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Figure 1. 

(a) Shoot biomass, (b) root biomass and (c) picture of plant pot experiments of alfalfa under different treatments. CK: 100% soil; 10%: 10% SMS; 20%: 20% SMS; 30%: 30% SMS; CF: 100% soil with chemical fertilizers added. Different lowercase letters above the columns represent significant differences among these treatments according to Duncan tests.

Figure 2. 

Number of alfalfa nodules under different treatments. CK: 100% soil; 10%: 10% SMS; 20%: 20% SMS; 30%: 30% SMS; CF: 100% soil with chemical fertilizers added. Different lowercase letters above the columns represent significant differences among these treatments according to Duncan tests.

Figure 3. 

Effects of different treatments on (a) soil dissolved organic carbon (DOC), (b) available P, (c) microbial biomass nitrogen (MBN), (d) microbial biomass carbon (MBC), (e) nitrate nitrogen ($\text{NO}_3^- $) and (f) ammonium nitrogen ($\text{NH}_4^+ $). Means ± S.E. CK: 100% soil; 10%: 10% SMS; 20%: 20% SMS; 30%: 30% SMS; CF: 100% soil with chemical fertilizers added. Different lowercase letters above the columns represent significant differences among these treatments according to Duncan tests.

Figure 4. 

The Redundancy Analysis (RDA) shows the effects of five experimental treatments on physical and chemical properties in substrate including pH, dissolved organic carbon (DOC), microbial biomass nitrogen (MBN), microbial biomass carbon (MBC), $\text{NH}_4^+ $ and $\text{NO}_3^- $, available P (P) and plant indicators including shoot biomass, root biomass, plant total nitrogen (TN), total carbon (TC), and carbon nitrogen ratio (CN). The red arrow in the figure represents plant growth indicators, and the blue arrow in the figure represents physical and chemical properties in substrate.

Figure 5. 

Correlation heatmaps of alfalfa growth indicators (shoot biomass and root biomass) and of nutrient concentration indicators in substrates ($\text{NO}_3^- $, $\text{NH}_4^+ $, MBC, MBN, DOC, pH and P). $\text{NO}_3^- $: nitrate nitrogen; $\text{NH}_4^+ $: ammonium nitrogen; MBC: microbial biomass carbon; MBN: microbial biomass nitrogen; DOC: dissolved organic carbon; pH: acidity and alkalinity of substrates; P: available P. The color intensity in each panel indicates the relative correlation between read numbers of two groups. Blue represents positive correlation, and red represents negative correlation. Statistically significant correlations are indicated with *p < 0.05, **p < 0.01, and ***p < 0.001.