Effects of Simulated Acid Rain on Soil Enzyme Activity and Related Chemical Indexes in Woodlands

Effects of Simulated Acid Rain on Photosynthesis in Pinus massoniana and Cunninghamia lanceolata in Terms of Prompt Fluorescence, Delayed Fluorescence, and Modulated Reflection at 820 nm

The effects of simulated acid rain (SAR) on the photosynthetic performance of subtropical coniferous species have not been thoroughly investigated. In this study, we treated two coniferous species, Pinus massoniana (PM) and Cunninghamia lanceolata (CL), with four gradients of SAR and then analyzed their photosynthetic activities through measurements of gas exchange, prompt fluorescence (PF), delayed fluorescence (DF), and modulated reflection at 820 nm (MR820). Gas exchange analysis indicated that the decrease in the net photosynthetic rate (Pn) in PM and CL was unrelated to stomatal factors. For the PF transients, SAR induced positive K-band and L-band, a significant reduction in photosynthetic performance index (PIABS), the quantum yield of electron transfer per unit cross-section (ETO/CSm), and maximal photochemical efficiency of photosystem II (Fv/Fm). Analysis of the MR820 kinetics showed that the re-reduction kinetics of PSI reaction center (P700+) and plastocyanin (PC+) became slower and occurred at later times under SAR treatment. For the DF signals, a decrease in the amplitude of the DF induction curve reduced the maximum value of DF (I1). These results suggested that SAR obstructed photosystem II (PSII) donor-side and acceptor-side electron transfer capacity, impaired the connectivity between PSII and PSI, and destroyed the oxygen-evolving complex (OEC). However, PM was better able to withstand SAR stress than CL, likely because of the activation of a protective mechanism.

Effects of nitric acid rain stress on soil nitrogen fractions and fungal communities in a northern subtropical forest, China

Acid rain has severely negatively impacted terrestrial ecosystems and biogeochemical cycles. However, the potential impacts of nitric acid rain (NAR) on soil nitrogen (N) fractions and fungal community diversity in northern subtropical forest soils remain largely unevaluated. In this study, treatments of NAR at pH = 4.5 (AR4.5), pH = 3.5 (AR3.5), and pH = 2.5 (AR2.5) were randomly sprayed in a typical Quercus acutissima Carruth. stand in northern subtropical China. The soil N fractions and soil fungal communities were analyzed after a 12-month experimental period. The results revealed that compared to the control, the soil total N (TN), microbial biomass N (MBN), hydrolysable ammonium N (HAN), amino-sugar N (ASN) and amino-acid N (AAN) contents decreased significantly by 19.61-13.07 %, 20.10-9.04 %, 60.41-28.87 %, 74.10-62.25 %, and 65.69-45.64 % under stronger acidity inputs (i.e., AR2.5 and AR3.5), respectively. Besides, the AR2.5 and AR3.5 treatments increased the α-diversity indices of soil fungal communities and altered the soil fungal community structure. Moreover, the NAR treatments represented an increase in the relative abundance of Ascomycota and Mortierellomycota and a decrease in that of Basidiomycota. Mortierella, Penicillium, and Tomentella can be used as indicator genera for changes in soil fungal community structures under NAR stress. Furthermore, AAN was the main environmental factor affecting soil fungal community at the phylum and genus levels. Cumulatively, findings from this research provide valuable insight into NAR's effects on N cycling and microbial communities in forest soils.

Effects of simulated acid rain on rhizosphere microorganisms of invasive Alternanthera philoxeroides and native Alternanthera sessilis

Acid rain not only has serious harm to the environment, but also has the same threat to plants, but the invasive plant Alternanthera philoxeroides still grows well compared to the native plant Alternanthera sessilis under acid rain stress. However, the underlying mechanism of resistance to the acid rain environment in invasive Alternanthera philoxeroides remains unclear. In the current study, we comparatively analyzed the plant physiological characteristics, soil physicochemical properties, and rhizosphere microbial communities of invasive A. philoxeroides and native A. sessilis under different pH condition. The simulated acid rain had a significant inhibitory effect on the morphological and physiological traits of A. philoxeroides and A. sessilis and reduced the soil nutrient content. However, A. philoxeroides was more tolerant of acid rain. Compared with CK, simulated acid rain treatment at pH 2.5 significantly increased the Chao1, ACE, and Shannon indexes of A. philoxeroides microorganisms. Under simulated acid rain treatment at pH 2.5, the fungal flora Chao1, ACE and Shannon index were significantly higher than those of CK by 14.5%, 12.4%, and 30.4%, respectively. The dominant bacterial phyla of soil bacteria were Proteobacteria, Actinobacteria, Bacteroidota, Actinobacteria, Firmicutes, Myxococcota, Chloroflexi, Patescibacteria, Gemmatimonadota, Verrucomicrobiota, and Armatimonadota. The dominant fungi were Ascomycota, Basidiomycota, Rozellomycota, and Olpidiomycota. The bacterial and fungal diversity and structure of A. philoxeroides and A. sessilis showed the greatest difference between the pH 2.5 treatment and CK. Redundancy analysis showed that electrical conductivity (EC) and total phosphorus (TP) were the main factors changing the bacterial communities, and available phosphorus (AP), organic matter (OM), EC, and pH were the main factors changing the fungal communities. This study contributes to the microbial community structure of the invasive plant A. philoxeroides and provides a theoretical basis for studying the invasion mechanism of invasive plants under acid rain.