Nitrogen fertilizer value of activated sewage derived protein: Effect of environment and nitrification inhibitor on NO 3 - release,soil microbial activity and yield of summer cabbage

Yield response of summer cabbage (Brassica oleracea varcapitata cv. Hispi F₁) to N applied as organic (activated sewage sludge derived protein [Protox] and dried blood) and inorganic (ammonium nitrate, ammonium sulphate, sodium nitrate and urea) fertilizers was compared in relation to the N availability characteristics of the materials. Effects of the nitrification inhibitor dicyandiamide (DCD) on N release, crop yield and N status were also assessed. In addition CO₂ efflux was measured from amended soil to determine effects of fertilizer application on soil microbial activity. The organic N sources were mineralized quickly on application to soil and exhibited similar patterns of NH₄-N depletion and NO₃-N accumulation as functions of thermal-time as with mineral fertilizers. However, the yield response to organic N was marginally smaller (though not significantly) compared with mineral forms; probably because less N was released to the crop. This was reflected in smaller total N concentrations and N recoveries in plants supplied with organic fertilizer. Applied DCD increased the thermal-time for complete nitrification of NH₄-N sources and raised the total N content of the crop, but had no overall effect on crop growth. In contrast to inorganic N sources which generally reduced CO₂ efflux from soil, application of protein-based fertilizers increased the rate of soil microbial activity directly by raising substrate availability. Sewage sludge derived protein provided an effective alternative to mineral fertilizers for the nutrition of summer cabbage whilst minimising stress of the soil environment which may occur following the application of conventional forms of inorganic N to the soil.     

Influence of nitrogen nutrition and metabolism on ammonia volatilization in plants

Barley (Hordeum vulgare L. cv. Golf) was grown in solution culture with controlled nitrogen availability in order to study the influence of nitrogen nutrition on ammonia emission from the leaves. Ammonia emission measured in cuvettes connected to an automatic NH₃ monitor was close to zero for nitrate grown plants but increased to 0.9–1.3 nmol NH₃ m^-2 leaf area s^-1 after 3–5 days of ammonium nutrition. Increasing concentrations from 0.5 to 10 mM NH₄ ⁺ in the root medium increased NH₃ emission from the shoots, root glutamine synthetase activity and NH₄ ⁺ concentrations in apoplast, xylem sap and bulk tissue, while apoplastic pH values decreased.Inhibition of glutamine synthetase in nitrate grown barley plants by addition of 1 mM methionine sulfoximine (MSO) to the root medium caused ammonia emission to increase 5 to 10-fold after 2–3 hours. At the same time shoot tissue ammonium concentrations started to increase. Addition of an inhibitor of photorespiration, 1 mM pyrid-2-yl hydroxymethane sulfonate (HPMS) reduced this increase in ammonia emission showing a relation between NH₃ emission and photorespiration.Oil seed rape (Brassica napus L. cv. Global) plants grown at 3 different nitogen levels (2N, 4N and 7N) in a sand/soil mixture showed increasing NH₃ compensation points with increasing N level. This increase was highly correlated with increasing NH₄ ⁺ concentrations in the leaf apoplast and total leaf tissue. The NH₃ compensation points could be succesfully predicted on basis of the pH and NH₄ ⁺ concentration in the leaf apoplast.     

Uptake and apparent utilization of urea and ammonium nitrate in wheat seedlings

Intact wheat (Triticum aestivum cv. Quern) seedlings that were grown in presence or absence of NH₄NO₃ were exposed to solutions containing CO(NH₂)₂, NH₄NO₃, CO(NH₂)₂ + NH₄NO₃, CO(NH₂)₂ + KNO₃ and CO(NH₂)₂ + (NH₄)₂SO₄ for consecutive periods of 3, 3, 6, 12 and 24h and N uptake determined by solution depletion measurements. Differences in ethanol-soluble N and ethanol-insoluble N content of roots and shoots of control (zero time) seedlings and seedlings exposed to CO(NH₂)₂, NH₄NO₃ and CO(NH₂)₂ + NH₄NO₃ for 48 h were used to characterize N utilization during/following uptake.Regardless of initial N status, uptake of N from CO(NH₂)₂ was less than one-third of that from NH₄NO₃. Relative absorption of the CO(NH₂)₂ and NH₄NO₃ was not substantially altered by acidity control of the uptake solutions. There was a reciprocal antagonism between uptake of CO(NH₂)₂ and uptake of NH₄NO₃. Whereas CO(NH₂)₂ inhibited NH₄ absorption in each set of seedlings, it decreased NO₃ uptake only in seedlings that had been pretreated with N. Simultaneous presence of KNO₃ enhanced CO(NH₂)₂ uptake but presence of (NH₄)₂SO₄ decreased it to the same extent as NH₄NO₃. All absorption processes involving CO(NH₂)₂ and NH₄ were substantially restricted by pretreatment of the seedlings with NH₄NO₃. The results suggest that apparent utilization of ambient N was dependent on initial N status of the seedlings and on the nature of the N species to which they were exposed.     

Effects of type and amount of applied nitrogen fertilizer on nitrous oxide fluxes from intensively managed grassland

Five field experiments and one greenhouse experiment were carried out to assess the effects of nitrogen (N) fertilizer type and the amount of applied N fertilizer on nitrous oxide (N₂O) emission from grassland. During cold and dry conditions in early spring, emission of N₂O from both ammonium (NH ₄ ⁺ ) and nitrate (NO ₃ ^– ) containing fertilizers applied to a clay soil were relatively small, i.e. less than 0.1% of the N applied. Emission of N₂O and total denitrification losses from NO ₃ ^– containing fertilizers were large after application to a poorly drained sand soil during a wet spring. A total of 5–12% and 8–14% of the applied N was lost as N₂O and via denitrification, respectively. Emissions of N₂O and total denitrification losses from NH ₄ ⁺ fertilizers and cattle slurry were less than 2% of the N applied. Addition of the nitrification inhibitor dicyandiamide (DCD) reduced N₂O fluxes from ammonium sulphate (AS). However, the effect of DCD to reduce total N₂O emission from AS was much smaller than the effect of using NH ₄ ⁺ fertilizer instead of NO ₃ ^– fertilizer, during wet conditions. The greenhouse study showed that a high groundwater level favors production of N₂O from NO ₃ ^– fertilizers but not from NH ₄ ⁺ fertilizers. Inereasing calcium ammonium nitrate (CAN) application increased the emitted N₂O on grassland from 0.6% of the fertilizer application rate for a dressing of 50 kg N ha^–1 to 3.1% for a dressing of 300 kg N ha^–1. In another experiment, N₂O emission increased proportionally with increasing N rate. The results indicate that there is scope for reducing N₂O emission from grasslands by choosing the N fertilizer type depending on the soil moisture status. Avoiding excessive N application rates may also minimize N₂O emission from intensively managed grasslands.     

Ammonium transformation in paddy soils affected by the presence of nitrate

Coupled nitrification and denitrification is considered as one of the main pathways of nitrogen losses in paddy soils. The effect of NO₃ ^– on NH₄ ⁺ transformation was investigated by using the ¹⁵N technique. The paddy soils were collected from Wuxi (soil pH 5.84) and Yingtan (soil pH 5.02), China. The soils were added with either urea (18.57 mol urea-N enriched with 60 atom% ¹⁵N excess) plus 2.14 mol KNO₃-N (natural abundance) per gram soil (U+NO₃) or urea alone (U). The KNO₃ was added 6 days after urea addition. The incubation was carried out under flooded condition in either air or N₂ gas headspace at 25°C. The results showed that in air headspace, ¹⁵NH₄ ⁺ oxidization was so fast that about 10% and 8% of added ¹⁵N in the treatment U could be oxidized during the incubation period of 73 hours after KNO₃ addition in Wuxi and Yingtan soil, respectively. The addition of KNO₃ significantly inhibited ¹⁵NH₄ ⁺ oxidation (p<0.01) in air headspace, while it stimulated ¹⁵NH₄ ⁺ oxidation in N₂ gas headspace, although the oxidation was depressed by the N₂ gas headspace itself. Therefore, the accumulation of NO₃ ^– would inhibit further nitrification of NH₄ ⁺ at micro-aerobic sites in paddy soils, especially in paddy soils with a low denitrification rate. On the other hand, NO₃ ^– would lead to oxidation of NH₄ ⁺in anaerobic bulk soils.     

Effect of nitrogen,phosphorus and molybdenum application on growth and symbiotic N2-fixation of groundnut in an acid sandy soil in Niger

Two field experiments were conducted in 1988 and 1989 on an acid sandy soil in Niger, West Africa, to assess the effect of phosphorus (P), nitrogen (N) and micronutrient (MN) application on growth and symbiotic N₂-fixation of groundnut (Arachis hypogaea L.). Phosphorus fertilizer (16 kg P ha^–1) did not affect pod yields. Addition of MN fertilizer (100 kg Fetrilon Combi 1 ha^–1; P + MN) containing 0.1% molybdenum (Mo) increased pod yield by 37–86%. Nitrogen concentration in shoots at mid pod filling (72 days after planting) were higher in P + MN than in P – MN fertilizer treatment. Total N uptake increased from 53 (only P) to 108 kg N ha^–1 by additional MN application. Seed pelleting (P + MoSP) with 100 g Mo ha^–1 (MoO₃) increased nitrogenase activity (NA) by a factor of 2–4 compared to P treatment only. The increase in NA was mainly due to increase in nodule dry weight and to a lesser extent to increase in specific nitrogenase activity (SNA) per unit nodule dry weight. The higher NA of the P + MoSP treatment was associated with a higher total N uptake (55%) and pod yield (24%). Compared to P + MoSP or P + MN treatments application of N by mineral fertilizer (60 kg N ha^–1) or farmyard manure (130 kg N ha^–1) increased only yield of shoot dry matter but not pod dry matter. Plants supplied with N decreased soil water content more and were less drought tolerant than plants supplied with Mo. The data suggest that on the acid sandy soils in Niger N deficiency was a major constraint for groundnut production, and Mo availability in soils was insufficient to meet the Mo requirement for symbiotic N₂-fixation of groundnut.     

Ammonia and nitrous oxide emissions from grass and alfalfa mulches

Ammonia (NH₃) and nitrous oxide (N_-2O) emissions were measured in the field for three months from three different herbage mulches and from bare soil, used as a control. The mulches were grass with a low N-content (1.15% N in DM), grass with a high N-content (2.12% N in DM) and alfalfa with a high N-content (4.33% N in DM). NH₃ volatilization was measured using a micrometeorological technique. N_-2O emissions were measured using closed chambers. NH₃ and N_-2O emissions were found to be much higher from the N-rich mulches than from the low-N grass and bare soil, which did not differ significantly. Volatilization losses of NH₃ and N_-2O occurred mainly during the first month after applying the herbage and were highest from wet material shortly after a rain. The extent of NH₃-N losses was difficult to estimate, due to the low frequency of measurements and some problems with the denuder technique, used on the first occasions of measurements. Nevertheless, the results indicate that NH₃-N losses from herbage mulch rich in N can be substantial. Estimated losses of NH₃-N ranged from the equivalent of 17% of the applied N for alfalfa to 39% for high-N grass. These losses not only represent a reduction in the fertilizer value of the mulch, but also contribute appreciably to atmospheric pollution. The estimated loss of N_-2O-N during the measurement period amounted to 1% of the applied N in the N-rich materials, which is equivalent to at least 13 kg N_-2O-N ha^-1 lost from alfalfa and 6 kg ha^-1 lost from high-N grass. These emission values greatly exceed the 0.2 kg N_-2O-N ha^-1 released from bare soil, and thus contribute to greenhouse gas emissions.     

The influence of formula modifications and additives on ammonia loses from surface-applied urea-ammonium nitrate solutions

Urea-ammonium nitrate (UAN) solution fertilizers are subject to N loss through ammonia (NH₃) volatilization. This loss may be reduced by manipulation of the proportion of urea and by use of additives to reduce urea hydrolysis or increase fertilizer solution acidity. This research was design to study the effect of urea proportion in UAN solutions, added ammonium thiosulfate (ATS), and aquechem liquor (an industry by-product) on NH₃ loss from N solutions surface-applied to a range of agricultural soils.NH₃ volatilization from urea (U), ammonium nitrate (AN), and UAN solutions surface-applied on six eastern Canadian soils was investigated. Ammonia loss from urea solutions ranged from 23 to 55% of the applied N. Increased AN-N in UAN solutions caused a reduction of NH₃ loss greater than the reduction in urea. Less volatilization was observed with N solutions of higher acidity. This effect was more pronounced on a sandy soil than on clay soil.When ATS was added to UAN solution, a further reduction of NH₃ losses was observed. This reduction ranged from 12 to 23.5% in Dalhousie clay and Ste. Sophie sand soils, respectively. Addition of aquachem liquor (AqL) to the UAN solution did not consistently reduce NH₃ loss.Supported by a grant from the Natural Sciences and Engineering Research Council of Canada, and Nitrochem Inc., Canada.     

Sources of nitrous oxide in soils

Research to identify sources of nitrous oxide (N₂O) in soils has indicated that most, if not all, of the N₂O evolved from soils is produced by biological processes and that little, if any, is produced by chemical processes such as chemodenitrification. Early workers assumed that denitrification was the only biological process responsible for N₂O production in soils and that essentially all of the N₂O evolved from soils was produced through reduction of nitrate by denitrifying microorganisms under anaerobic conditions. It is now well established, however, that nitrifying microorganisms contribute significantly to emissions of N₂O from soils and that most of the N₂O evolved from aerobic soils treated with ammonium or ammonium-yielding fertilizers such as urea is produced during oxidation of ammonium to nitrate by these microorganisms. Support for the conclusion that chemoautotrophic nitrifiers such as Nitrosomonas europaea contribute significantly to production of N₂O in soils treated with N fertilizers has been provided by studies showing that N₂O emissions from such soils can be greatly reduced through addition of nitrification inhibitors such as nitrapyrin, which retard oxidation of ammonium by chemoautotrophic nitrifiers but do not retard reduction of nitrate by denitrifying microorganisms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Wheat Responses in Semiarid Northern Ethiopia to N2 Fixation by Pisum Sativum Treated with Phosphorous Fertilizers and Inoculant

Nitrogen fixation (N₂) by leguminous crops is a relatively low-cost alternative to N fertilizers for smallholder farmers in Africa. Nitrogen fixation in pea (Pisum sativum L. cv. Markos) as affected by phosphorus (P) fertilization (0, 30 kg P ha⁻¹) and inoculation (uninoculated and inoculated) in the semiarid conditions of Northern Ethiopia was studied using the ¹⁵N isotope dilution method and locally adapted barley (Hordeum vulgare L. cv. Bureguda) as reference crop. The effect of pea fixed nitrogen (N₂) on yield of the subsequent wheat crop (Triticum aestivum L.) was also assessed. Phosphorus and inoculation significantly influenced nodulation at the late flowering stage and also significantly increased P and N concentrations in shoots, and P concentration in roots, while P and N concentrations in nodules were not affected. Biomass, pods m⁻² and grain yield responded positively to P and inoculation, while seeds pod⁻¹ and seed weights were not significantly affected by these treatments. Phosphorus and inoculation enhanced the percentage of N derived from the atmosphere in the whole plant ranging from 53 to 70%, corresponding to the total amount of N₂ fixed varying from 55 to 141 kg N ha⁻¹. Soil N balance after pea ranged from − 9.2 to 19.3 kg N ha⁻¹ relative to following barley, where barley extracted N on the average of 6.9 and 62.0 kg N ha⁻¹ derived from fertilizer and soil, respectively. Beneficial effects of pea fixed N₂ on yield of the following cereal crop were obtained, increasing the average grain and N yields of this crop by 1.06 Mg ha⁻¹ and 33 kg ha⁻¹, respectively, relative to the barley–wheat monocrop rotation. It can be concluded that pea can be grown as an alternative crop to fallow, benefiting farmers economically and increasing the soil fertility.     

Fertilizers, agronomy and N2O

N₂O is emitted from agricultural soils due to microbial transformation of N from fertilizers, manures and soil N reserves. N₂O also derives from N lost from agriculture to other ecosystems: as NH₃ or through NO ₃ ^- leaching. Increased efficiency in crop N uptake and reduction of N losses should in principle diminish the amount of N₂O from agricultural sources. Precision in crop nutrient management is developing rapidly and should increase this efficiency. It should be possible to design guidelines on good agricultural practices for low N₂O emissions in special situations, e.g. irrigated agriculture, and for special operations, e.g. deep placement of fertilizers and manures. However, current information is insufficient for such guidelines. Slow-release fertilizers and fertilizers with inhibitors of soil enzymatic processes show promise as products which give reduced N₂O emissions, but they are expensive and have had little market penetration. Benefits and possible problems with their use needs further clarification.     

Nitrogen fertigation of greenhouse-grown strawberries

Two greenhouse experiments were conducted with strawberries (Fragaria ananassa) grown in plastic pots filled with 12 kg of soil, and irrigated by drip to evaluate the effect of 3 N levels and 3 N sources. The N levels were 3.6, 7.2 or 10.8 mmol Nl^–1 and the N sources were urea, ammonium nitrate and potassium nitrate for supplying NH₄/NO₃ in mmol Nl^–1 ratios of 7/0, 3.5/3.5 or 0/7, respectively. Both experiments were uniformly supplied with micronutrients and 1.7 and 5.0 mmoll^–1 of P and K, respectively. The fertilizers were supplied through the irrigation stream with every irrigation. The highest yield was obtained with the 7.2 mmol Nl^–1 due to increase in both weight and number of fruits per plant. With this N concentration soil ECe and NO₃-N concentration were kept at low levels. Total N and NO₃-N in laminae and petioles increased with increasing N level. With the N sources the highest yield was obtained with urea due to better fruit setting. The N source had no effect on soil salinity and residual soil NO₃-N; residual NH₄-N in the soils receiving urea and ammonium nitrate were at low levels.     

Effects of N management on growth, N2 fixation and yield of soybean

Soybean (Glycine max) is one of the most importantfood and cash crops in China. Although soybean has the capacity to obtain alarge proportion of its N from N₂ fixation, it is commonfarmer's practice to apply an N top dressing to maximize grain yield. Afield experiment was conducted to study the effects of N application as urea atvarious stages during the vegetative and reproductive phases on crop biomass,N₂ fixation and yield of two soybean genotypes, Luyuebao and Jufeng.Starter N at 25 kg ha^–1 resulted in minimumbiomass and pod yield while starter N at 75 kgha^–1 had no significant effect and N top dressing, ateither the R1 or R5 stage, resulted in increased biomass and pod yield. Maximumbiomass and pod yield were obtained when a top dressing of 50 kgha^–1 was applied at the flowering stage. The effects oftop dressing on the capacity to fix N₂ were complex. The proportionof plant N derived from N₂ fixation (Pfix) was highest when onlystarter N at 25 kg ha^–1 was applied. Any topdressing reduced nodulation and Pfix, but increased biomass, so that totalN₂ fixed increased for top dressing at the flowering or pod fillingstage. Common farmer's practice of applying 75 kg Nha^–1 at the V4 stage, resulted in a significantreduction in N₂ fixation. To evaluate the application of Nfertilization at various stages ofdevelopment on growth, nodulation and N₂ fixation in more detail, anexperiment in nutrient solution with or without 20 mMNO₃ ^– was conducted with genotype Tidar. The N-freetreatment gave the lowest biomass and total N accumulation, as in the fieldexperiment. A continuous nitrate supply resulted in the highest biomass,associated with an increase in total leaf area per plant, maximum individualleaf area, branch and node number per plant, shoot/root ratio and leaf arearatio, compared to the N-free treatment. R1 was the most responsive stage fornitrate supply as well as for interruption of the nitrate supply. Since theresults from the field experiment were in agreement with thosefrom the experiment in nutrient solution in a greenhouse, the latter can beusedto predict crop performance in the field.     

Nitrogen assimilation by legumes - processes and ecological limitations

Legumes may feed on three different sources of nitrogen: nitrate, ammonium, and, due to symbiotic N₂ fixation, atmospheric dinitrogen. In all three cases ammonium is finally assimilated by the glutamine synthetase (GS) / glutamate synthase (GOGAT) system. NH ₄ ⁺ produced by nitrogenase in symbiosomes of legume nodules is released into the host cell cytosol where it is incorporated into amino acids and amides. The release of NH ₄ ⁺ into the cytosol appears to occur purely by diffusion. Therefore, the activity of the GS / GOGAT enzymes is decicive to avoid product inhibition of nitrogenase by NH ₄ ⁺ . No information is available on the mechanism of xylem loading with amides or ureides, a process that may play a key role in avoiding accumulation of amino acids in infected nodule cells. The same applies to phloem unloading of sucrose. Both transport processes, however, may determine the efficiency of N₂ fixation by legumes.There is no convincing evidence that N₂ fixation by legumes is generally limited by energy supply to nodules. On the other hand, N₂ fixation is often restricted by environmental constraints. Environmental stresses may limit N₂ fixation of legumes at four different levels:Rhizobium (Bradyrhizobium) multiplication in soil, rhizobial infection of roots, nodulation, and N₂ fixation. There is increasing evidence that, sufficient infection by effective rhizobial strains provided, N demand of the host plant determines the potential of N₂ fixation. Various environmental stresses and supply of mineral N reduce nodulation and nitrogenase activity without affecting total N concentration of the plant tissue. Stress-induced reduction of plant growth, however, results in an accumulation of free amino acids, amides, or ureides in shoots, roots, and nodules which may be responsible for the regulation of nodulation and nitrogenase activity via a feedback system. This implies that enhancement of N₂ fixation by legumes can be realized in two different ways: either by improvement of stress resistance and dry matter accumulation or by uncoupling of the feedback control.     

An assessment of N loss from agricultural fields to the environment in China

Using the 1997 IPCC Guidelines for National Greenhouse Gas Inventory Methodology, and statistical data from the China Agricultural Yearbook, we estimated that the direct N₂O emission from agricultural fields in China in 1990 was 0.282 Tg N. Based on micro-meteorological field measurement of NH₃ volatilization from agricultural fields in different regions and under different cropping systems, the total NH₃ volatilization from agricultural fields in China in 1990 was calculated to be 1.80 Tg N, which accounted for 11% of the applied synthetic fertilizer N. Ammonia volatilization from agricultural soil was related to the cropping system and the form of N fertilizer. Ammonia volatilization from paddy fields was higher than that from uplands, and NH₄HCO₃ had a higher potential of NH₃ volatilization than urea. N loss through leaching from uplands in north China accounted for 0.5–4.2% of the applied synthetic fertilizer N. In south China, the leaching of applied N and soil N from paddy fields ranged from 6.75 to 27.0 kg N ha^-1 yr^-1, while N runoff was between 2.45 and 19.0 kg N ha^-1 yr^-1.     

Chemical interactions between soil N and alkaline-hydrolysing N fertilizers

Chemical interactions between soil N and alkaline-hydrolysing N fertilizers labelled with¹⁵N were studied in the laboratory using twelve-irradiated soils. Fertilizer was recovered in the soil organic N fraction via the process of NH₃ fixation. NH₃ fixation at day 7 varied from 1.8 to 4.6% of the N added as aqua ammonia at 1000 mg kg^–1 soil. The amount of NH₃ fixed increased with increasing rates of application of NH_3(aq) and urea. The rate of NH₃ fixation decreased with time, with more than 55% of the total NH₃ fixation in 28 days occurring in the first week following application of 2000 mg urea-N kg^–1 soil. Soil pH and NH₃ fixation varied in response to N source, and increased in the order of di-ammonium phosphate 3 fixation, resulting in the release of unlabelled ammonium (deamination) and a real added nitrogen interaction in all but two of the soils studied. The release of NH ₄ ⁺ initially increased up to a pH of 7.5, was inhibited between pH 8.5 and 9.0, but increased thereafter. The balance (N_bal) between NH₃ fixation and deamination was either positive or negative, depending on the pH of the fertilized soil, which was directly related to N source and concentration for a given soil.     

Nitrogen dynamics in paddy field as influenced by free-air CO2 enrichment (FACE) at three levels of nitrogen fertilization

As part of a FACE (free-air CO₂ enrichment) experiment in a rice paddy field in Shizukuishi (Iwate Prefecture, Japan), studies were conducted to determine the effects of elevated CO₂ on N dynamics at three levels of N application. Rice plants were grown under ambient CO₂ or ambient + 200 ppmV CO₂ conditions throughout the growing season in an Andosol soil with each treatment having 4 replicated plots. Three levels of N fertilizer (high, standard and low; 15, 9 and 4 gN m^–2, respectively) were applied to examine different N availability under both CO₂ conditions. Soil samples were collected at 4 different times from upper and lower soil layers (0–1 cm and 1–10 cm soil depths, respectively) and analyzed for microbial biomass N (B_N), mineralizable N (Min. N) and NH₄ ⁺-N in soil. Plant sampling was also done at 3 different times during the growing season to determine the N uptake by plant. Elevated CO₂ significantly increased B_N and Min. N in the upper soil layer at harvest by 25–42% and 18–24%, respectively, compared to ambient CO₂, regardless of N application rate. In low N soil, these significant increases were also observed at the ripening stage. In addition, elevated CO₂ only significantly increased the NH₄ ⁺-N in the upper soil layer at harvest in low N soil compared to ambient CO₂. The N uptake was not significantly affected by CO₂ treatment. These results indicate that elevated CO₂ had significant positive influence on B_N and Min. N in the upper soil layer in paddy soil at the later period of the cropping season at all levels of N application rates, but only at low levels of application rate on NH₄ ⁺-N.     

The role of <Superscript>15</Superscript>N-depleted fertilizers as tracers in N cycling studies in agroecosystems

A search of the literature (principally Google Scholar, using the keywords given below) revealed that 120 research papers have been published where ¹⁵N-depleted (¹⁴N-enriched) fertilizers, including ammonium sulfate, ammonium nitrate, urea and ammonia forms were used as tracers. The studies included annual cereals, vegetables, a grain legume and a fibre crop, and perennial crops including grasses, shrubs (berries), trees (fruits and nuts) and N₂-fixing forage, grain and woody legumes and an actinorhizal species. Other minor examples include perennial ornamentals and trees harvested for wood. Applications included estimation of fertilizer recovery in crops, in post-harvest soil and estimation of N losses in the soil–plant system by mass balance. The residual value of ¹⁵N-depleted fertilizers and the recovery of ¹⁵N-depleted legume residues have also been reported. ¹⁵N depleted fertilizers have played an important role in differentiating the relative uptake of fertilizer N to N mobilized in storage tissues with respect to the N nutrition of deciduous horticultural trees, shrubs and vines in spring. The symbiotic dependence of N₂-fixing species and transfer to companion non-legumes has been indirectly determined using ¹⁵N-depleted fertilizers. Thus ¹⁵N-depleted fertilizers have shared the same applications as their ¹⁵N-enriched counterparts. Because of their relative cheapness compared with ¹⁵N-enriched fertilizers, larger and more representative unconfined plots can be employed in field studies.     

Ammonia losses from ammonium-containing and -forming fertilizers after surface application at different rates on alkaline soils

The objective of this investigation was to compare the susceptibility of different ammonium containing and forming fertilizers to NH₃ losses and to determine the effect of application rates on NH₃ volatilization. Losses of NH₃ from five fertilizers, namely (NH₄)₂SO₄, CAN, urea, MAP and DAP were determined. The fertilizers were surface-applied to a sandy clay loam Arniston soil and a clayey Gelykvlakte soil of which the pH values were respectively 9.0 and 8.9. The application levels were equivalent to 0, 15, 30, 60, 120 and 240 kg N ha^–1. After a contact period of 3 days NH₃ losses were determined. Ammonia was lost from both soils under all treatments. More NH₃ was lost from the clayey Gelykvlakte soil compared with the sandy clay loam Arniston soil. Loss of NH₃ from the various fertilizers was ranked as follows: Urea > DAP > (NH₄)₂SO₄ > MAP > CAN. Ammonia losses increased with increasing application rates, but the proportion of N lost, decreased. Losses of NH₃ may be reduced by selective choice of fertilizer type and application rate.     

Anthropogenic emissions of NOX, NH3 and N2O in India

Emissions of NO_x, NH₃ and N₂O from anthropogenic activities in India have been estimated based on actual field measurements as well as available default methodologies. The NO_x emissions are mainly from the transport sector and contribute about 5% of the global NO_x emission from fossil fuel. NH₃ emissions from urea seems to be highly uncertain. However, emissions of NH₃ from fertilizers and livestock are estimated to be 1175 Gg and 1433 Gg, respectively. N₂O emissions seem to be derived predominantly from fertilizer applications, resulting in the release of 199–279 Gg N₂O. Other sources of N₂O, viz. agricultural residue burning, biomass burning for energy and nitric acid production are estimated to be 3, 35–187 and 2–7 Gg, respectively.