Orychophragmus violaceus/cotton relay intercropping with reduced N application maintains or improves crop productivity and soil carbon and nitrogen fractions

The advantages of cover crops in enhancing soil organic C (SOC) sequestration and curbing reactive N losses have been verified. However, there is still a lack of related research evaluating the effects of cover crops and N fertilization on soil properties and crop performance as well as their potential relationships, especially in the continuous cotton cropland of the Yellow River Basin. Herein, we examined the impacts of two cropping systems (cotton (Gossypium hirsutum L.)-cotton (C-C) continuous cropping and Orychophragmus violaceus (O. violaceus)/ cotton (O/C) relay intercropping) and four N application rates (0 (N0), 112.5 (N1), 168.75 (N2), and 225 (N3) kg N ha-1) on soil chemical properties, cotton yield, biomass, N uptake and N use efficiency (NUE) during 2017-2019 in the Yellow River Basin. In addition, we determined important soil chemical properties and revealed the relationship between soil parameters and lint yield. Compared with the C-C system, the O/C relay intercropping system significantly improved the soil total N (STN), soil organic N (SON), microbial biomass N (MBN), lint yield (1312.84 kg ha-1), biomass (10130.04 kg ha-1) and N uptake (195.68 kg ha-1) by 2%, 2%, 8%, 6%, 7% and 7%, respectively. N application notably increased the soil chemical properties associated with C and N, lint yield, biomass and N uptake, while the NUE of N3 was significantly lower than that of N2. Moreover, the key driving factors that significantly influenced the lint cotton yield were particulate organic N (PON), dissolved organic N (DON), soil ammonia N (SAN), SON, SOC and soil nitrate N (SNN). DON, PON, and SOC were key factors that directly impacted lint yield. According to the Z score of soil parameters, cotton productivity and resource use efficiency, the combination of O/C relay intercropping and N2 yielded the highest total Z score (10.97). Our results demonstrated that O. violaceus/cotton relay intercropping with 75% N application (168.75 kg N ha-1) can maintain higher lint cotton yield, biomass accumulation, N uptake and NUE by increasing soil C and N fractions. Because non-legume crop straw requires a long period of time to decompose, it is necessary to evaluate the legacy impacts of O. violaceus cover cops in long-term experiments in the future.