Effects of different combinations of exogenous organic acids and biomass materials on the saline–alkaline dynamics of mudflat soil
Some differences were noted in the response values of soil pH, EC, and SS to the application of exogenous organic acids and biomass materials in different periods (Fig. 3). Organic acids significantly affected soil pH, EC, and SS at the seedling, elongation, and heading stages (P < 0.01), and biomass materials significantly affected soil EC and SS at the three periods and soil pH at the elongation stage (P < 0.01). In addition, the interaction effect between organic acids and biomass materials highly significantly influenced soil EC and SS in all periods (P ≤ 0.01) but not soil pH (P > 0.05). Soil pH in the CPN and CGC treatments was the lowest at the seedling stage and was not significantly different (P > 0.05) from that in the CCH, FPN, FGC, FCH and CCM treatments but was significantly lower (P ≤ 0.05) than that in the remaining treatments. Soil EC and SS values were also significantly low (P < 0.05) in the CPN and CGC treatments. Soil pH in the CPN treatment was the lowest at the elongation stage and was not significantly different (P > 0.05) from that in the FPN, CCM, FCM, and FCH treatments but was significantly lower (P ≤ 0.05) than that in the remaining treatments. Soil EC and SS in the CPN treatment were also significantly lower (P < 0.05) than those in the other treatments. Soil pH in the FPN, CPN, FCM, and CCM treatments was the lowest at the heading stage, and the difference between these treatments was not significant (P > 0.05). Soil EC and SS in the FPN treatment were the lowest and were not significantly different from those in the FCM treatment (P > 0.05) but were significantly lower than those in the other treatments (P ≤ 0.05).
Effects of different combinations of exogenous organic acids and biomass materials on soil nutrient content in tidal flats
Our results show highly significant differences (P < 0.01) in the response values of OM, AP, and AN to the application of organic acids and biomass materials in the mudflat soils from the seedling stage to the heading stage (Fig. 4). The interaction effects of organic acids and biomass materials reached a highly significant level (P < 0.01) for all indicators, except OM and AN values at the heading stage that were only significantly affected (P < 0.05). At the seedling stage, the soil OM, AP, and AN of the CPN treatment were significantly higher (P < 0.05) than those of the other treatments and increased by 44.17%, 66.73%, and 51.24%, respectively, compared with those of CK. Soil OM and AN in the FPN and CPN treatments were the highest at the elongation stage, and all indicators were significantly higher than those in the other treatments (P ≤ 0.05), except for soil OM that was not significantly different from that in the CCM treatment (P > 0.05). Soil OM in the FPN treatment was the highest at the heading stage and was not significantly different from those in the FCM treatment (P > 0.05) but was significantly higher than those in the other treatments (P ≤ 0.05). Soil AN value were also significantly high (P < 0.05) in the FPN and FCM treatments. Soil AP in the FPN treatment was also significantly higher (P < 0.05) than those in the other treatments at the elongation stage and heading stage.
Effects of different combinations of exogenous organic acids and biomass materials on the morphological indexes and biomass of sweet sorghum
The morphological indicators and biomass of sweet sorghum differed in their response to the application of exogenous organic acids and biomass materials at different growth stages (Fig. 5). From the seedling to heading stage, the PH, SD, LN, RL, and BP of sweet sorghum generally increased and showed highly significantly different changes (P < 0.01) in response to organic acids. Meanwhile, their responses to biomass materials also reached highly significant differences (P < 0.01), except for SD that only reached a significant difference level (P < 0.05) at the elongation stage. For the interaction effect between organic acids and biomass materials, a highly significant effect (P < 0.01) was observed on seedling BP, elongation PH, LN, RL, BP, and heading PH, SD, and BP; a significant effect (P ≤ 0.05) on seedling PH and heading RL; and an insignificant effect (P > 0.05) on the other treatments. At the seedling and elongation stages, PH, SD, LN, RL, and BP were higher in the CPN treatment than in the other treatments and increased from 62.71–93.08%, 38.41–73.63%, 58.73–85.72%, 59.35–209.11%, and 297.80–882.72%, respectively, compared with those in the CK. At the heading stage, the PH, LN, RL, and BP of the FPN treatment were significantly higher than those in the other treatments (P ≤ 0.05). The SD of the FPN treatment also reached the highest but was not significantly different from that of the FCM treatment (P > 0.05). Each of the indexes increased by 104.23%, 318.56%, 63.00%, 78.16%, and 115.20% compared with those in the CK.
Effects of different combinations of exogenous organic acids and biomass materials on the osmotic regulators, chlorophyll, and root activity of sweet sorghum
As shown in Fig. 6, the response values of Chl, RA, MDA, PRO, and DC of sweet sorghum to the application of exogenous organic acids and biomass materials reached highly significant levels (P < 0.01) from the seedling to heading stage. The interaction effect of organic acids with biomass materials had a highly significant effect (P < 0.01) on seedling Chl, RA, elongation RA, MDA, PRO, DC, and Chl, RA, and MDA values at the heading stage; a significant effect (P ≤ 0.05) on Chl at the elongation stage; and an insignificant effect (P > 0.05) on the other treatments. The CPN treatment had the highest Chl, PRO, and DC content at the seedling stage, followed by CGC; the difference between these two treatments was not significant (P > 0.05). RA content in the CPN was the highest and was significantly higher than that in the other treatments (P < 0.05). MDA content was significantly lower in the CPN, CGC, CCH, FPN, and FGC treatments than in the other treatments (P ≤ 0.05), but the differences were not significant (P > 0.05). Chl content was significantly higher (P ≤ 0.05) in the CPN treatment than in the other treatments at the elongation stage. RA content in the CPN treatment was also the highest but was not significantly different (P > 0.05) from that in the FPN, CCM, FCM, CCH, and FCH treatments. MDA content was significantly low (P ≤ 0.05) in the CPN and FPN treatments. PRO was the highest in the CPN treatment, but its values was not significantly different between FPN and CCM (P > 0.05) but was significantly different from that in the other treatments (P < 0.05). The DC values of CPN and FPN treatments were significantly higher than those of the other treatments (P < 0.05). Chl and RA were significantly higher (P < 0.05) and MDA content was significantly lower (P < 0.05) in the FPN treatment than in the other treatments at the heading stage. PRO and DC were the highest (P ≤ 0.05) in the FPN, FCM, and CPN treatments, but the differences among these three were not significant (P > 0.05).
Effects of different combinations of exogenous organic acids and biomass materials on the antioxidant enzyme system of sweet sorghum
The SOD, POD, and CAT activities of sweet sorghum reached their highest levels at the seedling stage and decreased followed by a small increase as the reproductive period progressed (Fig. 7). Highly significant differences (P < 0.01) in SOD, POD, and CAT were observed in response to the organic acid treatment for sweet sorghum from the seedling to heading stage. Except for SOD at the heading stage (P < 0.01), the response of other indexes to biomass materials was significantly different (P < 0.01). For the interaction between organic acids and biomass, extremely significant effects were observed on CAT at the seedling stage and on SOD, POD, and CAT at the elongation stage (P < 0.01); significant effects were also found on POD at the seedling stage (P < 0.05). SOD activity in the CPN and CGC treatments at the seedling stage was the highest; its value was not significantly different from that in the CCH and FPN treatments (P > 0.05) but was significantly higher than that in the other treatments (P ≤ 0.05). POD activity in the CPN treatment was significantly higher than that in the other treatments (P < 0.05). CAT values in the CPN treatment were the highest; these values were not significantly different from those in the CGC and CCH (P > 0.05) but were significantly different from those in the other treatments (P ≤ 0.05). The SOD values of CPN, FPN, CCM, CCH, FCM, and FCH treatments were not significant differently from each other (P > 0.05) but were significantly higher (P ≤ 0.05) than those of the other treatments. For POD and CAT activities, both were the highest in the FPN, CPN, FCM, and CCM treatments at the elongation stage. The SOD activity of FPN, FCM, CPN, CCM and FCH treatments at heading stage was significantly higher than that of other treatments (P < 0.05). POD activity was the highest in the FPN treatment but was not significantly different from that in the CPN treatment (P > 0.05). CAT activity was significantly higher in the FPN treatment than in the other treatments during this period (P ≤ 0.05).
Correlation analysis of soil improvement and sweet sorghum growth performance upon the application of combined exogenous organic acid and biomass
Spearman correlation analysis (Fig. 8) showed that after the addition of exogenous organic acid and biomass materials, the biomass and morphological indexes (PH, SD, LN, RL, and BP) of sweet sorghum in the three periods were significantly positively correlated with the physiological and biochemical indexes (Chl, PRO, SOD, CAT, POD, and DC) of sweet sorghum (P ≤ 0.01); negatively correlated with the MDA content (P ≤ 0.01); positively correlated with soil nutrient indexes (OM, AP, and AN) (P ≤ 0.05); and negatively correlated with soil saline–alkali indexes (pH, EC, and SS) (P ≤ 0.01). Meanwhile, the soil nutrient indexes (OM, AP, and AN) were negatively correlated with soil salinity indexes (pH, EC, and SS) (P ≤ 0.05); positively correlated with the physiological and biochemical indexes (Chl, PRO, SOD, CAT, POD, and DC) of sweet sorghum (P ≤ 0.05); and significantly negatively correlated with the MDA content (P ≤ 0.05).