Effects of different types of organic fertilizers on soil, growth and fruiting characteristics, and nut quality in walnut orchards

[Objective] The study aimed to explore the effects of different organic fertilizers on the physical and chemical properties of the soil, tree growth, fruiting characteristics, and nut quality in a walnut orchard. [Methods] A randomized block experimental design with a single factor was used to study 13-year-old Chulinbaokui walnut forests. Five organic fertilizers-plant-based Songta (ST), animal-based pig manure (ZF) and cow manure (NF), organic-inorganic compound fertilizer (HZ), and a combination of potassium humate and biological bacteria (FJ) were applied at consistent nitrogen levels over two years. Soil samples from 0-20 cm and >20-40 cm soil layers were collected and analyzed for physicochemical properties, alongside the assessment of tree growth and fruiting traits. EXCEL and SAS 8.1 softwares were used for data statistics, with analysis of variance to compare significant effects of the five fertilizers. Oringin2021 was used for principal component analysis to average the effects of the fertilizers. [Results] ①All the five organic fertilizer treatments improved the soil conditions in the walnut orchards. The soil porosity in the >20-40 cm soil layer, the available sulfur content in the 0-40 cm soil layer, and the available potassium content in the 0-20 cm soil layer were significantly increased in the five treatments (p<0.05, the same below). ②Different organic fertilizers exhibited varying effects on soil improvement. Among that, the soil bulk density from 0 to 20 cm was significantly reduced, but soil pH value and hydrolytic nitrogen content significantly increased in ST, NF, ZF, and HZ treatments. The organic matter content and available phosphorus content in the >20-40 cm soil layer were significantly increased in ST, NF, and HZ treatments, with ZF treatment showing a significant increase in available phosphorus content in the same layer. The available potassium content in the >20-40 cm soil layer was significantly increased in NF, ST, HZ, and FJ treatments. ③The thickening growth of the annual branches was significantly promoted in all the five treatments. The trunk ground diameter growth was significantly promoted by HZ, NF, and ZF treatments, with a 46.67% increase in HZ treatment compared to the control. In addition, the elongation growth of annual branches was significantly promoted by ZF, NF, and ST treatments. ④The dynamics of walnut fruit setting were impacted by organic fertilizer treatments after thirty days of flowering. One hundred and twenty days after flowering, the fruit setting rates in the four treatments excluding FJ were significantly higher than the control, with HZ treatment achieving the highest fruit setting rate of 88.19%. ⑤Different organic fertilizers had varying effects on nut weight, oil content, and fatty acid composition. Among them, the HZ treatment increased the weight of nuts by 8.90% and kernel oil content by 0.44%; the ST treatment significantly increased the proportion of saturated fatty acids in walnut kernels, and the NF and ZF treatments significantly increased the proportion of unsaturated fatty acids in walnut kernels. However, the FJ treatment had little effect on the fatty acid content, but significantly reduced the vitamin E content in walnut kernels. ⑥Principal component analysis showed that the cumulative contribution rate of the four principal components was 95.05%, which reflects the fertilization effect of each treatment. Coordinate analysis using Prin1 and Prin2 showed that HZ and NF treatments had similar fertilization effects, whereas the ST treatment showed significant differences compared to other treatments. A comprehensive evaluation model was constructed using four principal component eigenvalues and contribution rates for scoring and sorting. The result showed that the HZ treatment, with organic matter content of 24.00%, available ingredients N 6.46%, P2O5 4.16%, K2O 6.00%, and S 3.79%, had the highest overall score (40.94). [Conclusion] The effects of different fertilizer types on walnut orchards varied. Considering soil factors, tree growth, and fruiting characteristics comprehensively, organic-inorganic compound fertilizers are recommended.