Effect of Sesbania,Azolla and rice straw incorporation on the kinetics of NH4, K,Fe, Mn,Zn and P in some flooded rice soils

In a greenhouse study, with and without rice plants, of five flooded Philippine rice soils whose organic C (OC) content varied from 0.5 to 3.6%, incorporation ofSesbania rostrata, Azolla microphylla and rice straw affected the kinetics of soil solution NH ₄ ⁺ −N, K⁺, Fe²⁺, Mn²⁺, Zn²⁺, and P. Sesbania and Azolla increased NH ₄ ⁺ −N concentration above the control treatment, whereas rice straw depressed it. In all soils Azolla released less NH ₄ ⁺ −N than Sesbania. The apparent net N release depended on the soil and ranged from 44–81% for Sesbania and 27–52% for Azolla. These effects persisted throughout the growth of IR36. Soil solution and exchangeable NH ₄ ⁺ −N increased initially but levelled off between 30 to 80 days and between 20 to 40 days after flooding (DF), respectively. With rice, soil solution NH ₄ ⁺ −N concentration, reached a peak at 15–40 DF and declined to very low levels (<4mg L⁻¹). In the 3 soils of low OC content nitrogen derived from green manure ranged from 34–53% and the apparent revovery of added green manure N varied from 29–67%. Almost all N released from both Azolla and Sesbania were recovered in the rice plant in all soils except Concepcion with only 77%. The concentration of K⁺, Fe²⁺, Mn²⁺ and P in the soil solution were higher with rice straw than Sesbania and Azolla in all soils except Hanggan which showed no change in Fe²⁺ and Mn²⁺ but increased K⁺ and P. In general, rice straw, Sesbania and Azolla decreased Zn²⁺ concentration in all soils.     

Ammonium versus Nitrate Nutrition of Plants Stimulates Microbial Activity in the Rhizosphere

Using an alkaline calcareous soil, pot experiments were conducted to elucidate the effects of NH ₄ ⁺ vs. NO ₃ ⁻ nutrition (50 or 100 mg kg⁻¹ soil) of wheat and maize on microbial activity in the rhizosphere and bulk soils. Dicyandiamide was used as nitrification inhibitor to maintain NH ₄ ⁺ as the predominant N source for plants grown in NH ₄ ⁺ -treated soil. While maize grew equally well on both N sources, root and shoot growth of wheat was higher under NH ₄ ⁺ than under NO ₃ ⁻ nutrition. Bacterial population density on roots, but not in the rhizosphere soil, was higher under NH ₄ ⁺ than under NO ₃ ^– supplied at 150 mg N kg⁻¹ soil; whereas at both N levels applied, NH ₄ ⁺ compared to NO ₃ ⁻ nutrition of wheat and maize significantly increased microbial biomass in the rhizosphere soil. Under both plant species, NH ₄ ⁺ vs. NO ₃ ⁻ nutrition also increased aerobic and anaerobic respiration, and dehydrogenase activity in the rhizosphere. As microbial activity in the planted bulk and unplanted soils was hardly affected by the N-source, we hypothesize that the stimulation by NH ₄ ⁺ of the rhizosphere microbial activity was probably due to higher availability of root exudates under NH ₄ ⁺ than under NO ₃ ⁻ nutrition.     

Plant and microbial nitrogen use and turnover: Rapid conversion of nitrate to ammonium in soil with roots

Immobilization of ammonium (NH ₄ ⁺ ) by plants and microbes, a controlling factor of ecosystem nitrogen (N) retention, has usually been measured based on uptake of¹⁵NH ₄ ⁺ solutions injected into soil. To study the influence of roots on N dynamics without stimulating consumption of NH ₄ ⁺ , we estimated gross nitrification in the presence or absence of live roots in an agricultural soil. Tomato (Lycopersicon esculentum var. Peto76) plants were grown in microcosms containing root exclosures. When the plants were 7 weeks old,¹⁵N enriched nitrate (NO ₃ ⁻ ) was applied in the 0–150 mm soil layer. After 24 h, > 30 times more¹⁵NH ₄ ⁺ was found in the soil with roots than in the soil of the root exclosures. At least 18% of the NH ₄ ⁺ -N present at this time in the soil with roots had been converted from NO ₃ ⁻ . We estimated rates of conversion of NO ₃ ⁻ to NH ₄ ⁺ , and rates ofNH ₄ ⁺ immobilization by plants and microbes, by simulating N-flow of¹⁴⁺¹⁵N and¹⁵N in three models representing mechanisms that may be underlying the experimental data: Dissimilatory NO ₃ ⁻ reduction to NH ₄ ⁺ (DNRA), plant N efflux, and microbial biomass nitrogen (MBN) turnover. Compared to NO ₃ ⁻ uptake, plant NH ₄ ⁺ uptake was modest. Ammonium immobilization by plants and microbes was equal to at least 35% of nitrification rates. The rapid recycling of NO ₃ ⁻ to NH ₄ ⁺ via plants and/or microbes contributes to ecosystem N retention and may enable plants growing in agricultural soils to capture more NH ₄ ⁺ than generally assumed.     

Availability and utilization of fertilizer nitrogen by rice under alternate flooding

Summary A greenhouse pot experiment with three N sources—¹⁵N-depleted ammonium sulfate, urea and sulfur-coated urea (SCU), three N levels—0, 50 and 200 ppm, and three irrigation treatments was conducted with rice in an Alamo clay soil. The irrigation treatments consisted of continuous flooding, alternating flooded-saturated and alternating flooded-drained conditions. The kinetics of NH ₄ ⁺ and NH ₄ ⁺ +NO ₃ ⁻ (available N) in soil was investigated under rice culture. Alternating flooded-drained conditions maintained a significantly lower level of available N in the soil than the other irrigation treatments. Among the N sources evaluated, urea and A/S had similar effects but SCU maintained significantly lower levels of available N in the soil during the growth of rice. Appreciable quantities of NO ₃ ⁻ accumulated during the draining periods of alternating flooded-drained treatments were apparently lost from the soil system upon reflooding.     

The Effect of Biological Soil Crusts on Throughput of Rainwater and N into Lake Michigan Sand Dune Soils

Biological soil crusts composed of cyanobacteria, green algae, bryophytes, and lichens colonize soils in arid and semiarid ecosystems worldwide and are responsible for significant N input to the soils of these ecosystems. Soil crusts also colonize active sand dunes in more humid regions, but studies of structure and function of such sand dune crusts are lacking. We identified the cyanobacterial, algal, and bryophytic constituents and N production and leachates of biological soil crusts that colonize beach dunes at the Indiana Dunes National Lakeshore along southern Lake Michigan in Indiana, USA. To determine the role of these crusts in this system, we conducted a greenhouse experiment in which intact soil cores with biological crusts were subjected to artificial rainfall over a full growing season. The volume and N content of leachate from the cores were quantified in relation to degree of crust development, taxonomic composition, rainfall volume and intensity, light intensity, and the presence of plant litter. Net N throughput significantly exceeded N inputs to cores in rainwater. Net N outputs from crusts to subsurface soil ranged from 0. 01 to 0.19 g NH ₄ ⁺ -N m⁻² yr⁻¹ and 0.01 to 0.61 g NO ₃ ^– N m⁻² yr⁻¹. Thus, total inorganic N inputs associated with biological soil crusts ranged from 0.02 g N m⁻² yr⁻¹ to 0.8 g N m⁻² yr⁻¹. High volume (≥2 cm) rainfall resulted in more N leaching than low volume events, and plant litter added over the surface of crusted soil cores significantly increased the amount of N in leachate. Exploratory path analysis revealed direct and indirect linkages among environmental factors, crust development, and crust composition in regulating the throughput of H₂O and N from these intact soil cores. Biological soil crusts at this site, combined with other properties of the soil surface, substantially increase N inputs to this water- and nutrient-limited sand dune ecosystem.     

Spatial distributions and net deposition rates of mineral elements in the elongating wheat (Triticum aestivum L.) leaf under saline soil conditions

Wheat leaf growth is known to be spatially affected by salinity. The altered spatial distribution of leaf growth under saline conditions may be associated with spatial changes in tissue mineral elements. The objective of this study was to evaluate the spatial distributions of mineral elements and their net deposition rates in the elongating and mature zones of leaf 4 of the main stem of spring wheat (Triticum aestivum L. cv. Lona) during its linear growth phase under saline soil conditions. Plants were grown in an illitic-chloritic silty loam with 0 and 120 mM NaCl. Three days after emergence of leaf 4, sampling was begun at 3 and 13 h into the 16-h light period. Spatial distributions of fresh weight (FW), dry weight (DW), and Na⁺, K⁺, Cl⁻, NO⁻ ₃, Ca²⁺, Mg²⁺, total P, and total N in the elongating and mature tissues were determined on a millimeter scale. The patterns of spatial distribution of Na⁺, Cl⁻, K⁺, NO₃ ⁻, and Ca²⁺ in the growing leaves were affected by salinity, while those of Mg²⁺, total P, and total N were not. Sodium, K⁺, Cl⁻, Ca²⁺, Mg²⁺, and total N concentrations (mmol · kg⁻¹ FW) were consistently higher at 120 mM NaCl than at 0 mM NaCl along the leaf axis from the leaf base, whereas NO₃ ⁻ concentration was lower at 120 mM NaCl. Deposition rates of all nutrients were greatest in the elongation zone. The elongation zone was the strongest sink for mineral elements in the leaf tissues. Local net deposition rates of Na⁺, Cl⁻, Ca²⁺, and Mg²⁺ (mmol · kg⁻¹ FW · h⁻¹) in the most actively elongating zone were enhanced by 120 mM NaCl, whereas for NO₃ ⁻ this was depressed. The lower supply of NO⁻ ₃ to growing leaves may be responsible for the inhibition of growth under saline conditions. Higher tissue concentrations of Na⁺ and Cl⁻ may cause ion imbalance but probably did not result in ion toxicity in the growing leaves. Potassium, Ca²⁺, Mg²⁺, total P, and total N are less plausibly responsible for the reduction in leaf growth in this study. Higher tissue K⁺ and Ca²⁺ concentrations at 120 mM NaCl are probably due to the presence of high Ca²⁺ in the soil of this study. Received: 13 March 1997 / Accepted: 9 June 1997     

Fifteen-year Change in Forest Floor Organic and Element Content and Cycling at the Turkey Lakes Watershed

To assess the long-term effects of atmospheric deposition on forest floor chemical composition, we took quantitative samplings of L-(Oi), F-(Oe), and H-(Oa) layers at an old-growth sugar maple–yellow birch stand on a till soil at the Turkey Lakes Watershed near Lake Superior, Ontario, Canada, in 1981 and 1996. We then assessed these samples for contents of organic matter (OM), total N, K, Ca, Mg, S, and Na, and exchangeable NH₄ ⁺, NO₃ ⁻, K⁺, Ca²⁺, Mg²⁺, SO₄ ²⁻, and Na⁺. Over the 15-year period, total OM and element contents remained unchanged, with the exception of N, which increased significantly from 61.3 kmol/ha in 1981 to 78.4 kmol/ha in 1996. On an area basis, there were significant increases in exchangeable Ca²⁺ (from 3.8 to 4.6 kmol/ha) and Na⁺ (from 0.05 to 0.08 kmol/ha) and decreases in exchangeable NH₄ ⁺-N (from 1.41 to 0.95 kmol/ha) and SO₄ ²⁻-S (from 1.29 to 0.96 kmol/ha). There were no significant differences in average annual litterfall OM, N, Ca, Mg, S or Na inputs between 1980 and 1985 and between 1992 and 1997. Average annual wet-only SO₄ ²⁻-S deposition during 1981–86 was 0.30; during 1992–97, it was 0.21 kmol/ha. Annual wet-only NO₃ ⁻-N averaged 0.33 kmol/ha during 1981–86 and was similar during 1992–97. Throughfall was less rich in SO₄ ²⁻ and Ca²⁺, Mg²⁺, and Na⁺ during 1992–97 than earlier. Throughfall NH₄ ⁺ and NO₃ ⁻ fluxes were unchanged. Efflux of cations from the forest floor reflected reduced throughput of SO₄ ²⁻. Overall, the results suggest that in spite of atmospheric inputs, active biological processes—including litter input, fine-root turnover, and tree uptake—serve to impart stability to the mineral composition of mature sugar maple forest floor. Received 5 October 1999; accepted 25 October 2000.     

The effect of forest type on throughfall deposition and seepage flux: a review

Converting deciduous forests to coniferous plantations and vice versa causes environmental changes, but till now insight into the overall effect is lacking. This review, based on 38 case studies, aims to find out how coniferous and deciduous forests differ in terms of throughfall (+stemflow) deposition and seepage flux to groundwater. From the comparison of coniferous and deciduous stands at comparable sites, it can be inferred that deciduous forests receive less N and S via throughfall (+stemflow) deposition on the forest floor. In regions with relatively low open field deposition of atmospheric N (<10 kg N ha⁻¹ year⁻¹), lower NH₄⁺ mean throughfall (+stemflow) deposition was, however, reported under conifers compared to deciduous forest, while in regions with high atmospheric N pollution (>10 kg N ha⁻¹ year⁻¹), the opposite could be concluded. The higher the open field deposition of NH₄⁺, the bigger the difference between the coniferous and deciduous throughfall (+stemflow) deposition. Furthermore, it can be concluded that canopy exchange of K⁺, Ca²⁺ and Mg²⁺ is on average higher in deciduous stands. The significantly higher stand deposition flux of N and S in coniferous forests is reflected in a higher soil seepage flux of NO₃⁻, SO₄²⁻, K⁺, Ca²⁺, Mg²⁺ and Al(III). Considering a subset of papers for which all necessary data were available, a close relationship between throughfall (+stemflow) deposition and seepage was found for N, irrespective of the forest type, while this was not the case for S. This review shows that the higher input flux of N and S in coniferous forests clearly involves a higher seepage of NO₃⁻ and SO₄²⁻ and accompanying cations K⁺, Ca²⁺, Mg²⁺ and Al(III) into the groundwater, making this forest type more vulnerable to acidification and eutrophication compared to the deciduous forest type.     

Leaching of subterranean clover-derived N from a loam soil

The leaching of subterranean clover-derived N (¹⁵N) was investigated in a laboratory and a field experiment. In both experiments 30 cm i.d. ×50cm soil columns were used. In the laboratory experiment the clover material was buried in the soil in mesh bags, and leaching of clover-derived N was compared to leaching of added NH ₄ ⁺ −N and NO ₃ ⁻ −N over a period of 75 days at 20°C. During that time 75% of the clover-N was released from the mesh bags and 17% of the clover-N, 50% of the NH ₄ ⁺ −N and 70% of the NO ₃ ⁻ −N was leached through the soil column. In the field experiment 6 lysimeters and 7 control microplots were constructed. The clover material was buried in soil (to the soil of two control microplots within mesh bags) in October. During one year 2% of the added clover-N was leached. This was despite a release of 65% of the N from the mesh bag contents and despite a 26% loss of the clover-derived N in total from the controls.     

Seasonal patterns of pore-water nutrients, benthic chlorophyll a and sedimentary AVS in a macrobenthos-rich tidal flat

Monthly field investigations were carried out at controlled low-tide in an estuarine intertidal sandflat of the Seto Inland Sea (Japan) between January 1995 and April 1996. We assessed the spatial and temporal distribution of pore-water nutrient [NH ₄ ⁺ -N, (NO ₃ ⁻ + NO ₂ ⁻ )-N, PO ₄ ³⁻ -P and Si(OH)₄-Si], chlorophyll a (chl a) and acid-volatile sulphide (AVS) concentrations in the uppermost 0–10 cm sediment horizon, and evaluated their relationships with macrobenthic assemblages. Monthly hydrological data, nutrient and chl a concentrations in low-tide creek water adjacent to the flat were used as a complementary environmental characterisation of the study area. All different pore-water nutrients showed a 10 to > 30-fold variability between different layers and periods. NH ₄ ⁺ -N, PO ₄ ³⁻ -P and Si(OH)₄-Si concentrations were lowest in winter, progressively increased throughout spring and summer, and were highest between September and October, with a major increase at intermediate (4–8 cm) layers. In contrast, (NO ₃ ⁻ + NO ₂ ⁻ )-N concentrations, correlated positively with chl a, peaked in winter and sharply decreased with depth in all different seasons and in most occasions. Depth-integrated NH ₄ ⁺ -N, PO ₄ ³⁻ -P and Si(OH)₄-Si pools were correlated positively with biogenic nutrient flux calculated from the nutrient excretion rates of dominant bivalves Ruditapes philippinarum and Musculista senhousia. These relationships indicated rapid removal pathways of P, Si and N (i.e., every 0.1, 0.24 and 0.34 day, respectively), implying highly dynamic sediments. Based on the mass balance P = F + I, biogenic production (P) vs. diffusive flux (F) estimates suggested a predominant contribution of animal bioirrigation (I) to the upward flux of nutrients. AVS concentrations, correlated negatively with both (NO ₃ ⁻ + NO ₂ ⁻ )-N and chl a, showed a progressive increase from late summer throughout winter, subsequent to that of NH ₄ ⁺ -N, PO ₄ ³⁻ -P and Si(OH)₄-Si. The results indicate that in our study area the spatial and temporal distribution of pore-water nutrients, chl a and AVS in sediments are strongly interlinked and suggest that the metabolic processes (e.g., excretion, bio-deposition) and behavioural activities (e.g., particle mixing, bioirrigation) of abundant macrobenthos play an important role in the year-round biogeochemical processes occurring on this flat.     

Soil solution chemistry of two reclamation sites in the Lusatian lignite mining district as influenced by organic matter application

Due to a large reclamation (recultivation) demand in the Lusatian lignite mining district, efficient strategies for the rehabilitation of abandoned mine sites are needed. A field study was conducted for comparing the effects of three different fertilizer treatments (mineral fertilizer, sewage sludge and compost) on soil solution chemistry of both a lignite and pyrite containing spoil as well as a lignite and pyrite free spoil. The lignite and pyrite containing spoil was ameliorated with fly ash from a lignite power plant (17–21 t ha⁻¹ CaO), whereas the lignite and pyrite free site received 7.5 t ha⁻¹ CaO in form of limestone. Fertilizer application rates were: mineral fertilizer 120 N, 100 P and 80 K kg ha⁻¹. 19 t ha⁻¹ sewage sludge and 22 t ha⁻¹ compost were applied. Soil solution was sampled in 20, 60 and 130 cm depth for the period of 16 months. Solution was collected every fortnight and analysed for pH, EC, Ca²⁺, Mg²⁺, K⁺, Na⁺, Feⁿ⁺, Alⁿ⁺, Mn²⁺, Zn²⁺, NO₃ ⁻, NH₄ ⁺, SO₄ ²⁻, Cl⁻, PO₄ ³⁻, C_inorg and DOC. Lignite and pyrite containing spoil differed clearly from lignite and pyrite free spoil regarding soil solution concentrations and composition. Acidity (H⁺) produced by pyrite oxidation led to an enhanced weathering of minerals and, therefore, to at least 10 fold higher soil solution concentrations compared to the lignite and pyrite free site. Major ions in solution of the lignite and pyrite containing site were Ca²⁺, Mg²⁺, Feⁿ⁺, Alⁿ⁺ and SO₄ ²⁻, whereas soil solution at the lignite and pyrite free site was dominated by Ca2+, Mg²⁺ and SO₄ ²⁻. At both sites application of mineral fertilizer led to an immediate but short term (about 1 month) increase of NO₃ ⁻, NH₄ ⁺ and K+ concentrations in soil solution down to a depth of 130 cm. Application of sewage sludge caused a long term (about 16 months) increase of NO_{3 3} ⁻ in the topsoil, whereas NO₃ ⁻ concentrations in the subsoil were significantly lower compared to the mineral fertilizer plot. Compost application resulted in a strong long-term increase of K+ in soil solution, whereas NO₃ ⁻ concentrations did not increase. Concentrations of PO₄ ³⁻ in soil solution depend on solution pH and were not correlated with any treatment. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date.     

Micronutrients and nitrogen metabolism

A sand-culture experiment was conducted to study the influence of a deficiency of and an excess of micronutrients on the uptake and assimilation of NH ₄ ⁺ and NO ₃ ⁻ ions by maize. By studying the fate of¹⁵N supplied as¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃, it was demonstrated that in maize plants NH₄−N was absorbed in preference to NO ₃ ⁻ −N. The uptake and distribution of N originating from both NH ₄ ⁺ and NO ₃ ⁻ was considerably modified by deficiency of, or an excess of, micronutrients in the growth medium. The translocation of NH ₄ ⁺ −N from roots to shoots was relatively less than that of NO ₃ ⁻ −N. Deficiency as well as excessive amounts of micronutrients, in the growth medium, substantially reduced the translocation of absorbed N into protein. This effect was more pronounced in the case of N supplied as NO ₃ ⁻ . Amino-N was the predominant non-protein fraction in which N from both NH ₄ ⁺ and NO ₃ ⁻ tended to accumulate. The next important non-protein fractions were NO ₃ ⁻ −N when N was supplied as NO ₃ ⁻ and amide-N when NH ₄ ⁺ was the source. The relative accumulation of¹⁵N into different protein fractions was also a function of imposed micronutrient levels.     

Hillslope Nutrient Dynamics Following Upland Riparian Vegetation Disturbance

We investigated the effects of removing near-stream Rhododendron and of the natural blowdown of canopy trees on nutrient export to streams in the southern Appalachians. Transects were instrumented on adjacent hillslopes in a first-order watershed at the Coweeta Hydrologic Laboratory (35°03′N, 83°25′W). Dissolved organic carbon (DOC), K⁺, Na⁺, Ca²⁺, Mg²⁺, NO₃ ⁻-N, NH₄ ⁺-N, PO₄ ³⁻-P, and SO₄ ²⁻ were measured for 2 years prior to disturbance. In August 1995, riparian Rhododendron on one hillslope was cut, removing 30% of total woody biomass. In October 1995, Hurricane Opal uprooted nine canopy trees on the other hillslope, downing 81% of the total woody biomass. Over the 3 years following the disturbance, soilwater concentrations of NO₃ ⁻-N tripled on the cut hillslope. There were also small changes in soilwater DOC, SO₄ ²⁻, Ca²⁺, and Mg²⁺. However, no significant changes occurred in groundwater nutrient concentrations following Rhododendron removal. In contrast, soilwater NO₃ ⁻-N on the storm-affected hillslope showed persistent 500-fold increases, groundwater NO₃ ⁻-N increased four fold, and streamwater NO₃ ⁻-N doubled. Significant changes also occurred in soilwater pH, DOC, SO₄ ²⁻, Ca²⁺, and Mg²⁺. There were no significant changes in microbial immobilization of soil nutrients or water outflow on the storm-affected hillslope. Our results suggest that Rhododendron thickets play a relatively minor role in controlling nutrient export to headwater streams. They further suggest that nutrient uptake by canopy trees is a key control on NO₃ ⁻-N export in upland riparian zones, and that disruption of the root–soil connection in canopy trees via uprooting promotes significant nutrient loss to streams. Received 30 January 2001; accepted 25 July 2002.     

Assessment of methylammonium as an analog for ammonium in plant uptake from soil

We tested radioactive methylammonium (¹⁴CH₃NH _inf3 ^sup+ ) as a tracer for ammonium (NH₄ ⁺) in root uptake measurements from soil. Tomato (Lycopersicon esculentum Moll. cv T5) in 3 L pots filled with loamy sand soil received 40, 200, or 600 μmol ¹⁴CH₃NH₃ ⁺ or ¹⁵NH₄ ⁺. During a 4 h period, the plants absorbed ¹⁴CH₃NH₃ ⁺ at slower rates than ¹⁵NH₄ ⁺. Estimates of NH₄ ⁺ absorption based on ¹⁵NH₄ ⁺ absorption were 0.9–7.9 μmol NH₄ ⁺ g⁻¹ plant dry weight h⁻¹, whereas those based on ¹⁴CH₃NH₃ ⁺ absorption were 0.2–1.0 μmol NH₄ ⁺ g⁻¹ plant dry weight h⁻¹. After 4 h, approximately one-half of the applied ¹⁵N was not recovered in the plants or soil KCl extracts; apparently, this ¹⁵N was either immobilized or nitrified and denitrified by soil biota. By contrast, almost all the ¹⁴CH₃NH₃ ⁺ remained in the soil solution after 4 h, but after a 10 d incubation, approximately 20% had been released as ¹⁴CO₂. These differences in plant absorption rates and movement through soil pools indicate that CH₃NH₃ ⁺ cannot be used reliably as an NH₄ ⁺ analog in soil.     

Short-term nitrous oxide emissions from pasture soil as influenced by urea level and soil nitrate

Nitrogen excreted by cattle during grazing is a significant source of atmospheric nitrous oxide (N₂O). The regulation of N₂O emissions is not well understood, but may vary with urine composition and soil conditions. This laboratory study was undertaken to describe short-term effects on N₂O emissions and soil conditions, including microbial dynamics, of urea amendment at two different rates (22 and 43 g N m⁻²). The lower urea concentration was also combined with an elevated soil NO ₃ ⁻ concentration. Urea solutions labelled with 25 atom%¹⁵N were added to the surface of repacked pasture soil cores and incubated for 1, 3, 6 or 9 days under constant conditions (60% WFPS, 14 °C). Soil inorganic N (NH ₄ ⁺ , NO ₂ ⁻ and NO ₃ ⁻ ), pH, electrical conductivity and dissolved organic C were quantified. Microbial dynamics were followed by measurements of CO₂ evolution, by analyses of membrane lipid (PLFA) composition, and by measurement of potential ammonium oxidation and denitrifying enzyme activity. The total recovery of¹⁵N averaged 84%. Conversion of urea-N to NO ₃ ⁻ was evident, but nitrification was delayed at the highest urea concentration and was accompanied by an accumulation of NO ₂ ⁻ . Nitrous oxide emissions were also delayed at the highest urea amendment level, but accelerated towards the end of the study. The pH interacted with NH ₄ ⁺ to produce inhibitory concentrations of NH₃(aq) at the highest urea concentration, and there was evidence for transient negative effects of urea amendment on both nitrifying and denitrifying bacteria in this treatment. However, PLFA dynamics indicated that initial inhibitory effects were replaced by increased microbial activity and net growth. It is concluded that urea-N level has qualitative, as well as quantitative effects on soil N transformations in urine patches.     

The effect of fire on nutrients in a pine forest soil

The effect of a hot summer fire on soil nutrient contents in the upper 2 cm of Aleppo pine forest with a dense woody understory was studied from September 1985 to May 1986. In comparison with the adjacent unburned forest, total nitrogen decreased by 25% but available forms of nitrogen were much higher. In burned and unburned soils there was a similar trend to increase and decrease in NH ₄ ⁺ −N, However, while (NO ₂ ⁻ +NO ₃ ⁻ −N decreased in the unburned soil it rose rapidly in the burned ash soil. Total phosphorus increased by 300% after the fire but decreased again 2 months later. Also water-soluble P increased up to November and then decreased to the levels of the unburned soils. The same was true for electrical conductivity and pH, increasing immediately after the fire and then leveling off again. This increase in nutrient levels in the “ash soil” was reflected in the striking increase in shoot and root biomass and in the content of N, P, Mg, K, Ca, Zn and Fe in wheat and clover plants grown in pots in these soils. These nutrient levels were much higher in the wheat plants, which also produced 12 times more seeds in the “ash soil.” It seems that fire in these pine forests causes a short-term flush of the mineral elements in the upper “ash soil” layer which is reverted gradually via the herbaceous post-fire to the ecosystem.     

Organic matter quality and management effects on enrichment of soil organic matter fractions in contrasting soils in Zimbabwe

Spatial variability of soil total nitrogen (N), available N (KCl extractable NH₄⁺ and NO₃⁻), and spatial patterns of N mineralization and nitrification at a stand scale were characterized with geostatistical and univariate analysis. Two extensive soil spatial samplings were conducted in an evergreen broadleaf forest in Sichuan province, southwestern China in June and August 2000. In a study area of 90 × 105 m², three soil samples were collected from each 5 × 5 m² plot (n = 378) in June and August, and were analyzed for total N and available N contents. Net N mineralization and nitrification were measured by in situ core incubation and the rates were estimated based on the difference of NH₄⁺ and NO₃⁻ contents between the two sampling dates. Total N, NH₄⁺, and NO₃⁻ were all spatially structured with different semivariogram ranges (from high to low: NH₄⁺, NO₃⁻, and total N). The semivariograms of mineralization and nitrification were not as spatially structured as available N. NH₄⁺ was the dominant soil inorganic N form in the system. Both NH₄⁺ and NO₃⁻ affected spatial patterns of soil available N, but their relative importance switched in August, probably due to high nitrification as indicated by greatly increased soil NO₃⁻ content. High spatial auto-correlations (>0.7) were found between available N and NH4⁺, available N and NO₃⁻ on both sampling dates, as well as total N measurements between both sampling dates. Although significant, the spatial auto-correlation between NH₄⁺ and NO₃⁻ were generally low. Topography had significant but low correlations with mineralization (r = −0.16) and nitrification (r = −0.14), while soil moisture did not. The large nugget values of the calculated semivariograms and high-semivariance values, particularly for mineralization and nitrification, indicate that some fine scale (<5 m) variability may lie below the threshold for detection in this study.     

The effects of low,regulated supplies of nitrate and ammonium nitrogen on the growth and composition of perennial ryegrass

Summary Perennial ryegrass was grown in flowing solution culture with nitrogen supplied in amounts that increased exponentially,i.e. in parallel with the rate of increase in growth. Nitrogen was supplied as either NO ₃ ⁻ or NH ₄ ⁺ , and the amounts to be added were calculated on the basis of extrapolated values for dry weights obtained from fitted curves. There were two rates of addition for each form of N aimed at providing adequate (5.0 per cent) and less than adequate (2.75 per cent) contents in the plants in each case. Measured plant weights and N concentrations were in close agreement with predicted values over a four week experimental period. There was no effect of N-form at high N, and these plants produced 46 per cent more dry matter than the plants at low N. Only minor differences in overall growth occurred with NO ₃ ⁻ or NH ₄ ⁺ plants at low N, but the NH ₄ ⁺ plants had a greater shoot:root ratio. The absorption rate (m mol Ng⁻ root d⁻¹) for NH ₄ ⁺ -N was therefore greater than for NO ₃ ⁻ -N. The cation/anion composition of the plants was affected in a predicable way, and to a greater or lesser extent at high or low N, respectively, in NO ₃ ⁻ or NH ₄ ⁺ plants. The major changes in cation composition came through effects on potassium absorption. Plants with low NO ₃ ⁻ appeared to be under greater N stress than those with low NH ₄ ⁺ because of the lower shoot:root ratio and the greater C∶N ratio in the shoots.     

Inhibition of Nitrification Alters Carbon Turnover in the Patagonian Steppe

Human activities are altering biodiversity and the nitrogen (N) cycle, affecting terrestrial carbon (C) cycling globally. Only a few specialized bacteria carry out nitrification—the transformation of ammonium (NH ₄ ⁺ ) to nitrate (NO ₃ ⁻ ), in terrestrial ecosystems, which determines the form and mobility of inorganic N in soils. However, the control of nitrification on C cycling in natural ecosystems is poorly understood. In an ecosystem experiment in the Patagonian steppe, we inhibited autotrophic nitrification and measured its effects on C and N cycling. Decreased net nitrification increased total mineral N and NH ₄ ⁺ and reduced NO ₃ ⁻ in the soil. Plant cover (P < 0.05) and decomposition (P < 0.0001) decreased with inhibition of nitrification, in spite of increases in NH ₄ ⁺ availability. There were significant changes in the natural abundance of δ¹⁵N in the dominant vegetation when nitrification was inhibited suggesting that a switch occurred in the form of N (from NO ₃ ⁻ to NH ₄ ⁺ ) taken up by plants. Results from a controlled-condition experiment supported the field results by showing that the dominant plant species of the Patagonian steppe have a marked preference for nitrate. Our results indicate that nitrifying bacteria exert a major control on ecosystem functioning, and that the inhibition of nitrification results in significant alteration of the C cycle. The interactions between the C and N cycles suggest that rates of C cycling are affected not just by the amount of available N, but also by the relative availability for plant uptake of NH ₄ ⁺ and NO ₃ ⁻ .     

Effects of soil acidification and subsequent leaching on levels of extractable nutrients in a soil

Summary The effects of soil acidification (pH values from 6.5 to 3.8), and subsequent leaching, on levels of extractable nutrients in a soil were studied in a laboratory experiment. Below pH 5.5, acidification resulted in large increases in the amounts of exchangeable Al in the soil. Simultaneously, exchangeable cations were displayed from exchange sites and Ca, Mg, K and Na in soil solution increased markedly. With increasing soil acidification, increasing amounts of cations were leached; the magnitude of leaching loss was in the same order as the cations were present in the soil: Ca²⁺>Mg²⁺>K⁺>Na⁺. Soil acidification appeared to inhibit nitrification since in the unleached soils, levels of NO ₃ ⁻ clearly declined below pH 5.5 and at the same time levels of NH ₄ ⁺ increased greatly. Significant amounts of NH ₄ ⁺ and larger amounts of NO ₃ ⁻ , were removed from the soil during leaching. Concentrations of NaHCO₃-extractable phosphate remained unchanged between pH 4.3 and 6.0 but were raised at higher and lower pH values. No leaching losses of phosphate were detected. For the unleached soils, levels of EDTA-extractable Mn and Zn increased as the soil was acidified whilst levels of extractable Fe were first decreased and then increased greatly and those for Cu were decreased slightly between pH 6.5 and 6.0 and then unaffected by further acidification. Significant leaching losses of Mn and Zn were observed at pH values below 5.5 but losses of Fe were very small and those of Cu were not detectable.