Effects Of Simulated Acid Rain On Greenhouse Gas Emissions And Microbial Communities In Agricultural And Forest Soils

Acid rain,as a global environmental issue,directly or indirectly affect the carbon（C）and nitrogen（N）cycle of ecosystems.Microorganisms play an important role in regulating the global C and N balance and global changes.However,very few studies have assessed the impact of acid rain on the microbiota involved in N cycle,and it is still unclear whether and how the acid rain influences the structure and function of microbial communities.Herein,we conducted intact soil cores experiments to investigate the effects of acid rain with different acidity and frequency on greenhouse gas emissions in agricultural and forest soils.The mechanisms on the responses of soil greenhouse gas emissions to acid rain was explored from the changes in soil physicochemical properties and microbial community structure and function.The greenhouse gases were collected twice a month from October 2017 to April2018（total 36 times in 6 months）.At the end of the experiment,we measured the physicochemical properties,microbial communities biomass and structure,and microbial carbon source metabolism of the topsoil（0-10 cm）of agricultural and forest soils.1.We investigated the effects of acid rains of pH 5.0,4.0 and 3.0 on soil CO₂,CH₄,and N₂O fluxes,and microbial communities in an agricultural soil.We did not detect any effect of acid rain on CO₂ and N₂O fluxes as compared to the control;however,acid rain of p H 3.0significantly reduced the cumulative CH₄ flux from the soil.Most noticeably,both acid rains of p H 4.0 and p H 3.0 significantly increased the total amount of soil microbial phospholipid fatty acids（PLFAs）by increasing the PLFA contents of gram-positive bacteria,actinomycetes,fungi,and arbuscular mycorrhizal fungi,though all the acid rain treatments did not change the relative abundance of microbial groups.In addition,both CO₂ and CH₄fluxes negatively correlated with the total amount of soil microbial PLFAs;however,the N₂O flux positively correlated to soil NO₃^--N contents（p<0.05）.Moreover,amplicon sequencing of N-cycle functional genes（amo A,nir S and nos Z）in soil treated with acid rain of p H 4.0 was performed to explore the underlying mechanism on the response of N₂O emissions to acid rain from the changes in the structure and network of ammonia-oxidizing archaea（AOA）,ammonia-oxidizing bacteria（AOB）,nir S-and nos Z-harboring denitrifiers.Acid rain of p H 4.0（AR）did not alter the community structure of AOA,AOB,nir S-and nos Z-harboring denitrifiers.AOB community structure was significantly correlated with soil available P,while the community structure of nir S-and nos Z-harboring denitrifiers were both correlated with soil p H and available P.Soil N₂O emissions was mainly driven by the changes in microbial abundance of nir S-harboring denitrifiers.The networks of AOA and nir S-harboring denitrifiers in AR treatment were less complex with fewer nodes and lower connectivity compared to CK,while the network of nos Z-harboring denitrifiers in AR treatment had higher complexity and connectivity relative to CK.Supporting this,the abundance of keystone taxa under acid rain was lower in the network of AOA and nir S-harboring denitrifiers,but was higher in the network of nos Z-harboring denitrifiers.2.We tested the effect of acid rain frequency（0,30%and 100%）on soil CO₂ emission,microbial communities,and metabolism of various carbon sources in the agricultural and forest soils.We found idiosyncratic responses of soil microbial communities and carbon utilization to the changes in frequency of acid rain in two different soil ecosystems.The high-frequency acid rain（100%）substantially reduced soil microbial biomass,particularly those of gram-negative bacteria and arbuscular mycorrhizal fungi in forest soil,while this effect was not observed in the agricultural soil.The acid rain frequencies（30%and 100%）significantly reduced the utilization of amines/amides in agricultural soil,while they did not alter the carbon sources utilization in forest soil.However,surprisingly,the acid rain frequency did not affect the soil CO₂ emission in agricultural and forest soils.In addition,the cumulative CO₂ emission was negatively correlated with total,bacterial,gram-positive bacterial,actinomycetal and arbuscular mycorrhizal fungal PLFAs,whereas it was positively correlated with the microbial utilization of polymers,carbohydrates,carboxylic acids,and amino acids.Overall,during the experimental period,high-intensity acid rain suppressed the CH₄emissions while changes in acidity of acid rain did not affect CO₂ and N₂O emissions in an agricultural soil.Increasing the acidity of acid rain promoted the growth of gram-positive bacteria,actinomycete,fungi,and arbuscular mycorrhizal fungi,but did not change the relative proportions of microbial groups.Similarly,changes in frequency of acid rain did not affect soil microbial CO₂ emission in agricultural and forest soils.High-frequency acid rain inhibited the growth of gram-negative bacteria and arbuscular mycorrhizal fungi,while this effect was not observed in the agricultural soil.Increasing the frequency of acid rain reduced the microbial utilization of amines/amides in agricultural soil,while it did not alter microbial utilization of carbon sources in forest soil.Due to the complexity of the greenhouse gas emission process,there are still many uncertainties in its underlying response mechanism.Therefore,investigating the characteristics of long-term changes in greenhouse gas emissions under acid rain pollution,and clarifying its potential response mechanism,could provide a theoretical basis for predicting the dynamic changes of the C and N cycle of ecosystems under global changes,and can also be used to formulate reasonable acid rain control measures.