Non-Peer ReviewedEphemeral wetlands or depressions in hummocky landscapes have high levels of C, N, and soil moisture, often leading to high nutrient cycling activity. However, measuring soil nitrate and ammonium pools is typically a poor indication of N-cycling activity or of the soil N that is available for other processes such as N2O emissions. This study used stable 15N isotope dilution techniques in cultivated and uncultivated ephemeral wetlands in central Saskatchewan to quantify land use effects on gross mineralization and nitrification rates. In-field incubation experiments were repeated in early May, mid-June and late July. There was a clear land use effect on inorganic soil N levels, with significantly less NH4+ and more NO3- in the cultivated wetland soils. However, the rates of NH4+ mineralization and NO3 - nitrification were similar for both land uses, indicating similar substrate availability but different N-consuming processes. Both N pools turned over in as little as 1-2 d, highlighting the ineffectuality of measuring inorganic N pools as a predictor for N availability in these soils

Bedard-Haughn, A.

Pennock, D.

English

eCommons@USASK

Gross N-cycling rates in ephemeral wetlands

University of Saskatchewan Research Archive

 1  Gross N-cycling Rates in Ephemeral Wetlands   A. Bedard-Haughn and D. Pennock Department of Soil Science, University of Saskatchewan, Saskatoon, SK, S7N 5A8  Key Words: mineralization, nitrification, N turnover, 15N isotope dilution Abstract Ephemeral wetlands or depressions in hummocky landscapes have high levels of C, N, and soil moisture, often leading to high nutrient cycling activity. However, measuring soil nitrate and ammonium pools is typically a poor indication of N-cycling activity or of the soil N that is available for other processes such as N2O emissions. This study used stable 15N isotope dilution techniques in cultivated and uncultivated ephemeral wetlands in central Saskatchewan to quantify land use effects on gross mineralization and nitrification rates. In-field incubation experiments were repeated in early May, mid-June and late July. There was a clear land use effect on inorganic soil N levels, with significantly less NH4+ and more NO3- in the cultivated wetland soils. However, the rates of NH4+ mineralization and NO3- nitrification were similar for both land uses, indicating similar substrate availability but different N-consuming processes. Both N pools turned over in as little as 1-2 d, highlighting the ineffectuality of measuring inorganic N pools as a predictor for N availability in these soils.  Introduction Hummocky till landscapes are the dominant geomorphic surface in the Prairie Pothole region, which extends from East-Central Alberta through to North-Central Iowa. These landscapes contain a very high number of wetlands – for example, our research site near St. Denis, Saskatchewan, Canada contains 216 wetlands in an area of 3.84 km2 (Hogan and Conly, 2002) – interspersed within the cultivated upland portions of the landscape. Many of the wetlands have been cleared of natural vegetation and are now used for annual crop production. This conversion from native wetland to cultivated wetlands has presumably altered the rates of fundamental N processes but to date little or no information is available on the rates of N-cycling processes in these landscape positions.  Cropped landscapes receive nitrogen (N) inputs in the form of inorganic and/or organic fertilizers, and these inputs have a significant impact on local and regional N cycling, N gas losses, and N leaching (Burke et al., 2002). N-cycling processes of particular interest for both agricultural and environmental concerns are: 1) mineralization, which produces NH4+ from the soil organic N pool and 2) nitrification, which produces NO3- from the NH4+ and organic N pools and emits nitrous oxide (N2O) as a by-product. Nitrous oxide is a greenhouse gas with 296 times the global warming potential of CO2. In Canada, agriculture accounts for up to 70% of anthropogenic N2O emissions (Janzen et al., 1998).   2 Typical assessments of N pool status and activity include “snapshot” tests of soil NH4+ or NO3- pools at one or more times in the growing season, or measures of net mineralization and nitrification (i.e., the net increase or decrease in NH4+ or NO3- over a given period of time). However, researchers are increasingly recognizing that these measures provide limited information about N-cycling activity in the soil and are turning to stable isotope dilution techniques to measure gross rates of NH4+ and NO3- production (Booth et al., 2005; Davidson et al., 1991; Hart et al., 1994). In US ecosystems with more temperate climates than the Prairie Pothole region, lower slopes have been shown to have higher rates of both gross NH4+ mineralization and immobilization (Verchot et al., 2002) and lower rates of gross nitrification compared to upper slopes (Corre et al., 2002), emphasizing the significance of landform position in N-cycling processes. Research in the Pothole region of Saskatchewan has shown that lower slopes have higher SOC stores and lower N2O emissions in uncultivated soils compared to cultivated ones, emphasizing the overlying effects of land use on landform patterns (Corre et al., 1999; Pennock, 2003).  Examining N-cycling processes in the context of land use will improve our ability to manage agricultural landscapes to minimize N losses, advancing sustainable agriculture while addressing concerns surrounding greenhouse gas emissions and global change. This study used stable 15N isotope techniques in cultivated and uncultivated ephemeral wetlands in central Saskatchewan to quantify gross mineralization and nitrification by land use. These measurements will be compared with the more ubiquitous measurements of soil inorganic N levels. Methods This project was carried out in the St. Denis National Wildlife Area (SDNWA) (Latitude: 52°12', Longitude: 106°5') of central Saskatchewan, Canada. The SDNWA is in the Dark Brown soil climatic zone and is dominated by hummocky terrain with loamy unsorted glacial till (Weyburn association) parent materials; slope classes range from 10 to 15% (Miller et al., 1985). Within the SDNWA, five ephemeral wetlands were selected: 3 uncultivated wetlands (2 were cultivated prior to 1968) and 2 cultivated wetlands. Ephemeral wetlands are those depressions in hummocky landscapes that contain standing water in the spring, but typically dry up during the growing season (Hayashi et al., 1998). The cultivated wetlands were direct-seeded to wheat on May 8, 2005. Granular fertilizer containing 72% 46-0-0 and 28% 11-52-0 was placed with seed at a rate of 135 kg ha-1.  Each of the 5 wetlands was sampled three times throughout the 2005 growing season: May 4 (post-snowmelt), June 16 (early growing season), and July 26 (mid-growing season). Within each uncultivated wetland, sampling was focused in the riparian or willow ring soils, not the pond centers. For the cultivated wetlands, sampling was from the analogous concave footslope elements (Pennock et al., 1987). Three sub-sampling points were selected for each wetland (5×3 = 15 points in total). On a given sampling date, five intact cores were taken per sub-sampling point: one for bulk density and gravimetric soil moisture measurements and four for the isotope dilution field incubation experiment. Cores (5-cm I.D.) were taken to a 15-cm depth, corresponding to the average thickness of the A horizon, using a slide hammer sampler equipped with a removable plastic liner (AMS Signature Series).   3 Of the four isotope-dilution cores, one pair was labeled with (15NH4)2SO4 (for gross NH4 mineralization) and the other pair was labeled with K15NO3 (for gross nitrification); each solution contained 30 µg N ml-1 at 98% 15N enrichment. Labeling was done using seven 2-ml injections per core with an 18-gauge side-port spinal needle after Davidson et al. (1991). Solution injections increased the gravimetric soil moisture by 7±2%. With the exception of those uncultivated cores with the lowest initial concentrations of soil NO3- (where NO3- injections as much as doubled the soil NO3- levels) adding inorganic N did not significantly increase the soil N levels relative to the inherent background variability observed within each wetland. One core of each pair was broken up within 1 h of injection (t=0), sub-sampled and extracted using 2M KCl (on site).  The other core was capped and buried on site for 24 h before extraction. The KCl extracts were analyzed for inorganic N concentration colorimetrically using a Technicon AutoAnalyzer (Technicon Industrial Systems, 1978) and analyzed for 15NH4 and 15NO3 using the modified diffusion procedure outlined in Bedard-Haughn et al. (2004). The differences in N concentration and 15N content between the t=0 and t=24 h samples were used to calculate gross mineralization and nitrification according to Hart et al. (1994). Turnover rate, the rate at which a given inorganic N pool replaces itself, was calculated as the NH4+ pool size divided by the gross mineralization rate for NH4+ turnover, and as the NO3- pool size divided by the gross nitrification rate for NO3- turnover. Results  Soil NH4+ levels in the uncultivated wetlands (mg NH4+-N kg-1 soil) were significantly higher than cultivated wetlands for all experimental dates (Fig. 1). Over the course of the growing season, there was a small increase in the amount of NH4+ measured in the cultivated soils but no significant change in the uncultivated soils. This land use difference in soil NH4+ levels was not reflected in gross mineralization levels. Gross mineralization levels increased similarly for both land uses from May to July (Fig. 2). The NH4+-N turnover time reflected the land use differences in soil NH4+, with average turnover times of 1-2 days for smaller NH4+ pools in the cultivated soils and 2-5 days for the uncultivated soils. Turnover rates for both land uses tended to become more rapid through the season as both production and consumption rates increased (Table 1). Table 1. Turnover rates for NH4+ and NO3- pools in cultivated vs. uncultivated wetlands.  Days to turnover (S.D.)   NH4+  NO3-  May 2.8 (1.0) 0.5 (0.4) June 4.9 (4.1) 1.5 (1.2) Uncultivated July 2.7 (2.9) 1.2 (0.6) May 1.5 (1.1) 3.4 (1.0) June 1.6 (1.3) 1.8 (1.0) Cultivated July 0.6 (0.3) 1.7 (1.0)  4 JULYJUNEMAY35.030.025.020.015.010.05.00.0mg NH4-N/kg soilCultivatedUncultivatedmg NH4-N/kg soilFigure 1. Soil NH4+ levels in cultivated vs. uncultivated ephemeral wetlands. 30.025.020.015.010.05.00.0Mineralization (mg/kg/d)CultivatedUncultivatedJULYJUNEMAYMineralization (mg/kg/d)Figure 2. Gross NH4+ mineralization rates in cultivated vs. uncultivated ephemeral wetlands.  5 The land use effect for soil NO3- (mg NO3--N kg-1 soil) was inverse that observed for soil NH4+: cultivated soils had significantly greater soil NO3- than uncultivated soils (Fig. 3). Mean soil NO3- for cultivated soils was similar for May and June, but lower in July. Mean soil NO3- for uncultivated soils increased through the season, resulting in equal soil NO3- for both soils in July. There was no land use effect on gross nitrification levels and nitrification rates increased for both soils from May to July (Fig. 4). As was observed for NH4+, soil NO3- turnover reflected soil NO3- levels, with more rapid turnover (≤1 d) occurring in the uncultivated soils with their lower soil NO3- levels (Table 1). However, unlike the seasonal decrease observed for the NH4+ turnover, NO3- turnover became slower for the uncultivated soils (from 0.5 to 1.5 d) and more rapid for the cultivated soils (from 3.5 to 2 d) between the beginning and end of the growing season, reflecting soil NO3- levels over the same time period. 12.010.08.06.04.02.00.0mg NO3-N/kg soilCultivatedUncultivatedJULYJUNEMAYmg NO3-N/kg soil Figure 3. Soil NO3- levels in cultivated vs. uncultivated ephemeral wetlands.  6 8.06.04.02.00.0Nitrification (mg/kg/d)CultivatedUncultivatedJULYJUNEMAYNitrification (mg/kg/d) Figure 4. Gross nitrification rates in cultivated vs. uncultivated ephemeral wetlands. Discussion The N-production and -consumption rates reported for these ephemeral wetlands were within the range of previously-reported values for agricultural and grassland soils (Booth et al., 2005). However, there were significant differences in the size of the NH4+ and NO3- pools between cultivated and uncultivated soils despite similar gross mineralization and nitrification rates, suggesting mechanistic N-cycling differences under the two ecosystems. From the similar gross N-production rates, we can infer that neither mineralization nor nitrification is substrate-limited in either soil. From the different soil inorganic-N levels, however, we can infer that the fate or cycling mechanism for the produced N differs between soils.  For example, the higher levels of soil NH4+ in uncultivated soils may indicate a greater occurrence of heterotrophic nitrification under the associated willow ring or riparian grassland vegetation. Uncultivated soils are more likely to have a significant fungal biomass and hence, a greater tendency towards heterotrophic nitrification whereby the biomass oxidizes both organic and inorganic substrate to produce NO3- (Paul and Clark, 1996). In contrast, the cultivated wetlands will be dominated by bacterial biomass and hence autotrophic nitrification will dominate. Therefore for both soils to produce the comparable amounts of NO3- measured in the gross nitrification experiment, nitrifiers in the cultivated soil would consume more NH4+ whereas those in the uncultivated soil would consume both NH4+ and organic N to meet their needs.  Similarly, differences in the size of the soil NO3- pool may be attributable to differences in NO3- consuming processes. There was significantly higher gross NO3- consumption in the uncultivated soils throughout the growing season and consumption rates increased at a rate  7 similar to gross nitrification rates, resulting in minimal increases in extractable soil NO3- through the season. Given the absence of plant uptake or leaching as a loss mechanism in the incubated cores, NO3- reduction is likely the primary pathway for uncultivated NO3- consumption. Based on 15N measurements of the NH4+ extracted from 15NO3- labeled cores in the uncultivated soils, there was a small amount of dissimilatory nitrate reduction to ammonium (DNRA) taking place, but the dominant NO3- consumption mechanism was likely reduction by denitrification. Previous authors have also noted that NO3--N does not tend to accumulate under native grass vegetation (Malo et al., 2005).  Dividing the N pool size by its corresponding gross N-production rate gives an estimation of how rapidly that pool turns over or replaces itself. For these soils, the smaller the pool, the more rapidly it turned over. The NH4+ pool in the cultivated soils and the NO3- pool in the uncultivated soils both turned over in approximately 1 d, comparable to previously reported turnover rates in a range of soils (Booth et al., 2005). This rapid turnover further demonstrates the limited utility of a “snapshot” of extractable NH4+ and NO3- as indicators of either plant-available N for agricultural production or, in the case of NO3-, denitrification substrate for N2O emissions.  There was a clear seasonal effect for gross N-producing and N-consuming processes. Booth et al. (2005) noted a strong positive relationship between mineralization, NH4+ assimilation and nitrification rates across a range of ecosystems. A positive relationship was also noted in our soils when all data were considered. However, when examined on a date-by-date basis, the relationship did not persist. In these soils, the correlation was apparently more a function of a similar response of the N-producing processes to the increase in soil temperature through the growing season (Bengtson et al., 2005), with rates of both processes increasing from May to July. Booth et al. (2005) noted that the temperature-nitrification relationship depends in part on the availability of NH4+. Therefore, when gross mineralization increases with soil temperature, gross nitrification will also increase, as long as no other NH4+-assimilating or consuming process dominates. Schimel and Bennett (2004) suggest that N-producing, -consuming, and -emitting processes are most likely occurring in close proximity in soil microsites and are controlled in part by N availability. Nitrogen-producing and -consuming processes may be similarly activated by enzymes when the intracellular N content of associated microorganisms reaches critically low levels (Bengtson et al., 2005). Further investigation into the landform and land use impacts on these N-cycling feedback mechanisms is necessary. Conclusion Land use has had a significant impact on the soil NH4+ and NO3- levels in these ephemeral wetlands but no influence on gross mineralization and nitrification rates. Instead, the primary effect of land use on N-cycling processes appears to be in the mechanistic details, where each land use favors a different suite of processes carried out by a specific group of microorganisms. The assemblage of dominant microbial players present under a given land use appears to be more important than other environmental factors previously assumed to be decisive, including fertilization and WFPS. The complexity of the interrelationships among the N-producing, N-consuming, and N-emitting processes highlights the need for greater knowledge of the structure and lability of the soil organic N pool and of defining the microbial communities responsible for N cycling under different land use regimes.   8 References Bedard-Haughn, A., Tate, K.W., van Kessel, C., 2004. Using 15N to quantify vegetative buffer effectiveness for sequestering N in runoff. Journal of Environmental Quality 33, 2252-2262. Bengtson, P., Falkengren-Grerup, U., Bengtsson, G., 2005. Relieving substrate limitation-soil moisture and temperature determine gross N transformation rates. Oikos 111, 81-90. Booth, M.S., Stark, J.M., Rastetter, E., 2005. Controls on nitrogen cycling in terrestrial ecosystems: A synthetic analysis of literature data. Ecological Monographs 75, 139-157. Burke, I.C., Lauenroth, W.K., Cunfer, G., Barrett, J.E., Mosier, A., Lowe, P., 2002. Nitrogen in the central grasslands region of the United States. Bioscience 52, 813-823. Corre, M.D., Pennock, D.J., Van Kessel, C., Elliott, D.K., 1999. Estimation of annual nitrous oxide emissions from a transitional grassland-forest region in Saskatchewan, Canada. Biogeochemistry 44, 29-49. Corre, M.D., Schnabel, R.R., Stout, W.L., 2002. Spatial and seasonal variation of gross nitrogen transformations and microbial biomass in a Northeastern US grassland. Soil Biology & Biochemistry 34, 445-457. Davidson, E.A., Hart, S.C., Shanks, C.A., Firestone, M.K., 1991. Measuring gross nitrogen mineralization, immobilization, and nitrification by N-15 isotopic pool dilution in intact soil cores. Journal of Soil Science 42, 335-349. Hart, S.C., Stark, J.M., Davidson, E.A., Firestone, M.K., 1994. Nitrogen mineralization, immobilization, and nitrification. In: R. Weaver (Editor), Methods of Soil Analysis, Part 2. Microbiological and Biochemical Properties. SSSA Book Series, No. 5. 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Terrain attributes, landform segmentation, and soil redistribution. Soil & Tillage Research 69, 15-26. Pennock, D.J., Zebarth, B.J., Dejong, E., 1987. Landform classification and soil distribution in hummocky terrain, Saskatchewan, Canada. Geoderma 40, 297-315. Schimel, J.P., Bennett, J., 2004. Nitrogen mineralization: Challenges of a changing paradigm. Ecology 85, 591-602. Verchot, L.V., Groffman, P.M., Frank, D.A., 2002. Landscape versus ungulate control of gross mineralization and gross nitrification in semi-arid grasslands of Yellowstone National Park. Soil Biology & Biochemistry 34, 1691-1699.  

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Gross N-cycling rates in ephemeral wetlands

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