Combined effect of sheep manure and DAP fertilizer on growth attributes
Results concerning growth parameters including above and below parts of the plant are shown in (Fig. 3). The combined application of fertilization treatments (SM and DAP fertilizer) significantly (p ≤ 0.05) increased NL, RL, and SB of soybean compared to the control and single treatments. However, PH and NN did not differ statistically between fertilization treatments (Fig. 1A and Fig. 1E). The increased growth may be attributed to the improved soil physical and chemical properties, such as increased soil organic matter and available nutrients enhanced by SM and DAP fertilizer, which facilitated root growth and nutrient uptake. Correlation matrix analysis showed that PH and RL (r = 0.88*, p < 0.05) are highly correlated, i.e., the longer the roots of a plant, the more water, and nutrients it receives, increasing the height of the plant (Fig. 6). Alone SM (FT2), DAP fertilizer (FT3) and combination of both (FT4) increased NL in soybean (Fig. 3B). Because SM is an organic fertilizer that contains nutrients such as nitrogen, phosphorus, potassium, and micronutrients. When applied to soil, it can improve soil fertility and increase the availability of nutrients for plants. This can result in an increase in NL in soybean plants, as more nutrients can support the growth of more leaves. DAP fertilizer, on the other hand, is a synthetic fertilizer that contains high amounts of nitrogen and phosphorus. Its application to the soil can provide the necessary nutrients for plant growth and development. Hence, there was a strong positive correlation between the NL and SB (r = 0.85*, p < 0.05), and Tr (r = 0.83*, p < 0.05) (Fig. 6) showing that plants receiving both fertilizers can increase NL and SB of plants, as they provide the necessary nutrients for growth. However, the effect on transpiration rate may differ depending on the application rate and timing of the fertilizers. If the application rate of fertilizer is too high (FT4), it may cause water stress in plants and increase Tr (Fig. 4C). Generally, the more leaves a plant has, the higher amount of SB and the higher its Tr will be. This is because each leaf has tiny openings called stomata, which allow for gas exchange and water vapor to escape. Therefore, the greater the surface area of leaves, the more stomata a plant has, and the higher the transpiration rate. Similarly, except for control condition (FT1), the RL improved significantly across all fertilization treatments. The root length of soybean was significantly positively correlated with Pn (r = 0.85*, p < 0.05) and Tr (r = 0.95**, p < 0.01) (Fig. 6), indicating that when roots are longer, they can explore a larger volume of soil and absorb more water and nutrients. This can improve the plant's overall health and, in turn, increase its ability to perform photosynthesis. The length of roots can indirectly affect Pn by influencing the plant's ability to take up water and nutrients. Whilst plants with longer roots generally have a greater surface area for water uptake from the soil, allowing them to absorb more water and thus transpire more. The outcome in Fig. 3D. indicates that with the application of FT4 treatment, massive SB was produced followed by FT5, FT3 and FT2. The use of SM and DAP fertilizer resulted in a positive significant correlation between SB and SY (r = 0.81*, p < 0.05) (Fig. 6). This is because a balanced amount of SM and DAP provided the nutrient requirements of the crop as needed could help promote healthy plant shoot growth and improve SY. It is possible that the addition of SM and DAP fertilizer could affect NN in soybean plants. However, our results showed that none of the treatments, significantly affected NN (Fig. 3E).
Combined effect of sheep manure and DAP fertilizer on photosynthesis related traits
It can be seen from Fig. 4 that application of combined organic and inorganic fertilizer can significantly increase the net photosynthetic rate (Pn), stomatal conductance (Gs), and transpiration rate (Tr) of soybean. FT4 processed the highest Pn, Gs, and Tr. The Ci of each fertilization was less than FT1. The concentration of Ci treated by FT2, FT3, and FT6 was the largest. The Tr of FT4 reached a maximum. This may be related to a higher leaf area index (LAI). The application of SM can effectively improve the instantaneous water use efficiency of soybean. The instantaneous water use efficiency of FT5 treatment was the highest. Fertilization treatment was significantly different from FT1 (P < 0.05). The WUEp of leaves with FT5, FT4, FT2, FT6, and FT3 was higher than FT1. They were increased by 43.70%, 30.04%, 21.20%, 19.70%, 8.90%, respectively, compared with that of FT1. Overall, the combined application of SM and DAP fertilizer significantly increased the Pn rate, Gs, and Tr of soybean compared to the control and single treatments. The improved photosynthetic capacity may be attributed to the increased chlorophyll content and chlorophyll fluorescence parameters of soybean leaves, which indicates a higher photosynthetic efficiency. Pn had a significant positive correlation with Gs (r = 0.96**, p < 0.01) and Tr (r = 0.69**, p < 0.01), indicating that a higher Pn rate was associated with a higher Gs and Tr (Fig. 6). Pn also had a significant positive correlation with SY of soybean (r = 0.86**, p < 0.01), indicating that a higher Pn rate was associated with increased yield. Surprisingly, Ci had significant negative correlation with other photosynthetic related traits, for example, the reason for higher Pn rate but lower Ci in plants is due to the process of stomatal regulation. Stomata are tiny pores present on the surface of leaves that allow for gas exchange between the plant and the environment, including the uptake of CO2 for photosynthesis and release of O2. Plants regulate the opening and closing of stomata to optimize the balance between CO2 uptake and water loss through transpiration. When the stomata are open, more CO2 can enter the plant, allowing for a higher Pn rate. However, this also increases the amount of water that is lost through transpiration. To maintain the balance between CO2 uptake and water loss, plants can reduce the Ci by closing the stomata partially or completely, which limits the amount of CO2 that enters the plant but also reduces water loss. This strategy is especially important for plants growing in arid or drought-prone environments where water is a limiting factor. Therefore, even though the Ci is lower, the plant can still maintain a high Pn rate through the regulation of stomatal opening and closing. Hence, the results of the correlation analysis suggest that photosynthetic related traits of soybean were positively correlated with each other, except for Ci and were positively correlated with yield. This highlights the importance of improving soil fertility through combined application of SM and DAP fertilizer to enhance the photosynthesis efficiency of soybean, ultimately leading to increased yield.
Combined effect of sheep manure and DAP fertilizer on yield and its components
Neither single fertilization nor combined application significantly affected PN and NSP (Fig. 5A & Fig. 5B). However, there was significant difference (p < 0.05) between fertilization treatments in regards with soybean SY. The integrated use of SM and DAP fertilizer (FT4 and FT5) increased SY considerably as compared with single treatment and FT1 (control). FT4 and FT5 treatments increased the SY by 81.30% and 64.32%, respectively, compared to FT1 (Fig. 5C).
The results of the correlation analysis suggest that the growth and photosynthetic related traits of soybean were positively correlated with each other, and both were positively correlated with SY (Fig. 6). This highlights the importance of improving soil fertility through combined application of SM and DAP fertilizer to enhance the growth and photosynthetic related traits of soybean, ultimately leading to increased yield. We designed a descriptive model based upon the relationship in between growth, photosynthetic traits, and seed yield per plant with the application of SM and DAP fertilizer, as shown in Fig. 7. This model illustrates that the combined application of both fertilization increased growth rate and photosynthetic activity, which ended up resulting with higher seed yield.
Principal Component Analysis
A principal component analysis (PCA) was performed utilizing the study's data to further investigate the relations between soybean growth, photosynthesis related traits, and yield. According to the PCA, the first principal component (PC1) and the second principal component (PC2), respectively, accounted for 71.1% and 13.5% of the variance in the data. Therefore, the two components are enough to explain the variance. Gs, WUEp, and Pn have the largest loadings on PC1. This suggests that PC1 represents all features associated with the processes of photosynthetic activity. NSP, NL, and SB had the highest loadings on PC2. It implies that PC2 represents the characteristics of soybean growth and yield. RL, SY, and Ci are strongly correlated with PC1 in the loadings plot (values close to 1 or -1) but slightly correlated with PC2, as shown by the loadings plot. Overall, the PCA analysis shows that the combined application of SM and DAP fertilizer had a favorable impact on crop yield of soybeans, as well as growth and features associated to photosynthesis processes. This is further reinforced by the high Gs, WUEp, and Pn loadings on PC1 and the high NSP, NL, and SB loadings on PC2. According to the findings, applying DAP fertilizer along with SM may improve the growth and characteristics associated to photosynthetic attributes in soybean, eventually contributing to a higher yield. Furthermore, there is a high positive correlation between WUEp, Gs, Pn, NN, SY, RL, which means that photosynthesis related parameters enhance plant growth. However, Ci had a negative correlation with both photosynthesis and growth attributes (Fig. 8).