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It is not clear how the number of 15 N-injection needles affects the estimated results of gross N transformation rates in the presence of plants. To evaluate this, a 15 N tracing study of sandy loam soil and a silty loam soil in the presence of maize (Zea mays L., cv. "Zhengdan 958") was conducted and the Ntraceplant tool was used to quantify gross N transformation rates. Our results showed that the number of 15 N-injection needles could significantly influence the estimated results of gross N transformation rates in the studied soil–plant systems. There was no discernible difference in the gross rates of N mineralization (Min), the oxidation of NH4+ to NO3− (ONH4), the oxidation of recalcitrant organic-N to NO3− (ONrec), the NH4+ and NO3− plant uptake between 4-needle and 6-needle injections for both studied soils. However, for the 1-needle and 2-needle injections in the silty loam soil, Min was significantly lower than the 4-needle and 6-needle injections. While, in sandy loam soil, Min measured with 1-needle injection was significantly higher than the 4-needle and 6-needle injections. The ONH4 and ONrec also varied for the 1-needle and 2-needle injections, compared to the 4-needle and 6-needle injections. The total plant N uptake rate declined with the increase in the number of injection needles. Based on these results and considering the simplicity of the experimental procedure, we suggest that the 4-needle injection could be used in the 15 N tracing studies conducted in pot (diameter = 6.4 cm in this study) experiments with plants, i.e., one needle for about 8 cm2. [ABSTRACT FROM AUTHOR]

Nitrification can be driven by either ammonia-oxidising bacteria (AOB) or ammonia-oxidising archaea (AOA) and is a central process in the nitrogen cycle. However, to date, it is not clear how the ecological niche differentiation of AOB and AOA are affected by land use and management changes in strongly acidic soils in subtropical China. In this study, three different land-use acidic soils--forest, upland, and paddy soils--were collected and a DNA Stable-Isotope Probing experiment performed to determine the relative contributions of AOA and AOB to ammonia oxidation in these soils. The results showed that AOA, but not AOB, amoA genes were detected in [sup.13]C-labelled DNA in the forest and paddy soils; however, only AOB amoA genes were detected in [sup.13]C-labelled DNA in the upland agricultural soils. The growth and activity of AOA and AOB in the different land-use soils provided direct evidence for the shift in roles for AOA and AOB in ammonia oxidation. AOA played the predominant role in ammonia oxidation in acidic forest and paddy soils. However, AOB, not AOA, mainly regulated the ammonia oxidation in acidic upland agricultural soils. Phylogenetic analysis indicated that AOA members within the marine Group 1.la-associated lineage dominated nitrification in the forest and paddy soils. Ammonia oxidation in the upland soil was catalysed by Nitrosospira cluster 3-like AOB. The moisture condition was likely the main reason inducing the ecological niche differentiation between upland and paddy soils; and AOA was more suitable for growth in the flooded, low oxygen conditions. Additional keywords: ammonia oxidation, DNA Stable-Isotope Probing experiment, forest soil, paddy soil, upland soil., Introduction Two-step nitrification is a central process in the nitrogen (N) cycle, during which ammonia is first oxidised to nitrite and subsequently to nitrate (Alawi et al. 2009). Ammonia oxidation, [...]

Rationale: Nitrite is well known to be unstable, including during soil extraction with KCl (especially in acidic soils), but the source and fate of NO 2 - in the short duration of the extraction process remain unclear., Methods: A series of 15 N-tracing studies explored NO 2 - transformations during KCl extraction in acidic and alkaline soils. Tests considering multiple factors assessed the interactions of such factors as soil sterilization, extraction time, and pH adjustment. After addition of 15 NO 2 - , 15 NH 4 + , and 15 NO 3 - tracers, the concentrations and isotopic compositions of N 2 O, N 2 , NH 4 + , NO 3 - , NO 2 - , and dissolved organic nitrogen (DON) were measured to investigate the production and consumption of NO 2 - ., Results: Nitrite was stable in alkaline soils during KCl extraction. In contrast, changes did occur in acidic soils during KCl extraction: NO 2 - declined rapidly in the first 10 min of extraction although the subsequent rate of decrease lessened as the extraction time progressed. Significant dilution of 15 NO 2 - suggested high rates of NO 2 - production and even higher rates of consumption. The soil's organic N was the only source of NO 2 - and also its main destination. Soil sterilization showed that NO 2 - processes during extraction were chemical, not microbial. The pH adjustment of acidic soil stabilized its NO 2 - ., Conclusions: Overall, the pH adjustment of KCl solution appears favorable for investigating NO 2 - dynamics. For example, this work recommends an extraction solution comprising a 4:1 mixture of 2.5 M KCl solution and pH 8.4 buffer, which was more convenient to operate than the method reported by Stevens and Laughlin., (© 2020 John Wiley & Sons, Ltd.)

Nitrous oxide (N 2 O) is an important greenhouse gas and contributes to stratospheric ozone depletion. Increasing temperature generally exerts a positive effect on soil N 2 O production. However, not much is known on the temperature influence on individual N 2 O production pathways. In this study, both laboratory 15 N labelling experiments with an incubation temperature gradient (35 °C, 25 °C, 15 °C, 5 °C) and field 15 N labelling experiments carried out in different seasons were conducted in Korean pine forest (KF) and Redwood coniferous forest (RF) soils. The results showed that the contribution of denitrification was positively correlated with temperature in KF and negatively correlated with temperature in RF, while their N 2 O production rates via denitrification (N 2 O d ) all declined with decreasing temperature. The contribution of autotrophic nitrification in KF ranged from 11% to 21%, while the contribution in RF significantly increased with decreasing temperature (P < 0.05). However, the N 2 O production rates via autotrophic nitrification process (N 2 O a ) were significantly and positively correlated with incubation temperature (P < 0.05). In addition, the contribution of heterotrophic nitrification to N 2 O production showed a negative and positive relation with increasing temperature in KF and RF, respectively. Whereas, the N 2 O production rates via heterotrophic nitrification (N 2 O h ) showed a significantly positive correlation with temperature (P < 0.05), but a negative relation with gross heterotrophic nitrification rates. The results in the field experiments corresponded to the laboratory results, indicating that the methods applied in field experiments were suitable for the estimation and prediction of in situ N 2 O production. The response of calculated N 2 O production rates to seasonal temperature in KF during the year of 2015-2017 also confirmed the suitability of the field research methods. This novel in situ technique to determine N 2 O production in temperate forest soils should be validated for other ecosystems., (Copyright © 2019. Published by Elsevier B.V.)