A limited effect of cover crops on soil C-sequestration and nitrogen retention in dry continental climates due to water stress and N lockup

Cover crops (CC) provide ecosystem services such as nitrogen retention (NR) and increased soil organic carbon (SOC), but in dry continental climates their short-term benefits may be limited.

This study evaluated the effects of a conservation management system combining CC and non-inversion tillage (MT+CC) on NR, C:N ratio, and SOC over a full crop rotation (sunflower–winter wheat–corn–sunflower) in south-eastern Romania, compared with conventional ploughing tillage (PT). A randomized block design (three blocks × six plots × management) was followed on clay-loamy Luvisol. NR was estimated annually from soil mineral-Nitrogen and the biomass and N-content of CC and weeds prior to sunflower and corn sowing.

NR was significantly higher under MT+CC than PT during the first three years, averaging +20.30 kg/ha before corn and +26.67 kg/ha before sunflower. However, this difference declined over time, and in the fourth year, NR before sunflower sowing was higher under PT due to intensive weed biomass growth. Each year, MT+CC produced significantly lower yields for corn and sunflower (p<0.01) due to water competition and N lockup, but higher yields for winter wheat (p<0.001) than PT.

No significant SOC differences were observed in the first year; however, after four years SOC was significantly higher under MT+CC (1.42%) than PT (1.37%) (p<0.05). Total nitrogen remained unchanged, leading to an increased C:N ratio under MT+CC. In dry continental climates, N immobilization and drought slow soil organic matter accumulation, raising questions about the suitability of current crop rotation and CC mixtures with low legume proportions.