Decomposition characteristics of pruned branches of kiwifruit trees and their response to substrate quality

Abstract:【Objective】Fruit tree pruning is one of the most common management measures in orchards. However, very few studies focus on the decomposition process and nutrient release of pruned branches after returning to the field, and the mechanism of the effect of branch substrate quality on decomposition rate is not clear. The decomposition and nutrient release process of pruned kiwifruit branches were quantitatively analyzed to reveal the mechanism of the influence of branch substrate quality on decomposition rate, and to provide reference for scientific fertilization in kiwifruit orchard.【Methods】 The pruned branches of four varieties of kiwifruit (Hongyang, Jinyan, Jinkui, and Hort-16A) were taken as the experimental materials. The in-situ decomposition experiment of kiwifruit branches was carried out in a 10-year orchard covering an area of 15 ha using decomposition bag method. In December 2020, after the trees were pruned, the branches pruned off were collected, dried at 60 ℃ to constant weight, and 15 g branch samples were weighed and put into decomposition bags. The decomposition experiment was started in early January 2021. According to the property that the decomposition rate of litters declines with time, the decomposition bags were sampled in the first, second, fifth, eighth and twelfthmonths of decomposition. The decomposed residue with the sampled bags was dried at 60 ℃ to constant weight and weighed. At the same time, the nutrient content in the residue was analyzed. The contents of total carbon and total nitrogen were determined with an elemental analyzer; total phosphorus was determined with an ultraviolet spectrophotometer; total potassium was measured with a flame photometer. The contents of lignin and cellulose were determined with the ammonium ferrous sulfate titration method after potassium dichromate oxidation, and the content of hemicellulose with the copper-iodine method. Based on the residual amount and decomposition time, the Olson decomposition model was used to fit the decomposition process and calculate the decomposition rate. Based on the dynamics of the contents of nutrients, lignin, cellulose, and hemicellulose during the decomposition process, the residues of nutrients and other substances, the relative nutrient return index, and the degradation rate of lignin and other substances were calculated. Moreover, the correlation between substrate quality and decomposition rate was analyzed.【Results】The results showed that the decomposition rates of pruned branches of the four kiwifruit varieties were in the order from fast to slow of Hongyang＞Hort-16A＞Jinyan＞Jinkui. Specifically, the half-life of Hongyang branch being decomposed was 5.2 months, and the turnover period was 26.6 months. The half-life for Hort-16A branches was 6.2 months, and the turnover period 26.7 months. The half-life for Jinyan branches was 6.4 months, and the turnover period 29.7 months. The half-life for Jinkui branches was 7.2 months, and the turnover period 31.1 months. Among the four varieties, the contents of potassium and hemicellulose in Hongyang were the highest, while its contents of lignin and cellulose were the lowest. The contents of phosphorus in Jinyan, and nitrogen and lignin in Jinkui were the highest. The cellulose content of Hort-16A was the highest, while its contents of phosphorus and hemicellulose were the lowest. The initial nitrogen, potassium, lignin and hemicellulose contents of the litters were significantly correlated with the decomposition half-life. The initial content of carbon in the branches of the four varieties was similar, ranging from 36% to 38%. During the decomposition process, the carbon content was in a relatively stable state, indicating that carbon was gradually released with the decomposition of the dry matter. The initial content of nitrogen of Jinkui was 1.71%, and the content of nitrogen gradually decreased with the decomposition of the litters, suggesting release of nitrogen. The initial content of nitrogen of Hongyang, Jinyan, and Hort-16A was low, ranging from 0.7% to 0.9% . With the decomposition of the litters, the content of nitrogen increased gradually, indicating enrichment of nitrogen. The initial content of phosphorus of the four varieties was similar, ranging from 0.09% to 0.2%. With the decomposition of the litters, the content of phosphorus fluctuated, indicating that phosphorus release and accumulation occurred during the decomposition process. The initial content of potassium of Hongyang and Jinyan was higher than that in the other two varieties, ranging from 0.4% to 0.55%. With the decomposition process, potassium decreased gradually suggesting a gradual release of the element. The initial content of potassium of Jinkui and Hort-16A was 0.08%-0.14%. With the decomposition process, potassium of Jinkui gradually increased. The potassium content of Hort-16A decreased within the first month of decomposition experiment, and then remained relatively stable. After twelve months of decomposition, branches of the four kiwifruit varieties released about 50%-70% of carbon and nitrogen, and Hongyang and Jinyan branches had a phosphorus and potassium release rate of more than 80%. Lignin, cellulose, and hemicellulose degraded gradually, and the degradation rate was the highest in the first month. Lignin degradation in Hongyang was the fastest, followed by Hort-16A and Jinyan, and that of Jinkui was the slowest, consistent with the decomposition rate of dry matter.【Conclusion】The decomposition half-life of kiwifruit branches is about six months, and the turnover period is about twenty-nine months. The decomposition of branches of kiwifruit treesis an important supplement to soil fertility. The decomposition characteristics of branches of fruit trees differ among varieties. The substrate quality is the main factor affecting the decomposition rate, and the lignin content plays the largest role in limiting the decomposition rate. The lignin content in Hongyang branches was the lowest and the decomposition rate was the highest, while the lignin content in Jinkui branches was the highest and the decomposition rate was the lowest.