Combined turnover of carbon and soil aggregates using rare earth oxides and isotopically labelled carbon as tracers

Abstract This study used a combined tracer approach of isotopically labelled carbon (C) and rare earth oxides (REO) to determine soil aggregate transfer paths following input of organic matter. A model quantifying aggregate turnover rates over time was verified by a controlled incubation study. Four natural soil aggregate size ranges ( 13 C g −1 soil of 13 C-labelled glucose. There were four treatments: i) soil without REO and 13 C as a control, ii) soil labelled with REO, iii) soil without REO but amended with 13 C-glucose, and iv) soil labelled with REO and amended with 13 C-glucose. Aggregate stability, REO concentrations, soil respiration and 13 C were measured after 0, 7, 14 and 28 days incubation. REOs were found to not impact microbial activity ( P  > 0.05). Based on the 84%–106% recovery of REOs after wet sieving of aggregates, and a close 1:1 relationship between measured aggregates and model predictions, REOs were found to be an effective tracer for studies of aggregate dynamics. A greater portion of aggregates transferred between neighbouring size fractions. The turnover rate was faster for macroaggregates than for microaggregates, and slowed down over the incubation time. The new C was accumulated more but decomposed faster in macroaggregates than in microaggregates. A positive relationship was observed between the 13 C concentration in aggregates and the aggregate turnover rate ( P