Identification of environmental indices that influence SOC plasticity
To define the environmental factors influencing SOC, we examined the general trends of three key climate conditions during the growth season: temperature, precipitation, and solar radiation. Among the three experimental sites, Hefei (HF) exhibits the lowest cumulative growing-degree day (GDD) values, while Chengdu (CD) has the least amount of cumulative precipitation (Fig. 1b; Fig. S3). For solar radiation, Wuhan (WH) has higher levels of ultraviolet B (UV-B) irradiation during the leaf stage than the other sites. However, this trend diminishes and eventually reverses as the plants progress to inflorescence emergence and later stages (Fig. S4). In addition, variations in environmental conditions between different years at the same site also contributed to SOC plasticity (Fig. 1b, Fig. S3, S4). We identified days of cold stress (below -3.0°C) as a significant variable, with a noticeable disparity between years, with 33 days in HF2018 and 17 days in HF2017, suggesting that low temperature should be considered when assessing SOC differences (Fig. S3). These climate conditions are likely to be pivotal factors influencing SOC plasticity, but further investigation is needed to determine the key phases that affect SOC performance more explicitly.
To identify the explicit environmental indices that contribute to SOC plasticity, we used the CERIS package, a tool designed for analyzing phenotypic plasticity (Fig. S5). Through the exploration of various combinations of environmental parameters and growth windows, CERIS identified three factors: DTR183–192, PR166–195, and UVB144–186, showing strong correlation with environmental means, thus determining SOC plasticity in eight different environments (Fig. 2). DTR183–192 represents the average diurnal temperature range within the window of 183–192 days after planting. This window falls within the pod-filling stage, highlighting plant sensitivity to temperature changes during this stage. PR166–195 and UVB144–186 represent the average precipitation (PR) and average UV-B irradiation (UVB) in the windows of 166–195 and 144–186 days after planting, respectively. These two windows coincide with the flowering and pod-filling stages. To quantify the influence of these environmental indices on SOC, we conducted separate regressions of the environmental mean for SOC with each factor, revealing a 0.78% decrease in SOC for every 1-unit increment in DTR183–192, a 1.55% increase in PR166–195, and a 1.9% decrease in UVB144–186 (Fig. S6). These findings establish a quantitative link between environmental indices, growth stages, and SOC plasticity, thereby enhancing our understanding of the environmental basis of SOC plasticity.