Soil gravels and plant species configuration control vegetation restoration in Qinghai-Tibet Plateau
Yonggang Youa,b,c, Xinliang Wua,b,c, Lihe Hana,b,c,Yapei Lua,b,c, Jinxing Zhoua,b,c*, Ansa Rebia,b,c, Qian Donga,b,c , Lina Wanga,b,c, Pengcheng Zhanga,b,c
a.Jianshui Research Station, School of Soil and Water Conservation, Beijing Forestry University, Beijing 100083, China
b. Key Laboratory of State Forestry Administration on Soil and Water Conservation, Beijing Forestry University, Beijing 100083, China
c Engineering Research Center of Forestry Ecological Engineering, Ministry of Education, Beijing Forestry University, Beijing 100083, China
* Corresponding author. E-mail:bjfuzix@126.com (J.Zhou).Address: School of Water and Soil Conservation, Beijing Forestry University, East Qinghua Road No.35 in Haidian District, Beijing, 100083, China.
Note: Yonggang You and Xinliang Wu contribute equally to this paper.
Abstract: Soil gravel content strongly affects ecological restoration; however, the response and mechanism of plant traits to soil gravel content under the sensitive and fragile natural environment of Qinghai-Tibet Plateau remains unclear. Herein, soils with three gravel content (10%, 30%, 50%) in the southeastern Tibetan Plateau were selected, and three plant species ( one indigenous specie of Elymus dahuricus (Ed) , and two introduced ones of Festuca elata (Fe ) and Medicago sativa (Ms )) were used in seven planting patterns with different proportions (Fe , Ed , Ms , Fe +Ed(1:1), Fe +Ms (2:1), Ed +Ms (2:1),Fe +Ed +Ms (2:2:1)). Plant traits, phytochemical properties and soil stoichiometric characteristics were measured to explore the interactive effects of soil gravels and plant species on vegetation restoration. Average plant height, coverage, shoot biomass and total biomass were most affected by plant species (F=277~611, p<0.01), followed by gravel content (F=90~195, p<0.01) and their interaction (F=5~51, p<0.05); root biomass was most affected by gravel content (F=130, p<0.01). Among plant species, shoot and root biomass, total biomass overall decreased in the order of Fe+Ed+Ms˃Fe˃Fe+Ms˃Fe+Ed˃Ms˃Ms+Ed˃Ed . Plant total biomass, shoot biomass, root biomass and shoot/root ratio among different soils overall decreased in the order of low˃ high˃ medium gravel contents. All plant species were restricted by soil nitrogen except for Ed and Ed +Ms (N:P>14). In addition, average plant height, coverage, shoot biomass and total biomass were separately negatively and positively correlated with bulk density and total porosity (r=-0.88~-0.96 and 0.78~0.91, p<0.05), so did for total nitrogen, total phosphorus, organic carbon, C:N and N:P of shoot fraction and rhizosphere soils (|r|=0.69~0.97, p<0.05), indicating that gravel content affects plant growth through bulk density and nutrients. Therefore, optimizing the configuration of soil properties (mainly nitrogen and compactness) and plant species (isecologic niche plants) is an effective strategy for ecological restoration in the Qinghai-Tibet Plateau.
Keywords: Ecological restoration, Soil gravel, Plant species, Plant biomass, Tibetan Plateau.
Introduction
As the key component of soil texture, the content and distribution of gravel particles affect soil physical structure and hydrological processes such as soil water storage, runoff, water infiltration, solute transport and water flow (Smets et al.,2011; Qiu et al.,2015; Zhang et al.,2016;Wang et al.,2019; Mahinroosta et al., 2021); they also could reduce the mechanical resistance to root extension (Alameda et al.,2012), and may cause root aggregation and growth (Clark et al.,2003). But the impact and the underlying mechanism of soil gravels are relatively complex (Han et al.,2021; Li et al.,2020). In order to better comprehensively unravel the effect of soil on plant growth, the importance of gravel cannot be ignored (Du et al.,2022).
At present, a large number of studies have shown that gravels could promote cation exchange, and store water to facilitate plant growth (Certini et al.,2004). For instance, in the foothills of the Himalayas mountains, high gravel content of up to 40% can preserve rainwater and promote better economic benefits for leguminous plants or relatively drought-tolerant crops with vigorous rootstock systems (Grewal et al.,1984). Hubbert et al. (2001) reported that in stony soil, gravel in the soil can provide 70% water for plant growth. Other studies have found that the addition of a certain amount of waste gravel can improve soil physical properties, and is conducive to the improvement of soil quality and crop yield (Ye et al.,2021; Jin et al.,2022). In the arid area of northwest China, the addition and mulching of appropriate topsoil gravel can increase the retention of soil moisture (Qiu et al.,2021) and increase the yield of Malus pumila , Zea maysand other crops (Suo et al.,2019). For the pot culture of flue-cured tobacco, adding 10%-30% volcanic ash gravel significantly promoted the plant height and maximum leaf area of the overground part of the initial flue-cured tobacco, but the influence of volcanic ash gravel on the growth of tobacco plants became weaker with the extension of planting time (Shen et al.,2012). Less gravel inhibits the development of tobacco plants, but is beneficial to the accumulation of dry matter in the later period, while the opposite is true when gravel content is too high (Luo et al.,2014).
In general, soil gravel has different effects on plant growth at different growth stages. The content of gravel in the soil also leads to a decrease in soil water retention and the lack of root-soil contact area, and the lack of resources will restrict the growth of plants (Rytter et al., 2012; Ceacero et al., 2020). Studies on the Loess Plateau have found that high gravel content (50%) restricts plant growth and dry matter accumulation (Mi et al., 2016). For grassland in the Qinghai-Tibet Plateau, gravel content was found to negatively correlate with above-ground biomass and vegetation coverage (Yu et al., 2015), and soil organic carbon and total nitrogen reserves were the highest when gravel was covered by 40-50% (Wang et al., 2011). Additonally, soil gravel could reduce plant nitrogen and phosphorus contents, and inhibit plant growth and development (Masoni et al., 2008). A potted pot study on the adaptation of legume Sandalwood to soil gravels found that with the increase of gravel content, the root/shoot ratio of seedlings showed an increasing trend, and the biomass of stems transferred to roots and leaves, especially to low-grade roots (Liu et al., 2016). Therefore, the influence of gravel content on plant traits is species-specific, and the difference response of vegetation to soil gravels varys in different ecosystems.
Large-scale projects have disturbed soil surface texture and damaged native vegetation, and if timely ecological restoration is not carried out, it may cause serious ecological consequences such as land degradation and water and soil loss (Zhang et al.,2015; Si et al.,2020; Chen et al.,2020). After the ecological environment is disturbed by engineering, scientific and effective vegetation restoration is needed to ensure ecological security (Xie et al.,2021). However, the complex geological background and unique, sensitive and fragile natural environment of the Qinghai-Tibet Plateau make ecological restoration techniques scarce (Zhang et al.,2016b; Xu et al.,2022; Li et al.,2022). Gravel soils are widely distributed in the Tibetan Plateau (Pan et al.,2017). Therefore, it is urgent to understand the mechanism of soil gravel on plant traits to carry out the effective ecological restoration. The objectives of this study were to: (i) evaluate the effects of gravel content and plant species on plant growth; (ii) explore the mechanism of action of gravel content and plant species on plant growth based on stoichiometry.