Effects of 3-year organic farming management on soil antibiotic resistant genes and virulence factors in a double rice cropping system.

Investigating the antibiotic resistance genes (ARGs) and virulence factors (VFs) within soil microbial communities is crucial for understanding microbial ecology and the evolution of antibiotic resistance. However, the study of ARGs, VFs, and their predominant microbial hosts in soils under varying rice production management practices remains largely underexplored. To this end, a three-year field experiment was conducted under organic management within a double rice cropping system in South China. The study revealed that, in contrast to conventional management (CK), organic farming practices did not significantly alter the total reads of ARGs and VFs. However, there was a notable alteration in the ARGs abundance at the antibiotic class level, such as an increase (P < 0.05) in the abundance of Multidrug ARGs (by 1.7 %) and a decrease (P < 0.05) in Rifamycin (by 17.5 %) and Fosfomycin ARGs (by 15.3 %). Furthermore, a significant shift in VFs was observed under organic farming compared to CK, characterized by an increase (P < 0.05) in offensive VFs and a decrease (P < 0.05) in nonspecific VFs and the regulation of virulence-associated genes. Key microbial taxa identified as influencing ARGs and VFs in the tested soil samples, e.g., Proteobacteria. The findings highlight the need for more detailed attention to soil ecology within organic rice production systems in South China, particularly concerning the significant alterations observed in ARGs and VFs.

Effects of contrasting tillage managements on the vertical distribution of plant- and microbial-derived carbon in rice paddy.

Soil microbial necromass is considered a persistent component of soil organic carbon (SOC), constituting the final product of the microbial carbon pump (MCP). However, the mechanisms involved in the effects of tillage and rice residue managements on the vertical distribution of microbial necromass and plant residues in rice paddy soils remain unclear, limiting knowledge of SOC sequestration mechanisms. Therefore, we estimated microbial- and plant-derived C by biomarker amino sugars (AS) and lignin phenols (VSC) at the 0-30 cm soil depth, as well as their relationships with SOC contents and mineralization in a rice paddy soil under contrasting tillage practices, namely no-tillage (NT), reduced tillage (RT), and conventional tillage (CT). The results showed that the SOC contents in the rice paddy soil were positively correlated with soil AS and VSC contents. The NT resulted in significantly higher (P < 0.05) AS (expressed as per kilogram soil) at the 0-10 cm and 10-30 cm soil depths by 45-48 % than RT and CT. However, microbial-derived C contents and SOC mineralization were not significantly changed by NT. In contrast, the plant-derived C contents in the total SOC decreased significantly under the NT scenario, suggesting the consumption of plant-derived C even with more rice residue inputs (at the 0-10 cm soil depth). In summary, 5-year short-term NT management with more rice residue mulch on the soil surface in rice paddy maintained a low plant-derived C content (at a sampling date before rice transplanting), suggesting a different mode of C sequestration, except for the protection of plant-derived C under anaerobic conditions.

A 40 % paddy surface soil organic carbon increase after 5-year no-tillage is linked with shifts in soil bacterial composition and functions.

Soil organic carbon (SOC) is related to soil fertility, crop yield, and climate change mitigation. Paddy soil is a significant carbon (C) sink, but its C sequestration potential has not been realized as the various driving factors are still not fully understood. We performed a 5-year paddy field experiment in southern China to estimate tillage effects on SOC accumulation and its relation with soil bacteria. The C input from rice residue, SOC content, CO2 flux, soil bacterial community composition, and predicted functions were analyzed. No-tillage (NT) increased (p < 0.05) rice residue C inputs (by 12.6 %-15.9 %), SOC (by 40 % at the surface soil layer compared with conventional tillage, CT), and CO2 fluxes compared with reduced tillage (RT) and CT. Also, NT significantly altered the soil bacterial community. The random forest model showed that the predicted bacterial functions of "Degradation/Utilization/Assimilation Other", "C1 Compound Assimilation", and "Amin and Polyamine Degradation" were the most important functions associated with SOC accumulation. Analysis of metabolic pathway differences indicated that NT significantly decreased the BENZCOA-PWY (anaerobic aromatic compound degradation) and the AST-PWY (L-arginine degradation II). Therefore, the rapid paddy SOC increase is associated with both residue C input (from higher rice yields) and the degradation functions regulated by soil bacteria.

Impacts of climate change on climatic division for double cropping rice in Guangdong Pro-vince, China.

Based on the dataset of air temperature from 86 stations during 1961 to 2016, and DEM data derived from 1:250000 topographic maps, we analyzed the spatial-temporal changes of key climatic factors (safe period for double cropping rice, ≥10 ℃ active accumulated temperature) using the methods of linear regression, accumulative anomaly, and inverse distance weighted interpolation. The impacts of climate change on climatic division for double cropping rice was studied by combining with the changes of key climate factors for the periods of 1961-1990, 1971-2000, 1981-2010, before (1961-1997) and after (1998-2016) climatic mutation. The results showed that spatial distributions of safe period for double cropping rice and ≥10 ℃ active accumulated temperature were different from place to place in Guangdong. Generally, they were more or higher in south than in north part, in plain than in mountain regions. Under the background of global warming, the safe period for double cropping rice and ≥10 ℃ active accumulated tempera-ture showed a significantly increasing trend with the rate of 1.7 d and 43 ℃·d per decade, respectively. All climatic factor had mutation in the year of 1997. According to the climatic conditions of various maturing rice growth and development, climatic zoning for double cropping rice in Guangdong could be divided into three regions: early maturity with early maturity, early maturity with late maturity, late maturity with late maturity. Those regions were distributed in middle-subtropics of northern Guangdong, south-subtropics of middle Guangdong, and north-tropics of southern Guangdong, respectively. Due to the climate change, the area of late maturity with late maturity significantly increased, the area of early maturity with late maturity significantly decreased, and the area of early maturity with early maturity showed no significant change. Relative to that in 1961-1990, the areas of late maturity with late maturity in 1971-2000 and 1981-2010 increased 1.22×106 hm2 and 2.56×106 hm2, respectively, but the area of early maturity with late maturity decreased 1.13×106 hm2 and 2.56×106 hm2, respectively. The area of late maturity with late maturity was more than doubled, but that of early maturity with late maturity decreased about a half after 1997 than that before in Guangdong.