Impact of no-till and reduced tillage on aggregation and aggregate-associated carbon in Northern European agroecosystems

•Aggregate size decreased in the order of NT > RT > CT at all study sites.•Bulk soil SOC did not increase under NT or RT compared to CT in a decade.•NT increased SOC in the mM fraction only on coarse soil.•Lower crop yields under NT most likely cause for limited C sequestration.•Redistribution of SOC to larger aggregates indicates positive impact of NT practice.

Minimum tillage practices have been shown to enhance soil aggregation and soil organic carbon (SOC) stabilization. Carbon turnover rate slows down when soil aggregation increases and SOC is protected within stable microaggregates (53–250 μm). However, this has not been investigated in boreal soils. Therefore, the objective of this study was to quantify the long-term effects of no-till (NT) and reduced tillage (RT) on SOC stabilization in four soils typical for the boreal region. Distribution of SOC in different soil fractions in a 0–20 cm soil layer was analyzed by wet sieving and further isolation of microaggregates (mM) from large (>2000 μm, LM) and small (250–2000 μm, sM) macroaggregates. Aggregate size decreased in the order of NT > RT > CT at all study sites. In addition to increased mean weight diameter (MWD) under NT, a general trend of redistribution of SOC into these formed macroaggregates was found at all study sites, i.e., the LM fraction gained SOC. However SOC was lost in other fractions under NT compared to CT at some sites and none of the sites showed any significant changes in bulk soil SOC content under NT or RT. Also our hypothesis that there would be more SOC incorporated in mM fraction in NT and RT compared to CT was corroborated only at site 4 under NT. Thus, although the potential to accumulate SOC under NT or RT compared to CT seems to be limited in boreal agroecosystems, the redistribution of SOC to the more stable conditions within the aggregates indicates positive impacts of no-till practice.