Fire increases the risk of higher soil N₂O emissions from Mediterranean Macchia ecosystems

Highlights

• Fire increases the risk of N₂O emissions from Mediterranean Macchia ecosystems.

• Post-fire N₂O production was related to increased mineral N concentrations and pH.

• N₂O production rates not intimately linked with rates of biological N mineralisation.

• Post-fire effects on N₂O emissions decrease with time since fire.

Abstract

Intensification of droughts under climate change is projected to increase fire frequency in the Mediterranean region. Fires cause direct emission of greenhouse gases (GHG) such as carbon dioxide (CO₂) and nitrous oxide (N₂O), due to the combustion of organic matter, creating a positive feedback on climate change. However, the potential importance of indirect GHG emissions due to changes in soil biological and chemical properties after fire is less well known. Increased soil mineral nitrogen (N) concentrations after fire pose a risk for increased emissions of gaseous N, but studies on the post-fire N₂O production and soil N turnover rates (mineralization, nitrification, microbial immobilization, denitrification) are still rare. We determined N₂O production, rates of N turnover and pathways for N₂O production from the soil of burned and unburned plots of a Macchia shrubland in central Spain using a ¹⁵N labelling approach. Measurements were initiated before the controlled burning and continued for up to half a year after fire. Fire markedly increased the risk of N₂O emissions from soil through denitrification (N₂O production rate was 3 to ≈30 times higher in burned soils compared to control, with N₂O being produced solely from soil nitrate). In contrast, soil gross N cycling rates were not accelerated after fire. Thus, the increased N₂O production was not closely linked with N mineralization, but may be explained by increased mineral N availability from ash, increased pH in burned plots, and less competition for available N and C sources due to absence of plants.

Introduction

The Mediterranean area is considered to be especially vulnerable to climate change, because it lies at the transition between arid and humid regions of the world (Scarascia-Mugnozza et al., 2000). Climate change projections for the Mediterranean region include warmer temperatures and decreased precipitation, leading to intensification of water scarcity (droughts) (IPCC, 2013). Mediterranean Macchia shrubland ecosystems are naturally affected by fires, but models project increased occurrence of fires for the Mediterranean region with global warming (e.g. Carvalho et al., 2011). This is because reduced water availability in combination with higher temperatures increases fire risk and the length and severity of the fire season (Moreno et al., 2010). Severe fire events during recent years in Spain, Italy and Greece have been linked to temperature anomalies (Amraoui et al., 2013), and heat waves like this are projected to increase with global warming (IPCC, 2013).

Direct CO₂ production during fires significantly contributes to GHG emissions in Mediterranean countries, and thus further accelerates climate change, if the area burned increases due to global warming (Miranda et al., 1994). Burning biomass is also a source of other GHGs such as N₂O and other air pollutants (Crutzen et al., 1979, Laursen et al., 1992). However, the post-fire effects on N₂O emissions due to mid-to long term changes in soil biological and chemical properties are less well known. Only a few studies from Mediterranean ecosystems have concentrated on changes in N₂O production and N cycling after fire (Dannenmann et al., 2011, Fierro and Castaldi, 2011, Inclán et al., 2012). Therefore, the risk for increased post-fire N₂O emissions (Skiba and Smith, 2000) from these ecosystems, and the magnitude of the potential feedback to climate change remains uncertain. Emissions of N₂O could increase as a consequence of changes in C and N cycling after fire (Brown et al., 2012). Fire disturbance combined with other climate change drivers (elevated CO₂, heat, altered precipitation and enhanced N deposition) has been suggested to cause strongly positive interactions between these factors, leading to higher N₂O fluxes than would be expected based on single-factor studies (Niboyet et al., 2011, Brown et al., 2012). This highlights the importance of studying the post-fire effects on soil N₂O fluxes under a global change context.

Our understanding of how fire may affect soil microbial N cycling in Mediterranean ecosystems is still limited. Fire consumes a large proportion of total aboveground N (up to 80% depending on fire severity, Boby et al., 2010), but it also enhances mineral N availability, as N uptake is drastically diminished, and N remains available in the ash mainly as ammonium (Levine et al., 1988, Marion et al., 1991, Prieto-Fernández et al., 2004). However, the extent to which the soil mineral N concentrations could be affected indirectly by altered inorganic N production (mineralization of organic N to ammonium (${\text{NH}}_4^{+}$), and nitrification of ${\text{NH}}_4^{+}$ to nitrate (${\text{NO}}_3^{-}$)) and uptake rates by soil micro-organisms (immobilization of ${\text{NH}}_4^{+}$ or ${\text{NO}}_3^{-}$) is largely unknown (Dannenmann et al., 2011). Fire directly produces ${\text{NH}}_4^{+}$, but ${\text{NO}}_3^{-}$ is not formed in the fire, and requires nitrification to form (Knicker, 2007). Soil pH increases after fire, due to the high cation content of the ash (Jensen et al., 2001), and the increase in pH could stimulate nitrification (Ste-Marie and Paré, 1999) and denitrification (Skiba and Smith, 2000).

It is uncertain whether organic N and C availability to soil microbes increases or decreases after fire (Prieto-Fernández et al., 2004), as there are factors affecting into opposite directions. Microbial biomass in the surface soils decreases immediately after fire (Dumontet et al., 1996, Rutigliano et al., 2007:; Fontúrbel et al., 2012), and the released organic C and N from the micro-organisms killed by the heat could increase substrate availability for the surviving microbes (Andersson et al., 2004), and thus increase mineralization. Organic N content has been found to increase, decrease or stay the same after fire, but in each case with decreased average lability (Prieto-Fernández et al., 2004). Therefore, it is also possible that N mineralization decreases after fire, as the remaining organic N is mainly stored in recalcitrant heteroaromatic N structures formed in the fire (Knicker, 2007). Soil micro-organisms are important for determining the fate of the mineral N released in the fire: it could be either retained in the biomass (Bell and Binkley, 1989) or lost from the ecosystem through the microbial nitrification and denitrification (a process converting ${\text{NO}}_3^{-}$ to N₂O and N₂), both of which are known to be stimulated by N additions (e.g. Levine et al., 1988, Barnard et al., 2005). The fate of the N has also consequences for the plant community, as losses of N through leaching and denitrification could potentially reduce the labile pool of N available for plant growth slowing recovery (Knicker, 2007).

The aim of this study was to measure N₂O emissions, pathways of N₂O production (nitrification and denitrification) and rates of soil N turnover after a fire in a Mediterranean Macchia ecosystem. We hypothesised that N₂O emissions would increase after the fire, due to increased mineral N availability in the soil. We also hypothesised that the effect would decline with time since fire, as recovery of vegetation would increase competition for available N.