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Purpose: Physico-chemical mechanisms of soil organic carbon (SOC) protection in volcanic ash soils (Andisols) are well understood. However, this role of microbes in the process is unclear. This study was conducted to understand the microbial contribution to SOC accumulation in Andisols, in comparison with non-volcanic ash soils (Inceptisols). Methods: Soils were collected from agricultural land, coniferous forest, and deciduous forest (n = 7 per land-use for Andisol, n = 3 for Inceptisols). Soils were analyzed for SOC and physico-chemical properties related to SOC protection (clay content, aggregate size distribution, pH, and active Al and Fe concentration). Soil microbial biomass C (MBC), composition, respiration, and metabolic quotient (qCO2) were further analyzed. Results: Forest Andisols had greater SOC compared to agricultural Andisols and Inceptisols from all land-use types. The greater SOC content in forest Andisols was associated with the complexation of SOC with clay and active Al and Fe. The physico-chemical protection of SOC in forest Andisols was further related to lower percentage of total SOC respired as CO2 (% RCO2/SOC) and qCO2, which indicates that microbes assimilate more C than they respire. Microbial composition as well as the inter-correlation among qCO2, RCO2/SOC, and SOC content suggested that microbes in forest Andisols are likely to incorporate C derived from the decomposition of plant-derived C rather than indigenous SOC, contributing to SOC accumulation in the soils. Conclusions: Our study suggests that microbial contribution is critical for enhanced SOC in forest Andisols in addition to the inherent soil physico-chemical properties, but not in agricultural Andisols. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 NH 4 + 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. • Fly ash (FA) and zeolite (Z) reduced rice uptake of Cu and Zn in polluted soils. • FA and Z also reduced N and P availability via sorption and precipitation. • FA and Z decreased rice growth due to the nutrient limitation. • Both nutrient and metal immobilization by FA and Z should be considered. [ABSTRACT FROM AUTHOR]

Synthetic fertilizer, livestock manure, and green manure are the typical nitrogen (N) sources in agriculture. This study was conducted to investigate the effects of different N sources on soil chemical environment and N dynamics. Changes in pH, redox potential (Eh), and concentration and δ15N of dissolved N [ammonium (NH4+), nitrate (NO3−), organic N, and total N] of soils treated with urea (U), pig manure compost (PMC), and hairy vetch (HV) were investigated in an incubation experiment under waterlogged conditions. The patterns of pH, Eh, and N concentration reflected both a greater mineralization potential of N derived from U than that from HV and PMC and easier decomposability of HV than PMC. The δ15N further suggested that nitrification was more active for U than for HV- and PMC-treated soils and that N loss via NH3volatilization and denitrification would be greater for HV than U and PMC treatments. [ABSTRACT FROM PUBLISHER]

Global meta-analyses showed that biochar application can reduce N 2 O 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 N 2 O 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 N 2 O mitigation was evaluated by calculating N 2 O reduction index (R index , percentage reduction of N 2 O 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 N 2 O 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 N 2 O 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. • Nitrous oxide reduction (Rindex) of biochar (BC) was affected by experimental types. • Rindex was greater at a higher BC rate and in a shorter experimental duration. • Biochar feedstock and pyrolysis temperature affected Rindex. • Soil properties and fertilizer types rarely affected Rindex. • Unit Rindex per BC rate was rarely affected by biochar, soil, and fertilizer types. N 2 O reduction by biochar was dependent on its application rate and affected by experimental types, biochar types, and soil moisture conditions, but biochar-induced N 2 O reduction per unit biochar application rate was rarely affected by these factors in East Asia. [ABSTRACT FROM AUTHOR]