Excess of Organic Carbon in Mountain Spruce Forest Soils after Bark Beetle Outbreak Altered Microbial N Transformations and Mitigated N-Saturation

Abstract

Mountain forests in National park Bohemian Forest (Czech Republic) were affected by bark beetle attack and windthrows in 2004–2008, followed by an extensive tree dieback. We evaluated changes in the biochemistry of the uppermost soil horizons with the emphasis on carbon (C) and nitrogen (N) cycling in a near-natural spruce (Picea abies) mountain forest after the forest dieback, and compared it with an undisturbed control plot of similar age, climate, elevation, deposition, N-saturation level, and land use history. We hypothesised that the high litter input after forest dieback at the disturbed plot and its consequent decomposition might influence the availability of C for microorganisms, and consequently, N transformations in the soil. The concentrations of dissolved organic C (DOC) and N (DON) in soil water extracts rapidly increased at the disturbed plot for 3 yeas and then continually decreased. Net ammonification exhibited a similar trend as DOC and DON, indicating elevated mineralization. Despite the high ammonium concentrations found after the forest dieback (an increase from 0.5 mmol kg-1 to 2–3 mmol kg-1), net nitrification was stable and low during these 3 years. After the DOC depletion and decrease in microbial biomass 5 years after the forest dieback, net nitrification started to rise, and nitrate concentrations increased from 0.2–1 mmol kg-1 to 2–3 mmol kg-1. Our results emphasize the key role of the availability of organic C in microbial N transformations, which probably promoted microbial heterotrophic activity at the expense of slow-growing nitrifiers.

Water quality following extensive beetle-induced tree mortality: Interplay of aromatic carbon loading, disinfection byproducts, and hydrologic drivers

“…Averaged TSUVA values were 150% higher in the high impact sites compared to moderate and low impact watersheds, which tended to overlap throughout most of the study (Figure 3a, Figure SI-1b). The increased TSUVA in surface waters exhibited here at the catchment scale in high impacted watersheds supports prior findings of elevated soil digest TSUVA found below trees in beetle infested forest plots when contrasted with uninfested controls (Kaňa et al, 2015).…”

Factors Affecting the Leaching of Dissolved Organic Carbon after Tree Dieback in an Unmanaged European Mountain Forest

“…3a). 42 This NO 3 production increased concentrations of electron acceptors in the system, available for NO 3 reducing microbes (denitrification and dissimilatory nitrate reduction to ammonium) in anoxic soil micro-sites, which could further reduce the pool of DOC available for leaching. 25,26,38 Decreasing NO 3 availability in soils due to reduced excess N supply from mineralization of dead biomass and increasing N uptake by re-growing vegetation, and the reduced SO runoff events.…”

Riparian zones attenuate nitrogen loss following bark beetle‐induced lodgepole pine mortality

“…No change in soil water DOC is consistent with findings that soil heterotrophic respiration remained constant [ Borkhuu et al ., ] or decreased [ Moore et al ., ], possibly because decreased root exudation and mycorrhizal turnover may be temporarily balanced by increased litter decomposition [ Xiong et al ., ; Mikkelson et al ., ]. Our findings of elevated NO 3 but unchanged DOC support the idea that by the fourth and fifth years following beetle outbreak, DOC limitation reduced the capacity of fast‐growing heterotrophs to compete for available N, favoring nitrification, and NO 3 loss to groundwater [ Kaňa et al ., , ]. Increased NO 3 leaching to groundwater could also be facilitated both by temporal asynchrony between snowmelt periods of high soil water NO 3 and potential uptake by short regenerating vegetation that is more likely to be snow‐covered than is overstory [ Meixner et al ., ].…”

“…B‐[ Rhoades et al ., ]. C‐[ Knight et al ., ; Parsons et al ., ; Kaňa et al ., ]. D‐[ Hill , ; Hedin et al ., ; McClain et al ., ; Vidon and Hill , ; Hefting et al ., ; Näsholm et al ., ].…”

“…In the recent bark beetle infestation, several mechanisms have been proposed to explain why increased N inputs to the forest floor and N concentrations in soils [ Huber , ; Morehouse et al ., ; Clow et al ., ; Griffin et al ., ; Griffin and Turner , ] have not impacted stream water chemistry in large basins [ Clow et al ., ; Mikkelson et al ., ; Rhoades et al ., ]. First, N may be recycled by soil microbial communities, which appear to be resilient so long as sufficient carbon substrate remains available [ Ferrenberg et al ., ; Kaňa et al ., ]. Second, surviving vegetation may take up available N if it is scarce, as in many western conifer forests [ Fahey et al ., ].…”