Douglas fir stimulates nitrification in French forest soils

Abstract

Douglas fir trees presumable stimulate nitrification in the soil. We studied in 21 French Douglas fir forests if and how nitrification is modulated by soil properties, past land use and current forest management. Soil (0-10 cm depth) was collected and initial concentrations of N-NH₄⁺ and N-NO₃^-, potential net nitrogen mineralization (PNM) and net nitrification (PNN) rates and microbial biomass were measured. At 11 of the 21 sites, annual nitrate fluxes in the soil were measured using anion exchange resin bags. Soils contained between 2.3 to 29.4 mg N-NO₃^- kg soil^-1. About 86% (±14%) of mineral N was nitrate. The proportion of nitrate increased to almost 100% during incubation. PNN varied from 0.10 mg N kg soil^-1 day^-1 to 1.05 mg N kg soil^-1 day^-1 (21 sites). Neither the initial nitrate concentration nor PNN was related to soil chemistry (pH, % C, %N, P, CEC), microbial biomass, texture, past land use or thinning. In situ net nitrate accumulation (NNA) estimated with resins beds varied from 4 to 100 kg N-NO₃^- ha^-1 yr^-1 (11 sites). It was positively correlated with base saturation, clay content, ELLENBERG N, temperature and negatively with soil organic N, C/N ratio and precipitation.

Figure 1

Relationships between potential net nitrification (PNN) and (A) potential net mineralization (PNM), (B) pH, (C) soil organic N, (D) microbial biomass N (except MEY and SAL_B), (E) clay content, (F) base saturation (S/T), (G) past land use (1 = conifers, 2 = broadleaf, 3 = pasture or farmland) and (H) forest management (years since last thinning).

Figure 2

Relationships between annual net nitrate accumulation (NNA) and (A) base saturation (S/T), (B) clay content, (C) soil organic N, (D) C/N of the 0–10 cm soil), (E) Ellenberg N, (F) past land use, (G) mean annual precipitation, (H) number of days with T > 5 °C and (I) PNN (except Mey). Data obtained at 11 sites.

Figure 3

(a) distribution of the 17 parameters (soil chemistry, stand characteristics) (b) distribution of the sites according to the PNN group, (c) distribution of the sites according to the NNA group.

Figure 4

Relationships between PNM and PNN in French Douglas fir forests soils (red dots), in coniferous forests soils (black dots) (White et al., 1988; Gower and Son 1992; Menyailo et al., 2002; Nugroho et al., 2006; Colman and Schimel 2013; Urukawa et al., 2016) and in broadleaf forest soils (grey dots) (Morris and Boerner 1998; Verchot et al., 2001; Gower and Son; Menyailo et al., 2002; Andrianarisoa et al., 2009; Colman and Schimel 2013; Urukawa et al., 2016)