Tillage impacts on soil aggregation and aggregate-associated carbon and nitrogen after 49 years

Intensive tillage systems that feature a moldboard plow (MP) or a chisel-disk (CD) can degrade soil structure. No till (NT) can improve soil structure and can increase soil carbon (C) but the effect of alternating (AT; 2-yr no-till and 1-yr MP) on soil aggregation and soil C is less understood. This study evaluated the effects of four tillage practices including MP, CD, AT, and NT on soil physical and chemical properties and corn (Zea mays L.) yield. Percentage of dry medium (1-2 mm) aggregates was highest in NT, but contrary to our hypothesis, MP had a high percentage of dry large (2-4.75 mm) aggregates similar to NT. Increased tillage intensity decreased aggregate stability indicating that a high percentage of large sized aggregates in MP does not necessarily translate into aggregate stability. Aggregate-associated carbon (C) and nitrogen (N) percentages were higher in NT than other treatments but the C:N ratio was similar among all treatments at all depths. Disturbing the soil periodically (AT) disrupted C or N accrual compared with continuous NT while AT had the lowest amount of aggregate associated permanganate oxidizable C (POXC) accumulation. Therefore, practicing no-till and periodically deep-tilling can be as detrimental to soil structure as practicing yearly reduced tillage (CD), if not more. No-till greatly increased the C, N and POXC concentrations only in the topsoil layer (0-5 cm), highlighting a need for additional management tactics to increase C, N and POXC concentrations deeper in the soil profile. Tillage practices did not significantly influence corn yield. We concluded that AT offered improvements in aggregate size and stability, but was outperformed by CD, and that benefits of NT are limited to the topsoil. Therefore, integrated management practices are needed to build soil organic C (SOC) beyond topsoil. When a long-term NT system is disturbed, it could result in a greater loss of built soil resources such as soil C, N, and POXC. Future research should focus on evaluating the effect of alternating NT with CD on soil aggregation, and SOC buildup in combination with integrated practices with potential to build C beyond topsoil.

Automated summary

The study found that no tillage can improve soil structure and increase soil carbon, but alternating between no-till and deep-tillage may affect soil aggregation and carbon accumulation. Periodic deep-tillage can be as detrimental to soil structure as yearly reduced tillage, if not more.