ObjectivesTo provide a theoretical basis for reasonably applying fertilizers and alleviating the growth obstacles of replanted apple, a pot experiment was conducted to study effects of different nitrogen forms on growth of apple rootstock (Malus hupehensis Rehd.) seedlings and the replanted soil microorganisms.

MethodsThe experiment was carried out in 2015. M. hupehensis Rehd. Seedling was used in this study, and the biomass, photosynthetic parameters and soil microorganisms were determined in August 2015. Three N fertilizer treatments were designed as follows: ammonium nitrogen (T1), nitrate nitrogen (T2) and amide nitrogen (T3) (N 180 mg/L), and took the apple replanted orchard soil (CK1) and methyl bromide fumigation (CK2) as the control. M. hupehensis Rehd. seedlings were planted in pots with different treatments.

Results Compared with the replanted soil control (CK1), the biomass amounts of M. hupehensis Rehd. seedlings were significantly improved with the ammonium nitrogen (T1) and amide nitrogen (T3). The plant height, ground diameter, fresh weight and dry weight of the ammonium nitrogen (T1) were increased by 35.3%, 24.4%, 42.0%, 57.7%. Different nitrogen forms could increase the net photosynthesis stomatal conductance and root respiration rate. The net photosynthetic rate, stomatal conductance in leaves of the seedlings and root respiration rate of ammonium nitrogen of T1 were increased by 27.6%, 35.6% and 43.3% compared with CK1, respectively, but they did not reach the highest effect of the methyl bromide fumigation (CK2). Real-time quantitative PCR results showed that the ammonium nitrogen (T1) effectively reduced the gene copy number of Fusarium oxysporum, which was decreased by 41.3% compared with the replanted soil control (CK1). The copy numbers of the nitrate nitrogen (T2) and amide nitrogen (T3) had no significant differences with the soil control (CK1). T-RFLP results showed that soil fungal community structure was changed by the ammonium nitrogen (T1) and amide nitrogen (T3) greatly, the fungal community structure in the ammonium nitrogen (T1) was similar to that in the methyl bromide fumigation (CK2), and the structure of the nitrate nitrogen (T2) was similar to that in the soil control (CK1), while an independent fungal community was formed in the amide nitrogen (T3).

ConclusionsAmmonium nitrogen greatly improved the biomass of M. hupehensis Rehd. seedlings, net photosynthetic rate and root respiration rate, reduced the gene copy number of Fusarium oxysporum, and changed the community structure of soil fungi. Therefore, the appropriate dosages of ammonium nitrogen could be used as a measure to reduce the apple replanted disease.