Environmental Research Letters LETTER • OPEN ACCESS Global soil acidification impacts on belowground processes To cite this article: Cheng Meng et al 2019 Environ. Res. Lett. 14 074003 View the article online for updates and enhancements. This content was downloaded from IP address 182.130.23.18 on 01/07/2019 at 14:07 Environ. Res. Lett. 14 (2019) 074003 https://doi.org/10.1088/1748-9326/ab239c LETTER Global soil acidification impacts on belowground processes OPEN ACCESS Cheng Meng1,2,5 , Dashuan Tian1,5, Hui Zeng2, Zhaolei Li1, Chuixiang Yi3 and Shuli Niu1,4 RECEIVED 1 Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, 2 January 2019 Chinese Academy of Sciences, Beijing 100101, People’s Republic of China REVISED 2 Shenzhen Graduate School, Peking University, Shenzhen 518055, People’s Republic of China 18 May 2019 3 School of Earth and Environmental Sciences, Queens College of the City University of New York, NY 11367, United States of America ACCEPTED FOR PUBLICATION 4 Department of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China 22 May 2019 5 These authors contribute equally. PUBLISHED 1 July 2019 E-mail: [email protected] Keywords: acid deposition, soil cations, meta-analysis, soil pH, soil respiration, microbes Original content from this Supplementary material for this article is available online work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of Abstract this work must maintain With continuous nitrogen (N) enrichment and sulfur (S) deposition, soil acidification has accelerated attribution to the author(s) and the title of and become a global environmental issue. However, a full understanding of the general pattern of the work, journal citation fi and DOI. ecosystem belowground processes in response to soil acidi cation due to the impacting factors remains elusive. We conducted a meta-analysis of soil acidification impacts on belowground functions using 304 observations from 49 independent studies, mainly including soil cations, soil nutrient, respiration, root and microbial biomass. Our results show that acid addition significantly reduced soil pH by 0.24 on average, with less pH decrease in forest than non-forest ecosystems. The response ratio of soil pH was positively correlated with site precipitation and temperature, but negatively with initial + + + + + soil pH. Soil base cations (Ca2 ,Mg2 ,Na ) decreased while non-base cations (Al3 ,Fe3 ) increased with soil acidification. Soil respiration, fine root biomass, microbial biomass carbon and nitrogen were significantly reduced by 14.7%, 19.1%, 9.6% and 12.1%, respectively, under acid addition. These indicate that soil carbon processes are sensitive to soil acidification. Overall, our meta-analysis suggests a strong negative impact of soil acidification on belowground functions, with the potential to suppress soil carbon emission. It also arouses our attention to the toxic effects of soil ions on terrestrial ecosystems. Introduction cycles and impairs ecosystem function (Stevens et al 2010,Lianget al 2018). Therefore, understanding the Since the mid 20th century, acid rain has become a general patterns of ecosystem processes with acid serious global environmental problem due to rapid deposition across diverse environments will provide industrial development (Blank 1985,Duanet al 2016). valuable knowledge for predicting future ecosystem The main sources of acid and acidifying pollutants are dynamics under global change. To date, however, there sulfur dioxide (SO2), nitrogen oxides (NOx) and has been no systematic global synthesis of acid deposi- ammonia (NH3) emitted from fossil fuel combustion tion impact on ecosystem functions. and agricultural activities (Zhao et al 2009,Yanget al Acid deposition has complex effects on ecosystems, + 2012).ThoughSO2 and NOx emissions have been especially for belowground processes. First, more H 2- - reduced in Europe and North America, they are input to soil along with SO4 and NO3 induced by acid increasing in many developing countries due to coal deposition may directly affect microbial activities + combustion (Gao et al 2018).Soilacidification is a (Kuperman and Edwards 1997). Second, H from acid + natural process, which has been accelerated by increases deposition will compete with base cations (e.g. K , + + in N and S deposition associated with human activities Mg2 ,Ca2 ) for replacement, which increase base (Grieve 2001,Kunhikrishnanet al 2016).Human cation leaching out of soil. This further reduces soil acid accelerated soil acidification alters biogeochemical buffering capacity (Driscoll et al 2003) and nutrient © 2019 The Author(s). Published by IOP Publishing Ltd Environ. Res. Lett. 14 (2019) 074003 availability (Likens et al 1996). Third, increased soil acid deposition and ecosystem function. The searched acidification with continuous acid deposition has the keywords were: (acid deposition or S deposition or + + + potential to mobilize and release free Al3 and Fe3 to simulated acid rain) AND (soil cations (e.g. Na , + + soil solution. This accumulation of toxic elements in Mg2 ,Ca2 ), soil nutrient (e.g. SOC, STN), soil topsoil may eventually impair root growth and micro- respiration, fine root biomass, microbial biomass). bial activity (Godbold et al 1988,Kochian1995, The following criteria were employed to screen appro- Poschenrieder et al 2008), which will consequently priate studies for analysis: (1) only acid addition reduces soil respiration. experiments in the field were included, with the The rate and form of acid deposition, soil type, treatment duration lasting at least one growing season; environmental factor, and ecosystem type all may reg- (2) The control and acid addition treatments had ulate the effects of acid deposition on soil processes. to experience the same climate and soil condition; Normally, acid deposition rate should be a major fac- (3) examined variables were required to be clearly tor to drive soil acidification (Vanhala et al 1996). Dif- described by their means, sample sizes and standard ferent acid forms, e.g. H2SO4 and HNO3, may also deviation. contribute to the variable impacts of acid deposition To acquire as many observations as possible, we - due to their different adsorption mechanism. NO3 is gathered the data at each peak biomass stage of the adsorbed only through electrostatic attraction, while growing season during all measurement years. If sev- 2- SO4 can be specially adsorbed through ligand eral studies with different vegetation types or environ- exchange, especially in variable charge soils (Curtin mental conditions (i.e. annual temperature or and Syers 1990, Guadalix and Pardo 1991). This spe- precipitation) were reported in an article, each study cial adsorption may lead to a release of hydroxyl ions, was considered to be independent. Table-form data which could neutralize a part of the acids and retard were directly extracted, while graph-form data were soil acidification to some extent. Furthermore, soil obtained by the Engauge Digitizer software (Free Soft- type is a significant contributor to regulating soil ware Foundation, Inc., Boston, MA, USA). If climate acidification response. It is expected that soils with variables could not be obtained from the papers, we different initial pH may go through different acidifica- used the latitude and longitude of each study to extract tion buffering phases (Bowman et al 2008). Soil with these data from a global database (http://worldclim. a lower pH generally experiences greater acid- org/). Soil type data were acquired from the FAO data- weathering, which makes less sensitive to external acid base (http://fao.org/). Finally, a global dataset was input (De Vries et al 1989, Zhu 2017). High precipita- established with 49 independent studies from 45 tion accelerates the leaching of soil cations and further papers (figure S1 is available online at stacks.iop.org/ aggravates acidification (Lapenis et al 2004, Ling et al ERL/14/074003/mmedia). This dataset covered the 2007). Low temperature possibly depresses litter area with latitude range from 23.15 to 69.75° N and decomposition (Oulehle et al 2011, Liang et al 2013), elevation range from 10 to 1200 m. Mean annual leading to litter accumulation and then weakening soil temperature varied from −2 °C to 21.4 °C, and pre- acidification magnitude (Aerts 1997). The influence of cipitation from 130 to 2400 mm. Ecosystems included fi these abiotic and biotic factors in combination nally forest, grassland and peatland, but we sorted them causes different ecosystem response to acid deposi- into two groups for analysis (forest and non-forest). fl tion. It is a challenge but essential to quantify the in u- This is due to the lack of data from grassland and peat- fi ences of those factors on the soil acidi cation impacts land ecosystems. Experimental duration spanned 1 to across different experiments with different application 14 years. rates and acid agents to soils with varying buffering In our dataset, most data were related to below- capacities. ground processes. Response variables included soil ( + + + + + + + Here, we compiled a global dataset 304 observa- cations (K ,Na ,Mg2 ,Ca2 ,Zn2 ,Mn2 ,Al3 , ) + tions from 49 case studies and performed a meta- Fe3 ), soil nutrient (SOC-soil organic C, DOC- analysis to quantify belowground process dynamics in dissolved organic C, STN-soil total N, soil NH , soil fi 4 response to experimental