Tracing service crops' net carbon and nitrogen rhizodeposition into soil organic matter fractions using dual isotopic brush-labeling

Recent studies emphasize the importance of rhizodeposition in soil organic matter (SOM) formation. However, its quantification in different soil fractions is still uncommon. To estimate the net carbon (C) and nitrogen (N) rhizodeposition into particulate (POM) and mineral-associated (MAOM) organic matter, we conducted a pot experiment to trace plant-derived C and N into the soil. In this study, we first evaluated the effectiveness of the dual isotope (¹³C and ¹⁵N) brush-labeling method on oat and vetch plants, two species commonly sown as service crops, to investigate in situ plant-soil interactions. Our results indicate that foliar brushing successfully labeled plant tissues and traced plant-derived C and N into the soil, allowing us to make a conservative estimate of net rhizodeposition. Based on soil δ¹³C and δ¹⁵N changes, we show that most of the net C (81.5 and 100% for oats and vetch, respectively) and N (92.4 and 93.2% for oats and vetch, respectively) rhizodeposition contributes directly to the formation of MAOM and not POM. Both species contributed similar amounts of net C rhizodeposition to the MAOM fraction (about 0.59 g plant⁻¹), while oat net C rhizodeposition also formed a small amount of POM (0.138 g plant⁻¹). Because vetch had low root biomass, the net C rhizodeposition:root C ratio was much higher in vetch than in oats (12.3 and 0.65, respectively). For net N rhizodeposition, vetch contributions were greater than oats in both fractions (71.4% greater in MAOM and 50.4% greater in POM). Our estimates of net C and N rhizodeposition range from 15 to 40% of the total recovered C or N. Our results demonstrate the importance of incorporating rhizodeposition into belowground biomass estimates and SOM models, and suggest that service crops provide large amounts of C and N inputs from living roots that are mainly retained in MAOM.