Abstract
BackgroundChina has been increasingly subject to significant acid rain, which has negative impacts on forest ecosystems. Recently, the concentrations of NO3− in acid rain have increased in conjunction with the rapid rise of nitrogen deposition, which makes it difficult to precisely quantify the impacts of acid rain on forest ecosystems.MethodsFor this study, mesocosm experiments employed a random block design, comprised of ten treatments involving 120 discrete plots (0.6 m × 2.0 m). The decomposition of fine roots and dynamics of nutrient loss were evaluated under the stress of three acid rain analogues (e.g., sulfuric (SO42−/NO3− 5:1), nitric (1:5), and mixed (1:1)). Furthermore, the influences of soil properties (e.g., soil pH, soil total carbon, nitrogen, C/N ratio, available phosphorus, available potassium, and enzyme activity) on the decomposition of fine roots were analyzed.ResultsThe soil pH and decomposition rate of fine root litter decreased when exposed to simulated acid rain with lower pH levels and higher NO3− concentrations. The activities of soil enzymes were significantly reduced when subjected to acid rain with higher acidity. The activities of soil urease were more sensitive to the effects of the SO42−/NO3− (S/N) ratio of acid rain than other soil enzyme activities over four decomposition time periods. Furthermore, the acid rain pH significantly influenced the total carbon (TC) of fine roots during decomposition. However, the S/N ratio of acid rain had significant impacts on the total nitrogen (TN). In addition, the pH and S/N ratio of the acid rain had greater impacts on the metal elements (K, Ca, and Al) of fine roots than did TC, TN, and total phosphorus. Structural equation modeling results revealed that the acid rain pH had a stronger indirect impact (0.757) on the decomposition rate of fine roots (via altered soil pH and enzyme activities) than direct effects. However, the indirect effects of the acid rain S/N ratio (0.265) on the fine root decomposition rate through changes in soil urease activities and the content of litter elements were lower than the pH of acid rain.ConclusionsOur results suggested that the acid rain S/N ratio exacerbates the inhibitory effects of acid rain pH on the decomposition of fine root litter.
Highlights
Acid rain has emerged as one of the most important global scale environmental challenges (Li et al 2019), and China has become the third most seriously affected regions worldwide to its effects (Liang et al 2016)
No significant differences in the soil pH were observed between the different acid rain S O42−/ NO3− (S/N) ratios (p > 0.05)
Soil enzyme activities The variable trends in soil enzyme activities were similar to those of the soil pH over the four decomposition periods (Fig. 2), where significant differences were observed between decomposition periods (Additional file 1: Table S2, p < 0.001)
Summary
Acid rain has emerged as one of the most important global scale environmental challenges (Li et al 2019), and China has become the third most seriously affected regions worldwide to its effects (Liang et al 2016). Acid rain increases risks to the integrity of forest ecosystem, in terms of its changing compositions (Liu et al 2018). Previous studies revealed that the SO42−/NO3− ratio of acid rain is a significant factor that profoundly influences litter decomposition processes (Lv et al 2014; Liu et al 2017a). These earlier investigations set their primary focus on the decomposition of litters on the ground surface (Wang et al 2010; Tang et al 2019) while essentially ignoring the dynamics of fine root decomposition. The concentrations of NO3− in acid rain have increased in conjunction with the rapid rise of nitrogen deposition, which makes it difficult to precisely quantify the impacts of acid rain on forest ecosystems
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