Long-term effect of tillage and straw retention in conservation agriculture systems on soil carbon storage

The main principles of conservation agriculture (CA) are minimal soil disturbance, permanent soil cover, diversification of crop rotations, and retention of crop residues. These practices have been associated with improved soil quality, water use efficiency, biodiversity, and reduced climate impact through carbon (C) sequestration. No tillage has been implicated with reduced soil organic C (SOC) mineralization, but recently this has been contested. Residue retention and cover crops result in higher soil C inputs and are therefore expected to increase SOC. CA affects both the distribution of SOC in the profile as well as the bulk density and this complicates the analysis of total SOC stocks. Furthermore, these changes only manifests over a long time. Therefore, the effects of CA practices require long-term experiments and thorough sampling of the soil profile, and there are few studies in the literature. This study aimed to evaluate the effect of 17 years of residue retention and tillage on soil C storage in the soil profile under cool temperate humid climate conditions. The study used field trials established in 2002 on two Danish sandy loam soils with different clay and SOC contents and compared different tillage (plowing and no-till) and straw management practices (with and without straw retention). Soil samples from 0–50 cm were taken in winter 2002, 2009, and 2019 to quantify C content, and crop yields were measured annually. Straw retention reduced bulk density at both sites. It also increased the C concentration with 0.47 mg C g–1 dry soil on average from 2002 to 2019, and SOC stocks in 2019 with 23 tonne C ha–1 on average, at the site with higher initial soil C content. The effect of straw retention on soil C was higher when the soil was plowed, and no significant effect was observed in no-till soils. Straw retention did not significantly affect C concentration at the site with low initial soil C. Tillage altered the C distribution in the soil profile, but did not significantly increase total C concentration at either site, although there was a non-significant trend (P = 0.07) to increased C concentration across sites. Use of no-till and straw retention did not consistently affect crop yield across site-years, but the inter-annual variability was large, obscuring possible differences in yields. The limited effect of CA practices on soil C storage observed in this study may be due to the soil type and climatic conditions at these sites, but this needs further investigation. Core Ideas Straw retention reduced soil bulk density at sites with low or medium-high initial C level Straw retention increased C concentration and SOC stock at a site with higher C Tillage altered soil profile C distribution, but did not increase SOC storage Yield was not affected by 17 years of no-till and straw retention.