Ronald Amundson D.L. Brenner A.C. Cook C. Kendall J.W. Harden W.T. Baisden 2002 <p><span>We examine soil organic matter (SOM) turnover and transport using C and N isotopes in soil profiles sampled circa 1949, 1978, and 1998 (a period spanning pulse thermonuclear&nbsp;</span>¹⁴<span>C enrichment of the atmosphere) along a 3‐million‐year annual grassland soil chronosequence. Temporal differences in soil Δ</span>¹⁴<span>C profiles indicate that inputs of recently living organic matter (OM) occur primarily in the upper 20–30 cm but suggest that OM inputs can occur below the primary rooting zone. A three‐pool SOM model with downward transport captures most observed variation in Δ</span>¹⁴<span>C, percentages of C and N, δ</span>¹³<span>C, and δ</span>¹⁵<span>N, supporting the commonly accepted concept of three distinct SOM pools. The model suggests that the importance of the decadal SOM pool in N dynamics is greatest in young and old soils. Altered hydrology and possibly low pH and/or P dynamics in highly developed old soils cause changes in soil C and N turnover and transport of importance for soil biogeochemistry models.</span></p> application/pdf 10.1029/2001GB001823 en AGU A multiisotope C and N modeling analysis of soil organic matter turnover and transport as a function of soil depth in a California annual grassland soil chronosequence article