Climate, duration, and N placement determine N 2 O emissions in reduced tillage systems: a meta‐analysis

No‐tillage and reduced tillage ( NT / RT ) management practices are being promoted in agroecosystems to reduce erosion, sequester additional soil C and reduce production costs. The impact of NT / RT on N 2 O emissions, however, has been variable with both increases and decreases in emissions reported. Herein, we quantitatively synthesize studies on the short‐ and long‐term impact of NT / RT on N 2 O emissions in humid and dry climatic zones with emissions expressed on both an area‐ and crop yield‐scaled basis. A meta‐analysis was conducted on 239 direct comparisons between conventional tillage ( CT ) and NT / RT . In contrast to earlier studies, averaged across all comparisons, NT / RT did not alter N 2 O emissions compared with CT . However, NT / RT significantly reduced N 2 O emissions in experiments >10 years, especially in dry climates. No significant correlation was found between soil texture and the effect of NT / RT on N 2 O emissions. When fertilizer‐N was placed at ≥5 cm depth, NT / RT significantly reduced area‐scaled N 2 O emissions, in particular under humid climatic conditions. Compared to CT under dry climatic conditions, yield‐scaled N 2 O increased significantly (57%) when NT / RT was implemented <10 years, but decreased significantly (27%) after ≥10 years of NT / RT . There was a significant decrease in yield‐scaled N 2 O emissions in humid climates when fertilizer‐N was placed at ≥5 cm depth. Therefore, in humid climates, deep placement of fertilizer‐N is recommended when implementing NT / RT . In addition, NT / RT practices need to be sustained for a prolonged time, particularly in dry climates, to become an effective mitigation strategy for reducing N 2 O emissions.