Dicyandiamide has more inhibitory activities on nitrification than thiosulfate
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.
Nitrification inhibitors have been proved as an effective tool to reduce N loss and improve N use efficiency in last decades. According to their modes of action on nitrification, NIs are divided into two groups: one group of chemicals inhibit the oxidation of NH3 to NO2-, and another group of chemicals suppress the conversion of NO2- to NO3-. Among the chemicals available, DCD and thiosulfate are two different types of NIs, which retard the first and the second stage of nitrification, respectively. Previous work suggest that net nitrification rate was significantly related to the abundance of AOB not AOA, and DCD exerted more greater inhibition on the growth of AOB than AOA . It was observed that the AOB rather than AOA functionally dominate NH3 oxidation irrespective of N-rich grassland soil or high N input vegetable soil. AOB is also found to be inactive in acidic conditions. Thiosulfate inhibits NH3 oxidation by heterotrophic nitrifiers , however, very limited information is available on the distinct effects of thiosulfate on the growth of AOB or AOA in soil.
DCD has been evaluated as an effective NIs being widely used and studied because it is nonvolatile, nonhygroscopic, relatively water soluble and highly mobile. Application of DCD increased soil N immobilization, urea-N recovery and showed no effect on urea hydrolysis . It was found that DCD was investigated mainly in the cultivated land and grassland, where applications of DCD (10–30 kg ha-1) have been proved to be effective in reducing N2O emissions from nitrogen fertilizers, urine or livestock slurry , retaining N in soil in the less mobile ammonium (NH4+) form then decreasing soil NO3- leaching and increasing yields. In contrast, a few field studies showed that DCD was ineffective in mitigation on bovine urine N2O emission under Oxisol and yield improvement of summer barley, maize, winter wheat, potato and canola. The contrastive results indicated that the performance of DCD as NIs is not consistent and is affected by many factors. According to McGeough et al. (2016) , among nine contrasting UK soils tested, the lower efficacy of DCD on inhibition of NH4+ oxidation and NO3- production was observed in soils with high temperature, clay and organic matter content. In an analysis consisting 111 datasets from 39 studies, DCD was found to be effective with all fertilizer types (organic and chemical) except for nitrate-based fertilizers in different soils with irrespective of texture.
Thiosulfate has been identified as an active NIs, which is convenient for handling and is highly compatible with other nutrient sources, commonly used as a source of sulfur in fluid fertilizers. A field study with maize showed that addition of ammonium thiosulfate to urea ammonium nitrate (UAN) tended to increase grain yield and plant nitrogen efficiency. Potassium thiosulfate with application rate of 102 mg S kg?1 reduced N2O emissions by 48% indicating it is as effective as nitrification inhibitor N-Serve. Ammonium thiosulfate exhibited different efficacy as incorporated with NH4NO3, (NH4)2SO4, urea and poultry manure, and the highest inhibitory effect was with NH4NO3 which maintained 100% inhibition of nitrification during 12 weeks. Inhibition of nitrification induced by sodium thiosulfate ranged 55–80% with application of 32 mg S kg-1. During nitrification inhibition, thiosulfate resulted in the accumulation of NO2- and NO under aerobic conditions and showed no effect on reduction rate of NO3- under anaerobic environment. In addition, thiosulfate was found to retard soil urease activity when applied at rates of 2500 or 5000 μg g-1 soil. As a nitrification inhibitor, thiosulfate acts more efficiently than Azadirachta indica (neem) and calcium chloride, but not as DCD. The stronger inhibitory effect induced by DCD than thiosulfate was observed by Goos and Johnson (1992) under laboratory, field microplot, and field conditions. Kumar et al. (2000), however, found that DCD and thiosulfate reduced N2O emissions by a similar amount from urea (11 and 9% reduction, respectively) in a rice field. The different effects of DCD and thiosulfate on nitrification attribute to not only their distinct characteristics but also the various modes of action. Moreover, the efficacy of NIs can vary widely with the variation of environmental factors, while the literature relating to DCD and thiosulfate is extensive, there have been few studies comparing their ability on N transformations involving NO2--N dynamics and ammonia-oxidizing microbes under same environmental conditions, and the mechanism for the differentiation between the efficacy of DCD and DMPP is still unclear.
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.