New Methods of Investigating Physical Protection of Soil Organic Matter.

Positive relationships between soil aggregate stability and soil C are widely recognized but surprisingly little is known about how antecedent C and aggregation impact the turnover of new C. We evaluated several conceptual models using a new structural disruption method with a ball milling type of action, a new physical fractionation method that includes isolation of four fine fractions that vary in their susceptibility to dispersion, and two incubation experiments in which no-till (NT) and conventional till (CT) soils with comparable texture received varying levels of structural disruption and amendment. Soil fragmentation by the new disruption method was highly reproducible, particularly when longer durations of shaking were used. Results from the incubation experiments demonstrated the modulating effects of CT and NT management history on soil C and aggregate dynamics following structural disruption and residue addition. Positive effects of residue and structural disruption on aggregation were greatest in the CT soil. Residue decomposed more rapidly in the NT soil but had a greater priming effect on antecedent C in the CT soil. Nitrate addition had little effect on C or aggregate dynamics in either soil. Within 1 wk, a labeled nitrate source was detectable in slakeable microaggregates isolated from the NT soil and stable microaggregates within macroaggregates isolated from both the NT and CT soils but no significant changes occurred during the remaining 7 wk of incubation. Slower decomposition of residue C in the CT soil (with less antecedent C) fits the C saturation concept but may also result from less favorable physical properties in the CT soil (with and without structural disruption).