A growing number of ecosystems are experiencing large-scale degradation due to the increasing impact of climate change and unsustainable land use (Mao et al. 2018, Xie et al. 2020, Bardgett et al. 2021). Land degradation poses a serious threat to biodiversity loss, land desertification, human living environment and sustainable development of social and economic systems (Solomun et al. 2018, Winslow et al. 2011). Vegetation restoration can speed up the restoration process and is one of the common and effective ways to restore degraded land and ecosystem functions (Chen et al. 2005, Hu et al. 2022, Klopf et al. 2017).The purpose of vegetation restoration is not only to improve biodiversity, but also to improve soil health, including soil structure, soil nutrient and soil quality (Vaverkova et al. 2018, Xu et al. 2021, Zhao et al. 2020).
As the basic unit of soil structure formation, soil aggregates play an important role in soil structure and fertility (Pérès et al. 2013, Yao et al. 2019, Liu et al. 2023). Soil aggregate stability is one of the relevant indexes of soil stability, which refers to the ability of soil aggregates to resist damage when applying destructive force (water, wind), and is a good indicator of soil sensitivity to erosion (Barthes & Roose 2002, Oztas & Fayetorbay 2003). Soil mean weight diameter (MWD) and geometric mean diameter (GMD) is an important index to evaluate the stability of aggregates, and the larger the value of MWD and GMD, the stronger the stability and aggregation ability of aggregates (Padbhushan et al. 2016, Schaller & Stockinger 1953). Soil aggregation is a complex process, which is regulated by vegetation type (Duchicela et al. 2012, Dou et al. 2020), fertilization (Eviner & Chapin 2002) and management measures (Du et al. 2022, Ren et al. 2022), etc. For example, the distances between alfalfa and jujube at 0.5 m can significantly increase the amounts of macro-aggregates, and improved soil mechanical properties and aggregate stability among the other treatments in herbage-fruit tree intercropping systems (Chen et al. 2023). Long-term fertilization for 26 years significantly increased MWD and GMD, which meant that the stability of soil aggregates was improved (Mustafa et al. 2020). Previous studies have shown microbial changes driven by the change of soil nutrient might be the major factor affecting soil surface aggregate stability in the process of degradation succession in typical grassland (Ren et al. 2022). In addition, in the process of vegetation restoration, the selection of different plants has different effects on soil aggregates. For example, studies have shown that the effects of vegetation restoration measures on the stability of soil aggregates were different and the MWD and GMD were both highest in two soil layers only under the natural shrub restoration on the Loess Plateau, China (Dou et al. 2020). Conversely, a previous study showed that natural pasture restoration improved soil physical properties and water stable aggregate stability better than planting species (Wang et al. 2012). In northern China, Medicago sativa L. (alfalfa) and Bromus inermis Leyss. (smooth brome) are often used a community building species for degraded land restoration. Unlike the fibrous roots of smooth brome, alfalfa is a premium perennial leguminous plant that may be an improvement in texture and nutrients in deeper soils (Hafner & Kuzyakov 2016). Our previous studies indicated that short-term alfalfa planting could enhance surface soil fertility (Xu et al. 2022a). However, in the process of vegetation restoration, there are few reports on the influence of different herbaceous restoration methods on the soil aggregate stability and its driving factors (microbial community characteristics, stoichiometric characteristics and soil nutrients) in degraded land.
Soil carbon (C), nitrogen (N) and phosphorus (P) stoichiometry has been extensively studied around the world in recent decades (Bai et al. 2012, Cleveland & Liptzin 2007, Sun et al. 2022, Wang et al. 2022). Soil aggregate stability and its stoichiometric characteristics are the effective ways to evaluate the effect of soil restoration and various sized aggregates play different roles in the supply and transport of soil C, N, and P (Chen et al. 2021, Tang & Wang 2022). Therefore, in order to better understand the effects of different vegetation restoration on soil structure and function, it is necessary to study the stoichiometric characteristics of soil C, N and P at the aggregate scale (Smith et al. 2019, Zhao et al. 2015, Xu et al. 2022c). In recent years, some studies have investigated the distribution of soil stoichiometric characteristics in aggregates. For example, the increases in soil C:N, C:P, and N:P ratios were accompanied by the decreasing aggregate size in Chinese fir plantations (Tang & Wang 2022). The vegetation restoration only significantly reduced the value of N:P and C:P in different particle size aggregates and had no significant effect on the C:N in all particle size aggregates (Xu et al. 2022c). However, it is not known how the stability of soil aggregates is related to oil stoichiometric characteristics under different vegetation restorations.
The effects of planting alfalfa and brome after five years on soil aggregate composition, stoichiometry and stability in degraded wastelands in North China Plain were studied. The objectives of this study were to (1) analyze the influence of different restoration methods on the soil aggregate stability, (2) evaluate the changes of soil aggregate composition and structure after five years of vegetation restoration, (3) measure the stoichiometric characteristics of soil aggregate, and (4) explore the key driving factors (microbial community characteristics, stoichiometric characteristics and soil nutrients) affecting the soil aggregate stability.