Rapid immobilization of applied nitrogen in saline–alkaline soils

The dynamics of inorganic N are important in soil, and this applies particularly to the saline–alkaline soils of the former lake Texcoco in Mexico with high pH and salinity where a forestation program was started in the 1970s. In soils of lake Texcoco, in Mexico, more than 50% of applied N could not be accounted for one day after application of 200 mg kg^–1 soil along with glucose amendment. It was not clear whether this was due to abiotic or biotic processes, the form of inorganic N applied or the result of applying an easily decomposable substrate. We investigated this by adding glucose and 200 mg kg^–1 soil as (NH₄)₂SO₄-N or KNO₃-N to sterilized and unsterilized soil. The changes in inorganic and ninhydrin N, microbial biomass C and production of CO₂ were then monitored. Between the time of applying N and extraction with 0.5 M K₂SO₄, i.e., after ca 2 h, approximately 110 mg NH₄ ⁺-N kg^–1 dry soil could not be accounted for in the unsterilized and sterilized soil and that remained so for the entire incubation in the sterilized soil. After 1 day this increased to 140 mg NH₄ ⁺-N kg^–1 dry soil in the unsterilized control and 170 mg NH₄ ⁺-N kg^–1 dry soil in C amended soil. Volatilization of NH₃ accounted for 56 mg NH₄ ⁺-N kg^–1 so the rest appeared to be adsorbed on the soil matrix. The NH₃ volatilization and NH₄ ⁺ fixed in the soil matrix remained constant over time and no oxidation to NO₂ ^– or NO₃ ^– had occurred, so unaccounted N in unsterilized soil was probably incorporated into the microbial biomass in excess of what was required for metabolic activity. The unaccounted N was ca 70 mg NO₃ ^––N in nitrate amended soil after 3 days and 138 NO₃ ^––N when glucose was additionally added. Losses through abiotic processes were absent as inferred from changes in sterilized soil and the aerobic incubation inhibited possible losses through denitrification. It was inferred that NO₃ ^– that could not be accounted for was taken up by micro-organisms in excess of what was required for metabolic activity.     

Effects of different nitrogen forms and combination with foliar spraying with 6-benzylaminopurine on growth,transpiration, and water and potassium uptake and flow in tobacco

NH₄ ⁺-N can have inhibitory effects on plant growth. However, the mechanisms of these inhibitory effects are still poorly understood. In this study, effects of different N forms and a combination of ammonium + 6-benzylaminopurine (6-BA, a synthetic cytokinin) on growth, transpiration, uptake and flow of water and potassium in 88-days-old tobacco (Nicotiana tabacum L. K 326) plants were studied over a period of 12 days. Plants were supplied with equal amounts of N in different forms: NO₃ ^–, NH₄NO₃, NH₄ ⁺ or NH₄ ⁺+6-BA (foliar spraying every 2 days after onset of the treatments). For determining flows and partitioning upper, middle and lower strata of three leaves each were analysed. During the 12 days study period, 50% replacement of NO₃ ^–-N by NH₄ ⁺-N (NH₄NO₃) did not change growth, transpiration, uptake and flow of water and K⁺ compared with the NO₃ ^–-N treatment. However, NH₄ ⁺-N as the sole N-source caused: (i) a substantial decrease in dry weight gain to 42% and 46% of the NO₃ ^–-N and NH₄NO₃ treatments, respectively; (ii) a marked reduction in transpiration rate, due to reduced stomatal conductance, illustrated by more negative leaf carbon-isotope discrimination (¹³C) compared with the NO₃ ^– treatment, especially in upper leaves; (iii) a strong reduction both in total water uptake, and in the rate of water uptake by roots, likely due to a decrease in root hydraulic conductivity; (iv) a marked reduction of K⁺ uptake to 10%. Under NH₄ ⁺ nutrition the middle leaves accumulated 143%, and together with upper leaves 206% and the stem 227% of the K⁺ currently taken up, indicating massive mobilisation of K⁺ from lower leaves and even the roots. Phloem retranslocation of K⁺ from the shoot and cycling through the root contributed 67% to the xylem transport of K⁺, and this was 2.2 times more than concurrent uptake. Foliar 6-BA application could not suppress or reverse the inhibitory effects on growth, transpiration, uptake and flow of water and ions (K⁺) caused by NH₄ ⁺-N treatment, although positive effects by 6-BA application were observed, even when 6-BA (10^–8 M) was supplied in nutrient solution daily with watering. Possible roles of cytokinin to regulate growth and development of NH₄ ⁺-fed plants are discussed.     

Soil acidification as caused by the nitrogen uptake pattern of Scots pine (Pinus sylvestris)

Three-year-old Scots pine (Pinus sylvestris) trees were grown on a sandy forest soil in pots, with the objective to determine their NH₄/NO₃ uptake ratio and proton efflux. N was supplied in three NH₄-N/NO₃-N ratios, 3:1, 1:1 and 1:3, either as ¹⁵NH₄+¹⁴NO₃ or as ¹⁴NH₄+¹⁵NO₃. Total N and ¹⁵N acquisition of different plant parts were measured. Averaged over the whole tree, the NH₄/NO₃ uptake ratios throughout the growing season were found to be 4.2, 2.5, and 1.5 for the three application ratios, respectively. The excess cation-over-anion uptake value (C_a-A_a) appeared to be linearly related to the natural logarithm of the NH₄/NO₃ uptake ratio. Further, this uptake ratio was related to the NH₄/NO₃ ratio of the soil solution. From these relationship it was estimated that Scots pine exhibits an acidifying uptake pattern as long as the contribution of nitrate to the N nutrition is lower than 70%. Under field circumstances root uptake may cause soil acidification in the topsoil, containing the largest part of the root system, and soil alkalization in deeper soil layers.     

Response of grain sorghum to enhanced-ammonium supply

Two greenhouse experiments were conducted to examine the effects of increased levels of soil NH ₄ ⁺ on the growth and yield of grain sorghum (Sorghum bicolor (L.) Moench). Nitrogen was supplied as urea plus the nitrification inhibitor nitrapyrin (enhanced-NH ₄ ⁺ supply) or as a 41 molar ratio of CA(NO₃)₂ and Mg(NO₃)₂ at rates of 0 to 450 mg N kg^–1 soil in 37.5 mg N increments. Enhanced-NH ₄ ⁺ supply, in comparison to the NO₃ treatment, increased grain yield 15 and 18% in the two experiments. In one experiment this yield increase occurred through increased number of kernels and in a second experiment, through increased weight of kernels. During the first 28 days after plant emergence, the number of leaves, stalk width, plant weight, and plant N content were greater with enhanced-NH ₄ ⁺ supply than with NO ₃ ^– . However, at harvest total plant weight and plant N content were minimally affected by enhanced-NH ₄ ⁺ supply.     

Growth,chemical composition,and nitrate reductase activity of Rumex species in relation to form and level of N supply

Growth, chemical composition, and nitrate reductase activity (NRA) of hydroponically cultured Rumex crispus, R. palustris, R. acetosa, and R. maritimus were studied in relation to form (NH₄ ⁺, NO₃ ^-, or both) and level of N supply (4 mM N, and zero-N following a period of 4mM N). A distinct preference for either NH₄ ⁺ or NO₃ ^- could not be established. All species were characterized by a very efficient uptake and utilization of N, irrespective of N source, as evident from high concentrations of organic N in the tissues and concurrent excessive accumulations of free NO₃ ^- and free NH₄ ⁺. Especially the accumulation of free NH₄ ⁺ was unusually large. Generally, relative growth rate (RGR) was highest with a combination of NH₄ ⁺ and NO₃ ^-. Compared to mixed N supply, RGR of NO₃ ^-- and NH₄ ⁺-grown plants declined on average 3% and 9%, respectively. Lowest RGR with NH₄ ⁺ supply probably resulted from direct or indirect toxicity effects associated with high NH₄ ⁺ and/or low Ca²⁺ contents of tissues. NRA in NO₃ ^- and NH₄NO₃ plants was very similar with maxima in the leaves of ca 40 μmol NO₂ ^- g^-1 DW h^-1. ‘Basal’ NRA levels in shoot tissues of NH₄ ⁺ plants appeared relatively high with maxima in the leaves of ca 20 μmol NO₂ ^- g^-1 DW h^-1. Carboxylate to organic N ratios, (C-A)/N_org, on a whole plant basis varied from 0.2 in NH₄ ⁺ plants to 0.9 in NO₃ ^- plants. After withdrawal of N, all accumulated NO₃ ^- and NH₄ ⁺ was assimilated into organic N and the organic N redistributed on a large scale. NRA rapidly declined to similar low levels, irrespective of previous N source. Shoot/root ratios of -N plants were 50–80% lower than those from +N plants. In comparison with +N, RGR of -N plants did not decline to a large extent, decreasing by only 15% in -NH₄ ⁺ plants due to very high initial organic-N contents. N-deprived plants all exhibited an excess cation over anion uptake (net proton efflux), and whole-plant (C-A)/N_org ratios increased to values around unity. Possible difficulties in interpreting the (C-A)/N_org ratio and NRA of plants in their natural habitats are briefly discussed.     

Nitrogen nutrition of rice plants under salinity

Two rice (Oryza sativa L.) cultivars, Koshihikari and Pokkali, were grown in solution culture at three concentrations of NaCl or Na₂SO₄ [0 (S0), 50 (S1), and 100 (S2) mmol dm^–3] and three N contents [0.7 (N1), 7 (N2) and 14 (N3) mmol dm^–3]. Salinity significantly decreased dry matter of both cultivars. Pokkali had better growth than Koshihikari under both saline and non-saline conditions. Applications of N enhanced development of shoot dry mass under S0 and S1 treatments up to N2. Under S2, N application had no effect on shoot dry mass of both cultivars. Root dry mass of both cultivars decreased with increasing N application at S1 and S2. Shoot and root NO₃-N content in both rice cultivars increased with increasing N concentration in the nutrient solutions. The absorption of NO₃-N was less in Koshihikari than Pokkali plants, and also was much less in Cl^– than SO₄ ^2– salinity suggesting the antagonism between Cl^– and NO₃ ^–. In addition a significant negative correlation between concentrations of NO₃-N and Cl^– in the shoots or roots was observed in both cultivars     

Nitrate reduction in sediments of lowland tropical streams draining swamp forest in Costa Rica: An ecosystem perspective

Nitrate reduction and denitrification were measured in swamp forest streams draining lowland rain forest on Costa Rica's Atlantic slope foothills using the C₂H₂-block assay and sediment-water nutrient fluxes. Denitrification assays using the C₂H₂-block technique indicated that the full suite of denitrifying enzymes were present in the sediment but that only a small fraction of the functional activity could be expressed without adding NO₃ ^–. Under optimal conditions, denitrification enzyme activity averaged 15 nmoles cm^–3 sediment h^–1. Areal NO₃ ^– reduction rates measured from NO₃ ^– loss in the overlying water of sediment-water flux chambers ranged from 65 to 470 umoles m^–2 h^–1. Oxygen loss rates accompanying NO₃ ^– depletion averaged 750 umoles m^–2 h^–1. Corrected for denitrification of NO₃ ^– oxidized from NH₄ ⁺ in the sediment, gross NO₃ ^– reduction rates increase by 130 umoles m^–2 h^–1, indicating nitrification may be the predominant source of NO₃ ^– for NO₃ ^– reduction in swamp forest stream sediments. Under field conditions approximately 80% of the increase in inorganic N mass along a 1250-m reach of the Salto River was in the form of NO₃ ^– with the balance NH₄ ⁺ . Scrutiny of potential inorganic N sources suggested that mineralized N released from the streambed was a major source of the inorganic N increase. Despite significant NO₃ ^– reduction potential, swamp forest stream sediments appear to be a source of inorganic N to downstream communities.     

不同氮处理下速生柳对水体氮的吸收、分配及生理响应

采用水培法,研究了旱柳苗在外源添加不同氮水平(贫氮、中氮、富氮、过氮)的铵态氮(NH+4-N)和硝态氮(NO-3-N)的生长、氮吸收、分配和生理响应。结果表明:一定范围氮浓度的增加能够促进旱柳苗的生长,但过量氮会抑制其生长,且NH+4-N的抑制作用大于NO-3-N;两种氮处理下,旱柳表现出对NH+4-N的吸收偏好,在同一氮水平时,旱柳各部位氮原子百分含量Atom%15N(AT%)、15N吸收量和来自氮源的N%(Ndff%)均为NH+4-N处理大于NO-3-N处理,且随着氮浓度的增加,差异增大,且在旱柳各部位的分布为根﹥茎﹥叶;2种氮素过量和不足均会对旱柳根和叶生理指标产生不同的影响,其中在过氮水平时,NH+4-N和NO-3-N处理下根系活力比对照减少了50.61%和增加了19.53%;在过氮水平时,NH+4-N处理柳树苗根总长、根表面积、根平均直径、根体积和侧根数分别对照下降了30.92%、29.48%、19.44%、27.01%和36.41%,NO-3-N处理柳树苗相应的根系形态指标分别对对照下降了1.66%、5.65%、1.49%、5.06%和25.72%。可见,高浓度NH+4-N对旱柳苗的胁迫影响大于NO-3-N,在应用于水体氮污染修复时可通过改变水体无机氮的比例,削弱其对旱柳的影响,从而提高旱柳对水体氮污染的修复效果。     

Effects of Feedstock and Pyrolysis Temperature on Biochar Adsorption of Ammonium and Nitrate

Biochar produced by pyrolysis of biomass can be used to counter nitrogen (N) pollution. The present study investigated the effects of feedstock and temperature on characteristics of biochars and their adsorption ability for ammonium N (NH₄ ⁺-N) and nitrate N (NO₃ ⁻-N). Twelve biochars were produced from wheat-straw (W-BC), corn-straw (C-BC) and peanut-shell (P-BC) at pyrolysis temperatures of 400, 500, 600 and 700°C. Biochar physical and chemical properties were determined and the biochars were used for N sorption experiments. The results showed that biochar yield and contents of N, hydrogen and oxygen decreased as pyrolysis temperature increased from 400°C to 700°C, whereas contents of ash, pH and carbon increased with greater pyrolysis temperature. All biochars could sorb substantial amounts of NH₄ ⁺-N, and the sorption characteristics were well fitted to the Freundlich isotherm model. The ability of biochars to adsorb NH₄ ⁺-N followed: C-BC>P-BC>W-BC, and the adsorption amount decreased with higher pyrolysis temperature. The ability of C-BC to sorb NH₄ ⁺-N was the highest because it had the largest cation exchange capacity (CEC) among all biochars (e.g., C-BC400 with a CEC of 38.3 cmol kg⁻¹ adsorbed 2.3 mg NH₄ ⁺-N g⁻¹ in solutions with 50 mg NH₄ ⁺ L⁻¹). Compared with NH₄ ⁺-N, none of NO₃ ⁻-N was adsorbed to biochars at different NO₃ ⁻ concentrations. Instead, some NO₃ ⁻-N was even released from the biochar materials. We conclude that biochars can be used under conditions where NH₄ ⁺-N (or NH₃) pollution is a concern, but further research is needed in terms of applying biochars to reduce NO₃ ⁻-N pollution.     

天山林区不同类型群落土壤氮素对冻融过程的动态响应

季节性冻融过程对北方温带森林土壤氮素的转化与流失具有重要影响,但不同类型群落对冻融过程响应的差异尚不明确。通过在林地、草地、灌丛上设置系列监测样地,采用原位培养的方法,利用林冠遮挡形成的自然雪被厚度差异,监测分析了冻融期天山林区不同群落表层土壤(0—15 cm)的氮素动态及净氮矿化速率间的差异。结果表明:(1)不同类型群落土壤的铵态氮(NH+4-N)含量、微生物量氮(MBN)含量基本与土壤(5 cm)温度呈正相关,深冻期林地土壤铵态氮含量低于其他群落类型而硝态氮含量高于其他群落类型;(2)硝态氮(NO-3-N)为天山林区季节性冻融期间土壤矿质氮的主体,占比达78.4%。灌丛土壤硝态氮流失风险较大,融化末期较融化初期灌丛土壤硝态氮含量下降了64.6%;(3)冻融时期对整体氮素矿化速率影响显著,群落类型对氨化速率影响显著;(4)天山林区土壤氮素在冻结期主要以氮固持为主。通过揭示不同类型群落土壤氮素对冻融格局的响应,能够助益于对北方林区冬季土壤氮素循环的认识。     

Effects of macro elements and nitrogen source on adventitious root growth and ginsenoside production in ginseng (Panax ginseng C. A. Meyer)

We investigated the effects on ginseng adventitious root growth and ginsenoside production when macro-element concentrations and nitrogen source were manipulated in the culture media. Biomass growth was greatest in the medium supplemented with 0.5-strength NH₄PO₃, whereas ginsenoside accumulation was highest (9.90 mg g^-1 DW) in the absence of NH₄PO₃. At levels of 1.0-strength KNO₃, root growth was maximum, but a 2.0 strength of KNO₃ led to the greatest ginsenoside content (9.85 mg g^-l). High concentrations of MgSO₄ were most favorable for both root growth and ginsenoside accumulation (up to 8.89 mg g^-1 DW). Root growth and ginsenoside content also increased in proportion to the concentration of CaCI₂ in the medium, with the greatest accumulation of ginsenoside (8.91 mg g^-1 DW) occurring at a 2.0 strength. The NH₄/NO₃ ^-- ratio also influenced adventitious root growth and ginsenoside production; both parameters were greater when the NO₃ ^- concentration was higher than that of NH₄ ⁺. Maximum root growth was achieved at an NH₄ ⁺/NO₃ ^- ratio of 7.19/18.50, while ginsenoside production was greatest (83.37 mg L^-1) when NO₃ ^- was used as the sole N source.     

Foliar N application reduces soil NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack>-N leaching loss in apple orchards

A comparison of the effects of foliar and soil N application was made in field-grown mature fruiting Gala/M9 apple trees (Malus domestica Borkh) in 2001 and 2002 growing seasons under Pacific Northwest growing conditions in southern British Columbia, Canada. The trees, six years old at the start of the experiment, were treated: (1) with 5 g/l urea sprays supplied every two weeks (7 times) from mid May to mid August (total about 50 g N/tree/year), (2) with the same amount of N applied to the soil with the same timing and quantity as for the foliar treatment, and (3) with no N (control). Leaf color (as SPAD readings) and N concentrations (mg/g), and soil NH₄⁺-N and NO₃^–-N were measured periodically throughout the two seasons. Leached NO₃^–-N was monitored monthly via an anion exchange probe from June to October in 2001 and from May to November in 2002. Shoot length was measured in October and N concentration of one-year-old wood and roots was determined in December of each growing s eason. Soil N application significantly increased shoot length relative to control or foliar N application. Leaf color, leaf N, and N concentration of one-year-old wood and roots were similarly increased relative to control by both soil and foliar N application. These treatments also increased fruit yield relative to control. There was no significant difference in yield and fruit quality between soil and foliar N applications. Soil N application increased soil NH₄⁺-N and NO₃^–-N content in the root zone, and also increased the NO₃^– leaching loss below the root zone especially late in the growing season. Our results suggested that tree N status and yield and fruit quality could be maintained by multiple urea sprays during the growing season in apple orchards, and foliar N application will reduce the risk of soil NO₃^–-N leaching.     

Combination Effect of NaCl Salinity and Nitrogen Form on Mineral Composition of Sunflower Plants

The effect of two N-forms (NH₄ ⁺ and NO₃ ^–) and NaCl on pattern of accumulation of some essential inorganic nutrients was examined in sunflower (Helianthus annuus L.) cv. Hisun 33. Eight-day-old plants of were subjected for 21 d to Hoagland's nutrient solution containing 8 mM N as NH₄ ⁺ or NO₃ ^·, and salinized with and addition of NaCl to the growth medium had no significant effect on total leaf N. However, root N of NH₄-supplied plants decreased significantly with increase in NaCl concentration, whereas that of NO₃-supplied plants remained unaffected. There was no significant effect of NaCl on leaf or root P, but the NO₃-supplied plants had significa concentration of leaf P than that of NH₄-supplied plants at varying salt treatments. Salinity of the rooting med did not show any significant effect on Na⁺ concentrations of leaves or roots of plants subjected to two differen N. NH₄-treated plants generally had greater concentrations of Cl^– in leaves and roots and lower K⁺ content in leaves than NO₃-supplied plants. Ca²⁺ concentrations of leaves and roots and Mg²⁺ concentrations of leaves decreased in NH₄-supplied plants due to NaCl, but they remained unaffected in NO₃-treated plants.     

马尾松组培苗对氮素形态的生长响应

马尾松属高氮需求树种,然而在苗木培育中马尾松对氮素,尤其是不同形态氮素的需求尚不明确。该文以马尾松组培苗为试验材料,采用基质培养方法,针对硝态氮、铵态氮两种氮素形态均分别设置了2、4、8、16 mmol·L~(-1)4个处理,以不添加氮素为对照,对苗木的高径生长、根构型参数(总根长、总表面积、总体积、平均直径和根尖数)以及生物量的变化进行了分析。结果表明:(1)在2～8 mmol·L~(-1)硝态氮处理下,除根冠比外,苗高、地径、根构型参数、生物量均不低于对照,其中以2 mmol·L~(-1)水平下苗木生长效果最好,苗高、地径、根构型参数、生物量均高于对照;在16 mmol·L~(-1)硝态氮处理下,苗高、总根长与根尖数低于对照。(2)在2～16 mmol·L~(-1)供试范围内,铵态氮处理下的苗木根冠比小于对照,但其苗高、地径、根构型参数、生物量均不低于对照,整体上以4 mmol·L~(-1)处理下的苗木生长表现最佳。(3)在任一供氮水平,除根冠比和2mmol·L~(-1)处理下的根总表面积与根尖数在两种氮素形态间无明显差异外,铵态氮处理下的苗木生长情况显著优于硝态氮处理,这说明马尾松组培苗偏好于吸收和利用铵态氮。综上结果表明,外施硝态氮、铵态氮均能促进马尾松组培苗生长,但需控制在适宜浓度范围内,其中以2 mmol·L~(-1)硝态氮和4 mmol·L~(-1)铵态氮处理效果较佳。高浓度硝态氮会抑制苗木高度及根系发育,且在相同施肥水平下,对苗木生长的促进效果大多弱于铵态氮。因此,今后为达到培育优质壮苗和提高肥效、减少肥害的目的,可考虑使用铵态氮肥。     

The kinetics of NH4 + and NO3 − uptake by Douglas fir from single N-solutions and from solutions containing both NH4 + and NO3 −

The kinetics of NH₄ ⁺ and NO₃ ⁻ uptake in young Douglas fir trees (Pseudotsuga menziesii [Mirb.] Franco) were studied in solutions, containing either one or both N species. Using solutions containing a single N species, the V_max of NH₄ ⁺ uptake was higher than that of NO₃ ⁻ uptake. The K_m of NH₄ ⁺ uptake and K_m of NO₃ ⁻ uptake differed not significantly. When both NH₄ ⁺ and NO₃ ⁻ were present, the V_max for NH₄ ⁺ uptake became slightly higher, and the K_m for NH₄ ⁺ uptake remained in the same order. Under these conditions the NO₃ ⁻ uptake was almost totally inhibited over the whole range of concentrations used (10–1000 μM total N). This inhibition by NH₄ ⁺ occurred during the first two hours after addition. ei]{gnA C}{fnBorstlap}     

Effects of forest fertilization on nitrogen leaching and soil microbial properties in the Northern Calcareous Alps of Austria

Several boreal and alpine forests are depleted in nutrients due to acidification. Fertilization may be a remedy, but rapidly-soluble salts (N, P, K, Mg) may pose nitrate problems for the groundwater or decrease microbial activity.With the aim to investigate potential nitrogen leaching after fertilization we set up an experiment employing intact soil cores (11 cm diameter, 20–40 cm long) from a mixed forest and a Picea abies stand (soil type Rendsina) in the Northern Calcareous Alps of Austria. The cores were fertilized with a commercial NPK fertilizer or a methylene-urea-apatite-biotite (MuAB) fertilizer at a rate corresponding to 300 kg N ha^-1 and incubated for 28 weeks together with unfertilized controls. Both soil water (retrieved 5 cm below the soil surface) and leachate were analyzed for nitrate and ammonium in regular intervals. After the incubation, soil microbial biomass and basal repiration were determined and a nitrogen mineralization assay was performed.For the control, in the soil water and leachate maximum NH₄ ⁺ and NO₃ ^- concentrations of 5 and 11 mg N L^-1, respectively, were found. Compared to that, MuAB fertilizer resulted in a slow increase of NH₄ ⁺ and NO₃ ^- in the soil water (up to 11 and 35 mg N L^-1 respectively) and in the leachate (4 mg NH₄ ⁺-N L^-1 and 44 mg NO₃ ^--N L^-1). Highest nitrogen loads were found for the fast release NPK fertilizer, with NH₄ ⁺ and NO₃ ^- concentrations up to 170 and 270 mg N L^-1, respectively, in the soil water. NH₄ ⁺-N levels in the leachate remained below 5, while NO₃-N levels were up to 190 mg L^-1. Fast- release NPK caused a significant decrease of microbial biomass and basal respiration. These parameters were not affected by MuAB fertilizer.The results suggest that the MuAB fertilizer may be an ecologically appropriate alternative to fast-release mineral fertilizers for improving forest soils.     

Transformation of15N-labelled urea and its modified forms in tropical wetland rice culture

Summary The fate of 100 kg N ha^–1 applied as¹⁵N-urea and its modified forms was followed in 4 successive field-grown wetland rice crops in a vertisol. The first wet season crop recovered about 27 to 36.6% of the applied N depending upon the N source. In subsequent seasons the average uptake was very small and it gradually decreased from 1.4 to 0.5 kg N ha^–1 although about 18 to 20, 12 to 17 and 14 to 18 kg ha^–1 residual fertilizer N was available in the root zone after harvest of first, second and third crops, respectively. The average uptake of the residual fertilizer N was only 7.6% in the second crop and it decreased to 4.5% in the third and to 3.2% in the fourth crop although all these crops were adequately fertilized with unlabelled urea. The basal application of neem coated urea was more effective in controlling the leaching loss of labelled NH₄+NO₃–N than split application of uncoated urea. In the first 3 seasons in which¹⁵N was detectable, the loss of fertilizer N through leaching as NH₄+NO₃–N amounted to 0.5 kg ha^–1 from neem-coated urea, 1.5 kg from split urea and 4.1 kg from coal tar-coated urea. At the end of 4 crops, most of the labelled fertilizer N (about 69% on average) was located in the upper 0–20 cm soil layer showing very little movement beyond this depth. In the profile sampled upto 60 cm depth, totally about 13.8 kg labelled fertilizer N ha^–1 from neem-coated urea, 12.7 kg from coal-tar coated urea, and 11.8 kg from split urea were recovered. The average recovery of labelled urea-N in crops and soil during the entire experimental period ranged between 42 and 51%. After correcting for leaching losses, the remaining 47 to 56% appeared to have been lost through ammonia volatilization and denitrification.     

Effects of pH,phosphate and ammonia on the rate of uptake of nitrate and ammonia by freshwater phytoplankton

The effect of phosphate (PO₄ ⁺³) and pH in regulating nitrate (NO₃) and ammonia (NH₃ ⁺) uptake by phytoplankton was investigated in two Oklahoma lakes using ¹⁵N tracers. Addition of PO₄ ⁺³ above ambient concentrations had a negligible effect on the rate of uptake of NO₃ ^– or NH₃ ⁺. Manipulation of pH of lake water had little effect on uptake of either NO₃ ^– or NH₃ ⁺. A correlation analysis suggested that NO₃ ^– is not used by phytoplankton when NH₃ ⁺ concentrations exceed about 210 µg NH₃ ⁺-N(1)^–1.     

In vitro weathering of phlogopite by ectomycorrhizal fungi

Oxalate accumulation in external medium under hyphal mats of two ectomycorrhizal species is strongly stimulated (1.7 to 35 fold) by a simultaneous depletion of available K⁺ and Mg²⁺. Pisolithus tinctorius strain 441 accumulates oxalate both on NH₄–N and on NO₃–N whereas Paxillus involutus strain COU only accumulates oxalate on NO₃–N. On NO₃–N, under a simultaneous K⁺ and Mg²⁺ deficiency, P. involutus COU is a very active oxalate producer compared to P. tinctorius 441. The present results could explain the various mineralogical evolutions of a phlogopite mica previously recorded under P. involutus COU or P. tinctorius 441 and suggest a key role for fungal oxalic acid during mineral weathering in response to nutrient deficiency.     

Functional role of DNRA and nitrite reduction in a pristine south Chilean <Emphasis Type="Italic">Nothofagus</Emphasis> forest

Nitrite (NO₂ ⁻) is an intermediate in a variety of soil N cycling processes. However, NO₂ ⁻ dynamics are often not included in studies that explore the N cycle in soil. Within the presented study, nitrite dynamics were investigated in a Nothofagus betuloides forest on an Andisol in southern Chile. We carried out a ¹⁵N tracing study with six ¹⁵N labeling treatments, including combinations of NO₃ ⁻, NH₄ ⁺ and NO₂ ⁻. Gross N transformation rates were quantified with a ¹⁵N tracing model in combination with a Markov chain Monte Carlo optimization routine. Our results indicate the occurrence of functional links between (1) NH₄ ⁺ oxidation, the main process for NO₂ ⁻ production (nitritation), and NO₂ ⁻ reduction, and (2) oxidation of soil organic N, the dominant NO₃ ⁻ production process in this soil, and dissimilatory NO₃ ⁻ reduction to NH₄ ⁺ (DNRA). The production of NH₄ ⁺ via DNRA was approximately ten times higher than direct mineralization from recalcitrant soil organic matter. Moreover, the rate of DNRA was several magnitudes higher than the rate of other NO₃ ⁻ reducing processes, indicating that DNRA is able to outcompete denitrification, which is most likely not an important process in this ecosystem. These functional links are most likely adaptations of the microbial community to the prevailing pedo-climatic conditions of this Nothofagus ecosystem.