Mitigation of N2O emission from an alluvial soil by application of karanjin

An incubation experiment was conducted to study N₂O emissions from a Typic Ustochrept, alluvial soil, fertilized with urea and urea combined with different levels of two nitrification inhibitors, viz karanjin and dicyandiamide (DCD). Karanjin [a furano-flavonoid, obtained from karanja (Pongamia glabra Vent.) seeds] and DCD were incorporated at rates of 5, 10, 15, 20 and 25% of applied urea-N (100 mg kg^-1 soil), to the soil adjusted to field capacity moisture content. The highest N₂O flux (366 µg N₂O-N kg^-1 soil day^-1) was obtained on day 1 after incubation from soil fertilized with urea without any inhibitor. The presence of the inhibitors appreciably reduced the mean N₂O flux from the urea-treated soils. The application of karanjin resulted in a higher mitigation of total N₂O-N emission (92-96%) compared to DCD (60-71%). Rates of N₂O flux ranged from 0.9 to 140 µg N₂O-N kg^-1 soil day^-1 from urea combined with different levels of the two inhibitors (coefficient of variation=24-272%). Karanjin (62-75%) was also more effective than DCD (9-42%) in inhibiting nitrification during the 30-day incubation period.     

Nitrous oxide emissions from the wheat-growing season in eighteen Chinese paddy soils: an outdoor pot experiment

To identify the key soil parameters influencing N₂O emission from the wheat-growing season, an outdoor pot experiment with a total of 18 fertilized Chinese soils planted with wheat was conducted in Nanjing, China during the 2000/2001 wheat-growing season. Average seasonal N₂O-N emission for all 18 soils was 610 mg m^-2, ranging from 193 to 1,204 mg m^-2, approximately a 6.2-fold difference between the maximum and the minimum. Correlation analysis indicated that the seasonal N₂O emission was negatively correlated with soil organic C (r²=0.5567, P<0.001), soil total N (r²=0.4684, P<0.01) and the C:N ratio (r²=0.4530, P<0.01), respectively. A positive dependence of N₂O emission on the soil pH (r²=0.3525, P<0.01) was also observed. No clear relationships existed between N₂O emission and soil texture, soil trace elements of Fe, Cu and Mg, and above-ground biomass of the wheat crop at harvest. A further investigation suggested that the seasonal N₂O-N emission (E, mg m^-2) can be quantitatively explained by E=1005-34.2SOC+4.1S_a (R²=0.7703, n=18, P=0.0000). SOC and S_a represent the soil organic C (g kg^-1) and available S (mg kg^-1), respectively.     

Effects of dicyandiamide, farmyard manure and irrigation on crop yields and ammonia volatilization from an alluvial soil under a rice (Oryza sativa L.)-wheat (Triticum aestivum L.) cropping system

Volatilization of NH₃ from soil is a major N-loss mechanism that reduces the efficiency of applied N fertilizers, and causes environmental pollution. Strategies are needed to reduce the loss. The influences of dicyandiamide (DCD), farmyard manure (FYM) and irrigation on NH₃ volatilization from an alluvial soil in rice (Oryza sativa L.)-wheat (Triticum aestivum L.) cropping system was studied using the acid trap method. The loss of NH₃ in the rice-wheat system ranged from 38.6 kg N ha^-1 from the unfertilized soil to 69.0 kg N ha^-1 in the treatment with urea+DCD. Substitution of 50% N provided through urea by FYM reduced NH₃-N volatilization by 10% in rice and wheat as compared to the urea treatment. Application of DCD increased NH₃ volatilization in wheat by 7% but in rice it had no effect. The irrigation level had no effect on NH₃ volatilization in rice but fewer irrigations with fewer splits of N in wheat resulted in higher NH₃ volatilization. Application of DCD and FYM with urea had similar effects on grain yield and N uptake by rice and wheat as that of the urea treatment. The study showed that integrated use of organic manure and chemical fertilizer has the potential to reduce the loss of N due to volatilization and thereby minimize environmental pollution. Nitrification inhibitors, which are reported to be useful in increasing the N-use efficiency by reducing the leaching and denitrification losses of N, however, may increase N loss due to volatilization.     

Nitrous oxide concentrations in an Andisol profile and emissions to the atmosphere as influenced by the application of nitrogen fertilizers and manure

A field experiment was conducted to determine N₂O concentrations in the soil profile and emissions as influenced by the application of N fertilizers and manure in a typical Japanese Andisol, which had been under a rotation of oat and carrot for the previous 3 years. The treatments include ammonium sulphate (AS), controlled-release fertilizer (CRF) and cattle manure (CM) in addition to a control; all the fertilizers were applied either at 150 kg N ha^-1 or 300 kg N ha^-1 at the time of sowing carrot. N₂O emissions from the soil surface were measured with closed-chamber techniques, while N₂O concentrations in the soil profile were measured using stainless steel sampling probes inserted into the soil at depths of 10, 20, 40, 60, 80 and 100 cm. Moreover, soil water potential, soil temperature and rainfall data were also recorded. The results indicated that N₂O concentrations in the soil profile were always greater than in the atmosphere, ranging from 0.36 µl N₂O-N l^-1 to 5.3 µl N₂O-N l^-1. The relatively large accumulation of N₂O in the lower profiles may be a significant source for N₂O flux. Taking the changes of soil mineral N into consideration, most emissions of N₂O were probably produced from nitrification. The accumulation of N₂O in the soil profile and emissions to the atmosphere were differently influenced by the amendments of N fertilizers and manure, being consistently higher in CRF than in CM and AS treatments at the corresponding application rates, but no significant difference existed with respect to the various N sources.     

Overwinter soil denitrification activity and mineral nitrogen pools as affected by management practices

During freeze-thaw events, biophysical changes occurring in soils can affect processes such as mineralization, nitrification and denitrification which control inorganic N balances in agro-ecosystems. To evaluate the impact of these climatic events on soil biochemical properties, a study was conducted comparing soil denitrification enzyme activity (DEA), dissolved organic C (DOC) and inorganic N levels before and after the winter season in plots under: (1) continuous corn (Zea mays L.) (CC) with annual chisel plow and disking, (2) corn-soybean (Glycine max L.) (CS) rotation with chisel plow every other year prior to planting soybean, and (3) corn-soybean-wheat (Triticum aestivum L.)/hairy vetch (Vicia villosa Roth) (CSW-V) with ridge tillage during the corn and soybean crops, and dairy manure application during the corn year. Soil cores were collected in late autumn and immediately after spring thaw at 0-5, 5-10, 10-15, and 15-30 cm depths. Regardless of management practices, freeze-thaw events resulted in significant (2-10 times) increases in NH₄⁺-N, NO₃^--N (P<0.001) and DOC (P<0.01) levels at all soil depths. Following freeze-thaw, DEA remained unchanged in the 5-30 cm depth but dropped significantly (P<0.01) in the 0-5 cm soil layer. In that layer, soils which had been chisel plowed during the previous growing season lost 78-84% of the DEA recorded during the fall, whereas in the plots amended with manure during the previous season, the loss of activity was 40-45%. These data indicate that frequent tillage, compared with manure additions, is more conducive to overwinter loss of DEA in surface layers of soils subject to freeze-thaw cycles.     

Denitrification, N2O and CO2 fluxes in rice-wheat cropping system as affected by crop residues, fertilizer N and legume green manure

Use of renewable N and C sources such as green manure (GM) and crop residues in rice-wheat cropping systems of South Asia may lead to higher crop productivity and C sequestration. However, information on measurements of gaseous N losses (N₂O+N₂) via denitrification and environmental problems such as N₂O and CO₂ production in rice-wheat cropping systems is not available. An acetylene inhibition-intact soil core technique was employed for direct measurement of denitrification losses, N₂O and CO₂ production, in an irrigated field planted to rice (Oryza sativa L.) and wheat (Triticum aestivum L.) in an annual rotation. The soil was a coarse-textured Tolewal sandy loam soil (Typic Ustochrept) and the site a semi-arid subtropical Punjab region of India. Wheat residue (WR, C:N=94) was incorporated at 6 t ha^-1 and sesbania (Sesbania aculeata L.) was grown as GM crop for 60 days during the pre-rice fallow period. Fresh biomass of GM (C:N.=18) at 20 or 40 t ha^-1 was incorporated into the soil 2 days before transplanting rice. Results of this study reveal that (1) denitrification is a significant N loss process under wetland rice amounting to 33% of the prescribed dose of 120 kg N ha^-1 applied as fertilizer urea-N (FN); (2) integrated management of 6 t WR ha^-1 and 20 t GM ha^-1 supplying 88 kg N ha^-1 and 32 kg FN ha^-1 significantly reduced cumulative gaseous N losses to 51.6 kg N ha^-1 as compared with 58.2 kg N ha^-1 for 120 kg FN ha^-1 alone; (3) application of excessive N and C through applying 40 t GM ha^-1 (176 kg N ha^-1) resulted in the highest gaseous losses of 70 kg N ha^-1; (4) the gaseous N losses under wheat were 0.6% to 2% of the applied 120 kg FN ha^-1 and were eight- to tenfold lower (5-8 kg N ha^-1) than those preceding rice; (5) an interplay between the availability of NO₃^- and organic C largely controlled denitrification and N₂O flux during summer-grown flooded rice whereas temperature and soil aeration status were the primary regulators of the nitrification-denitrification processes and gaseous N losses during winter-grown upland wheat; (6) the irrigated rice-wheat system is a significant source of N₂O as it emits around 15 kg N₂O-N ha^-1 year^-1; (7) incorporation of WR in rice and rice residue (C:N=63) in wheat increased soil respiration, and increased CO₂ production in WR- and GM-amended soils under anaerobic wetland rice coincided with enhanced rates of denitrification; and (8) with adequate soil moisture, most of the decomposable C fraction of added residues was mineralized within one crop-growing season and application of FN and GM further accelerated this process.     

Nitrous oxide and nitric oxide emissions from maize field plots as affected by N fertilizer type and application method

N₂O and NO emissions from an Andisol maize field were studied. The experimental treatments were incorporation of urea into the plough layer at 250 kg N ha^-1 by two applications (UI250), band application of urea at a depth of 8 cm at 75 kg N ha^-1 plus incorporation of urea into the plough layer at 75 kg N ha^-1 (UB150), band application of polyolefin-coated urea at a depth of 5 cm at 150 kg N ha^-1 (CB150), and a control (without N application). N₂O fluxes from UI250 and UB150 peaked following the incorporation of supplementary fertilizer, and declined to the background level after that, while the N₂O flux from CB150 was relatively low but remained at a constant level until shortly after harvest. Accordingly, the total N₂O emissions during the whole cultivation period from the three treatments were not significantly different. The fertilizer-derived N₂O-N losses from UI250, UB150 and CB150 were 0.15%, 0.27% and 0.28% of the applied N, respectively. However, it was suggested that, due to the low plant N recovery, UI250 had a significantly larger potential for indirect N₂O emission than the other treatments. On the other hand, NO emissions from UI250 and UB150 were 12 times higher than that from CB150, and the fertilizer-derived NO-N losses from the three treatments were 0.16%, 0.27% and 0.026% of the applied N, respectively. Significant NO fluxes were detected only when urea-N fertilizer was surface-applied and incorporated into plough-layer soil.     

Dehydrogenase activity, redox potential, and emissions of carbon dioxide and nitrous oxide from Cambisols under flooding conditions

Samples from topsoils (0-10 cm) of 16 Polish arable Cambisols developed from different parent materials (sand, silt, sandy gravel, loess, loam and clay), were incubated under flooded conditions with NO₃^-. Dehydrogenase activity, redox potential (Eh), and emissions of CO₂ and N₂O were measured. According to dehydrogenase activity, the soils were divided into two groups: those of low activity (I), where the final dehydrogenase activity was <0.03 nmol triphenylformazan (TPF) g^-1 min^-1, and those with high final dehydrogenase activity (II), >0.03 nmol TPF g^-1 min^-1. Generation of CO₂ and of N₂O under flooded conditions was shown to be significantly related to dehydrogenase activity. Soil dehydrogenase activity increased curvilinearly with organic matter content, showed a maximum at pH 7.1, and decreased curvilinearly with Eh. The final cumulative CO₂ production increased linearly with soil organic matter content and curvilinearly with dehydrogenase activity and decreased linearly with Eh. The most significant relationship was found with dehydrogenase activity (R²=0.74, P<0.001). The final cumulative N₂O production decreased linearly with Eh and increased curvilinearly with pH and dehydrogenase activity but linearly with organic matter content; the most significant relation being found with dehydrogenase activity (R²=0.69, P<0.001). The CO₂:N₂O ratio in the gases evolved increased curvilinearly with Eh and decreased with dehydrogenase activity and N₂O and CO₂ production.     

Effect of rice straw management on nitrogen balance and residual effect of urea-N in an annual lowland rice cropping sequence

Two field experiments were conducted in 1999 (wet season) and 2000 (dry season) on a Ustic Endoaquerts in central Thailand to examine the impact of rice straw management practices on rice yield, N uptake and fertilizer-N use efficiency. Treatments included a combination of urea broadcast at a rate of 70 kg N ha^у with either straw or compost which were incorporated at a rate of 5 Mg ha^у. At maturity of the wet season rice, ¹⁵N recovery by the grain was low (11-14%) as well as straw-N derived from labeled N (5-7%). After harvest, 25-29% of applied N still remained in the soil, mainly in the 0 to 5-cm layer. Large amounts of fertilizer-N (53-55%) were lost (unaccounted for) from the soil/plant system during the first crop. Residual fertilizer-N recovery in the second rice crop was less than 3% from the original application. During both fallow seasons NO₃^m-N remained the dominant form of mineral N (NO₃^m + NH₄⁺) in the soil but its concentration was low. In the wet season grain yield response to N application was significant (P =0.05). Organic material sources did not significantly change grain yield and N accumulation in rice. In terms of grain yield and N uptake at maturity, there was no significant residual effect of fertilizer-N on the subsequent rice crop. These results indicated that the combined use of organic residues with urea did not decrease total N losses or increase crop yield or uptake of N compared to urea alone.     

Naidid worms (Oligochaeta, Naididae) in paddy soils as affected by the application of legume mulch and/or tillage practice

Reproduction, intrinsic rate of natural increase and population density of naidid worms were investigated in submerged paddy fields and the laboratory. No tillage plus legume-mulching increased the population density of naidid worms. Soil treatments with neither tillage nor legume mulch, and tillage practice alone, did not increase the number of worms. Dero dorsalis Ferronnière was dominant in soil of the no-tillage treatment. In laboratory experiments, legume-mulching with the proper amount of dissolved O₂ accelerated asexual reproduction of D. dorsalis through zooid budding. For the legume and aeration treatment, (N_i+1-N_i) N_i^-1 values (where N_i and N_i+1 are the populations at times t=i and t=i+1) were plotted against N_i+1. Utilizing this linear relation, this data fitted the logistic curve (r²=0.885, P<0.05). Based on the linear relation, the intrinsic rate of natural increase (r), carrying capacity (K), and doubling time (T) were calculated as 0.2125 day^-1, 12,666 m^-2, and 3.26 days, respectively. The amounts of legumes applied were highly correlated with the population of D. dorsalis, indicating that the weight of legume is a limiting factor with respect to carrying capacity. A literature review indicated a significant correlation (P<0.01) between intrinsic rate of natural increase and maximum body length of naidids with temperature conversion of the growth rate. Sexually mature worms were rarely found in submerged paddy fields. Sexual reproduction seems to be adopted in response to soil desiccation after paddy field drainage.     

Cultivable heterotrophic N<Subscript>2</Subscript>-fixing bacterial diversity in rice fields in the Yangtze River Plain

Heterotrophic N₂-fixing bacteria are a potentially important source of N₂ fixation in rice fields due to the moist soil conditions. This study was conducted at eight sites along a geographic gradient of the Yangtze River Plain in central China. A nitrogen-free solid malate-sucrose medium was used to isolate heterotrophic N₂-fixing bacteria. Numbers of the culturable N₂-fixing bacteria expressed as CFU (colony forming units) ranged between 1.41ǂ.42᎒⁶ and 1.24ǂ.23᎒⁸ in the sampled paddy field sites along the plain. Thirty strains with high ARA (acetylene reduction activity) were isolated and purified; ARA of the strains varied from 0.9 to 537.8 nmol C₂H₄ culture^-1 h^-1, and amounts of ¹⁵N fixed ranged between 0.008 and 0.4866 mg·culture^-1·day^-1. According to morphological and biochemical characteristics, 14 strains were identified as the genus Bacillus, 2 as Burkholderia, 1 as Agrobacterium, 4 as Pseudomonas, 2 as Derxia, 1 as Alcaligenes, 1 as Aeromonas, 2 as Citrobacter, and 3 strains belonged to the corynebacter-form group.     

Cultivar-specific dinitrogen fixation in Vicia faba studied with the nitrogen-15 natural abundance method

N fixation by different faba bean (Vicia faba) cultivars was studied using the natural abundance method. The delta ¹⁵N ('¹⁵N) values of the faba beans and the reference plants differed by 4.6-7.0‰. The non-nodulating V. faba cv. F48 seems to be the best reference plant for nodulated and N₂-fixing V. faba. Significant differences occurred in the quantity of N₂ fixation of six V. faba cultivars. The average fraction of N derived from air (FNdfa) estimated from leaf material ranged between 69 and 80%. Shoot-based estimates of N fixation varied between 200 and 360 kg N ha^-1. N fixation was affected more by differences in FNdfa than by differences in total N accumulation. Fixation data calculated with the non-nodulated reference plant V. faba cv. F48 were lower than those calculated with cabbage (Brassica oleracea) and ryegrass (Lolium perenne) as reference plants. Of all reference plants, non-N₂-fixing V. faba cv. F48 has a root system and temporal pattern of N assimilation that is the one most similar to that of N₂-fixing V. faba plants. Cv. F48 showed senescence as did the other V. faba cultivars after pod-fill was complete, whereas cabbage, ryegrass and camomile had a later senescence period. N fixation during pod-filling appears more important for a good yield than N₂ fixation abilities in the earlier growth period. The best V. faba cultivars left about 100 kg N ha^-1 in residual material on the field as fertilization for the following crops.     

Biological dinitrogen fixation and soil microbial biomass carbon as influenced by free-air carbon dioxide enrichment (FACE) at three levels of nitrogen fertilization in a paddy field

An experiment was conducted in an Andosol paddy field in Shizukuishi (Iwate Prefecture, Japan) to determine the effects of free-air CO₂ enrichment (FACE) on biological N₂-fixation activity and soil microbial biomass C at three levels of N application. Rice (Oryza sativa L. cv. Akitakomachi) plants were grown under ambient CO₂ or FACE (ambient +200 µmol mol^-1 CO₂) conditions throughout the growing season with each treatment having four replicated plots. Three levels of N fertilizer (high, standard and low; 15, 9 and 4 g N m^-2, respectively) were applied to examine the effect of different N availability under both CO₂ conditions. Soil samples were collected at four different times from upper and lower soil layers (0-1-cm and 1-10-cm soil depths, respectively) and analysed for biological N₂-fixation (BNF) activity and microbial biomass C (MBC) by the acetylene reduction and chloroform fumigation-extraction methods, respectively. The amounts of chlorophyll-type compounds (Chls), an index of algal growth, and soil available C were also determined. Compared to the ambient CO₂ treatment, the FACE treatment had significantly higher BNF activity in both the upper and lower soil layers at ripening only in low-N soil and at harvest at all three levels of N fertilization rates. MBC was significantly increased by FACE in both the upper and lower soil layers from the middle to later period of the growing season compared to the ambient CO₂ treatment. The FACE treatment increased the Chls in the upper soil layers at ripening only in low-N soil and at harvest at all three levels of N fertilization rates. The amount of soil available C was not significantly different between FACE and ambient CO₂ treatments in both the upper and lower soil layers throughout the cropping season. From these results it can be concluded that the FACE treatment had a significantly positive influence on BNF activity, MBC and Chls at different levels of N fertilization rates in paddy field during the cropping season.     

Carbon partitioning in plant and soil,carbon dioxide fluxes and enzyme activities as affected by cutting ryegrass

The effect of a single cut (simulated grazing) and regrowth of Lolium perenne on CO₂ efflux from soil (loamy Haplic Luvisol), on below-ground C translocation and on the distribution of plant C among different soil particle size fractions was investigated under controlled conditions with and without N fertilization by pulse labelling with ¹⁴C 7 times (four before and three after the cutting). The amount of ¹⁴C respired from the rhizosphere of Lolium decreased by a factor of about 3 during 1 month of growth. At the same time the amount of ¹⁴C stored in soil increased. Cut and non-fertilized plants respired less C in the rhizosphere compared to the uncut plants and cut fertilized plants. About 80% of the root-derived CO₂ efflux originated from the C assimilated after defoliation, and 20% originated from the C assimilated before cutting. N fertilization decreased the below-ground C losses (root respiration and exudation) during regrowth. The shoot is the main sink of assimilated C before and after the defoliation. N fertilization led to higher C incorporation into the shoot parts growing after defoliation compared to unfertilized plants. A lower incorporation of ¹⁴C was observed in the roots of N fertilized plants. The relative growth rates (expressed as ¹⁴C specific activity) of roots and stubble were minimal and that of shoot parts growing after defoliation was maximal. Twelve percent of ¹⁴C was found in the newly grown leaves after regrowth; nevertheless, 4.7% and 2.4% of ¹⁴C in the new shoot parts were translocated from the root and shoot reserves of unfertilized and fertilized plants, respectively. Most of the C retranslocated into the new Lolium leaves originates from the stubble and not from the roots. Between 0.5% and 1.7% of ¹⁴C recovered in shoots and below-ground C pools was found in the soil microbial biomass. Cutting and fertilization did not change ¹⁴C incorporation into the microbial biomass and did not affect xylanase, invertase, and protease activities. Tracing the assimilated C in particle size fractions revealed maximal incorporation for the sand and clay fraction.     

Short-term patterns of carbon and nitrogen mineralisation in a fallow field amended with green manures from agroforestry trees

The mineralisation of green manure from agroforestry trees was monitored with the objective to compare the temporal dynamics of mineralisation of litter from different species. Green manures from five agroforestry tree species were used on a fallow field during the long rainy season of 1997 (March-August) and from two species in the following short rainy season (September-January) in western Kenya. Different methods, i.e. measurements of isotopic ratios of C in respired CO₂ and of soil organic matter (SOM) fractions, soil inorganic N and mass loss from litterbags, were used in the field to study decomposition and C and N mineralisation. Soil respiration, with the separation of added C from old soil C by using the isotopic ratio of ¹³C/¹²C in the respired CO₂, correlated well with extractable NH₄⁺ in the soil. Mineralisation was high and very rapid from residues of Sesbania sesban of high quality [e.g. low ratio of (polyphenol+lignin)/N] and low and slow from low quality residues of Grevillea robusta. Ten days after application, 37% and 8% of the added C had been respired from Sesbania and Grevillea, respectively. Apparently, as much as 70-90% of the added C was respired in 40 days from high quality green manure. Weight losses of around 80%, from high quality residues in litterbags, also indicate substantial C losses and that a build-up of SOM is unlikely. For immediate effects on soil fertility, application of high quality green manure may, however, be a viable management option. To achieve synchrony with crop demand, caution is needed in management as large amounts of N are mineralised within a few days after application.     

Different behavioral patterns of the earthworms Octolasion tyrtaeum and Diplocardia spp. in tallgrass prairie soils: potential influences on plant growth

This study addressed differences between Diplocardia spp. (a native North American earthworm) and Octolasion tyrtaeum (an introduced European species), with respect to behavior, influence on soil microbial biomass, and plant uptake of N in tallgrass prairie soils. We manipulated earthworms in PVC-encased soil cores (20 cm diameter) over a 45-day period under field conditions. Treatments included: (1) control with no earthworms, (2) Diplocardia spp. only, and (3) O. tyrtaeum only. Prior to addition of earthworms, seedlings of Andropogon gerardii (a dominant tallgrass) were established in each core, and a dilute solution of ¹³C-labeled glucose and ¹⁵N-labeled (NH₄)₂SO₄ was added to the soil to facilitate examination of earthworm/microbe/plant interactions. We found that Diplocardia spp. were significantly more active than O. tyrtaeum, and quickly assimilated ¹³C and ¹⁵N from the tracer. Individuals of Diplocardia spp. were present at shallower soil depths than O. tyrtaeum throughout the study. Contrary to expectation, this greater activity of Diplocardia spp. did not result in increased plant productivity. Rather, the activity of Diplocardia spp. was associated with less plant growth and smaller amounts of N acquired by A. gerardii seedlings compared to controls or O. tyrtaeum treatments. We observed few significant influences of earthworm treatments on microbial biomass C or N pool sizes, but the microbial C/N ratio was consistently greater in the presence of Diplocardia spp. relative to O. tyrtaeum. Results of this study indicate that activity of earthworms may enhance competition for N between microbes and plants during the growing season in tallgrass prairie.     

Nitrification and denitrification as sources of atmospheric nitrous oxide – role of oxidizable carbon and applied nitrogen

Laboratory incubation experiments were conducted to study the influence of easily oxidizable C (glucose) and mineral N (NH₄⁺ and NO₃^-) on N₂O emission, evolution of CO₂ and consumption of O₂. A flush of N₂O was always observed during the first few hours after the start of soil incubation, which was significantly higher with NH₄⁺ compared to NO₃^- applications. The increase in N₂O emission was attributed mainly to enhanced soil respiration and subsequent O₂ limitation at the microsite level. Application of NH₄⁺ helped to develop denitrifying populations since subsequent additions of NO₃^- and a C source significantly enhanced N₂O emissions. In soils treated with NH₄⁺, N₂O emissions declined rapidly, which was related to decreasing concentrations of easily oxidizable C. Addition of glucose in different amounts and pre-incubation of soil for different lengths of time (to create variation in the amount of easily oxidizable C) changed the pattern of N₂O emissions, which was ascribed to changes in soil respiration.     

Correlations between pesticide transformation rate and microbial respiration activity in soil of different ecosystems

Cecil sandy loam soils (ultisol) from forest (coniferous and deciduous), pasture, and arable ecosystems were sampled (0-10 cm) in the vicinity of Athens, Georgia, USA. Soil from each site was subdivided into three portions, consisting of untreated soil (control) as well as live and sterile samples treated with the fungicide metalaxyl and the herbicide propachlor at 10 mg kg^-1 soil. Pesticide transformation rate, basal respiration (basal) and substrate-induced respiration (SIR) rates, and microbial metabolic quotient (qCO₂) were measured for the initial application of metalaxyl [methyl-N-(2,6-dimethylphenyl)-N-(metoxyacetyl)-DL-alaninate] or propachlor (2-chloro-N-isopropyl-acetanilide) at 22°C and 60% water holding capacity. Positive correlations were found for the following: metalaxyl transformation rate constant (K_met) and basal (r=0.73); K_met and SIR (r=0.83); propachlor transformation rate constant (K_pr) and basal (r=0.89); and K_pr and SIR (r=0.91). Regression analysis of pesticide transformation rate and soil respiration activity, coupled with specific soil properties (pH, C_org, and clay content), revealed a positive correlation between K and SIR for C_org (r=0.88 and 0.98, for metalaxyl and propachlor, respectively). qCO₂s were not significantly different (P=0.05) in propachlor-amended and pesticide-free soils. Metalaxyl amendment resulted in a change in the ecophysiological status of the soil microbial community as expressed by qCO₂. The qCO₂ values in metalaxyl-amended soils were significantly greater (P=0.05) in pine forest (by 25%) and arable and pasture (by 20%) soils compared to unamended soils. Differences in qCO₂ values may represent the magnitude of pesticide-induced disturbance. The duration of this disturbance was greater in the pine forest soil (48 days) compared to arable and pasture soils (21 and 15 days, respectively).     

Influence of soil parameters on the effect of 3,4-dimethylpyrazole-phosphate as a nitrification inhibitor

Nitrification inhibitors specifically retard the oxidation of NH₄⁺ to NO₂^- during the nitrification process in soil. In this study, the influence of soil properties on the nitrification-inhibiting effect of 3,4-dimethylpyrazole-phosphate (DMPP), a newly developed nitrification inhibitor, has been investigated. Based on short-term incubation experiments, where the degradation of DMPP could be largely disregarded, the oxidation of the applied NH₄⁺ was more inhibited in sandy soils compared with loamy soils. The influence of soil parameters on the relative NO₂^- formation could be described by a multiple regression model including the sand fraction, soil H⁺ concentration and soil catalase activity (R²=0.62). Adsorption studies showed that the binding behaviour of DMPP was influenced markedly by soil textural properties, viz. the clay fraction (r²=0.61). The adsorption of DMPP was found to be an important factor for the inhibitory effect on NH₄⁺ oxidation in a short-term incubation (r²=0.57). It is concluded that the evaluated soil properties can be used to predict the short-term inhibitory effect of DMPP in different soils. The significance of these results for long-term experiments under laboratory and field conditions needs further investigation.     

Influence of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) in comparison to dicyandiamide (DCD) on nitrous oxide emissions, carbon dioxide fluxes and methane oxidation during 3 years of repeated application in field experiments

In a 3-year field experiment, the effect of the nitrification inhibitor (NI) 3,4-dimethylpyrazole phosphate (DMPP) on the release of N₂O, CO₂, and on CH₄ oxidation, was examined in comparison to that of dicyandiamide (DCD) on N-fertilized and unfertilized experimental sites. Soil samples were analysed simultaneously for the concentrations of N₂O retained in the soil body, NH₄⁺, NO₂^-, NO₃^-, and for the degradation kinetics of DMPP as well as DCD. DMPP decreased the release of N₂O on fertilized plots by 41% (1997), 47% (1998) and 53% (1999) (on average by 49%) while DCD reduced N₂O emissions by 30% (1997), 22% (1998) and 29% (1999) (on average by 26%). In addition, the NIs seemed to decrease the CO₂ emissions of each fertilized treatment. DCD reduced the release of CO₂ by an average of 7% for the 3 years (non-fertilized 10%), and DMPP reduced it by an average of up to 28% (non-fertilized 29%). Furthermore, both NIs failed to affect CH₄ oxidation negatively. The plots that received either DCD or DMPP even seemed to function as enhanced sinks for atmospheric CH₄. DMPP apparently stimulated CH₄ oxidation of N-fertilized plots by ca 28% in comparison to the control. In total, DCD and DMPP reduced the global warming potential of N-fertilized plots by 7% and 30%, respectively. Further, DCD and DMPP diminished the amount of N₂O retained in the soil by 52% and 61%, respectively. The concentrations of NH₄⁺ remained unaffected by both NIs, whereas the amounts of NO₂^- diminished in the plots treated with DCD by 25% and with DMPP by 20%. In both NI treatments NO₃^- concentrations in the soil were 23% lower than in the control. DMPP and DCD did not affect the yields of summer barley, maize and winter wheat significantly. DCD was mineralized more rapidly than DMPP.