Organic management increases soil nitrogen but not carbon content in a tropical citrus orchard with pronounced N2O emissions

Abstract

The use of organic amendments is important for the sustainability of organic farming, with implications for soil organic matter turnover, nutrient cycling and greenhouse gases (GHGs) emissions to the atmosphere. Here, we investigated how long-term citrus organic farming influenced carbon sequestration and GHG emissions under organic and conventional management. We assessed the effects of management systems on soil organic matter dynamics and GHG emissions, focusing on N₂O direct emissions from fertilizers. Soil stable isotope C and N compositions (0-100 cm) were used as parameters to assess changes in soil organic matter dynamics, with native forest as the reference. After the conversion from forest to orange orchard, stocks of soil C increased approximately 40 Mg ha^-1, whereas stocks were similar in the organic and conventional treatments. Enrichment of ¹³C through the entire soil profile showed that organic matter from fertilizer replaced the original soil C by at least 20%, considering that poultry was fed only with C4 plants. By contrast, organic farming increased soil N stocks and inorganic N. Nitrogen emission factors for inorganic and organic fertilizers were 1.47 and 3.14, respectively. Organic management increased soil GHG emissions, primarily N₂O emissions. Carbon emissions either as CO₂ or CH₄ were greater at the mid-rows than those under the crop canopy. We conclude that organic management did not promote C sequestration after six years of management. Moreover, organic management increased N₂O emissions, and the GHG balance was more negative for organic than that for conventional farming when the ratio between crop harvest and emissions was determined.

Keywords: Biological farming; Carbon sequestration; Greenhouse gases; Orange orchard; Stable isotopes.