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.