In the Virginia Coastal Plain of the US, growers have practiced rotational no-tillage (RT) and continuous no-tillage (NT) for over 20 years to improve soil quality, but the long-term effects of these practices on soil organic carbon (SOC) and soil nitrogen (SN) stocks remain largely unquantified. We sampled 48 grower’s fields representing the regions’ three most cultivated soil series; Altavista (fine-loamy Aquic Hapludult), Bojac (coarse-loamy,Typic Hapludult), and Kempsville (fine-loamy, Typic Hapludult).  

Within each soil series, fields under RT and NT with or without biosolids application (in 2001) were sampled from 0-120 cm in 30 cm increments.  Surface soil C (0-30 cm) accounted for 41-65% of the profile SOC; ranging from 37.9 to 57.2 Mg C ha^-1. There were no significant treatment effects for SOC or SN in the 0-30 cm depth, but both tillage type and biosolid application had significant effects in the 30-60 and 60-90 cm depths, respectively.  Surprisingly in the sandy Bojac soil series, RT with biosolid application significantly increased SOC stocks in the 30-60 cm depth resulting in the highest SOC stocks for all the treatments, including NT, and significantly greater SOC stocks for the 0-120 cm depth. There was no significant management effect for the other two soil series over the entire 0-120 cm depth and confirms other studies that found that tillage redistributes SOC throughout the profile when deeper soil layers are taken into account.

In these soils, the incorporation of biosolids by a deep tillage event into the 30-60 cm depth appears to have protected SOC from decomposition in comparison to the NT treatment, where SOC is still available for decomposition in this warm, wet climate. These results highlight the need for deep soil sampling to accurately quantify SOC stocks and fluxes and particularly to account for variability in producer managed fields where historical management practices may have a significant influence.