Long-term nitrogen addition decreases carbon leaching in a nitrogen-rich forest ecosystem

• Published in: Biogeosciences 2013-06, Vol.10 (6), p.3931-3941
• Main Authors: Lu, X, Gilliam, F. S, Yu, G, Li, L, Mao, Q, Chen, H, Mo, J
• Format: Article
• Language: English
• Subjects: Biological pest control; Ecosystems; Environmental aspects; Forest ecology; Forest soils; Leaching; Old-growth forests
• ISSN: 1726-4189; 1726-4170; 1726-4189
• DOI: 10.5194/bg-10-3931-2013

Dissolved organic carbon (DOC) plays a critical role in the carbon (C) cycle of forest soils, and has been recently connected with global increases in nitrogen (N) deposition. Most studies on effects of elevated N deposition on DOC have been carried out in N-limited temperate regions, with far fewer data available from N-rich ecosystems, especially in the context of chronically elevated N deposition. Furthermore, mechanisms for excess N-induced changes of DOC dynamics have been suggested to be different between the two kinds of ecosystems, because of the different ecosystem N status. The purpose of this study was to experimentally examine how long-term N addition affects DOC dynamics below the primary rooting zones (the upper 20 cm soils) in typically N-rich lowland tropical forests. We have a primary assumption that long-term continuous N addition minimally affects DOC concentrations and effluxes in N-rich tropical forests. Experimental N addition was administered at the following levels: 0, 50, 100 and 150 kg N ha−1 yr−1, respectively. Results showed that seven years of N addition significantly decreased DOC concentrations in soil solution, and chemo-physical controls (solution acidity change and soil sorption) rather than biological controls may mainly account for the decreases, in contrast to other forests. We further found that N addition greatly decreased annual DOC effluxes from the primary rooting zone and increased water-extractable DOC in soils. Our results suggest that long-term N deposition could increase soil C sequestration in the upper soils by decreasing DOC efflux from that layer in N-rich ecosystems, a novel mechanism for continued accumulation of soil C in old-growth forests.