Aggregation and Organic Matter Protection Following Tillage of a Previously Uncultivated Soil

Understanding the effects of tillage on soils following years or decades of no‐till is critical for developing C conservation strategies. To date, short‐term responses to tillage in previously uncultivated or other long‐term no‐till soils have primarily focused on total C changes, which are difficult to detect. Tillage effects on soil conservation and C permanence may be better predicted by changes in more readily detected factors known to affect C storage such as aggregation and physically protected C. We annually plowed replicated plots in a previously uncultivated midsuccessional field between 2002 and 2004 and investigated changes in the distribution of aggregates, physically protected C, and light fraction (LF) organic matter. Within 60 d of initial cultivation, soil aggregates in the 2000‐ to 8000‐μm size class declined from 0.47 to 0.15 g g −1 at 0‐ to 7‐cm soil depth and from 0.32 to 0.23 g g −1 at 7 to 20 cm. Lower levels of aggregation persisted through the winter and spring of the following year. Inter‐aggregate, unprotected light fraction (LF) increased following cultivation, as did particulate C in soil fractions with densities < 1.9 g cm −3 Changes in the mass of total soil C were not detectable after 3 yr but the vertical distribution of all soil C pools was altered by plowing. Our study demonstrates that plowing once immediately and substantially alters aggregation and LF and particulate C dynamics and that these conditions persist. Results suggest that no‐till soils need to be continuously maintained to protect aggregation and physically stabilized C pools.