Reduced Lignin Decomposition and Enhanced Soil Organic Carbon Stability by Acid Rain: Evidence from 13C Isotope and 13C NMR Analyses

Jianping Wu,Qi Deng,Hongou Zhang,Minghui Hu,Huiling Zhang,Xuan Wang,Yongxian Su,Dafeng Hui,Mengdi Zhao,Deqiang Zhang,Xin Xiong,Guowei Chu

doi:10.3390/f11111191

Abstract

Due to the emissions of air pollutants, acid rain in southern China poses a great threat to terrestrial ecosystems. However, its influences on ecological processes such as litter decomposition and soil organic carbon (SOC) accumulation are still not clear. The aim of this study was to understand the potential mechanisms of carbon sequestration change in response to long-term acid rain in a subtropical forest. A field experiment with simulated acid rain (SAR) treatment was conducted in a monsoon evergreen broadleaf forest in southern China. Four levels of SAR treatment were implemented by irrigating the plots with water of different pH values (4.5 as a control, 4.0, 3.5, and 3.0). The results showed that the concentration of SOC and recalcitrant index for the SAR pH = 3.0 treatment were significantly higher compared to the control. Lignin fractions in litter residue layers were significantly increased, while soil microbial biomass carbon and soil ligninolytic enzyme activities were reduced under the SAR treatment. The concentration of SOC and recalcitrant index had positive relationships with the litter residue lignin fraction, but negative relationships with soil ligninolytic enzyme activity. These findings indicate that soil carbon accumulation could be enhanced with more stable lignin input under prolonged acid rain in forest ecosystems in southern China.

Highlights

With the increase in anthropogenic activity in recent decades, such as coal combustion, industrial air pollution, and automobile exhaust gases emissions, China has suffered from severe acid deposition, especially in the southern regions [1,2]
Two lines of evidence showed that the lignin fraction increased significantly as decomposition Two lines of evidence showed that the lignin fraction increased significantly as decomposition proceeded (Table 1). (1) The decrease in δ13 C of the litter residue proceeded (Table 1). (1) The decrease in δ13C of the litter residue was greater in the lower layer (H) where the concentration of the lignin fraction was higher than in was greater in the lower layer (H) where the concentration of the lignin fraction was higher than in the upper layers (L and F)
After more than seven years of the simulated acid rain (SAR) treatment in a subtropical forest in southern China, we found a significant increase in the soil organic carbon concentration and recalcitrant index under heavy SAR treatment

Summary

Introduction

With the increase in anthropogenic activity in recent decades, such as coal combustion, industrial air pollution, and automobile exhaust gases emissions, China has suffered from severe acid deposition, especially in the southern regions [1,2]. The prevalence of acid rain has increased public concern about the widespread impacts on terrestrial ecosystems in southern China [5]. Understanding the soil organic matter (SOM) dynamics in forest ecosystems is important because soils are the largest pool of carbon (C) on Earth [6,7]. As a key ecological process responsible for the transfer of organic matter from vegetation to soil, plant litter decomposition plays a critical role in global C cycling [8,9,10]. Plant organic matter fixed by photosynthesis is eventually deposited in soils, decomposed and utilized by soil organisms, partially transformed into humic substances, and contributed to the SOM [11]. On the basis of previous studies, prolonged acid rain would change the quality and quantity of SOM into the soil, inhibits the decomposition of SOM, and alter the sequestration of soil C [12,13,14,15]

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