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.     

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.     

Relative effects of ammonia and nitrite on the germination and early growth of aerobic rice

Recent studies have documented adverse affects of urea on the establishment and growth of aerobic rice when applied at seeding. The following experiments were conducted to examine the relative importance of ammonia and nitrite (NO$ _2^- $ ) toxicities as mechanisms contributing to poor germination and early growth of aerobic rice. Soil was collected from an experiment in the Philippines where aerobic rice was grown continuously for 7 years. Subsamples of the soil were: (1) pretreated with sulfuric acid (0.5 M H₂SO₄ added at 75 mL kg^–1), (2) oven‐heated at 120°C for 12 h, or (3) left untreated. In a greenhouse study N was applied to the untreated, acidified, and oven‐heated soils as either urea or ammonium sulfate (0.0 or 0.3 g N kg^–1). Plant height, root length, total biomass, and number of seminal roots were evaluated after 10 d. Microdiffusion incubations were used to assess the effects of soil pretreatment, N source, and N rate (0, 0.5, 1.0, 1.5 g N kg^–1) on ammonia (NH₃) volatilization and germination. Nitrite incubations were conducted to establish a critical level for NO$ _2^- $ toxicity and measure the extractable NO$ _2^- $ and germination trends as affected by soil pretreatment, N source, and N rate. On untreated soil, urea reduced early growth and germination while ammonium sulfate caused no adverse effects. Progressively higher rates of urea increased NH₃ volatilization and inhibited germination, while oven‐heating and acidification minimized the adverse effects. All treatment combinations (soil pretreatment, N source, N rate) had extractable NO$ _2^- $ levels below the critical level of 0.2 g N kg^–1, suggesting that ammonia and not NO$ _2^- $ toxicity was the principal cause of inhibition. Since the risk of NH₃ toxicity is highest just following urea hydrolysis, strategies to optimize the timing and placement of urea should be considered.     

Effects of earthworms and plant growth–promoting rhizobacteria (PGPR) on availability of nitrogen,phosphorus, and potassium in soil

Both earthworms and plant growth–promoting rhizobacteria (PGPR) are ubiquitous and important for promoting circulation of plant macronutrients. Two series of laboratory experiments were conducted to investigate the effects of earthworm casts and activities on the growth of PGPR, and the inoculation of earthworms and PGPR on the availability of N, P, and K in soils, respectively. During a short incubation period (0–34 h), the extracts of earthworm (Pheretima guillelmi)‐worked soil significantly (p < 0.05) increased the abundance of the three species of PGPR, including N‐fixing bacteria (NFB) (Azotobacter chroococcum HKN‐5), phosphate‐solubilizing bacteria (PSB) (Bacillus megaterium HKP‐1), and K‐solubilizing bacteria (KSB) (B. mucilaginous HKK‐1), in Luria‐Bertani (LB) broth. There were synergistic effects of dual inoculation of earthworms and PGPR on increasing the concentrations of NH$ _4^+ $ ‐N, (NO$ _3^- $ + NO$ _2^- $ )‐N, NaHCO₃‐extractable P, and NH₄OAc‐extractable K in the corresponding soils. Bioavailable N (the sum of NH$ _4^+ $ ‐N and [NO$ _3^- $ + NO$ _2^- $ ]‐N) in the dual inoculation was 4 to 24 times those inoculated with earthworms or NFB alone, respectively. The significantly higher concentrations of bioavailable N and P in the dual inoculation of earthworms and NFB or PSB may be due to the higher abundance of PGPR and/or higher activities of urease and acid phosphatase than those of single inoculation of NFB or PSB, respectively. Dual inoculation of earthworms and PGPR would be most effective in reducing the need for chemical fertilizers in agriculture.     

Nitrogen transformation and plant growth in response to different urea‐application methods and the addition of DMPP

The differences in soil inorganic‐nitrogen (N) concentration and distribution, plant biomass, and root growth in the presence or absence of the nitrification inhibitor 3,4‐dimethylpyrazole phosphate (DMPP) under different urea‐application methods (placement versus homogeneously applied) were explored in a short‐term microcosm experiment. Spring wheat (Triticum aestivum L.) was grown in a microcosm with six different treatments: no amendment (CK), DMPP homogeneously applied (DMPP‐hom), urea homogeneously applied (Urea‐hom), urea with DMPP homogeneously applied (Urea + DMPP‐hom), urea placement (Urea‐place), and urea with DMPP placement (Urea + DMPP‐place). After 28 d, plant biomass, soil inorganic nitrogen content, distribution of soil inorganic nitrogen and plant roots in the soil were analyzed. The soil inorganic N and plant roots tended to be distributed asymmetrically in the placement treatment but were distributed symmetrically in the homogeneous treatment. DMPP addition significantly increased the soil NH$ _4^+ $ ‐N content and decreased the NO$ _3^- $ ‐N content, especially near the fertilized zones in the placement treatment. Compared to the urea‐only treatments, DMPP application significantly increased the shoot biomass and root lengths of the wheat in the homogeneous treatment but decreased them in the placement treatment. Therefore, homogeneously applied urea and DMPP may produce a more uniform nutrient distribution, leading to greater nitrogen retention in the soil and thus accelerating wheat growth.     

Water quality under intensive banana production and extensive pastureland in tropical Mexico

The effects of intensive banana production with high mineral‐fertilizer application and of extensive pastures were compared regarding water quality in a lowland region of SE Mexico. We monitored NO$ _3^- $ , NO$ _2^- $ , and PO₄^3– concentrations in groundwater (80 m depth), subsurface water (5 m depth), and surface water (open‐ditch drainage) at monthly intervals for a one‐year period. Irrespective of the land use, the NO$ _3^- $ concentrations in all water bodies were lower than the threshold value for drinking water and aquatic life. Particularly in areas with intense banana production, the NO$ _2^- $ contents in water exceeded the safety thresholds for drinking water of 1.0 mg L^–1 (WHO, 2006) and aquatic ecosystems of 0.2 mg L^–1 (OATA, 2008). Water from pastureland showed significantly higher PO₄^3– concentration than that from the banana plantation, indicating a high risk of eutrophication. There is a need to provide recommendations for optimal time and amount of N application in commercial banana production and for limitation of P inputs in pasturelands to avoid further contamination of water bodies.     

Does the source of nitrogen affect the response of subterranean clover to prolonged root hypoxia?

Nitrogen (N) is taken up by most plant species in the form of nitrate (NO ) or ammonium (NH ). The plant response to continuous ammonium nutrition is species‐dependent. In this study, the effects of the source of N nutrition (NO , NH , or the mixture of NO and NH ) on the response of clover (Trifolium subterraneum L. cv. 45C) plants to prolonged root hypoxia was studied. Under aerobic conditions, plant growth was strongly depressed by NH , compared to NO or mixed N nutrition, as indicated by the significant decrease in root and shoot‐dry‐matter production (DW), root and shoot water contents (WC), leaf chlorophyll concentration, and chlorophyll fluorescence parameters (F₀, F_v/F_m). However, the N source had no effect on chlorophyll a–to–chlorophyll b ratio. Under hypoxic conditions, the negative effects of root hypoxia on plant‐growth parameters (DW and WC), leaf chlorophyll concentration, and chlorophyll fluorescence parameters were alleviated by NH rather than NO supply. Concomitantly, shoot DW–to–root DW ratio, and root and leaf NH concentrations were significantly decreased, whereas root and leaf carbohydrate concentrations, glutamine synthetase activities, and protein concentrations were remarkably increased. The present data reveal that the N source (NO or NH ) is a major factor affecting clover responses to hypoxic stress, with plants being more tolerant when NH is the N form used. The different sensitivity is discussed in terms of a competition for energy between nitrogen assimilation and plant growth.     

Plant‐available nitrogen in the soil: Relationships between pre‐plant and pre‐sidedress nitrate tests for corn production

Corn (Zea mays L.) producers in the rainfed regions sometimes sidedress fertilizer N according to pre‐plant–nitrate test (PPNT) results based on the assumption that there is a linear relationship between pre‐sidedress nitrate test (PSNT) and the PPNT. There has been no report on such relationship in Ontario (Canada) and elsewhere in the nonirrigated corn‐growing regions. A field study was conducted near Ottawa, Canada for 7 y to (1) determine changes in soil available N from pre‐planting to shortly after the sidedress stage (late June) for corn and (2) establish a quantitative relationship between PPNT and PSNT. In each year, soil samples from fields of three to four plot experiments with different cropping histories, soil textures, and management levels, taken at 7 to 10 d intervals, and from on‐farm trials taken at pre‐planting and pre‐sidedress, were extracted with 2 M KCl. The concentrations of NO ‐N were determined colorimetrically. It was found that soil NO ‐N concentration of PSNT was a linear function of PPNT with an average slope of 1.7. However, the slope of the regression equations differed dramatically among cropping sequences, and to a lesser extent, soil textures. The NO ‐N concentration after planting to pre‐sidedress was influenced by air temperature and precipitation during this period of time. Both PPNT and PSNT positively correlated with corn‐grain yield. Our data suggest that cautions must be taken when deciding the rate of fertilizer N for sidedress application to corn based on PPNT test, especially under more humid northern climate conditions.     

Effects of hydrochar application on the dynamics of soluble nitrogen in soils and on plant availability

Before hydrochars can be applied as soil amendments in agriculture, information about how hydrochar application affects soil nutrient cycles and plant growth are necessary. In this study, incubation experiments were performed to investigate hydrochar effects on N concentrations (NO$ _3^- $ , NH$ _4^+ $ ) in soils with different N pools (soil N, fertilizer N). A set of pot trials with three crop species (barley, phaseolus bean, leek) was conducted to determine hydrochar effects on plant N availability and biomass production after mineral‐N fertilization. Results of the incubation experiments show that hydrochar reduced the concentration of mineral N in soil within the first week after incorporation, especially that of nitrate. This was particularly evident, when hydrochars with high C : N ratio, high DOC and low mineral‐N contents were applied. Hydrochars promoted biomass production of barley and phaseolus bean in pot trials, which can be partly attributed to an increase in soil pH after hydrochar incorporation. Dry‐matter yield of leek tended to decrease after hydrochar application. Hydrochars with high C : N ratio decreased the plant's N content, an effect that was strongest with increased hydrochar concentration. Hydrochars with low C : N ratio did not affect the crop's N uptake. Our results show that the use of hydrochars as amendment in arable field or horticultural pot production will require an adjustment of N‐mineral‐fertilization strategies.     

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.     

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.     

Nitrogen‐fertilizer forms affect the nitrogen‐use efficiency in fertigated citrus groves

The fertigated area of the Brazilian citrus industry has grown rapidly during recent years, and an efficient management of nitrogen (N) application at these sites is required for sustainable citrus production. Therefore, a field trial with Valencia orange trees [Citrus sinensis (L.) Osbeck] on Swingle citrumelo rootstock (Citrus paradise Macfad. x Poncirus trifoliata L. Raf.) was conducted for 8 years to evaluate the effects of N rates (80, 160, 240 and 320 kg ha^–1 y^–1) applied by fertigation, either as ammonium nitrate (AN) or calcium nitrate (CN), on soil solution dynamics, fruit yield, nutritional status, and N‐use efficiency (NUE) of trees. The maximum fruit yield was reached with 240 kg N ha^–1 for AN, whereas a linear response and greater fruit yield was observed for N supplied as CN. The NUE was reduced for both N forms with increasing N rates. However, the NUE for CN was 14 to 38% greater than the NUE for AN. The lower fruit yield and NUE for AN compared to CN‐treated trees was associated with the increased acidification of the soil solution with increased AN rates (pH ≤ 4.0). This limited nitrification resulted in a high ammonium (NH$ _4^+ $ ) concentration in the soil solution and a reduction in the net absorption of cations by the trees, particularly calcium (Ca). Due to the improved ion balance as well as the higher pH of the soil solution (pH ≥ 6.3) and diminished NH$ _4^+ $ availability, gains in both fruit yield and NUE in fertigated citrus groves in tropical soils can be obtained with the use of CN as a source of N.     

Nitrous oxide emissions and dynamics of soil nitrogen under 15N‐labeled cow urine and dung patches on a sandy grassland soil

Grazing animals highly influence the nutrient cycle by a direct return of 80% of the consumed N in form of dung and urine. In the autumn‐winter period, N uptake by the sward is low and rates of seepage water in sandy soils are high, hence high mineral‐N contents in soil and in seepage water as well as large losses of N₂O are expected after cattle grazing in autumn. The objective of this study was the quanitfication of N loss deriving from urine and dung leaching and by N₂O emission. Therefore the deposition of urine and dung patches was simulated in maximum rates excreted by cows by application of ¹⁵N‐labeled cow urine and dung (equivalent to 1030 kg N ha^–1 and 1052 kg N ha^–1, respectively) on a sandy pasture soil in N Germany. Leachate was collected in weekly intervals from free‐draining lysimeters, and ¹⁵N‐NO , ¹⁵N‐NH , and ¹⁵N‐DON (dissolved organic N) were monitored over 171 d. Furthermore, the ¹⁵N‐N₂O emission rates and the dynamics of inorganic ¹⁵N in the upper soil layer were monitored in a field trial, adjacent to the lysimeters. After 10 d following the urine application, the urea was completely hydrolyzed, shown by a 100% recovery of urine‐N in the soil NH . The following decrease of ¹⁵N‐NH in the soil was higher than the increase of ¹⁵N‐NO , and some N loss was explained by leaching. Amounts of 51% and 2.5% of the applied ¹⁵N were found in leachate as inorganic N, 2.4% and 0.7% as DON derived from urine and dung, respectively. Release of N₂O from urine and dung patches applied to the pasture was low, with losses of 0.05% and 0.33% of the applied ¹⁵N, respectively. Overall loss of dung‐derived N was very low, but as the bulk dung N remained in the soil, N loss after mineralization of the dung needs to be investigated.     

Nutrient limitation of alpine plants: Implications from leaf N : P stoichiometry and leaf δ15N

Nitrogen (N) deposition can affect grassland ecosystems by altering biomass production, plant species composition and abundance. Therefore, a better understanding of the response of dominant plant species to N input is a prerequisite for accurate prediction of future changes and interactions within plant communities. We evaluated the response of seven dominant plant species on the Tibetan Plateau to N input at two levels: individual species and plant functional group. This was achieved by assessing leaf N : P stoichiometry, leaf δ¹⁵N and biomass production for the plant functional groups. Seven dominant plant species—three legumes, two forbs, one grass, one sedge—were analyzed for N, P, and δ¹⁵N 2 years after fertilization with one of the three N forms: NO$ _3^- $ , NH$ _4^+ $ , or NH₄NO₃ at four application rates (0, 7.5, 30, and 150 kg N ha^–1 y^–1). On the basis of biomass production and leaf N : P ratios, we concluded that grasses were limited by available N or co‐limited by available P. Unlike for grasses, leaf N : P and biomass production were not suitable indicators of N limitation for legumes and forbs in alpine meadows. The poor performance of legumes under high N fertilization was mainly due to strong competition with grasses. The total above‐ground biomass was not increased by N fertilization. However, species composition shifted to more productive grasses. A significant negative correlation between leaf N : P and leaf δ¹⁵N indicated that the two forbs Gentiana straminea and Saussurea superba switched from N deficiency to P limitation (e.g., N excess) due to N fertilization. These findings imply that alpine meadows will be more dominated by grasses under increased atmospheric N deposition.     

Nitrogen turnover in bare soil planted subsequently with grass as investigated by electro‐ultrafiltration (EUF)

Changes of EUF‐extractable nitrogen (N) (nitrate, ammonium, organic N) in 20 arable bare soils, subsequently planted with ryegrass (Lolium multiflorum L.) and cutting three times were investigated in pot experiments. All 20 soils responded qualitatively in the same way. During the period of bare soil, there was a significant increase of EUF‐extractable nitrate (EUF NO ), while extractable ammonium (EUF NH ) remained on the same level and organic N (EUF N_org) decreased. This decrease, however, was not significant. From sowing until the first cutting of the grass, EUF‐NO concentration decreased to almost zero. This low EUF‐NO level was maintained throughout the subsequent experimental period (three cuttings of grass). During the growth of the first cutting, EUF N_org decreased while EUF NH remained constant, however, on a low level. EUF NH fell during the growth of the second and third cutting. In this period, however, the N supply of the grass was insufficient. EUF N_org decreased during the growth of the second cutting, but increased during the growth of the third cutting. This shows that the EUF‐N_org fraction represents a transient pool, which gains and loses N. EUF NO , EUF NH , and EUF N_org correlated with the N uptake of the grass. Strongest correlation for EUF NO was found for the first cutting (p < 0.001), and for EUF NH and EUF N_org for the second and third cutting (p < 0.001). Total soil N was not correlated with the N uptake of the grass. EUF N_org was only about 2% of the total N. This relatively small EUF‐N_org fraction, however, is relevant for the mineralization of organic soil N, and the N quantity indicated by EUF N_org is in the range of the N amount mineralized in arable soils within a growing season.     

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.     

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.     

The effect of nitrification inhibitors on the nitrous oxide (N2O) release from agricultural soils—a review

The use of nitrification inhibitors (NI) is a technique which is able to improve N fertilizer use efficiency, to reduce nitrate leaching and to decrease the emission of the climate‐relevant gas N₂O simultaneously, particularly in moderately fertilized agricultural systems adapted to plant N demand. The ammonia monooxygenase (AMO) is the first enzyme which is involved in the oxidation of NH$ _4^+ $ to NO$ _3^ - $ in soils. The inhibition of the AMO by NIs directly decreases the nitrification rate and it reduces the NO$ _3^- $ concentration which serves as substrate for denitrification. Hence, the two main pathways of N₂O production in soils are blocked or their source strength is at least decreased. Although it has been shown that archaea are also able to oxidize NH₃, results from literature suggest that the enzymatic activity of NH₃ oxidizing bacteria is the most important target for NIs because it was much stronger affected. The application of NIs to reduce N₂O emissions is most effective under conditions in which the NI remains close to the N ‐ fertilizer. This is the case when the NI was sprayed on mineral ‐ N fertilizer granules or thoroughly mixed with liquid fertilizers. Most serious problems of spatial separation of NI and substrate emerge on pasture soils, where N₂O hotspots occur under urine and to a lesser extent under manure patches. From the few studies on the effect of different NI quantities it seems that the amount of NI necessary to reduce N₂O emissions is below the recommendations for NI amounts in practice. NIs can improve the fertilizer value of liquid manure. For instance, the addition of NIs to slurry can increase N uptake and yield of crops when NO$ _3^ - $ ‐ N leaching losses are reduced. It has clearly been demonstrated that NIs added to cattle slurry are very effective in reducing N₂O as well as NO emissions after surface application and injection of slurry into grassland soils. In flooded rice systems NIs can reduce CH₄ emission significantly, whereas the effect on CO₂ emission is varying. On the other hand, as an effect of the delay of nitrification by NIs, NH₃ emission might increase when N fertilizers are not incorporated into the soil. As compared to other measures NIs have a high potential to reduce N₂O emissions from agricultural soils. Further, no other measure has so consistently been proofed according its efficiency to reduce N₂O emissions. From the published data [Akiyama et al. ( 2010 ) and more recent data from the years 2010–2013; 140 data sets in total] a reduction potential of approx. 35% seems realistic; however, further measurements in different management systems, particularly in regions with intense frost/thaw cycles seem necessary to confirm this reduction potential. These measurements generally should cover a whole annual cycle.