Long-term fertilizer application mitigates the impacts of climate conditions on soil nitrogen activation

• Nitrogen activation rate, i.e., slope of correlation between available nitrogen and total nitrogen, declined at later period of fertilization • Soil properties were the main factors regulating nitrogen activation • Impact of climate on soil nitrogen activation was attenuated under long-term fertilization • Interaction between climate and soil had an enhanced impact on nitrogen activation in the later period. Soil nitrogen activation (N act ) is pivotal for the global N cycle, influencing crop N availability and environmental N losses. However, it remains unclear how climate and fertilization individually, or jointly, affect soil N act over multiple decades. Here, we examined the dynamic shifts in N act between the first decade and later period in soils treated with non-fertilizer, mineral fertilizer with/without manure, mineral fertilizer with stover return, at 1982/1990, in six typical agricultural zones. Results revealed that soil total N (TN) and available N (AN) increased at rates of 10.1–58.2 mg kg –1 yr –1 and 1.41–4.13 mg kg –1 yr –1 , respectively, by manure application at five sites, suggesting that manure enhanced both soil N storage and availability. The GZL site exhibited the highest annual change rate (ACR) in TN (48.3–58.2 mg kg –1 yr –1 ), while the ZY site had the lowest (10.1 mg kg –1 yr –1 ). Conversely, the YL site showed the highest ACR in AN (3.65–4.13 mg kg –1 yr –1 ), whereas the ZZ site exhibited the lowest (1.41–2.23 mg kg –1 yr –1 ). Notably, the N act rates with manure application were higher in the first decade (42–181 mg g –1 ) than in the later period (33–92 mg g –1 ) at all sites. Overall, the average N act of all treatments in the first decade (79–105 mg g –1 ) across six study sites was higher than in the later period (30–78 mg g –1 ). Variance portioning analysis indicated that soil properties’ contribution to N act increased from 35% to 45% over time, while climatic conditions’ effect decreased from 19% to 8%. Structural equation modeling confirmed a direct impact of annual temperature on N act , with path coefficients of 0.86 in the first decade and 0.45 in the later period. These results show that the impacts of climate on soil N ac t were attenuated under long-term fertilizer application, while the interactions between climate and soil had an enhanced impact on N act in the later period of fertilization. This context-specific insight can guide soil management strategies to enhance N availability, modulate the effects of climate change on agricultural production, and minimize environmental N losses.