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 2000to 8000-mm size class declined from0.47 to 0.15 g g21 at 0to 7-cm soil depth and from0.32 to 0.23 g g 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. 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 LFand 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. CHANGES in soil organic matter (SOM) and other soil properties following the conversion of native ecosystems to agriculture are well known. Over time, SOM declines, is redistributed between surface and subsurface horizons, and is released from protected microsites (e.g., Del Gado et al., 2003; DeGryze et al., 2004). Accompanying changes include increased soil temperature, modified trace gas fluxes (e.g., Smith et al., 2001; Smith and Conen, 2004), and altered microbial community structure and function (e.g., Cavigelli and Robertson, 2000; Buckley and Schmidt, 2001), among others. Typically, SOM declines over time to 30 to 60% of original values (e.g., Davidson and Ackerman, 1993; Buyanovsky et al., 1997; West and Post, 2002; Lal, 2003). In tropical ecosystems change occurs rapidly, and within months of initial cultivation measurable declines in soil C concentration can sometimes be detected (Houghton et al., 1985; Detwiler, 1986; Brown and Lugo, 1990). In temperate ecosystems it is more difficult to generalize because data for the first 10 yr of cultivation are not currently available (Davidson and Ackerman, 1993; West and Post, 2002; Miller et al., 2004). Theory and a handful of relevant studies suggest that immediate C losses from temperate region soils may be less dramatic than from tropical ecosystems (Bowman et al., 1990). Tiessen and Stewart (1983) found no difference in total C 4 yr after initially cultivating a silt loam. Other authors (e.g., Pierce et al., 1994; VandenBygaart and Kay, 2004) have also failed to find an effect within the first few years of cultivating long-term no-till soils, perhaps because of detection difficulties. Short-term total soil C changes are notoriously difficult to detect because of high background C concentrations in most arable soils, spatial variability, and the redistribution of aboveground pools on cultivation (Robertson et al., 1988; Sollins et al., 1999). Consequently, soil C permanence may be better predicted by changes in more readily detected factors known to affect soil C storage such as aggregation, the vertical distribution of C, and the dynamics of specific SOM fractions. Macroaggregates (.250-mm soil particles) may be especially good predictors of potential C responses to tillage because of their importance for protecting recently deposited, labile, LForganic matter (Angers and Giroux 1996; Jastrow et al., 1996). Isotope studies, including those using CCP/MASNMRspectroscopy, havedemonstrated that newly incorporated crop residues, root derived C, and young SOM are found in macroaggregates (Golchin et al., 1994; Six et al., 1998; Gale et al., 2000a; 2000b). Physical protection of LF and other compounds within macroaggregates may limit its oxidation by 50% ormore (Balesdent et al., 2000). In ecosystems with frequent soil disturbance, accelerated turnover rates of macroaggregates limit the physical stabilization of labile SOM compounds such as LF C within aggregates (Six et al., 1999). These and other studies documenting tillage effects on aggregation and SOM, however, were conducted in soils with a long history of cultivation. The immediate and short-term effects of tillage on soil-aggregate distribution and SOM protection still need to be resolved. Our overall objective was to investigate short-term responses of aggregate-size distribution and SOM to tillage independent of other factors. We removed the potentially confounding effects of historical tillage and fertilization by cultivating a previously undisturbed soil profile, and minimized plant community changes that can also confound studies of agricultural conversion by leaving the site fallow after tillage. This approach provides a uniquely unobstructed view into the potential consequences of tillage on soil structure and themechanisms controllingCpersistence. A.S. Grandy and G.P. Robertson, W.K. Kellogg Biological Station and Dep. of Crop and Soil Sciences,Michigan StateUniv., Hickory Corners, MI 49060. A.S. Grandy currently at: University of Colorado–Boulder, Dep. of Geological Sciences, Boulder, CO 80309-0399. Received 21 Sept. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:1398–1406 (2006). Soil & Water Management & Conservation doi:10.2136/sssaj2005.0313 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: DI, deionized water; LF, light fraction organic matter; MWD, aggregate mean weight diameter; NAPT, sodium polytungstate; POM, particulate organic matter (.53 mm); SOM, soil organic matter. R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 1398 Published online June 21, 2006