Rapid global warming significantly threatens global food security. The Intergovernmental Panel on Climate Change's Sixth Assessment Report indicates that ongoing warming reduces crop yields and negatively impacts crop biomass, which is crucial for human food and animal feed. With a projected world population of 10 billion by 2050, global food demand is expected to increase substantially, making the decline in crop biomass due to warming an increasingly serious challenge. Most research focuses on the impacts of changes in mean annual temperatures. However, winter warming is more pronounced, particularly in northern mid- and high-latitude areas, increasing at a rate exceeding 0.5 °C per decade—nearly 1.8 times faster than the rise in mean annual temperatures. This accelerated winter warming can reduce winter crop yields by disrupting dormancy, advancing phenology, shortening the growing season, impacting photosynthesis, and exacerbating pests and pathogens. While less studied for non-winter crops, winter warming alters soil temperature and moisture, influencing soil fertility and subsequently crop growth. Soil fertility, particularly soil organic matter (SOM), is fundamental to plant growth, affecting crop biomass by maintaining soil moisture and nutrient availability. Winter warming can stimulate soil respiration, accelerating SOM decomposition. Therefore, understanding the mechanism by which winter warming affects crop biomass through soil processes is crucial.

Existing literature extensively covers the impact of mean annual temperature changes on crop yields. However, research specifically addressing the effects of winter warming on crop biomass, particularly for non-winter crops, is limited. Studies highlight the accelerated rate of winter temperature increase in high-latitude regions, exceeding the rate of mean annual temperature increase. While the impacts of this accelerated winter warming on winter crops are acknowledged, its effects on non-winter crops through soil-mediated processes remain under-investigated. The existing literature emphasizes the role of soil organic matter (SOM) in maintaining soil fertility and supporting plant growth. However, the specific mechanisms through which winter warming affects SOM decomposition and, consequently, crop biomass, require further investigation. Previous research points toward the potential for winter warming to stimulate soil respiration, leading to increased SOM decomposition. This aspect, combined with the lack of focused research on the impact of winter warming on non-winter crop biomass, motivates the current study.

This study integrated global observational data with a decade-long field experiment to assess the impacts of winter warming on crop biomass carbon. A global database was compiled, comprising 309 observations of straw carbon (C) and 1358 observations of grain C contents from 161 field sites worldwide. This database was used to investigate the impact of winter warming on crop biomass through soil processes. A decade-long field experiment was conducted across three distinct climatic zones in China (cold temperate, warm temperate, and mid-subtropical) to further explore this relationship and its mechanism. The experiment involved an in situ study and a soil translocation study. The in situ study investigated the impacts of winter warming on crop biomass C across different climatic zones, while the soil translocation study simulated accelerated SOM decomposition by relocating soils from high-latitude regions to mid- and low-latitude areas. The hypothesis was that warmer winters reduce SOM, diminishing the soil's nutrient retention capacity and leading to decreased crop biomass C content, especially at mid-to-high latitudes. Data analysis involved linear mixed-effects models, random forest models, structural equation modeling (SEM), and hierarchical partitioning. The global-scale meta-analysis utilized a linear mixed-effects model with crop biomass C as the dependent variable, winter soil temperature as a fixed effect, and climate type as a random effect. Random forest models were used to identify key variables affecting crop biomass C, while SEM evaluated the effects of soil physicochemical properties, mineral protection, micronutrient availability, enzymatic activity, and microbial diversity on changes in crop biomass C. The field experiment involved measuring winter soil temperature and moisture, collecting soil and plant samples, and analyzing various soil properties (SOM, micronutrients, enzymatic activity, microbial diversity). Statistical analysis included linear regression, ANOVA, Tukey's post-hoc test, Duncan's New Multiple Range Test, and bootstrapping.

Global-scale meta-analysis revealed a significant negative correlation between winter soil temperature and crop biomass carbon. For every 1 °C increase in winter soil temperature, the global average C content in straw and grain decreased by 6.62 ± 1.65 g kg⁻¹ and 10.21 ± 2.31 g kg⁻¹, respectively (P < 0.05). Maize and rice showed more significant negative impacts than wheat. The negative correlation persisted across fertilization treatments. The field experiments corroborated the meta-analysis results, showing a significant negative correlation between winter soil temperature and crop biomass C, with a more pronounced decline at mid-to-high latitudes. Random forest models indicated that SOM is a secondary influential factor on crop biomass C after temperature changes. The soil translocation experiment showed that the decline rates in SOM for translocated soil were significantly higher than those observed in local soils, and the decline rates of straw C and grain C in the translocated Mollisols were notably higher than those in the local soils (P < 0.05). Structural equation modeling (SEM) indicated that the interaction among winter soil temperature, soil geochemistry, and microbial characteristics could account for a significant portion of the variability in crop biomass C across latitudes. An increase in winter soil temperature resulted in a significant decrease in soil mineral activity, reducing the mineral protection of SOM and leading to a decrease in micronutrients, negatively impacting crop biomass C content. The influence of soil moisture content varied across latitudes, possibly due to freeze-thaw cycles. Projected losses of total crop C due to winter warming were estimated to range from 4% to 19%, with higher losses at mid-to-high latitudes.

This study's findings highlight the significant adverse impact of winter warming on crop biomass carbon, particularly in mid-to-high latitude regions. This impact is primarily attributed to the accelerated degradation of SOM and consequent loss of micronutrients crucial for plant growth. The accelerated SOM decomposition is linked to increased microbial activity and reduced mineral protection under warmer winter conditions. This research demonstrates the importance of considering winter warming in agricultural productivity models and climate change adaptation strategies. The results suggest a more significant reduction in crop productivity due to future global warming than previously estimated. The study's findings underscore the necessity of incorporating winter soil temperatures rather than only air temperatures in future food production projections. This is because winter soil temperatures act as an amplifier for air temperature fluctuations, and soil temperature is influenced by soil moisture and varies significantly across soil depths. The observed reduction in SOM due to winter warming also has implications for the global carbon cycle, suggesting that winter warming might reduce the soil's capacity as a carbon sink.

This study provides compelling evidence for the significant negative impact of winter warming on crop biomass carbon production, especially in mid-to-high latitude regions. This reduction is primarily driven by the degradation of soil organic matter and subsequent micronutrient loss, highlighting the need to incorporate winter warming effects into agricultural models and adaptation strategies. Future research should investigate the role of organic fertilizers in mitigating these effects and explore crop breeding strategies to enhance resilience to winter warming.

The study primarily focuses on the effects of inorganic fertilizers, potentially overlooking the role of organic fertilizers in mitigating the negative effects of winter warming. While the study uses a global dataset and a decade-long field experiment, the generalizability of these findings to other regions and crop types may require further investigation. The study's focus on three specific regions in China might not fully represent the diverse range of soil types and climatic conditions across the globe.