Nitrous oxide emission
The results of the analysis of variance show that there are highly significant differences in the nitrous oxide emission fluxes per day, due to the effect of the treatments. In Fig. 1, generated from the data obtained as daily emission fluxes, it can be observed that the highest emissions occur on days 1 and 2 after the second fertilization event; then, approximately 15 days after the two fertilization events, the emission flux stabilizes; it is also observed that the differences are much more noticeable mainly between treatment 3 and treatment 5,6 7 and control.
In relation to the source of nitrogen, the highest emissions occur with the use of uncoated urea with fractional application (treatment 2), followed by the treatment of urea with NBPT in total application (treatment 4) and in third place uncoated urea in total application (treatment 2); On the other hand, the lowest values are presented in treatment 1 or control, followed by treatment 5, i.e. use of urea + NBPT with fractional application, followed by treatment 6 and 7; in this order of ideas, the values reported are considered high, according to Yang et al., (2021) and coincide with those reported by Wang et al., (2016a); Wang et al., (2016b).
This variability in N2O emissions occurs according to D Signor et al (2013) regardless of whether the origin of the N2O is nitrification or denitrification, on the other hand, Davidson et al (1996), mention that heterogeneity in N2O emissions can be explained by a variation in nitrification potential, which, in turn, is associated with differences in the distribution of populations of nitrifying microorganisms, emphasizing that these considerations can also be applied to denitrification due to microbial activity and biomass. According to Treseder, (2008), they state that the highest emissions occur with urea, as in the case of this research, since urea stimulates N2O emissions, however, they emphasize that emissions are reduced when high rates of fertilizer are applied, which in our opinion is a bit contradictory.
On the other hand, and as shown in Fig. 2, a highly positive correlation was found, when performing the modeled regression analysis, it is shown that as CO2 increases, there is also a tendency to emit more N2O (R2 0.95, y = 400x − 133.33), this behavior is valid between days 7 and 32 after fertilization, and tends to stabilize between days 45–62, after which this behavior disappears, It is possible that the first applications stimulate soil microbial activity and biomass and favor the emission of nitrous oxide, however, Ramirez et al (2010) also mention that the addition of nitrogen can have a negative effect on soil biomass and microbial respiration, and therefore can have a negative effect on N2O fluxes in the soil that are generated by microorganisms.
The ANOVA results for the cumulative emission fluxes and the comparison of daily means 62 presented in Table 2 show significant differences between treatments, being more evident between 3 and 7.6 and 5. For the fractional or total application type, no significant differences are found between the control and the fractional application of urea + NBPT (T4) and urea + NBPT + DCD (T6).
The results in the treatments with total application show how urea + NPBT + DCD can reduce gas emission compared to urea without additive up to 1.6 times, however, for urea + NBPT it generates higher emission if compared to urea without additive. For the case of treatments with fractional application, it is found that urea + NBPT can reduce up to 3.7 times the emission if compared to urea without additives, on the other hand, urea + NBPT + DCD emits 2.24 times less than urea without additives, the above results agree with data reported by Emerson et al, (2021), Souza et al., (2019), in the same order of ideas, authors such as Degaspari et al., (2019) show how the addition of these and other inhibitors such as DMPP to urea reduce up to 90% N2O emission.
Table 2
Comparison of means of accumulated fluxes at 62 days (LSD Fisher), p-val 0.05.
Treatment | Average | | | | |
T1 | 1,91 | A | | | |
T5 | 4,50 | A | B | | |
T6 | 6,18 | A | B | C | |
T7 | 7,42 | A | B | C | |
T2 | 10,12 | | B | C | |
T4 | 12,14 | | | C | D |
T3 | 16,69 | | | | D |
In the same vein, results reported by Mateo-Marín et al, (2020), state that the use of inhibitors, in addition to reducing N2O emission fluxes, in some cases also reduces nitrogen losses in the form of nitrate by leaching, coinciding with Morales-Morales et al (2019), who state that urea with N-(n-butyl thiophosphoric triamide) (NBPT), is an effective inhibitor of nitrogen leaching, who state that urea with N-(n-butyl) thiophosphoric triamide (NBPT) is a urease inhibitor, which temporarily prevents the enzymatic degradation of urease and minimizes the loss by volatilization of NH3, thus increasing the uptake of N from the fertilizer by the crop.
Figure 3 shows the behavior of each treatment accumulated over time, where it is shown that N2O is stimulated and the emission flux increases when there is an input of nitrogen in the soil, in this case nitrogen fertilization. Regardless of the type of fertilizer or the amount of it, in each of the treatments the activity of the microorganisms is impacted and the emission is increased, these results, can induce, as stated by Tian et al., (2019) and Shcherbak et al., (2014) that with synthetic nitrogen applications to the soil, the soil microbiology causing the emission of the gas is always going to be stimulated.
The figure also shows how urea without additive and in fractional application is the main emitter of gas, emitting 14.78 kg/ha more than treatment one or control or the lowest emission, and emitting 4.55 kg/ha more than treatment four, which is the second highest in cumulative emission flow. On the other hand, urea in fractional application ranked third highest in emission, being below the application with urea + NBPT in total application with 2.02 kg less accumulated emission, this may indicate that not only the use of urease inhibitors is sufficient, but also the number of nitrogen units applied in each fertilization event should be considered.
Figure 4 shows the effect of the addition of inhibitors to the urea molecule, in particular the difference between the emission flux between urea with a single inhibitor, NBPT, lower than the flux presented with the application of urea with two inhibitors, NBPT + DCD.
The urease inhibitor, NBPT, has the function of inhibiting the activity of the urease enzyme present in the soil, this inhibitor delays the hydrolysis of nitrogen, thus the nitrogen entering the soil can better meet the nitrogen demand of the crop, and the losses of nitrogen as NH3 by volatilization are reduced (Ju et al., 2009). The use of these inhibitors does not avoid the conversion of NH4+ to NO3− through nitrification, so N2O emissions and nitrate leaching are generated, it is necessary to use nitrification inhibitors such as DCD, which inhibits the nitrification process in the soil, avoiding the conversion of NH4+ to NO2− and NO3− and reducing the loss of nitrogen as nitrate in leaching (Zhao et al., 2017).
Consequently, for this trial and as shown in the previous figure, there was a lower emission flux for the treatments with urea + NBPT, i.e., only with the use of an inhibitor. This result is possibly due to the differences in the concentrations of the inhibitor applied to the urea molecule in each fertilizer; in that sense, it is likely that the concentration of the additive in the molecule coated only with NBPT is higher than the concentration of the additives in the molecule containing the two inhibitors, NBPT + DCD, urease and nitrification inhibitor respectively the results obtained, agree with those reported by Vivian et al. (2018).
Nitrogen emission factor (EF).
The results of the emission factor show that there are highly significant differences in the Emission Factor due to the effect of the application of the different treatments, presenting values ranging from 1.6–9.18%.
As shown in Table 3, the best performing treatment, or the most environmentally friendly, since it presents lower EF values and therefore less percentage of N2O emitted per hectare per kg of nitrogen applied is T5 (Urea + NBPT in fractional application), followed by T6 (urea + NBPT + DCD in total application). On the other hand, the treatment with the highest EF and therefore generating more N2O per hectare per kg of nitrogen applied is T3 (Urea in fractional application), followed by T4 (Urea + NBPT in total application). In general, it could be stated that the EF tends to increase as the doses of nitrogen applied increase, but the use of nitrification inhibitors changes this behavior, according to the results shown.
Table 3
Emission factor per treatment.
Treatment | Average |
T1 | |
T2 | 5,1 |
T3 | 9,18 |
T4 | 6,36 |
T5 | 1,61 |
T6 | 2,65 |
T7 | 3,42 |
In that order of ideas, the values shown for the case of the treatments applied with urea without inhibitors (T2-T3), can be considered high percentages, if compared with the results reported by Degaspari I., et al, (2019), who found emission factors ranging from 0.76–1.13%, likewise when compared to the results exposed by Harty et al., (2016) who found emission factor values for urea in the range of 0.15–0.49%, and emission factors of 0.21–0.69% for urea + NBPT. For the T5 emission factor, we report a value of 1.6% which is within the values assigned by IPCC (2019) to the study area.
On the other hand, it is important to take into account that these values may be underestimating the emissions due to the measurement technique based on closed static chambers, which according to Merbold et al. (2021), does not allow identifying some high peaks of gas emission and the number of samples is limited, which prevents a more complete understanding of the complex and dynamic behavior of nitrogen in the soil. On the other hand, this emission factor represents the emissions of the crop during two months of its development, which represent the fertilization period; it is possible that the first month of crop development and the 10 months following fertilization present emissions close to those found in T1 or treatment without fertilization.