Effects of different nitrogen fertilizers on the rhizosphere of apple seedlings

Malus hupehensis Reld(MH), H. prunifolia Borkh(MP) and M. sieversii Roem(MS) apple rootstock seedlings were used in the experiment. The seed of the three rootstocks was stratified at 0-2degreesC for 60 days, and was sown into a mix 1/3 loam soil + 1/3 silt sand + 113 compost manure. The seedlings grew in the mix until the three-leaf stage, and then were transplanted into the rhizobags with 4 plants in each bag. The bags were put into containers with the same soil as in the bag. After the seedlings revived, (NH4)(2)SO4, urea and KNO3 were, respectively, applied into each container at the rate of 2g N per container with no nitrogen treatment as control (CK). Plants were harvested 60 days after nitrogen application, and rhizosphere and bulk soils were sampled. Effects of different nitrogen fertilizers on the rhizosphere were determined. (NH4)(2)SO4 reduced the rhizosphere pH by 0.22-0.29 unit, urea by 0.08-0.18 unit, and the treatment with KNO3 had an increase of rhizosphere pH compared with CK (no N) treatment but it was still lower in rhizosphere than in bulk soil. The redox potentials (Eh) were lower in rhizosphere at all treatments and no direct relationships between Eh and nitrogen forms were found. The nitrogen levels in both rhizosphere and bulk soil were raised by fertilization but still showed depletion in rhizosphere. Phosphorus always showed depletion in rhizosphere and the depletion zone was larger in the treatment with (NH4)(2)SO4 than KNO3. The responses of K, Ca, Mg, Cu, Zn, Mn and Fe concentrations in the rhizosphere to nitrogen fertilizers varied with species of seedlings and treatments.