Stimulation of anaerobic organic matter decomposition by subsurface organic N addition in tundra soils

Highlights

• Field and lab experiments measured the effect of N addition on CH₄ production.

• N addition increased fermentation in all experiments.

• Increasing Fe(II), DIC, and CH₄ indicated increasing anaerobic C mineralization.

• Non-aromatic DOC was selectively degraded following N addition.

• Increasing N availability could increase tundra CH₄ emissions.

Abstract

Increasing nitrogen (N) availability in Arctic soils could stimulate the growth of both plants and microorganisms by relieving the constraints of nutrient limitation. It was hypothesized that organic N addition to anoxic tundra soil would increase CH₄ production by stimulating the fermentation of labile substrates, which is considered the rate-limiting step in anaerobic C mineralization. We tested this hypothesis through both field and lab-based experiments. In the field experiment, we injected a solution of ¹³C- and ¹⁵N-labeled glutamate 35 cm belowground at a site near Nome on the Seward Peninsula, Alaska, and observed the resulting changes in porewater geochemistry and dissolved greenhouse gas concentrations. The concentration of free glutamate declined rapidly within hours of injection, and the ¹⁵N label was recovered almost exclusively as dissolved organic N within 62 h. These results indicate rapid microbial assimilation of the added N and transformation into novel organic compounds. We observed increasing concentrations of dissolved CH₄ and Fe(II), indicating rapid stimulation of methanogenesis and Fe(III) reduction. Low molecular weight organic acids such as acetate and propionate accumulated despite increasing consumption through anaerobic C mineralization. A laboratory soil column flow experiment using active layer soil collected from the same site further supported these findings. Glutamate recovery was low compared to a conservative bromide tracer, but concentrations of NO₃⁻ and NH₄⁺ remained low, consistent with microbial uptake of the added N. Similar to the field experiment, we observed both increasing Fe(II) and organic acid concentrations. Together, these results support our hypothesis of increased fermentation in response to organic N addition and suggest that increasing N availability could accelerate CH₄ production in tundra soils.

Introduction

Experiments, observations, and model simulations reveal that Arctic ecosystems are being transformed through the combined effects of rising temperature and by increasing N availability. There are a number of complementary and competing pathways through which this could happen. First, higher temperatures increase organic matter decomposition, releasing some of the N stored within and increasing its availability (Nadelhoffer et al., 1991; Hobbie, 1996; Schaeffer et al., 2013; Salmon et al., 2016). Second, thawing permafrost can liberate previously inaccessible inorganic and labile organic N (Keuper et al., 2012). Third, improved landscape drainage associated with permafrost thaw in some parts of the Arctic (Liljedahl et al., 2016) could increase ecosystem N availability by increasing aerobic N mineralization and decreasing N loss via denitrification (Heikoop et al., 2015; Newman et al., 2015). Finally, warming is causing northward expansion of shrub vegetation, including alder species (Alnus spp.) that harbor N-fixing symbionts (Tape et al., 2006; Myers-Smith et al., 2011).

The increase in N availability will potentially affect both inputs and outputs of soil C, and the net impact is uncertain. It is well established that plant growth in tundra soils is N-limited and C inputs to the soil increase in response to increasing N availability (Shaver and Chapin, 1980, 1986; Hobbie and Chapin, 1998). However, the effects on C mineralization are contradictory. Some studies indicate N addition can cause large increases in organic matter decomposition. For example, a 20-year fertilization experiment at Toolik Lake, Alaska, resulted in net loss of soil C despite a large increase in soil C inputs via increased plant growth (Mack et al., 2004). This appeared to be driven by increasing microbial production of C-degrading exoenzymes in response to fertilization (Koyama et al., 2013). In contrast, others have observed that N fertilization had no effect or reduced total C mineralization due to reduced N mining from recalcitrant or mineral-associated organic matter (Neff et al., 2002; Craine et al., 2007; Allison et al., 2008).

Permafrost, due to low permeability of ice in frozen soils, results in poor drainage of many Arctic landscapes, so understanding the effects of increasing N availability on anaerobic C mineralization and hence CH₄ emissions is particularly important for predicting the permafrost-C feedback. Global meta-analysis indicates that fertilization experiments typically result in increased CH₄ emissions from wetland soils, both through stimulation of methanogens and inhibition of methane oxidation (Liu and Greaver, 2009). However, most of these studies were conducted on temperate and tropical soils, and studies of high-latitude ecosystems were focused on peatlands.

Studies that evaluate long-term fertilization of peatlands report conflicting effects on CH₄ emissions (Saarnio and Silvola, 1999; Keller et al., 2005, 2006; Lund et al., 2009; Eriksson et al., 2010; Juutinen et al., 2018). When emissions increase, it appears to be driven by the replacement of Sphagnum with sedge and dwarf shrub vegetation, which provide higher quality litter, increase the soil pH, and facilitate soil-atmosphere gas exchange through aerenchyma (Saarnio and Silvola, 1999; Eriksson et al., 2010; Juutinen et al., 2018). This mechanism is not relevant to tundra ecosystems with low moss cover. In addition, these studies generally apply inorganic fertilizer as an N source. Inorganic nitrogen as nitrate is a high potential electron acceptor, and ammonia can be oxidized aerobically or anaerobically, which could alter both soil pH and redox potential. These factors make it difficult to extend insights from peatland fertilization experiments to predicting the potential impact of increases in subsurface N availability on anaerobic C mineralization and ultimately CH₄ emissions in tundra soils.

The extent to which N addition will stimulate anaerobic C mineralization likely depends on its effect on soil organic matter (SOM) decomposition and fermentation rates. Laboratory incubations indicate CH₄ production exhibits an initial lag phase after thawing, until low-molecular weight organic acids (the primary substrates for methanogenesis in tundra soils) have accumulated (Herndon et al., 2015; Roy Chowdhury et al., 2015; Throckmorton et al., 2015; Vaughn et al., 2016). Models of anaerobic SOM decomposition incorporating insights from these experiments identify the hydrolysis and fermentation of more complex organic matter into labile organic acids as the rate-limiting step (Tang et al., 2016; Zheng et al., 2018b). Therefore, the effect of N fertilization on CH₄ production will depend in part on changes in the supply of labile C substrates through fermentation.

We performed both field and laboratory experiments to investigate changes in the pathways of anaerobic organic matter degradation following subsurface organic N addition. This approach allowed frequent monitoring of the porewater geochemistry, and analysis of the production and consumption of fermentation products. In both sets of experiments, we used the amino acid glutamate as a labile source of N. Due to scarcity and strong competition for N, amino acids such as glutamate are the predominant form of available N in many Arctic soils, and are assimilated within hours of introduction (Kielland, 1995; Jones and Hodge, 1999; Jones and Kielland, 2002). We hypothesized that N addition would increase anaerobic C mineralization and specifically CH₄ production by stimulating the production of labile organic acids through fermentation.