3.1.3 Main mineral elements
With different N addition rates increasing, the total content of mineral elements in L. chinensis shoot showed an increasing trend (Fig.2). Specifically, the contents of K+ and Ca2+ in shoot increased significantly with N addition rate increasing (P <0.05), on the contrary, the content of Na+ in shoot decreased significantly with the increase of N addition (P <0.05), and the content of Mg2+ in shoot hardly changed. The total contents of mineral elements in shoot in the treatments with 150 kg N ha-1 and 180 kg N ha-1 increased 27.6% and 34.2% compared with that in the control treatment, respectively. While the content of K+ in shoot in the treatments with 150 kg N ha-1 and 180 kg N ha-1 increased 71.9% and 99.3% compared with that in the control treatment, the content of Ca2+ in shoot in the treatments with 90 kg N ha-1 to 210 kg N ha-1 increased 45.0% to 80.7% compared with that in the control treatment, and the difference were all significant (P <0.05). Instead, the content of Na+ in shoot in the treatment with 120 kg N ha-1 to 180 kg N ha-1 decreased significantly 32.3%, 30.9% and 40.8% compared with that in the control treatment (P <0.05), respectively.
With the increase of N addition level, the ratio of K+/Na+ and the ratio of Ca2+/Na+ in shoot of L. chinensis also increased constantly (Fig.3). When N addition level reached 90 kg N ha-1 to 210 kg N ha-1, the ratio of K+/Na+ in shoot increased 1.26 to 3.37 times compared with that in the control treatment, and the ratio of Ca2+/Na+ in shoot increased 1.56 to 2.98 times compared with that in the control treatment, and the differences all reached significant levels (P <0.05). However, when N addition level was 210 kg N ha-1, the both ratios decreased, but still higher than those in the control treatment. Comprehensive mineral elements content, K+/Na+ and Ca2+/Na+ in shoot, when N addition rates were 120 kg ha-1 to 180 kg ha-1 the mineral quality of L. chinensisreached the best state, and there was a significant difference compared with those in the control treatment.