The impact of tillage practices on the distribution of humified organic carbon in clay loam soil

A field experiment was carried out at the Joniklis Experimental Station of Lithuanian Research Centre for Agriculture and Forestry in 2016-2018 to ascertain the effects of reduced tillage and the combinations of reduced tillage with soil improvement practices on soil organic carbon (SOC) and quantity and quality of humus in a clay loam. The following tillage systems were investigated: 1) deep ploughing (DP) at 21-23 cm depth (control), 2) shallow ploughing at 15-17 cm depth (SP), 3) ploughless tillage (at 10-12 cm depth) (PT), 4) ploughless tillage and aftereffect of lime sludge last incorporated in 2014 (PT + aLS), 5) ploughless tillage with cover crops for green manure (PT + GM) and 6) no-tillage with cover crop for winter mulch (NT + WM). The reduction of tillage intensity increased the contents of SOC and C in humic substances (HS) in the upper (0-10 cm) soil layer. The incorporation of organic sources in the form of green manure and winter mulch promoted microbiological activity and the mineralization of the soil organic matter (SOM) already present in it. However, the SOC and total nitrogen (N tot ) content did not increase in proportion to the fresh organic matter incorporated. The tillage systems applied had influence on the changes in the humus fractional composition. The increased content of plant residues under PT + GM and NT + WM treatments resulted in higher amounts of mobile HS in the upper soil layer, meanwhile the PT + aLS treatment increased the amount of agronomically most valuable Ca-bound HS. Higher amounts of HS bound with clay minerals were found under PT + aLS treatment, as calcium (Ca) is an important factor for SOC sequestration in a clay. The humification degree increased in 0-30 cm soil layer under all reduced tillage systems investigated compared to the DP (control). The use of soil improvers had a more significant effect than just reduction of tillage. However, due to the complexity of SOM a clear relationship between SOM qualitative characteristics and tillage practices applied was not identified.