Nitric acid rain (NAR) seriously affects the biogeochemical cycles of forest communities’ ecosystems. However, the effects of NAR on the composition and diversity of the soil bacterial community remain unclear. In this study, a typical subtropical forest of Quercus acutissima was selected and simulated spraying of NAR at pH 2.5 (AR2.5), 3.5 (AR3.5), and 4.5 (AR4.5) was implemented to investigate the response of the forest soil bacterial communities to NAR. The results showed that the total number of OTUs of soil bacteria in AR2.5 and AR3.5 treatments was 1.11 and 1.23 times that in the control treatment without NAR (CK), respectively. Acidobacteria, Proteobacteria, and Actinobacteria were the dominant phyla in the subtropical forest, accounting for more than 80% of the community’s relative abundance. Concurrently, simulated NAR changed the relative abundance of Rhodanobacter significantly, which could be an indicator of soil bacterial community structure under NAR stress. Moreover, the Chao1, Shannon, and Simpson indices of strong acid rain treatments (i.e., AR2.5 and AR3.5) increased by 9.55%–22.5%, 3.6%–7.43%, and 0.15%–0.26%, respectively, compared to CK. Redundancy and correlation analysis illustrated that the phylum level structure of the bacterial community was significantly affected by soil total carbon, total nitrogen, and ammonium nitrogen. Our findings contribute to a better understanding of the effects of NAR on soil microbial communities and potential soil element cycling in north subtropical forests.
With the comprehensive emissions of fossil fuel combustion and transportation waste gas, the concentrations of nitrogen oxides (NOX) in the environmental atmosphere increase significantly, leading to nitric acid rain (NAR) pollution. However, the effects of NAR on soil enzyme activities and soil microbial metabolism are unclear. In this study, the Quercus acutissima Carruth. forest in the Yangtze River Delta of China was selected as the experimental subject, and was exposed to the simulated spraying of NAR with pH values of 2.5, 3.5, and 4.5 to study the response of the forest soil enzyme activities and soil microbial metabolism to NAR. The results showed that compared to the non-NAR treatment, the activities of β-1,4-glucosidase (BG), L-leucine aminopeptidase (LAP), and β-1,4-N-acetylglucosidase (NAG) decreased by 56.48%–42.24%, 44.57%–38.20%, and 56.13%–48.11% under the AR2.5 and AR3.5 treatments, respectively. Moreover, there was no significant change in the Vector Length (VL) under different gradients of NAR. The Vector Angle (VA) increased with the decrease of the pH value and reached the maximum value with the AR2.5 treatment, indicating that the strong acid type NAR had a greater phosphorus-limiting effect on the soil microorganisms. The RDA analysis results showed that the dissolved organic carbon (DOC) was a significant factor affecting the soil enzyme activity and stoichiometric ratio, with interpretation rates of 40.2%. In conclusion, we believe that in the restoration of acidified soil, attention should be paid to the regulation of soil pH, reducing scour.
It is of agronomic importance to apply nitrogen (N), but it has high environmental risks in reclaimed saline soils. Therefore, we should apply N fertilizer at an appropriate rate to increase crop yield but decrease N losses. In this soil column experiment, rice yield, N uptake, and ammonia (NH3) and nitrous oxide (N2O) losses were measured in four treatments with no N application (control) and with N applications of 160, 200, and 240 kg/ha (N160, N200, and N240, respectively). The results show that grain yield, spike number, and thousand-kernel weight increased with increases in N application rate, but there was no significant difference in grain yield between N200 and N240. However, the kernels per spike increased first and then decreased with the increase in N application, of which N200 was recorded to have the highest kernels per spike value, which was 16.8 and 9.8% higher than those of N160 and N240, respectively. Total NH3 volatilization of the rice season increased with increasing N input, especially during the first and second supplementary fertilization stages. The NH4+-N concentration of overlying water was relatively lower under the N200 treatment in these two stages, and the yield-scaled NH3 volatilization and the emission factor were the lowest in N200, which were 26.2–27.8% and 4.0–21.0% lower than those of N160 and N240, respectively. Among the three N-applied treatments, N2O losses and the emission factor as well as the yield-scaled N2O emissions were the lowest under the N200 treatment, which had 34.7% and 78.9% lower N2O emissions and 57.8% and 83.5% lower emission factors than those of the N160 and N240 treatments, respectively. Moreover, the gene copies of AOA and AOB amoA, nirS, and nirK in cultivated layer soils all reached the minimum under the N200 treatment. According to the comprehensive effects of N fertilizer on rice grain yield and NH3 and N2O losses, we recommend applying 200 kg/ha to reclaimed saline soil to ensure crop yield and reduce N losses.
In order to explore the influence of climate warming on soil microbial metabolism in the ecosystem and reveal the relationship between soil microbial metabolism limitation and environmental factors, in this study, the effects of warming on soil enzyme activities and nutrient availability were investigated by setting underground heating cables at 2 °C and 4 °C soil warming in a typical Quercus acutissima forest in the northern subtropics, and enzyme stoichiometric models were used to evaluate the limits of soil microbial metabolism. The results showed that soil warming significantly increased the activities of β-1,4-glucosidase (BG) and L-leucine aminopeptidase (LAP), and significantly increased the contents of nitrate nitrogen (NO3−-N) and available phosphorus (AP) in soil. The soil warming increased soil microbial C limitation and alleviated soil microbial P limitation. Our study showed that the change of soil microbial C and P limitation caused by warming may cause a large amount of SOM decomposition in a short period, leading to a large fluctuation of soil carbon turnover, which is not conducive to the stability of the soil C pool. This study provides important insights linking microbial metabolism to soil warming and improves our understanding of C cycling in forest systems.
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