4.3 The optimum of N addition and its effects on multifunctionality
Our previous study determined the optimal N addition rate was 90 to 120 kg N ha-1 in MSSL according to the hay yield and N use efficiency (Huang et al. , 2015). The overall results of this study strongly support this conclusion again from the quality ofL. chinensis and soil characteristics. Although L. chinensis may have the maximum seed yield when N addition rate was 150 kg N ha-1 in MSSL(Huang et al. , 2019) and other higher quality indicators, such as protein contents and K+/Na+ ratio in this study, comprehensive analysis indicated that this recommendation was still the best. The relationship between L.chinensis quality and soil chemical characteristics also fully illustrated that soil desalination was consistent with the improvement of soil fertility and plant quality (Fig.6), and N addition was beneficial to achieve the goal.
In recent years, there has seen a surge of interest in ecosystem multifunctionality which including ‘ecosystem function multifunctionality’ and ‘ecosystem service multifunctionality’ (Manninget al. , 2018). As a whole, the EMF has been widely studied in natural ecosystem (Pfisterer & Schmid, 2002). In this study, the effect of N addition on the EMF of L. chinensis showed a quadratic curve relationship, and there was a positive correlation between the EMF and N addition rates of more than 90 kg N ha-1(Fig.7). When N addition rate was 120 to 210 kg N ha-1, there were no significant changes on the EMF among different N treatments, indicating that the EMF basically reached a steady state. Consequently, N addition of 90-120 kg N ha-1 was also suitable in moderately saline-sodic grassland of L. chinensis from the perspective of the EMF.