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Fertilizations affect soil organic carbon (SOC) content but the relative influences of the edaphic and climate factors on SOC storage are rarely studied across wide spatiotemporal scales. This study synthesized long-term datasets of fertilization experiments in six typical Chinese croplands, and calculated annual C input from crops and manure amendments, changes in SOC storage (ΔSOC) and C sequestration efficiency (i.e. the percentage of soil C change per unit of C input, hereafter referred as CSE) in 0–20 cm soil over three decades. Three fertilization treatments include no fertilization (CK), chemical nitrogen, phosphorus and potassium fertilizers (NPK) and combined chemical fertilizers and manure (NPKM). Results showed significant fertilization effects on C input and ΔSOC (NPKM>NPK>CK), and significantly higher CSE in Qiyang at Hunan than Zhengzhou at Henan and Heihe at Heilongjiang. The variance partitioning analysis (VPA) showed more variance of CSE can be explained by edaphic factors (up to 39.7%) than other factors. Furthermore, soil available N content and pH were identified as the major soil properties explaining CSE variance. This study demonstrated key controls of soil fertility factors on SOC sequestration and informs the need to develop strategic soil management plan to promote soil carbon sequestration under long-term intensive fertilization.

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Core Ideas Nitrite concentration was highly correlated with N2O emissions within two distinct water content ranges. Soil moisture was the most important environmental factor affecting N2O emissions. Nitrous oxide emissions increased exponentially as the N application rate increased. Biochar and N transformation inhibitors showed great potential to reduce N2O emissions. A better understanding of the factors and processes affecting N2O emissions is essential for developing mitigation strategies. This research aimed to examine the factors and processes affecting N2O emissions and N dynamics. Laboratory incubation experiments examined the effects of N (urea) application rate (0–150 mg N kg‐1); soil water content (5–30%, w/w); temperature (10–40°C); and incorporation of biochar (1%, w/w), a urease inhibitor (Agrotain Ultra), and a nitrification inhibitor (N‐Serve 24) on N2O emissions and N transformation dynamics in a Hanford sandy loam soil. Nitrous oxide emissions, soil pH, and mineral N species were monitored for 35 d. Peak emission rates and cumulative losses of N2O increased more than the N application rate increased. Soil water content at 20 and 30% [above the water holding capacity (WHC) of 12%] resulted in much higher total N2O emissions (9.3 and 8.0% of total soil inorganic N, respectively) than that from 5 and 10% water content (0.2 and 0.3%, respectively). Increasing soil water content above WHC caused higher N2O emissions than increasing the soil temperature, which demonstrates that soil moisture is more significant in affecting the process. Nitrite concentration was highly correlated with N2O emissions within two water content ranges (above or below WHC). Amendment with biochar, Agrotain Ultra, and N‐Serve 24 reduced N2O emissions by 74, 78, and 74%, respectively. This research provide further understanding of the processes affecting N2O emissions from soil, which can assist in developing management practices.