Root or Shoot: Tracing the Pathways of Soil Organic Carbon Accumulation Through Plant‐Specific Contributions

Abstract

Soil organic carbon (SOC) in agricultural soils is a cornerstone of soil health and climate change mitigation, and its accumulation is largely dependent on composition and addition rates of plant residues. Our objectives were to determine the stabilization pathways of SOC derived from root and shoot residues of different plant species and to assess the impact of shoot input rates on these pathways. We cultivated in pots three species selected for their different C:N ratios—wheat ( Tricitum aestivum ), crimson clover ( Trifolium incarnatum ) and pea ( Pisum sativum ) – and exposed to weekly 13 CO 2 pulse labeling until the flowering stage. At harvest, shoot and root were collected for biochemical and morphological characterization, and 13 C tracing in soil was used to estimate rhizodeposition‐derived C. To assess the contributions of root and shoot degradation to SOC, pots containing soil and intact root systems, and soil amended with different shoot residues rates (1, 2, or 6 Mg C ha −1 ) were incubated under controlled conditions for 2 and 12 months. Soils incubated with shoot and root residues were particle size fractionated into particulate organic matter (POM), and mineral‐associated organic matter (MAOM) consisting of fine silt and clay fractions. A greater amount of SOC was derived from rhizodeposition and root degradation by wheat (0.58 and 0.70 g kg −1 , respectively) than legumes (0.22 and 0.001 g kg −1 , respectively). On the other hand, a greater contribution of legume‐ than wheat‐ shoot residues was observed to SOC (0.71 vs. 0.29 g kg −1 ) and to clay‐C fraction (0.90 vs. 0.63 g kg −1 ), particularly at 2 and 6 Mg C ha −1 . The high content of recalcitrant compounds such as hemicellulose and cellulose of wheat roots explains the strong contribution to SOC and their preferential accumulation in POM. Conversely, the preferential contribution of legume shoots to MAOM is likely due to the higher water‐soluble compounds and N contents. Our results suggest that both grasses and legumes can enhance SOC content by targeting different organic matter pools. Similar studies conducted in the field could contribute to emphasize the role of plant‐specific residue composition in shaping C stabilization pathways in the soil.