Effects of enriched CO₂, temperature, and irrigation on soil properties in greenhouse apple tree cultivation

The significant contributions of anthropogenic activity to global warming are evidenced by the increasing average atmospheric temperatures and CO₂ concentration. To date, data on the impact of global warming on crops, particularly field-scale fruit growth and the soil environment, remain lacking. Therefore, we conducted a 4-year monitoring experiment on semi-closed apple tree populations cultivated under the present or near-future climate to assess the effects of atmospheric temperature and CO₂ concentrations on soil moisture, temperature, and inorganic ion concentration. Apple saplings were grown in three independent environment-controlled greenhouses, namely greenhouses A (GH_A; control), B (GH_B; high-temperature), and C (GH_C; high-temperature and high CO₂), representing climate modification experiments. The pH increased by 0.02, 0.20, and 0.12/year for GH_A, GH_B, and GH_C, respectively. In particular, GH_B (standard irrigation plot; STD) showed a significant pH increase of 0.25/year. The EC increased by 0.014 mS/cm/year for GH_A, decreased by -0.010 mS/cm/year for GH_B, and increased by 0.006 mS/cm/year for GH_C, showing no clear overall increasing or decreasing trend. The NO₃ concentration increased by 1.2 mg/L/year for GH_A, decreased by 10.2 mg/L/year for GH_B and decreased by 0.7 mg/L/year for GH_C. The apparent activation energy (E_a) ranged from 85.3 to 214.4 (kJ/mol NH₄⁺) and, regardless of watering treatment, GH_A > GH_B > GH_C. The values in STD plots were 1.4 to 1.7 times those in abundant irrigation plots. Using GH_A (STD) as a standard, the soil temperature dependence of the nitrification rate did not change overall at high or low air temperatures with increasing watering (E_a = 150.5 kJ/mol) or heating (E_a = 154.1 kJ/mol). The soil temperature dependence of the nitrification rate did not vary under heating and increased watering (E_a = 89.5 kJ/mol) or under heating, increased watering, and increased CO₂ (E_a = 85.3 kJ/mol). This study provides novel insights into determining the environmental impacts of apple tree community soils under near-future climatic conditions.