应用15N示踪技术研究土壤水分对氮素有效性的影响

在有防雨设施的试验田里,设置不同的土壤水分处理,应用     

应用^15N示踪技术土壤水分对氮素有效性的影响

在有防雨设施的试验田里,设置不同的土壤水分处理,应用＾１５Ｎ示踪技术研究土壤水分对氮素有效性的影响。试验结果表明：冬小麦对肥料氮素的利用率随土壤水分提高而提高;土壤供应的有效性氮素（Ａ值）在土壤水分由田间持水量的５０％提高到６０％时出现“跃迁”,土壤水分超过田间持水量的６０％以后,Ａ值差异不显著,表明土壤氮素有效性对土壤水分存在一个阈值反映。节水、节肥高效的土壤水分下限应控制在土壤田间持水量的６０     

不同保水剂对土壤水分和氮素保持的比较研究

保水剂应用对土壤水肥利用效率具有重要影响。本文采用土柱模拟试验方法,以不施保水剂处理为对照,比较3种保水剂——聚丙烯酸盐类保水剂(A)、有机–无机复合保水剂(B)、腐植酸型多功能保水剂(C)对土壤水分和两种氮肥(尿素、硝酸铵)的保持效应,筛选保水剂与氮肥的合理施用配合。8次土壤淋溶结果表明:3种保水剂对土壤水分和两种氮肥都有保持作用,但差异明显。在保水方面,A、B保水剂土壤水分保持效果较好且保水效果相近,C保水剂相对较差;随浇水次数增加,3种保水剂的保水效果均有所降低。在保肥方面,C保水剂对两种氮素的保持效果显著优于对照,且对硝酸铵保持效果优于对尿素的保持效果;A保水剂对尿素的保持效果明显,但对硝酸铵的保持效果很小,淋溶8次后,甚至对氮素淋溶有促进作用;B保水剂对尿素的保持效果8次淋溶后与C保水剂相近,对硝酸铵的保持效果介于其他两种保水剂之间。此外,保水剂对土壤脲酶活性有一定影响,其变化与氮素转化有关;施用尿素的土壤中,保水剂对土壤脲酶活性的影响为B保水剂C保水剂A保水剂,而施用硝酸铵的土壤中为A保水剂B保水剂C保水剂。     

土壤保墒性能与土壤水分有效性综述

土壤保墒性能与土壤水分有效性多年来一直是土壤物理学领域中最为活跃的研究课题之一,也是半干旱地区农业研究中迫切需要解决的问题.土壤水分是植物需水的主要来源,其保蓄和储存在一定程度上决定于土壤的保墒性能.而植物从土壤中吸水的容易程度决定于土壤水分有效性.随着SPAC理论的发展,土壤与植物水分关系的概念已发生了根本变化;本文拟将土壤保墒性能与土壤水分有效性研究进展做一综述.     

基于15N示踪技术的烟田肥料氮素再利用分析

为研究后季烤烟对首季残留氮肥的吸收利用情况,2011年利用蒸渗仪设计了不同灌水量(600、800和1 000 mm)和施氮量(15N双标记NH_4NO_3,90和120 kg/hm~2)试验,并于2012-2014年对烤烟不同器官中15N-N(来源于2011年施入的肥料氮素)累积量、土壤中15N-N含量进行了跟踪观测,同时分析了烤烟对首季肥料氮素再利用率的影响因素。结果表明:1)后季烤烟叶、茎和根中来源于首季的肥料氮素累积量,随首季施氮量的增加而增加,但随首季灌水量增加总体上有所下降;2)后三季烤烟对首季施入肥料氮素的总再利用率为10.79%~14.58%,首季灌水量600 mm、施氮量90 kg/hm~2处理最有利于后季烤烟对其残留肥料氮素的吸收;3)后三季烤烟对首季施入的肥料氮素的平均再利用率,与首季灌水量呈极显著负相关(P0.01),与0~20 cm土壤中首季残留肥料氮素量呈显著正相关(P0.05)。首季灌水主要通过改变其肥料氮素在不同土层中的分配格局,尤其是通过改变0~20 cm土层的肥料氮素含量,来影响后季烤烟对首季施入肥料氮素的再利用,但其具体影响机理仍需进一步明确。综上,后季烤烟能吸收相当比例的前季残留肥料氮素,合理制定前季灌水和施氮制度,对于后季烤烟肥料氮素再利用率的提高至关重要。研究结论可为烟区土壤生态环境的改善及烟草农业的可持续发展提供有益参考。     

缓释复合肥料对土壤和黑麦草氮素营养的影响

采用恒温培养和盆栽生物试验研究了非包膜缓释复合肥料对土壤氮素养分(铵态氮、硝态氮、碱解氮、微生物量氮和固定态铵)和黑麦草氮素营养的效应。结果表明,在恒温培养条件下,各施肥处理土壤铵态氮含量随培养时间均表现为先升高后降低的趋势,且中后期缓释复合肥料处理(SRCF)高于普通复合肥料处理(CCF);缓释复合肥料SRCF1处理土壤硝态氮含量始终低于普通复合肥料CCF1处理,缓释复合肥料SRCF2处理土壤硝态氮含量在初期较高、中后期较低;SRCF各处理土壤碱解氮和微生物量氮含量变化均表现为增加-降低-增加-降低趋势,分别在培养第21d和105d时出现峰值;土壤固定态铵含量变化较小,但总体上以SRCF处理高于CCF处理。在等养分比例盆栽试验中,SRCF1-3处理黑麦草株高、生物量、干重和氮素养分吸收量均高于CCF1处理,SRCF有利于提高黑麦草产量、氮素吸收和利用率;黑麦草氮素吸收量与培养土壤铵态氮、微生物量氮含量呈负相关,而与硝态氮和固定态铵含量呈正相关。     

N2O emissions from humid tropical agricultural soils: effects of soil moisture,texture and nitrogen availability

We studied soil moisture dynamics and nitrous oxide (N₂O) fluxes from agricultural soils in the humid tropics of Costa Rica. Using a split-plot design on two soils (clay, loam) we compared two crop types (annual, perennial) each unfertilized and fertilized. Both soils are of andic origin. Their properties include relatively low bulk density and high organic matter content, water retention capacity, and hydraulic conductivity. The top 2–3 cm of the soils consists of distinct small aggregates (dia. <0.5 cm). We measured a strong gradient of bulk density and moisture within the top 7 cm of the clay soil. Using automated sampling and analysis systems we measured N₂O emissions at 4.6 h intervals, meteorological variables, soil moisture, and temperature at 0.5 h intervals. Mean daily soil moisture content at 5 cm depth ranged from 46% water filled pore space (WFPS) on clay in April 1995 to near saturation on loam during a wet period in February 1996. On both soils the aggregated surface layer always remained unsaturated. Soils emitted N₂O throughout the year. Mean N₂O fluxes were 1.04±0.72 ng N₂O-N cm⁻² h⁻¹ (mean±standard deviation) from unfertilized loam under annual crops compared to 3.54±4.31 ng N₂O-N cm⁻² h⁻¹ from the fertilized plot (351 days measurement). Fertilization dominated the temporal variation of N₂O emissions. Generally fluxes peaked shortly after fertilization and were increased for up to 6 weeks (‘post fertilization flux’). Emissions continued at a lower rate (‘background flux’) after fertilization effects faded. Mean post-fertilization fluxes were 6.3±6.5 ng N₂O-N cm⁻² h⁻¹ while the background flux rate was 2.2±1.8 ng N₂O-N cm⁻² h⁻¹. Soil moisture dynamics affected N₂O emissions. Post fertilization fluxes were highest from wet soils; fluxes from relatively dry soils increased only after rain events. N₂O emissions were weakly affected by soil moisture during phases of low N availability. Statistical modeling confirmed N availability and soil moisture as the major controls on N₂O flux. Our data suggest that small-scale differences in soil structure and moisture content cause very different biogeochemical environments within the top 7 cm of soils, which is important for net N₂O fluxes from soils.     

Quantification and potential availability of non-symbiotically fixed 15N in soil

Summary Non-symbiotic N₂ fixation was studied under laboratory conditions in two soils from Pakistan (Hafizabad silt loam and Khurrarianwala silt loam) and one from Illinois, USA (Drummer silty clay loam) incubated in a ¹⁵N-enriched atmosphere. N₂ fixation was greatest with the Drummer soil (18–122 g g^–1 soil, depending upon the soil treatment) and lowest with the Khurrarianwala soil (4–81 g g^–1 soil). Fixation was increased by the addition of glucose, a close correlation being observed between the amount of glucose added and the amount of N₂ fixed in the three soils (r = 0.96). Efficiency of N₂ fixation varied with soil type and treatment and was greatest in the presence of added inorganic P. Application of Mo apparently had a negative effect on the amount and efficiency of N₂ fixation in all the soils. The percentage of non-symbiotically fixed ¹⁵N in potentially mineralizable form (NH ₄ ⁺ -N released in soil after a 15-day incubation period under anaerobic conditions) was low (2%–18%, depending upon the soil treatment), although most of the fixed N (up to 90%) was recovered as forms hydrolysable with 6N HCl. Recovery in hydrolysable forms was much greater for the fixed N than for the native soil N, indicating that the former was more available for uptake by plants.     

Citric acid influence on soil phosphorus availability

Citric acid is a low-molecular-weight organic acid exuded by the plant roots. Organic anions derived from this acid compete for phosphorus (P) adsorption sites in clay minerals. The objective was to evaluate the P availability by application of P and citric acid tothe soil. For this purpose, an experiment in green house was conducted at the rates of 0, 45, 90, and 180 mg dm^?3 P in Entisol and 0, 100, 200, and 400 mg dm^?3 P in Oxisol, combined with 0, 0.5, 1.0, and 2.0 mg dm^?3 citric acid in pot. In the Entisol, 115 mg dm^?3 of P and 0.7 mg dm^?3 of citric acid resulted in maximum corn growth. However, in the Oxisol, the estimated rates were 299 mg dm^?3 of P and 1.3 mg dm^?3 of acid citric. Citric acid use in Entisol and Oxisol increased P availability of corn plants.     

The influence of temperature and moisture on the effects of the herbicide dalapon on nitrogen transformations in soil

The herbicide dalapon, at 2660 parts/10⁶ in four soils, generally inhibited nitrification and stimulated mineralization of N. These effects of dalapon cannot be used to judge the likely effect of the chemical in the field in view of the abnormal concentration used. However, the modification of these effects by environmental factors can be validly assessed.The results were modified by soil characteristics, moisture content and temperature. Different effects were found in samples from different fields on the same soil and in similar crop rotations. The degradation of the herbicide was markedly affected by soil type.The significance of the results is discussed in relation to the requirements of the chemical registration and approval authorities for information on the side-effects of agricultural chemicals on the soil microflora.     

Changes in 15N abundance and amounts of biologically active soil nitrogen

 Estimation of the capacity of soils to supply N for crop growth requires estimates of the complex interactions among organic and inorganic N components as a function of soil properties. Identification and measurement of active soil N forms could help to quantify estimates of N supply to crops. Isotopic dilution during incubation of soils with added ¹⁵NH₄ ⁺ compounds could identify active N components. Dilution of ¹⁵N in KCl extracts of mineral and total N, non-exchangeable NH4₄ ⁺, and N in K₂SO₄ extracts of fumigated and non-fumigated soil was measured during 7-week incubation. Samples from four soils varying in clay content from 60 to 710 g kg^–1 were used. A constant level of ¹⁵N enrichment within KCl and K₂SO₄ extracted components was found at the end of the incubation period. Total N, microbial biomass C and non-exchangeable NH₄ ⁺ contents of the soils were positively related to the clay contents. The mineralized N was positively related to the silt plus clay contents. The active soil N (ASN) contained 28–36% mineral N, 29–44% microbial biomass N, 0.3–5% non-exchangeable NH₄ ⁺ with approximately one third of the ASN unidentified. Assuming that absolute amounts of active N are related to N availability, increasing clay content was related to increased N reserve for crop production but a slower turnover. Received: 7 July 1998     

Symbiotic nitrogen fixation and soil N availability under legume crops in an arid environment

Purpose  

Legume crops often present an important option to maintain and improve soil nitrogen (N) quality and fertility in a dryland agroecosystem. However, the work on the integral assessment of the symbiotic N₂ fixation (N_fix) and their effects on soil N availability under field conditions is scare.     

不同水分状况下施氮对夏玉米水分利用效率的影响

通过盆栽试验采用五因素五水平通用旋转组合设计(1/2实施)方案,研究了不同水分状况下氮肥的用量和施用时期对夏玉米水分利用效率的影响。结果表明,施氮对夏玉米水分利用效率的影响大于土壤含水量,但子粒产量和生物产量水分利用率(WUE子粒和WUE生物)对施氮时期的要求不尽相同,苗期和灌浆期施氮对WUE子粒的影响较显著,而苗期和拔节期施氮对WUE生物的影响则更显著。从单因素效应看,并非施氮量和土壤含水量越高越好。水氮高效配合的关键期是拔节期,且存在阈值反应,其阈值是N0.2g/kg,土壤含水量为21%。低于阈值水平,水氮交互作用不明显,高于阈值水平,水氮互作效应显著。     

Natural N abundances of inorganic nitrogen in soil treated with fertilizer and compost under changing soil moisture regimes

This study was conducted to examine whether the applications of N-inputs (compost and fertilizer) having different N isotopic compositions (δ¹⁵N) produce isotopically different inorganic-N and to investigate the effect of soil moisture regimes on the temporal variations in the δ¹⁵N of inorganic-N in soils. To do so, the temporal variations in the concentrations and the δ¹⁵N of NH₄⁺ and NO₃⁻ in soils treated with two levels (0 and 150 mg N kg⁻¹) of ammonium sulfate (δ¹⁵N=−2.3‰) and compost (+13.9‰) during a 10-week incubation were compared by changing soil moisture regime after 6 weeks either from saturated to unsaturated conditions or vice versa. Another incubation study using ¹⁵N-labeled ammonium sulfate (3.05 ¹⁵N atom%) was conducted to estimate the rates of nitrification and denitrification with a numerical model FLUAZ. The δ¹⁵N values of NH₄⁺ and NO₃⁻ were greatly affected by the availability of substrate for each of the nitrification and denitrification processes and the soil moisture status that affects the relative predominance between the two processes. Under saturated conditions for 6 weeks, the δ¹⁵N of NH₄⁺ in soils treated with fertilizer progressively increased from +2.9‰ at 0.5 week to +18.9‰ at 6 weeks due to nitrification. During the same period, NO₃⁻ concentrations were consistently low and the corresponding δ¹⁵N increased from +16.3 to +39.2‰ through denitrification. Under subsequent water-unsaturated conditions, the NO₃⁻ concentrations increased through nitrification, which resulted in the decrease in the δ¹⁵N of NO₃⁻. In soils, which were unsaturated for the first 6-weeks incubation, the δ¹⁵N of NH₄⁺ increased sharply at 0.5 week due to fast nitrification. On the other hand, the δ¹⁵N of NO₃⁻ showed the lowest value at 0.5 week due to incomplete nitrification, but after a subsequence increase, they remained stable while nitrification and denitrification were negligible between 1 and 6 weeks. Changing to saturated conditions after the initial 6-weeks incubation, however, increased the δ¹⁵N of NO₃⁻ progressively with a concurrent decrease in NO₃⁻ concentration through denitrification. The differences in δ¹⁵N of NO⁻₃ between compost and fertilizer treatments were consistent throughout the incubation period. The δ¹⁵N of NO₃⁻ increased with the addition of compost (range: +13.0 to +35.4‰), but decreased with the addition of fertilizer (−10.8 to +11.4‰), thus resulting in intermediate values in soils receiving both fertilizer and compost (−3.5 to +20.3‰). Therefore, such differences in δ¹⁵N of NO₃⁻ observed in this study suggest a possibility that the δ¹⁵N of upland-grown plants receiving compost would be higher than those treated with fertilizer because NO₃⁻ is the most abundant N for plant uptake in upland soils.     

Analysis of soil microbial biomass nitrogen and 15N with a high background of labeled mineral N

Abstract

Microbial biomass is determined from the excess of extractable organic N released from fumigated soil samples. In the presence of relatively high contents of labeled mineral N, small differences in organic N and N may not be detectable. Two approaches were tested to determine organic N content and its N enrichment in the presence of considerably greater concentrations of labeled mineral N: (i) Removal of mineral N from mixed solutions of alanine and NH₄ or NO₃ by reduction and boiling under alkaline conditions, prior to Kjeldahl digestion. (ii) Including mineral N in Kjeldahl N analysis, by reduction under acidic conditions prior to digestion and calculating organic N and N content by subtracting mineral N and N. The removal of mineral N was either incomplete‐ particularly regarding labeled mineral N, or partly destroyed organic N as well. When mineral N was included in the digest, the recovery of N and ¹⁵N was sufficiently accurate to obtain good results of organic N and ¹⁵N by subtracting the known quantities of mineral N, even when organic N was only 20% of the total Kjeldahl N. This procedure was used to determine the flush of Kjeldahl N in fumigated soils that were incubated with labeled mineral N and cellulose.     

Changes in variability of soil moisture alter microbial community C and N resource use

Grassland ecosystems contain ∼12% of global soil organic carbon (C) stocks and are located in regions where global climate change will likely alter the timing and size of precipitation events, increasing soil moisture variability. In response to increased soil moisture variability and other forms of stress, microorganisms can induce ecosystem-scale alterations in C and N cycling processes through alterations in their function. We explored the influence of physiological stress on microbial communities by manipulating moisture variability in soils from four grassland sites in the Great Plains, representing a precipitation gradient of 485-1003 mm y⁻¹. Keeping water totals constant, we manipulated the frequency and size of water additions and dry down periods in these soils by applying water in two different, two-week long wetting-drying cycles in a 72-day laboratory incubation. To assess the effects of the treatments on microbial community function, we measured C mineralization, N dynamics, extracellular enzyme activities (EEA) and a proxy for substrate use efficiency. In soils from all four sites undergoing a long interval (LI) treatment for which added water was applied once at the beginning of each two-week cycle, 1.4-2.0 times more C was mineralized compared to soils undergoing a short interval (SI) treatment, for which four wetting events were evenly distributed over each two-week cycle. A proxy for carbon use efficiency (CUE) suggests declines in this parameter with the greater soil moisture stress imposed in LI soils from all four different native soil moisture regimes. A decline in CUE in LI soils may have been related to an increased effort by microbes to obtain N-rich organic substrates for use as protection against osmotic shock, consistent with EEA data. These results contrast with similar in situ studies of response to increased soil moisture variability and may indicate divergent autotrophic vs. heterotrophic responses to increased moisture variability. Increases in microbial N demand and decreases in microbial CUE with increased moisture variability observed in this study, regardless of the soils’ site of origin, imply that these systems may experience enhanced heterotrophic CO₂ release and declines in plant-available N with climate change. This has particularly important implications for C budgets in these grasslands when coupled with the declines in net primary productivity reported in other studies as a result of increases in precipitation variability across the region.