Factors driving potential ammonia oxidation in Canadian arctic ecosystems: does spatial scale matter?

Abstract

Ammonia oxidation is a major process in nitrogen cycling, and it plays a key role in nitrogen limited soil ecosystems such as those in the arctic. Although mm-scale spatial dependency of ammonia oxidizers has been investigated, little is known about the field-scale spatial dependency of aerobic ammonia oxidation processes and ammonia-oxidizing archaeal and bacterial communities, particularly in arctic soils. The purpose of this study was to explore the drivers of ammonia oxidation at the field scale in cryosols (soils with permafrost within 1 m of the surface). We measured aerobic ammonia oxidation potential (both autotrophic and heterotrophic) and functional gene abundance (bacterial amoA and archaeal amoA) in 279 soil samples collected from three arctic ecosystems. The variability associated with quantifying genes was substantially less than the spatial variability observed in these soils, suggesting that molecular methods can be used reliably evaluate spatial dependency in arctic ecosystems. Ammonia-oxidizing archaeal and bacterial communities and aerobic ammonia oxidation were spatially autocorrelated. Gene abundances were spatially structured within 4 m, whereas biochemical processes were structured within 40 m. Ammonia oxidation was driven at small scales (<1m) by moisture and total organic carbon, whereas gene abundance and other edaphic factors drove ammonia oxidation at medium (1 to 10 m) and large (10 to 100 m) scales. In these arctic soils heterotrophs contributed between 29 and 47% of total ammonia oxidation potential. The spatial scale for aerobic ammonia oxidation genes differed from potential ammonia oxidation, suggesting that in arctic ecosystems edaphic, rather than genetic, factors are an important control on ammonia oxidation.

Fig 1

Geographic location of three experimental sites in circumpolar arctic region: Truelove Lowland (75°40′N, 84°35′W), Simpson Lake (68°35′N, 91°57′W), and Ross Point (68°31′N, 111°10′W). Adapted from the Toolik-Arctic Geobotanical Atlas (www.arcticatlas.org; Alaska Geobotany Center).

Fig 2

Log gene copy numbers of ammonia-oxidizing functional groups (archaeal amoA and bacterial amoA) (A) and overall ammonia oxidation potential (AOP) and heterotrophic ammonia oxidation potential (HAOP) (B) at three arctic sites: Truelove Lowland, Simpson Lake, and Ross Point.

Fig 3

Semivariogram of bacterial amoA showing spatial variability at Truelove Lowland compared to experimental error associated with extracting and quantifying genes in soil. The qPCR variability (semivariance = 0.17; n = six independent DNA extracts, with 10 qPCR replicates each) and DNA extraction variability (semivariance = 0.12; n = 10 independent extractions [0.5 g, fresh weight] of the same 300-g soil sample, which were then analyzed by qPCR in triplicate) are indicated as a long dash and a dotted line, respectively.

Fig 4

Semivariograms showing differential spatial structure of bacterial amoA abundance (AOB), archaeal amoA abundance (AOA), and potential ammonia oxidation (AOP) at three arctic sites: Truelove Lowland (TR), Simpson Lake (SL), and Ross Point (RP). Spatial dependency (SPD) was considered from fine scale (10 cm) to large scale (300 m). The values of SPD vary from 0 (no spatial dependence) to 1 (strong spatial dependence). Range indicates the zone of spatial dependency. Different models (Gaussian, spherical, and exponential) were fitted (solid line) to each semivariogram. Semivariograms are shown up to the specific lag distance for clarity of the spatial patterns near the origin.