Drought reinforces the relation between microbial activities and soil respiration but depresses the linkage between microbial activities and soil C and N: Evidence from rhizosphere in rapeseed (Brassica napu)

Soil respiration (SR), the second-largest terrestrial carbon flux, plays a pivotal role in regulating global carbon cycling under climate change. However, drought-induced changes in SR dynamics, particularly in the rhizosphere, remain poorly understood due to methodological limitations. This study employed in situ rhizosphere partitioning (root bag system) and high-frequency diurnal SR monitoring to compare drought responses between two rapeseed (Brassica napus) cultivars with contrasting drought tolerance. Soil organic carbon (SOC), total nitrogen (TN), enzyme index (SEI) and microbial carbon source utilization (MCSU) were assessed in both bulk and rhizosphere area to quantify root-derived effects. The interactions among these factors in the rhizosphere were further explored using structural equation model (SEM). Result showed drought significantly reduced SR, SOC, C:N ratio and MCSU in the rhizosphere for both cultivars (p < 0.05) while had minimal effects on bulk soil. Bulk soil exhibited a 6-h hysteresis in peak SR compared to the rhizosphere. Drought significantly suppressed root-derived SR in the drought-sensitive CY36, while enhancing it in the drought-tolerant YY57. Additionally, rhizosphere effect play a positive role in SR but a negative role in SOC and TN, with this negative effect aggravated by drought. SEM revealed drought depressed the effects of SOC and TN on C:N ratio, SEI and MCSU, while positively reinforced the direct effect of MCSU on SR in the rhizosphere (p < 0.05). Our results highlight the importance of accounting for sensitive changes in the rhizosphere induced by drought when predicting ecosystem carbon and nutrient balance responses to future drought events.