Differences in isotopic fractionation of nitrogen in water-saturated and unsaturated soils

Temporal variations in δ¹⁵N of NH₄⁺ and NO₃⁻ in water-saturated and unsaturated soils were examined in a laboratory incubation study. Ammonium sulfate (δ¹⁵N=−2.6‰) was added to 25 g samples of soil at concentrations of 160 mg N kg⁻¹. Soils were then incubated under unsaturated (50% of water holding capacity at saturation, WHC) or saturated (100% of WHC) water conditions for 7 and 36 d, respectively. During 7 d incubation of unsaturated soil, the NH₄⁺-N concentration decreased from 164.8 to 34.4 mg kg⁻¹, and the δ¹⁵N of NH₄⁺ increased from −0.4 to +57.2‰ through nitrification, as evidenced by corresponding increase in NO₃⁻-N concentration and lower δ¹⁵N of NO₃⁻ (product) than that of NH₄⁺ (substrate) at each sampling time. In saturated soil, the concentration of NH₄⁺-N decreased gradually from 162.4 to 24.2 mg kg⁻¹, and the δ¹⁵N values increased from +0.8 to +21.0‰ during 36 d incubation. However, increase in NO₃⁻ concentration was not observed due to loss of NO₃⁻ through concurrent denitrification in anaerobic sites. The apparent isotopic fractionation factors (α_s/p) associated with decrease in NH₄⁺ concentration were 1.04 and 1.01 in unsaturated and saturated soils, respectively. Since nitrification is likely to introduce greater isotope fractionation than microbial immobilization, the higher value for unsaturated soil probably reflected faster nitrification under aerobic conditions. The lower value for saturated soil suggests that immobilization and subsequent remineralization of NH₄⁺ were relatively more dominant than nitrification under the anaerobic conditions.     

Pathways and dynamics of NO3 and NH4 applied in a mountain Picea abies forest and in a nearby meadow in central Switzerland

To evaluate the pathways and dynamics of inorganic nitrogen (N) deposition in previously N-limited ecosystems, field additions of ¹⁵N tracers were conducted in two mountain ecosystems, a forest dominated by Norway spruce (Picea abies) and a nearby meadow, at the Alptal research site in central Switzerland. This site is moderately impacted by N from agricultural and combustion sources, with a bulk atmospheric deposition of 12 kg N ha⁻¹ y⁻¹ equally divided between NH₄⁺ and NO₃⁻. Pulses of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ were applied separately as tracers on plots of 2.25 m². Several ecosystem pools were sampled at short to longer-term intervals (from a few hours to 1 year), above and belowground biomass (excluding trees), litter layer, soil LF horizon (approx. 5-0 cm), A horizon (approx. 0-5 cm) and gleyic B horizon (5-20 cm). Furthermore, extractable inorganic N, and microbial N pools were analysed in the LF and A horizons. Tracer recovery patterns were quite similar in both ecosystems, with most of the tracer retained in the soil pool. At the short-term (up to 1 week), up to 16% of both tracers remained extractable or entered the microbial biomass. However, up to 30% of the added ¹⁵NO₃⁻ was immobilised just after 1 h, and probably chemically bound to soil organic matter. 16% of the NH₄⁺ tracer was also immobilised within hours, but it is not clear how much was bound to soil organic matter or fixed between layers of illite-type clay. While the extractable and microbial pools lost ¹⁵N over time, a long-term increase in ¹⁵N was measured in the roots. Otherwise, differences in recovery a few hours after labelling and 1 year later were surprisingly small. Overall, more NO₃⁻ tracer than NH₄⁺ tracer was recovered in the soil. This was due to a strong aboveground uptake of the deposited NH₄⁺ by the ground vegetation, especially by mosses.     

Variation in NH4 mineralization and microbial communities with stand age in lodgepole pine (Pinus contorta) forests, Yellowstone National Park (USA)

Soils and vegetation were analyzed in 20 lodgepole pine (Pinus contorta) forest stands, varying in age from 50 to 350 years, that had initiated following stand-replacing fire. Our goal was to determine how nitrogen availability (NH₄⁺-N) and microbial community composition varied with stand age-class and to determine whether differences could be explained by canopy, soil, or understory characteristics. Gross NH₄⁺ mineralization was measured using laboratory isotopic pool dilution, and microbial community composition was evaluated using microbial membrane lipids. The microbial community composition of stands in the 300-350 age class was distinct from stands in younger age classes. Microbial community composition among sites varied with pH, % organic matter, and phosphorus. Gross NH₄⁺ mineralization rates averaged 1.45±0.07 mg NH₄⁺ kg soil⁻¹ d⁻¹ while consumption averaged 1.37±0.20 mg NH₄⁺ kg soil⁻¹ d⁻¹, resulting in low net NH₄⁺ mineralization rates (0.08±0.18 mg NH₄⁺ kg soil⁻¹ d⁻¹), but rates were not significantly different with stand age-class at p<0.05. At p<0.10, net NH₄⁺ mineralization was significantly higher in the 300-350 age class compared to the 125-175 age class. None of the measured variables significantly explained NH₄⁺ consumption and net mineralization patterns. However, gross NH₄⁺ mineralization rates were best explained by information on microbial community structure (i.e. lipids). Variation among stands within a given age-classes was high, indicating that patterns of N cycling across landscapes reflect substantial heterogeneity among mature stands.     

Impacts of anthropogenic N additions on nitrogen mineralization from plant litter in exotic annual grasslands

Urban regions of southern California receive up to 45 kg N ha^-1 y^-1 from nitrogen (N) deposition. A field decomposition study was done using ¹⁵N-labelled litter of the widespread exotic annual grass Bromus diandrus to determine whether elevated soil N is strictly from N deposition or whether N mineralization rates from litter are also increased under N deposition. Tissue N and lignin concentrations, which are inversely related in field sites with high and low N deposition, determine the rate at which N moves from plant litter to soil and becomes available to plants. The effect of soil N on N movement from litter to soil was tested by placing litter on high and low N soil in a factorial experiment with two levels of litter N and two levels of soil N. The litter quality changes associated with N deposition resulted in faster rates of N cycling from litter to soil. Concentrations of litter-derived N in total N, NH₄⁺, NO₃⁻, microbial N and organic N were all higher from high N/low lignin litter than from low N/high lignin litter. Litter contributed more N to soil NH₄⁺ and microbial N in high N than low N soil. At the end of the study, N mineralized from high N litter on high N soil accounted for 46% of soil NH₄⁺ and 11% of soil NO₃⁻, compared to 35% of soil NH₄⁺ and 6% of soil NO₃⁻ from low N litter on low N soil. The study showed that in high N deposition areas, elevated inorganic soil N concentrations at the end of the summer N deposition season are a result of N mineralized from plant litter as well as from N deposition.     

Role of soil drying in nitrogen mineralization and microbial community function in semi-arid grasslands of north-west Australia

We examined effects of wetting and then progressive drying on nitrogen (N) mineralization rates and microbial community composition, biomass and activity of soils from spinifex (Triodia R. Br.) grasslands of the semi-arid Pilbara region of northern Australia. We compared soils under and between spinifex hummocks and also examined impacts of fire history on soils over a 28 d laboratory incubation. Soil water potentials were initially adjusted to −100 kPa and monitored as soils dried. We estimated N mineralization by measuring changes in amounts of nitrate (NO₃⁻-N) and ammonium (NH₄⁺-N) over time and with change in soil water potential. Microbial activity was assessed by amounts of CO₂ respired. Phospholipid fatty acid (PLFA) analyses were used to characterize shifts in microbial community composition during soil drying. Net N mineralized under hummocks was twice that of open spaces between hummocks and mineralization rates followed first-order kinetics. An initial N mineralization flush following re-wetting accounted for more than 90% of the total amount of N mineralized during the incubation. Initial microbial biomass under hummocks was twice that of open areas between hummocks, but after 28 d microbial biomass was<2 μ g⁻¹ ninhydrin N regardless of position. Respiration of CO₂ from soils under hummocks was more than double that of soils from between hummocks. N mineralization, microbial biomass and microbial activity were negligible once soils had dried to −1000 kPa. Microbial community composition was also significantly different between 0 and 28 d of the incubation but was not influenced by burning treatment or position. Regression analysis showed that soil water potential, microbial biomass N, NO₃⁻-N, % C and δ¹⁵N all explained significant proportions of the variance in microbial community composition when modelled individually. However, sequential multiple regression analysis determined only microbial biomass was significant in explaining variance of microbial community compositions. Nitrogen mineralization rates and microbial biomass did not differ between burned and unburned sites suggesting that any effects of fire are mostly short-lived. We conclude that the highly labile nature of much of soil organic N in these semi-arid grasslands provides a ready substrate for N mineralization. However, process rates are likely to be primarily limited by the amount of substrate available as well as water availability and less so by substrate quality or microbial community composition.     

Comparative Effect of Nitrogen Forms on Nitrogen Uptake and Cotton Growth Under Salinity Stress

The effects of nitrogen (N) forms (ammonium- or nitrate-N) on plant growth under salinity stress [150 mmol sodium chloride (NaCl)] were studied in hydroponically cultured cotton. Net fluxes of sodium (Na⁺), ammonium (NH₄⁺), and nitrate (NO₃^?) were also determined using the Non-Invasive Micro-Test Technology. Plant growth was impaired under salinity stress, but nitrate-fed plants were less sensitive to salinity than ammonium-fed plants due mainly to superior root growth by the nitrate-fed plants. The root length, root surface area, root volume, and root viability of seedlings treated with NO₃-N were greater than those treated with NH₄-N with or without salinity stress. Under salinity stress, the Na⁺ content of seedlings treated with NO₃-N was lower than that in seedlings treated with NH₄-N owing to higher root Na⁺ efflux. A lower net NO₃^? efflux was observed in roots of nitrate-fed plants relative to the net NH₄⁺ efflux from roots of ammonium-fed plants. This resulted in much more nitrogen accumulation in different tissues, especially in leaves, thereby enhancing photosynthesis in nitrate-fed plants under salinity stress. Nitrate-N is superior to ammonium-N based on nitrogen uptake and cotton growth under salinity stress.     

Simultaneous measurement of S, macronutrients, and heavy metals in the soil microbial biomass with CHCl3 fumigation and NH4NO3 extraction

The study was carried out to investigate whether 1 M NH₄NO₃ extraction is a useful alternative to 10 mM CaCl₂ extraction for estimating soil microbial biomass S and whether the data of CHCl₃-labile NH₄NO₃-extractable macronutrients and heavy metals are useful and in agreement with the available data on element concentrations in soil microorganisms. Microbial biomass C was followed by microbial biomass S after CaCl₂ extraction with an average C/S ratio of 82, and by microbial biomass S after NH₄NO₃ extraction with an average C/S ratio of 57. The mean contribution of CHCl₃-labile metals in relation to the NH₄NO₃-extractable fraction from non-fumigated soils ranged from 0.1 to 112% in the order potassium < magnesium < cadmium < sodium < zinc + nickel < manganese < copper. The mean contribution of CHCl₃-labile metals in relation to the microbial biomass C ranged from 0.03 to 22‰ in the order cadmium < nickel < zinc < manganese < magnesium < copper < sodium < potassium. These relative contributions varied within the different metals from a 4-fold (Na⁺) to a more than 200-fold range (Cu²⁺). Significant positive correlations with microbial biomass C were observed for CHCl₃-labile zinc, sodium and especially potassium. The concentration of all elements except copper in relation to microbial biomass C were in the range known from the limited literature on fungi grown on heavy metal contaminated soils.     

不同形态氮素比例对不同小白菜品种生物量和硝酸盐含量的影响

利用NO-3 N∶NH 4 N为 10 0∶0、5 0∶5 0和 0∶10 0三个硝铵配比的营养液对 12个不结球小白菜品种进行水培试验。结果表明 :不同的硝铵配比对不同品种小白菜的生物量、叶绿素SPAD值、硝酸盐积累量等有着显著的影响 ,同一氮源培养下不同的小白菜品种间也表现出显著的差异 ;12个小白菜品种叶绿素SPAD值随营养液中的NH 4 N比例的增加而升高 ,两者间存在着显著的正相关 ;单株生物量除亮白叶 1号和五月慢在全硝培养中生物量较其他两种配比大外 ,其他 10个品种均在 5 0∶5 0硝铵营养液中表现最好 ;供试的 12个小白菜品种中有 9个品种的硝酸盐积累量随着NH 4 N比例的增加而下降 ,表明适当地配施铵态氮较纯硝营养液能获得更好的产量、更高的叶绿素SPAD值和较低的硝酸盐积累量。     

Animal treading stimulates denitrification in soil under pasture

The effects of animal treading on denitrification in a mixed ryegrass-clover pasture were studied. A single treading event of moderate or severe intensity was applied in plots during spring by using dairy cows at varying stocking rates (4.5 cows 100 m⁻² for 1.5 or 2.5 h, respectively). Treading caused a significant short-term 21 days) increase in denitrification. Denitrification rates reached a maximum of 52 g N₂O-N ha⁻¹ day⁻¹ at 8 days after severe treading compared to 2.3 g N₂O-N ha⁻¹ day⁻¹ under nil treading. Thereafter, denitrification rates declined, and were similar to non-trodden control plots after 28 days. Soil aeration, was significantly reduced by treading as expressed by water-filled porosity. In addition, soil NH₄⁺-N and NO₃⁻-N concentrations were also increased by treading. We propose that the underlying processes involved in increasing denitrification under treading were two-fold. Firstly, treading caused a temporary (e.g. 3 days after treading) reduction in soil aeration through soil physical damage, and secondly, reduced soil N utilisation prompted by reduced plant growth led to increased soil NH₄⁺-N and NO₃⁻-N availability. This study shows that treading, without the influence of other grazing animal factors (e.g. excretion), can cause a large short-term stimulation of denitrification in grass-clover pastures.     

Impact of land-use types on soil nitrogen net mineralization in the sandstorm and water source area of Beijing,China

Changes of land-use type (LUT) can affect soil nutrient pools and cycling processes that relate long-term sustainability of ecosystem, and can also affect atmospheric CO₂ concentrations and global warming through soil respiration. We conducted a comparative study to determine NH₄⁺ and NO₃⁻ concentrations in soil profiles (0–200 cm) and examined the net nitrogen (N) mineralization and net nitrification in soil surface (0–20 cm) of adjacent naturally regenerated secondary forests (NSF), man-made forests (MMF), grasslands and cropland soils from the windy arid and semi-arid Hebei plateau, the sandstorm and water source area of Beijing, China. Cropland and grassland soils showed significantly higher inorganic N concentrations than forest soils. NO₃⁻-N accounted for 50–90% of inorganic N in cropland and grassland soils, while NH₄⁺-N was the main form of inorganic N in NSF and MMF soils. Average net N-mineralization rates (mg kg⁻¹ d⁻¹) were much higher in native ecosystems (1.51 for NSF soils and 1.24 for grassland soils) than in human disturbed LUT (0.15 for cropland soils and 0.85 for MMF soils). Net ammonification was low in all the LUT while net nitrification was the major process of net N mineralization. For more insight in urea transformation, the increase in NH₄⁺ and, NO₃⁻ concentrations as well as C mineralization after urea addition was analyzed on whole soils. Urea application stimulated the net soil C mineralization and urea transformation pattern was consistent with net soil N mineralization, except that the rate was slightly slower. Land-use conversion from NSF to MMF, or from grassland to cropland decreased soil net N mineralization, but increased net nitrification after 40 years or 70 years, respectively. The observed higher rates of net nitrification suggested that land-use conversions in the Hebei plateau might lead to N losses in the form of nitrate.     

不同铵硝比对菠菜生长、安全和营养品质的影响

通过水培试验,研究了等氮条件下5种不同铵硝比对菠菜生长和品质的影响。结果表明:(1)从铵硝比100∶0到0∶100,菠菜地上部鲜重不断增加,铵硝比为0∶100时,菠菜的鲜重达最大值;但铵硝比25∶75和0∶100两个处理菠菜的干物重没有显著差异(p<0.05)。(2)随着铵硝比的降低,菠菜茎叶中硝酸盐、亚硝酸盐的含量均表现为线性增加;菠菜茎叶中可溶性草酸的含量和营养液中铵硝比之间呈现出二次曲线相关,在铵硝比为25∶75时,菠菜茎叶中草酸含量最低。适当增施铵态氮有利于降低菠菜硝酸盐、亚硝酸盐及草酸的含量。(3)增铵可以提高菠菜Vc含量,铵硝比为50∶50的处理菠菜Vc含量最高;随着铵硝比的下降,菠菜茎叶中可溶性糖的含量逐渐降低,而粗蛋白的含量则以铵硝比25∶75处理最高。     

Tropical savannah woodland: effects of experimental fire on soil microorganisms and soil emissions of carbon dioxide

Burning of the vegetation in the African savannahs in the dry season is widespread and may have significant effects on soil chemical and biological properties. A field experiment in a full factorial randomised block design with fire, ash and extra grass biomass as main factors was carried out in savannah woodland of the Gambella region in Ethiopia. The microbial biomass C (C_mic) was 52% (fumigation-extraction) and 20% (substrate-induced respiration) higher in burned than unburned plots 12 d after burning. Both basal respiration and potential denitrification enzyme activity (PDA) immediately responded to burning and increased after treatment. However, in burned plots addition of extra biomass (fuel load) led to a reduction of C_mic and PDA due to enhanced fire temperature. Five days after burning, there was a short-lived burst in the in situ soil respiration following rainfall, with twice as high soil respiration in burned than unburned plots. In contrast, 12 d after burning soil respiration was 21% lower in the burned plots, coinciding with lower soil water content in the same plots. The fire treatment resulted in higher concentrations of dissolved organic C (24-85%) and nitrate (47-76%) in the soil until 90 d after burning, while soil NH₄⁺-N was not affected to the same extent. The increase in soil NO₃⁻-N but not NH₄⁺-N in the burned plots together with the well-aerated soil conditions indicated that nitrifying bacteria were stimulated by fire and immediately oxidised NH₄⁺-N to NO₃⁻-N. In the subsequent rainy season, NO₃⁻-N and, consequently, PDA were reduced by ash deposition. Further, C_mic was lower in burned plots at that time. However, the fire-induced changes in microbial biomass and activity were relatively small compared to the substantial seasonal variation, suggesting transient effects of the low severity experimental fire on soil microbial functioning.     

Ammonium immobilisation during chloroform fumigation

The assumption in using the chloroform fumigation technique for microbial biomass determination is that microbes are killed or at least inactivated by the treatment. Problems associated with transformations of the N released on or during fumigation have so far only been associated with the fumigation-incubation method. A laboratory and a field study were carried out to investigate the possible N transformations during biomass determination by the fumigation-extraction method. Labelled NH₄NO₃ (either the NO₃⁻, NH₄⁺ or both pools were ¹⁵N enriched) was applied to the soil and biomass determinations made at intervals subsequently. The size and enrichment of the ammonium (NH₄⁺), and nitrate (NO₃⁻) pools were determined before and after chloroform fumigation. The ¹⁵N enrichment of the NH₄⁺ pool after fumigation could only be explained if immobilisation of ammonium occurred at some time during the 24 h fumigation period. The extent of this immobilisation was calculated. In addition, there was evidence that nitrification occurred during the fumigation procedure at the start of the laboratory study and throughout the field study. The laboratory and field study differed mainly in the dynamics related to NO₃⁻ uptake and release. There was evidence for uptake of NO₃⁻ by the microbial biomass with and without utilization. We conclude that the ¹⁵N enrichment in the microbial biomass cannot be accurately determined when N transformations and release of non-utilized N occurs during fumigation. The possible immobilisation of mineral N during fumigation will affect the magnitude of the factor used to convert measured microbial biomass N to actual microbial biomass N in soil.     

Microbial immobilization of ammonium and nitrate in relation to ammonification and nitrification rates in organic and conventional cropping systems

Agricultural systems that receive high or low organic matter (OM) inputs would be expected to differ in soil nitrogen (N) transformation rates and fates of ammonium (NH₄⁺) and nitrate (NO₃⁻). To compare NH₄⁺ availability, competition between nitrifiers and heterotrophic microorganisms for NH₄⁺, and microbial NO₃⁻ assimilation in an organic vs. a conventional irrigated cropping system in the California Central Valley, chemical and biological soil assays, ¹⁵N isotope pool dilution and ¹⁵N tracer techniques were used. Potentially mineralizable N (PMN) and hot minus cold KCl-extracted NH₄⁺ as indicators of soil N supplying capacity were measured five times during the tomato growing season. At mid-season, rates of gross ammonification and gross nitrification after rewetting dry soil were measured in microcosms. Microbial immobilization of NO₃⁻ and NH₄⁺ was estimated based on the uptake of ¹⁵N and gross consumption rates. Gross ammonification, PMN, and hot minus cold KCl-extracted NH₄⁺ were approximately twice as high in the organically than the conventionally managed soil. Net estimated microbial NO₃⁻ assimilation rates were between 32 and 35% of gross nitrification rates in the conventional and between 37 and 46% in the organic system. In both soils, microbes assimilated more NO₃⁻ than NH₄⁺. Heterotrophic microbes assimilated less NH₄⁺ than NO₃⁻ probably because NH₄⁺ concentrations were low and competition by nitrifiers was apparently strong. The high OM input organic system released NH₄⁺ in a gradual manner and, compared to the low OM input conventional system, supported a more active microbial biomass with greater N demand that was met mainly by NO₃⁻ immobilization.     

Acetylene and N-serve effects upon N2O emissions from NH4+ and NO3− treated soils under aerobic and anaerobic conditions

N₂O emissions from soils treated with NH₄⁺-N under aerobic conditions in the laboratory were 3- to 4-fold higher than those from controls (no extra N added) or when NO₃^?-N was added. Although the emission of N₂O-N in these field and laboratory experiments represented only 0.1–0.8% of the applied fertilizer NH₄⁺-N and are therefore not significant from an agronomic standpoint, these studies have conclusively demonstrated that the oxidation of applied ammoniacal fertilizers (nitrification) could contribute significantly to the stratospheric N₂O pool.Like N-serve, acetylene was shown to be a potent inhibitor of nitrification as it stopped the oxidation of NH₄⁺-N to (NO₃⁺-N + NO₂^?)-N and hence reduced the evolution of N₂O from nitrification within 60 min after its addition.Although high amounts of NO₃^?-N were present, the rate of denitrification was very low from soils with moisture up to 60% saturation. The further increase in the degree of saturation resulted in several-fold increase of denitrification which eventually became the predominant mechanism of gaseous N losses under anaerobic conditions.     

Long-term N addition effects on the C mineralization and DOC production in mor humus under spruce

This study was based on laboratory incubations of mor humus from two N fertilized stands of Norway spruce in Sweden (Skogaby and Stråsan), which had received repeated N additions (100 kg N ha⁻¹ yr⁻¹ as (NH₄)₂SO₄ at Skogaby and 35, 73 and 108 kg ha⁻¹ yr⁻¹ as NH₄NO₃ at Stråsan) during 8 and 24-29 years, respectively. The aim was to investigate long-term N effects on the mineralization of C and production of DOC. Mor humus (Oe and Oa) was incubated in columns at 20 °C for 49 days. Columns were leached once a week with artificial throughfall solution, which was analyzed for DOC, total N, NH₄⁺-N and NO₃⁻-N. Prior to each leaching event, CO₂ evolution from the columns was determined. C-to-N ratios in the N-treated Oe layers at Stråsan (21-24) and Skogaby (24) were significantly lower than those of the controls (Stråsan, 32; Skogaby, 28). The cumulative amount of CO₂-C showed a significant treatment effect in the Oe layer at Skogaby, i.e. 18 and 29 mg C g⁻¹ C in the N treatment and control, respectively. At Stråsan, the cumulative CO₂-C was significantly lower in the N3 treatment compared to the control in both layers (33 compared to 74 mg C g⁻¹ C in the Oe layer and 16 compared to 35 mg C g⁻¹ C in the Oa layer). Neither the DOC nor the DON production showed any significant treatment effects at the two sites. However, DOC was lower in the fertilized Oe layer at Skogaby throughout the incubation. The leaching of DON was highest in the Oe layers at both sites, and DON increased with time at Skogaby while there was a decreasing trend at Stråsan. The DOC-to-DON ratio tended to be lower in the fertilized Oe layers at both sites. The NH₄⁺ leaching at Skogaby decreased in the N-treated Oe and Oa layers. At Stråsan, NH₄⁺ from the Oe layer increased in N2 and control. The NO₃⁻ leaching was low and constant in both Skogaby layers. At Stråsan, NO₃⁻ increased in the Oe layer of N1. Cumulative CO₂ was positively correlated to C-to-N ratio (r2=0.71,p<0.01) and to cumulative DOC (r2=0.63,p<0.05) in the Oe layer at Stråsan. Our conclusion was that long-term N additions caused decreased C-to-N ratios and decreased CO₂ evolution rates. The correlation between CO₂ and C-to-N ratio in the Oe layers at Stråsan may be due to a changed quality of the fertilized forest floor material and presence of more N efficient microorganisms.     

Surface/atmosphere exchange and chemical interactions of reactive nitrogen compounds above a manured grassland

Manure application to managed grassland is a common agricultural practice. There are, however, limited studies looking at the fluxes and interactions of reactive N compounds and aerosols following fertilisation with manure. In this study, state-of-the-art chemical analysers (GRAEGOR, QCLAS, PTRMS) were used to investigate concentrations, fluxes and chemical interactions of reactive nitrogen containing trace gases (NH₃, HNO₃, HONO) and aerosols (NO₃⁻) above a grassland fertilised with 164 kg N ha⁻¹ of cattle slurry. Emissions of NH₃ peaked at >67 μg m⁻² s⁻¹, based on a 30 min average. The estimated overall loss of total ammoniacal nitrogen (TAN) from the applied slurry through NH₃ emissions in the first 5 days was 33.5%. The average trimethylamine flux in the first 31 h following the first slurry application was 40 ng m⁻² s⁻¹ and amounted to 0.38% of the NH₃-N emissions. Apparent nitrate aerosol emissions were observed following the slurry application peaking at 13.0 ng m⁻² s⁻¹. This suggests formation of NH₄NO₃ from reaction of the emitted NH₃ with atmospheric HNO₃, consistent with the observation of gaseous concentration products exceeding the dissociation constants of ammonium nitrate. Fluxes of total nitrate (HNO₃ + NO₃⁻) were bi-directional and positive during the mid-day period after fertilisation, suggesting that the slurry acted as a net source for these compounds. There is evidence of small HONO emission following fertilisation (up to 1 ng m⁻² s⁻¹), although the production process is currently not identified. By contrast, all compounds showed deposition to the adjacent unfertilised grassland.     

Effects of repeated fertilizer and cattle slurry applications over 38 years on N dynamics in a temperate grassland soil

The effects of repeated synthetic fertilizer or cattle slurry applications at annual rates of 50, 100 or 200 m³ ha⁻¹ yr⁻¹ over a 38 year period were investigated with respect to herbage yield, N uptake and gross soil N dynamics at a permanent grassland site. While synthetic fertilizer had a sustained and constant effect on herbage yield and N uptake, increasing cattle slurry application rates increased the herbage yield and N uptake linearly over the entire observation period. Cattle slurry applications, two and four times the recommended rate (50 m³ ha⁻¹ yr⁻¹, 170 kg N ha⁻¹), increased N uptake by 46 and 78%, respectively after 38 years. To explain the long-term effect, a ¹⁵N tracing study was carried out to identify the potential change in N dynamics under the various treatments. The analysis model evaluated process-specific rates, such as mineralization, from two organic-N pools, as well as nitrification from NH₄⁺ and organic-N oxidation. Total mineralization was similar in all treatments. However, while in an unfertilized control treatment more than 90% of NH₄⁺ production was related to mineralization of recalcitrant organic-N, a shift occurred toward a predominance of mineralization from labile organic-N in the cattle slurry treatments and this proportion increased with the increase in slurry application rate. Furthermore, the oxidation of recalcitrant organic-N shifted from a predominant NH₄⁺ production in the control treatment, toward a predominant NO₃⁻ production (heterotrophic nitrification) in the cattle slurry treatments. The concomitant increase in heterotrophic nitrification and NH₄⁺ oxidation with increasing cattle slurry application rate was mainly responsible for the increase in net NO₃⁻ production rate. Thus the increase in N uptake and herbage yield on the cattle slurry treatments could be related to NO₃⁻ rather than NH₄⁺ production. The ¹⁵N tracing study was successful in revealing process-specific changes in the N cycle in relationship to long-term repeated amendments.     

Short-term competition between crop plants and soil microbes for inorganic N fertilizer

Agricultural systems that receive high amounts of inorganic nitrogen (N) fertilizer in the form of either ammonium (NH₄⁺), nitrate (NO₃⁻) or a combination thereof are expected to differ in soil N transformation rates and fates of NH₄⁺ and NO₃⁻. Using ¹⁵N tracer techniques this study examines how crop plants and soil microbes vary in their ability to take up and compete for fertilizer N on a short time scale (hours to days). Single plants of barley (Hordeum vulgare L. cv. Morex) were grown on two agricultural soils in microcosms which received either NH₄⁺, NO₃⁻ or NH₄NO₃. Within each fertilizer treatment traces of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ were added separately. During 8 days of fertilization the fate of fertilizer ¹⁵N into plants, microbial biomass and inorganic soil N pools as well as changes in gross N transformation rates were investigated. One week after fertilization 45-80% of initially applied ¹⁵N was recovered in crop plants compared to only 1-10% in soil microbes, proving that plants were the strongest competitors for fertilizer N. In terms of N uptake soil microbes out-competed plants only during the first 4 h of N application independent of soil and fertilizer N form. Within one day microbial N uptake declined substantially, probably due to carbon limitation. In both soils, plants and soil microbes took up more NO₃⁻ than NH₄⁺ independent of initially applied N form. Surprisingly, no inhibitory effect of NH₄⁺ on the uptake and assimilation of nitrate in both, plants and microbes, was observed, probably because fast nitrification rates led to a swift depletion of the ammonium pool. Compared to plant and microbial NH₄⁺ uptake rates, gross nitrification rates were 3-75-fold higher, indicating that nitrifiers were the strongest competitors for NH₄⁺ in both soils. The rapid conversion of NH₄⁺ to NO₃⁻ and preferential use of NO₃⁻ by soil microbes suggest that in agricultural systems with high inorganic N fertilizer inputs the soil microbial community could adapt to high concentrations of NO₃⁻ and shift towards enhanced reliance on NO₃⁻ for their N supply.     

不同铵硝配比对弱光下白菜氮素吸收及相关酶的影响

以黑色遮阳网覆盖模仿弱光环境, 使光照强度为自然光的20%左右, 以自然光照为对照, 采用精确控制水培溶液氮素营养, 研究NH₄⁺-N/NO₃^--N 比例分别为0/100、25/75、50/50、75/25、100/0 对弱光下白菜氮代谢及硝酸还原酶和谷氨酰胺合成酶活性的影响。结果表明, 弱光下, 白菜的鲜重及叶片总氮量以NH₄⁺-N/NO₃^--N 比为25/75 时最大, NH₄⁺-N/NO₃^--N 比为100/0 时最低。随弱光处理的进行, 白菜叶片中硝酸还原酶活性及谷氨酰胺合成酶活性均呈下降趋势, 但NH₄⁺-N/NO₃^--N 比为25/75 时, 可维持叶片内较高的硝酸还原酶活性及谷氨酰胺合成酶活性。试验表明, NH₄⁺-N/NO₃^--N 比25/75 是白菜在弱光下生长的较适宜氮素形态配比。