Anthropogenic nitrogen enrichment increased the efficiency of belowground biomass production in a boreal forest

Highlights

• High N addition doubled Pinus sylvestris fine-root production.

• High N addition reduced autotrophic soil respiration by one third.

• Low N additions had no effect on fine-roots or soil respiration.

• Neither low or high N addition had any effect on EMF biomass or production.

• High levels of N deposition enhance fine-root production efficiency.

Abstract

Anthropogenic nitrogen (N) enrichment in boreal forests has been shown to enhance aboveground net primary production and downregulate soil respiration, but it is not well understood if these effects are driven by reduced belowground C allocation or shifts between biomass production and respiration in fine-roots and ectomycorrhizal fungi (EMF). We utilized an experiment in a Pinus sylvestris (L.) forest simulating anthropogenic N enrichment with additions of low (3, 6, and 12 kg N ha⁻¹ yr⁻¹) and high (50 kg N ha⁻¹ yr⁻¹ × 12 yr) doses of N (n = 6) and measured the production of needles, fine-roots, and EMF mycelium during the 12th and 13th year of the experiment. We created a biomass production efficiency index by relating the biomass production rate to root-associated respiration, including both root and EMF respiration. The high N treatment enhanced the production of both needles and fine-roots, with a relatively larger increase in fine-roots, and strongly increased fine-root biomass production efficiency but had no effect on the fungal biomass in fine-roots or the production of EMF mycelium. The low N treatments had no effect on any of the measured variables. These results show that high levels of N enrichment drive shifts in the use of C allocated below ground, with less C going towards metabolic functions that result in rapid C emissions, and more C going towards the production of new tissues.

Introduction

Nitrogen (N) is an essential component of plant biomass and often limits net primary production in boreal forests (LeBauer and Treseder, 2008). The N cycle has been profoundly enhanced by human activities during the past century (Galloway et al., 2008; Steffen et al., 2015), and N deposition makes an important contribution to the N input in boreal forests (Gundale et al., 2011; Binkley and Högberg, 2016). Soils in boreal forests store a significant amount of the global carbon (C), and even small changes in their C balance can enhance or offset the ongoing rise in atmospheric C concentration (Tarnocai et al., 2009; Ciais et al., 2019; Crowther et al., 2019). To predict how boreal forests will contribute to the global greenhouse gas balance in the future, it is therefore critical to understand the impact of increasing N availability on the input and output of C to these soils.

Carbon enters soils through two distinct pathways; via aboveground litter, and through roots (Moore and William Hunt, 1988; Davidson et al., 2002). While it is well understood that N enrichment can increase above ground litter inputs (Berg and McClaugherty, 2014), less is known about its impact on the root C input. The root C input consists of photosynthates transported below ground via the phloem, and may account for as much as half of the gross primary production, and may be twice as large as the input of C by litterfall (Davidson et al., 2002; Litton et al., 2007; Chen et al., 2014). A portion of the root C is used to produce root biomass, and eventually enters the soil as root litter (Finer et al., 2011). Another portion of the root C is used for other functions, including transfer to symbiotic fungi and exudation into the root environment (Smith and Read, 2008; Bahr et al., 2013; Canarini et al., 2019), stimulating microbial activity and decomposition, processes with a negative impact on soil C storage (i.e. priming) (Craine et al., 2007; Kuzyakov, 2010; Kuyper, 2017). The collective respiration from roots and these root-associated microbes and processes (hereafter referred to as root-associated respiration) is a large C flux in terrestrial ecosystems, typically responsible for half of the soil respiration (Högberg et al., 2002; Litton et al., 2007; Chen et al., 2014), and is often strongly suppressed after N enrichment (Olsson et al., 2005; Janssens et al., 2010; Forsmark et al., 2020). The mechanisms behind this reduction remain unclear, and while some studies ascribe this effect to reductions in belowground C allocation (Chapin, 1980; Landsberg and Waring, 1997; Litton et al., 2007; Mäkelä et al., 2008), other studies emphasize shifts in C use between respiration and transfer to root-associated microbes, versus growth of root biomass (Vicca et al., 2012; Chen et al., 2014). Few studies have, however, measured multiple components of the below-ground C flux and related them to the release of C by respiration and the production of biomass above ground (McCormack et al., 2012), and it thus remains to be determined if N enrichment leads to a general reduction in all below ground C fluxes, or whether different components respond in different directions to changes in N availability.

The growing tip of fine-roots is an important sink for C allocated below ground that determines if C will be used for root growth, exuded into the surrounding root-zone, exported into microbial biomass, or respired (Jones et al., 2004; Canarini et al., 2019). Ectomycorrhizal fungi (EMF) live in symbiosis with most of the dominating tree species in boreal forests, where they are the main group of root-associated microbes and play a key role in nutrient acquisition and decomposition (Smith and Read, 2008). As much as 80% of the total N uptake may occur via EMF (Hobbie, 2006), and as a consequence of this relationship, EMF is also an important sink for tree photosynthates, possibly receiving as much as 60%, but perhaps on average 15% of the photosynthates (Simard et al., 2002; Zak et al., 2019). By using some of this C, EMF produce extensive mycelia that enables exploration of a large volume of soil (Smith and Read, 2008), which often comprise a significant part of the microbial biomass in boreal forest soils (Högberg and Högberg, 2002), and is an important source of necromass (Rasse et al., 2005; Clemmensen et al., 2013; Ekblad et al., 2013). However, some of the C transferred to EMF is used to support respiration and the production of extracellular enzymes and organic acids that are released into the soil environment and are involved in the degradation of soil organic matter and mobilization of N (Bödeker et al., 2014; Kuyper, 2017). Thus, C allocated to EMF play a dual role in the C balance of boreal forest soils. Despite the obvious relevance for soil C and nutrient dynamics (Vicca et al., 2012; Zak et al., 2019), it is still unclear how gradual changes in N availability impact key mycorrhizal properties such as the degree of colonization of fine-roots (Treseder and Allen, 2002; Franklin et al., 2014), the production of EMF biomass (Lilleskov et al., 2002; Treseder, 2008; Kjøller et al., 2012; Bahr et al., 2013), and respiration rates in root and EMF biomass (Hasselquist et al., 2012). These uncertainties are especially notable in boreal forests, where both plants and microbes often are limited by N (Näsholm et al., 2013; de Vries et al., 2014).

In this study, we utilized a long-term (12 years) N addition experiment with low (3, 6, and 12 kg N ha⁻¹ yr⁻¹) and high (50 kg N ha⁻¹ yr⁻¹) N addition rates (n = 6) simulating a gradient of N deposition. A recent study at this site found a strong increase in soil C storage at the highest N addition rate (Forsmark et al., 2020), which coincided with increased above-ground growth (From et al., 2016; Lim et al., 2017) and decreased root-associated respiration (Supporting Information Fig. S1) (Forsmark et al., 2020). Similar responses have been reported for a wide range of forests in both the temperate and boreal zone, e.g. by Janssens et al. (2010), Pregitzer et al. (2008), and Hyvönen et al. (2008). In this study, we simultaneously measured the production of needles and fine-roots, as well as the production of EMF biomass in the soil and EMF colonization of fine-roots, to better understand how N changes the allocation of C between above- and below-ground biomass production, and the partitioning of C between root and EMF production. We further related these measurements to root-associated respiration measured in a previous study (Forsmark et al., 2020) to better understand how N influences the partitioning of C between biomass production and respiration. First, we hypothesized that N additions would alter C allocation between above and below ground, such that the production of fine-roots would decrease in relation to the production of needles, while we anticipated that the production of fine-roots in absolute terms could either increase or decrease (Chen et al., 2014). Second, we hypothesized that N enrichment would downregulate the allocation of C to EMF (Vicca et al., 2012), which would decrease fungal biomass in fine-roots, decrease the production of EMF mycelium in the soil, and increase the production of fine-roots relative to root-associated respiration since biomass-specific respiration in fungal hyphae are higher than in roots. By directly measuring the impact of N on the most dynamic C fluxes above and below ground across a gradient of N treatments spanning the entire range of N deposition in the boreal region, we aimed to more clearly elucidate the mechanisms through which N deposition regulates soil C accumulation of boreal forests.