Fig. 1

Conceptual model of changes in the rate of biomass accumulation, N leaching, N availability and foliar δ¹⁵N during forest secondary succession. Stage (I): The early stage. In the beginning of secondary succession after disturbances by forest fire or logging, there was little aboveground biomass. The soil surface temperature increased as did the rate of soil mineralization and nitrification. The plant N demand was much lower relative to the soil N supply, i.e., high N availability. With the rapid recovery of tree biomass, N leaching, N availability and δ¹⁵N decreased rapidly from the initial maximum due to the great N demand for plant growth. Stage (II): Plant biomass accumulation (but the accumulation rate declined), soil organic matter accumulation, and soil C/N ratio increased in this stage. Although soil mineralization and nitrification were still higher than those before disturbances, a portion of soil N was immobilized in organic matter and the N demand for plant growth increased gradually. Therefore, soil N supply could not meet the N demand for plants. Soil N leaching progressively declined to extremely low levels, and N availability and foliar δ¹⁵N also decreased. Stage (III): The mature stage. Forest biomass accumulation still increased, although the accumulation rate continued to decrease. At this point, the N demand for plants began to decline with the re-accumulation of soil N. The soil N supply exceeded the plant N demand. Nitrogen availability, foliar δ¹⁵N and N leaching began to increase. Stage (IV): The late stage of succession. The ecosystem gradually reached a new steady state as plant biomass accumulation ceased. Ecosystem N input approximately equaled the output. Nitrogen was no longer the limiting nutrient for plant growth while base cation limitation increased. The horizontal dashed lines in b, c and d represent the N input equal output, pre-disturbance N availability and pre-disturbance δ¹⁵N, respectively