Elevated active nitrogen (N) deposition is occurring in many areas around world, especially in China. The effects of elevated N deposition on forest soil carbon (C) pools and cycles have been widely documented; soil C pools are generally found to be enhanced under N addition, which is associated with higher C input derived from above-and/or below-ground litter and lower C-CO2 loss. However, leaching of dissolved organic carbon (DOC), an important component in the C cycle that plays a significant role in soil C transport, transformation and formation in the mineral soil, is less documented, and the responses of DOC to N addition are debatable and unclear.
Here, in a montane evergreen forest located at the eastern edge of the Tibetan Plateau, 2 levels of N were applied over 2 years to determine the effects of N addition on DOC release from organic (O layer) and mineral soil. Frequent sampling revealed that high levels of N addition could decrease the concentration of DOC and the flux from the O layer but not from mineral soil and that moderate N addition had no effect on DOC leaching from either the O or mineral layer. The effect of N addition on DOC leaching from the O layer was seasonally dependent, showing a significant reduction in DOC leaching during autumn/winter but no changes during summer and spring. This seasonally different response of DOC to N addition affected the seasonal pattern of DOC leaching.
Soil and leachate pH were not influenced by N addition in the short term, indicating that there was not enough difference in DOC retention by mineral soil to significantly affect DOC leaching under N addition. In contrast, N addition-derived reduction in DOC leaching was likely to be due to suppressed fresh litterfall–derived DOC production during autumn/winter; this speculation was supported by lower values of O layer water-extractable organic carbon and microbial biomass carbon as well as lower saccharase and cellulose activities found with high N addition. These results suggested that the processes in control of DOC leaching and their responses to N addition were different for O and mineral soil and that short-term N addition could decrease O-layer DOC leaching, which is likely associated with decreased DOC production rather than greater DOC retention.
This study has been supported by the National Key Research and Development Program of China (2016YFC0502105), the Youth Innovation Promotion Association CAS (2018406), and other funds (QYZDJ-SSW-DQC006, SDS-135-1702, 41301219).
This study entitled Nitrogen addition reduces dissolved organic carbon leaching in a montane forest has been published online in Soil Biology and Biochemistry.
Figure 1 Seasonal patterns of dissolved organic carbon(DOC) and total dissolved nitrogen (TDN) concentrations (A) and fluxes (B) from the organic layer and mineral layer (O layer+ the top 20？cm of mineral soil) under different treatments. Values are the mean ± 1SE. The black dashed line indicates the date of N addition (Image by CHANG Ruiying).
Dr. CHANG Ruiying
Institute of Mountain Hazards and Environment, Chinese Academy of Sciences
Chengdu, Sichuan, 610041, China