(red) and final seedling length on the right x-axis (blue) against seeded and non-seeded soil treatments, y-axis. Error bars denotes the standard deviation of three replicates of each soil treatments and five points measurement of the seedling height on the final day of full germination.
*NH4+ was below detection limit for all the treatments ( DL, 0.01g N gw-1), reason for its absence.
6.0 DISCUSSION
The result from the gas fluxes revealed that the horse manure had the most significant HONO, NO, and N2O emissions when compared to hay field and grassland. This can be traced to (1) ammonium oxidation, due to the notable highest concentration of NH4+
, (2) hydrolysis of nitrite because of the traceable highest NO2- concentration, and (3) reduction of the highly present NO3- concentration.
This suggests that nitrogen availability in the horse dung has positive correlation with traceable nitrogen input in the horse feeds. The gaseous emissions can also be traced to low pH and the presence of NO3- and NO2-. However, a disappearance in NO2- concentration as HONO, NO and N2O increased was noticed throughout the sampling points. Previous studies have shown that NO2
-pool has the potential to form NO and HONO gases, as most of their emissions have been linked to the presence of NO2- (Bhattarai et al., 2018).
Although, the hay field and grassland treatments showed very similar total nitrogen gas emission rates and nutrients concentrations between each other, yet more nitrogen appeared to be available in the hay field than in the grassland. This acknowledges the effect of higher nitrogen input in the hay field through the 24.6% N of 315 kg/ha fertilizer applied, shortly before the sampling collection. Also, the emissions of all the gases have a positive correlation with decreasing soil pH and the presence of NO2- and NO3- concentration across the sampling times. Our results show that N2O emissions through denitrification process correlates with soil available NO3
-concentration, and nitrification process through conversion of NH4+ to NO3- availability (Bhattarai et al., 2018). It is also logical to say that the NO and HONO gases behave similarly while N2O emissions followed a different emission pathway within the tree soil treatments. An observation
40
from the last sampling point was that, as the soil moisture decreases, pH decreases and the N-gaseous emissions rates increase (Henault et al., 1998).
In the seed experiment, only two soil samples (hay field and horse paddock) were compared. In both the hay field (Fig. 21, A), and the horse paddock (Fig. 21, C) seeded treatments, it was observed that the NO emission decreases with increasing seed length. Nitric oxide has been identified as a dormancy-releasing candidate in seed germination by releasing major nitrogen nutrients such as NO3- and NO2- which are essential for seedling growth (Arc et al., 2013). We assume that majority of NO concentration has been converted into soil NO3- and NO2- for seed germination and seedling growth. However, in the HONO emissions we observed a positive correlation with the seedling length in both the hay field (Fig. 21, B) and horse paddock (Fig. 21, D) treatments. HONO emission increased totally as the seedling length increased, but the emission rates differ in the two soil treatments. The horse paddock had the highest emission rate of about 5.5 g Nm-2 h-1 and seedling length (of about 11.7cm), while the hay field had lower emission rate of about 2.7 g Nm-2 h-1 and seedling length (of about 8.8cm) respectively (Fig. 21 B, D). Past studies have shown that HONO emissions have been linked to soil NO2- and pH (Su et al., 2011).
Therefore, we noticed a high presence of soil NO2- concentration in both the hay field and the horse paddock seeded cores (Fig. 22, B). However, the effect of N-input can still be noticed due to the higher concentration of soil NO3- and NO2- in the hay field treatment compared to the horse paddock (Fig. 22). When comparing this study with recent research, where maximum HONO emission was noticed in early stage of germinating wheat seed with decreasing emission rate as the shoot length increases (Bhattarai et. al 2019), this result showed that experimenting with a different seed and perhaps at different timing has clearly influenced the HONO emission rates. We hope that further studies would help to clarify the effect of N-input on HONO emissions in seed varieties.
The soil nutrient concentration after the collection of biomasses from the seedling growth shows the uptake of soil NO3- and NO2- for seedling growth (Fig. 22). In fact, it was clear that seedling length increases with the reduction of NO3- concentration when comparing the seeded and non-seeded treatments of each soil types. The disappearance of soil NO3- supports the literature review that most of the soil NO3- have been used up during the seedling growth and some were converted into NO and N2O. However, NO2- showed some irregular variation with a sharp increasing concentration in S-Hay and a slight decreasing concentration in S-Hp. This may be a
41
good reason why the S-Hp has more NO and HONO emission rates compared to S-Hay, because previous results have shown that HONO emissions have strong correlation with soil NO2
-availability (Su et al., 2011). Majority of the soil NO2- concentration in S-Hp may have been converted into increasing the concentrations of NO and HONO in the soil.
Finally, clearly well-germinated, taller and greener seedling length in the horse paddock cores can be observed when compared to the hay filed (Fig. 12) This indicates that the horse paddock has more available nitrogen for seedling assimilation, with stronger affinity to emit more greenhouse gases (especially N2O), HONO and NO emissions when compared to the hay field. The biomass yield from the horse manure seed treatment was at least twice the size of that from hay field, and thicker. We assume that the biomass yield responded to the rate of nitrogen input in the horse paddock. The organic C and N concentration in N-fertilized soils tends to increase through high biomass yield and crop residues (Russell et al., 2005)
42 7.0 CONCLUSIONS
Greenhouse gas (especially N2O), HONO and NO emissions increased with increasing available nitrogen in a horse paddock, hay field and grassland. The emission rates correlate positively with soil NO3- and NO2- concentration and reduced soil pH.
NO and HONO gases behaved the same way in all the soil treatments with response to soil NO2- across the sampling times, while N2O emission followed a different pathway with response to soil NO3-.
• The horse paddock had the most N2O, HONO and NO emission rate amongst the three closely-related agricultural soils, with respect to soil pH.
• HONO emission increased with seedling length in both hay field and horse paddock treatments, with respect to soil NO2-.
• More nitrogen nutrients were available for plant assimilation in the horse paddock, than in the hay field.
43
ACKNOWLEDGEMENTS
I am grateful to God for his mercy and sufficient grace to complete my studies in the University of Eastern Finland.
I appreciate every effort from my Research Supervisor, Associate Professor Marja Maljanen for her supervisory role, intellectual contribution and words of encouragement in making this work a success.
I acknowledge the effort of Hem Raj Bhattarai (MSc.), who unrelentingly put me through all laboratory activities and challenging data analysis. I also thank Minna Kivimaenpaa for her decent role on this project.
Finally, to every individual who have in one way or the other contributed to this academic success, I am grateful.
Thank you.
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