Cover cropping with reduced nitrogen fertilization rate enhances soil carbon but not nitrogen in the malt barley–pea rotation

Reduction in N fertilization rate due to N supplied by pea (Pisum sativum L.) may enhance soil C and N in the malt barley (Hordeum vulgare L.)–pea rotation. The effect of winter cover cropping (oat [Avena sativa L.] vs. none) and N fertilization rate (0, 40, 50, 60, 70, and 80 kg N ha⁻¹) for malt barley (NR) was evaluated on soil C and N stocks at the 0- to 120-cm depth from 2013 to 2019 in the dryland malt barley–pea rotation in the US northern Great Plains. Carbon and N stocks were soil total C (STC), soil organic C (SOC), soil inorganic C (SIC), and soil total N (STN). Cover crop C and N were greater for 40 kg N ha⁻¹ than other NRs and crop residue C and N were greater in 2016 than other years. The STC and SOC at 0–15 and 15–30 cm increased by 0.02 and 0.06 Mg C ha⁻¹ kg⁻¹ N in 2016 and 2019, respectively (p ≤ 0.10). The STC at 0–15 and 15–30 cm was greater at 40 than 50 and 70 kg N ha ⁻¹ in 2019. The STN at 15–30 cm increased by 0.004 Mg N ha⁻¹ kg⁻¹ N with than without cover cropping. The STC, SOC, and STN at 0–15 and 15–30 cm also increased at greater rates from 2013 to 2019 with than without cover cropping (p ≤ 0.10). Cover cropping with reduced N fertilization rate for malt barley (NR) can enhance soil C but not soil N stock at surface layers in the malt barley–pea rotation.