3.5 Correlation between N2O emission and the
fungal community and soil properties
Associations between N2O emission, soil properties and
community structures of the fungal phyla and genera were analyzed by
RDA. For the fungal phyla, axis1 and axis2 explained 60.7% and 7.5% of
the total variation, respectively, in the community structure (Fig. 6a).
N2O emission was positively correlated with fungalAscomycota , with the highest correlation (R = 0.726, P = 0.008).
The dominant phylum of fungi in the tested soil was Ascomycota, which
accounted for 21.1-36.6% of the total fungal microbial abundance. The
trend of change in the abundance of Ascomycota in the four treatments
was consistent with that of N2O emission, which meant
that Ascomycota was an important contributor to N2O
emission.
For the fungal genus community structure, RDA analysis explained 73.2%
of the total variation, and axis1 and axis2, respectively, explained
46.6% and 26.6% of the total variation (Fig. 6b). N2O
emission showed a positive correlation with the abundance ofPyrenochaetopsis , Myrothecium , Zopfiella ,Humicola , Bullera , and Conocybe . The generaPyrenochaetopsis , Myrothecium , Zopfiella , andHumicola belong to the phylum Ascomycota, and Bullera andConocybe belong to the phylum Basidiomycota. Further quantitative
analysis of the correlation between N2O emission and the
relative abundance of the fungal genera revealed that Myrothecium(R2 = 0.556, P = 0.005), Pyrenochaetopsis (R2 = 0.478, P =
0.013), and Humicola (R2 = 0.372, P = 0.035) were positively
correlated with N2O emission, while Cryptococcus(R2 = 0.551, P = 0.006) was negatively correlated with
N2O emission (Fig. 7). Pyrenochaetopsis ,Myrothecium , and Humicola were also found to be the main
functional fungi in N2O emission. However,Zopfiella , Bullera , and Conocybe were not
significantly correlated with N2O emission.
In addition, N2O emission was positively correlated with
soil NO3–N and negatively correlated with soil pH and
NH3+-N (Fig. 6). The influence of soil properties on
fungal community structure decreased in the order of pH >
NO3–N > NH3+-N
> SOC > TN. N fertilizer use led to changes in
soil properties and in the fungal community structure, and ultimately
affected N2O emission.
4. Discussion
In this study, we specifically studied the effects of different
fertilization amounts on N2O emission from lawn soil and
microbes, and further analyzed the community changes within the major
functional microorganisms and their correlation with N2O
emissions. The results showed that N fertilizer significantly increased
the N2O emission from lawn soil (Fig. 2b). This was
consistent with most studies reporting that exogenous nitrogen addition
significantly enhanced soil N2O emission[28-31]. The contribution of fungi to the
N2O emissions in lawn soil was the highest of the three
evaluated microbial communities, accounting for 45%, which was
significantly higher than that of bacteria (31%) and other
microorganisms (24%) (Fig. 3). Therefore, our results showed that fungi
played a larger role than bacteria in N2O emission in
lawn soil. This result supported that soil N2O was
produced primarily by fungi and emphasized the importance of fungi on
N2O emissions in the lawn soil. This result was also
similar to results found by other studies. For instance, it has been
found that fungi are the major contributors to N2O
emission of soil in grazing grasslands in Tibet[17,41], grazing grasslands in New Zealand[35], tea in southern China[36], and croplands in China[42]. Meanwhile, our result was consistent with
results from the traditional agricultural ecosystem, crop-livestock
integrated ecosystem, organic agricultural ecosystem, and the artificial
forest ecosystem in which fungi contributed 40-51% to
N2O emission [17,42]. Therefore,
it is of great significance to explore the microbial mechanism of
N2O production by fungi for reducing N2O
emission from lawn soil.
In addition, fungi are better adapted to the environment than bacteria.
N fertilizer significantly increased the NO3–N
concentration and decreased pH in lawn soil (Table. S1). Bacteria
preferred ammonia oxidation at high NH3+-N
concentrations [21]. This also proved that fungi
contributed more to the N2O emissions in the lawn soil.
In our lawn soil, the NH4+- N was low, while the
NO3–N concentration was high, indicating that the
lawn soil environment was more suitable for the growth of fungal
communities.
Our results showed that the N2O emissions of the N300
treatment which represented the highest level of N fertilizer addition,
did not produce the highest emission levels, and they were instead
significantly lower than that of the N225 treatment (Fig. 2b). The
accumulation of N2O began to decline after the highest
N225 treatment, indicating that high N fertilizer application could
effectively promote N2O emission, but it was not the
case that the higher the nitrogen application, the higher the
N2O emission. We speculated that it might be related to
soil microorganisms with N transformation function. Thus, increasing the
number of microorganisms involved should result in increased production
of N2O emissions. Further analysis revealed that the
dominant fungi in the lawn soil accounted for the top three fungal
communities, namely Ascomycota, Basidiomycota, and Mucoromycota.
Nitrogen fertilizer significantly increased the relative abundance of
Ascomycota, while it significantly decreased the relative abundance of
Basidiomycota (Fig. 4a). We found a positive correlation between
N2O emission and Ascomycota through RDA analysis (Fig.
6a). Moreover, the growing trend of Ascomycota during the four nitrogen
fertilizer treatments was consistent with the N2O
emission trend in lawn soil. N2O emissions reached their
highest levels in the N225 treatment, rather than the N300 treatment. We
hypothesized that this might be related to the microbial biomass and
nitrogen transformation in lawn soil. We found that the relative
abundance of Ascomycota in the N225 treatment (36.6%) was higher than
that of the N300 treatment (35.1%). This result not only explained why
N2O emission in the N225 treatment was higher than that
in the N300 treatment, but also indicated that Ascomycota played an
important role in the N2O emissions in lawn soil. It has
been reported that among fungi, Ascomycota and Basidiomycota preferred
to use soil nitrate for denitrification and released N2O[32]. 90% percent of the fungi reported to
produce N2O belong to the phylum Ascomycota, followed by
fungi in the Basidiomycota and Mucoromycota which account for 7% and
3%, respectively. Representative N2O-producing
Ascomycota [33,34]. Ascomycota preferred to grow
with nitrogen than Basidiomycota. Meanwhile, high nitrogen could inhibit
N2O reductase activity [35]. These
results supported our conjecture.
At the fungal genus level, the relative abundance ofPyrenochaetopsis , Myrothecium , Zopfiella , andHumicola increased significantly after nitrogen fertilizer
treatment, while the relative abundance of Chaetomium ,Simplicillium , Cryptococcus , Mortierella andPhoma significantly decreased (Fig. 4b). Myrothecium ,Zopfiella , Pyrenochaetopsis , Humicola of Ascomycota
and Bullera and Conocybe of Basidiomycota were positively
correlated with N2O emission (Fig. 6b). Regression
analysis showed Pyrenochaetopsis , Myrothecium , andHumicola of Ascomycota were positively correlated with
N2O emission, while Bullera and Conocybeof Basidiomycota were not significantly correlated with
N2O emission. These results also indicated that
Ascomycota might be the key microbial population driving nitrogen
transformation in lawn soil. Moreover, we found that Myrotheciumwas the fungal genus with the highest correlation coefficient for
N2O emission in lawn soil through correlation analysis
(Fig. 7). According to the results of previous studies,Myrothecium had a strong ability to produce N2O.
The N2O production capacity of Myrothecium was
21.4 nmol N2O mL-1d-1, and the efficiency was far higher than that ofPyrenochaetopsis and Humicola , respectively 3.5 and 5.1
nmol N2O mL-1d-1[34]. Meanwhile, the relative abundance ofMyrothecium in Ascomycota was the highest in the N225 treatment
(1.88%) and significantly higher than that in the other three
treatments (0.37%–0.82%). We found that the adaptability ofMyrothecium was stronger under the condition of nitrogen
addition, but the condition of high nitrogen decreased, which was
consistent with the previous research results[32,34]. We concluded that Myrotheciumplayed an important role in the increased N2O emission
in lawn soil. Therefore, it is of strategic significance to study the
mechanisms related to Myrothecium and resultant
N2O emissions in order to reduce increases in
N2O emissions from the urban lawn soil in future.
During the incubation period, we found that N fertilizer increased
N2O flux in the lawn soil (Fig. 2a). Further, the
N2O flux of the N225 and N300 treatment showed the
highest emission peaks on the 5th day, while N0 and N150 treatments did
not show emission peaks, indicating that high nitrogen fertilizer input
could significantly stimulate the N2O emission of lawn
soil, which was consistent with the results of previous studies[36-37]. This indicated that N fertilizer
increased the N2O emissions in lawn soil, and also
strengthened the soil nitrogen stimulation effect. Meanwhile, some
laboratory experiments showed that N2O could rapidly
reach the emission peak within a short period after N fertilizer
addition, and the cumulative emissions accounted for more than half of
the total emissions and then rapidly declined[8,38]. Mate analysis showed that the proportion
of nitrogen addition was linearly correlated to N2O
emissions [39]. However, the low N fertilizer
application (Urea) did not significantly enhance N2O
emission, which was because the fact that the lawn was irrigated after
low urea application, leading to urea hydrolysis into inorganic nitrogen
which was directly used by plants [7,40].
5. Conclusions
Nitrogen fertilizer significantly promoted N2O emissions
in lawn soil, although this result was not linearly related to the
amount of fertilizer applied. When the amount of fertilizer applied was
225 kg·ha·yr-1, N2O emission was the highest, but it
decreased when the amount of fertilizer applied increased to 300
kg·ha·yr-1. N fertilizer significantly altered the soil microbial
community structure. Through biological inhibitor treatment, we found
that fungi were the main contributors to N2O emission in
lawn soil, accounting for 45% of the total N2O
emissions. Pyrenochaetopsis , Myrothecium , andHumicola of Ascomycota were significantly positively correlated
with N2O emission and were the predominant contributors
to N2O emissions within lawn soil. These findings will
help to draw up appropriate measures for mitigation of
N2O emissions in lawn soil.
Funding information This research was supported by the
Liaoning
Province Grassland Plant Resources Special Investigation Fund
(1103-01044415001)
in China.
Author Contributions:Z.X.
and L.B. conceived and designed the experiments; Z.X. and W.Y. performed
the experiments; Z.X., J.L. and B.R. analyzed the data; Z.X. wrote the
paper.
Acknowledgements: We gratefully acknowledge Dr. Feng Zhou
(Shenyang Institute of Applied Ecology, Chinese Academy of Sciences) for
providing valuable comments and suggestions. We acknowledge Bowen Duan
and Yifan Lin (Shenyang Agricultural University) for their assistance
during our experiments. We gratefully acknowledge the time and expertise
devoted to reviewing this manuscript by the reviewers and the members of
the editorial board.
Conflicts of interest: The authors declare no conflict of
interest.
Data Availability Statement :
- Relative abundances of the main fungal phyla and genus in the lawn
soil of all treatments, and soil properties at 0 - 20 cm soil depth
sampling: Dryad Doi https://doi.org/10.5061/dryad.nvx0k6dnv.
-All data that support the findings of this study are available from the
corresponding author upon reasonable request.
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