**3. Characteristics of paddy soils**

Paddy soils are mostly alluvial soils and low humic gley soils (or Entisols and Inceptisols). In addition, vertisols, reddish-brown earths or Alfisols, red-yellow podzolic soils or Ultisols, and latosols or Oxisols are utilized for paddy rice cultivation. Paddy soils are found mainly on alluvial lands such as deltas and flood plains of big rivers, coastal plains, fans, and lower terraces. In general, paddy soils are resistant to erosion when they are terraced and there are ridges around the field, as measures to retain surface water. Paddy fields in the lowlands receive new sediments deposited from run-off that carries eroded topsoil down from the uplands, thus sustaining soil fertility and productivity. The paddy soils have medium to high organic matter (1.5–3.97 g/kg), available phosphorus (11.7–19.9 mg/kg), available potassium (61.6–132.9 mg/kg), and cation exchange capacity (15.5–33.1 cmol/kg). The most common practice in paddy rice cultivation is flooding or temporary water logging of the land surface. Soil redox potential (Eh) or electron activity in soil gradually decreases after flooding, which causes significant methane production at around −200 mV [16], and creates high risk of gaseous N losses through denitrification (**Figure 2**).

**Figure 2.** Paddy soil redox status, sequential reduction and oxidation of inorganic nitrogen, manganese, and iron in flooded soil, and methane gas formation [16].

In paddy fields, the kinetics of the reduction processes are strongly affected by the composition and texture of soil and its content of inorganic electron acceptors. After flooding, microbial reduction processes sequentially use NO3 <sup>−</sup>, Mn4+, Fe3+, and SO<sup>4</sup> <sup>2</sup>− as electron acceptors, accompanied by the emission of the trace gases N<sup>2</sup> O, N<sup>2</sup> , H<sup>2</sup> S, and CH<sup>4</sup> due to reduction-induced increasing pH-NH<sup>3</sup> (**Figure 2**). Microorganisms drive redox reactions in soil by using organic carbon and electron acceptors for their metabolic activities. Methane is produced at the terminal step under anaerobic decomposition of organic matter and due to the reduction of CO<sup>2</sup> into CH4 in wetland soils. Soil Eh values decreased rapidly after flooding within 5–7 weeks then stabilized toward −200 to −240 mV and produced significant amount of methane [17]. High concentrations and fluxes of dissolved organic matter (DOM) in paddy soils from plant debris trigger microbial activity and thus the emission of greenhouse gases. Therefore, the objectives of this thematic topic are to highlight the feasible field management practices for sustainable rice production and recommend appropriate strategies to mitigate GHG emissions from paddy soils in the changing climate.
