Optimizing water and organic fertilizer use to enhance seedling growth and soil health in Chinese cabbage (Brassica rapa spp. pekinensis) cultivation

Water and fertilizer are the two indispensable elements in agriculture. The rational use of soil water and fertilizer can not only improve crop yield and quality rate, but also alleviate land pollution and water shortage. Water and fertilizer management in Chinese cabbage (Brassica rapa spp. pekinensis) involves in improper irrigation and fertilization. This experiment was conducted to explore he shoot growth, root traits, nutrient contents, soil microbial population and soil enzyme activities of Chinese cabbage in response to different irrigation and organic fertilizer application rates. The experiment consisted of three irrigation levels (A1-A3: 90%-70% ± 5% of field water holding capacity), five organic fertilizer application rates (B1-B5: 10%-50% cattle manure with nitrogen replacement) and CK treatments (CK1: regular fertilizer, CK2: 100% cattle manure with nitrogen replacement). The results showed that irrigation rate A2 was the better irrigation rate. At this irrigation rate B2 treatment significantly increased shoot fresh weight by 22.65% and 31.09% compared to CK1 and CK2, respectively. Organic fertilizer application increased the SPAD value of Chinese cabbage, but the trend was not consistent under different irrigation and organic fertilizer levels. All irrigation levels showed more soil available K content in CK2. Compared to other treatments, CK2 was enhanced by 20.70%-317.42% under A1, 7.34%-235.68% under A2, and 28.16%-188.42% under A3 irrigation. Adequate irrigation is prerequisite to enhance root fresh weight, while lower irrigation and organic fertilizer helps to promote root length and root surface area. Soil enzyme activity and bacteria were significantly higher in A1 and A2 treatments than A3, while irrigation had no significant differences on soil actinomycetes and fungi. Under A1 irrigation, the B4 treatment had higher acid phosphatase and sucrase activities, 11.31%-22.46% and 4.30%-63.50%, respectively, compared to other treatments. In conclusion, the effect of organic fertilizer on soil enzyme activity and soil microbial population varied according to the irrigation levels, but generally organic fertilizer were beneficial to boost up the soil enzyme activity and soil microbial population.