No-till and carbon stocks: Is deep soil sampling necessary? Insights from long-term experiments

Confusion persists regarding the potential for no-till (NT) farming to accumulate soil organic C (SOC) and mitigate climate change. This uncertainty could be partly due to shallow sampling (<40 cm soil depth) employed in most previous SOC publications. The specific objective of this study was to gain additional knowledge of SOC accumulated by a wide range of tillage systems in surface and deeper soil profiles in the long term (> 34 yr). We quantified SOC stock (Mg ha(-1)) on an equivalent mass basis for the 0-60 or 100 cm soil depths for NT, disk, double disk, chisel plow, and moldboard plow under continuous corn and corn-soybean on silt clay loams after 34 yr and 39 yr in the western US Corn Belt. Results showed that NT did not increase SOC stocks near the surface but it did with cumulative depth relative to moldboard plow. No-till accumulated SOC at a rate of 0.68 Mg ha(-1) yr(-1) relative to inversion tillage (moldboard plow). However, NT did not store more SOC than common conventional (chisel plow) or reduced (disk) till practices for the whole soil profile. Results indicate that depth of soil sampling can affect conclusions about tillage system impacts on SOC stocks. For example, if we had sampled to 20 cm soil depth only, we would have incompletely concluded that NT does not significantly accumulate more SOC than moldboard plow. Results after 34 and 39 yr of tillage management also indicate that non-inversion tillage, particularly disk, can be as effective as NT for accumulating SOC. Overall, deep soil sampling can tell a different story about NT impacts on SOC stocks from shallow sampling.