Fly ash (FA) and zeolite (Z) are known to reduce bioavailability of metals in soils via immobilization; however, these amendments may not only immobilize metals such as copper (Cu) and zinc (Zn), but also reduce nitrogen (N) and phosphorus (P) availability in the soils via sorption (for N by Z) and precipitation (for P by FA). This study was conducted to evaluate the effects of application of FA and Z (0, 5, and 10% rate) on the availability of nutrients as well as metals in the metal-contaminated soils cultivated with paddy rice (Oryza sativa L.). Both FA and Z reduced Cu and Zn uptake by rice while increasing metal retention in the soils regardless of the application rates. However, reduced uptake of metals did not translate into increase in rice growth, especially at the 10% of amendment rates, due to decreased nutrient uptake as indicated by higher NH4+ and available P concentration in the soils amended with Z and FA, respectively, which inhibited tillering in the early rice growth period and thus reduced biomass accumulation at maturity. Our results suggest that FA and Z may reduce Cu and Zn uptake by rice in the soils contaminated with the metals; however, the availability of N and P is likely to be co-decreased. We suggest that the capacities of FA and Z to immobilize nutrients as well as metals need to be considered prior to using the amendments in metal-contaminated rice paddies.

Global meta-analyses showed that biochar application can reduce N₂O emission. However, no relevant review study is available for East Asian countries which are responsible for 70% of gaseous N losses from croplands globally. This review analyzed data of the biochar-induced N₂O mitigation affected by experimental conditions, including experimental types, biochar types and application rates, soil properties, and chemical forms and application rates of N fertilizer for East Asian countries. The magnitude of biochar-induced N₂O mitigation was evaluated by calculating N₂O reduction index (R_index, percentage reduction of N₂O by biochar relative to control). The R_index was further standardized against biochar application rate by calculating R_index per unit of biochar application rate (ton ha^-1) (Unit R_index). The R_index averaged across different experimental types (n = 196) was -21.1 ± 2.4%. Incubation and pot experiments showed greater R_index than column and field experiments due to higher biochar application rate and shorter experiment duration. Feedstock type and pyrolysis temperature also affected R_index; either bamboo feedstock or pyrolysis at > 400 °C resulted in a greater R_index. The magnitude of R_index also increased with increasing biochar rate. Soil properties did not affect R_index when evaluated across all experimental types, but there was an indication that biochar decreased N₂O emission more at a lower soil moisture level in field experiments. The magnitude of R_index increased with increasing N fertilizer rate up to 500-600 kg N ha^-1, but it decreased thereafter. The Unit R_index averaged across experimental types was -1.2 ± 0.9%, and it was rarely affected by experimental type and conditions but diminished with increasing biochar rate. Our results highlight that since N₂O mitigation by biochar is affected by biochar application rate, R_index needs to be carefully evaluated by standardizing against biochar application rate to suggest the best conditions for biochar usage in East Asia.

The properties of secondary salt-affected soils developed from improper irrigation and drainage management and their effects on rice growth and yield are well documented. However, relevant information on coastal reclaimed tideland (RTL) soils, which are classified as primary salt-affected soils developed through salt-accumulated sediments is lacking. In this paper, we reviewed the physical and chemical properties of RTL soils in comparison with non-RTL soils and analyzed the relationship between rice production and soil salinity in RTL to suggest agricultural management practices for sustainable rice production and soil carbon sequestration in RTL. Similar to the secondary salt-affected soils, RTL soils were characterized by high alkalinity, salinity, and sodicity, and rice yield was negatively correlated with salinity. However, it was also found that lower fertility (e.g., organic matter and phosphorus) of RTL soils than non-RTL soils might also hamper rice growth and thus carbon input via plant residues in RTL soils. Correlation between years after reclamation and soil properties of RTL showed that cultivation of rice with annual fertilization and organic matter inputs increased soil fertility but salinity and sodicity did not show a significant tendency of change, suggesting that natural desalinization in RTL soils is hard to be achieved with conventional rice cultivation. Therefore, it is suggested that fertilization management as well as salinity management via drainage, gypsum application, tillage, and proper irrigation may be necessary to improve rice production and carbon sequestration in RTL soils.