Dicyandiamide (DCD) and thiosulfates are two type of nitrification inhibitors (NIs) that have been widely used in agriculture to improve nitrogen (N) fertilizer use efficiency and mitigate negative effect of N on environment. Little information is available concerning the comparison of the efficacy of DCD and thiosulfate on N transformations in soil. The aim of this study was to compare the effects of DCD and thiosulfate (K2S2O3) on changes of NH4+-N, nitrification inhibition and N recovery in a latosolic red soil. An incubation experiment was conducted with four treatments of control (CK), N, N+DCD, and N+K2S2O3. Soil samples were collected periodically over 50 d to determine concentrations of mineral N, and the amoA gene abundance of ammonia monooxygenase (AMO) for ammonia-oxidizing bacteria (AOB) was estimated by qPCR after 10 d incubation. In the N treatment, 67.8% of the applied N as NH4+-N disappeared from the mineral N pool and only 2.7% and 30.8% of the applied N was accumulated as NO2--N and NO3--N, respectively. Addition of DCD and thiosulfate to the soil prevented NH4+-N disappearance by 63.0% and 13.6%, respectively. DCD suppressed the production of NO2--N by 97.41%, whereas thiosulfate increased accumulation of NO2--N by 14.6%. Application of N along with DCD and thiosulfate inhibited nitrification, respectively, by 72.6% and 33.1%, resulting in the delay of the nitrification process for 30 days and 10 days, respectively. Apparent N recovery in N treatment was 66.2%, which increased by 55.2% and 4.8% by DCD and thiosulfate, respectively. Numbers of AOB amoA gene copy was significantly inhibited by both DCD and thiosulfate, and the stronger inhibition induced by DCD than thiosulfate was recorded. Results indicated that both DCD and thiosulfate were effective inhibitors for NH4+-N oxidation, NO3--N production, mineral N losses and AOB growth. DCD showed a more pronounced effect on nitrification inhibition than thiosulfate.

Nitrogen is an essential element for plant growth and crop productivity in agroecosystems and undergoes a series of microbial transformations in soils. During N transformation, the nitrification plays a key role in regulating soil N loss in relation to nitrate leaching and oxynitride emissions to the environment. Conversion of ammonia to nitrite is the first and rate-limiting step in nitrification and three different groups of microorganisms including ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA) and comammox bacteria, which all possess a pivotal enzyme-ammonia monooxy-genase (AMO) enzyme, conduct this pathway. Oxidation of ammonia is considered as the main contributor to the ammonium: nitrate balance in terrestrial ecosystems, receiving much attention in difficulties relating to the chemical reactive nature of NO2--N. Isobe et al. (2012) observed the simultaneous production and consumption of NO2-, which exhibited a faster conversion rate than NH4+ and NO3-, suggesting that rapid NO2- turnover could be a major driving force for N transformation in forest soil. It is generally accepted that NO2- rarely accumulates in terrestrial ecosystems. However, NO2- accumulation may occur as the consumption rate is lower than the production rate.

In this study, both DCD and thiosulfate (K2S2O3) were chosen because of their importance as NIs in agriculture and horticulture. We hypothesized that DCD and thiosulfate may impose different effects on N transformation based on their different mechanisms related to nitrification inhibition. Few studies have been examined the impact of thiosulfate on the abundance of ammonia-oxidizing microbes, which limits our comprehensive understanding of its potential in nitrification inhibition and N management. The abundance of AOB in different treatments was investigated because of its dominance during ammonia oxidation in vegetable soils as reported before. The objectives of this study were to: i) test if thiosulfate is as effective as DCD acting as a nitrification inhibitor, ii) assess the different effect between DCD and thiosulfate on NO2--N accumulation, NO3--N production and the abundance of bacteria (AOB), iii) ascertain whether thiosulfate imposes inhibitory effect on ammonium oxidation.

Effects of application of lime nitrogen and dicyandiamide on nitrous oxide emissions from green tea fieldsThe aim of this study was to assess the mitigating effects of lime nitrogen (calcium cyanamide) and dicyandiamide (DCD) application on nitrous oxide (N2O) emissions from fields of green tea [Camellia sinensis (L.) Kuntze]. The study was conducted in experimental tea fields in which the fertilizer application rate was 544 kg nitrogen (N) ha?1 yr?1 for 2 years. The mean cumulative N2O flux from the soil between the canopies of tea plants for 2 years was 7.1 ± 0.9 kg N ha?1 yr?1 in control plots. The cumulative N2O flux in the plots supplemented with lime nitrogen was 3.5 ± 0.1 kgN ha?1, approximately 51% lower than that in control plots. This reduction was due to the inhibition of nitrification by DCD, which was produced from the lime nitrogen. In addition, the increase in soil pH by lime in the lime nitrogen may also be another reason for the decreased N2O emissions from soil in LN plots. Meanwhile, the cumulative N2O flux in DCD plots was not significantly different from that in control plots. The seasonal variability in N2O emissions in DCD plots differed from that in control plots and application of DCD sometimes increased N2O emissions from tea field soil. The nitrification inhibition effect of lime nitrogen and DCD helped to delay nitrification of ammonium-nitrogen (NH4+-N), leading to high NH4+-N concentrations and a high ratio of NH4+-N /nitrate-nitrogen (NO3–-N) in the soil. The inhibitors delayed the formation of NO3–-N in soil. N uptake by tea plants was almost the same among all three treatments.

In recent years, the amount of fertilizer applied to tea fields in Japan has been reduced in an effort to reduce environmental impacts. Some new methods of fertilizer application have been proposed to increase the efficiency of N uptake by tea plants. Among the new methods of fertilizer application, application of lime nitrogen and dicyandiamide (DCD), one of the nitrification inhibitors used in tea cultivations in Japan, are promising methods to increase the N use efficiency of tea plants. Lime nitrogen consists primarily of calcium cyanamide; it contains approximately 20% N and 50% lime in the form of calcium oxide (CaO) (Klasse 1996). In the soil, calcium cyanamide is broken down into urea and DCD. DCD inhibits ammonia oxidation by microbes and is also degraded by some microbes. The inhibition of nitrification by DCD allows ammonium-nitrogen (NH4+-N) to remain the soil for longer. This condition is favorable for the growth of tea plants because tea plants prefer NH4+-N to nitrate-nitrogen (NO3–-N) (Ishigaki 1978) and because the amount of N leached from the soil is reduced. In addition, N2O emissions are also reduced by directly limiting the nitrification process and by indirectly limiting the denitrification process by suppressing rapid formation of NO3–-N (Aulakh et al. 1984; Bhatia et al. 2010).

In a previous study, application of lime nitrogen reduced N2O emissions from tea field soil (Tokuda 2005; Yamamoto et al. 2014). In those studies, the effects of lime nitrogen on N2O emissions were, however, evaluated based on the results of 1 year of monitoring. Interannual variations of N2O emissions have not been assessed. In addition, there is no report on the effect of DCD application on N2O emissions from tea field soil. Thus, further detailed information was required to clarify the effects of lime nitrogen and DCD on N2O emission from tea fields. Therefore, in this study, we assessed the mitigating effects of lime nitrogen and DCD on N2O emissions from tea fields based on detailed results obtained in a 2-year monitoring experiment.