Seasonal Dynamics of Soil Microbial Biomass C and N along an Elevational Gradient on the Eastern Tibetan Plateau, China

RESEARCH ARTICLE Seasonal Dynamics of Soil Microbial Biomass C and N along an Elevational Gradient on the Eastern Tibetan Plateau, China Xiaolin Gou☯, Bo Tan☯, Fuzhong Wu, Wanqin Yang*, Zhengfeng Xu, Zhiping Li, Xitao Zhang Key Laboratory of Ecological Forestry Engineering, Institute of Ecology & Forestry, Sichuan Agricultural University, No. 211, Huimin Road, Wenjiang District, Chengdu 611130, P. R. China ☯ These authors contributed equally to this work. * Abstract OPEN ACCESS Citation: Gou X, Tan B, Wu F, Yang W, Xu Z, Li Z, et al. (2015) Seasonal Dynamics of Soil Microbial Biomass C and N along an Elevational Gradient on the Eastern Tibetan Plateau, China. PLoS ONE 10(7): e0132443. doi:10.1371/journal.pone.0132443 Editor: Annamaria Bevivino, ENEA Casaccia Research Centre, ITALY Received: April 2, 2014 Accepted: June 16, 2015 Published: July 6, 2015 Copyright: © 2015 Gou et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Little information is available on the seasonal response of soil microbial biomass to climate warming even though it is very sensitive to climate change. A two-year field experiment was conducted in the subalpine and alpine forests of the eastern Tibetan Plateau, China. The intact soil cores from 3,600 m site were incubated in three elevations (3,000 m, 3,300 m and 3,600 m) to simulate climate warming. Soil microbial biomass carbon (MBC) and nitrogen (MBN) were measured at different periods (early growing season [EG], late growing season [LG], onset of soil freezing period [OF], deep soil frozen period [DF] and soil thawing period [ET]) from May 2010 to August 2012. Average air temperature and soil temperature increased with the decrease of elevation during the experimental period. MBC and MBN showed a sharp decrease during the OF and ET in both organic layer and mineral layer at the three sites. Additionally, a relatively high MBC was observed during the DF. MBC and MBN in the soil organic layer decreased with the decrease of elevation but the opposite was true in the mineral soil layer. Warming had stronger effects on soil microbial biomass in the organic layer than in the mineral soil layer. The results indicated that future warming would alter soil microbial biomass and biogeochemical cycling in the forest ecosystems on the eastern Tibetan Plateau. Data Availability Statement: All relevant data are within the paper. Introduction Funding: The project was financially supported by the National Natural Science Foundation of China (No. 31170423; No. 31270498; No. 31200474), http:// www.nsfc.gov.cn/Portal0/default166.htm; National Key Technologies R&D Program (No. 2011BAC09B05), http://www.most.gov.cn; and Program of Sichuan Youth Sci-tech Foundation (No. 2012JQ0008; No.2012JQ0059), http://xmgl.scst.gov. cn/. A growing body of evidence has demonstrated that climate change is occurring more intensely at higher elevations and latitudes in cold regions [1]. Soil microbes play critical roles in C and nutrient transformation in forest soils [2]. Soil microbial biomass not only lays their important roles as driving force in soil processes (e.g. N mineralization), but also acts as sensitive bio-indicator to ongoing climate change [3]. Generally, temperature is one of the main factors limiting the activity and reproduction of soil microbes in cold bioregions. However, studies of soil microbial biomass and its response to climate change have produced inconsistent results. Many studies observed that climate warming resulted in an increase in soil microbial biomass [4], but others PLOS ONE | DOI:10.1371/journal.pone.0132443 July 6, 2015 1 / 11 Soil Microbial Biomass along an Elevational Gradient of Alpine Forest Competing Interests: The authors have declared that no competing interests exist. declared soil microbial biomass decreased when temperature increasing [5]. Moreover, a few studies even documented that climatic warming did not affect microbial biomass [6]. These inconsistent observations may attribute to at least two related causes. On the one hand, various methods have been applied in experimental warming studies in cold bioregions. For example, either open top chambers or heating lamps have been used to stimulate climate warming [7, 8], but whether these findings can be reproduced under field conditions is uncertain. In contrast, temperature increases with change in elevational gradient in the field may provide a better way to mimic ongoing climate change [9, 10]. On the other hand, numerous studies have focused on the effects of long-term temperature changes on microbial biomass, but microbial characteristics are affected by changing temperatures in cold bioregions through at least three periods during winter as described below [11]. First, onset of the soil freezing period (OF), which is characterized by frequent freeze-thaw events as soil temperatures fall to the freezing point repeatedly until the first snowfall when the soil completely frozen. Second, deep soil frozen period (DF) which is characterized by soil temperatures remain below freezing point. Third, soil thawing period (ET) is also characterized by frequent freeze-thaw events as soil temperatures completely above the freezing point during early spring. These three periods might affect the response of soil microbial biomass to climatic warming in cold regions [12]. Moreover, the insulation of snow cover can prevent soil temperature from paralleling air temperature [13]. As a result, decreased in snow cover under climate warming will promote colder soil temperatures [1, 13], more frequent freeze-thaw cycles as well as decreased overall microbial activity [5, 14]. This may cause soil microbial biomass change with temperature increases in frost-free regions (the tropics) or seasons (growing season). Unfortunately, available studies on the subject have not adequately addressed this particular winter stage, making the relationship between soil microbial biomass and temperature unclear. The alpine forests of Western China locate in the transitional area between the Qinghai– Tibet Plateau and the Sichuan Basin [15]. Soil in these forests consists of a thick organic layer but a thin mineral soil layer [16]. Previous studies have showed that the dynamics surrounding freezing and snowpack development and subsequent thawing often last about half a year [15]. Moreover, the magnitude of global warming on the Tibet Plateau is projected to be larger relative to other temperate regions at the same latitude [1]. The structure and activity of microbial community are significantly controlled by the seasonal freeze-thaw cycle in this region [11, 16, 17]. However, little attention has been paid to the responses of soil microbial biomass to climatic warming in this region. In this study, we test the hypothes (...truncated)