Landscape position, sampling time, and tillage, but not legume species, affect labile carbon and nitrogen fractions in a 4-yr-old rejuvenated grazed pasture

Abstract

Termination by tillage is one strategy used for regenerating pasture stands. Yet, research gaps exist on how tillage affects carbon (C) and nitrogen (N) forms and amounts in western Canadian soils. We measured total soil organic C (SOC), dissolved organic C (DOC), total dissolved N (TDN), light fraction organic C (LFOC) and light fraction organic N (LFON), microbial biomass C (MBC) and microbial biomass N (MBN), and inorganic N as indicators of soil organic matter (SOM) dynamics. After tillage termination in fall 2018, we sampled soils (0–10 cm; 0–15 cm) under three legume species (alfalfa, cicer milkvetch, and sainfoin) three times (spring, summer, and fall of 2019) across three landscape positions. Legume species did not affect the measured parameters. Over time, tillage affected DOC, TDN, and inorganic N. Averaged across three pasture legumes and three landscape positions, tillage increased DOC 29% by summer. Fall-applied tillage led to 59% and 33% higher TDN in the succeeding summer and fall. Inorganic N increased by 14% and 40% across landscape positions and sampling after tillage. Averaged across landscape positions, MBC decreased by 31% from spring to summer and increased by 51% from summer to fall. However, MBN increased by 53% and decreased by 5% within the same period. The seasonal fluctuations in MBC and MBN reflected variations in moisture, temperature, and substrate quality. Total SOC, LFOC, and LFON increased on the upper slopes and fall sampling time. Although single intensive tillage did not affect total SOC, several tillage operations could accelerate SOM loss and reduced total C storage over time.