Soil N₂O Emissions under Different N Rates in an Oil Palm Plantation on Tropical Peatland

• Main Authors: Auldry Chaddy, Lulie Melling, Kiwamu Ishikura, Ryusuke Hatano
• Format: Article
• Language: English
• Published: MDPI AG 2019-10-01
• Series: Agriculture
• Subjects: ground water level (gwl); water-filled pore space (wfps); oil palm yield; nitrification; denitrification
• Online Access: https://www.mdpi.com/2077-0472/9/10/213

(1) Background: Nitrogen (N) fertilization on drained tropical peatland will likely stimulate peat decomposition and mineralization, enhancing N₂O emission from the peat soil. (2) Methods: A field experiment was conducted to quantify the N₂O emissions from soil in an oil palm plantation (<i>Elaeis guineensis</i> Jacq.) located in a tropical peatland in Sarawak, Malaysia, under different rates of N fertilizers. The study was conducted from January 2010 to December 2013 and resumed from January 2016 to December 2017. Nitrous oxide (N₂O) flux was measured every month using a closed chamber method for four different N rates; control&#8212;without N (T1), 31.1 kg N ha^&#8722;1 yr^&#8722;1 (T2), 62.2 kg N ha^&#8722;1 yr^&#8722;1 (T3), and 124.3 kg N ha^&#8722;1 yr^&#8722;1 (T4); (3) Results: Application of the N fertilizer significantly increased annual cumulative N₂O emissions for T4 only in the years 2010 (<i>p</i> = 0.017), 2011 (<i>p</i> = 0.012), 2012 (<i>p</i> = 0.007), and 2016 (<i>p</i> = 0.048). The highest average annual cumulative N₂O emissions were recorded for T4 (41.5 &#177; 28.7 kg N ha^&#8722;1 yr^&#8722;1), followed by T3 (35.1 &#177; 25.7 kg N ha^&#8722;1 yr^&#8722;1), T1 (25.2 &#177; 17.8 kg N ha^&#8722;1 yr^&#8722;1), and T2 (25.1 &#177; 15.4 kg N ha^&#8722;1 yr^&#8722;1), indicating that the N rates of 62.2 kg N ha^&#8722;1 yr^&#8722;1 and 124.3 kg N ha^&#8722;1 yr^&#8722;1 increased the average annual cumulative N₂O emissions by 39% and 65%, respectively, as compared to the control. The N fertilization had no significant effect on annual oil palm yield (<i>p</i> = 0.994). Alternating between low (deeper than &#8722;60 cm) and high groundwater level (GWL) (shallower than &#8722;60 cm) enhanced nitrification during low GWL, further supplying NO₃^&#8722; for denitrification in the high GWL, and contributing to higher N₂O emissions in high GWL. The emissions of N₂O ranged from 17 &#181;g N m^&#8722;2 hr^&#8722;1 to 2447 &#181;g N m^&#8722;2 hr^&#8722;1 and decreased when the water-filled pore space (WFPS) was between 70% and 96%, suggesting the occurrence of complete denitrification. A positive correlation between N₂O emissions and NO₃^&#8722; at 70&#8722;96% WFPS indicated that denitrification increased with increased NO₃^&#8722; availability. Based on their standardized regression coefficients, the effect of GWL on N₂O emissions increased with increased N rate (<i>p</i> &lt; 0.001). Furthermore, it was found that annual oil palm yields negatively correlated with annual N₂O emission and NO₃^&#8722; for all treatments. Both nitrification and denitrification increased with increased N availability, making both processes important sources of N₂O in oil palm cultivation on tropical peatland.; and (4) Conclusions: To improve understanding of N₂O mitigation strategies, further studies should consider plant N uptake on N₂O emissions, at least until the completion of the planting.