This study explores the complex relationship between soil electricity generating capacity, bacterial community dynamics, and soil chemical and physical properties across diverse regions of Japan. First, soil samples were systematically collected and analyzed. Subsequent investigations evaluated soil microbial biomass carbon, dissolved organic carbon (DOC), and total dissolvable iron (DFeT) concentrations. In the experiments, soil samples underwent a rigorous 60-day microbial fuel cell trial, wherein power density and total energy output were measured. Significant variations in power density were observed among different soil samples; specifically, a sugarcane field designated as Okinawa-3 and a peach orchard soil as Nagano-2 demonstrated relatively high total energy output. Analysis of soil bacterial community structures identified some families which showed positive correlations with increased electricity generation capabilities. Correlation analyses revealed associations between these bacterial communities and key soil parameters, particularly with DOC and DFeT concentrations. Redundancy analysis revealed intricate connections between soil properties and electricity generation capacities. Particularly noteworthy was the positive correlation between Acidobacteriaceae and DOC, as well that between Sphingomonadaceae and electricity generation, highlighting the crucial roles of soil microbial communities and chemical compositions in driving electricity generation processes.

Biochar combined with other applications has gained attention to improve crop productivity, however, the effect of biochar application on soil nitrogen (N) dynamics and sorghum productivity in dry tropics remains underexplored. A field experiment was conducted on sorghum cultivation in the cropland of south India to evaluate the effect of biochar (15 Mg ha –1 ) and biochar combined with chemical fertilizer (CF) or farmyard manure (FYM) application (100 kg N ha −1 yr −–1 ) on soil N dynamics (inorganic N and microbial biomass N [MBN]), soil moisture, and crop productivity (grain yield, total N uptake, and nitrogen use efficiency [NUE]). Biochar sole application improved the soil moisture condition for approximately 3 years, though did not increase soil N dynamics and crop productivity. Grain yield and total N uptake in biochar with CF treatment were larger than that in control in the rainy years but were not different from that in the CF treatment. CF application increased the soil inorganic N until the flowering stage but did not affect MBN. FYM application increased MBN and inorganic N only during the early growth stage, resulting in no clear improvement in crop productivity. Considering the highest NUE for 3 years in biochar with CF treatment, our results indicate that both improved soil water condition by biochar application and better inorganic N availability until the flowering stage through N application would be required to improve crop productivity and consequently, NUE, in the nutrient-poor soils of south India.

Biochar application is currently considered to be an effective soil organic carbon (SOC) management to prevent land degradation by enhancing SOC stock. However, quantitative information on the impact of biochar application on carbon dioxide (CO2) flux and associated microbial responses is still scarce, especially in degraded tropical agroecosystems. Here, we evaluated the impact of land management (control (C), biochar (B; 8.2 Mg C ha−1), farmyard manure (FYM) (M; 1.1 Mg C ha−1 yr−1), and a mixture of both (BM; 8.2 Mg biochar-C ha−1 and 1.1 Mg FYM-C ha−1 yr−1)) on CO2 flux, SOC stock, microbial biomass C (MBC), and metabolic quotient (qCO2) in degraded tropical alkaline cropland of southern India, based on a 27-month field experiment. Cumulative CO2 flux over the experiment was 2.4, 2.7, 4.0, and 3.7 Mg C ha−1 in the C, B, M, and BM treatments, respectively. Biochar application increased soil moisture and SOC stock, though did not affect CO2 flux, MBC, and qCO2, indicating the limited response of microbes to increased soil moisture because of small amount of SOC. Combined application of biochar and FYM did not increase CO2 flux compared with FYM alone, due to little difference of microbial responses between the M and BM treatments. Additionally, SOC increment (8.9 Mg C ha−1) and the rate of C-input retention in soil (0.78) was most significant in the BM treatment. Hence, the combined application of biochar and FYM could be sustainable land management by efficient increase of SOC stock in the tropical degraded cropland.

Deforestation of native tropical forests has occurred extensively over several decades. The plantation of fast-growing trees, such as Acacia spp., is expanding rapidly in tropical regions, which can contribute to conserve the remaining native tropical forests. To better understand belowground biogeochemical cycles and the sustainable productivity of acacia plantations, we assessed the effects of vegetation (acacia plantations vs. native forests) and soil types (Oxisols vs. Ultisols) on soil properties, including the diversity and community structures of bacteria- and fungi-colonizing surface and subsurface roots and soil in the Central Highlands of Vietnam. The results in surface soil showed that pH was significantly higher in acacia than in native for Oxisols but not for Ultisols, while exchangeable Al was significantly lower in acacia than in native for Ultisols but not for Oxisols. Bacterial alpha diversity (especially within phylum Chloroflexi) was higher in acacia than in native only for Oxisols but not for Ultisols, which was the same statistical result as soil pH but not exchangeable Al. These results suggest that soil pH, but not exchangeable Al, can be the critical factor to determine bacterial diversity. Acacia tree roots supported greater proportions of copiotrophic bacteria, which may support lower contents of soil inorganic N, compared with native tree roots for both Oxisols and Ultisols. Acacia tree roots also supported greater proportions of plant pathogenic Mycoleptodiscus sp. but appeared to reduce the abundances and diversity of beneficial ECM fungi compared with native tree roots regardless of soil types. Such changes in fungal community structures may threaten the sustainable productivity of acacia plantations in the future.

An understanding of the mechanisms of soil organic carbon (SOC) stabilization is essential to develop the appropriate management for C sequestration and soil health. In southern India, where neutral-alkaline soils are mainly distributed, soil C stocks are inherently low in cropland, despite relatively high clay contents (Clay>ca. 30%, OC-1 soil). To consider this reason of low SOC in this area, we evaluated the fractionated C contents and its controlling factors, by measuring the particulate organic matter (POM). The objective of this study was to evaluate the effect of land management on the amount and composition of each fraction of soil in southern India. We collected the surface soils (0-10 cm) from two representative sites of southern India; Vertisols with alkaline soil pH (8.4-8.8) and Alfisols with neutral soil pH (6.0-7.0). At each site, two different land management were selected; forest and cropland of Vertisols, and cropland with no organic matter application (no-OM) and with manure application (with-OM) of Alfisols. Soils were separated into the four fractions; (1) Light Fraction; LF (<1.7 g cm-3) , (2) Coarse POM; cPOM (>1.7 g cm-3, 250-2000 µm), (3) Fine POM; fPOM(>1.7 g cm-3, 53-250 µm), and (4) Silt+Clay; S+C (>1.7 g cm-3, <53 µm). Each fraction was analyzed by elemental analysis (C, N) and CPMAS 13C NMR spectroscopy. In Vertisols, C contents of cPOM, fPOM, S+C were significantly higher in forest (0.65, 0.91, 4.8 g kg-1 soil, respectively) than those of cropland (0.17, 0.22, 4.1 g kg-1 soil, respectively), causing the higher total SOC in forest (7.8 g kg-1 soil) than in cropland (4.5 g kg-1 soil). C concentration of cPOM, fPOM, and S+C fractions were also significantly higher in forest (3.7, 7.6, 6.7 g kg-1 fraction, respectively) than those of cropland (1.0, 2.7, 5.4 g kg-1 fraction, respectively). In particular, increasing rates in cPOM and fPOM (180-280 %) were greater than S+C (24 %), possibly suggesting that forest management should increase the relatively active and intermediate SOC pools through the C accumulation in cPOM and fPOM fractions of Vertisols. In Alfisols, C contents in LF and S+C were significantly higher in with-OM (1.1 and 5.2 g kg-1 soil, respectively) than in no-OM (0.76 and 4.7 g kg-1 soil, respectively). C concentration of S+C fraction was significantly higher in with-OM (14 g kg-1 fraction) than in no-OM (11 g kg-1 fraction), but not of cPOM and fPOM fractions. It suggests that the OM application to cropland should increase the slow SOC pool through the C accumulation in S+C fractions of Alfisols. These results indicate that different fraction may contribute to SOC stabilization between Vertisols and Alfisols in southern India.

Soils in the dry tropical croplands of south India are inherently low in soil carbon (C) stock, and it is essential to accumulate the soil C for sustainable soil management. Biochar is generally considered to be a useful material that enhance the soil C stock, though its real effect on soil C dynamics is still unclear especially in the dry tropical croplands such as south India. Thus, our objective was to evaluate the effect of biochar application on soil C dynamics for optimal soil management in south India. Field experiment was conducted in Tamil Nadu state (Inceptisols) from Sep. 2017 to Apr. 2019 (1.5 years), which include two times sorghum cultivation (each 4 months) with six treatment plots (control (C), biochar (B) (8.2 Mg C ha-1), farmyard manure (FYM) (F) (1.1 Mg C ha-1), chemical fertilizer (CF) (100 kg N; 40 kg P ha-1), biochar and FYM (B+F), and biochar and chemical fertilizer (B+CF)). We applied biochar once at the beginning of the experiment to evaluate the effective duration of biochar in soil after application, while we applied FYM every year before crop cultivation. We periodically measured the CO2 efflux rate (29 times totally) with continuous environmental data including soil moisture (0-15 cm) and soil temperature (5 cm), and estimated the total CO2 flux as C output, based on the relationship between the CO2 efflux rate and environmental data. We found that the CO2 efflux rate in the B+F plot tended to be lower than the F plot throughout the experimental period, though the significant difference between the B+F plot and F plot was only in the cultivation period of the 1st year, in case of using the analysis of variance for each cultivation period separately. We found that cumulative CO2 flux in the B+F plot (2.2 Mg C ha-1 1.5 year-1) was also lower than the F plot (2.5 Mg C ha-1 1.5 year-1), and that biochar and FYM application decreased ca. 0.3 Mg C ha-1 1.5 year-1 decomposition compared to the application of FYM alone. This might be because combined application of biochar and FYM decreased the soil microbial activity, resulting in the lower FYM decomposition in the B+F plot. Our results indicate that biochar combined with FYM application would effective for soil C sequestration, and hence for sustainable soil management in the dry tropical cropland.

Soils in tropical croplands are becoming degraded because of soil carbon (C) depletion. Local farmers in South India use a specific management of traditional cultivation, i.e., broadcast seeding. However, for sustainable C management, there is no quantitative data on the CO2 flux under this management. Our objectives were to (1) estimate the annual CO2 flux, and (2) evaluate the effect of traditional cultivation management (seeding rate) on the CO2 flux. Our field experiment was conducted in South India, from 2015 to 2017, including two cultivation periods with four cultivation management treatments (traditional cultivation management plot (T), fixed density plot (FD), no thinning plot (NT), and bare plot (B)). The seeding rate in the FD plot was ca. 50% of the T plot. We applied 1.1 Mg C ha−1 farmyard manure just before the experiment as a C input. We found that broadcasting, thinning, and cultivation increased soil moisture, while the CO2 efflux rate showed no significant difference between treatments throughout the experimental period. This indicates that cultivation management did not affect the CO2 flux. The total CO2 fluxes for two years were estimated at 2.2–2.7 Mg C ha−1. Our results indicate that it is necessary to apply larger or more frequent C inputs to prevent C depletion.