hydroponically for 30 d at low (0.5 mM) and sufficient (2.5 mM) levels of NO$ _3^ - $ . Ammonium was added at 0.05 mM and 0.25 mM to both levels of NO$ _3^ - $ . At low NO$ _3^ - $ levels, addition of NH$ _4^+ $ was found to improve the growth of maize plants. This increased plant growth was accompanied by an increase in total N uptake, as well as total phosphorus, sulphur and other micronutrients in the shoot. Ammonium influx was higher than NO$ _3^ - $ influx for all the plants and decreased as the total N in the nutrient medium increased. This study shows that agriculturally relevant proportions of NH$ _4^+ $ supplied in addition to NO$ _3^ - $ can increase growth of maize.     

Nitrogen fertilization and nitrate leaching into groundwater on arable sandy soils

Nitrate leaching depending on N fertilization and different crop rotations was studied at two sites with sandy soils in N Germany between 1995 and 2000. The leaching of NO was calculated by using a numerical soil‐water and N model and regularly measured N_min values as input data. Also the variability of N_min values on the sandy soils was determined along transects. They reveal the high variability of the N_min values and show that it is not possible to confirm a significant N_min difference between fertilizer treatments using the normal N_min‐sampling intensity. Nitrate‐leaching calculations of five leaching periods showed that even strongly reduced N‐fertilizer applications did not result in a substantially lower NO leaching into the groundwater. Strong yield reductions of even more than 50%, however, were immediately measured. Mean NO concentrations in the groundwater recharge are >50 mg L^–1 and are mainly due to mineralization from soil organic matter. Obviously, the adjustment of the N cycle in the soil to a new equilibrium and a reduced NO ‐leaching rate as a consequence of lower N inputs need a much longer time span. Catch crops are the most efficient way to reduce the NO concentrations in the groundwater recharge of sandy soils. Their success, however, strongly depends on the site‐specific development possibilities of the catch crop. Even with all possible measures implemented, it will be almost impossible to reach NO concentrations <50 mg L^–1 in sandy soils. The only way to realize this goal on a regional scale could be by increasing areas with lower nitrate concentrations in the groundwater recharge like grassland and forests.     

Influence of nitrogen forms and mycorrhizal colonization on growth and composition of Chinese bunching onion

In recent years, interest has grown in cultivating Allium species with enhanced health benefits and/or distinct flavor. Concentrations of phytochemicals determining these desired characteristics may be influenced by nitrogen forms (ammonium or nitrate) and arbuscular mycorrhizal (AM) fungi. We examined these relations with the test plant bunching onion (Allium fistulosum L.). Three different ammonium‐to‐nitrate (NH : NO ) ratios were supplied in combination with or without inoculation with an AM fungus (Glomus mosseae). The plants were evaluated for dry weight, leaf number, and content of nutrients (N, NO , P, S), sugars (glucose, fructose, and sucrose), and organosulfur compounds (measured as pyruvic acid). The experiment was carried out under controlled conditions in a greenhouse. Plants were grown on perlite amended twice a day with nutrient solution. In nonmycorrhizal plants, the application of nutrient solution with predominant NO or NH₄NO₃ as N source supported adequate growth of Allium fistulosum while predominant NH supply resulted in decreased growth and occurrence of wilting symptoms. Mycorrhizal inoculation significantly increased dry weight and leaf number of predominantly NH ‐fed or NH₄NO₃‐fed plants. While shoot P concentration increased with higher NH supply, shoot N concentration increased in predominantly NH ‐fed plants only. Nitrogen form and AM colonization had little effect on shoot S or sugar concentrations. The total content in organosulfur compounds was significantly affected by both, N form and AM colonization. The optimal growth condition for a high formation of organosulfur compounds in this experiment was a nutrient solution with predominant NO supply, but when supported by AM fungi, Allium fistulosum produced similar amounts of pyruvic acid in NH₄NO₃‐fed plants.     

N form–dependent growth retardation of Arabidopsis thaliana seedlings as revealed from physiological and microarray studies

Ammonium (NH ) nutrition causes retardation of growth in many plant species. In Arabidopsis grown with NH as the sole N source, growth retardation occurs already at early stages before photosynthesis has come to its full power. In order to describe the peculiarities of these retarded plants, they were compared with nitrate (NO )‐grown plants of the same age of 15 d. Photosynthetic activity as measured by CO₂ uptake per unit chlorophyll is half as high in NH ‐grown seedlings as in NO ‐grown ones. This finding is confirmed by the analysis of chlorophyll fluorescence. Chloroplasts of NO ‐grown, but not of NH ‐grown, seedlings show starch deposits after 5 h of illumination with 40 μmol m^–2 s^–1. Gene‐expression analysis based on cDNA microarray and on Northern blots provide a clue about the biochemical background. After the first 2 weeks of growth, it seems that NO ‐grown seedlings subsist mainly on normal photosynthesis, whereas NH ‐grown seedlings still use lipids from the seeds stored in oleosomes. Corresponding to this observation, the mRNAs for enzymes of β‐oxidation are more strongly expressed in NH ‐grown seedlings. Different carbohydrate sources for sucrose synthesis are indicated by different gene expression. Higher gene expression of fructose bisphosphate aldolase (cytosolic isoform) in NO ‐grown seedlings indicates the dependence on photosynthesis, whereas a higher gene expression of PEP carboxykinase in NH ‐grown seedlings points to a prominent role of β‐oxidation of storage lipids still present.     

Nitratammonifizierung im Boden mit Abwasserverrieselung

Nitrate Reduction to Ammonium in a Soil with Wastewater Irrigation Flooding with wastewaters including 48 mg/l NH-N, 15 mg/l organic N and 63 mg/l K¹⁵NO-N has led to strong nitrogen losses by denitrification from a sandy Cambisol. Beside this ¹⁵NH was formed also. Possible reasons of the nitrate reduction to ammonium are discussed and conclusions for practical wastewater irrigation are drawn.     

Mikrobiologische und biochemische Charakterisierung landwirtschaftlich genutzter Böden. I. Mitteilung: Die Ermittlung einer Bodenmikrobiologischen Kennzahl

Microbiological and biochemical characterization of agricultural soils Part. I. Determination of a soil microbiological index The intensity of soil microbial activity was determined by measuring the total microbial biomass, the activity of cellfree hydrolases (saccharase, protease and alkaline phosphatase) as well as mainly cellbound reductases (dehydrogenase and catalase). Testing 33 mixed samples of 11 different or differently treated, agricultural soils in Bavaria, a highly significant correlation (r = 0.77–0.99) between all individual microbiological soil properties was found. No correlations could be found with the additionally determined plate counts of bacteria or with nitrifications tests. This close relation between the tested biomass and enzyme activities could be used for calculating an over-all index (Bodenmikrobiologische Kennzahl) of soil microbial activity by transforming and combining the individual analytical soil properties according to the following equation:      

Species‐specific differences in nitrogen uptake and utilization by six European tree species

Species‐specific uptake and allocation mechanisms for N are scarce, in particular when trees are cultivated in potted soil under more natural conditions than in hydroponic culture. The objective of this study was to compare specific N‐uptake rates for economically and ecologically important tree species in Central European forests: pine (Pinus sylvestris), spruce (Picea abies), oak (Quercus petraea), beech (Fagus sylvatica), lime (Tilia cordata), and ash (Fraxinus excelsior) when they grow in mineral soil from an old fallow site with a pH of 6. We used an ¹⁵N‐labeling method to measure tree seedling ¹⁵N uptake in potted soils (Humic Cambisol) when both N forms NH$ _4^+ $ and NO$ _3^- $ were simultaneously present in the soil solution for interspecies comparison and assessment of relationships between specific ¹⁵N‐uptake rates and amino acid–accumulation rates or relative growth rates (RGR). The results demonstrate that tree species varied significantly in their capacity to take up NH$ _4^+ $ or NO$ _3^- $ into roots, stems, or leaves, but indicate only marginal differences in their preference for NH$ _4^+ $ or NO$ _3^- $ when they grow in mineral soil. The ranking of specific ¹⁵N‐uptake rates for NH$ _4^+ $ and NO$ _3^- $ was oak < beech < spruce < pine < lime < ash. Fine roots of all species had the highest specific ¹⁵N‐uptake rates for both N forms, followed by total roots, leaves/needles, and stems. As regards tree seedling species, we found negative relationships between glutamine (Gln)‐accumulation rates in leaves/needles and total ¹⁵N‐uptake rates in fine roots. Noteworthy was the fact that, at high Gln‐accumulation rates, the N‐uptake system in fine roots of ash was probably lower under feedback inhibition by the amino acid.     

Changes in soil nitrogen availability due to stand development and management practices on semi‐arid sandy lands,in northern China

Soil nitrogen (N) availability is one of the limiting factors for plant growth on sandy lands. Little is known about impacts of afforestation on soil N availability and its components in southeastern Keerqin sandy lands, China. In this study, we measured N transformation under sandy Mongolian pine (Pinus sylvestris var. mongolica Litv.) plantations of different ages (grassland, young, middle‐aged, close‐to‐mature) and management practices (non‐grazing and free‐grazing) during the growing seasons using the ion exchange resin bag method. Results showed that, for all plots and growing season, soil NH‐N, NO‐N, mineral N, and relative nitrification index, varied from 0·18 to 1·54, 0·96 to 22·05, 1·23 to 23·58 µg d⁻¹ g⁻¹ dry resin, and 0·76 to 0·97, respectively, and NO‐N dominated the available N amount due to intense nitrification in these ecosystems. In general, the four indices significantly increased in the oldest plantation, with corresponding values in non‐grazing sites lower than those in free‐grazing sites (p < 0·05). Our studies indicated that it is a slow, extended process to achieve improvement in soil quality after afforestation of Mongolian pine in the study area. Copyright © 2009 John Wiley & Sons, Ltd.