Yield-scaled greenhouse gas emissions in subtropical paddy rice treated with stabilized nitrogen fertilizers

Abstract

In this work, the hypothesis that stabilized nitrogen fertilizers (SNF) might be useful to boost paddy rice production with a decreased impact on nitrous oxide (N₂O) and methane (CH₄) emissions from soil was tested by conducting a field experiment on a subtropical typic Albaqualf in Southern Brazil over two rice growing seasons (GS). The experiment comprised the nitrogen fertilizers common urea, urea + NBPT [N-(n-butyl)thiophosphoric triamide], urea + NBPT + DCD (dicyandiamide), urea + Cu B (copper and boron), urea + S (sulfur) and urea-10% zeolite alongside a Control treatment with no nitrogen fertilizer. Field plots were arranged according to completely randomized block design with three replicates. Methane and N₂O fluxes were measured by using the static chamber technique and gases analyzed by gas chromatography. Seasonal pGWP values were calculated as the combination of those for the individual gases in each season multiplied by the respective radiative forcing potentials (viz., 28 for CH₄ and 265 for N₂O), and yield-scaled greenhouse gas (GHG) emissions were taken to be the ratios of pGWP to rice grain yields. Soil N₂O emissions in GS1 were 0.9 kg ha^− 1 on average; also, they were similar among treatments and whether or not N fertilization was applied. In GS2, soil N₂O emissions were lower with the Control treatment, all SNF leading to emission levels comparable to those of common urea (average 4.5 kg ha^− 1). There were no differences in soil CH₄ emissions, which peaked at 474 kg ha^− 1, among nitrogen fertilizers in GS1. On the other hand, urea + NBPT reduced CH₄ emissions relative to urea + S and urea-10% zeolite in GS2. pGWP averaged at 11 216 kg eq.CO₂ ha^− 1 across the two GS without N fertilization (Control treatment) and at 11 803 kg eq.CO₂ ha^− 1 with fertilization. The increase in grain yield resulting from nitrogen fertilization was similar irrespective of nitrogen source, with a mean of 9300 kg ha^− 1 in GS1 and one of 8200 kg ha^− 1 in GS2. Yield-scaled pGWP was influenced by fertilization but only in GS1, where urea + NBPT and urea-10% zeolite reduced it relative to no fertilization. The starting hypothesis was thus rejected since SNF affected rice grain yield and soil GHG emissions similarly to common urea over two growing seasons in subtropical paddy rice ecosystems of Southern Brazil.