Total and fresh organic carbon distribution in aggregate size classes and single aggregate regions using natural 13C/12C tracer

Soil structure is known to stabilise organic carbon (Corg), as it acts as physical barrier between the decomposing microorganisms and the substrates. It is, however, not fully understood how the organic carbon (Corg) and especially fresh material from plants is distributed within the soil structure. The aim of the current study is to investigate the long- and short-term accumulation of Corg in soil macro-aggregates following the 2 main soil structure formation models: hierarchical and gradient development around plant debris. Two types of differently vegetated and tilled silty loam soil were selected for the examination of Corg and δ13C signals within 4 aggregate size classes (< 2, 2–5, 5–8, and 8–12 mm across) and 3 different concentric regions (exterior, transitional and interior) of single macro-aggregates. The total and fresh Corg distribution was identified in soil i) 24 years following the crop conversion from C3 to C4 and ii) 6 weeks greenhouse plant growth after conversion from C3 to C4 and vice-versa. The results show that maize soil 24 years after C3 to C4 plant conversion has (9–27%) higher Corg content and the least negative δ13C signal in the aggregate size class < 2 mm in the topsoil, and 30–40% higher Corg content in the subsoil and is the only size fraction enriched in the fresh Corg while larger aggregates contain old carbon characteristic for the C3 vegetation. Although without statistical significance, a consistent pattern of higher Corg content and less negative δ13C values in the aggregate exteriors vs. the interior are found in the original soil. The aggregates from the greenhouse experiment 6 weeks after crop conversion are enriched by 6–15% in the fresh organic carbon from the recently grown plant. The fresh Corg, however, is rather randomly distributed within the aggregates showing no particular enhancement in any of the aggregate regions.