Wild boar grubbing causes organic carbon loss from both top- and sub-soil in an oak forest in central China
Introduction
Soils store large quantities of organic carbon (C) and the CO2 released from the soil is one of the largest fluxes in the global C cycling (Xie et al., 2014, Wan et al., 2015, Crowther et al., 2016). Both soil C storage and soil respiration have been shown to be a function of many soil characteristics such as microclimate (Davidson and Janssens, 2006, Chen et al., 2017, Liu et al., 2019a), physico-chemical properties (Ding et al., 2016, Miao et al., 2019, Wang et al., 2019), and biological factors (Karhu et al., 2014, Prescott et al., 2017, Liu et al., 2019b). Therefore, any changes in one or several of those factors can elevate or reduce organic matter decomposition and soil respiration, thus have substantial impact on the terrestrial C cycling.
Wild boar (Sus scrofa L.), native to Eurasia, is now one of the globally most widely distributed invasive ungulates (Long, 2003, Barrios-Garcia and Ballari, 2012). In China, wild boars occur across all regions except the Tibetan Plateau and Gobi Desert. Population of wild boar has increased rapidly in recent decades, reaching approximately 2 million across China (National Forestry Administration, 2009), probably due to the following reasons. First, folk hunting was completely forbidden and hunting of wildlife has been banned since 1994 in China. Second, wild boar has been added to the State Terrestrial Wildlife Protection Directory since 2000, which regulates that hunting any wildlife with beneficial or important economic or scientific values to humans is illegal. In addition, lack of predator is another important reason for the continuous increase in wild boar population.
The impacts of wild boar as an invasive species are well known around the world, not only for its destruction on crop production in agricultural ecosystems (Slg et al., 2009, Frauendorf et al., 2016), but also for its extensive influences on aboveground plant growth and regeneration (Dovrat et al., 2014, Burrascano et al., 2015, Bongi et al., 2017), as well as belowground soil physico-chemical properties (e.g. soil organic C (SOC) and total nitrogen (TN)) (Mohr et al., 2005, Bueno et al., 2013) in forest ecosystems. Wild boar gains most of its food by rooting (grubbing) activity in the soil searching for plant seeds, roots, and invertebrate and vertebrate animals (Baber and Coblentz, 1987, Wirthner et al., 2011). Compared to the sub-soil, wild boar would prefer to overturn the top-soil due to the greater probability in the occurrence of plant seeds and soil micro-animals (Wang et al., 2019). Although wild boar rooting activity was believed to have substantial impact on soil C cycling (Mitchell et al., 2007), surprisingly little research has focused on how wild boar foraging affect soil CO2 emission and subsequent soil C storage (Moody and Jones, 2000, Mohr et al., 2005).
In central China, natural forest resources are mainly distributed in the Dabie Mountains and Funiu Mountains, both of which are mainly dominated by broadleaf tree species. Broadleaf forest, especially oak forest, has been reported more suitable for wild boars compared with coniferous forest because of the high production of acorn (Fernández-Llario, 2004, Torres-Porras et al., 2015). Therefore, wild boar number has increased rapidly due to its vigorous reproduction and the suitable environmental condition in past several years in central China. Physical disturbance of introduced wild boars on soils can alter soil biogeochemical properties, and thus further affect soil C storage.
The aim of this study is to determine the influences of wild boar grubbing on soil C cycle in deciduous forest in central China, by comparing soil respiration, SOC, and microbial activity within and outside areas disturbed by wild boars. The objectives of this study were to (1) estimate the effects of grubbing disturbance of wild boars on soil respiration and its temperature sensitivity, and (2) quantify the influences of boar disturbance on SOC and microbial biomass across 2-year investigation.
Section snippets
Study site
The study site is located within the Jigong Mountains National Nature Reserve (31°46′-31 °50′N, 114 °01′-114 °06′), Henan province, central China, within a transitional zone from subtropical climate to warm temperate climate (Hu and Wan, 2019, Xia et al., 2019). The altitude ranges from 110 m to 810 m a.s.l.. Long-term mean annual precipitation (1968–2018) is 1068 mm with 60% occurring from May to September. Mean annual temperature is 15.7 °C with monthly mean temperature ranging from 2.1 °C in
Soil temperature and moisture
The seasonal dynamics of soil temperature displayed a monotonic peak in the mid-growth season in both 2015 and 2016 (T: P < 0.01, Table 1; Fig. 2a, b). On average, the soil temperature was significantly higher by 0.72 °C in 2015 and 0.69 °C in 2016 in the grubbed plots than in the non-grubbed plots (G: P < 0.05, Table 1; Fig. 2a, b). Soil moisture varied with month in both 2015 and 2016 (T: P < 0.05, Table 1). However, soil moisture showed no differences between the grubbed and non-grubbed
Grubbing effect on soil respiration
Our study exhibited that grubbing disturbance had a strong positive effects on soil respiration as we found significantly greater emission rates on the grubbed plots compared with the non-grubbed plots across our 2-year experiment. In accordance with our results, Risch et al. (2010) found an average 23.1% more CO2 was released from grubbed plots during 2-year measurement in hardwood forests in Switzerland. However, in the Monte Desert, Cuevas et al. (2012) reported a negative effect of wild
Conclusions
This study documented that disturbance of wild boars in temperate oak forest influences a suite of variables related to soil C cycling, with an overall increase in soil respiration and soil organic C. Specifically, results showed that grubbing activity by boars continuously stimulated soil respiration across our two-year experiment period. However, SOC and TN were only enhanced in the first year after disturbance of wild boars probably due to the decreased soil substrate availability during the
CRediT authorship contribution statement
Yanchun Liu: Conceptualization, Investigation, Writing - original draft, Funding acquisition. Xiaojing Liu: Methodology, Software. Zhongling Yang: Validation, Formal analysis. Guoyong Li: Visualization, Data curation. Shirong Liu: Writing - review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the China National Science Foundation (No. 31971454, 31930078), Chinese Postdoctoral Science Foundation (2018T110722 and 01702050), and Scientific Research Fund Project of Henan University, China (yqpy20170053). We are grateful to anonymous reviewers who provided us with important suggestions for improving this manuscript. We thank Yixi Ju and Guangwei Jing of Jigong Mountain Natural Reserve Administration for their field support during the experiment.
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