The conversion of tropical forest to pasture and cropland has fundamentally altered the global C cycle (Baccini et al., 2012; Houghton, 2005; Houghton et al., 2012). Driven by an increasing global demand for beef and grain from a growing human population (Boucher et al., 2012; Brown, 2009), tropical deforestation to date has largely been for conversion to pasture and the C consequences of such conversions have been extensively documented (De Camargo et al., 1999; Fujisaki et al., 2015; Neill et al., 1997; Powers et al., 2011). Although pasture remains a widespread use of tropical land, in the past two decades land has been increasingly converted to highly mechanized agriculture of commodity crops like soybeans (Galford et al., 2011; Laurance et al., 2014; Morton et al., 2016; Nepstad et al., 2006) and little is known about the transition to industrial soy production which now covers large swaths of the southern Amazon and Cerrado. This transition, which is playing out on deep, highly weathered soils may be a substantial C source to the atmosphere, but little work has attempted to quantify soil C changes, particularly the changes that occur deeper in the soil profile. Conversion of tropical forests to agriculture can influence soil C storage and SOM dynamics in several ways. First, the amount, depth distribution, and quality of litter inputs to soil C can change as annual crops or pasture grasses replace trees (Davidson et al., 1995; Fisher et al., 1994; Harrison et al., 1993). Second, decomposition rates may increase because of higher temperatures in agricultural soils and physical soil disturbance from management practices such as tillage which mixes soils and breaks up soil aggregates (Harrison et al., 1993; Matson et al., 1997; Murty et al., 2002; Six et al., 1999). The decomposition of fastcycling young C often increases with conversion to agriculture and old forest-derived C is often destabilized after cultivation (Dieckow et al., 2009; Don et al., 2011; Houghton, 2003; Powers et al., 2011; Trumbore, 1997). Third, there is less water transpired from agricultural compared with forest soils (Bosch & Hewlett, 1982; Coe et al., 2011; Hayhoe et al., 2011; Hibbert, 1967; Neill et al., 2013), which could reduce water limitation of decompositio al., 2016), though the fate of this C downstream of farm fields is not well understood (Huth et al., 2012).n. Finally, physical removal of SOM by increased erosion can remove C and nutrients, particularly in tilled soils (Harden et al., 1999; Lal et al., 2004; Quinton et al., 2010; Smith et