A method to assess whether ‘preferential use’ occurs after N ammonium addition; implication for the N isotope dilution technique

The robustness of the assumption of equilibrium between native and added N during ¹⁵N isotope dilution has recently been questioned by Watson et al. (Soil Biol Biochem 32 (2000) 2019-2030). We re-analyzed their raw data using equations that consider the added and native NH₄⁺ and NO₃⁻ pools as separate state variables. Gross mineralization rates and first-order rate constants for NH₄⁺ and NO₃⁻ consumption were obtained by combining analytical integration of the differential equations with a non-linear fitting procedure. The first-order rate constants for NH₄⁺ consumption and NO₃⁻ immobilization for the added NH₄⁺ and NO₃⁻ pool were used to estimate gross mineralization rates and first-order rate constants for nitrification of native NH₄⁺. The latter were 2-4 times lower than the first-order rate constants derived from the added N pool. This discrepancy between first-order rate constants for nitrification implies that one or more process rates estimated for the added N pools cannot be applied to the native N pools. Preferential use of the added N resulted in an overestimation of the gross mineralization by 1.5-2.5-fold, emphasizing the need for critical evaluation of the assumption of equilibrium before gross mineralization rates are calculated.     

Maize straw is more effective than maize straw biochar at improving soil N availability and gross N transformation rate

Soil nitrogen (N) transformation is vital in determining farmland N availability. Although many studies have investigated the effect of biochar on N retention and loss via leaching and gaseous emissions, few have determined the dynamics of gross N transformation during crop growth in long-term biochar-amended soils and compared the effect of the biochar with that of its feedstock. In this study, we conducted a five-time field measurement of soil gross N turnover rates via ¹⁵N isotope pool dilution during maize growth in 2021. Three treatments were employed, including no amendment, biochar and straw applied annually at rates of 2.63 and 7.50 t ha⁻¹, respectively, since 2013. The results showed that biochar did not change the rate of gross N mineralisation when compared with no amendment, but straw increased it by 139% in August, resulting in significantly higher cumulative gross N mineralisation than biochar and no amendment (701 vs 489 and 499 mg kg⁻¹ in 200 d). The inconsistent influence was attributed to the fact that inherent biochar-N was recalcitrant and could not be mineralized like the straw. The gross nitrification rate was decreased by 72.9% and 77.4% by biochar and straw application, respectively, in June relative to no amendment, but then it increased from July to August in the straw treatment as a result of the elevated gross N mineralisation rate. The decreased nitrification in the biochar treatment was an outcome of the synergetic effect of a low ammonium pool (−59.4%) and a high gross ammonium immobilisation rate (+263%), which was likely due to excessive fertilizer N loss and abiotic adsorption to biochar. Meanwhile, biochar amendment inhibited bacterial 16S and fungal ITS genes, as well as ureC and bacterial and archaea-amoA gene copies. In conclusion, straw is more effective than biochar at improving soil N transformation and availability in the long term.     

Nitrogen isotope analysis of free glycine in soil using isotope dilution mass spectrometry

To enable the estimation of production and consumption rates of free glycine in soils through ¹⁵N isotope dilution experiments, an isotope dilution mass spectrometric method was developed. The method, which enabled high precision N isotope ratio determination of glycine in soil extracts at δ¹⁵N levels up to 4000‰ and concentrations from approximately 2 μM, is based on the following steps: (i) addition of glycine spike to the soil extract, (ii) removal of humic substances and pre-concentration of glycine using solid phase extraction, (iii) derivatization of amino acids, (iv) separation of the derivatives using gas chromatography (GC), (v) their combustion to yield sample N₂ gas, and (vi) finally the use of N isotope ratio mass spectrometry (IRMS). Judging by uncertainty budget calculations, the precision obtained (SD=0.01-0.06 at% ¹⁵N) is sufficient for detecting differences in N isotopic ratios obtained in ¹⁵N isotope dilution experiments.     

Gross N transformation rates and net N mineralisation rates related to the C and N contents of soil organic matter fractions in grassland soils of different age

We investigated the relationship between soil organic matter (SOM) content and N dynamics in three grassland soils (0-10 and 10-20 cm depth) of different age (6, 14 and 50 y-old) with sandy loam textures. To study the distribution of the total C and N content the SOM was fractionated into light, intermediate and heavy density fractions of particulate macro-organic matter (150-2000 μm) and the 50-150 μm and <50 μm size fractions. The potential gross N transformation rates (mineralisation, nitrification, NH₄⁺ and NO₃⁻ immobilization) were determined by means of short-term, fully mirrored ¹⁵N isotope dilution experiments (7-d incubations). The long-term potential net N mineralisation and gross N immobilization rates were measured in 70-d incubations. The total C and N contents mainly tended to increase in the 0-10 cm layer with increasing age of the grassland soils. Significant differences in total SOM storage were detected for the long-term (50 y-old) conversion from arable land to permanent grassland. The largest relative increase in C and N contents had occurred in the heavy density fraction of the macro-organic matter, followed by the 50-150 and <50 μm fractions. Our results suggest that the heavy density fraction of the macro-organic matter could serve as a good indicator of early SOM accumulation, induced by converting arable land to permanent grassland. Gross N mineralisation, nitrification, and (long-term) gross N immobilization rates tended to increase with increasing age of the grasslands, and showed strong, positive correlations with the total C and N contents. The calculated gross N mineralisation rates (7-d incubations) and net N mineralisation rates (70-d incubations) corresponded with a gross N mineralisation of 643, 982 and 1876 kg N ha⁻¹ y⁻¹, and a net N mineralisation of 195, 208 and 274 kg N ha⁻¹ y⁻¹ in the upper 20 cm of the 6, 14 and 50 y-old grassland soils, respectively. Linear regression analysis showed that 93% of the variability of the gross N mineralisation rates could be explained by variation in the total N contents, whereas total N contents together with the C-to-N ratios of the <50 μm fraction explained 84% of the variability of the net N mineralisation rates. The relationship between long-term net N mineralisation rates and gross N mineralisation rates could be fitted by means of a logarithmic equation (net m=0.24Ln(gross m)+0.23, R²=0.69, P<0.05), which reflects that the ratio of gross N immobilization-to-gross N mineralisation tended to increase with increasing SOM contents. Microbial demand for N tended to increase with increasing SOM content in the grassland soils, indicating that potential N retention in soils through microbial N immobilization tends to be limited by C availability.     

Comparison of NH4 pool dilution techniques to measure gross N fluxes in a coarse textured soil

We compared gross N fluxes by ¹⁵N pool dilution in a coarse-textured agricultural soil when ¹⁵N was applied to the soil NH₄⁺ pool by either: (i) mixing a ¹⁵NH₄NO₃ solution into disturbed soil or (ii) injection of ¹⁵NH₃ gas into intact soil cores. The two techniques produced similar results for gross N mineralization rates indicating that NH₄⁺ production in soil was not altered by soil disturbance, method of application (gas vs. solution), or amount of N applied. This was not the case for immobilization rates, which were twofold higher when ¹⁵N label was applied to the soil NH₄⁺ pool with the mixing technique compared to the injection technique. This was attributed to the fact that more NH₄⁺ was applied with the mixing technique. Estimates of gross nitrification were accompanied by large error terms meaning differences between ¹⁵N labeling methods could not be accurately assessed for this process rate.     

Gross N transfer rates in field soils measured by 15N-pool dilution

Abstract

A micro-plot ¹⁵N-tracer experiment was established in three different soils of a long-term soil fertility field experiment. The nutrient-poor loam sand has been subjected to various treatments over the years and this has resulted in different organic C (0.35% – 0.86%), microbial biomass (38.3 – 100.0 µg C _mic g^?1 soil), clay and fine silt contents. Using the ¹⁵N-pool dilution technique, we assessed gross N-transfer rates in the field. Gross N mineralization rates varied strongly among the three plots and ranged between 0.4 and 4.2 µg N g^?1 soil d^?1. Gross nitrification rates were estimated to be between 0 and 2.1 µg N g^?1 soil d^?1. No correlation between gross N mineralization rates and the organic matter content of the soils was established. However, gross nitrate consumption rates increased with increasing soil C content. The ¹⁵N-pool dilution technique was successfully used to measure gross N transfer rates directly in the field.     

采用15N同位素稀释法研究不同层次土壤氮素总转化速率

采用15N同位素稀释方法,开展短期(7天)室内培养实验,估算了一水稻土0~20、20~60和60~90 cm土层土壤主要N素转化过程的总转化速率,结果表明,标记N溶液加入后2 h内各土层土壤的总矿化、硝化、固定速率显著高于其他时间段(p<0.01)。2 h后,矿化速率在小范围内起伏。0~20 cm土层土壤N素的硝化速率随培养时间延长而降低,另外两层土壤则基本保持稳定,硝化速率的变化与硝化作用底物NH4+-N浓度的变化呈显著正相关。值得注意的是,外源无机N溶液加入后2 h内,大量NH4+-N和NO3--N被固定,并认为N素的非生物固定起主导作用。2 h后,出现了N素在固定与再矿化间反复转换的现象。实验结果表明,与净转化速率相比总转化速率能更好地描述单个N素转化过程,但由于外源N加入对N素转化的影响、再矿化作用以及忽略了N素转化过程中的气体损失、DNRA(硝态氮异化还原为铵)过程等,本研究结果与真实值间存在一定差异。     

15N isotope dilution methodology for evaluating the dynamics of biologically fixed N in legume-non-legume associations

We examined the theoretical basis for estimating the transfer of N₂ fixed by legumes to companion cereals or grasses in intercropping or pasture systems using ¹⁵N isotope dilution methodology. A method was developed to calculate the symbiotic dependence of the legume in a mixed stand based on ¹⁵N enrichment of the associated non-legume and the estimate of fixed N transfer. Published field data were used to illustrate the application of the method. Complementary treatments for verifying N transfer and options for increasing the accuracy of estimates of N transfer are discussed.     

Technique of soil sample pretreatment for analysis of 15N:14N isotope ratio

Abstract

The technique of simultaneous quantitative determination of mineral N soil forms (nitrates, exchangeable and non‐exchangeable ammonium, and total amount of these compounds) and sample pretreatment for the analysis of ¹⁵N:¹⁴N ratio is suggested. The technique is based on the selective association of NH₄ ⁺‐ions into indophenol complex and subsequent ethyl‐acetate extraction of this complex from solution. The mineralization of indophenol is carried out in alkaline medium with simultaneous NH₃ distillation into H₂SO₄ titrant. The application of given technique allows us to shorten significantly the time required for analysis and to increase the accuracy of analytical determination.     

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.     

Effects of decreasing water potential on gross ammonification and nitrification in an acid coniferous forest soil

Changes in the soil water regime, predicted as a consequence of global climate change, might influence the N cycle in temperate forest soils. We investigated the effect of decreasing soil water potentials on gross ammonification and nitrification in different soil horizons of a Norway spruce forest and tested the hypotheses that i) gross rates are more sensitive to desiccation in the Oa and EA horizon as compared to the uppermost Oi/Oe horizon and ii) that gross nitrification is more sensitive than gross ammonification. Soil samples were adjusted by air drying to water potentials from about field capacity to around −1.0 MPa, a range that is often observed under field conditions at our site. Gross rates were measured using the ¹⁵N pool dilution technique. To ensure that the addition of solute label to dry soils and the local rewetting does not affect the results by re-mineralization or preferential consumption of ¹⁵N, we compared different extraction and incubation times.T₀ times ranging from 10 to 300 min and incubation times of 48 h and 72 h did not influence the rates of gross ammonification and nitrification. Even small changes of water potential decreased gross ammonification and nitrification in the O horizon. In the EA horizon, gross nitrification was below detection limit and the response of the generally low rates of gross ammonification to decreasing water potentials was minor. In the Oi/Oe horizon gross ammonification and nitrification decreased from 37.5 to 18.3 mg N kg⁻¹ soil d⁻¹ and from 15.4 to 5.6 mg N kg⁻¹ soil d⁻¹ when the water potential decreased from field capacity to −0.8 MPa. In the Oa horizon gross ammonification decreased from 7.4 to 4.0 mg N kg⁻¹ soil d⁻¹ when the water potential reached −0.6 MPa. At such water potential nitrification almost ceased, while in the Oi/Oe horizon nitrification continued at a rather high level. Hence, only in the Oa horizon nitrification was more sensitive to desiccation than ammonification. Extended drought periods that might result from climate change will cause a reduction in gross N turnover rates in forest soils even at moderate levels of soil desiccation.     

好氧条件下两种水稻土的氮素总转化速率比较

Although to date individual gross N transformations could be quantified by ~(15)N tracing method and models,studies are still limited in paddy soil.An incubation experiment was conducted using topsoil(0-20 cm) and subsoil(20-60 cm) of two paddy soils,alkaline and clay(AC) soil and neutral and silt loam(NSL) soil,to investigate gross N transformation rates.Soil samples were labeled with either ~(15)NH4_NO_3 or NH_4~(15)NO_3,and then incubated at 25 °C for 168 h at 60%water-holding capacity.The gross N mineralization(recalcitrant and labile organic N mineralization) rates in AC soil were 1.6 to 3.3 times higher than that in NSL soil,and the gross N nitrification(autotrophic and heterotrophic nitrification) rates in AC soil were 2.4 to 4.4 times higher than those in NSL soil.Although gross NO_3~- consumption(i.e.,NO_3~- immobilization and dissimilatory NO_3~- reduction to NH_4~+ rates increased with increasing gross nitrification rates,the measured net nitrification rate in AC soil was approximately 2.0 to 5.1 times higher than that in NSL soil.These showed that high NO_3~- production capacity of alkaline paddy soil should be a cause for concern because an accumulation of NO_3~- can increase the risk of NO_3~- loss through leaching and denitrification.     

Landscape-scale estimates of dinitrogen fixation by Pisum sativum by nitrogen-15 natural abundance and enriched isotope dilution

Topography and slope position influence the soil and environmental factors that affect N₂ fixation by legumes. The present study was conducted to (1) estimate N₂ fixation by field peas in a gently rolling farm field using the natural ¹⁵N abundance and the ¹⁵N-enriched isotope dilution techniques and (2) identify soil and environmental factors that influence N₂ fixation at the landscape scale. Whereas soil available water capacity, available NH _inf4 ^sup+ , total crop yield, and percent N derived from N₂ fixation (% Ndfa) estimated using enriched N were significantly affected by landform patterns, soil NO _inf3 ^sup- levels, seed yield, and the % Ndfa estimated using natural abundance did not follow landform patterns. The % Ndfa using natural abundance was correlated with NH _inf4 ^sup+ but not with available soil water, pH, electrical conductivity, NO _inf3 ^sup- , or particle size. Estimates of the % Ndfa using enriched ¹⁵N ranged from 0 to 92.8%. The highest median value (68.6%) for % Ndfa using enriched N occurred on the divergent footslopes, with the lowest value (28.1%) on the convergent shoulders. Estimates of % Ndfa using natural abundance ranged from 13.2% to 96.9%. Smaller fluctuations during the growing season in the ¹⁵N of the available N pool may have resulted in less variability for % Ndfa using natural abundance compared to enriched ¹⁵N. Despite similar mean values for % Ndfa using natural abundance (44.5) and enriched ¹⁵N (49.6), no significant correlation between the two estimates was found. These results suggest that although topography may exert gross controls on N₂ fixation, large variations in N₂ fixation at the microsite level may preclude correlations between individual estimates and limit detection of landscape scale patterns of N₂ fixation.Contribution No. R754 of the Saskatchewan Center of Soil Research     

Availability of soil and fertilizer nitrogen to wheat (Triticum aestivum L.) following rice-straw amendment

Summary A pot experiment was conducted to study the N availability to wheat and the loss of ¹⁵N-labelled fertilizer N as affected by the rate of rice-straw applied. The availability of soil N was also studied. The straw was incorporated in the soil 2 or 4 weeks before a sowing of wheat and allowed to decompose at a moisture content of 60% or 200% of the water-holding capacity. The wheat plants were harvested at maturity and the roots, straw, and grains were analysed for total N and ¹⁵N. The soil was analysed for total N and ¹⁵N after the harvest to determine the recovery of fertilizer N in the soil-plant system and assess its loss. The dry matter and N yields of wheat were significantly retarded in the soil amended with rice straw. The availability of soil N to wheat was significantly reduced due to the straw application, particularly at high moisture levels during pre-incubation, and was assumed to cause a reduction in the dry matter and N yields of wheat. A significant correlation (r=0.89) was observed between the uptake of soil N and the dry matter yield of wheat with different treatments. In unamended soil 31.44% of the fertilizer N was taken up by the wheat plants while 41.08% of fertilizer N was lost. The plant recovery of fertilizer N from the amended soil averaged 30.78% and the losses averaged 45.55%     

Use of 15N isotope dilution for quantification of N2 fixation associated with roots of kallar grass (Leptochloa fusca (L.))

Summary Leptochloa fusca (L.) Kunth (kallar grass) has previously been found to exhibit high rates of nitrogen fixation. A series of experiments to determine the level of biological nitrogen fixation using ¹⁵N isotopic dilution were carried out in nutrient solution and saline soil. In the nutrient solution, E. coli inoculated plants were taken as non-nitrogen-fixing control. It was observed that nearly 60%–80% of the plant N was derived from atmospheric fixation. Estimations based on the N difference method gave much lower values (18%–35%). In experiments with saline soil which was initially sterilized with chloroform fumigation, a mixed culture of N₂-fixing rhizospheric isolates from kallar grass roots was inoculated and planted to kallar grass. Uninoculated treatments were regarded as controls. The soil was previously labelled with ¹⁵N by adding cellulose and (¹⁵NH₄)₂SO₄. The results of these studies showed fixation values of 6%–32% when estimated by ¹⁵N dilution, whereas by the N difference method 54% of the plant N was estimated to be derived from fixation. This discrepancy is due to the increase in root proliferation due to inoculation, which results in greater uptake of soil N. The distribution of ¹⁵N in different fractions of the soil-N indicted isotopic dilution due to bacterial fixation of atmospheric N₂.     

Nitrogen turnover rates in a riparian fen determined by 15N dilution

Summary Gross rates of N mineralization, assimilation, nitrification, and NO _in3 ^sup- reduction were determined in soil from a wet riparian fen by 1-day incubations of soil cores and slurries with ¹⁵N-labelled substrates. N mineralization transformed 0.1% of the total organic N pool daily in the soil cores, of which 25% was oxidized through autotrophic nitrification and 53%–70% was incorporated into microorganisms. N mineralization and nitrification were markedly inhibited below 5 cm in soil depth. At least 80% of the NO _in3 ^sup- reduction in aerated cores occurred through dissimilatory processes. Dissimilatory reduction to NH _in4 ^sup+ (DNRA) occurred only below 5 cm in depth. The results show that NH _in4 ^sup+ oxidation was limited by available substrate and was itself a strong regulator of NO _in3 ^sup- -reducing activity. NO _in3 ^sup- reduction was significantly increased when the soil was suspended under anaerobiosis; adding glucose to the soil slurries increased NO _in3 ^sup- reduction by 2.4–3.7 times. Between 3% and 9% (net) of the added NO _in3 ^sup- was reduced through DNRA in the soil slurries. The highest percentage was observed in soil samples from deeper layers that were pre-incubated anaerobically.     

Gross N mineralization rates after application of composted grape marc to soil

Grape marc is a common waste product of the wine production industry. When partially composted and applied to soil it may contain enough N to affect vine growth and hence wine quality. Yet little is known about the quantity and timing of N release from composted grape marc. A laboratory incubation was conducted where composted grape marc amended and non-amended soils were periodically sampled over 148 days at 15 °C for gross N mineralization rates, C mineralization and microbial biomass-C. Gross N mineralization rates were determined by ¹⁵N pool dilution using both analytical equations and the numerical model FLUAZ (Mary, B., Recous, S., Robin, D., 1998. A model for calculating nitrogen fluxes in soil using 15N tracing. Soil Biology & Biochemistry 30, 1963-1979.). Both analytical and FLUAZ determined gross N mineralization rates were in close agreement in the control soil. However, in composted grape marc amended soils there was a discrepancy between the two solutions. Findings indicate that composted grape marc caused a net immobilization of N for the first 50-days of incubation, after which enough N was released to require consideration in fertilizer-N strategies.     

Inhibition of nitrogen uptake by rice after wheat straw application determined by tracer NH4 +-15N

This experiment was carried out to determine whether the delay in rice growth associated with wheat straw application, especially at the early stage, was due to the acceleration of N assimilation or N uptake inhibition.

Tracer ¹⁵N was used for rice plants cultivated in pots. After 24 h of tracer application the plants and soils were sampled for analysis. Seventeen days after transplanting, N uptake of rice decreased and the amount of unavailable tracer remaining in soil increased by wheat straw application. At the booting stage, 6 d before heading, N uptake was larger and the amount of remaining tracer was lower in the plots in which wheat straw was applied than in the control.

It was obvious that the decrease of N uptake by wheat straw application was caused by N uptake inhibition and not by N starvation for a period of time at the early stage. The inhibition was removed at the booting stage.     

川东北麦秆还田对水稻氮素吸收转运的影响

为合理利用秸秆,于2014和2015年两个水稻生长季,在大田条件下,以当地平均施肥量为标准,设置不同量的化肥配施秸秆处理,研究秸秆还田下水稻氮素吸收转运特征。结果表明:较纯化肥处理,秸秆替代一部分氮肥处理对水稻产量、地上部氮素积累量、氮素收获指数及氮肥生产效率无显著影响(P0.05);在不同程度上降低氮素在穗部的分配比例、营养器官吸收氮素向穗部转运量、转运率、转运氮对籽粒氮素贡献率;在不同程度上提高成熟期营养器官氮素积累量,显著提升抽穗后氮素吸收量及其对籽粒氮素贡献率(P0.05)。综合氮素吸收转运及利用效率,川东北稻麦轮作区水稻季在化肥减施30%基础上,麦秆还田量以6 000 kg/hm~2为宜。