Priming effects of 15N-labelled ammonium nitrate on uptake of soil N by wheat (Triticum aestivum L.) under field conditions

Summary The uptake of labelled and unlabelled N by wheat was measured in a field experiment using ¹⁵N-labelled ammonium nitrate fertilizer. The dry matter yield and N yields were significantly increased with fertilizer N application compared to those from unfertilized soil. The uptake of applied N by wheat ranged between 25 and 34%. Fertilizer N application increased the uptake of unlabelled soil N which was attributed to a positive priming effect or added N interaction. The added N interaction observed by applying 20, 60, and 120 kg fertilizer N was 11.4, 19.1, and 27.9 kg, corresponding to 26, 44 and 64%, respectively of the N taken up from unfertilized soil. The A values did not alter with the increase in fertilizer N application. The observed added N interaction may have been the result of pool substitution whereby added labelled fertilizer N stood proxy for unlabelled soil N. A significant correlation coefficient (r=0.996^**) between the uptake of soil N and the dry matter yield showed that soil N was more important than fertilizer N in wheat production.     

Nitrogen immobilization and mineralization during initial decomposition of15N-labelled pea and barley residues

The immobilization and mineralization of N following plant residue incorporation were studied in a sandy loam soil using¹⁵N-labelled field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) straw. Both crop residues caused a net immobilization of soil-derived inorganic N during the complete incubation period of 84 days. The maximum rate of N immobilization was found to 12 and 18 mg soil-derived N g^–1 added C after incorporation of pea and barley residues, respectively. After 7 days of incubation, 21% of the pea and 17% of the barley residue N were assimilated by the soil microbial biomass. A comparison of the¹⁵N enrichments of the soil organic N and the newly formed biomass N pools indicated that either residue N may have been assimilated directly by the microbial biomass without entering the soil inorganic N pool or the biomass had a higher preference for mineralized ammonium than for soil-derived nitrate already present in the soil. In the barley residue treatment, the microbial biomass N was apparently stabilized to a higher degree than the biomass N in the pea residue treatment, which declined during the incubation period. This was probably due to N-deficiency delaying the decomposition of the barley residue. The net mineralization of residue-derived N was 2% in the barley and 22% in the pea residue treatment after 84 days of incubation. The results demonstrated that even if crop residues have a relative low C/N ratio (15), transient immobilization of soil N in the microbial biomass may contribute to improved conservation of soil N sources.     

Interaction of 15N-labelled ammonium nitrate,urea and ammonium sulphate with soil N during growth of wheat (Triticum aestivum L.) under field conditions

The effects of ¹⁵N-labelled ammonium nitrate, urea and ammonium sulphate on yield and uptake of labelled and unlabelled N by wheat (Triticum aestivum L. cv. Mexi-Pak-65) were studied in a field experiment. The dry matter and N yields were significantly increased with fertilizer N application compared to those from unfertilized soil. The wheat crop used 64.0–74.8%, 61.5–64.7% and 61.7–63.4% of the N from ammonium nitrate, urea and ammonium sulphate, respectively. The fertilizer N uptake showed that ammonium nitrate was a more available source of N for wheat than urea and ammonium sulphate. The effective use of fertilizer N (ratio of fertilizer N in grain to fertilizer N in whole plant) was statistically similar for the three N fertilizers. The application of fertilizer N increased the uptake of unlabelled soil N by wheat, a result attributed to a positive added N interaction, which varied with the method of application of fertilizer N. Ammonium nitrate, urea and ammonium sulphate gave 59.3%, 42.8% and 26.3% more added N interaction, respectively, when applied by the broadcast/worked-in method than with band placement. A highly significant correlation between soil N and grain yield, dry matter and added N interaction showed that soil N was more important than fertilizer N in wheat production. A values were not significantly correlated with added N interaction (r=0.719). The observed added N interaction may have been the result of pool substitution, whereby added labelled fertilizer N stood proxy for unlabelled soil N.     

Effects of 15N-labelled ammonium nitrate and urea on soil nitrogen during growth of wheat (Triticum aestivum L.) under field conditions

We studied the effects of ¹⁵N-labelled ammonium nitrate and urea on the yield and uptake of labelled and unlabelled N by wheat (Triticum aestivum L., cv. Mexi-Pak-65) in a field experiment. The dry matter and N yields were significantly increased with fertilizer N application compared to those from unfertilized soil. The wheat crop used 33.6–51.5 and 30.5–40.9% of the N from ammonium nitrate and urea, respectively. Splitting the fertilizer N application had a significant effect on the uptake of fertilizer N by the wheat. The fertilizer N uptake showed that ammonium nitrate was a more available source of N for wheat than urea. The effective use of fertilizer N (ratio of fertilizer N in grain to fertilizer N in whole plant) was statistically similar for the two N fertilizers. The application of fertilizer N increased the uptake of unlabelled soil N by wheat, a result attributed to a positive added N interaction, which varied according to the fertilizer N split; six split applications gave the highest added N interaction compared to a single application or two split applications for both fertilizers. Ammonium nitrate gave 90.5, 33.5, and 48.5% more added N interaction than urea with one, two, and six split N applications. A values were not significantly correlated with the added N interaction (r=0.557). The observed added N interaction may have been the result of pool substitution, whereby added labelled fertilizer N replaced unlabelled soil N.     

氮肥施用对作物吸收土壤氮的影响——兼论作物氮肥利用率

在15N标记肥料微区试验中采取了抑制土壤中氮生物固持作用的措施——用长期不施肥试验地、小麦拔节前施15N标记KNO3,成功显示了影响施氮作物吸收土壤氮的另一个过程:肥料氮对土壤氮库的稀释作用(或作物吸收养分过程中肥料氮和土壤氮的库替换作用)的存在;试验中施氮作物较不施氮作物少吸收土壤氮,呈现负的ANI(加入氮交互作用)。生物固持作用和肥料氮对土壤氮库的稀释作用是两个作用相反且交织在一起的过程,其相互抵消、平衡后的最终结果可以是正ANI,也可以是负ANI,决定于两者影响力的强弱。由于受上述过程的影响,用田间试验估测作物氮肥利用率,无论15N标记法或传统的差值法,均不可能获得可信的结果。作为替代方法,比值法可避免上述过程的影响,经本试验检验,结果良好。     

Estimating N rhizodeposition of grain legumes using a N in situ stem labelling method

Grain legumes in crop rotations cause significant increases in yield for succeeding non-legumes, which cannot be explained simply by the small effect that legumes have on the soil nitrogen balance, as found in the analysis of N in crop residues. Besides known positive non-N-effects, other effects, mainly rhizodeposition and its contribution to the N balance and nitrogen dynamics after harvesting the grain, are poorly understood. In this study, N rhizodeposition, defined as root-derived N in the soil after removal of visible roots, was measured in faba bean (Vicia faba L.), pea (Pisum sativum L.) and white lupin (Lupinus albus L.). In a pot experiment the legumes were pulse labelled in situ with ¹⁵N urea using a cotton wick method. About 84% of the applied ¹⁵N was recovered for the three legume species at maturity. The ¹⁵N was comparatively uniformly distributed among plant parts. The N rhizodeposition constituted 13% of total plant N for faba bean and pea and 16% for white lupin at maturity, about 80% of below ground plant N, respectively. Some 7% (lupin)-31% (pea) of the total N rhizodeposits were recovered as micro-roots by wet sieving (200 μm) the soil after all visible roots had been removed. Only 14-18% of the rhizodeposition N was found in the microbial biomass and a very small amount of 3-7% was found in the mineral N fraction. In pea, 48% and in lupin 72% of N rhizodeposits could not be recovered in the mentioned pools and a major part of the unrecovered N was probably immobilised in microbial residues. The results of this study clearly indicate that N rhizodeposition from grain legumes represent a significant pool for N balance and N dynamics in crop rotations.     

Calcium concentrations of soil affect suppressiveness against Aphanomyces root rot of pea

The potential for field soils to cause Aphanomyces root rot of pea (Pisum sativum) was estimated for a large number of samples from commercial pea fields over a period of 5 years, using a greenhouse bioassay. The aim of the research project was to gain a mechanistic understanding of soil suppressiveness to the disease. Regression analysis showed that of the measured soil variables (Ca, Mg, K, P, pH), soil Ca concentrations had the strongest (negative) correlation with disease prevalence, and also a significant negative correlation with disease severity in samples with confirmed presence of the disease. Greenhouse bioassays using a set of non-infested soils inoculated with artificially produced oospore inoculum of the casual organism Aphanomyces euteiches, showed a similar negative correlation between soil Ca content and disease severity. Disease severity was not consistently affected by soil sterilisation, but was lowered by the addition of two different Ca salts. In contrast, addition of sodium bicarbonate to two soils lowered the content of water-soluble Ca in the soils and increased disease severity. Studies of cultures of A. euteiches exposed to varying Ca concentrations in vitro showed that zoospore production was inhibited at submillimolar concentrations, while mycelial growth was stimulated or unaffected. We conclude that free Ca is a major variable controlling the degree of soil suppressiveness against A. euteiches, and that inhibition of zoospore production from oospores is a possible mechanism.     

Nitrate removal from drained and reflooded fen soils affected by soil N transformation processes and plant uptake

Knowledge about nitrate transformation processes and how they are affected by different plants is essential in order to reduce the loss of valuable N fertiliser as well as to prevent environmental pollution due to nitrate leaching or N₂O emission after fertilisation or the reflooding of degraded fens with nitrate-containing municipal sewage. Therefore four microcosm ¹⁵N tracer experiments were performed to evaluate the effect of common wetland plants (Phalaris arundinacea, Phragmites australis) combined with different soil moisture conditions (from dry to reflooded) on nitrate turnover processes. At the end of experiment, the total formation of gaseous N compounds was calculated using the ¹⁵N balance method. In two experiments (wet and reflooded soil conditions) the N₂O and N₂ emissions were also directly determined.Our results show that in degraded fen soils, which process mainly takes place—denitrification or transformation into organic N compounds—is determined by the soil moisture conditions. Under dry soil moisture conditions (water filled pore space: 31%) up to 80% of the ¹⁵N nitrate added was transformed into organic N compounds. This transformation process is not affected by plant growth. Under reflooded conditions (water filled pore space: 100%), the total gaseous N losses were highest (77-95% of the ¹⁵N-nitrate added) and the transformation into organic N compounds was very low (1.8% of ¹⁵N nitrate added). Under almost all soil conditions plant growth reduced the N losses by 20-25% of the ¹⁵N nitrate added due to plant uptake. The N₂ emissions exceeded the N₂O emissions by a factor of 10-20 in planted soil, and as much as 30 in unplanted soil. In the treatments planted with Phragmites australis, N₂O emission was about two times higher than in the corresponding unplanted treatment. 15% of the N₂O and N₂ formed was transported via the Phragmites shoots from the soil into the atmosphere. By contrast, Phalaris arundinacea did not affect N₂O emissions and no emission via the shoots was observed.     

连续翻压紫云英对福建单季稻产量与化肥氮素吸收、分配及残留的影响

【目的】紫云英翻压后在一定程度上可改善土壤理化性状,并提高后作水稻的产量,但是该机理是由于紫云英翻压矿化后提供的氮素还是由于与翻压紫云英后化肥氮素利用率的提高有关尚不清楚,因此,本项目通过连续4年紫云英翻压还田的定位试验与原状土柱模拟及15N示踪,研究了福建单季稻区紫云英压青回田对水稻产量与化肥15N吸收、 分配及残留的影响。【方法】采用单季稻田间定位试验,设5个处理: 1）对照,不翻压紫云英,不施化肥（CK）;2）不翻压紫云英,常规化肥施用量（100%H）;3）紫云英＋常规化肥用量（Z+100%H）;4）紫云英＋60%的常规化肥（Z+60%H）;5）只翻压紫云英,不施化肥（Z）。常规化肥用量（100%H）为施氮量N 135 kg/hm2,N∶P2O5∶K2O=1∶0.4∶0.7,每年紫云英翻压量为18000 kg/hm2。每个处理3次重复,小区面积15 m2,种植水稻为每小区2015丛。于定位试验的第4年,在田间定位试验小区中,采用15N-尿素（丰度10%）示踪法与原状土柱模拟水稻植株的氮素吸收及分配情况。PVC管直径25 cm,长35 cm,其中压入田面下20 cm,每小区埋两个土柱,每个土柱中种植两株水稻。【结果】紫云英年翻压18000 kg/hm2并结合施用100%化肥（Z+100%H）,水稻子粒4年平均产量比单施100%化肥（100%H）增产6.5%,同时在18000 kg/hm2 的紫云英翻压量下,主作物水稻化肥减量40%（Z+60%H）的产量与100%H的处理基本相当。Z+100%H处理对提高水稻分蘖期植株氮含量最为明显,尤其是茎叶氮含量较100%H提高7.0%,差异显著。虽然不同施肥处理水稻生育期的化肥氮素利用率无明显变化,但Z+100%H处理分蘖期与成熟期植株氮素吸收量分别较100%H提高23.0%与18.0%,说明绿肥与化肥配施有利于水稻植株吸收外源氮素,且植株吸收氮的差异主要来自于紫云英矿化的氮源。Z+60%H 与100%H处理的分蘖期与成熟期植株氮素吸收量则基本相当。不同施肥处理均有提高土壤全氮含量的趋势;Z+60%H 处理的耕层土壤化肥氮素的残留率最高,并显著高于Z+100%H处理。【结论】连续4年翻压紫云英明显提高了福建单季稻区黄泥田的农田生产力,在减少40%常规化肥用量的情况下仍可维持产量稳定。翻压绿肥减肥增效的主要机制之一是紫云英矿化的养分替代了化肥。     

The release and plant uptake of nitrogen from some plant and animal manures

Summary Field and laboratory experiments were used to examine the efficiency of N uptake from various manure forms, and at different rates of application. In a field experiment, wheat was grown on soils with different amounts of ¹⁵N-labelled legume residues. The amount of N taken up by the crop was directly proportional to the amount applied, with a recovery of between 15% and 23% of the legume N. In a second field experiment, inorganic N was applied at rates varying from 0 to 120 kg N ha^-1 in the presence and absence of poultry manure. The uptake of N by barley was 11 kg ha^-1 greater in the manured plots when no inorganic N was applied, and 23 kg ha^-1 greater when N was applied at the top rate. N uptake in a pot experiment was again shown to be directly proportional to the rate of manure application, but the amount of N taken up was strongly related to the N content of the manure. An incubation experiment demonstrated that net N mineralisation reached a maximum where residue concentrations were 1,5%. The significance of added nitrogen interactions in the context of manure-N additions is discussed.     

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.     

Natural N abundance of plant and soil inorganic-N as evidence for over-fertilization with compost

To test the hypothesis that N isotope composition can be used as evidence of excessive compost application, we measured variation in patterns of N concentrations and corresponding δ¹⁵N values of plants and soil after compost application. To do so, a pot experiment with Chinese cabbage (Brassica campestris L. cv. Maeryok) was conducted for 42 days. Compost was applied at rates of 0 (SC0), 500 (SC1), 1000 (SC2), and 1500 mg N kg⁻¹ soil (SC3). Plant-N uptake linearly increased with compost application (r² = 0.956, P < 0.05) with an uptake efficiency of 76 g N kg⁻¹ of compost-N at 42 days after application, while dry-mass accumulation did not show such linear increases. Net N mineralized from compost-N increased linearly (r² = 0.998, P < 0.01) with a slope of 122 g N kg⁻¹ of compost-N. Plant-δ¹⁵N increased curvilinearly with increasing compost application, but this increase was insignificant between SC2 and SC3 treatments. The δ¹⁵N of soil inorganic-N (particularly NO₃⁻-N) increased with compost application. We found that plant-δ¹⁵N reflected the N isotope signal of soil NO₃⁻-N at each measurement during plant growth, and that δ¹⁵N of inner leaves and soil NO₃⁻-N was similar when initial NO₃⁻ in the compost was abundant. Therefore, we concluded that δ¹⁵N of whole plant (more obviously in newer plant parts) and soil NO₃⁻-N could reveal whether compost application was excessive, suggesting a possible use of δ¹⁵N in plants and soil as evidence of excess compost application.     

Fate of nitrogen derived from 15N-labeled plant residues and composts in rice-planted paddy soil

Pot experiments that lasted for 3 y were conducted to investigate the dynamics of nitrogen derived from plant residues (rice root, hull, straw, corn root, and rapeseed pod-wall), and composts (rice straw compost, cattle manure compost, and cattle manure sawdust compost), which were labeled with ¹⁵N. The rates of nitrogen uptake by rice (=N efficiency), denitrification, and immobilization derived from the organic materials incorporated before the first year of cultivation were investigated throughout 3 y of cultivation. At the end of the first year of cultivation, relatively high rates of N efficiency were obtained for rapeseed pod-wall (24.6%), rice straw (19.1%), and rice hull (18.6%), while corn root and cattle manure sawdust compost displayed a noticeably high denitrification rate. Corn root, cattle manure sawdust compost, rice hull, and rapeseed pod-wall exhibited remarkably high N mineralization rates ranging from 60 to 75% of the organic materials N applied. Cumulative rates of N efficiencies from the organic materials applied before the first year of cultivation fitted well to a first-order kinetic model and their asymptotes were compared among the organic materials. The asymptotic rates of N efficiency tended to depend on the rates at the end of the first year of cultivation.     

The effect of macro-invertebrates and plant litter of different quality on the release of N from litter to plant on alpine pastureland

The effect of ¹⁵N-labelled litter of different quality (Luzula sylvatica, a grass species, Vaccinium gaultheroides, a deciduous dwarf shrub, and Calluna vulgaris, a hardy dwarf shrub) and the presence of macro-decomposers (Lumbricus rubellus, Lumbricidae, and Enantiulus nanus, Diplopoda) on the growth of Dactylis glomerata (Poaceae), a grass species abundant on alpine pastureland, was investigated. After 4 months, the presence of soil animals significantly increased litter mass loss of L. sylvatica, V. gaultheroides and C. vulgaris by 27%, 11% and 40%, respectively. Soil animals generally reduced microbial biomass but significantly increased it in treatments where either L. sylvatica or C. vulgaris was present. The presence of soil animals significantly increased shoot and root biomass of D. glomerata by 48% and 64%, respectively. L. rubellus increased the transfer of ¹⁵N from the litter into plants. We conclude that macro-decomposers increased nutrient mobilization and plant uptake of nutrients mineralized from recalcitrant litter materials. Litter of L. sylvatica contributed most to the ¹⁵N uptake by D. glomerata, suggesting that litter quality is crucial for the cycling of nutrients on abandoned alpine pastureland.     

Factors controlling decomposition of soil organic matter in fallow systems of the high tropical Andes: A field simulation approach using C- and N-labelled plant material

N-rich (C:N=27) and N-poor (C:N=130) wheat straw, labelled with ¹⁴C and ¹⁵N, was incubated for 2 yr in two major ecosystems of the upper elevation belt of cultivation in the high Andes: the moist Paramo (precipitation=1329 mm, altitude=3400 m asl, Andes of Merida, Venezuela) and the dry Puna (precipitation=370 mm, altitude=3800 m asl, Central Altiplano, Bolivia). The experiment was installed in young (2 yr) and old (7 yr) fallow plots. The following soil analyses were performed at nine sampling occasions: soil moisture, total-¹⁴C and -¹⁵N, and Microbial Biomass (MB)-¹⁴C and -¹⁵N. The measured data were fitted by the MOMOS-6 model (a process based model, with five compartments: labile and stable plant material, MB, and labile (HL) and stable humus (HS)) coupled with the SAHEL model (soil moisture prediction) using daily measured and/or predicted meteorological data. The aim was to understand how (1) the climatic conditions, (2) the quality of plant material, (3) the fallow age and (4) the soil properties affect the cycling of C and N within the soil organic matter system.The fallow age (2 and 7 yr) did not affect the measured data or the model predictions, indicating that in these systems the decomposition potential is not affected by fallow length. During the short initial active decomposition phase, the labile plant material was quickly exhausted, enabling a build up of MB and of HL. During the low activity phase, that covered 4/5 of the time of exposure, the MB size decreased slowly and the HL pool was progressively exhausted as it was reused by the MB as substrate. The HL compartment was directly or indirectly the major source for the inorganic ¹⁵N production. If the C:N ratio of the added plant material increased, the model predicted (1) a reduction of the decomposition rates of the plant material (essentially the stable plant material) and (2) an increased mortality of the MB which increased the production of HL (microbial cadavers and metabolites). Thus the essential effect of the slower decomposition due to the N-poor plant material was a higher accumulation of C and N in the HL and its slower recycling by the MB during the low activity phase. The labelling experiment allows to understand the higher soil native organic matter content in Paramo soils compared to Puna. The large sequestration of organic matter generally observed in the Paramo soils can be explained by two abiotic factors: the unfavourable soil microstructure and the accumulation of free aluminium linked to the climatic and acid soil conditions, inhibiting the microbial activity physically and chemically.     

Differential effects of soil disturbance and plant residue retention on function of arbuscular mycorrhizal (AM) symbiosis are not reflected in colonization of roots or hyphal development in soil

The effects of soil disturbance and residue retention on the functionality of the symbiosis between medic (Medicago truncatula L.) and arbuscular mycorrhizal fungi (AMF) were assessed in a two-stage experiment simulating a crop rotation of wheat (Triticum aestivum L.) followed by medic. Plants were inoculated or not with the AMF, Glomus intraradices and Gigaspora margarita, separately or together. The contribution of the arbuscular mycorrhizal (AM) pathway for P uptake was determined using ³²P-labeled soil in a small hyphal compartment accessible only to hyphae of AMF. In general AM colonization was not affected by soil disturbance or residue application and disturbance did not affect hyphal length densities (HLDs) in soil. At 4 weeks disturbance had a negative effect on growth and phosphorus (P) uptake of plants inoculated with G. margarita, but not G. intraradices. By 7 weeks disturbance reduced growth of plants inoculated with G. margarita or AMF mix and total P uptake in all inoculated plants. With the exception of plants inoculated with G. margarita in disturbed soil at 4 weeks, the AM pathway made a significant contribution to P uptake in all AM plants at both harvests. Inoculation with both AMF together eliminated the negative effects of disturbance on AM P uptake and growth, showing that a fungus insensitive to disturbance can compensate for loss of contribution of a sensitive one. Application of residue increased growth and total P uptake of plants but decreased ³²P in plants inoculated with the AMF mix in disturbed soil, compared with plants receiving no residue. The AMF responded differently to disturbance and G. intraradices, which was insensitive to disturbance, compensated for lack of contribution by the sensitive G. margarita when they were inoculated together. Colonization of roots and HLDs in soil were not good predictors of the outcomes of AM symbioses on plant growth, P uptake or P delivery via the AM pathway.     

Effect of long-term fertilization on 15N uptake and retention in soil

We did a pot experiment with three different fertilized soils (no fertilizer (No-F), inorganic fertilizer nitrogen, phosphorus and potassium (NPK), manure plus inorganic fertilizer (MNPK)) from a 19-year fertilizer trial. Three N treatments, (1) no N, (2) 100 mg/kg urea-¹⁵N (N), (3) 50 mg/kg urea-¹⁵N + 50 mg/kg corn straw-N (1/2N + 1/2S), were applied to each soil. The residual soil from the same treatments was used to grow second wheat crop. The MNPK soil had significantly higher nitrogen use efficiency (NUE) in the first growing season, and lower N loss than the NPK, and No-F soils. The 1/2N + 1/2S treatment decreased NUE on each soil, even though the MNPK soil still had highest NUE and lowest N loss. The residual ¹⁵N use efficiency (RNUE) in 1/2N + 1/2S treatment of MNPK soil was higher than NPK and No-F soils. We concluded that long-term application of manure plus inorganic fertilizer increased NUE and decreased N loss.     

Comparative effects of applying leguminous and non-leguminous green manures and inorganic N on biomass yield and nitrogen uptake in flooded rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

The beneficial role of green manures in rice production is generally ascribed to their potential of supplying plant nutrients, particularly nitrogen (N). However, the mechanisms through which green manures enhance the crop productivity are poorly understood. Pot experiments were conducted using a ¹⁵N-tracer technique: (1) to compare the biomass production potential of sesbania (Sesbania aculeata Pers.) and maize (Zea mays L.) as green manuring crops for lowland rice and (2) to compare the effect of the two types of green manure and inorganic N on the dry matter accumulation and N uptake by two rice (Oryza sativa L.) cultivars, viz. IR-6 and Bas-370. Although maize produced three times higher shoot biomass compared with sesbania, the latter showed higher N concentration; and thus the total N yield was similar in the two types of plants. Applying the shoot material of the two plants to flooded rice significantly enhanced the dry matter yield and N uptake by the two rice cultivars, the positive effects generally being more pronounced with sesbania than with maize amendment. The difference in the growth-promoting potential of the two plant residues was related more to an increased uptake of the native soil N rather than to their direct role as a source of plant-available N. A positive added nitrogen interaction (ANI) was observed due to both plant residues, the effect was much more pronounced with the application of sesbania than with maize residues. In both rice cultivars, inorganic N also caused a substantial ANI, particularly at higher application rate. Losses from the applied N were 2–3 times lower from sesbania, compared with maize treatment. Green manuring with sesbania also caused much lower N losses than the inorganic N applied at equivalent or higher rates. The overall benefit of green manuring to rice plants was higher than inorganic N applied at comparable rates. The two rice cultivars differed in their response to green manuring, IR-6 generally being more responsive than Bas-370.     

Plant nitrate use in deciduous woodland: the relationship between leaf N, N natural abundance of forbs and soil N mineralisation

Our aim was to study whether the in situ natural abundance ¹⁵N (δ¹⁵N)-values and N concentration of understory plants were correlated with the form and amount of mineral N available in the soil. Also to determine whether such differences were related to earlier demonstrations of differences in biomass increase in the same species exposed to nutrient solutions with both and or to alone. Several studies show that the δ¹⁵N of in soil solution generally is isotopically lighter than the δ¹⁵N of due to fractionation during nitrification. Hence, it is reasonable to assume that plant species benefiting from in ecosystems without significant leaching or denitrification have lower δ¹⁵N-values in their tissues than species growing equally well, or better, on We studied the δ¹⁵N of six understory species in oak woodlands in southern Sweden at 12 sites which varied fivefold in potential net N mineralisation rate The species decreased in benefit from in the following order: Geum urbanum, Aegopodium podagraria, Milium effusum, Convallaria majalis, Deschampsia flexuosa and Poa nemoralis. Four or five species demonstrated a negative correlation between and leaf δ¹⁵N and a positive correlation between and leaf N concentration. In wide contrast, only D. flexuosa, which grows on soils with little nitrification, showed a positive correlation between and the leaf N concentration and δ¹⁵N-value. Furthermore, δ¹⁵N of plants from the field and previously obtained indices of hydroponic growth on relative to were closely correlated at the species level. We conclude that δ¹⁵N may serve as a comparative index of uptake of among understory species, preferably in combination with other indices of N availability. The use of δ¹⁵N needs careful consideration of known restrictions of method, soils and plants.     

Water limitation and plant inter-specific competition reduce rhizosphere-induced C decomposition and plant N uptake

Plants can affect soil organic matter decomposition and mineralization through litter inputs, but also more directly through root-microbial interactions (rhizosphere effects). Depending on resource availability and plant species identity, these rhizosphere effects can be positive or negative. To date, studies of rhizosphere effects have been limited to plant species grown individually. It is unclear how belowground resources and inter-specific interactions among plants may influence rhizosphere effects on soil C decomposition and plant N uptake. In this study, we tested the simple and interactive effects of plant diversity and water availability on rhizosphere-mediated soil C decomposition and plant N uptake. The study was conducted in the greenhouse with five semi-arid grassland species (monocultures and mixtures of all five species) and two water levels (15 and 20% gravimetric soil moisture content). We hypothesized that microbial decomposition and N release would be less in the low compared to high water treatment and less in mixtures compared to monocultures. Rhizosphere effects on soil C decomposition were both positive and negative among the five species when grown in monoculture, although negative effects prevailed by the end of the experiment. When grown in mixture, rhizosphere effects reduced soil C decomposition and plant N uptake compared to monocultures, but only at the low-water level. Our results suggest that when water availability is low, plant species complementarity and selection effects on water and N use can decrease soil C decomposition through rhizosphere effects. Although complementarity and selection effects can increase plant N uptake efficiency, plant N uptake in the mixtures was still lower than expected, most likely because rhizosphere effects reduced N supply in the mixtures more than in the monocultures. Our results indicate that rhizosphere effects on C and N cycling depend on water availability and inter-specific plant interactions. Negative rhizosphere effects on soil C decomposition and N supply in mixtures relative to monocultures of the component species could ultimately increase soil C storage and possibly influence how plant communities in semi-arid grasslands respond to global climate change.