Affect Nitrogen Dynamics Research Articles

Carbohydrate addition strategy affects nitrogen dynamics, budget and utilization, and its microbial mechanisms in biofloc-based Penaeus vannamei culture

Biofloc technology (BFT) is a microbial biotechnology that has shown great potential in water quality control and nutrient re-utilization towards sustainable aquaculture. Carbohydrate addition is the main operation means for the application of BFT; however, we are still unclear how it affects microbial-driven nitrogen (N) transformation and utilization in BFT systems. In this study, autotrophic- and heterotrophic-dominated BFT systems (AS vs. HS) in triplicates were operated by two molasses addition strategies for a 13-week production trial of P. vannamei. Throughout the trial, more amount of bioflocs (in terms of VSS) were produced in HS than in AS. Higher concentrations of NH4+-N, NO2−-N, NO3−-N, TIN and TN while lower concentrations of TON were monitored in AS compared to HS. N budget analysis showed that the harvested shrimp deposited >34.4% of input N; and higher N retention in the final culture water while lower N loss and N discharge from the systems were found in AS compared to HS. Higher efficiencies of gross and feed N utilization were achieved in HS (36.6% and 38.8%) than in AS (34.5% and 34.9%). Metagenomic analysis revealed that abundant and diverse N-cycling microbial communities and functional genes in bioflocs constituted a complete and complex N-cycling network, which was the driving force of N transformation and recycling in the culture water of systems. The lower abundances of nitrifying bacteria and associated genes led to the inhibition of nitrification under high molasses addition in HS, which accounted for the differences in N dynamics, budget and utilization efficiency between AS and HS. Together, a quantitative model of N migration and transformation driven by N-cycling microbial communities of bioflocs was depicted in closed BFT systems for shrimp aquaculture. This study firstly revealed the N-cycling microbiome and functional effects in BFT systems, which provides theoretical and technical foundations for efficient N management in intensive aquaculture.

Plant functional type affects nitrogen dynamics in urban park soils similarly to boreal forest soils

PurposeAlthough plant functional type can modulate soils and their processes in natural, nitrogen (N)-limited ecosystems, little is known about their ability to influence soil N dynamics in urban ecosystems that have high excess N input. We investigated whether i) plant functional type effects on soil N dynamics in urban parks follow the same pattern as those in undisturbed natural/semi-natural forests, and ii) park age influences plant functional type effects on soil N dynamics under boreal climate.MethodsWe selected 13 urban parks of varying ages (young: 10 to 15, old: > 70 years), and 5 undisturbed natural/semi-natural forests (> 80 years) in southern Finland. In these parks and forests, we measured soil total N concentration, availability of inorganic N, nitrous oxide (N2O) flux and earthworm biomass under three plant functional types (evergreen tree, deciduous tree, lawn).ResultsOur results showed that plant functional type influenced N dynamics also in urban greenspace soils, which may relate to the clear effect of plant functional type on earthworm biomass. Evergreen trees tended to have the highest ability to foster N accumulation and reduce N2O emissions in urban parks. Moreover, with increasing park age, N accumulation increased under trees but decreased under lawns, further emphasising the role of vegetation in affecting soil N dynamics in urban greenspaces.ConclusionsOur results show that, similar to natural/semi-natural forests, plant functional type, irrespective of park age, can influence soil N dynamics in urban parks.

Influence of cyanobacterial inoculants, elevated carbon dioxide, and temperature on plant and soil nitrogen in soybean.

Climate change affects nitrogen dynamics in crops and diazotrophic microorganisms with carbon dioxide (CO2 ) sequestering potential such as cyanobacteria can be promising options. The interactions of three cyanobacterial formulations (Anabaena laxa, Calothrix elenkinii and Anabaena torulosa-Bradyrhizobium japonicum biofilm) on plant and soil nitrogen in soybean, were investigated under elevated CO2 and temperature conditions. Soybean plants were grown inside Open Top Chambers under ambient and elevated (550 ± 25 ppm) CO2 concentrations and elevated temperature (+2.5-2.8°C). Interactive effect of elevated CO2 and cyanobacterial inoculation through A. laxa and Anabaena torulosa-B. japonicum biofilm led to improved growth, yield, nodulation, nitrogen fixation, and seed N in soybean crop. Nitrogenase activity in nodules increased in A. laxa and biofilm treatments, with an increase of 55% and 72%, respectively, over no cyanobacterial inoculation treatment. Although high temperature alone reduced soil microbial biomass carbon, dehydrogenase activity, and soil available N, the combined effect of CO2 and temperature were stimulatory; cyanobacterial inoculation further led to an increase under all the conditions. The highest seed N uptake (758 mg plant-1 ) was recorded with cyanobacterial biofilm inoculation under elevated CO2 with control temperature conditions. The positive interactions of elevated CO2 and cyanobacterial inoculation, particularly through A. laxa and A. torulosa-B. japonicum biofilm inoculation highlights their potential in counteracting the negative impact of changing climate along with enhancing plant and soil N in soybean.

Long-term nutrient management affects nitrogen dynamics and crop yield in rice (Oryza sativa)-wheat (Triticum aestivum) system

The present study was conducted to assess the in-season dynamics of nitrogen (N) in an Inceptisol under long-term fertilizer management in rice (Oryza sativa L.)-wheat (Triticum aestivum L.) cropping system at Punjab Agricultural University, Ludhiana. The treatments comprised combination of chemical fertilizers (100% NPK and 150% NPK) and organic manures (100% NPK+FYM, 100% NPK+GM and 100% NPK+SI) along with unfertilized control laid out in randomized complete block design with three replications. Soil samples were collected at 30, 60, 90 and 120 days after transplanting/sowing and at harvest of rice and wheat, respectively and analysed for available N, ammonical N, nitrate N and total N. Plant samples were also collected at corresponding time period to determine the N content, uptake and biomass. Application of 100% NPK+FYM had significantly higher contents of different N forms during growth of both rice and wheat compared to rest of the treatments. At harvest of rice, combined use of 100% NPK and FYM improved soil available N and total N over 100% NPK by 20 and 22%; respectively. Whereas at wheat harvest; the increase in available and total N amounted to 29 and 23%, respectively due to corresponding treatments. The gradual decrease in N forms was observed till crop harvest. The grain and straw yield, content and total uptake of N by rice and wheat increased significantly under the integrated fertilizer management. Integrated nutrient management practice improved the relative proportion of plant-available N forms and achieved higher rice-wheat productivity.

How Foliar Fungal Diseases Affect Nitrogen Dynamics, Milling, and End-Use Quality of Wheat.

Foliar fungal diseases may cause important losses on yield and quality of wheat (Triticum aestivum L.). They may impact crop growth rate differently, modifying nitrogen (N) dynamics and carbohydrate accumulation in the grain. The relationship between N and carbohydrates accumulation determines the grain protein concentration, which impacts the gluten concentration and rheological properties of the wheat flour. In addition, types of fungicides and N fertilization can influence the intensity of foliar diseases and have an effect on the milling and end-use quality, depending on the bread-making aptitude of the genotypes, the nutritional habit of the pathogen involved, the amount and time of infection, environmental factors, and interactions between these factors. In that way, N fertilization may modify the severity of the diseases according to the nutritional habit of the pathogen involved. Some fungicides, such as strobilurins and carboxamides, produce high levels of disease control and prolong the healthy leaf area duration, which translates into important yield responses, potentially compromising the grain protein concentration by additional carbohydrate production, with consequences in the bread-making quality. Furthermore, infections caused by biotrophic pathogens can be more damaging to N deposition than to dry matter accumulation, whereas the reverse has been generally true for diseases caused by necrotrophic pathogens. The time of infection could also affect yield components and N dynamics differentially. Early epidemics may reduce the number of grains per area and the N remobilization, whereas late epidemics may affect the thousand kernel weight and mainly the N absorption post-flowering. A review updating findings of the effects of infections caused by foliar fungal pathogens of different nutritional habits and the incidence of several factors modifying these effects on the above-ground biomass generation, N dynamics, protein and gluten concentration, milling, rheological properties, loaf volume, and other quality-related traits is summarized. Three main pathogens in particular, for which recent information is available, were taken as representative of biotrophic (Puccinia triticina), necrotrophic (Pyrenophora tritici-repentis), and hemibiotrophic (Zymoseptoria tritici) nutritional habit, and some general models of their effects are proposed. New challenges for researchers to minimize the impact of foliar diseases on end-use quality are also discussed.

Characteristics of denitrification genes and relevant enzyme activities in heavy-metal polluted soils remediated by biochar and compost

Denitrification is an important process affecting nitrogen dynamics in soils. In this study, abundances of denitrification genes (narG, nirK, nirS, and nosZ) and activities of nitrite reductase (S-NiR), nitrate reductase (S-NR) were measured in heavy-metal polluted soils with different amendments of biochar and compost. The relationships between physical-chemical parameters, denitrification gene abundance, and enzyme activity were analyzed by Pearson correlation method. Results showed that compost addition significantly increased the abundances of functional genes (nirS, nosZ, narG), and the abundances of nirK and nirS might be sensitive to compost and biochar addition. Compost addition and its combination with biochar significantly decreased the S-NiR enzyme activity and stimulated the S-NR enzyme activity. Negative relationships were obtained between S-NiR activity and electric conductivity (EC), water soluble carbon (WSC), nitrate, ammonium, nirK, narG gene abundances. While S-NR activity significantly positively correlated with soil EC, WSC and nirK gene abundance. Biochar and compost amendments can alter soil nitrogen cycling by changing denitrifying functional gene and relevant enzyme activities in soils polluted by heavy metals.

Examining nitrogen dynamics in the unsaturated zone under an inactive cesspit using chemical tracers and environmental isotopes

This study evaluates the dynamics of nitrogen compounds generated by infiltration of wastewater from an inactive cesspit in the unconfined and sedimentary Adamantina Aquifer in Urânia, Brazil. A monitoring station, consisting of an 11.2 m well (1.8 m in diameter) with an array of 12 tensiometers and 12 suction lysimeters, was installed to monitor the shallow unsaturated zone from 0.5 to 9 m depth. A monitoring well was also installed below the water level to monitor the shallow aquifer. High amounts of ammonium (up to 96 mg/L NH4+-N) and nitrate (up to 458 mg/L NO3−-N) were observed in the unsaturated zone porewater which is comparable to active septic systems effluents. The distribution of NO3−, Cl− and Na+, typical constituents of sewage effluents, varied seasonally and spatially, which is correlated with changes in infiltration rates between the wet and dry seasons and with hydraulic conductivity variations in interlayered sandy and clayey sediments. A detailed monitoring of porewater geochemistry demonstrated the occurrence of several important reactions affecting nitrogen dynamics in the unsaturated zone: i) oxidation of organic matter, ii) ammonification, iii) nitrification, iv) methanogenesis, v) denitrification and likely, vi) sulfate reduction. The changes in nitrogen compound distribution and δ15NNO3 and δ18ONO3 values in porewater, in association with the N2O concentration and δ15NN2O and δ18ON2O signatures in gas samples, indicate the occurrence of nitrification and denitrification, suggesting the coexistence of reducing and oxidizing microsites in the unsaturated zone. This study indicated that cesspits can generate a significant amount of nitrate even a few years after being inactivated which can represent a potential long-term source of nitrate to groundwater in highly populated areas.

Long-term grazing negatively affects nitrogen dynamics in Northern Patagonian wet meadows

Wet meadows are seasonally saturated wetlands that can develop in arid and semi-arid areas. In the semi-arid region of North Patagonia, Argentina wet meadows are islands of high net primary productivity surrounded by low productivity steppe vegetation. Because of this, Patagonian wet meadows have been heavily grazed by livestock for more than a century. It has been shown that overgrazed wet meadows have lower soil carbon, however, little is known about how overgrazing impacts N dynamics. To this end, we measured ammonium (NH4+), nitrate (NO3−), and total nitrogen (Nt) in paired wet meadows that varied in long-term grazing intensity (heavy vs. light). Results indicate that heavily grazed sites had on average 43% less Nt and 57% less NH4+ than lightly grazed sites. Soil NH4+ concentrations were always significantly greater than NO3− concentrations, being the later often below detection levels. In addition, NH4+ concentrations were correlated with soil moisture in lightly grazed sites, but not in heavily grazed sites. These findings indicate that implementation of better management practices are currently needed to promote more sustainable grazing in semi-arid wet meadows of North Patagonia.

Evidence from the real world: 15N natural abundances reveal enhanced nitrogen use at high plant diversity in Central European grasslands

Summary Complementarity that leads to more efficient resource use is presumed to be a key mechanism explaining positive biodiversity–productivity relationships but has been described solely for experimental set‐ups with controlled environmental settings or for very short gradients of abiotic conditions, land‐use intensity and biodiversity. Therefore, we analysed plant diversity effects on nitrogen dynamics across a broad range of Central European grasslands. The 15N natural abundance in soil and plant biomass reflects the net effect of processes affecting ecosystem N dynamics. This includes the mechanism of complementary resource utilization that causes a decrease in the 15N isotopic signal. We measured plant species richness, natural abundance of 15N in soil and plants, above‐ground biomass of the community and three single species (an herb, grass and legume) and a variety of additional environmental variables in 150 grassland plots in three regions of Germany. To explore the drivers of the nitrogen dynamics, we performed several analyses of covariance treating the 15N isotopic signals as a function of plant diversity and a large set of covariates. Increasing plant diversity was consistently linked to decreased δ15N isotopic signals in soil, above‐ground community biomass and the three single species. Even after accounting for multiple covariates, plant diversity remained the strongest predictor of δ15N isotopic signals suggesting that higher plant diversity leads to a more closed nitrogen cycle due to more efficient nitrogen use. Factors linked to increased δ15N values included the amount of nitrogen taken up, soil moisture and land‐use intensity (particularly fertilization), all indicators of the openness of the nitrogen cycle due to enhanced N‐turnover and subsequent losses. Study region was significantly related to the δ15N isotopic signals indicating that regional peculiarities such as former intensive land use could strongly affect nitrogen dynamics. Synthesis. Our results provide strong evidence that the mechanism of complementary resource utilization operates in real‐world grasslands where multiple external factors affect nitrogen dynamics. Although single species may differ in effect size, actively increasing total plant diversity in grasslands could be an option to more effectively use nitrogen resources and to reduce the negative environmental impacts of nitrogen losses.

Relating Freshwater Flow with Estuarine Water Quality in the Southern Everglades Mangrove Ecotone

Florida Bay is more saline than it was historically, and reduced freshwater flows may lead to more phosphorus inputs to the mangrove ecotone from the marine end-member. This is important given plans to restore freshwater flow into eastern Florida Bay. We investigated the relationships between salinity, nutrients, and hydrologic variables in the mangrove ecotone of Taylor Slough. We expected that total phosphorus (TP) would increase with salinity, reflecting a downstream marine source, while total nitrogen (TN) would increase with flow in the mangrove ecotone. Despite expectations of increased flows improving the ecological health of lower Taylor Slough and Florida Bay, total nitrogen (TN) and total phosphorus (TP) dynamics may shift in response to new conditions of flow and salinity as well as organic carbon, N, and P availability. Our results showed that TP concentrations are more discharge-driven while TN is more variable and potentially derived from different sources along the flow path from the freshwater Everglades marshes to Florida Bay. Increased flow of freshwater through Taylor Slough will likely decrease TP concentrations in this historically oligotrophic and P-limited ecosystem. However, more studies along the mangrove ecotone is needed to understand how increased flows will affect nitrogen dynamics relative to phosphorus.

Grass-clover undersowing affects nitrogen dynamics in a grain legume–cereal arable cropping system

A field experiment was carried out in an arable organic cropping system and included a sequence with sole cropped fababean (Vicia faba L.), lupin (Lupinus angustifolius L.), pea (Pisum sativum L.), oat (Avena sativa L.) and pea–oat intercropping with or without an undersown perennial ryegrass (Lolium perenne L.) – white clover (Trifolium repens L.) catch crop followed by a first crop of spring wheat (Triticum aestivum L.) and second crop of winter triticale (Triticale hexaploide L.). The rotation sequence was repeated twice. Natural 15N abundance techniques were used to determine grain legume N2 fixation and 15N labeling technique to determine the fate of pea and oat residue N recovery in the subsequent crop. The subsequent spring wheat and winter triticale crop yields were not significantly affected by the previous main crop, but a significant effect of catch crop undersowing was observed. A higher soil mineral N content in the soil profile without undersown grass-clover increased the spring wheat yield. This effect was circumvented in the subsequent winter triticale, where yields in the treatments with catch crops undersown were significantly greater. The grass-clover catch crop after grain legumes had a higher grass proportion before incorporation as compared to grass-clover after oat, which had the greatest clover proportion. The dynamic response of interspecific interactions in the catch crop to the soil mineral N levels is moderating the preceding effect of main crops in the subsequent cereal – and sometimes to a higher degree than the main crop effect. For research involving rotation principles it is recommended to evaluate cumulative effects over several years and not only single seasons.

Rice field drainage affects nitrogen dynamics and management

Many California rice growers are now using foliar-active herbicides that require fields to be drained before application. Current regulations limit aerial herbicides and they must be applied by ground, requiring a soil surface dry enough to support application equipment. Our research showed that draining rice fields for a prolonged period early in the season led to a buildup of nitrate in the soil. About 60% of this nitrogen was lost when the field was reflooded, reducing nitrogen-use efficiency and uptake, and lowering grain yields. Nitrate-nitrogen accumulated at a rate of about 1.8 pounds per acre daily, and accumulation began about 4 days after the field was drained. During a typical drain of 10 to 14 days, about 20 pounds of nitrate-nitrogen per acre can be lost. Field experiments showed that incorporating fertilizer nitrogen into the subsurface soil increases nitrogen-use efficiency. Based on this research, we recommend that growers incorporate as much of their preplant nitrogen as possible below the soil surface and limit the drain period as much as possible.

低タンパク質飼料を給与した肥育豚のふん尿の堆肥化過程における窒素動態とアンモニアの発生状況

低タンパク質飼料を給与した肥育豚のふん尿の堆肥化過程における窒素動態とアンモニアの発生状況

RETRACTED: Influence of aeration rate on nitrogen during blackwater composting

The paper aimed to study the influence of the aeration rate on nitrogen dynamics during the composting of Blackwater sludge with sawdust. The increased aeration was responsible for an increase in ammonia emissions and for the decrease in nitrogen losses through leaching. At high aeration rates, losses of nitrogen in leachates and as ammonia in exhaust gas accounted for 90.99% of the nitrogen removed from the material. At low aeration rates, those accounted for 47.85% of the nitrogen removed from the material. The nitrification rate was found to be proportional to the mean content of NH 4 + /NH 3 in the material, i.e., initial NH 4 + /NH 3 plus NH 4 + /NH 3 released by ammonification minus the losses in leachates and in exhaust gas. The aeration rate was shown to be a main parameter affecting nitrogen dynamics during composting, since it controlled the ammonification, the ammonia emission and the nitrification processes.

Influence of aeration rate on nitrogen dynamics during composting

The paper aimed to study the influence of aeration rate on nitrogen dynamics during composting of wastewater sludge with wood chips. Wastewater sludge was sampled at a pig slaughterhouse 24h before each composting experiment, and mixtures were made at the same mass ratio. Six composting experiments were performed in a lab reactor (300 L) under forced aeration. Aeration flow was constant throughout the experiment and aeration rates applied ranged between 1.69 and 16.63 L/h/kg DM of mixture. Material temperature and oxygen consumption were monitored continuously. Nitrogen losses in leachates as organic and total ammoniacal nitrogen, nitrite and nitrate, and losses in exhaust gases as ammonia were measured daily. Concentrations of total carbon and nitrogen i.e., organic nitrogen, total ammoniacal nitrogen, and nitrite and nitrate were measured in the initial substrates and in the composted materials. The results showed that organic nitrogen, which was released as NH4+/NH3 by ammonification, was closely correlated to the ratio of carbon removed from the material to TC/N(org) of the initial substrates. The increase of aeration was responsible for the increase in ammonia emissions and for the decrease in nitrogen losses through leaching. At high aeration rates, losses of nitrogen in leachates and as ammonia in exhaust gases accounted for 90-99% of the nitrogen removed from the material. At low aeration rates, those accounted for 47-85% of the nitrogen removed from the material. The highest concentrations of total ammoniacal nitrogen in composts occurred at the lowest aeration rate. Due to the correlation of ammonification with biodegradation and to the measurements of losses in leachates and in exhaust gases, the pool NH4+/NH3 in the composting material was calculated as a function of time. The nitrification rate was found to be proportional to the mean content of NH4+/NH3 in the material, i.e., initial NH4+/NH3 plus NH4+/NH3 released by ammonification minus losses in leachates and in exhaust gases. The aeration rate was shown to be a main parameter affecting nitrogen dynamics during composting since it controlled the ammonification, the ammonia emission and the nitrification processes.

Large woody debris influences vegetation zonation in an oligohaline tidal marsh

The amount of large woody debris (LWD) in Pacific Northwest estuaries has declined dramatically since Euro-American settlement in the mid 19th century. Little is known about the ecological significance of estuarine LWD. This ignorance impairs protection and restoration of habitat critical to threatened Chinook salmon (Oncorhynchus tshawytscha), as well as other fish and wildlife. This study investigates whether LWD affects the distribution of estuarine shrubs, particularly nitrogen-fixingMyrica gale L. (sweetgale), which dominates the tidal shrub community of the Skagit River estuary, Washington, U.S.A. LWD,M. gale, and other shrubs were surveyed along line transects in an oligohaline tidal marsh and in abandoned agricultural land whose dikes failed more than 50 years ago and which has reverted to marsh. The results demonstrate a strong association between LWD andM. gale. M. gale was very rare on LWD<30 cm in diameter, increasingly more common for LWD between 30 and 75 cm, and always present on LWD≥75 cm. The marsh surface was generally 45 cm below mean higher high water (MHHW), suggesting LWD benefitsM. gale by providing a growth platform at an elevation near MHHW and reducing flooding stress. The largest and most abundant tree in the marsh,Picea sitchensis, averaged only 35.8 cm in diameter, which suggests LWD recruitment from upstream sources is necessary to sustainM. gale populations in the geomorphologically dynamic Skagit marsh. By affecting the distribution and abundance ofM. gale in the estuary, LWD may indirectly affect nitrogen dynamics in the marsh and secondary production of detritivores and herbivores.

Catch crops affect nitrogen dynamics in organic farming systems without livestock husbandry—Simulations with the DAISY model

This paper presents simulations of the soil–plant–atmosphere model DAISY based on an organic crop rotation with incorporation of different catch crops following pea as a leguminous cash crop. Special emphasis was placed on the simulation of nitrogen (N)-mineralisation/immobilisation and of soil microbial biomass N. The DAISY model was able to simulate soil mineral N and soil microbial biomass N after soil incorporation of catch crop plant residues to some extent only. Several processes need further attention and should be integrated into the DAISY model: (1) soil tillage induced mobilisation of organic material including considerable amounts of organic N; (2) winter killing of sensitive plant species and varieties; (3) decomposition of plant residues at the soil surface; (4) decomposition of easily decomposable plant residues at low temperatures, both with respect to the temperature modifier function and to the linkage of C and N turnover; (5) furthermore, reliable criteria for the subdivision of green plant residues into an easily decomposable pool and a more recalcitrant pool have to be developed.

How plant diversity and legumes affect nitrogen dynamics in experimental grassland communities.

Positive relationships between species richness and ecosystem processes such as productivity or nitrogen cycling can be the result of a number of mechanisms. We examined how species richness, biomass, and legume presence, diversity, and abundance explained nitrogen dynamics in experimental grassland plots in northern Sweden. Nitrogen concentrations and δ15N values were measured in plants grown in 28 mixtures (58 plots) including 1, 2, 4, 8 or 12 local grassland species over four years. Values for δ15N declined over time for all three functional groups (grasses, legumes, and non-leguminous forbs), suggesting greater reliance on N fixed by legumes over time by all species. Above ground percent nitrogen (%N) also declined over time but root %N and total N did not. Path analysis of above ground data suggested that two main factors affected %N and the size of the N pool. First, higher plant diversity (species richness) increased total N through increased biomass in the plot. Although in the first two years of the experiment this was the result of a greater probability of inclusion of at least one legume, in the last two years diversity had a significant effect on biomass beyond this effect. Second, percent legumes planted in the plots had a strong effect on above ground %N and δ15N, but a much smaller effect on above ground biomass. In contrast, greater plant diversity affected N in roots both by increasing biomass and by decreasing %N (after controlling for effects mediated by root biomass and legume biomass). Increased legume biomass resulted in higher %N and lower δ15N for both non-legume forbs and grasses in the first year, but only for grasses in the third year. We conclude that a sampling effect (greater probability of including a legume) contributed towards greater biomass and total N in high-diversity communities early on in the experiment, but that over time this effect weakened and other positive effects of diversity became more important.

Effect of pruning on nitrogen dynamics within crowns of Pinus radiata.

Stem injection of (15)N-labeled ammonium sulfate was used to determine effects of pruning on canopy nitrogen dynamics in open-grown Pinus radiata D. Don in New Zealand. Trees were planted in July 1990 and the isotope introduced in December 1994. Tree crowns were divided into three zones: base section, from which branches of pruned trees were removed; mid section, between the pruned zone and the height of the trees at the start of the year in which they were pruned; and top section, which grew predominantly after the isotope was applied. Pruning removed 32% of the green crown length, representing 75% of foliage biomass. Needles were sampled from each region of the crown until July 1996. Branch growth was used to predict foliage biomass for each sampling occasion. Approximately 45% of the applied isotope was recovered from needles sampled in December 1994 (1 week after application and immediately before pruning), two-thirds of which occurred in needles in the base section. Thereafter, changes in isotope content of needles in the base section of unpruned trees largely reflected foliage biomass fluctuations and dilution of the isotope by continued uptake from the unlabeled soil nitrogen pool. Recovery of isotope in needles from the mid-crown section increased by 58 and 86% from December 1994 to July 1995 in control and pruned trees, respectively. Within this crown section, there was evidence of isotope translocation from old to new needles, with both isotope dilution and efflux observed in the needle cohorts that had been present at the time the isotope was applied. Therefore, isotope dynamics did not reflect the dynamics of the total nitrogen pool in the mid-crown section. By July 1996, a small proportion of the applied isotope was recovered from the new foliage formed in the top section of the crown. Within the top section, isotope dynamics closely matched total nitrogen fluxes. Pruning the lower crown did not affect nitrogen dynamics elsewhere in the crown for the following 18 months.

Effects of the zebra mussel on nitrogen dynamics and the microbial community at the sediment-water interface

A flow-through expenment was conducted on intact cores of sedunents from Saginaw Bay, Lake Huron, to examine how trophic interactions between filter-feeding bivalve mussels and microbial populations could affect nitrogen dynamics at the sediment-water interface. The zebra mussels used in this experiment removed a large proportion of protozoa and phytoplankton from the overlying water, particularly heterotrophic nanoplankton (up to 82 %), while bacterial population~ showed less change. A 3-fold decrease in the protozoan to bacterial carbon ratio corresponded to a 2.5-fold increase in relative ammonium removal rates as estimated from the dark loss of '5N-ammonium. Excretion by the bivalves also increased net ammonium flux to the water, thus elevating the total calculated area1 ammonium removal rates to about 6-fold over rates observed in the control treatment. These data suggest that filter-feedng bivalves may significantly affect nitrogen transformation rates near the sediment-water interface by excreting ammonium and altering the microbial food web structure at the sediment-water interface.