Plant Soil (2012) 352:289–301 DOI 10.1007/s11104-011-0997-2
REGULAR ARTICLE
Soil moisture effects on gross nitrification differ between adjacent grassland and forested soils in central Alberta, Canada Yi Cheng & Zu-cong Cai & Jin-bo Zhang & Man Lang & Bruno Mary & Scott X. Chang
Received: 26 April 2011 / Accepted: 9 September 2011 / Published online: 24 September 2011 # Springer Science+Business Media B.V. 2011
M. Lang College of Applied Meteorology, Nanjing University of Information Science & Technology, Nanjing 210044, China
content (65 vs. 100% water holding capacity, WHC) on gross N transformation rates using the 15N tracing technique (calculated by the numerical model FLUAZ) in adjacent grassland and forest soils in central Alberta, Canada. Results Gross N mineralization and gross NH4+ immobilization rates were not influenced by soil moisture content for both soils. Gross nitrification rates were greater at 100 than at 65% WHC only in the forest soil. Denitrification rates during the 9 days of incubation were 2.47 and 4.91 mg N kg-1 soil d-1 in the grassland and forest soils, respectively, at 100% WHC, but were not different from zero at 65% WHC. In the forest soil, both the ratio of gross nitrification to gross NH4+ immobilization rates (N/IA) and cumulative N2O emission were lower in the 65 than in the 100% WHC treatment, while in the grassland soil, the N/IA ratio was similar between the two soil moisture content treatments but cumulative N2O emission was lower at 65% WHC. Conclusions The effect of soil moisture content on gross nitrification rates differ between forest and grassland soils and decreasing soil moisture content from 100 to 65% WHC reduced N2O emissions in both soils.
B. Mary INRA, Unit Agro-Impact, Site de Laon, Pole du Griffon, 02000 Barenton-Bugny, France
Keywords 15N tracing technique . FLUAZ . Gross N mineralization . Gross nitrification . Soil moisture content . Denitrification
Abstract Background and aims Changes in soil moisture availability seasonally and as a result of climatic variability would influence soil nitrogen (N) cycling in different land use systems. This study aimed to understand mechanisms of soil moisture availability on gross N transformation rates. Methods A laboratory incubation experiment was conducted to evaluate the effects of soil moisture Responsible Editor: Per Ambus. Y. Cheng : Z.-c. Cai (*) : J.-b. Zhang State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China e-mail:
[email protected] Y. Cheng : S. X. Chang (*) Department of Renewable Resources, University of Alberta, 442 Earth Sciences Building, Edmonton T6G 2E3, Canada e-mail:
[email protected]
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Introduction Soil moisture availability is a major factor controlling the survival and activity of microorganisms (Borken and Matzner 2009) and can thus strongly influence soil N transformations (Bengtson et al. 2005; Chen et al. 2011). The influence of soil moisture content on net N transformation rates has been extensively studied (Owen et al. 2003; Vernimmen et al. 2007; Yu and Ehrenfeld 2009); however, net N transformation is the outcome of several concurrent N cycling processes such as N mineralization and immobilization. To quantify the response of each of those N cycling processes to soil moisture availability, gross N transformation rates need to be determined. Previous studies have demonstrated that net N transformation rates were generally lower than gross rates both in situ (Davidson et al. 1991; Hart et al. 1994; Verchot et al. 2001) and under laboratory incubation (Lang et al. 2010); therefore, the response of net and gross rates to changes in soil moisture content may be different. Few studies have been conducted to investigate the effects of soil moisture content on gross N transformation rates: increased gross mineralization and gross nitrification rates with increasing soil moisture content below field capacity has been reported for arable (Nishio et al. 1985), pasture (Zaman et al. 1999) and forest soils (Bengtson et al. 2005; Chen et al. 2011). One study reported that gross N mineralization rates continued while gross immobilization rates were inhibited under saturated conditions (Nishio et al. 1994). In general gross nitrification rates gradually increased with soil moisture content and then declined in both mineral (Zaman et al. 1999; Breuer et al. 2002) and organic soil layers (Brüggemann et al. 2005; Stange 2007), as neither low soil moisture content that limits the diffusive transport of solutes nor high moisture content that results in insufficient oxygen concentration can be conducive for nitrification. Forest and grassland are two important land use types in temperate regions that can have very different physical and chemical properties due to differences in organic matter input from both aboveground plant residues and root exudates (Cookson et al. 2007). For example, forest soils are characterized by lower pH relative to grassland soils (Booth et al. 2005). Such differences in soil properties are likely to impact soil N cycling (Cote et al. 2000; Chen et al. 2004; Grenon et al. 2004). Gross N mineralization rates in forest
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soils have been shown to be significantly greater than those in grassland soils in central Alberta, Canada, while the reverse was true for gross nitrification rates (Lang et al. 2010). However, it is not clear whether the effect of soil moisture content on gross N transformation rates change with land use type. The objective of this laboratory incubation experiment was to examine the effect of soil moisture content on gross N transformation rates in soils under two land use systems in central Alberta, Canada. A numerical model, FLUAZ, based on the 15N tracing technique was used to evaluate short-term (9 days) effects of soil moisture content (65 vs. 100% water holding capacity, WHC) on gross N transformation rates, in adjacent grassland and forest soils in central Alberta, Canada. Although short-term laboratory studies do not necessarily reflect field conditions, as cold storage, sieving, 15N addition, and temperature manipulation could change the true N transformation rates in situ (Booth et al. 2005), our laboratory study aimed to provide a mechanistic understanding of the dynamics of soil gross N transformations in response to soil moisture content change.
Materials and methods Site description and soil sampling A detailed description of the research site and the experimental plots as well as soil properties is presented elsewhere (Lang et al. 2010). Briefly, forest and adjacent grassland soils (0–20 cm), classified as Dark Gray Luvisols (Soil Classification Working Group 1998), were collected near Linaria (54°12'N and 114°8'W) in central Alberta, Canada. The forest site consisted mostly of native aspen (Populus tremuloides Michx.) and the grassland site was seeded with a mixture of tall fescue (Festuca arundinacea Schreb.), orchardgrass (Dactylis glomerata L.), and red clover (Trifolium pratense). The site has an annual mean temperature of 3°C and annual mean precipitation of 463 mm (Environment Canada 2007). In this paper, we use land use system (forest vs. grassland) and soil type (forest soil vs. grassland soil) interchangeably. For each land use type, three sampling plots (15 x 15 m) were established. After removal of the litter, one composite sample of the top 20 cm of the soil was collected from each plot. The soils were air-dried at
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room temperature, passed through a 2 mm sieve and stored at 4°C until use. Our data indicate that of the sampled mineral soil organic C, organic N concentrations and C/N ratios were significantly greater in the forest than in the grassland soil, whereas pH was significantly lower in the forest soil, with bulk density not different between the two soils (Table 1). Incubation experiment For each soil, a series of flasks were prepared, each with 30 g of air-dried soil placed inside. Deionized water was added evenly over the soil surface with a pipette to bring the moisture content to 50% WHC. The method used for determining WHC is described below. The flasks were sealed with rubber stoppers and pre-incubated at 25°C for 1 week. After preincubation, each soil was applied with 2 mL of either 15 N-enriched 15 NH 4 NO 3 (5 atom%) or NH415NO3 (5 atom%) solution by pipetting solutions uniformly over the soil surface, resulting in an equivalent addition of 40 mg NH4+ -N and 40 mg NO3--N kg-1 soil. Subsequently, the final moisture content of each labeled sample was adjusted to either 65 or 100% WHC by adding deionized water. Based on the bulk density and the gravimetric water content of the soils at 100% WHC, the water filled pore space (WFPS) was calculated to be 79 and 70% for the grassland and forest soils, respectively. The WFPS at 65% WHC was 51 and 45% for the grassland and forest soils, respectively. The flasks were sealed with rubber stoppers and incubated in the dark for 9 days. Gas samples were taken from the headspace of the flasks on days 1, 2, 5 and 9 (noted below as d1, d2, d5 and d9, respectively). At each gas sampling event the headspace was sealed for 24 h using a silicone sealant. Before sampling, the headspace gas was mixed by withdrawing and back injecting headspace gas five times using a 25 mL gastight syringe with a stopcock. Then a 12 mL gas sample
was collected from the headspace of each flask and transferred into evacuated 12 mL vials for N2O analysis on a gas chromatograph (GC, Varian CP-3800). During the incubation, the flasks were opened for 30 min each day to renew the atmosphere inside each flask. The moisture content of the incubated soil samples was maintained by adding water every 3 days when necessary to compensate for the amount of water lost through evaporation. After 0.5 h and 2, 5, 9 days of incubation, three replicate 15NH4NO3- and NH415NO3labeled flasks for each soil type and soil moisture content treatment were sampled destructively and extracted in a 2 mol L-1 KCl solution at a soil:solution ratio of 1:5 (w:v). The mixture was shaken on a 250 rpm mechanical shaker for 1 h at 25°C and the filtrates were stored at 4°C for less than 1 day before analysis. The residual soil was subsequently washed with distilled water, oven-dried at 60°C till constant weight, and ground to pass a 0.15 mm sieve for 15N analysis of insoluble organic N. Analysis of soil physical and chemical properties Soil pH was measured in deionized water (1:2.5 w:v ratio) using a digital type DMP-2 mV/pH meter (Thermo Orion). Water holding capacity (WHC) was determined as described in detail by Fierer and Schimel (2002). Soil organic N and organic C concentrations were determined using a CN analyzer (NA Series 2, CE Instruments, Italy). A portion of the KCl extract was steam-distilled with MgO to determine NH4+ concentrations by a steam distillation system (Vapodest 20, C. Gerhardt, Königswinter, Germany), thereafter the sample in the flask was distilled again after the addition of Devarda’s alloy to determine NO3- concentrations (Lu 2000). After titration to determine the NH4+ and NO3- concentrations, the distillates were acidified and dried in an oven at 60°C to determine 15N abundance. The 15N abundance of insoluble organic N and NH4+ and NO3-
Table 1 Some physical and chemical properties of the studied grassland and forest soils collected from two adjacent land use systems in central Alberta, Canada (mean±SD, n=3) Land use
Bulk density (g cm-3)
pH
Organic C (g kg-1)
Organic N (g kg-1)
C/N
Forest
1.06 (0.11) a
5.08 (0.06) a*
59.0 (2.8) a
3.5 (0.2) a
16.9 (0.7) a
Grassland
1.23 (0.03) a
5.77 (0.14) b
27.5 (0.9) b
2.3 (0.1) b
11.8 (0.3) b
*: Different letters indicate significant differences between soils of the two different land use systems (P
