Seasonal patterns of soil nitrogen availability in moist acidic tundra

Abstract

Our ability to predict effects of changing soil nitrogen (N) in Arctic tundra has been limited by our poor understanding of the intra-annual variability of soil N in this strongly seasonal ecosystem. Studies have shown that microbial biomass declines in spring accompanied by peaks in inorganic nutrients. However, subsequent to this early pulse, there are few high temporal resolution measurements during the growing season. We hypothesized that (1) low N would be maintained throughout the growing season, (2) peaks of total free primary amines (TFPA), ammonium (NH 4 + ), and nitrate (NO 3 − ) would follow a sequential pattern driven by mineralization and nitrification, and (3) a peak in soil N would occur as plants senesce. We conducted weekly measurements of TFPA, NH 4 + , and NO 3 − in two tundra sites, from soil thaw in spring to freeze in fall. At each site, NH 4 + peaks were followed by smaller peaks in NO 3 − , supporting the hypothesis that excess NH 4 + would be nitrified. Furthermore, peaks in NH 4 + were observed both shortly after leaf expansion and at plant senescence. The variation in timing between sites and the peaks in NH 4 + subsequent to thaw indicates that nutrient limitation in these ecosystems is more dynamic and spatially variable than previously thought.Key words: nitrogen availability, nitrogen mineralization, seasonality, moist acidic tundra, total free primary amines.Résumé : Notre capacité à prévoir les effets de l'azote (N) changeant dans les sols de la toundra arctique a été limitée par notre compréhension insuffisante de la variabilité au cours d'une même année de N des sols dans cet écosystème fortement saisonnier. Les études ont montré des baisses de biomasses microbiennes au printemps accompagnées de pics dans les substances nutritives inorganiques. Cependant, subséquemment à cette première pulsation, il y a peu de mesures de résolution temporelle élevée pendant la saison de croissance. Nous avons formulé l'hypothèse (1) qu'un faible niveau de N serait maintenu au cours de la saison de croissance, (2)

Limited effects of early snowmelt on plants, decomposers, and soil nutrients in Arctic tundra soils

“…Nitrogen (N) is an essential plant macronutrient in Arctic tundra ecosystems, where it often, though not always, limits both plant production and microbial activity (Mack, Schuur, Bret‐Harte, Shaver, & Chapin, ; Melle, Wallenstein, Darrouzet‐Nardi, & Weintraub, ; Nowinski, Trumbore, Schuur, Mack, & Shaver, ; Weintraub & Schimel, ). The main source of N in Arctic soils is the large stocks of organic N, which can be mined enzymatically for amino acids and sugars, and other organic N monomers, which can ultimately be mineralized to NO 3 − or NH 4 + (McLaren, Darrouzet‐Nardi, Weintraub, & Gough, ). During the snow‐covered season (approximately September through May), N accumulates in soil bacteria and fungi, which then release it in a large pulse during soil thaw in association with microbial turnover (Figure a; Lipson, Schmidt, & Monson, ).…”

“…(c) If microbes are reliant on plant inputs (e.g., via rhizodeposition), all biotic activity may slow, leading to a delayed crash in nutrient availability. (d) If microbes continue to thrive, they may immobilize nutrients and take up C from damaged roots leading to a phenological "mismatch" can ultimately be mineralized to NO 3 − or NH 4 + (McLaren, Darrouzet-Nardi, Weintraub, & Gough, 2017). During the snow-covered season (approximately September through May), N accumulates in soil bacteria and fungi, which then release it in a large pulse during soil thaw in association with microbial turnover (Figure 1a; Lipson, Schmidt, & Monson, 1999).…”

Above- and belowground responses to long-term herbivore exclusion

“…The fact that we did not detect any changes in soil nutrient pools twenty-one years after treatment began (Supplemental Table 3) indicates that influences of herbivores may be transient. Many of the variables we examined (e.g., available nutrients) show strong variation seasonally (McLaren, Darrouzet-Nardi, et al 2018) and between years (Edwards and Jefferies 2013), and with a single sampling it is possible that we missed transient effects that occurred during other parts of the growing season or during other years. Alternatively, studies have found that herbivore impacts may increase through time (Mallen-Cooper, Nakagawa, and Eldridge 2019) and, due to slow ecosystem processes in the Arctic, it may take greater than twenty years to see effects.…”

Changes in the understory plant community and ecosystem properties along a shrub density gradient

“…Nutrient availability may be highest in the tundra immediately following spring thaw (McLaren et al 2018) and this nutrient pulse has been found to differ between shrub and nonshrub communities with differences attributed to variation in snow accumulation, soil moisture, and soil temperature (Buckeridge and Grogan 2010). Our measurements were taken during peak growing season, when the majority of nutrients in the system are likely immobilized by microbes and vegetation is readily absorbing nutrients as they become available.…”

“…The potential effects of shrubs on soil properties are complex, and teasing apart the mechanisms driving these differences in ecosystem properties between the different shrub habitats will be difficult, especially given the conflicting results of studies done on different time frames [e.g., short-term vs. long-term decomposition (Hobbie and Gough 2004;DeMarco et al 2014)], or in different seasons [e.g., postthaw effects vs. summer effects (Buckeridge and Grogan 2010;McLaren et al 2018)]. Due to the observational nature of this study, it is important to note that our findings are limited in the cause-and-effect paradigm as observed differences may be driven by unquantified variables that covary along the shrub density gradient.…”