Soil Nitrogen Five Years after Bark Beetle Infestation in Lodgepole Pine Forests

Abstract

The unprecedented scale of recent mountain pine beetle (Dendroctonus ponderosae, MPB) infestation in lodgepole pine (Pinus contorta ex Loudon var. latifolia Engelm. ex S. Watson) forests of western North America may significantly impact the role of forest soils as important sinks for global C and N. The aim of this study was to explain soil and litter N responses in stands experiencing total mortality during the first 5 yr of MPB outbreak. Four-year monitoring (2009–2012) of needlefall deposition, soil water-filled pore space (WFPS), NH₄, NO₃, potentially mineralizable N (PMN), dissolved organic N (DON), N₂O, and CH₄ was established in Medicine Bow National Forest in stands of different infestation histories (infested in 2007, 2008, and uninfested). Results suggest that total stand mortality demonstrated strong temporal dynamics. First-year soil responses included increases in PMN and were followed by WFPS and NH₄ increases in the second year, NO₃ increase in the third year, and DON increase in in the fourth year of the infestation. Litter NH₄ increased within 1 yr, but litter PMN, NO₃, and DON increased 3 yr after the infestation, suggesting enhanced litter decomposability. Soil N₂O and CH₄ fluxes also started to increase 4 yr after the infestation. These changes were indicative of ongoing disequilibrium for the period of at least 5 yr after MPB infestation. The magnitude of N mineralization and N₂O and CH₄ fluxes may continue to increase beyond the initial 5 yr of monitoring because litter-derived labile N will probably contribute to the second pulse of N mineralization.