Knowledge on ecosystem nitrogen status is necessary for assessing ecosystem carbon sequestration capacity under global change and for predicting the direction and velocity of secondary succession. Although several ecological restoration projects have been implemented over the past 20 years, whether ecological restoration is limited by nitrogen remains unresolved in the karst region of southwest China. Additionally, it is still not clear whether the nitrogen status differs between karst and non-karst forests. Here, we synthesized the main findings obtained by our research group during the past several years. Studies based on a post-agricultural succession sequence or across a karst region show that soil total nitrogen pool can accumulate rapidly following agricultural abandonment. Due to the tight coupling of carbon and nitrogen cycles, soil organic carbon pool can also accumulate rapidly with the relative accumulation rate of soil organic carbon greater than that of total nitrogen. Across the post-agricultural succession sequence, plants and microbes are limited by nitrogen in grassland stage, but the secondary forests show clear symptoms of nitrogen saturation. Several lines of evidence supports that nitrogen saturation is a unique characteristic for karst forests, since the surrounding nonkarst forests are limited by nitrogen. Our findings suggest that (1) nitrogen is the limiting nutrient only in a short period of the early succession stage following agricultural abandonment in the karst region of southwest China, (2) sufficient nitrogen supply ensures relatively high carbon sequestration after the ecological restoration project implementation in the karst region, (3) due to their unique nitrogen biogeochemical cycling, the responses and adaptation of karst forests to global change may be different from other types of forests.