Soil organic C and N stock changes in grass-clover leys: Effect of grassland proportion and organic fertilizer

Grass-clover leys in crop rotations on dairy farms may contribute to mitigation of climate change through soil organic carbon (SOC) sequestration. However, accurately quantifying SOC sequestration potential of grass-clover leys is a challenge as long-term experiments with regular soil sampling and crop rotations with varying grassland proportion are extremely rare. Based on a long-term organic dairy crop rotation experiment at Foulumgaard Experimental Station (Denmark) initiated in 1987 with contrasting crop rotations (2 or 4 years of grass-clover in a six-course rotation) and cattle slurry input from 2006 to 2018, we examined the effect of grassland proportion (31 to 69% grass-clover) and slurry C-input (0 to 1.45 Mg C ha−1 yr−1) on SOC storage in the top soil layer (0–20 cm). At steady-state conditions, the SOC stock based on equivalent soil mass (SOC stockFM) increased by 5.0 Mg C ha−1 when converting soil with a prehistory of cereal dominated cropping to a crop rotation with 1/3 grass-clover. The steady-state condition was established after 20 years, and the rate of change was largest in the initial years. A multiple regression model including grassland proportion, slurry C-input and the initial SOC stockFM explained 47% of the variation in the SOC stock from 2005 to 2018. The SOC stockFM increased 4.2 Mg C ha−1 when increasing the grassland proportion from 1/3 to 2/3 during the 13 years (0.32 Mg C ha−1 yr−1). Of the applied slurry-C, 11% was retained in soil, which means that an annual increase in cattle slurry input of 1.5 Mg C ha−1 (150 kg total-N ha−1) will increase SOC stockFM by 2.2 Mg C ha−1 (0.17 Mg C ha−1 yr−1) during the 13 years. Further, our study showed that soil nitrogen is stored along with SOC in equal proportions.