High Rates Of Nitrogen Fertilizer Research Articles

BILL E. KUNKLE INTERDISCIPLINARY BEEF SYMPOSIUM: Coping with tall fescue toxicosis: Solutions and realities.

Beef cattle consume more tall fescue [ (Schreb.) Darbysh.] than any other perennial cool-season grass in North America. Tall fescue forms a mutualistic relationship with the fungus (Morgan-Jones and W. Cams) C.W. Bacon & Schardl, comb. nov.; these plants are often referred to as endophyte-infected tall fescue. Agronomically, endophyte infection is good; endophyte-infected plants resist biotic and abiotic stresses better than do endophyte-free plants. However, tall fescue plants infected with a common toxic endophyte produce the ergot alkaloids that cause tall fescue toxicosis. Several agronomic practices can eliminate or reduce the effects of tall fescue toxicosis. Renovating tall fescue fields infected with a common toxic endophyte to new cultivars infected with a novel (or nontoxic) endophyte eliminates the problem for that field. Although the cost to renovate currently exceeds US$600/ha, most economic analyses show it to be a good long-term investment, especially on fields with high forage production potential. Less effective, but popular, methods of dealing with tall fescue toxicosis include adding clovers ( spp.) or other forage species to the pastures to dilute the effects of the toxins, avoiding the use of high rates of nitrogen fertilizers, feeding supplements, and rotating cattle to warm-season forages in summer. Also, the alkaloid concentration in forage can be reduced by making it (drying) into hay. Recent research shows that animal tolerance to tall fescue toxicosis is possible, although there is still much to learn about this topic. Tall fescue toxicosis is likely a reality in the beef industry for some time to come. We contend that renovation of the most productive fields to cultivars infected with a novel endophyte coupled with the prospect of using tolerant livestock in scenarios where renovation is not possible will give innovative producers a competitive edge.

Quality of Groundwater in an Area with Intensive Agricultural Activity

The main objectives of this study were the application of principal component analysis (PCA) for variable selection of an aquifer with a large amount of data record and assessment of the overall quality and the extent of groundwater contamination in Andimeshk, Iran. For this purpose, 17 groundwater quality variables in 46 sampling wells and springs during an eight-year time period (2006–2013) were considered. A PCA was conducted for dry and wet seasons to find out associations between variables, thus reducing the dimensionality of the data. PCA resulted in the reduction of original data to nine variables in wet season encompassing 94.35 % of the total variance and eight variables in dry season accounting for 92.03 % of the total variance. A Procrustes Analysis on the reduced data set showed a high similarity between the original and reduced variables. Groundwater quality index indicated that the most polluted parts of the study area are located in areas of intensive agricultural activities. The results of water quality proved that the effect of geological formations on groundwater quality is mainly due to the influence of EC in terms of TDS, Mg, Ca, Sulfate, and Cu rather than other parameters. The application of animal manure at excessively high rates or the combined application of high rates of manure and nitrogen fertilizer has increased nitrate levels in recent years. Fertilizers may also have an impact on the levels of copper in ground water as it is one of the impurities in these inorganic fertilizers.

The core root microbiome of sugarcanes cultivated under varying nitrogen fertilizer application.

Diazotrophic bacteria potentially supply substantial amounts of biologically fixed nitrogen to crops, but their occurrence may be suppressed by high nitrogen fertilizer application. Here, we explored the impact of high nitrogen fertilizer rates on the presence of diazotrophs in field-grown sugarcane with industry-standard or reduced nitrogen fertilizer application. Despite large differences in soil microbial communities between test sites, a core sugarcane root microbiome was identified. The sugarcane root-enriched core taxa overlap with those of Arabidopsis thaliana raising the possibility that certain bacterial families have had long association with plants. Reduced nitrogen fertilizer application had remarkably little effect on the core root microbiome and did not increase the relative abundance of root-associated diazotrophs or nif gene counts. Correspondingly, low nitrogen fertilizer crops had lower biomass and nitrogen content, reflecting a lack of major input of biologically fixed nitrogen, indicating that manipulating nitrogen fertilizer rates does not improve sugarcane yields by enriching diazotrophic populations under the test conditions. Standard nitrogen fertilizer crops had improved biomass and nitrogen content, and corresponding soils had higher abundances of nitrification and denitrification genes. These findings highlight that achieving a balance in maximizing crop yields and minimizing nutrient pollution associated with nitrogen fertilizer application requires understanding of how microbial communities respond to fertilizer use.

Growth retardant and nitrogen levels in wheat agronomic characteristics

Facing the use of high rates of nitrogen (N) fertilizer in wheat crop, the importance of the use of plant growth regulators is observed, and it has become important technology to improve the production effi­ciency of various species of plants. Thus, we aimed at evaluating the effect of N fertilization, with Sulfammo ® , and the use of growth reducer on the agronomic traits of wheat cultivars. The work was conducted in the experimental area of the Federal University of Santa Maria, Frederico Westphalen campus, in the period from 06 June to 20 October 2012. The soil in the area is classified as dystrophic Red Latosol and the experimental design was a randomized complete block in a 3x5x2 factorial, with three cultivars (TBIO Mestre, TBIO Iguacu e TBIO Itaipu), five rates of N (0, 40, 80, 120 and 160 kg ha -1 ) and application or not of growth reducer (trinexapac-ethyl) at a dose of 100 g ha -1 , with three replications. The growth reducer reduces the length of the second and third internode to the TBIO Itaipu and TBIO Iguacu, the total biomass of the plant, increases thousand grain weight and increases the number of spikelets per spike in TBIO Itaipu. Increasing the rates of N as Sulfammo® allows increasing the length of the first, second and third internodes, culm length, ear length and mass of the spike.

Influence of Bacillus subtilis on the physiological state of wheat and the microbial community of the soil under different rates of nitrogen fertilizers

The effects of inoculation with bacteria Bacillus subtilis strain No. 2 (hereinafter, B. subtilis 2) and of the physical properties of the soil on the physiological state of wheat (Triticum aestivum L.) plants and the soil microbial community under different rates of nitrogen fertilizers are studied. In the field, the physiological state of wheat was evaluated using the optical vegetation index. It was found that (1) the impact of B. subtilis 2 on plants decreases with an increase in the rate of fertilizers and soil bulk density, (2) the inoculation of wheat with bacteria enhances the resistance of the plant-microbial system to the adverse impact of high rates of nitrogen fertilizers due to the rearrangement of bacteria in rhizosphere ecological niches, and (3) the highest agronomic efficiency of nitrogen fertilizers is observed in wheat inoculation with B. subtilis 2 at the rate of nitrogen fertilization of 120 kg/ha.

Meta-analysis of the effect of urease and nitrification inhibitors on crop productivity and nitrogen use efficiency

Nitrification and urease inhibitors are proposed as means to reduce nitrogen losses, thereby increasing crop nitrogen use efficiency (NUE). However, their effect on crop yield is variable. A meta-analysis was conducted to evaluate their effectiveness at increasing NUE and crop productivity. Commonly used nitrification inhibitors (dicyandiamide (DCD) and 3,4-dimethylepyrazole phosphate (DMPP)) and the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) were selected for analysis as they are generally considered the best available options. Our results show that their use can be recommended in order to increase both crop yields and NUE (grand mean increase of 7.5% and 12.9%, respectively). However, their effectiveness was dependent on the environmental and management factors of the studies evaluated. Larger responses were found in coarse-textured soils, irrigated systems and/or crops receiving high nitrogen fertilizer rates. In alkaline soils (pH≥8), the urease inhibitor NBPT produced the largest effect size. Given that their use represents an additional cost for farmers, understanding the best management practices to maximize their effectiveness is paramount to allow effective comparison with other practices that increase crop productivity and NUE.

Relationship Between Leaf C/N Ratio and Insecticidal Protein Expression in Bt Cotton as Affected by High Temperature and N Rate

Expression of insecticidal protein for transgenic Bacillus thuringiensis (Bt) cotton is unstable and related to nitrogen metabolism. The objective of this study was to investigate the relationship between leaf carbon nitrogen ratio (C/N) and insecticidal efficacy of two Bt cotton cultivars. C/N ratio and Bt protein content were both measured at peak square period and peak boll period respectively under 5-7 d high temperature and different nitrogen fertilizer rates on the Yangzhou University Farm and the Ludong Cotton Farm, China. All plants were grown in field. The results showed that the C/N ratio enhanced slightly and the Bt protein content remained stable at peak square period, but significant increases for the C/N ratio and decreases markedly for the leaf Bt protein concentration were detected at the peak boll period. The similar patterns at the two growth periods were found for the leaf C/N ratio and Bt protein content by different N fertilizer treatments. When nitrogen rate was from 0 to 600 kg ha−1, the C/N ratio was reduced by 0.017 and 0.006 for Sikang 1 and Sikang 3 at peak square period, compared to the 1.350 to 1.143 reduction for Sikang 1 and Sikang 3 at peak boll period, respectively. Correspondingly, the leaf Bt protein contents were bolstered by 2.6-11.8 and 26.9-36.9% at the two different growth periods, respectively. The results suggested that enhanced C/N ratio by high temperature and nitrogen application may result in the reduction of insectiocidal efficacy in Bt cotton, especially in peak boll period.

A new species of Burkholderia isolated from sugarcane roots promotes plant growth

Sugarcane is a globally important food, biofuel and biomaterials crop. High nitrogen (N) fertilizer rates aimed at increasing yield often result in environmental damage because of excess and inefficient application. Inoculation with diazotrophic bacteria is an attractive option for reducing N fertilizer needs. However, the efficacy of bacterial inoculants is variable, and their effective formulation remains a knowledge frontier. Here, we take a new approach to investigating diazotrophic bacteria associated with roots using culture-independent microbial community profiling of a commercial sugarcane variety (Q208A) in a field setting. We first identified bacteria that were markedly enriched in the rhizosphere to guide isolation and then tested putative diazotrophs for the ability to colonize axenic sugarcane plantlets (Q208A) and promote growth in suboptimal N supply. One isolate readily colonized roots, fixed N2 and stimulated growth of plantlets, and was classified as a new species, Burkholderia australis sp. nov. Draft genome sequencing of the isolate confirmed the presence of nitrogen fixation. We propose that culture-independent identification and isolation of bacteria that are enriched in rhizosphere and roots, followed by systematic testing and confirming their growth-promoting capacity, is a necessary step towards designing effective microbial inoculants.

Effect of nitrogen fertilization on nitrate leaching in relation to grain yield response on loamy sand in Sweden

High rates of nitrogen (N) fertilizer may increase N leaching with drainage, especially when there is no further crop response. It is often discussed whether leaching is affected only at levels that no longer give an economic return, or whether reducing fertilization below the economic optimum could reduce leaching further. To study nitrate leaching with different fertilizer N rates (0–135kgNha−1) and grain yield responses, field experiments in spring oats were conducted in 2007, 2008 and 2009 on loamy sand in south-west Sweden. Nitrate leaching was determined from nitrate concentrations in soil water sampled with ceramic suction cups and measured discharge at a nearby measuring station. The results showed that nitrate leaching per kg grain produced had its minimum around the economic optimum, here defined as the fertilization level where each extra kg of fertilizer N resulted in a 10kg increase in grain yield (85% DM). There were no statistically significant differences in leaching between treatments fertilized below this level. However, N leaching was significantly elevated in some of the treatments with higher fertilization rates and the increase in nitrate leaching from increased N fertilization could be described with an exponential function. According to this function, the increase was <0.04kgkg−1 fertilizer N at and below the economic optimum. Above this fertilization level, the nitrate leaching response gradually increased as the yield response ceased and the increase amounted to 0.1 and 0.5kgkg−1 when the economic optimum was exceeded by 35 and 100kgNha−1, respectively. The economic optimum fertilization level depends on the price relationship between grain and fertilizer, which in Sweden can vary between 5:1 and 15:1. In other words, precision fertilization that provides no more or no less than a 10kg increase in grain yield per kg extra N fertilizer can be optimal for both crop profitability and the environment. To predict this level already at fertilization is a great challenge, and it could be argued that rates should be kept down further to ensure that they are not exceeded due to overestimation of the optimum rate. However, the development of precision agriculture with new tools for prediction may reduce this risk.

Nitrogen Loss from Paddy Field with Different Water and Nitrogen Managements in Taihu Lake Region of China

High rates of nitrogen (N) fertilizer were applied to a paddy field in the Taihu Lake region of China to maximize crop production. Excessive N input has resulted in serious agricultural nonpoint pollution. Water and N management are two important approaches to regulating N loss from paddy fields. This study aimed to determine N losses through ammonia volatilization, runoff, and leaching from a paddy field during the rice-growing season in Taihu Lake region. Field experiments with two water and two N managements were conducted. The N exported to the environment through ammonia volatilization, runoff, and leaching from the paddy field was 37.2 kg N ha−1 to 102 kg N ha−1, with ammonia volatilization accounting for 69.6% to 83.5% of N loss. Ammonium and dissolved organic N significantly contributed to N loss through runoff and leaching. Controlled irrigation and site-specific N management (CS) significantly decreased N losses through ammonia volatilization, runoff, and leaching. Compared with the N and irrigation water inputs in traditional water and N management, those generated by controlled irrigation and site-specific N management were reduced by 34.6% to 43.0% and 59.2% to 63.3%, respectively. Moreover, the reduction in N and water input in the CS paddy field enabled the maintenance of high rice yield; it significantly increased N use efficiency by 15.1% to 34.9% and decreased the N exported to the environment by ammonia volatilization, runoff, and leaching by 53.1% to 56.1%. Therefore, the joint application of controlled irrigation and site-specific N management efficiently reduces agricultural nonpoint pollution through N loss from paddy fields.

Changa in the structure and activity of a soil microbial community caused by inorganic nitrogen fertilization

The changes in the structure and activity of a soil microbial community caused by addition of moderate and high rates of the mineral nitrogen fertilizer (KNO3) were studied in a laboratory incubation experiment. The structure of the microbial community was evaluated from the phospholipid fatty acid (PLFA) profile; specific growth rate of the microorganisms was determined by the method of the kinetics of substrate-induced respiration; the total pool of microbial carbon was estimated by the fumigation-extraction method. The amounts of nitrogen fertilizer applied in three treatments of the experiment were 0 (control), 100, and 2000 μg N/g soil. Even in the absence of additional sources of organic carbon, a considerable portion of the added 15N (up to 74%) was immobilized. No significant increase in the amount of microbial carbon was observed during incubation. The specific growth rate of the microbial community in soil supplemented with glucose decreased twofold after addition of 2000 μg N/g soil. In this treatment, the ratio of cyclic fatty acids to their monoenoic precursors also increased, indicating the adaptation of microbial cells to extremely high amounts of nitrogen fertilizer. Moreover, considerable changes in the structure of the soil microbial community, such as an increase in the ratio of fungalto bacterial markers and a decrease in the ratio between PLFA of gram-positive and gram-negative bacteria, were observed in the treatment with addition of 2000 μg N/g soil. Our data clearly indicate that mineral nitrogen fertilization of soil under carbon limitation has a pronounced impact on the structure and activity of soil microbial communities.

Contribution of legumes to wheat productivity in Mediterranean environments of central Chile

The potential benefits of using a legumes have not been evaluated in the rainfed, cropping systems of central Chile where wheat (Triticum aestivum) is typically grown in rotation with oats (Avena sativa) using high rates of nitrogen (N) fertilizer. Two experiments were initiated to compare unfertilized legume-wheat cropping sequences to the current oat–wheat system without and with applications of N fertilizer (160 or 207kgNha−1). One experiment was located in the interior dryland of central Chile (average annual rainfall 650mm), and the other in the Andean foothills (average annual rainfall 1200mm). Treatments were established in different areas of the same two experimental sites in consecutive years (2008 and 2009) resulting in four separate trials evaluating legume effects on wheat yield (2009 or 2010). Estimates of N2 fixation determined using the 15N natural abundance technique indicated that around 21kg shoot N was fixed for every tonne (t) of shoot dry matter accumulated by either narrow-leaf lupin (Lupinus angustifolius), field peas (Pisum sativum), yellow lupin (Lupinus luteus), or white lupin (Lupinus albus) grown for grain, and by vetch (Vicia atroporpurea) grown in association with oats for green manure. When the amounts of N2 fixed were adjusted to account for N in the nodulated roots, narrow-leaf lupin was calculated to have returned the lowest average net inputs of fixed N to the system following grain harvest (26kgNha−1) and field pea the highest (192kgNha−1). By comparison vetch in the green manure treatments where all above-ground biomass were incorporated into the soil returned 67kgfixedNha−1. Depending upon location and year, growing grain legumes increased the subsequent N uptake by wheat by up to 60kgNha−1 (average 35kgNha−1) and vetch+oats green manure up to 103kgNha−1 (average 50kgNha−1). Wheat yields after grain legumes ranged from 2.4 to 3.0tha−1 in the interior dryland and 5.4–6.4tha−1 in the Andean foothills which were respectively 72–110% (90% average) and 69–83% (75% average) of the yields achieved by N fertilized wheat. Wheat yields after the vetch+oats green manure were 1.8 and 7.9tha−1 representing 60% and 102% of the yield of N fertilized wheat. In the absence of N fertilizer or legumes wheat yields after oats were around half that of the N fertilized wheat. It was concluded that grain legumes or legume-based green manures provide opportunities to greatly reduce the reliance of wheat production upon N fertilizer.

Bottom-up effects of Brassica napus nutrition on the oviposition preference and larval performance of Ceutorhynchus obstrictus (Coleoptera: Curculionidae)

The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), is a serious pest of canola, Brassica napus L. and Brassica rapa L., both in Europe and North America. Nitrogen and sulfur levels can be readily manipulated by growers and could potentially be exploited to improve current pest management strategies. In an effort to better understand the relationship between host plant nutrition and C. obstrictus oviposition preference and its larval developmental biology, we exposed gravid females to host plants grown under differing regimes of nitrogen and sulfur. Results indicated that plants grown with a higher supply of nitrogen were preferred as hosts. Plants with a higher sulfur supply were also preferred but only in plants grown at lower nitrogen levels. In contrast, larval development time increased with increasing nitrogen levels although larval dry weights were unaffected. The results were inconsistent with the preference-performance hypothesis. The inconsistencies between oviposition choice and larval growth and development may be due to a conflict between maternal and larval fitness. Observed changes in larval development time may not represent a significant fitness cost for larval C. obstrictus, as no corresponding change in larval weight was observed. Possible causal mechanisms for the increase in development time include differences in oil, protein and glucosinolate contents of the seed. Fertilizer management regimes currently recommended were considered to be optimal for management of C. obstrictus as the yield benefits from higher rates of nitrogen fertilization would more than compensate for increased level of infestation.

Tilletia barclayana: biologia e manejo em arroz irrigado

A cárie-do-arroz (Tilletia barclayana), doença considerada secundária na cultura do arroz irrigado, vem ganhando importância nos últimos anos devido aos níveis elevados de infestação e à redução do rendimento. A doença é favorecida por temperatura e umidade do ar elevadas, doses altas de nitrogênio e semeadura tardia. Para o manejo, recomenda-se uso de resistência varietal, semeadura na época recomendada e adubação equilibrada.

Nitrate leaching assessment in a long-term experiment under supplementary irrigation in humid Argentina

Applying high rates of nitrogen (N) fertilizer to crops has two major disadvantages: (1) the low N fertilizer use efficiency and (2) the loss of N by leaching, which may cause groundwater nitrate (NO 3 −) pollution, especially in humid areas. The objectives of this study were to adjust and validate the LEACH-W model simulations with data observed in the field; to quantify nitrate concentrations in the soil solution; to estimate N loss by leaching; and to determine the moments during the year when greatest nitrate transport events occur beyond the rooting profile. A randomized complete block design with four replications was established on a typic Argiudoll. Crop fertilization treatments consisted of three N rates (0, 100, and 200 kg N ha −1) using urea and ammonium nitrate solution (UAN) as the N source. Corn ( Zea mays L.) was planted and ceramic soil–water suction samplers were installed to depths of 1, 1.5 and 2 m. Drainage was estimated by the LEACH-W model, which adjusted very well the actual volume of water in the soil profile. Nitrogen losses were statistically analyzed as repeated measure data, using the PROC MIXED procedure. Losses of nitrate-nitrogen (NO 3 −-N) during the study increased as the rate of N applied increased. At all depths studied, statistically significant higher values were found for 200 N compared to 100 N and 0 N, and for 100 N compared to 0 N ( p < 0.001). The greatest NO 3 −-N losses through leaching occurred during crop growth. Significant differences ( p < 0.05) were found between cropping and fallow in the three treatments and depths studied for seasons 4 and 5; these two seasons produced the highest drainage volumes at all depths.

Identification of denitrifying rhizobacteria from bentgrass and bermudagrass golf greens.

As high rates of nitrogen fertilization are used in turfgrass management, there is a great potential for nitrogen loss. Research on identification of denitrifiers in turfgrass has been limited. Therefore, the aim was to identify denitrifier species and genes from turfgrass roots. Rhizobacteria were isolated from roots of bentgrass and bermudagrass in sand-based United States Golf Association (USGA) golf greens and used for denitrification biochemical analysis. Seventeen per cent (34 isolates) were identified as denitrifiers, 47% were classified as nitrate-reducers and 36% were nondenitrifiers. Identification of species of the denitrifiers was performed by chromatography fatty acid methyl ester (GC-FAME) and16S rDNA analyses. Bacillus and Pseudomonas were the major turfgrass denitrifiers. The two methods showed a 60% agreement at the genus level. Nitrite reductase genes nirK and nirS were detected in 74 and 15% of the denitrifiers, respectively, but not in nondenitrifiers. The nosZ gene encoding nitrous oxide reductase was detected in all the denitrifiers, but also in some nondenitrifiers. To our knowledge, this is the first report for identification of denitrifiers and denitrification-related genes associated with turfgrass roots. These results provide valuable data for future denitrification studies that seek to improve turfgrass nitrogen management for maximum efficiency.

Colorado potato beetle control by application of the entomopathogenic nematode Heterorhabditis marelata and potato plant alkaloid manipulation

AbstractControl of the Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae), with the entomopathogenic nematode Heterorhabditis marelata Liu and Berry (Nematoda: Heterorhabditidae) was examined in the laboratory and in potato fields in north central Oregon. This research tested the hypothesis that varying nitrogen fertilizer levels would affect foliar alkaloid levels, which would stress the host, and allow increased nematode reproduction and long‐term control of the CPB. Laboratory results indicated that nematodes tended to reproduce more readily in CPB fed on potato plants with high levels of fertilizer. Field trials tested CPB population responses to four treatments: application of nematodes vs. no nematodes, with application of low vs. high rates of nitrogen fertilizer. The higher nitrogen application rate increased field foliar levels of the alkaloids solanine by 35%, and chaconine by 41% over the season. Nematodes were applied twice during the season, causing a 50% reduction in adult CPB populations, and producing six times as many dead prepupae in nematode‐treated soil samples as in the untreated samples. However, no reproducing nematodes were found in the 303 dead prepupae and pupae collected from nematode‐treated plots. Nitrogen fertilizer levels, and their related alkaloid levels, did not affect nematode infection rates or reproduction in the field. Foliar alkaloid levels of plants from the growth chamber were 3–6‐fold as high as those in the field, which may explain the variation in nematode response to nitrogen applications to host plants of the CPB. Heterorhabditis marelata is effective for controlling CPB in the field, and does not have negative non‐target effects on one of the most common endemic CPB control agents, Myiopharus doryphorae (Riley) (Diptera: Tachinidae), but the low rate of nematode reproduction cannot be manipulated through alkaloid stress to the beetle. Until H. marelata can be mass‐produced in an inexpensive manner, it will not be a commercially viable control for CPB.

Midseason Pest Status of the Cotton Aphid (Homoptera: Aphididae) in California Cotton - Is Nitrogen a Key Factor?

Field experiments were conducted from 1996 to 1998 to assess the effects of nitrogen fertilization rates and planting dates on the population dynamics of the cotton aphid, Aphis gossypii Glover, during the midgrowing season of California cotton. Cotton aphids reached higher densities in high nitrogen fertilized plants (227 kg N/ha, rate currently used by cotton growers) than in low nitrogen fertilized plants (57 kg N/ha). In addition, late-planted cotton (May–June), which had more nitrogen content, also harbored higher aphid populations than early plantings (April). Overall, aphid abundance was positively correlated with plant nitrogen content. In a moderate aphid pressure year (1996), planting the cotton early (April) was effective in keeping the aphid population below the midseason economic threshold. However, in a high aphid pressure year (1997), it was necessary to drastically reduce the nitrogen fertilization to 57 kg N/ha to maintain the aphid density under this threshold. Recent cultural practices in California cotton include higher rates of nitrogen fertilization, which increases nitrogen content of plants. The current data suggest that this practice (i.e., high fertilization) is an important factor contributing to the increased severity of the cotton aphid as a pest of California cotton during the midseason.

Flower nitrogen status and populations of Frankliniella occidentalis feeding on Lycopersicon esculentum

AbstractIn a 2‐year study, we manipulated flower chemistry of field grown Lycopersicon esculentum in North Florida and examined subsequent effects on populations of Frankliniella occidentalis. We examined variation in amino‐acid profiles and total nitrogen concentrations of flowers, flower number and plant size seasonally and in response to variable rates of nitrogen fertilization (202 kg ha−1 and 309 kg ha−1). Populations of F. occidentalis sampled every 6 to 10 days were then related to seasonal and treatment effects on plant variables. For both years, F. occidentalis was abundant on L. esculentum tomatoes for only short intervals (2–3 weeks), and populations were significantly higher on hosts receiving higher rates of nitrogen fertilization. Seasonal trends in F. occidentalis on L. esculentum were correlated to number of flowers per host plant, as well as concentrations of total nitrogen in flowers. Moreover, treatment (fertilization) effects were consistent with changes in the nitrogen status of flowers. Plants subjected to higher fertilization rates produced flowers that had higher nitrogen content as well as variations in amino‐acid profiles during the period of peak thrips populations. Abundance of F. occidentalis (particularly adult females) were most highly correlated to flower concentrations of phenylalanine during population peaks. Our results are consistent with beneficial effects of high dietary nitrogen to thrips populations, and also support the hypothesis that aromatic amino acids (e.g., phenylalanine) play a central role in the nutritional ecology of F. occidentalis.

Whole-season grass response to and recovery of nitrogen applied at various rates and distributions in a high rainfall environment

High rates of nitrogen (N) fertilizer are often used on perennial grass in the coastal region of British Columbia and the Pacific Northwest, but there is little information on optimum rates for abundant high-quality yields and on their environmental implications. A field trial was conducted in each of 3 yr to determine the effect of rates and distributions of N on whole-season herbage yield and quality, and pre- and post-season extractable inorganic N in the soil. Yearly rates were 100, 200 and 400 kg N ha−1 applied to each of four cuts in the following distributions: 1.00/0/0/0, 0.50/0.25/0.25/0 and 0.25/0.25/0.25/0.25. Whole-season yield was increased by increasing rates of N in all three trials, but the increase varied from 17% (Trial 1) to 127% (Trial 3). Distributing the N uniformly through the season resulted in only a 5% increase in yield compared with applying all of the N at the beginning of the season. Rate of N had a substantial effect on average herbage crude protein and nitrate concentrations, but the distribution effect was greater on herbage N constituents than on yield. Increasing rates of N consistently increased average herbage nitrate concentrations, and crude protein in two of three trials. Applying all of the N at the beginning of the season increased average herbage crude protein and nitrate concentrations more than distributing it evenly through the season. Extractable inorganic N in the soil at the end of the season increased only at 400 kg N ha−1 rate and was not affected by distribution. Although distribution pattern influenced herbage yield and nitrogen concentrations, distribution did not influence total herbage N uptake or recovery in herbage plus soil. Rates and distributions of N on grass influenced herbage yield and quality, and soil extractable inorganic N in different ways; therefore, compromises in N management are required to optimize forage yield and quality, and soil nitrate concentrations. Autumn soil inorganic N testing may be useful as feedback information for fertilizer recommendations in the subsequent season. Spring soil inorganic N testing was a poor predictor of crop response to fertilizer in the high rainfall environment of the study. Key words: Nitrogen, plant crude protein, plant nitrate, forage yield, forage quality, apparent N recovery, residual soil N, soil N test