Severe nitrogen leaching and marked decline of nitrogen cycle-related genes during the cultivation of apple orchard on barren mountain

Abstract

Limited available lands have led to the widespread practice of establishing orchards on barren mountains to expand apple cultivation in China. However, improper nitrogen (N) management strategies in intensive orchards escalate costs and also contribute to environmental contamination. A long-term field experiment was conducted in a representative apple-producing area in Shandong Province, China, to assess the environmental risks associated with newly established orchards. This research utilized lysimeters and soil metagenomic sequencing to investigate N leaching loss and the functional microbial profiles associated with the N cycle during apple cultivation on barren mountains. Five treatments were implemented: no inorganic N fertilizer, traditional farmer's fertilization, drip irrigation with a water-soluble fertilizer, resin-coated urea, and grass interplanting. Among treatments involving inorganic N, the total N leaching rate ranged from 16.5% to 30.7% of the total N fertilizer input. Notably, in treatments with equivalent N fertilizer applications, the water-soluble fertilizer treatment exhibited the highest N leaching loss rate -(20.2–29.0%), while the grass interplanting and resin-coated urea treatments showed significantly lower rates of N leaching loss (17.0–24.9% and 15.2–24.5%, respectively). The abundance of N cycle-related genes experienced a marked decline (17.1–18.8%) during the cultivation of orchards on barren mountains. The water-soluble fertilizer treatment displayed the highest potential for nitrification, contributing to elevated N leaching. Both resin-coated urea and traditional farmer's fertilization treatments exhibited the highest abundance of genes associated with N2O emission risk to the atmosphere. However, the grass interplanting treatment, while effectively reducing N leaching, also triggered the most pronounced soil acidification. Spearman correlation analysis revealed that the decline in pH, coupled with an increase in soil NO3--N concentration and electrical conductivity (EC), were the primary factors contributing to the reduction in the abundance of key N cycle functions. In summary, excessive use of inorganic N fertilizer in orchard environments led to increased N leaching, altered soil physicochemical properties, and significant impacts on N cycle-related genes. The elevated ionic strength in soils might be responsible for both the high N leaching loss and the decrease in N cycle function abundance. In intensive orchard cultivation, each N fertilization strategy mentioned above comes with its environmental drawbacks. Further investigation is warranted to achieve a balance between productivity and environmental protection.