Carbon and nitrogen footprints of apple orchards in China's Loess Plateau under different fertilization regimes

To comprehensively understand the effects of different fertilization regimes on carbon sequestration and greenhouse gas (GHG) emissions in apple orchard production systems, we assessed the carbon (C) emissions and nitrogen (N) losses along with the life cycle assessment (LCA) to quantify the carbon footprint (CF) and nitrogen footprint (NF) under different fertilization regimes in the Loess Plateau, China. The field experiment was comprising four N fertilizer regimes such as i) single inorganic fertilizer with 800 kg N ha-1 (N800); ii) single inorganic fertilizer with 400 kg N ha-1 (N400); iii) inorganic and organic fertilizers each with 200 kg N ha-1 (N200+O200) as well as without N fertilizer treatment as control (CK). Data regarding the soil organic carbon (SOC), nitrate N (NO3--N), ammonium N (NH4+-N) and emissions of carbon oxide (CO2) and nitrous oxide (N2O) were measured. Organic fertilizer substitution (OFS) boosted SOC while decreased NO3 --N and NH4+-N concentrations. The cumulative CO2 emissions and emission factors (EFs) of N2O did not change significantly amongst fertilization strategies; however, as compared to N800, N400 and N200+O200 significantly reduced N2O emissions by 41.8%-45.7% and 43.5%-46.6% respectively. The NF was dominated by N residues (73.0%- 97.4%) and organic fertilizer (24.37%), with a significant difference detected between N400 and N200+N200 treatments. Meanwhile, compared to inorganic plots, OFS significantly reduced CF (345.9%-368.0%), which was influenced by SOC sequestration. The CF from N plots in apple orchard was dominated by fertilizer (21.7%- 39.6%), pesticides (4.3%-6.2%) and N2O (5.6%-11.8%). In addition, our results were further supported by a meta-analysis with 22 published studies, which showed that organic fertilizer and chemical fertilizer + organic fertilizer significantly reduced regional CF by 388.8% and 78.2%, respectively. However, regional NF ranged from 441.4 kg N-eq ha-1 to 453.9 kg N-eq ha-1, with no significant difference between fertilizer strategies. Therefore, reducing the amount of N fertilizer and partial OFS is a viable long-term strategy for mitigating climate change through C sequestration, reduction of N residues, and GHG mitigation, which could be beneficial to low C apple production in the Loess Plateau, China.