Guohua Dai

Protection by metal (hydr) oxides is one of the key mechanisms for the long‐term stabilization of soil organic carbon (SOC). However, the source and turnover of (metal‐) bound organic carbon (OC) in soils are poorly constrained. Here we present the first large‐scale study on the ¹³C and ¹⁴C characteristics of bound OC in 15 wetland and upland soil profiles. We find that bound OC has similar δ¹³C as SOC, suggesting no preference for plant‐ or microbe‐derived carbon. However, bound OC Δ¹⁴C is more negative than SOC in wetland but not upland mineral soils, and decreases with increasing reactive minerals. Hence, in contrast to the conventional assumption, bound OC is better preserved relative to SOC in wetlands with high contents of reactive metals. Our finding highlights the dynamic exchange of bound OC with SOC in upland soils and calls for a better recognition of reactive metals in stabilizing OC in wetlands.

Root‐borne microbial necromass carbon (MNC) is an under‐investigated contributor to soil organic carbon (SOC) in grasslands. Here we conduct a benchmark assessment of root‐borne MNC based on amino sugars in the fresh roots of 27 dominant species and mixed roots (including dead ones) of mixed species in Inner Mongolian grasslands. We find that mixed roots contain 9.1−62.4 mg g⁻¹ MNC, which may contribute ∼10% of MNC in surface soils (0−10 cm) if all enters the soil without degradation. Root (and arbuscular mycorrhizal fungi; AMF) turnover rather than AMF biomass controls root‐borne MNC accumulation. Based on a microcosm decomposition experiment of mixed roots in model soils under optimal conditions (∼22°C; 60% of maximum water holding capacity), we further estimate that 43%−75% of root‐borne MNC remains after 2‐year decomposition, implying that root‐borne MNC is relatively stable. Hence, root‐borne MNC may be an overlooked potential source of soil MNC and SOC in grasslands.