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Purpose: Microorganisms play important ecological roles in NO pool production, e.g., through autotrophic nitrification (the oxidation of NH to NO) and heterotrophic nitrification (the oxidation of organic N to NO). Previous studies have mainly focused on nitrifying microorganisms capable of autotrophic nitrification; however, the contribution of microorganisms to heterotrophic nitrification has not been well investigated. The aim of this study was to identify the contribution of fungi and bacteria to autotrophic nitrification and heterotrophic nitrification in a subtropical coniferous forest soil of China. Materials and methods: A N tracing experiment was conducted in a subtropical forest soil, which was treated with CK (no antibiotics), cycloheximide (fungal inhibitor), and streptomycin (bacterial inhibitor), respectively. Soil was incubated at 25 °C with 60 % water holding capacity (WHC) for 144 h after being labeled with NHNO or NHNO at a 20 atom% N excess. Autotrophic and heterotrophic nitrification rates were calculated using a full process-based N cycle model. Results and discussion: The net nitrification rates were negative in all treatments, ranging from −0.346 mg N kg day in the cycloheximide treatment to −0.179 mg N kg day in the streptomycin treatment. The autotrophic nitrification was almost negligible (only 0.001 mg N kg day) in the soil from subtropical coniferous forest, while heterotrophic nitrification rate was 0.993 mg N kg day. Streptomycin reduced heterotrophic nitrification rate by 22.1 %, while cycloheximide nearly completely inhibited this process. Conclusions: Laboratory results showed that heterotrophic nitrification is a pivotal process producing NO and that fungi rather than bacteria may dominate heterotrophic nitrification in the subtropical coniferous forest soil. [ABSTRACT FROM AUTHOR]

The potential denitrification (PD) rate, NO, NO, and N emission were determined after treatment with 50 mg NO−N kg soil using the acetylene inhibition method, and meanwhile abundance of four denitrifying genes (i.e., narG, nirK, norB, nosZ) was also investigated in subtropical soils of China. Soil samples were collected from conifer forest (C), shrub forest, and farmland. These soils were derived from Quaternary red earth and granite. The PD rate and N gas emissions significantly ( p < 0.05) differed between forest and farmland soils; abundance of denitrifying genes was also significantly affected by the land-use change. Correlation and multiple stepwise regression analyses showed that the PD rate was significantly ( p < 0.05) and positively correlated with soil pH but not with soil organic C and total N contents ( p > 0.05). The norB gene copies in farmland soils were significantly higher than in conifer and shrub forest soils ( p < 0.01). Both norB and nosZ gene copies were linearly correlated with soil pH, and the PD rate and N emission rate were significantly correlated with the abundance of norB ( p < 0.05). Probably, soil pH affected denitrifiers targeted by the norB gene, thus decreasing the reduction of NO and NO. [ABSTRACT FROM AUTHOR]

Purpose: Nitrous oxide (NO) production and reduction rates are dependent on the interactions with each other and it is therefore important to evaluate them within the context of simultaneously operating NO emission and reduction. The objective of this study was to quantify the simultaneously occurring NO emission and reduction across a range of subtropical soils in China, to gain a mechanistic understanding of potential NO dynamics under the denitrification condition and their important drivers, and to evaluate the potential role of the subtropical soils as either sources or sinks of NO through denitrification. Materials and methods: Soils (45, from a range of different land uses and soil parent materials) were collected from the subtropical region of Jiangxi Province, China, and tested for their potential capacity for NO emission and NO reduction to N during denitrification. NO emission and reduction were determined in a closed system under N headspace after the soils were treated with 200 mg kg NO-N and incubation at 30 °C for 28 days. The soil physical and chemical properties, the temporal variations in headspace NO concentration, and NO-N and NH-N concentrations in the soil slurry were measured. Results and discussion: Variations in NO concentration ( N) over incubation time ( t) were consistent with an equation in which average R = 0.84 ± 0.11 ( p < 0.05): $$ N = A \times \left( {1 - \exp \left( { - {k_1} \times t} \right)} \right) - B \times \exp \left( {{k_2} \times t} \right) $$, where A is the total NO emission during the incubation, B is a constant, and k and k are the NO emission constant and reduction constants, respectively. The results of the simulation showed that k was greater than k. The reduced amount of NO-N in the first 7 days of incubation and the NO emission rate (the percentage of A value relative to the amount of NO-N reduced during the 28-day incubation, R) were able to explain 82.9 % ( p < 0.01) of the variation in total NO emission ( A) during the incubation for the soil samples studied, indicating that the total amount of NO emitted was determined predominately by denitrification capacity. Soil organic carbon content and soil nitrogen mineralization are the key factors that determine differences in the amounts of reduced NO-N among the soil samples. The R value decreased with increasing k ( p < 0.01), indicating that soils with higher NO reduction capacity under these incubation conditions would emit less NO per unit of denitrified NO-N than the other soils. Results are valuable in the evaluation of net NO emissions in the subtropical soils and the global N budget. Conclusions: In a closed, anaerobic system, variations in NO concentration in the headspace over the incubation time were found to be compatible with a nonlinear equation. Soil organic carbon and the amount of NH-N mineralized from the organic N during the first 7 days of incubation are the key factors that determine differences in the NO emission constant ( k), the NO reduction constant ( k), the total NO emission during the incubation ( A) and the NO emission rate ( R). [ABSTRACT FROM AUTHOR]

In this study, a 15 N tracing incubation experiment and an in situ monitoring study were combined to investigate the effects of different N fertilizer regimes on the mechanisms of soil N dynamics from a long-term repeated N application experiment. The field study was initiated in 2003 under a wheat-maize rotation system in the subtropical rain-fed purple soil region of China. The experiment included six fertilization treatments applied on an equivalent N basis (280 kg N ha −1 ), except for the residue only treatment which received 112 kg N ha −1 : (1) UC, unfertilized control; (2) NPK, mineral fertilizer NPK; (3) OM, pig manure; (4) OM-NPK, pig manure (40% of applied N) with mineral NPK (60% of applied N); (5) RSD, crop straw; (6) RSD-NPK, crop straw (40% of applied N) with mineral NPK (60% of applied N). The results showed that long-term repeated applications of mineral or organic N fertilizer significantly stimulated soil gross N mineralization rates, which was associated with enhanced soil C and N contents following the application of N fertilizer. The crop N offtake and yield were positively correlated with gross mineralization. Gross autotrophic nitrification rates were enhanced by approximately 2.5-fold in the NPK, OM, OM-NPK, and RSD-NPK treatments, and to a lesser extent by RSD application, compared to the UC. A significant positive relationship between gross nitrification rates and cumulative N loss via interflow and runoff indicated that the mechanisms responsible for increasing N loss following long-term applications of N fertilizer were governed by the nitrification dynamics. Organic fertilizers stimulated gross ammonium (NH 4 + ) immobilization rates and caused a strong competition with nitrifiers for NH 4 + , thus preventing a build-up of nitrate (NO 3 − ). Overall, in this study, we found that partial or complete substitution of NPK fertilizers with organic fertilizers can reduce N losses and maintain high crop production, except for the treatment involving application of RSD alone. Therefore, based on the N transformation dynamics observed in this study, organic fertilizers in combination with mineral fertilizer applications (i.e. OM, OM-NPK, and RSD-NPK treatments) are recommended for crop production in the subtropical rain-fed purple soils in China. [ABSTRACT FROM AUTHOR]