Forest regrowth and plantation have been found to be the major drivers of global C sink in land ecosystems. In China, the largest area of plantation forest in the world has been achieved by implementing several national-scale forestry projects, including the so-called "Grain for Green"project (GGP) since the end of the 20th century. Due to the short period of GGP implementation, it is hard to predict long-term N dynamics based on investigation on restored sites. Yet soil nitrogen (N) dynamics are crucial for ecosystem development and carbon sequestration. It is necessary to explore N dynamics following agricultural abandonment in order to predict the velocity of secondary succession and the capacity of ecosystem C sequestration.

In order to detect how N status change along with succession, a team of researchers in the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA) investigated soil N stock and N cycling indicators (e.g.soil dissolved inorganic N concentration, foliar N concentration, 15N of foliar and soil etc.) in cropland, grassland, shrubland, secondary forest and primary forest in a large portion of the karst region in southwest China with the grassland, shrubland and secondary forest developed spontaneously from abandoned cropland.

The researchers found that soil N accumulated with an average rate of 12.4 g N m ^-2 yr ^-1 across the succession sequence and soil N stock recovered to the primary forest level in about 67 years following agricultural abandonment. They also found that the rates of net N mineralization and nitrification increased with succession. δ¹⁵N of soil, leaves and litter was significantly elevated in the later three succession stages relative to the grassland, indicating that the N cycle became more open with succession. N release from bedrock weathering was likely a potential N source in addition to atmospheric N deposition and biological N fixation. In addition, the idea of N would not be the limiting nutrient for secondary succession and ecological restoration in the karst region of southwest China was suggested.

This study was supported by the National Key Research and Development Program of China (2016YFC 0502404), the National Key Basic Research Program of China (2015CB452703), the Chinese Academy of Sciences through its One-Hundred Talent Program (Y523101030, Y351025090), and the National Natural Science Foundation of China (41571295, 31400462, 31270555).

The study entitled "Rapid recuperation of soil nitrogen following agricultural abandonment in a karst area, southwest China" has been published in Volume 129, Issue 2016 of Biogeochemistry, details could be found at http://link.springer.com/article/10.1007/s10533-016-0235-3

Contact: LI Dejun

E-mail: dejunli@isa.ac.cn

Institute of Subtropical Agriculture, Chinese Academy of Sciences