不同质地土壤上烤烟氮素积累、分配及利用率的研究

【目的】土壤质地能概括反映土壤内在的肥力特征,对土壤养分供应具有调控作用,是影响农田中土壤氮素供应和氮肥利用的重要因素。本试验通过在皖南烟区3种质地(壤土、黏壤、砂壤)土壤上施用等量氮肥来研究其对烤烟不同生育期的氮素吸收、积累及利用特征的影响,旨在为烟田土壤改良及烤烟合理施肥提供理论依据。【方法】在皖南烟区现代农业科技园的典型壤土、黏壤和砂壤土上分别建立田间试验,采用15N田间微区试验和室内分析相结合的研究方法,在烤烟的团棵期(移栽后38 d)、现蕾期(移栽后53 d)、平顶期(移栽后64d)和成熟期(移栽后103 d),采集长势一致的烟株样品,测定烟株各部位的生物量,并采用凯氏定氮法检测其全氮含量,采用ZHTO2型同位素质谱仪测定其15N丰度。【结果】皖南烟区壤土和黏壤土上烤烟总氮和肥料氮积累均随生育期呈单峰变化,在烤烟平顶期达最大,总氮积累量分别为4.25 g/plant和3.96 g/plant,肥料氮积累量分别为2.34 g/plant和2.54 g/plant,而砂壤土上烤烟到成熟期其总氮和肥料氮的积累量达到最大,分别是5.64 g/plant和2.73 g/plant,均显著高于同时期的壤土和黏壤;壤土、黏壤和砂壤土上烤烟均以叶部肥料氮占总氮比例及氮素分配率较高,茎部次之,根部最低;不同质地土壤上烤烟氮肥利用率与肥料氮的积累动态具有一致的变化趋势,其中壤土和黏壤在平顶期最大,分别为34.5%和40.7%,之后壤土利用率缓慢下降,黏壤下降幅度较大,而砂壤土上烤烟氮肥利用率在生育期内呈上升趋势,至成熟期最大,为43.7%。【结论】不同质地土壤上烟株对氮素的吸收利用顺序为砂壤壤土黏壤,黏壤土在烤烟生育期内供氮能力较弱,应合理调控土壤氮的矿化及增加肥料氮的供应;砂壤土氮肥利用率较高,应严格控制氮肥的施用量。     

Nitrogen availability to grain sorghum from organic and inorganic sources on sandy and clayey soil surfaces in a greenhouse pot study

In a greenhouse pot study, we examined the availability of N to grain sorghum from organic and inorganic N sources. The treatments were¹⁵N-labeled clover residues, wheat residues, and fertilizer placed on a sandy clay loam and loamy sand soil surface for an 8-week period. Soil aggregates formed under each soil texture were measured after 8 weeks for each treatment. Significantly greater ¹⁵N was taken up and recovered by grain sorghum in sandy clay loam pots compared with loamy sand pots. Greater ¹⁵N recovery was consistently observed with the inorganic source than the organic sources regardless of soil texture or time. Microbial biomass C and N were significantly greater for sandy clay loam soil compared with the loamy sand. Microbial biomass ¹⁵N was also significantly greater in the sandy clay loam treatment compared to the loamy sand. The fertilizer treatment initially had the greatest pool of microbial biomass ¹⁵N but decreased with time. The crop residue treatments generally had less microbial biomass ¹⁵N with time. The crop residues and soil texture had a significant effect on the water-stable aggregates formed after 8 weeks of treatments. Significantly greater water-stable aggregates were formed in the sandy clay loam than the loamy sand. Approximately 20% greater water-stable aggregates were formed under the crop residue treatments compared to the fertilizer only treatment. Soil texture seemed to be one of the most important factors affecting the availability of N from organic or inorganic N sources in these soils.Contribution from the MissouriAgricultural Experiment Station, Journal Series No.12131     

Transformation of 15N-labelled leguminous plant material in three contrasting soils

Summary Two soils from Pakistan (Hafizabad silt loam and Khurrarianwala silt loam) and one from Illinois, USA (Drummer silty clay loam) were incubated with ¹⁵N-labelled soybean tops for up to 20 weeks at 30°C. Mineralization of soybean ¹⁵N was slightly more rapid in the Pakistani soils, and after 20 weeks of incubation, 50%, 53%, and 56% of the applied ¹⁵N was accounted for as (NH₄ ⁺+NO₃ ^–)-N in Drummer, Hafizabad, and Khurrarianwala soils, respectively. Potentially mineralizable N (determined by anaerobic incubation) varied between 1.5% and 10% of the applied ¹⁵N in the three soils at different stages of incubation; somewhat higher percentages were mineralizable in the Pakistani soils than in the Drummer soil. From 3.7% to 9% of the applied ¹⁵N was accounted for in the microbial biomass. From 10% to 32% of the applied N was recovered in the humic acid and fulvic acid fractions of the organic matter by sequential extraction with Na₄P₂O₇ and NaOH; from 12% to 49% was recovered in the humin fraction. Of the three soils, Drummer soil contained more ¹⁵N as humic and fulvic acids. In all cases, the ¹⁵N was approximately equally distributed between the humic and fulvic acid fractions. A significant percentage of the humin ¹⁵N (52%–78%, equivalent to 8%–34% of the applied ¹⁵N) occurred in non-hydrolyzable (6 N HCl) forms. Of the hydrolyzable ¹⁵N, 42%–51% was accounted for as amino acid-N followed in order by NH₃ (17%–30%), hydrolyzable unknown forms (20%–22%), and amino sugars (6%–2%). The recovery of applied ¹⁵N for the different incubation stages was 87±22%. Recovery was lowest with the Khurrarianwala soil, presumably because of NH₃ volatilization losses caused by the high pH of this soil.     

Uptake of carbon and nitrogen through roots of rice and corn plants,grown in soils treated with 13C and 15N dual-labeled cattle manure compost

Nitrogen and carbon dynamics in paddy and upland soils for rice cultivation and in upland soil for corn cultivation was investigated by using ¹³C and ¹⁵N dual-labeled cattle manure compost (CMC). In a soil with low fertility, paddy and upland rice took up carbon and nitrogen from the CMC at rates ranging from 0.685 to 1.051% of C and 17.6–34.6% of N applied. The ¹³C concentration was much higher in the roots than in the plant top, whereas the ¹⁵N concentration differed slightly between them, indicating that organic carbon taken up preferentially accumulated in roots. The ¹³C recovery in the plant top tended to be higher in upland soil than in paddy soil, whereas ¹⁵N applied was recovered at the same level in both paddy and upland soils. In the experiment with organic farming soil, paddy rice took up C and N from the CMC along with plant growth and the final recovery rates of ¹³C and ¹⁵N were 2.16 and 17.2% of C and N applied. In the corn experiment, a very large amount of carbon from the CMC was absorbed, accounting for at least 7 times value for rice. The final uptake rates of ¹³C and ¹⁵N reached about 13 and 10% of C and N applied, respectively. Carbon emission from the CMC sharply increased by 2 weeks after transplanting and the nitrogen emission was very low. It is concluded that rice and corn can take up an appreciable level of carbon and nitrogen from the CMC through roots.     

Chemical fixation of ammonia to soil organic matter after application of urea

Chemical fixation of NH₃ to soil organic matter was studied in two Swedish soils with different contents of organic matter: a clay soil with 2.3% C and an organic soil with 36.6% C. ¹⁵N‐labelled urea was applied at different rates to both sterilized and non‐sterilized soils. After 10 days, the soils were extracted and washed with K₂SO₄ and determined for total N and atom% ¹⁵N excess. Urea N was recovered as non‐extractable N in sterilized soil corresponding to 9.7% of supplied ^l5N‐labelled urea in the organic soil and 2.2% in the clay soil. Since no biological immobilization is thought to occur in the sterile soil, this non‐extractable N is suggested to be chemically fixed to soil organic matter. Owing to urea hydrolysis in the clay soil, pH increased from 6.3 to 9.3 and in the organic soil from 5.7 to 6.9 and 8.8, respectively, at the low and high urea supply.     

Availability of soil and fertilizer nitrogen to wetland rice following wheat straw amendment

Summary A pot experiment was conducted to study the availability of soil and fertilizer N to wetland rice as influenced by wheat straw amendment (organic amendment) and to establish the relative significance of the two sources in affecting crop yield. Straw was incorporated in soil at 0.1, 0.2, and 0.3% before transplanting rice. Inorganic N as ¹⁵N-ammonium sulphate was applied at 30, 60, and 90 g g^-1 soil either alone or together with wheat straw in different combinations. After harvesting the rice, the plant and soil samples were analyzed for total N and ¹⁵N. Straw incorporation significantly decreased the dry matter and N yield of rice, the decrease being greater with higher rates of straw. The reduction in crop yield following the straw incorporation was attributed mainly to a decrease in the uptake of soil N rather than fertilizer N. The harmful effects of organic matter amendment were mitigated by higher levels of mineral N addition. The uptake of applied N increased and its losses decreased due to the straw incorporation. Mineral N applied alone or together with organic amendment substantially increased the uptake of unlabelled soil N. The increase was attributed to a real added N interaction.     

Fate of residual 15N-labeled fertilizer in dryland farming systems on soils of contrasting fertility

Abstract

Up to 50% of nitrogen (N) fertilizer can remain in soil after crop harvest in dryland farming. Understanding the fate of this residual fertilizer N in soil is important for evaluating its overall use efficiency and environmental effect. Nitrogen-15 (¹⁵N)-labeled urea (165 kg N ha^?1) was applied to winter wheat (Triticum aestivum L.) growing in three different fertilized soils (no fertilizer, No-F; inorganic nitrogen, phosphorus and potassium fertilization, NPK; and manure plus inorganic NPK fertilization, MNPK) from a long-term trial (19 years) on the south of the Loess Plateau, China. The fate of residual fertilizer N in soils over summer fallow and the second winter wheat growing season was examined. The amount of the residual fertilizer N was highest in the No-F soil (116 kg ha^?1), and next was NPK soil (60 kg ha^?1), then the MNPK soil (43 kg ha^?1) after the first winter wheat harvest. The residual fertilizer N in the No-F soil was mainly in mineral form (43% of the residual ¹⁵N), and for the NPK and MNPK soils, it was mainly in organic form. The loss rate of residual ¹⁵N in No-F soil over summer fallow was as high as 48%, and significantly (P < 0.05) higher than that in the NPK soil (22%) and MNPK soil (19%). The residual ¹⁵N use efficiency (RNUE) by the second winter wheat was 13% in the No-F soil, 6% in the NPK soil and 8% in the MNPK soil. These were equivalent to 9.0, 2.0 and 2.2% of applied ¹⁵N. The total ¹⁵N recovery (¹⁵N uptake by crops and residual in 0–100 cm soil layer) in the MNPK and NPK soils (84.5% and 86.6%, respectively) were both significantly higher than that in the No-F soil (59%) after two growing seasons. The ¹⁵N uptake by wheat in two growing seasons was higher in the MNPK soil than in NPK soil. Therefore, we conclude that a high proportion of the residual ¹⁵N was lost during the summer fallow under different land management in dryland farming, and that long-term combined application of manure with inorganic fertilizer could increase the fertilizer N uptake and decrease N loss.     

A 15N tracing technique-based analysis of the fate of fertilizer N: a 4-year case study in eastern China

The fate of fertilizer N applied with different irrigation amounts in tobacco fields was quantitatively studied by applying ¹⁵N double-labelled NH₄NO₃ in lysimeters. The ¹⁵N (fertilizer N originating from the fertilizer applied in 2011) in tobacco plants, ¹⁵N in soils and ¹⁵N loss were observed continuously from 2011 to 2014. The results showed that 21.6% of ¹⁵N was utilized by tobacco plants, 72.1% remained in the 0–60 cm soil layer and 6.3% was lost from the soil–plant system after the first season’s harvest (2011) of flue-cured tobacco. During the four seasons from 2011 to 2014, cumulative utilization of ¹⁵N by tobacco plants was 34.3%, while 54.2% remained in the 0–60 cm soil layer, and 11.5% was lost via mechanisms such as leaching and volatilization. The fate of ¹⁵N in terms of accumulation in plants and soils or losses from the soil–plant system from 2012 to 2014 was greatly affected by the fertilizer and irrigation management strategies in 2011. The results of this investigation suggest that the major amount of fertilizer N applied during the first season remains available in the soil for utilization by tobacco plants after 4 years.     

麦-稻轮作系统有机无机肥料配施协同氮素转化的机制研究Ⅱ.小麦季残留~(15)N对水稻的有效性分析

以15氮(N)标记的硫酸铵和兔粪作为无机肥和有机肥氮源,采用土培试验,研究了小麦-水稻轮作(W-R)系统小麦季残留的氮素对下茬水稻(O.sativaL.cv.Wuyujing7)生长的影响。结果表明:1)小麦季残留的氮素对水稻生长具有显著影响,有机无机肥料配施(IOF)处理的水稻籽粒产量均高于单一无机氮肥施用(IF)处理,黏壤土和黏土上分别高49.3%和14.9%;2)与IF处理相比较,IOF处理的矿质氮含量在水稻季均保持较高水平,且IOF处理显著增加了黏壤土上穗期和黏土上分蘖期、拔节期、穗期土壤矿质氮中来自于有机肥料15N和无机肥料15N的比例。IOF处理还显著增加了水稻分蘖期、拔节期、穗期土壤微生物量氮(MBN)中来自于无机肥料15N的比例;3)与IF处理相比较,IOF处理有机肥料氮促进了小麦季残留无机肥料15N在水稻生殖器官籽粒的分配比例,黏壤土和黏土上分别增加60%和52.6%,降低了其在叶片等营养器官的分配比例,降幅均在30%以上;与单一有机氮肥(OF)处理相比较,IOF处理无机肥料氮降低了残留有机肥料15N在水稻生殖器官籽粒的分配比例,黏壤土和黏土上降幅分别为20%和22.7%,增加了其在根系的分配比例,黏壤土和黏土增幅分别为90%和240%;4)与IF或OF处理相比较,IOF处理有机肥料氮促进了无机肥料15N在土壤的残留,而无机肥料氮增加了有机肥料15N在植株和土壤的回收率。有机肥料氮和无机肥料氮的协同降低了双方在水稻季向环境的损失,黏壤土和黏土上无机肥料15N损失率(NLR)分别降低17%和16%,而有机肥料15N的NLR分别降低15%和56%。IOF通过提高有机、无机肥料氮在土壤-作物系统的回收减少了肥料氮在W-R轮作系统的损失。     

Effects of Slow‐Release Fertilizers on Tomato Growth and Nitrogen Leaching

Tomatoes (Lycopersicon esculentum Mill.) were grown in 9.46‐L plastic pots in a glasshouse for evaluation of their growth and nitrogen (N) losses through leaching. Plants were fertilized with either ammonium nitrate (AN) or one of three slow‐release N fertilizers. The slow‐release N fertilizers were Georgia Pacific liquid 30‐0‐0 (L30), Georgia Pacific granular 42‐0‐0 (N42), and Georgia Pacific granular 24‐0‐0 (N24). Each fertilizer was applied at 112 low N rate (L) and 224 high N rate (H) kg N ha^?1. The pots were filled with either a sandy soil from Florida or a loam soil from Georgia. Increasing the N rate did not influence shoot biomass at 19 days after transplanting (DAT) and increased biomass production at 77 DAT. Shoot biomass differed significantly among fertilizer treatments. The accumulation of N in shoots was significantly influenced by fertilizer source, rate, and soil type. The plants grown in the loam soil accumulated significantly more N than those grown in the sandy soil with the same treatment. In the loam soil, the highest and lowest N accumulations occurred in the N42‐H and N24‐L treatments, respectively; and in the sandy soil the corresponding treatments were AN‐H and N24‐L. The amount of N leached varied with the different fertilizers, soils, and time. The net leaching of N ranged from ?0.4% to 6.3% of the fertilizer N applied for the loam soil and 6.5% to 32.9% for the sand soil. The net amount of N leached from the loam soil at both high and low application rates declined in the following order: AN > N24 > N42 > L30; the corresponding order for the sandy soil was AN‐H > N42‐H > L30‐H > N24‐H. L30 had the least leaching potential, and ammonium nitrate had the most. Slow‐release fertilizers had significantly less leaching N than did ammonia nitrate.     

Fertilizer placement to maximize nitrogen use by fescue

The method of fertilizer nitrogen (N) application can affect N uptake in tall fescue and therefore its yield and quality. Subsurface-banding (knife) of fertilizer maximizes fescue N uptake in the poorly-drained clay-pan soils of southeastern Kansas. This study was conducted to determine if knifed N results in greater N uptake than the conventional top-dress application method in a deep, well-drained soil of east-central Kansas. The experiment, conducted in a Smolan silty clay loam soil, was a split-plot with fertilizer nitrogen rates 0, 140 and 280 kg N ha^?1 applied as urea-ammonium nitrate (UAN, 28% N), knifed or top-dressed. Soil inorganic N [ammonium (NH₄)- and nitrate (NO₃–N)] and N in roots and plant tops were measured at various times during the growing season. At final harvest, most of the knifed N (99.7%) was accounted for in plant tissue (roots and tops) and soil, with more than half of the knifed N remaining as soil inorganic N. With the top-dressed method, 27% was unaccounted for and presumed lost in gaseous form. Knifing fertilizer N in fescue fields of east-central Kansas will maximize the availability of N, reduce potential N losses, and increase forage quality.     

Fate of Fertilizer-15N to a Maize Crop Grown in Northern Zambia

Abstract

A field study with maize (Zea mays L.) was conducted in the 1988/89 cropping season to investigate the fate of ¹⁵NO₃-N-labelled NH₄ ¹⁵NO₃ applied at 40, 80 and 120 kg N ha^?1 (unlabelled N applied at 0, 80, 160 and 240 N ha^?1) with and without lime. The investigations were conducted in northern Zambia at Misamfu Regional Research Centre, Kasama on a Misamfu red sandy loam soil. The experimental design was a split plot arrangement with four replications with main plots receiving 0 and 2 Mg ha^?1 dolomitic limestone, while subplots received fertilizer N at various rates. Significant (p < 0.001) grain and DM yield responses to applied N up to 160 kg ha^?1 were observed. At higher rates little or no crop responses were observed and fertilizer use efficiency declined. Partitioning of amounts of total N and ¹⁵N in plants was in the order of seed = tassel > leaf> cob = earleaf> stem. Fertilizer N rates showed a highly significant (p < 0.001) effect on plant uptake of labelled N. Lime and its interaction with N rates had no effect on all measured parameters. Leaching of NO₃-N fertilizer to lower soil depths was in proportion to the rate of N applied, with highly significant (p < 0.001) differences among soil depths. Although higher concentrations of fertilizer-¹⁵N were recovered in the 0–20 cm depth the recovered portion at lower soil depths was still significant. Total recovery of labelled N by plant and by soil after crop harvest averaged 75, 55 and 54% of originally applied fertilizer-¹⁵N at 40, 80 and 120 kg N ha^?1, respectively. Corresponding unaccounted for ¹⁵N was 25, 45 and 46%. The most probable loss mechanism could have been by leaching to depths greater than 60 cm, gaseous losses to the atmosphere and root assimilation.     

Extractability of 15N-labeled corn-shoot tissue in a sandy and a clay soil by 0.01 MCaCl2 method in laboratory incubation experiments

Management of N fertilization depends not only on the mineral N measured at the beginning of the growing season but also on the status of the low-molecular-weight organic-N fraction. Our study was conducted to analyze how much of the ¹⁵N applied in labeled cornshoot tissue would be recovered in 0.01 M CaCl₂-extractable ¹⁵N fractions and wheter a decrease in the CaCl₂-extractable ¹⁵N fraction quantitatively followed the trend in net mineralization of the ¹⁵N applied in corn-shoot tissue during an incubation period. The effects of adding ¹⁵N-labeled young corn-shoot tissue to a sandy soil and a clay soil were investigated for 46 days in an aerobic incubation experiment at 25°C. The application of 80 mg N kg^-1 soil in the form of labeled corn-shoot tissue (24.62 mg ¹⁵N kg^-1 soil) resulted in a significant initial increase, followed by a decrease the labeled organic-N fraction in comparison with the untreated soils during the incubation. The labeled organic-N fraction was significantly higher in the sandy soil than in the clay soil until the 4th day of incubation. The decrease in labeled organic N in the sandy soil resulted in a subsequent increase in ¹⁵NO _inf3 ^sup- during the incubation. Ammonification of applied plant N resulted in a significant increase in the 1 M HCl-extractable non-exchangeable ¹⁵NH _inf4 ^sup+ fraction in the clay soik, owing to the vermiculite content. The ¹⁵N recovery was analyzed by the 0.01 M CaCl₂ extraction method; at the beginning of the incubation experiment, recovery was 37.0% in the sandy soil and 36.7% in the clay soil. After 46 days of incubation, recovery increased to 47.2 and 43.8% in the sandy and clay soils, respectively. Net mineralization of the ¹⁵N applied in corn-shoot tissue determined after the 46-day incubation was 6.60 mg ¹⁵N kg^-1 soil (=34.9% of the applied organic ¹⁵N) and 4.37 mg ¹⁵N kg^-1 soil (=23.1% of the applied organic ¹⁵N) in the sandy and the clay soils, respectively. The decrease in the labeled organic-N fraction extracted by 0.01 M CaCl₂ over the whole incubation period was 3.14 and 2.33 mg ¹⁵N kg^-1 soil in the sandy and clay soil, respectively. These results indicate that net mineralization of ¹⁵N was not consistent with the decrease in the labeled organic-N fraction. This may have been due to the inability of 0.01 M CaCl₂ to extract or desorb all of the applied organic ¹⁵N that was mineralized during the incubation period.     

苏州地区平田黄泥土氮素供应过程的特点及其与氮肥施用方法的关系

氮肥对作物的增产效果,决定于作物对氮肥的吸收率(即氮肥的利用率)和作物体内累积的氮素转化成经济产量的效率。水稻对氮肥的利用率一般显著低于旱作,未被吸收利用的部分从土壤中的损失一般也较旱地多。     

Interactive effects from combining fertilizer and organic residue inputs on nitrogen transformations

Concerns about sustainability of agroecosystems management options in developed and developing countries warrant improved understanding of N cycling. The Integrated Soil Fertility Management paradigm recognizes the possible interactive benefits of combining organic residues with mineral fertilizer inputs on agroecosystem functioning. However, these beneficial effects may be controlled by residue quality. This study examines the controls of inputs on N cycling across a gradient of (1) input, (2) residue quality, and (3) texture. We hypothesized that combining organic residue and mineral fertilizers would enhance potential N availability relative to either input alone. Residue and fertilizer inputs labeled with ¹⁵N (40–60 atom% ¹⁵N) were incubated with 200 g soil for 545 d in a microcosm experiment. Input treatments consisted of a no-input control, organic residues (3.65 g C kg⁻¹ soil, equivalent to 4 Mg C ha⁻¹), mineral N fertilizer (100 mg N kg⁻¹ soil, equivalent to 120 kg N ha⁻¹), and a combination of both with either the residue or fertilizer ¹⁵N-labeled. Zea mays stover inputs were added to four differently textured soils (sand, sandy loam, clay loam, and clay). Additionally, inputs of three residue quality classes (class I: Tithonia diversifolia, class II: Calliandra calothyrsus, class III: Z. mays stover) were applied to the clay soil. Available N and N₂O emissions were measured as indicators for potential plant N uptake and N losses. Combining residue and fertilizer inputs resulted in a significant (P < 0.05) negative interactive effect on total extractable mineral N in all soils. This interactive effect decreased the mineral N pool, due to an immobilization of fertilizer-derived N and was observed up to 181 d, but generally became non-significant after 545 d. The initial reduction in mineral N might lead to less N₂O losses. However, a texture effect on N₂O fluxes was observed, with a significant interactive effect of combining residue and fertilizer inputs decreasing N₂O losses in the coarse textured soils, but increasing N₂O losses in the fine textured soils. The interactive effect on mineral N of combining fertilizer with residue changed from negative to positive with increasing residue quality. Our results indicate that combining fertilizer with medium quality residue has the potential to change N transformations through a negative interactive effect on mineral N. We conclude that capitalizing on interactions between fertilizer and organic residues allows for the development of sustainable nutrient management practices.     

Microbial biomass and N transformations in two soils cropped with annual and perennial species

A field study was undertaken to determine the effects of different plant species on soil microbial biomass and N transformations in a well drained silty clay loam (Typic Dystrochrept) and a poorly drained clay loam (Typic Humaquept). The crop treatments were faba bean (Vicia faba L.), alfalfa (Medicago sativa L.), timothy (Phleum pratense L.), bromegrass (Bromus inermis L.), reed canarygrass (Phalaris arundinacea L.), and wheat (Triticum aestivum L.). Measurements of microbial biomass C, denitrification capacity, and nitrification capacity were performed periodically in the top 2–10 cm of soil. On most sampling dates, all three parameters were higher under perennial than under annual species. The nitrification capacity was positively affected by the level of N applied to each species (r=0.65^** for the silty clay loam and 0.84^*** for the clay loam) and not directly by the plant. The differences found in microbial biomass C were significantly correlated with the water-soluble organic C present under each plant species (r=0.74^*** for the silty clay loam and 0.90^*** for the clay loam), suggesting differences in C deposition in the soil among plant species. In the silty clay loam, the denitrification capacity was positively related to the amount of organic C found under each plant species, while in the clay loam, it was dependent on the amount of N applied to each species. There was less denitrification activity per unit biomass under legume species than under graminease, suggesting that, depending on their composition, root-derived materials may be used differently by soil microbes.     

Leaching of dissolved organic carbon in soil following anhydrous ammonia application

The transport of anhydrous NH₃-solubilized soil organic matter from surface to subsurface soils may affect subsurface microbial activity. In the present study we determined the impact of anhydrous NH₃-N fertilizer on organic C solubilization and the propensity of solubilized C to leach with percolating water. In fertilized treatments, anhydrous NH₃ was subsurface-banded at 20g N m^-2 in ridge or valley areas of a ridge tillage system. In contol treatments, 0g N m^-2 was banded into the valley area of a ridge tillage system. Rainfall (17 cm) was applied with a drop-type artificial rainfall simulator 3, 10, and 24 days after the fertilizer application. The treatments were replicated twice. Grid lysimeters (15 by 15 cm) were placed 75 cm below the soil surface of a Brandt silty clay loam (fine-silty over sandy or sandy skeletal mixed Pachic Udic Haploboroll). Lysimeters were used to collect percolating water temporally and spatially. The application of N fertilizer increased dissolved organic C concentrations in percolating water when rainfall was applied 3 days after the fertilizer application. However, when the rainfall was applied 24 days after the fertilizer application the dissolved organic C concentrations in percolating water was not influenced by anhydrous NH₃ application. The smaller dissolved organic C concentrations in percolating water with a longer incubation time were most likely the result of microbial assimilation or respiration of solubilized C.     

Fate and interaction with native soil N of ammonium N applied to wetland rice (Oryza sativa L.) grown under saline and non-saline conditions

Summary The effect of salts on the balance of fertilizer N applied as ¹⁵N-labelled ammonium sulphate and its interaction with native soil N was studied in a pot experiment using rice (Oryza sativa L.) as a test crop. The rice crop used 26%–40% of the applied N, the level of applied N and salts showing no significant bearing on the uptake of fertilizer N. Losses of fertilizer N ranged between 54% and 68% and only 5%–8% of the N was immobilized in soil organic matter. Neither the salts nor the rate of N application had any significant effect on fertilizer N immobilization. The effective use of fertilizer N (fertilizer N in grain/fertilizer N in whole plant) was, however, better in the non-saline soil. The uptake of unlabelled N (N mineralized from soil organic matter and that originating from biological N₂ fixation in thes rhizosphere) was inhibited in the presence of the salts. However, in fertilized soil, the uptake of unlabelled N was significantly enhanced, leading to increased A values [(1-% Ndff/% Ndff)x N fertilizer applied, where Ndff is N derived from fertilizer], an index of interaction with the added N. This added N interaction increased with increasing levels of added N. Since the extra unlabelled N taken up by fertilized plants was greater than the fertilizer N immobilized, and the root biomass increased with increasing levels of added N, a greater part of the added N interaction was considered to be real, any contribution by an apparent N interaction (pool substitution or isotopic displacement) to the total calculated N interaction being fairly small. Under saline conditions, for the same level of fertilizer N addition, the added N interaction was lower, and this was attributed to a lower level of microbial activity, including mineralization of native soil N, rootdriven immobilization of applied N, and N₂ fixation.     

Response of soil exchangeable and crop potassium concentrations to variable fertilizer and cropping regimes in long-term field experiments on different soil types

Annual potassium (K) balances have been calculated over a 40‐year period for five field experiments located on varying parent materials (from loamy sand to clay) in south and central Sweden. Each experiment consisted of a number of K fertilizer regimes and was divided into two crop rotations, mixed arable/livestock (I) and arable only (II). Annual calculations were based on data for K inputs through manure and fertilizer, and outputs in crop removal. Plots receiving no K fertilizer showed negative K balances which ranged from 30 to 65 kg ha^?1 year^?1 in rotation I, compared with 10–26 kg ha^?1 year^?1 for rotation II. On sandy loam and clay soils, the K yield of nil K plots (rotation I) increased significantly with time during the experimental period indicating increasing release of K from soil minerals, uptake from deeper soil horizons and/or depletion of exchangeable soil K (K_ex). Significant depletion of K_ex in the topsoil was only found in the loamy sand indicating a K supply from internal sources in the sandy loam and clay soils. On silty clay and clay soils, a grass/clover ley K concentration of ～2% (dry weight) was maintained during the 40‐year study period on the nil K plots, but on the sandy loam, loam and loamy sand, herbage concentrations were generally less than 2% K.     

Residue Decomposition and Fate of Nitrogen‐15 in a Wheat Crop under Different Previous Crops and Tillage Systems

Abstract

Nitrogen (N) management may be improved by a thorough understanding of the nutrient dynamics during previous‐crop residue decomposition and its impact on fertilizer N fate in the soil–plant system. An experiment was conducted in the Argentine Pampas to evaluate the effect of maize and soybean as previouscrops and plow‐till and no‐till methods on N dynamics and ¹⁵N‐labeled fertilizer uptake during a wheat growing season. Maize and soybean residues released N under both tillage treatments, but N release was faster from soybean residues and when residues were buried by tillage. Net immobilization of N on decomposing residues was not detected. A regression model that accounted for 92% of remaining N variability included time, previous crop, and tillage treatment as independent variables. The rapid residue decomposition with N release was attributed to the high temperatures of the agroecosystem. The recovery of ¹⁵N‐labeled fertilizer in the wheat crop, soil organic matter, and decomposing residues was not statistically different between previous crop treatments or tillage systems. Crop uptake of fertilizer N averaged 52% across treatments. Forty percent of fertilizer N was removed in grains. Immobilization of labeled N on soil organic matter was substantial, averaging 34% of the ¹⁵N‐labeled fertilizer retained, but was very small on decomposing residues, averaging 0.2–3.0%. Fertilizer N not accounted for at harvest in the soil–plant system was 12% and was ascribed to losses. Previous crop or tillage system had no impact on wheat yield, but when soybean was the previous crop, N content of grain and straw+roots increased. Discussion is presented on the potential availability of N retained in wheat straw, roots, and soil organic matter for future crops.