Characteristics of NO₃^- absorption and utilization in Malus hupehensis Rehd. seedlings under different phosphorus levels

In recent years, excessive application of nitrogen and phosphate fertilizers has not only wasted fertilizer, but also brought a high potential risk of environmental pollution. In addition, an unreasonable fertilization over the long-term has damaged the physical and chemical properties of soil, including soil porosity and nutrients contents. Therefore, it is crucial for sustainable fruit tree production to promote scientific utilization of nutrient, increase fertilizer utilization rate, reduce eluviation, volatilization and loss of nitrogen. In order to determine the key factors influencing nitrogen utilization ratio under different phosphorus levels, ¹⁵N-labeled tracer and non-invasive micro-test techniques were used to investigate NO₃^- absorption and utilization in Malus hupehensis Rehd. seedlings under different phosphorus levels (0 mmol·L^-1, 1.0 mmol·L^-1, 2.0 mmol·L^-1, 3.0 mmol·L^-1, 4.0 mmol·L^-1, 6.0 mmol·L^-1, 8.0 mmol·L^-1, 12.0 mmol·L^-1, 16.0 mmol·L^-1 H₂PO₄^-). The study aimed to increase nitrogen fertilizer utilization and reduce nitrogen fertilizer loss, which could provide theoretical basis for scientific and efficient utilization of phosphate fertilizer in apple orchard. The results showed that root length, root surface area and root tip quantity were lower in seedlings under phosphorus deficiency (0-1.0 mmol·L^-1). With the addition of 2.0-4.0 mmol·L^-1 of phosphorus, the biomass of single plant, root length, root surface area and root tip quantity increased over seedlings under other treatments. Also root growth was restrained in seedlings under excess phosphorus (6.0-16.0 mmol·L^-1). The absorption of NO₃^- in M. hupehensis seedlings was significantly different under different phosphorus levels. The non-invasive micro-test technique showed significant absorption of NO₃^- by M. hupehensis seedlings under 3.0-6.0 mmol·L^-1 phosphorus with the highest rate of absorption under 3.0 mmol·L^-1 treatment. While 0-2 mmol·L^-1 and 8.0-16.0 mmol·L^-1 phosphorus applications showed efflux effect of NO₃^-by M. hupehensis seedlings. With the addition of phosphorus, Ndff (percent of nitrogen derived from fertilizer) and nitrogen utilization efficiency initially increased and then decreased. The highest nitrogen use efficiency (42.24%) was observed under the phosphorus treatment of 4.0 mmol·L^-1. Then there was a significant reduction under phosphorus application in excess of 4.0 mmol·L^-1 treatments. Leaf nitrate reductase activity was very low under phosphorus deficiency, but it had significantly higher levels under 1.0-3.0 mmol·L^-1 phosphorus application. There was a marked decrease in leaf nitrate reductase activity when the phosphorus concentration exceeded 4.0 mmol·L^-1. In conclusion, phosphorus level had significant effect on NO₃^- absorption and utilization by M. hupehensis seedlings. Root growth and nitrogen absorption increased with appropriate phosphorus application. With phosphorus overdose, root growth and nitrate reductase activity decreased significantly. This resulted in a decrease in the absorption and utilization of nitrogen. The analysis showed that 3.0-4.0 mmol·L^-1 of phosphorus was beneficial to the efficient growth of M. hupehensis seedlings, nitrogen absorption and utilization.