Nitrogen uptake and nitrogen use efficiency above and below ground along a topographic gradient of soil nitrogen availability.
Nitrogen (N) uptake and nitrogen use efficiency (NUE) are closely related through feedback mechanisms to soil N availability and N cycling in forested ecosystems. We investigated N uptake and NUE not only at the leaf, litterfall, and aboveground levels but also belowground and whole stand levels along a topographic gradient of soil N availability in a cool temperate deciduous forest in Japan. In this study, we addressed how whole stand level N uptake and NUE affect C and N cycling in forested ecosystems. At the leaf, litterfall, and aboveground levels, N uptake decreased and NUE increased with decreasing soil N availability. This pattern resulted from decreasing leaf N concentrations and increasing N resorption efficiencies as soil N availability declined. Low N concentrations in litterfall may have resulted in little soil N being available to plants, due to microbial immobilization. In contrast, when belowground components were included, N uptake and NUE were not correlated with soil N availability. This was mainly due to higher levels of fine root production when soil N availability was low. Higher fine root allocation can result in a high input of detritus to decomposer systems and, thus, contribute to accumulation of soil organic matter and immobilization by microbes, which may result in further soil N availability decline. Our results suggest that allocation to the fine root rather than whole stand level NUE is important for C and N cycling in forested ecosystems, as is the feedback mechanism in which litterfall level NUE shifts with changes in the N concentration of litterfall.
Tateno
Ryunosuke
R
Graduate School of Agriculture, Kyoto University, Kyoto, Japan. tateno@agri.kagoshima-u.ac.jp
Takeda
Hiroshi
H
D002244:Carbon / Q000378:metabolism; D004784:Environmental Monitoring; D007564:Japan; D009584:Nitrogen / Q000378:metabolism*; D018515:Plant Leaves / Q000254:growth & development / Q000378:metabolism; D018517:Plant Roots / Q000254:growth & development / Q000378:metabolism; D012987:Soil / Q000032:analysis*; D014197:Trees / Q000145:classification / Q000254:growth & development / Q000378:metabolism*
D012987:Soil; D002244:Carbon; D009584:Nitrogen