| Several studies have hypothesized that turnover of soil aggregates promotes soil organic matter losses under cultivation. Other studies suggest that the protection of organic matter requires occlusion into aggregates. However, few observations or quantifications of aggregate turnover are reported. We selected ceramic spheres for use as tracers and dysprosium as a label for detection by instrumental neutron activation analysis. From characterization of the tracers we concluded that mass and diameter variability was insufficient to affect their quantification. Tracer spheres were applied to field plots to measure aggregate turnover in contrasting field soils. Soil aggregation was cyclical and reset by the over-winter period and spring tillage. Tracers were rapidly incorporated into macroaggregate size fractions, demonstrating rapid soil aggregate turnover. Data from the field study were used to generate a quantitative model of aggregate dynamics consisting of four macroaggregate size fractions and first-order flows between them. Mean residence times of aggregate size compartments ranged from 4 to 95 days, and were generally shorter in the Breton versus Malmo soils. Higher aggregate turnover, higher relative soil respiration, and lower soil organic matter content in the Breton soil all suggest a lower capacity to physically protect organic matter, leaving more C available for mineralization. However, observations of aggregate turnover in Malmo soil suggest that turnover may also provide the opportunity for organic matter occlusion. A laboratory study measuring the decomposition of added particulate organic matter and aggregate dynamics under differing frequencies of simulated tillage supported the findings of the field study. Total soil respiration in incubated soils was higher in a once-tilled treatment, suggesting that more frequent tillage promoted the retention of organic matter. While aggregate MWD was lower, frequently tilled samples showed greater incorporation of tracer spheres. We hypothesized that there may exist threshold rates of aggregate turnover that will occlude, rather than release organic matter. The work reported in this study represents a first attempt to observe and quantify gross soil aggregate turnover. Quantification of soil aggregate turnover is essential to testing hypotheses that aggregate turnover controls the accrual or loss of soil organic matter. |
