Tracking in-situ soil aggregate turnover under raindrop impact and wetting-drying cycles using rare earth elements

Raindrop impact and wetting-drying cycles of soil strongly influence aggregate breakdown and reformation, but these mechanisms have not been examined simultaneously under field conditions. This study examined soil breakdown and aggregation in situ following rain events and inter-event soil moisture variations (referred herein as wetting-drying cycles) using rare earth elements (REEs) as tracers. Four natural aggregate size ranges (8–19 mm, 2–8 mm, 0.075–2 mm, and <0.075 mm) of soil from an agricultural field were tagged with different REEs and returned to the designated plot in the field. Soil samples were collected from the plot following rain events three days and twenty-six days after aggregate application and separated into the original four size ranges to analyze their REE concentrations. Using the change in REE concentrations from the original tagging, it was determined that there were greater proportions of aggregate breakdown (33% ± 11%) than formation (9% ± 4%) during the first three days resulting from relatively high rain power (0.051 ± 0.004 W m−2). In the next twenty-three days, more aggregates were found to reform (16% ± 22%) than break (10 ± 12%) as soil water content decreased from 33% to 23% then increased to 38% following relatively small rain events (rain power was 0.002 ± 0.005 W m−2). The turnover rate was faster for the 2–8 mm (0.014 day−1) and 0.075–2 mm (0.019 day−1) aggregates than for the 8–19 mm and <0.075 mm fractions (0.009 day−1). For a given aggregate size fraction, the turnover rate slowed down over the experiment time. This study demonstrates how REEs can track the aggregate life cycle in situ and advances the fundamental understanding of aggregate dynamics in agroecosystems in the context of raindrop impact and wetting-drying cycles.