Nitrogen fertilization and tillage reversal affected water-extractable organic carbon and nitrogen differentially in a Black Chernozem and a Gray Luvisol

•WEOM dynamics in two contrasting soil types in Western Canada.•N fertilization and tillage reversal increased soil TOC and TN in the Gray Luvisol.•N fertilization was the main factor affecting the dynamics of WEOC.•Different responses of WEOC stability to N fertilization in the two soils.

Reversing land management from no tillage to conventional tillage (tillage reversal) to deal with weed infestation and accumulation of crop residue in long-term no tillage systems may dramatically alter soil carbon (C) dynamics. We studied the impact of nitrogen (N) fertilization and tillage reversal on the quantity and quality of water-extractable organic C (WEOC) and N (WEON) in the 0–10 cm soil layer in two contrasting soil types located at Ellerslie (high organic matter content) and Breton (low organic matter content) in central Alberta, Canada. We used a split-plot design with N assigned to the main plot and tillage to the subplot. Each treatment had two levels which included addition of 0 (N0) vs. 100 kg N ha−1 yr−1 (N100) N fertilizer and long-term no tillage (NT) vs. tillage reversal (TR); straw was retained on site in all treatments as part of the management regime. Our results showed that soil organic C and N storage were not affected by long-term N fertilization or tillage reversal at Ellerslie but were increased at Breton. Soil WEOC was significantly higher under N100 than under N0 at both sites. Soil WEOC was TR < NT at Breton but was not affected by tillage at Ellerslie. Soil WEON was influenced by the interaction effects of N fertilization and tillage reversal at both sites. The highest WEON concentration was in the N100–TR treatment combination (17.8 ± 1.5 and 10.5 ± 0.7 μg g−1 at Ellerslie and Breton, respectively). Nitrogen fertilization decreased the aromaticity of WEOC at both sites but had different effects on WEOC condensation between Ellerslie and Breton. Nitrogen fertilization increased non-aromatic compounds in WEOC and the stability of WEOC at Breton but not at Ellerslie. Neither tillage nor tillage × fertilizer interaction affected the quality of WEOC in either soil. Therefore, N fertilization was the main factor controlling the quality and quantity of WEOC in the studied soils.