Release of nutrients during the decomposition of green manure in different levels of the soil in an apple orchard in Weibei highland

Abstract：【Objective】Green manure is a universal organic fertilizer, and white clover and ryegrass are the most commonly used green manure in apple orchards. The mixed sowing of white clover and ryegrass not only facilitates the growth of each other, but also complements the nutrient demand during the growth of fruit trees. The purpose of the study is to explore the nutrient release pattern in green manure composed of 75% white clover and 25% ryegrass at different depths of soil in order to find a suitable green manure bury depth and to improve the orchard ecological environment and quality of fruit.【Methods】The apple variety for testing was‘Red Fuji'grown in a seventeen-year-old orchard. The field experiment was conducted from October of 2011 to May of 2012. 120 fine mesh nylon bags filled with mixed green manure with 75% white clover and 25% ryegrass were horizontally placed between therows of apple trees, with 40 bags placed at each depth of 0 cm (surface layer) , 10 cm, and 25 cm. When covering the soil, try not to damage the original soil structure. Bag samples were collected at 0, 7, 14, 21, 35, 50, 80, 155, 185 and 215 days after bag bury. After removing the surface debris, the green manure sample were removed from the bag and placed in a 60 ℃ oven for 48 hours. The fine soil particles that entered the bags were carefully removed. After collecting the dry matter weight, the sample was crushed for the determination of plant nutrients including carbon, nitrogen, phosphorus, potassium, calcium, magnesium and trace elements. Dry matter residues and nutrient residues were then calculated.【Results】The decomposition speed of the green manure at different depths during the whole period was in the order of 10 cm> 25 cm> 0 cm. At all the three depths, decomposition of the green manure was faster in the early period than in the late period. During the first 35 days decomposition dry matter residue reduced to only 36% to 50%, while during 35 to 215 days decomposition was slow, and the decomposition rate was only 12.5%, 13.4% and 13.3% at 0, 10 and 25 cm, respectively. The change pattern of residual rate of organic carbon was similar to that of dry matter. The release of the same nutrient at different depths was different. The nutrient residual rate at the three depths was in the order of 0 cm > 25 cm> 10 cm. At the end of the experiment, the residue rates of nitrogen at the depths of 0 cm, 10 cm and25 cm were 33.9%, 24.2% and 26.3% respectively; those of phosphorus 47.7%, 19.8% and 44.3%, respectively; those of potassium 13.6%, 6.1%, 9.1%; those of calcium 30.8%, 26.3% and 27.3%, respectively. C/N ratio was in the order of 0 cm (13.8) > 10 cm (13.4) > 25 cm (11.9) . C/N remained in the range of 10-16 throughout the experiment in all the depths. The release rates of nitrogen, phosphorus, potassium and magnesium from the green manure were faster in the early period than in the late period.There were differences in the release of different nutrients. The fast release was potassium, which was almost completely released after 80 days at all the three depths. Magnesium release pattern was similar to potassium release. Calcium, iron, manganese, copper and zinc showed different degrees of enrichment in different decomposing stages. The main enrichment period appeared in the second year, in which there was scarce rainfall from January to April. pre-enrichment occurred at depths of 10 cm and 25 cm. After 215 days, the green manure at surface had the highest residual rate of copper (64.81%) .The residual rates of calcium at the depths of 10 cm and 25 cm were the highest, being 89.95% and 84.48%, respectively.【Conclusion】Green manure of 3 portions of white clover and one portions of ryegrass decomposes fast with fast nutrient release at the depths of 10 cm. However, the roots of fruit trees are mainly distributed in the soil layer of 20-40 cm. If the green manure is maintained at a depth of 10 cm, the released nutrients cannot be maximally utilized. This causes the roots to grow upward, which is negative to resistance to cold and drought. Considering the characteristics of root growth, we would like to recommend soil depth of 10 cm and 25 cm for green manure bury.