Effect of apple replanting on soil microorganism and nitrogen absorption, distribution, utilization of Malus hupehensis Rehd. seedlings

Objectives Apple replanting disease (ARD) is a frequently occurring plant disease, which inhibits growth of trees seriously. Under replanted condition, absorption, distribution and utilization of ¹⁵N-urea in Malus hupehensis Rehd. seedlings were studied, to reveal mechanism of ARD and provide a theoretical basis for prevention and control of ARD.

Methods A pot experiment was carried out using Malus hupehensis Rehd. seedlings as the material. Four treatments, including replanted soil fumigated by methyl bromide (T1), replanted soil pasteurized (T2), wheat field soil (T3) and replanted soil (CK) were carried out to explore the effect of ARD on absorption, distribution and utilization of ¹⁵N, using isotope tracer technique. The biomass, root, nitrogen and microorganism were measured in August and September.

Results Continuous cropping inhibited the growth and root architecture of Malus hupehensis Rehd. seedlings significantly. Compared to T1, T2 and T3, the fresh weight treated with replanted soil was reduced by 46.77%, 46.50%, 27.38%, respectively. The plant height was reduced significantly, by 41.97%, 41.95%, 23.51%, respectively. In addition, the root surface area of seedlings decreased by 56.21%, 55.72%, 48.04%, respectively, compared to T1, T2 and T3. Continuous cropping also deteriorated the environment of soil microorganisms. The fungal biomass in replanted soil was increased significantly, which was 1.76 times of the control. However, the bacterial biomass accounted for 78.77% in the wheat soil. The ratio of bacteria and fungi was also decreased significantly. The contribution of nitrogen derived from fertilizer (Ndff) in replanted organs was also decreased, due to continuous cropping. In September, the Ndff of replanted leaves was reduced by 61.34%, 58.65%, 57.36%, respectively, compared to T1, T2 and T3. Moreover, continuous cropping affected the distribution ratio of ¹⁵N in different organs seriously. 42.11% ¹⁵N was measured in root of replanted seedlings, which was higher than other treatments in September. However, the ¹⁵N distribution ratio in replanted leaves was lower. With increase of replanted time, the ¹⁵N distribution ratio in leaves became less. Only 29.25% ¹⁵N of replanted leaves was measured in September. Furthermore, the ratios of ¹⁵N utilization was reduced significantly under replanted condition. In September, the ratios of ¹⁵N utilization was only 13.33% and reduced by 67.19%, 67.68%, 60.39%, respectively, compared to T1, T2 and T3. In addition, continuous cropping affected the root system and function seriously. The root activity was decreased (39.71%, 40.64%, 26.80%, respectively) and H⁺-ATPase activity of mitochondrial plasmalemma was also reduced (41.44%, 38.24%, 25.78%, respectively), compared to T1, T2 and T3.

Conclusions Soil microorganism was the main factor for apple replanted disease. Continuous cropping not only inhibited the growth of Malus hupehensis Rehd. seedlings and root architecture, but also affected root functions. Furthermore, nitrogen absorptivity and utilization ratio were reduced significantly. Continuous cropping also affected nitrogen distribution in different organs, leading to root consuming more nutrition and reducing supply to other organs.