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1.
For estimating turnover rates of soil microbial populations or for energy balance studies maintenance coefficients of microorganisms grown in vitro have generally been used. To study maintenance carbon requirements under in situ conditions the biomasses of two agricultural soils (Cambisol, Chernozem) and a beech forest soil (Rendzina) were activated metabolically and the CO2 production rate at 22°C recorded every hour. In soil samples treated with increasing amounts of glucose the concentration was determined that still yielded a maximal initial respiratory response but where the CO2 rate remained constant over 8–13 h. Decrease of CO2 production after that time was directly related to glucose exhaustion. The amount of glucose-C which kept the CO2-C rate at the previous level was regarded as the maintenance ration. Values for the maintenance coefficient m (mg glucose-C mg?1 biomass-C) were 0.012 h?1 for the two agricultural soils and 0.03 h?1 for the forest soil. The metabolic quotient qCO2 and maintenance values were identical for both agricultural soils and the qCO2 values were the same in the three soils used. No net growth was observed during the experimental period using bacterial plate counts and nalidixic acid treatment as test measurements.The determined m values reflect all individual maintenance requirements of species belonging to the total active biomass pool in these particular soils and correspond to known values from in vitro studies. The relationship between annual input of carbon and the maintenance requirements of actively-metabolizing biomasses are discussed.  相似文献   

2.
Photochemical formation rates and sources of the hydroxyl (OH) radical were determined in dew water formed on the surface of Japanese red pine (Pinus densiflora) needles of declining (NO2 polluted area) and healthy pine stands at Mt. Gokurakuji located west of Hiroshima city in western Japan. The measured OH radical photoformation rates in dew water (n=10), which were normalized to the rate at midday on May 1 at 34°N, ranged from 0.67 to 5.18 µM h?1 (1M=1mol L?1). The mean value (2.69 µM h?1) was higher than that in dew water collected on a Teflon board and higher than the mean value in rain water published previously. Of the total OH radical formation rate observed in dew water on the pine needles, 16.4 % was estimated to originate from N (III) (NO2 ? and HNO2) and 24.6 % was estimated to originate from NO3 ?. There were other sources of OH radical photochemical formation in dew water on the pine needles besides photolysis of NO2 ? and NO3 ?.  相似文献   

3.
The rate of loss in weight during 4 weeks for 9 earthworm species was linear with time. Milligrams live wt lost mg?1 initial wt day?1 (QWL) for single species did not conform to the surface principle of metabolism for most species examined, but conformed for the species collectively in a plot of × QWL for species vs × initial wt of species. The points for Lumbricus rubellus and Allolobophora longa at both 15° and 25°C departed significantly from the regression lines, suggesting that these species are less and more capable of retaining weight, respectively, than the other species tested. qwl for Eisenia foetida was significantly different at temperatures of 15, 18, 20, 25 and 30°C, but a significantly reduced slope between 20 and 25°C in a plot of QWL vs temperature corroborated previous workers' findings of a metabolic zone of thermal compensation in this interval. Milligrams live wt gained mg?1 initial wt day?1 (QWG) for single species, in contrast to QWL, conformed to the surface principle for all species. A plot of × QWG vs × initial wt for 7 species showed that Amynthas spp gained significantly more slowly and L. terrestris significantly more rapidly than predicted by the regression equation. A comparison of rates of WL to WG showed that small species generally recover weight more efficiently than large ones. A study of rate of WL on cellulose vs ashed loam showed that earthworms derive a nutritional benefit from cellulose.  相似文献   

4.
Gas exchange rates over soils were measured in a closed, flowing-gas system. 14CO was rapidly oxidized to 14CO2 with only a minor loss in atmospheric radioactivity. Incorporation of 14C into the soil was slight and was via 14CO2 rather than 14CO. CO oxidation was a microbial process and no oxidation occurred when soils had been autoclaved. The rate of CO depletion was concentration dependent and followed Michaelis-Menten kinetics. The rate constants Km and Vmax ranged from 18 to 51 μ 1?1 CO and from 0.58 to 4.35 mg C kg?1 dry soil h?1 respectively. The maximum rate of reaction for Hubbard Brook soil was about an order of magnitude greater than any soil previously reported. The oxidation reaction was accompanied initially by a reduction in net soil respiration. This was then followed by a period of high respiration which continued until CO levels were reduced to about 5μll?1. Thereafter respiration fell below the pretreatment rate and only returned to that rate 45 min after CO had been depleted from the atmosphere. The data suggest that at high CO concentrations (40–100 μll?1CO) autotrophic carboxydobacteria comprise the main component of the CO-oxidizing population and, as the concentration declines towards ambient levels they are replaced by heterotrophic microorganisms possessing a cometabolic process.  相似文献   

5.
In order to characterise the term microbial ?activity”? three different microbial populations belonging to a luvisol (I), a phaeozem (II) and a rendzina (III) were used for studying kinetic parameters such as substrate affinity, growth rate, yield and turnover time and the metabolic quotient of basal respiration. Glucose was used as a carbon source. Specific growth rate values (μ) varied between 0.0037 and 0.015 h?1 depending on soil type and glucose concentration and were far below the potential μmax. The calculated turnover time was 3–11 days, respectively. The yield coefficient was in the range between 0.37 and 0.53. The maximal uptake rate of glucose–C of soil population (II) was 0.041 g C g?1 biomass-C h?1. The determined affinity constant (Km) was 57 μg C g?1 soil. The affinity to glucose was higher for the glucose-mediated CO2 evolution with Km values of 15.2 and 17.5 than for the glucose uptake system itself. The observed qCO2 values of the basal respiration at temperature increments from 0 to 45° C were almost identical for the soils (I) and (II). The calulated Q10 lay in the range between 1.4 and 2.0.  相似文献   

6.
Continuous culture methods were used to isolate bacteria from sediment from Lake Ontario. These mixed cultures were grown in chemostats at different dilution rates and the glucose concentration in the culture vessel, the optical density, the biomass of cells, and the number and types of bacteria present were monitored for at least 80 generations. Two bacterial types, bothPseudomonas spp., were present at all dilution rates in significant quantities. The mixed cultures exhibited a reciprocal relationship between dilution rate and biomass (and number of bacteria). When Hg was added to the growth medium at a concentration of 5 mg 1?1, the bacteria tolerated that concentration at a dilution rate of 0.117 h?1 substantial changes in the population were noted at a concentration of 10 mg 1?1 Hg. One of the isolates from the mixed culture would not grow at 5 or 10 mg 1?1 of Hg in continuous culture at a dilution rate of 0.066 h?1. In the mixed continuous culture the same isolate showed only minimal response to a Hg concentration of 10 mg 1?1.  相似文献   

7.
Soil respiration throughout an annual cycle was measured at three different stands in a tropical grassland situated at Kurukshetra at 29°58' N lat. and 76°51' E long. Rates of CO2 evolution were measured by alkali absorption using 13 cm dia × 23 cm aluminium cylinders inserted 10 cm into the ground. Both movable and permanently-fixed cylinders were used. The CO2 evolution rates for the three stands were: Stand I (dominated by Sesbania bispinosa) 49–358 mg CO2 m?2 h?1; Stand II (mixed grasses) 55–378 mg CO2m?2 h?1; and Stand III (dominated by Desmostachya bipinnata) 55–448 mg CO2 m?2 h?1. A positive significant relation existed between rate of CO2 evolution and soil water content (r = 0.59?0.740), and between soil respiration and temperature (r = 0.58?0.69). A statistical model developed on the basis of the relationship between CO2 evolution rates and certain abiotic environmental factors showed 69% comparability between the calculated and observed values of soil respiration. The contribution of root and root-associated microorganisms to total soil respiration was estimated at 42% using the relationship between root biomass and CO2 output from movable cylinders.  相似文献   

8.
Azaarenes are one of several classes of organic compounds which contain mutagenic and carcinogenic substances that are found in synthetic fuels effluents. This study investigated the potential for a mutagenic azaarene, acridine, to accumulate in freshwater fish (Pimephales promelas) via four possible pathways: (1) direct uptake from water, (2) uptake via interaction with contaminated sediments, (3) uptake via ingestion of contaminated zooplankton (Daphnia pulex), and (4) uptake via ingestion of benthic invertebrates (Chironomus tentans) living in contaminated sediments. The results showed that acridine was rapidly accumulated from water by fathead minnows. Equilibrium concentration was attained within 24 h at a concentration factor ([acridine]fish, wet wt/[acridine]water) of 125±10. Depuration was rapid and appeared to occur in two stages, with a net elimination rate of 0.23 h?1 [acridine]fish at equilibrium. Equilibrium concentration factors of 51±5, 30±2, and 874±275 were observed forChironomus, Daphnia, and sediment, respectively. The calculated rates of uptake of acridine via ingestion of contaminated invertebrates (0.02 μg g?1 h?1) and ingestion of sediment (0.01 μg g?1 h?1) were negligible compared with direct uptake from water (1.40 μg g?1 h?1) in a hypothetical system with all compartments in equilibrium.  相似文献   

9.
Biological treatment systems such as biofilters offer a potential alternative to the existing physicochemical techniques for the removal of volatile organic compounds from gaseous emissions. In this experimental work, continuous phase biofiltration of xylene vapors were performed in a laboratory scale compost biofilter that was inoculated with a xylene-acclimatized consortium. The performance was assessed by continuously monitoring the removal efficiency (RE) and elimination capacity (EC) of the biofilter at loading rates varying between 2–220 g?m?3?h?1. The steady-state removal efficiencies were maintained between 60% and 90% up to a loading rate of 80 g?m?3?h?1. The removal efficiency decreased significantly at loading rates higher than 100 g?m?3?h?1. The pressure drop values were consistently less and insignificant in affecting the performance of the system. The present study also focuses in evaluating the stability of biofilter during shut down, restart, and shock-loading operations. An immediate restoration of biological activity after few days of starvation indicated their capability to handle discontinuous treatment situations which is more common to industrial biofilters. The sensitiveness of the biofilm to withstand shock loads was tested by abruptly increasing/decreasing the loading rates between 9–55 g?m?3?h?1, where, removal efficiencies between 60–90% were achieved. These results prove the resilience of the biomass and the stability of the compost biofilter. Anew, results from kinetic analysis reveal that, steady-state xylene removal in the biofilter can be adequately represented by Michaelis–Menten type kinetics, and the kinetic constants namely, ECmax (120.4 g?m?3?h?1) and K s (2.21 g?m?3) were obtained.  相似文献   

10.
Increasing nitrogen deposition due to human activity might have a serious impact on ecosystem functions such as the nitrogen transformations conducted by microbes. We therefore focused on nitrous oxide (N2O) production as an indicator of soil microbial activity. The rates of N2O emission from the forest floor were measured every two weeks in two forest stands in the central part of Japan: a red pine stand at Kannondai and a deciduous stand at Yasato. Nitrogen deposition rates by throughfall were 30.6 kg N ha?1 y?1 at Kannondai and 15.7 at Yasato. The rates of N2O emission ranged from 0.5 to 14.2 µg N m?2 h?1 (mean 4.5) at Kannondai and from 0.2 to 7.0 µg N m?2 h?1 (mean 2.3) at Yasato. The N2O emission rate showed significant positive relationships with soil temperature and nitrogen deposition during the preceding two weeks. The annual emission rates of N2O were 0.38 kg N ha?1 y?1 at Kannondai and 0.20 at Yasato. As a the annual nitrogen deposition, these rates were 1.23% at Kannondai and 1.27% at Yasato.  相似文献   

11.
Anaerobic ammonium oxidation (anammox process) widely occurs in paddy soil and may substantially contribute to permanent N removal; however, little is known about the factors controlling this process. Here, effects of temperature, pH, organic C, and substrates on potential rate of anammox and the relative contribution of anammox to total N2 production in a paddy soil were investigated via slurry incubation combined with 15N tracer technique. Anammox occurred over a temperature range from 5 to 35 °C with an optimum rate at 25 °C (1.7 nmol N g?1 h?1) and a pH range from 4.8 to 10.1 with an optimum rate at pH 7.3 (1.7 nmol N g?1 h?1). The presence of glucose and acetate (5–100 mg C L?1) significantly inhibited anammox activities and the ratio of anammox to total N2 production. The response of potential rates of anammox to ammonium concentrations fitted well with Michaelis-Menten relationship showing a maximum rate (Vmax) of 4.4 nmol N g?1 h?1 and an affinity constant (Km) of 6.3 mg NH4+-N L?1. Whereas, nitrate addition (5–15 mg 15NO3?-N L?1) significantly inhibited anammox activities and the ratio of anammox to total N2 production. Our results provide useful information on factors controlling anammox process and its contribution to N loss in the paddy soil.  相似文献   

12.
The effects of aluminium on plant nutrition in small birch plants (Betula pendula Roth) were investigated. By using relative addition rate (r A, g g?1 d?1) of nutrients as the growth-controlling variable, it was possible to grow the plants at very low external nutrient concentrations and to simulate plant requirements at two different fertility levels. Before aluminium addition the plants were at steady-state relative growth rate, (R G, g g?1 d?1). The two addition rates were free access of nutrients with R G ≈ 0.215 d?1, or nutrient-limited, R Aand R G=0.10 d?1. Internal concentrations of calcium and magnesium decreased with increasing Al3+ conncentration in the nutrient solution while nitrogen concentrations in the plants remained unchanged or increased. It was demonstrated in both nutrition treatments that calcium and magnesium decrease per se does not reduce plant growth and that uptake has to be considered in relation to plant requirement at different growth rates. The interpretation of the effects of aluminium on Ca and Mg uptake and plant biomass development suggested that processes other than disturbances in Ca and Mg uptake are the cause of the decrease in growth.  相似文献   

13.
To understand spatial and temporal variations of nitrous oxide (N2O) fluxes, we chose to measure N2O emissions from three plant stands (Kobresia tibetica, Carex muliensis, and Eleocharis valleculosa stands) in an open fen on the northeastern Qinghai?CTibetan plateau during the growing seasons from 2005 to 2007. The overall mean N2O emission rate was about 0.018?±?0.056?mg?N?m?2?h?1 during the growing seasons from 2005 to 2007, with highly spatiotemporal variations. The hummock (K. tibetica stand) emitted N2O at the highest rate about 0.025?±?0.051?mg?N?m?2?h?1, followed by the hollow stands: the E. valleculosa stand about 0.012?±?0.046?mg?N?m?2?h?1 and the C. muliensis stand about 0.017?±?0.068?mg?N?m?2?h?1. Within each stand, we also noted significant variations of N2O emission. We also observed the significant seasonal and inter-annual variation of N2O fluxes during the study period. The highest N2O emission rate was all recorded in July or August in each year from 2005 to 2007. Compared with the mean value of 2005, we found the drought of 2006 significantly increased N2O emissions by 104 times in the E. valleculosa stand, 45 times in K. tibetica stand, and 18 times in the C. muliensis stand. Though there was no significant relation between standing water depths and N2O emissions, we still considered it related to the spatiotemporal dynamics of soil water regime under climate change.  相似文献   

14.
Abstract

Methane emission rates from plots with and without fertilizer and rice straw application, and growth of two rice varieties (an improved variety, IR74 or IR64, and a local variety, Krueng Aceh) in two Indonesian paddy fields (Inceptisol and Alfisol soils of volcanic ash origin) were measured every week throughout the growth period in the first and the second cropping seasons, 1994. The CH4 emission rates from the fields were similar between the two varieties. The effect of chemical fertilizer on the increase of the emissions was observed only in the Tabanan paddy field for the plots treated with rice straw. Application of rice straw increased the CH4 emission rates. The mean rates of CH4 emission were 1.37-2.13 mg CH4?C m?2 h?1 for the plots without rice straw and 2.14–3.62 mg CH4?C m?2 h?1 for the plots with rice straw application in the Alfisol plots, and 2.32–3.32 mg CH4 -C m-2 h-1 for the plots without rice straw and 4.18–6.35 mg CH4?C m?2 h?1 for the plots with rice straw application in the Inceptisol plots, respectively. Total amounts of CH4 emitted during the growth period were 3.9–6.8 and 2.6–3.3 g CH4?C m?2 for the Alfisol plots and 6.9–10.7 and 4.2–5.8 g CH4?C m?2 for the Inceptisol plots with and without rice straw application, respectively. These findings suggested that CH4 emission from tropical paddy fields with soils of volcanic ash origin is low.  相似文献   

15.
Soil heterotrophic respiration and its temperature sensitivity are affected by various climatic and environmental factors.However,little is known about the combined effects of concurrent climatic and environmental changes,such as climatic warming,changing precipitation regimes,and increasing nitrogen(N)deposition.Therefore,in this study,we investigated the individual and combined effects of warming,wetting,and N addition on soil heterotrophic respiration and temperature sensitivity.We incubated soils collected from a temperate forest in South Korea for 60 d at two temperature levels(15 and 20℃,representing the annual mean temperature of the study site and 5℃warming,respectively),three moisture levels(10%,28%,and 50%water-filled pore space(WFPS),representing dry,moist,and wet conditions,respectively),and two N levels(without N and with N addition equivalent to 50 kg N ha-1year-1).On day 30,soils were distributed across five different temperatures(10,15,20,25,and 30℃)for 24 h to determine short-term changes in temperature sensitivity(Q10,change in respiration with 10℃increase in temperature)of soil heterotrophic respiration.After completing the incubation on day 60,we measured substrate-induced respiration(SIR)by adding six labile substrates to the three types of treatments.Wetting treatment(increase from 28%to 50%WFPS)reduced SIR by 40.8%(3.77 to 2.23μg CO2-C g-1h-1),but warming(increase from 15 to 20℃)and N addition increased SIR by 47.7%(3.77 to 5.57μg CO2-C g-1h-1)and 42.0%(3.77 to 5.35μg CO2-C g-1h-1),respectively.A combination of any two treatments did not affect SIR,but the combination of three treatments reduced SIR by 42.4%(3.70 to 2.20μg CO2-C g-1h-1).Wetting treatment increased Q10by 25.0%(2.4 to 3.0).However,warming and N addition reduced Q10by 37.5%(2.4 to 1.5)and 16.7%(2.4 to 2.0),respectively.Warming coupled with wetting did not significantly change Q10,while warming coupled with N addition reduced Q10by 33.3%(2.4 to 1.6).The combination of three treatments increased Q10by 12.5%(2.4 to 2.7).Our results demonstrated that among the three factors,soil moisture is the most important one controlling SIR and Q10.The results suggest that the effect of warming on SIR and Q10can be modified significantly by rainfall variability and elevated N availability.Therefore,this study emphasizes that concurrent climatic and environmental changes,such as increasing rainfall variability and N deposition,should be considered when predicting changes induced by warming in soil respiration and its temperature sensitivity.  相似文献   

16.
Foliar exchange of mercury vapor: Evidence for a compensation point   总被引：1，自引：0，他引：1  
Historical studies for crop and weed species documented elemental Hg vapor (Hg°) deposition to foliage, but they used Hg° concentrations that were orders of magnitude higher than levels now known to occur under background conditions, possibly creating artificially high gradients between the atmosphere and landscape surfaces. Measurements of Hg° exchange with white oak (Quercus alba L.), red maple (Acer rubrum L.), Norway spruce (Picea abies L.), and yellow-poplar (Liriodendron tulipifera L.) foliage were conducted in an open gas exchange system that allows for simultaneous measurements of CO2, H2O and Hg° exchange under controlled environmental conditions. When Hg° concentrations were held at 0.5 to 1.5 ng m?3, red maple (Acer rubrum L.), Norway spruce (Picea abies L.), yellow-poplar (Liriodendron tulipifera L.), and white oak (Quercus alba L.) foliage exhibited mean Hg° emissions of 5.5, 1.7, 2.7, and 5.3 ng m?2 h?1, respectively. At Hg° concentrations between 9 and 20 ng m?3 little net exchange of Hg° was observed. However at concentrations between 50 and 70 ng m?3 the Hg° was deposited to foliage at rates between 22 and 38 ng m?2 h?1. These data suggest that dry foliar surfaces in terrestrial forest landscapes may be a dynamic exchange surface that can function as a source or sink dependent on the magnitude of current Hg° concentrations. These data provide evidence of species-specific compensation concentrations (or compensation points) for Hg° deposition to seedling foliage in the 10–25 ng m?3 range.  相似文献   

17.
Investigations of diurnal and seasonal variations in soil respiration support modeling of regional CO2 budgets and therefore in estimating their potential contribution to greenhouse gases. This study quantifies temporal changes in soil respiration and their driving factors in grassland and arable soils located in Northern Germany. Field measurements at an arable site showed diurnal mean soil respiration rates between 67 and 99 mg CO2 m–2 h–1 with a hysteresis effect following changes in mean soil temperatures. Field soil respiration peaked in April at 5767 mg CO2 m–2 day–1, while values below 300 mg CO2 m–2 day–1 were measured in wintertime. Laboratory incubations were carried out in dark open flow chambers at temperatures from 5°C to 40°C, with 5°C intervals, and soil moisture was controlled at 30%, 50%, and 70% of full water holding capacity. Respiration rates were higher in grassland soils than in arable soils when the incubating temperature exceeded 15°C. The respiration rate difference between them rose with increasing temperature. Monthly median values of incubated soil respiration rates ranged from 0 to 26.12 and 0 to 7.84 µg CO2 g–1 dry weight h–1, respectively, in grassland and arable land. A shortage of available substrate leads to a temporal decline in soil respiration rates, as indicated by a decrease in dissolved organic carbon. Temporal Q10 values decreased from about 4.0 to below 1.5 as temperatures increased in the field. Moreover, the results of our laboratory experiments confirmed that soil temperature is the main controlling factor for the Q10 values. Within the temperature interval between 20°C and 30°C, Q10 values were around 2 while the Q10 values of arable soils were slightly lower compared to that of grassland soils. Thus, laboratory studies may underestimate temperature sensitivity of soil respiration, awareness for transforming laboratory data to field conditions must therefore be taken into account.  相似文献   

18.
In-season diagnosis of crop nitrogen(N) status is crucial for precision N management. Critical N(N_c) dilution curve and N nutrition index(NNI) have been proposed as effective methods to diagnose N status of different crops. The N_c dilution curves have been developed for indica rice in the tropical and temperate zones and japonica rice in the subtropical-temperate zone, but they have not been evaluated for short-season japonica rice in Northeast China. The objectives of this study were to evaluate the previously developed N_c dilution curves for rice in Northeast China and to develop a more suitable N_c dilution curve in this region. A total of17 N rate experiments were conducted in Sanjiang Plain, Heilongjiang Province in Northeast China from 2008 to 2013. The results indicated that none of the two previously developed N_c dilution curves was suitable to diagnose N status of the short-season japonica rice in Northeast China. A new N_c dilution curve was developed and can be described by the equation N_c = 27.7 W~(-0.34) if W ≥ 1 Mg dry matter(DM) ha~(-1) or N_c = 27.7 g kg~(-1) DM if W 1 Mg DM ha~(-1), where W is the aboveground biomass. This new curve was lower than the previous curves. It was validated using a separate dataset, and it could discriminate non-N-limiting and N-limiting nutritional conditions. Additional studies are needed to further evaluate it for diagnosing N status of different rice cultivars in Northeast China and develop efficient non-destructive methods to estimate NNI for practical applications.  相似文献   

19.
Kinetically controlled release of uranium from soils   总被引：1，自引：0，他引：1  
Although trade element uptake on and release from solid phases are fundamental controls on the migration of the elements in the environment, the controls are incompletely understood. The extraction of uranium from two soils, both of which have been labelled naturally with uranium, was therefore studied using a cation resin exchange technique. One soil was a peat from the Needle's Eye natural analogue site, Scotland, and the other was a calcareous brown earth from Derbyshire, England. The effects of different exchanging cations, solution pH and the presence of complexing anions (Cl?, CO32?, SO42?) in solution on uranium extraction were assessed. The extraction could be described by a simple, first-order kinetic model with up to three rate constants being identifiable in individual experiments. In both soils no single reaction pathway appeared to dominate, and extraction was slow, with rate constants of 10?3?10?4 h?1 in acid conditions and around 10?6 h?1 in neutral conditions. Half-times for uranium release in the experiments were in the range 30–60 days in acid and around 10 years in neutral conditions; in the field they are therefore expected to be several years at both sites. Incorporation of kinetic factors into a simple one-dimensional migration model illustrates that their overall effect is to retard migration. Ideally, therefore, reaction rates should be taken into account in predictive modelling of element transport.  相似文献   

20.

Purpose

Rice-paddy-dominated watersheds in eastern China are intensively cultivated, and lands with two crops receive as much as 550–600 kg?ha–1?year–1 of nitrogen (N), mainly through the addition of N-based fertilizers. However, stream N concentrations have been found to be relatively low. Waterways in the watersheds are assumed to be effective “sinks” for N, minimizing its downstream movement. We directly measured net sediment denitrification rates in three types of waterways (ponds, streams/rivers, and a reservoir) and determined the key factors that control net sediment denitrification. Such information is essential for evaluating the impact of the agricultural N cycle on the quality of surface water.

Materials and methods

The pond–stream–reservoir continuum was sampled every 2 months at nine sites in an agricultural watershed between November 2010 and December 2011. Net sediment N2 fluxes/net sediment denitrification rates were determined by membrane inlet mass spectrometry and the N2/Ar technique. A suite of parameters known to influence denitrification were also measured.

Results and discussion

Net denitrification rates ranged between 28.2?±?18.2 and 674.3?±?314.5 μmol N2–N?m–2?h–1 for the streams, 23.7?±?23.9 and 121.2?±?38.7 μmol N2–N?m–2?h–1 for the ponds, and 41.8?±?17.7 and 239.3?±?49.8 μmol N2–N?m–2?h–1 for the reservoir. The mean net denitrification rate of the stream sites (173.2?±?248.4 μmol N2–N?m–2?h–1) was significantly higher (p?<?0.001) than that of the pond sites (48.3?±?44.5 μmol N2–N?m–2?h–1), and the three types of waterways all had significantly higher (p?<?0.01) mean net denitrification rates in summer than in other seasons. Linear regression and linear mixed effect model analysis showed that nitrate (NO3 ?–N) concentration in surface water was the primary controlling factor for net sediment denitrification, followed by water temperature. Using monitoring data on NO3 ?–N concentrations and temperature of the surface water of waterways and an established linear mixed effect model, total N removed through net sediment denitrification in the pond–stream–reservoir continuum was estimated at 46.8?±?24.0 t?year–1 from July 2007 to June 2009, which was comparable with earlier estimates based on the mass balance method (34.3?±?12.7 t?year–1), and accounted for 83.4 % of the total aquatic N. However, the total aquatic N was only 4.4 % of the total N input to the watershed, and thus most of the surplus N in the watershed was likely to be either denitrified or stored in soil.

Conclusions

High doses of N in a rice-paddy-dominated watershed did not lead to high stream N concentrations due to limited input of N into waterways and the high efficiency of waterways in removing N through denitrification.  相似文献   

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