Anthropogenic nitrogen deposition enhances carbon sequestration in boreal soils

Abstract

It is proposed that carbon (C) sequestration in response to reactive nitrogen (Nr ) deposition in boreal forests accounts for a large portion of the terrestrial sink for anthropogenic CO2 emissions. While studies have helped clarify the magnitude by which Nr deposition enhances C sequestration by forest vegetation, there remains a paucity of long-term experimental studies evaluating how soil C pools respond. We conducted a long-term experiment, maintained since 1996, consisting of three N addition levels (0, 12.5, and 50 kg N ha(-1) yr(-1) ) in the boreal zone of northern Sweden to understand how atmospheric Nr deposition affects soil C accumulation, soil microbial communities, and soil respiration. We hypothesized that soil C sequestration will increase, and soil microbial biomass and soil respiration will decrease, with disproportionately large changes expected compared to low levels of N addition. Our data showed that the low N addition treatment caused a non-significant increase in the organic horizon C pool of ~15% and a significant increase of ~30% in response to the high N treatment relative to the control. The relationship between C sequestration and N addition in the organic horizon was linear, with a slope of 10 kg C kg(-1) N. We also found a concomitant decrease in total microbial and fungal biomasses and a ~11% reduction in soil respiration in response to the high N treatment. Our data complement previous data from the same study system describing aboveground C sequestration, indicating a total ecosystem sequestration rate of 26 kg C kg(-1) N. These estimates are far lower than suggested by some previous modeling studies, and thus will help improve and validate current modeling efforts aimed at separating the effect of multiple global change factors on the C balance of the boreal region.

Initial shifts in nitrogen impact on ecosystem carbon fluxes in an alpine meadow: patterns and causes

“…However, in the second year, soil microbial respiration declined under high N addition levels, in combination with the low root respiration, resulting in decreases in SR under N16 and N32 treatments. This decrease in SR was also observed in other ecosystems under long-term or high levels of N addition (Yan et al, 2010;Maaroufi et al, 2015). We are fully aware that there are some limitations for the partitioning technique, in which we used deep versus shallow collars to partition root from microbial respiration.…”

“…Similarly, net ecosystem CO 2 exchange (NEE) and its components of gross ecosystem production (GEP) and ecosystem respiration (ER) may also respond nonlinearly to increasing N loading rates (Fleischer et al, 2013;Gomez-Casanovas et al, 2016;Tian et al, 2016). In the N-limited stage, low rates of N addition could stimulate ecosystem productivity (Aber et al, 1989), GEP (Fleischer et al, 2013;Gomez-Casanovas et al, 2016), and ER (Hasselquist et al, 2012;Zhu et al, 2016), while in the N saturation stage, high doses of N addition could have negative effects on GEP and ER (Treseder, 2008;Janssens et al, 2010;Maaroufi et al, 2015). The unbalanced responses of GEP and ER may lead to changes in NEE.…”

Are nitrate production and retention processes in subtropical acidic forest soils responsive to ammonium deposition?

“…Our previous field investigations have shown that fungal biomass and fungi to bacteria ratios (Wang et al, 2015) and decomposition of lower-order roots and needles (Kou et al, 2015a(Kou et al, , 2015b) exhibited a decreasing trend at high NH 4 þ additions. Moreover, long-term elevated N input has been found to enhance chemical stabilization of organic matter into recalcitrant compounds that are resistant to microbial decay (Neff et al, 2002;Swanston et al, 2004), thereby potentially impairing fungal metabolism (Maaroufi et al, 2015). With increasing additions of NH 4 þ , microbial NH 4 þ cycling shifted from a state of decoupling (gross NH 4 þ immobilization rates were incomparable to gross N mineralization rates) to coupling (gross NH 4 þ immobilization rates were comparable to gross N mineralization rates).…”

Genotypic variability in Populus tremula L. affects how anthropogenic nitrogen enrichment influences litter decomposition

“…While impacts of N enrichment on aboveground properties are well studied, relatively little is known about how soil processes respond to N enrichment (but see e.g. Janssens et al 2010;Maaroufi et al 2015), and in particular whether genetic variation within plant species modifies how soil processes respond to eutrophication (Sundqvist et al 2012;Kumordzi et al 2014).…”