Soil organic carbon (SOC) is crucial for ecosystem health and climate regulation. However, agricultural practices have significantly depleted SOC levels globally. Conservation agriculture (CA), characterized by no-till farming and residue retention, offers potential for SOC accrual. Climate change, particularly warming, introduces a significant challenge, as increased temperatures can stimulate microbial decomposition and potentially offset SOC gains from CA. This study aimed to address the limited understanding of the interactive effects of CA and warming on long-term SOC dynamics. The research question focused on how warming differentially affects SOC accrual under CA versus conventional agriculture (CONV) and how this relates to shifts in microbial community structure and function over a decade. Hypotheses posited that CA would increase SOC through increased carbon inputs and enhanced microbial efficiency, leading to greater microbial necromass accumulation, particularly under warming conditions. The study used a 10-year field experiment comparing CA and CONV under ambient and warmed conditions, measuring SOC, microbial communities (DNA sequencing and metagenomics), physiological traits (CUE, necromass), and functional genes. The significance lies in determining if CA can maintain or enhance SOC sequestration under future climate scenarios, informing sustainable agricultural practices and climate change mitigation strategies.

Previous research highlights the potential of conservation agriculture to increase soil organic carbon (SOC) levels. However, the interaction between conservation agriculture and warming remains poorly understood. Studies have shown that warming can stimulate microbial respiration, potentially leading to SOC loss, particularly in soils with high organic carbon content. While conservation agriculture practices aim to increase SOC by enhancing carbon input through residue retention and reducing soil disturbance, the response of this increased SOC to warming remains uncertain. Microbial carbon use efficiency (CUE), a key factor in SOC dynamics, is affected by both temperature and management. Warming can decrease CUE, while conservation practices can potentially increase it through improved resource availability and reduced energy expenditure on nutrient acquisition. The role of microbial community composition, particularly the fungal-to-bacterial ratio, in mediating the effects of warming and management on SOC remains an area needing further investigation. Existing research lacks long-term studies examining the interactive effects of CA, CONV, and warming across the microbial spectrum; hence this study fills an important knowledge gap.

A long-term field experiment was conducted at the Yucheng Comprehensive Experiment Station in North China using a wheat-maize cropping system. The experiment incorporated a randomized complete block design with four treatments: conservation agriculture (CA) under ambient and warmed conditions, and conventional agriculture (CONV) under ambient and warmed conditions. Warming was implemented using infrared heaters, maintaining soil temperatures 2°C above ambient levels. Soil samples were collected every two years from 2010 to 2020. SOC, total nitrogen (TN), and dissolved organic carbon (DOC) were measured using standard laboratory methods. Microbial communities were analyzed using high-throughput sequencing of 16S rRNA and ITS regions, providing bacterial and fungal community profiles. Metagenomic sequencing was used to assess functional gene abundances. Microbial carbon use efficiency (CUE) was determined using the 18O-H2O tracer method, measuring CO2 production and isotopic composition. Microbial necromass carbon was estimated using amino sugar biomarkers (glucosamine, galactosamine, and muramic acid). Root biomass and root exudation carbon input were measured using a modified culture-based cuvette system. Statistical analyses included mixed models for repeated measures ANOVA, linear regressions, Cohen’s d effect sizes, principal component analysis, non-parametric multivariate statistical tests (ADONIS, ANOSIM, MRPP), linear mixed-effect models, and structural equation modeling (SEM).

The 10-year experiment revealed that conservation agriculture significantly increased SOC content compared to conventional agriculture, regardless of warming treatment. Warming further increased SOC content under CA by 3.1% compared to the ambient control, but not under CONV. The positive effect of warming on SOC under CA was stronger over time, significantly accelerating after the fifth year. CA moderated the warming effects on soil temperature and moisture, creating cooler and wetter conditions compared to CONV. Warming increased aboveground biomass under CA, but not under CONV. Both root biomass and root exudation carbon increased with warming in both CA and CONV. Under CA, warming increased microbial CUE, growth, and carbon uptake by 12%, 43%, and 24%, respectively, with the differences between warmed and ambient conditions increasing over time. Total microbial necromass carbon increased by 77% with warming in CA, driven mainly by an increase in fungal necromass. Fungal necromass carbon emerged as the strongest predictor of SOC content in the structural equation model (SEM), accounting for 36% of total SOC and with ~86% of fungal origin. Warming accelerated fungal community temporal turnover under CA, with faster turnover rates compared to bacteria. The differences in microbial community structure between warmed and ambient plots increased linearly over time only under CA, and this effect was more pronounced for fungi than bacteria. SEM analysis indicated that increased carbon input from plants, mediated through changes in the fungal community, was a key driver of the enhanced microbial CUE and necromass accumulation, ultimately leading to increased SOC under CA and warming.

The results demonstrate that conservation agriculture enhances soil carbon sequestration under warming conditions. The increased SOC accrual under CA with warming is attributed to several interacting factors. The continuous soil cover from crop residues in CA mitigated the negative impacts of warming on soil temperature and moisture, providing a more favorable environment for microbial activity. The increase in plant carbon input, encompassing aboveground and belowground components, provided abundant substrates for microbial growth and activity, resulting in higher microbial CUE. The shift towards a greater proportion of fungal biomass, driven by increased resource availability and less soil disturbance, was crucial for enhancing the microbial carbon pump, leading to the accumulation of fungal necromass and increased SOC. The accelerated fungal community turnover under CA with warming highlights the dynamic nature of microbial communities in response to environmental changes and their role in shaping SOC dynamics. The study's findings support the growing understanding of the critical role of microbial communities and their physiology in SOC stabilization and climate change mitigation.

This decade-long field experiment provides compelling evidence that conservation agriculture can significantly enhance soil organic carbon (SOC) accumulation under warming conditions. Increased plant carbon input, mediated by shifts in fungal community structure and function, plays a crucial role in driving higher microbial carbon use efficiency and necromass accumulation. Fungal necromass is identified as a key driver of stable SOC accrual. The findings highlight the importance of considering microbial processes in developing effective climate-smart agricultural strategies. Future research should investigate the generalizability of these findings across diverse agro-ecosystems and climatic regions, particularly focusing on the impact of water availability. Further investigation into the specific functional roles of dominant fungal taxa under these conditions is crucial for refining management practices to optimize SOC sequestration.

The study was conducted at a single location in North China, potentially limiting the generalizability of the findings to other regions with different soil types, climates, and cropping systems. While the study covered a decade, longer-term investigations may reveal additional insights into long-term SOC dynamics. The experimental warming (+2°C) might not fully reflect the complexity of future climate change scenarios, which could include other factors like altered precipitation patterns and extreme weather events. The study's focus on microbial communities and functions provides crucial insights, but other factors influencing SOC dynamics, such as soil physical properties and chemical interactions, warrant further investigation.