1. Introduction
Soil salinization is a worldwide environmental degradation problem. According to the statistics of FAO/UNESCO, the total area of salt affected soils is up to 100 million hectares, accounting for about 6% of the total land area in the world (Wong et al. , 2009; Hajiboland, 2013; Singh, 2015). With the global climate changing and increasing of the population, the area of salt affected soil is also increasing rapidly (Setia et al. , 2013).Due to soil salinization aggravating, many plants are facing unprecedented challenges. Taking Songnen Plain in as an example, soil salinization had resulted in a sharp decrease of grassland area by 64% from 1954 to 2000, most of which became saline-sodic wasteland (Wang et al ., 2009).
Leymus chinensis (Trin.) Tzvel.(Poaceae) is a perennial rhizome grass and the natural dominant species in the western grassland of Songnen Plain in (Zhu, 2004). It has strong drought resistance, saline-alkali tolerance and low temperature resistance (Wang et al ., 1994; Bai et al. , 2004; Jin et al ., 2008). However, with the increasing of human interference in this area, overgrazing, reclaiming farmland, collecting soil and other production activities further aggravated soil salinization and sodicization, which to result to the continuous decline of soil fertility and further degradation of L. chinensisgrasslands (Gao et al. , 2015; Qin et al ., 2016).
Numerous studies have shown that nutrients addition was an important measure to improve soil fertility and maintain high productivity of grassland, especially nitrogen (N) fertilizer addition was widely used in pasture land (Han et al ., 2007; Clark & Tilman, 2008). Also, N fertilizer addition can be used as soil restoration method in infertile or salt-affected soils (Murtaza et al ., 2000). As one of the key nutrient elements, N addition can promote leaf chlorophyll (Song et al ., 2016) and crude protein contents of L. chinensis (Hu et al ., 2017), and increase the hay yield and quality of L. chinensis (Qi et al ., 2013; Song et al ., 2016). Meanwhile, high soil N mineralization (Zhang et al ., 2009) and nitrification rates (Qin et al ., 2016) were observed in the pasture which N addition. However, Zhang et al. (2008) found that excessive N addition led to inefficient utilization of N fertilizers, and increased nitrate leaching and high environmental risks (Qin et al ., 2016). According to different soil types and plant species, the reasonable amount of N addition determined the crop yield and ecosystem stability (Vitousek et al ., 2009). The effects of N addition on the characteristics of plant or soil were often demonstrated lively in terms of ecosystem mutifunctionality (EMF) . Therefore, the study of EMF driven by plant or soil characteristics had become a recently hot spot in various ecosystem researchs (Hector & Bagchi, 2007; Manning et al ., 2018; Jing et al ., 2015), while the research on the EMF affected by salts had received little attention.
Given the acknowledged importance of rational nutrients addition on ecologically fragile area, studying the effects of N addition onL. chinensis performance and ecosystem stability would provide a new insight on ecosystem productivity and saline-sodic land restoration. So we had established a field experiment about the effects of N addition on L. chinensis in two kinds saline-sodic soils since 2009, and the effects of continuous application of N on hay yield and seed yield of L. chinensis were also reported successively, and the previous studies demonstrated that reasonable N addition could improve the biomass and seed yield of L. chinensis (Huang et al ., 2015 & 2019). However, the effects of long-term continuous N addition on the quality of L. chinensis , the characteristics of soil salt and the stability of ecosystem need to be further studied. Thus, the purposes of this study were (i) to reveal the effect of long-term continuous N addition on the nutritional quality of L. chinensis , (ii) to clarify the effect of long-term N addition on the salt characteristics of saline-sodic soil, (iii) to analyze comprehensively the roles of N addition on the restoration of saline-sodic grassland and EMF in Northeast China.