Canopy Nitrogen Addition and Soil Warming Affect Conifer Seedlings’ Phenology but Have Limited Impact on Growth and Soil N Mineralization in Boreal Forests of Eastern Canada

Charles Marty,Nelson Thiffault,Daniel Houle,Émilie Dussault-Chouinard,Myrna J. Simpson,Joanie Piquette,Hubert Morin,Maxime C. Paré,Rock Ouimet,Robert L. Bradley

doi:10.3389/ffgc.2020.581363

Abstract

Changes in atmospheric N deposition and the global warming associated with human activities may impact soil organic matter mineralization rate, with possible consequences on ecosystem functioning and productivity. In this study, soil temperature was increased by +2–4 °C from April to July with heating cables and artificial rain was applied above the canopy of mature stands from June to September to simulate a 50% increase in N concentration of atmospheric deposition for a period of nine years (2008–2016) in two eastern Canada boreal forests (BER and SIM). We assessed the effects of these treatments on the rate of organic N mineralization and on the growth, phenology and N sources of Abies balsamea (L.) Mill. (balsam fir; BF) and Picea mariana (Mill.) BSP (black spruce; BS) seedlings growing below the mature tree’s canopies. The soil warming (SW) treatment had no significant effect on both net and gross N mineralization rates, whereas canopy N addition (CNA) decreased gross N mineralization rate by 23% and forest floor’s alkyl/O-alkyl C ratio by ~15% as compared to unfertilized plots. This decrease likely resulted from a reduced mining activity by soil microorganisms due to increased inorganic N availability. Foliar δ15N in the control plots was markedly lower in BS than in BF and at BER than at SIM (–4.8‰ and –2.9‰ in BS at BER and SIM, respectively; 1.6‰ and 3.8‰ in BF at BER and SIM, respectively). This was interpreted as a higher contribution of 15N-depleted N derived from mycorrhizal fungi in BS and at BER, the colder and the more N-depleted site. The SW and CNA treatments had nonsignificant effects on seedling growth and foliar chemistry. In contrast, SW caused a premature bud outbreak and faster bud development for both species at both sites. Overall, our results show that increased soil temperature and N deposition in boreal forests may not impact soil fertility and vegetation growth as much as previously thought, but climate warming, by initiating earlier and faster bud development, may however expose seedlings to late spring frosts in the future.

Highlights

Soil organic matter of Canadian temperate and boreal forest soils store large amounts of N, in the mineral horizons of the soil (Marty et al, 2017), N is one of the main factors limiting the growth of trees in boreal forests (Schulte-Uebbing and de Vries, 2018), because most of this N is in its organic forms due to low mineralization rates, and not directly and readily available to support plant development and growth
The very low NO3− concentrations we measured in the soil of both study sites agree with what is generally observed in boreal forests of the region, where N-NH4+ is the main inorganic N form in the soil solution (Ste-Marie and Houle, 2006)
At the beginning of the growing season, NH4+ concentration in the forest floor was more than twice higher at SIM than at BER (Table 1), which is consistent with the higher net N mineralization rate observed at SIM (Figure 3A)

Summary

Introduction

Nitrogen mineralization is crucial in determining soil fertility and forest productivity because plant N nutrition largely relies upon NH4+, especially in cold ecosystems such as taiga and boreal forests (Schimel and Chapin, 1996; Aerts and Chapin, 2000; McFarland et al, 2002; Persson et al, 2003; Schimel and Bennett, 2004; Shenoy et al, 2013), several plants of these habitats can absorb small organic-N compounds such as amino acids (Persson and Näsholm, 2001; McFarland et al, 2002; Persson et al, 2003). It is expected that increased soil temperature and changes in N cycling will impact boreal forests because these habitats are characterized by low temperatures, low N availability and high C:N ratio litter (Marty et al, 2017). These changes may increase N availability with subsequent effects on forest regeneration and growth (Hobbie et al, 2002; D’Orangeville et al, 2013a; Blaško et al, 2015). Significant shifts in tree species performances (e.g., survival, regeneration, photosynthetic and growth rates) under a warmer climate may, for instance, impact forest productivity and require adapting silvicultural and management practices (e.g., assisted migration) (Benomar et al, 2018; Otis Prud’Homme et al, 2018)

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