Evidence for the role of soil C/N ratio in shaping plant responses to root-knot nematode infection

INTRODUCTION: Root-knot nematodes (RKNs) pose a major threat to global crop production. Soil properties influence plant responses to RKN infestation, but the specific soil factors that are most influential in determining these responses remain poorly understood. OBJECTIVE: This study aims to identify the key soil factors that influence plant responses to Meloidogyne incognita, develop a dynamic model to quantify plant disease severity in response to variations in these soil factors, and further elucidate the underlying mechanisms driving this interaction. METHODS: We collected 28 soils with diverse physicochemical and microbial properties at a national scale and conducted nematode infection experiments under controlled environmental conditions, using cucumber plants (a typical susceptible host for M. incognita) to assess disease severity. Based on the resulting dataset, a Mantel test was applied to identify the key soil factor influencing M. incognita infection. To further validate these findings, we performed organic carbon (C) addition experiments and M. incognita chemotaxis assays. RESULTS: We found that 28 soils exhibit a broad range of plant performance indices (PPI) and disease indices (DI). The Mantel test revealed that soil carbon/nitrogen (C/N) ratio is the strongest correlate of both plant performance and disease symptoms. The DI follows an inverted hump-shaped response curve with increasing soil C/N ratio, indicating the existence of an optimal soil C/N ratio (about 8.0) for minimizing DI. This finding is further supported by the fact that organic C addition decreases DI in soils with low initial C/N ratio, but increases DI in soils with high initial C/N ratio. The shaping effects of soil C/N ratio are underpinned by its regulation of overall soil quality and plant resistance to M. incognita chemotaxis. CONCLUSION: Optimizing C/N ratio reduces soil sensitivity and suppresses RKN infestation, offering valuable insights for sustainable agricultural management practices.