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 (C-org), as it acts as physical barrier between the decomposing microorganisms and the substrates. It is, however, not fully understood how the organic carbon (C-org) 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 C-org 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 C-org and delta 13C 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 C-org 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 C-org content and the least negative delta 13C signal in the aggregate size class <2 mm in the topsoil, and 30-40% higher C-org content in the subsoil and is the only size fraction enriched in the fresh C-org while larger aggregates contain old carbon characteristic for the C3 vegetation. Although without statistical significance, a consistent pattern of higher C-org content and less negative delta 13C 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 C-org, however, is rather randomly distributed within the aggregates showing no particular enhancement in any of the aggregate regions. (C) 2011 Elsevier B.V. All rights reserved.