Turnover and storage of C and N in five density fractions from California annual grassland surface soils

Washington D.C.

2002

64.1-64.16

8 Abb., 5 Tab., 54 Lit. Ang.

Bandaufführung

10401S

153529

[1] We measured 14C/12C in density fractions from soils collected before and after atmospheric thermonuclear weapons testing to examine soil organic matter (SOM) dynamics along a 3 million year California soil chronosequence. The mineral-free particulate organic matter (FPOM; <1.6 g cm-3) mainly contains recognizable plant material, fungal hyphae, and charcoal. Mineral-associated light fractions (1.6-2.2 g cm-3) display partially or completely humified fine POM, while the dense fraction (>2.2 g cm-3) consists of relatively OM-free sand and OM-rich clays. Three indicators of decomposition (C:N, δ13C, and δ15N) all suggest increasing SOM decomposition with increasing fraction density. The Δ14C-derived SOM turnover rates suggest that >90% of FPOM turns over in <10 years. The four mineral-associated fractions contain 69-86% stabilized (decadal) SOM with the remainder assumed to be passive (millenial) SOM. Within each soil, the four mineral-associated fractions display approximately the same residence time (34-42 years in 200 kyr soil, 29-37 years in 600 kyr soil, and 18-26 years in 1-3 Myr soils), indicating that a single stabilized SOM pool exists in these soils and may turn over primarily as a result of soil disruption.