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.