Effect of controlled-release urea fertilizers for oilseed rape (Brassica napus L.) on soil carbon storage and CO2 emission

Fertilizer-induced CO₂ emission is a primary driver of global warming. The experiment was used to study whether controlled-release urea (CRU) application in winter oilseed rape can play a positive role in mitigating CO₂ emission and promoting C utilization by soil microorganisms. Five fertilizer types consisted of N0 (0 g N plant⁻¹), conventional CRU application (CRU100%), monotypic CRU at the 80% of conventional rate (CRU80%), co-application of CRU with uncoated urea (CRC), and organic fertilizer (CRO). Results showed that soil CO₂ fluxes were significantly affected by N fertilizer types after the start of the stem growing (P < 0.05). CO₂ emissions typically peaked during the seed filling period, with the highest emission of 1.99 μmol m⁻² s⁻¹ being registered for CRU100%. CRU100% had 25.00%, 30.60%, and 4.17% greater CO₂ emissions than CRU80%, CRC, and CRO practices by harvest, respectively. Compared to the conventional CRU treatment, CRU80% led to a lower root volume and root mass ratio than CRU100%, which could partly contribute to the reduced CO₂ emission. Conversely, CRU80% performed better in N agronomic efficiency than that of CRU100% treatment. Also, C source utilization by soil microbiomes as well as microbial diversity indices following CRU80% along with CRO applications was substantially higher than that under the conventional CRU supply. These observations suggest that opportunity exists to maintain N balance by N fertilization practices to mitigate CO₂ emission from cropland. Further, a close and positive relationship between soil total nitrogen and CO₂ emission also supports this. CRO-treated soils substantially elevated the contents of total carbon and readily oxidation carbon over CK. Moreover, the enzyme activity of β-glucosidase in CRO soil was about twice as high as the CRU100%. Consequently, CRU amendments by decreasing CRU rate application and the incorporation of organic fertilizer into CRU have the potential for mitigating of CO₂ emission and positive effect on the soil microbial functional diversity to improve nitrogen use efficiency of rapeseed.