4.1 Effect of N addition on the quality of L. chinensis
Our previous studies have demonstrated that N addition can increase hay yield of L. chinensis and inhibit grassland degradation in different saline-sodic soils, and determined the optimum N addition rate was 90 to 120 kg N ha-1 in MSSL according to the yield and N use efficiency (Huang et al ., 2015). Many other researchers have also reported that N addition improved the quality of various herbages (Chen & Carran,1990;Dong et al .,2008; Tan et al. ,2014;Hu et al. , 2017). However, the effect of N addition on the quality of L. chinensis was rarely studied in saline-sodic grassland. It is very worth studying how long-term continuous N application will affect the quality of L. chinensis in MSSL.
The results of this experiment indicated that the protein and fat contents of L. chinensis were significantly increased by N addition for 10 years in MSSL, and the coarse fiber and ash contents did not change significantly under different N addition levels (Fig.1). In saline or alkali condition, Leymus chinensis usually has high protein content (Shi et al. , 2008; Zhou et al. , 2009). Generally, N addition increased the protein and fat contents and decreased the coarse fiber content of L. chinensis , which has been confirmed by many studies (Dong et al. , 2014; Bai et al. , 2017; Hu et al. , 2017). Previous studies also demonstrated that N addition did not affect the ash content of more plants (Brownet al. , 1932; Dong et al. , 2014).
Furthermore, N addition had a positive effect on the optimization of ion-allocated proportion in L. chinensis plants, and the content of K+ and Ca2+ in L. chinensis plants increased, while the content of Na+decreased with N addition rates increasing (Fig.2). The ratio of K+/Na+ and Ca2+/Na+ in L. chinensisshoot also increased with N addition rates increasing (Fig.3). The ratio of K+/Na+ and Ca2+/Na+ in shoot of plants was usually regarded as an important index of salt tolerance (Huang et al ., 2008). Under N addition, both of them increasing indicated that the toxicity of sodium ions to L. chinensis was reduced (Yinet al. , 1993) and salt tolerance of L. chinensis was enhanced (Li et al. ,2002). Therefore, the reasonable N addition not only improved the quality of L. chinensis , but also improved its salt tolerance and reduced the degradation of saline-sodic grassland.