The mechanism underlying plant nitrogen limitation following vegetation restoration in karst regions

The unique geological structure combined with human activities cause serious rocky desertification in fragile karst regions, which restricts regional social and economic developments. Vegetation restoration is the key practice of comprehensive administration of rocky desertification, but this process is extremely slow, especially in some special karst geomorphic units. Nitrogen (N) element has been suggested to be a critical limiting factor for vegetation growth, but the characteristics of soil N supply and plant N acquisition remain largely unknown in karst regions. This hinders our better understanding of vegetation restoration of karst rocky desertification as well as its restoration effects. We chose natural succession sequences with different vegetation restoration stages in karst peak-cluster depression and faulted basin regions. Vegetation survey and data collection were conducted, and the N/phosphorus (P) ratio, N content and δ15N values of plant leaf were used to reflect the degree of plant N limitation. In addition, 15N labeling techniques were employed to investigate soil gross N transformation rates, available N supply capacity and N acquisition characteristics of the dominate plant species during vegetation succession. We found that plants were severely limited by N in the early stages of vegetation restoration, which was more seriously in the karst fault basin. As vegetation recovered, plants were no longer limited by N but by P. This difference was mainly attributed to the changes in soil N supply capacity and plant N utilization strategies. In the early stages of vegetation restoration, the rates of soil N supply processes including mineralization and nitrification was weak and inorganic N was mainly ammonium. In the later stages, soil inorganic N supply capacity increased significantly, resulting in higher inorganic N content dominated by nitrate. In such N condition, plants can adjust their own root functional traits to develop different N utilization strategies. Plants develop larger specific root length and specific surface area in the early stages to increase ammonium utilization, but plants improve nitrate utilization in the later stages. Overall, our results unraveled the mechanism underlying reduced plant N limitation following vegetation restoration through increasing soil inorganic N supply and adjusting plant N utilization strategy. The present study provided a scientific basis for ecological restoration and reconstruction of karst rocky desertification.Keywords: Rocky desertification; Nitrogen availability; Plant N limitation; Plant N utilization strategy; Gross N transformation rates