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.