The Impact of Water-Soluble Inorganic Ions in Particulate Matter (PM_2.5) on Litter Decomposition in Chinese Subtropical Forests

• Main Authors: Yanli Ji, Qiang Li, Rumeng Ye, Kai Tian, Xingjun Tian
• Format: Article
• Language: English
• Published: MDPI AG 2020-02-01
• Series: Forests
• Subjects: leaf litter decomposition; ecotoxicity; wsii-pm_2.5; extracellular enzyme activity; microbial biomass; isopods; carbon flux
• Online Access: https://www.mdpi.com/1999-4907/11/2/238

Although numerous studies have demonstrated the toxic effects of fine particulates less than 2.5 &#181;m (PM_2.5) on the health of humans, little information is available on the ecotoxicity of PM_2.5. Water-soluble inorganic ions (WSII, including Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, Cl^&#8722;, NO₃^&#8722;, and SO₄^2&#8722;) can compose more than 60% of PM_2.5. To better understand the possible impacts of WSII-PM_2.5 on leaf litter decomposition, we conducted an experiment in which two leaf litters from oak (<i>Quercus variabilis</i>) and pine (<i>Pinus massoniana</i>) dominant forests in subtropical China were incubated in microcosms containing their respective forest soils and treated with WSII-PM_2.5. Our results showed that, after six-months of decomposition, the WSII-PM_2.5 treatments inhibited leaf litter decomposition rates, carbon and nitrogen loss, microbial biomass, and enzyme activities in the two forests. In addition, higher WSII-PM_2.5 concentration led to stronger negative effects. Comparative analysis showed that the negative effects of WSII-PM_2.5 on oak forest were greater than on pine forest, relating to the higher susceptibility to changes of soil microenvironment in oak forests. WSII-PM_2.5 may influence decomposition through soil acidification and salinization, which could also cause a sub-lethal depression in soil isopod activity. However, in the first month of decomposition, mass loss of the oak and pine leaf litters under the low concentration WSII-PM_2.5 were 21.63% and 35.64% higher than that under the control, respectively. This suggests that transitory low concentrations of WSII-PM_2.5 have a promoting effect on decomposition. Long-term PM_2.5 exposure, therefore, may have profound ecosystem consequences by altering the balance of ecosystem carbon flux, nutrient cycling, and humus formation in the future.