Anthropogenic nitrogen enrichment enhances soil carbon accumulation by impacting saprotrophs rather than ectomycorrhizal fungal activity

Abstract

There is evidence that anthropogenic nitrogen (N) deposition enhances carbon (C) sequestration in boreal forest soils. However, it is unclear how free‐living saprotrophs (bacteria and fungi, SAP) and ectomycorrhizal (EM) fungi responses to N addition impact soil C dynamics. Our aim was to investigate how SAP and EM communities are impacted by N enrichment and to estimate whether these changes influence decay of litter and humus. We conducted a long‐term experiment in northern Sweden, maintained since 2004, consisting of ambient, low N additions (0, 3, 6, and 12 kg N ha−1 year−1) simulating current N deposition rates in the boreal region, as well as a high N addition (50 kg N ha−1 year−1). Our data showed that long‐term N enrichment impeded mass loss of litter, but not of humus, and only in response to the highest N addition treatment. Furthermore, our data showed that EM fungi reduced the mass of N and P in both substrates during the incubation period compared to when only SAP organisms were present. Low N additions had no effect on microbial community structure, while the high N addition decreased fungal and bacterial biomasses and altered EM fungi and SAP community composition. Actinomycetes were the only bacterial SAP to show increased biomass in response to the highest N addition. These results provide a mechanistic understanding of how anthropogenic N enrichment can influence soil C accumulation rates and suggest that current N deposition rates in the boreal region (≤12 kg N ha−1 year−1) are likely to have a minor impact on the soil microbial community and the decomposition of humus and litter.

Resistant Soil Microbial Communities Show Signs of Increasing Phosphorus Limitation in Two Temperate Forests After Long-Term Nitrogen Addition

“…There is growing consensus that added N generally reduces soil microbial biomass (Treseder, 2008;Liu and Greaver, 2010;Lu et al, 2011;Zhou et al, 2017), which in turn drives reductions in decomposition and soil respiration (Janssens et al, 2010;Riggs and Hobbie, 2016). Yet common, this response is not universal (Waldrop et al, 2004a;Keeler et al, 2009;Riggs et al, 2015;Heuck et al, 2018) but contingent on, for example, ecosystem type (Yue et al, 2016), experimental duration (Treseder, 2008), N load (Maaroufi et al, 2019), or sampling year (Gutknecht et al, 2012;Contosta et al, 2015). For instance, in a recent meta-analysis MBC increased under low N loads (<50 kg N ha −1 yr −1 , similar levels as applied at both study sites here) but did not respond above that levels (Yue et al, 2016).…”

Only Minor Changes in the Soil Microbiome of a Sub-alpine Forest After 20 Years of Moderately Increased Nitrogen Loads

“…The decline in fungal biomass was largely attributed to a decrease in ectomycorrhiza, which form associations with trees, suggesting that fungi are more sensitive to long-term N fertilization than bacteria. Most previous research focused on soil bacterial communities in forests (Shen et al, 2010;Turlapati et al, 2013), and we are aware of only a few studies that examined changes in soil fungal community composition following increased N loads (Boxman et al, 1998;Allison et al, 2007;Edwards et al, 2011;Maaroufi et al, 2019).…”

Long‐term nitrogen enrichment does not increase microbial phosphorus mobilization in a northern coniferous forest

“…For the humus substrate, we observed a net C gain (Figure 2f), which is somewhat unusual in decomposition studies. However, humus has not been used as often as needles in decomposition studies, and previous studies using humus as a decomposition substrate have shown it is very resistant to decomposition, with mass loss typically below 10% per year (Bödeker et al, 2016;Maaroufi et al, 2019).…”

“…For the humus substrate, we observed a net C gain (Figure 2f), which is somewhat unusual in decomposition studies. However, humus has not been used as often as needles in decomposition studies, and previous studies using humus as a decomposition substrate have shown it is very resistant to decomposition, with mass loss typically below 10% per year (Bödeker et al., 2016; Maaroufi et al., 2019). In addition to the recalcitrance of the humus in general, the specific humus we used had a relatively high nutrient content compared to the surrounding soil matrix (Table 1; Figure 1), which may have stimulated fungal ingrowth causing this substantial C accrual.…”

“…Ingrowth cores are simple tools that have been used extensively in ecosystem studies to measure the production of fine roots and fungal mycelium (Addo‐Danso et al., 2016; Vogt et al., 1998; Wallander et al., 2012), and decomposition (Berg & McClaugherty, 2014). By choosing different substrates, the ingrowth core can provide valuable information about different ecosystem processes (Bödeker et al., 2016; Maaroufi et al., 2019). We constructed cylindrical 10 cm long by 2 cm wide ingrowth cores using fine nylon mesh (50 μm, SinTab), which prevents ingrowth of roots, including the majority of Ericaceous hair roots (Bonfante‐Fasolo & Gianinazzi‐Pearson, 1979).…”