Soil, groundwater and surface water chemistry of nitrogen

Transcription

1 Soil, groundwater and surface water chemistry of nitrogen Shaun Watmough Trent University

2 Nitrogen Deposition Past and Present mg N/m 2 /yr Galloway and Cowling, 2002; Galloway et al., 2002b

3 Some basic information on nitrogen: Forms and functions of N in plants N required by plants in large amounts Usually the limiting nutrient in unfertilized systems Amino acids, amides, amines Building blocks, intermediary compounds Proteins, chlorophyll, nucleic acids Proteins/enzymes regulate biochemical reactions N is an integral part of chlorophyll structure Pale green/yellow color of N deficient plants Nitrogenous bases of DNA, RNA N mobility Translocated from older leaves to younger leaves Deficiencies occur first on lower, older leaves

4 Ammonium and nitrate dominate: Ammonium (NH 4+ ) Does not have to be reduced Conserves energy Rhizosphere ph Roots release H + Nitrate (NO 3- ) Must be reduced before synthesis of amino acids, etc. Rhizosphere ph Roots release HCO 3- (OH - )

5 Nitrogen dynamics are complex:

6 The Nitrogen Cycle N transformations: Mineralization Immobilization Nitrification Denitrification Volatilization N Fixation

7 Mineralization 1. Release of organic N as plant available inorganic NH 4 -N 2. Heterotrophic bacteria and fungi 3. Soil OM ~5% N (considerable variation) 4. ~1 to 4% of organic N mineralized each year (considerable variation)

8 Immobilization (assimilation) 1. Reverse of mineralization 2. Uptake of inorganic N from the soil and incorporation into organic N forms by microbes Can be NH 4+ or NO 3-3. Mass balance studies often show vast majority of N is retained in soils

9 Nitrification 1. Conversion of NH 4+ to NO 3-2. Two-step process 3. Primarily performed by two types of autotrophic bacteria Obtain energy from N oxidation

10 Denitrification 1. Gaseous loss of N Conversion of NO 3- to N 2 and N 2 O 2. Anaerobic bacteria Pseudomonas, Bacillus, and other species Facultative anaerobes Use oxidized N as their O 2 source for respiration Waterlogged soils or low O 2 microsites in soil e.g. around roots or decomposing residues

11 Volatilization Gaseous loss of NH 3 Primarily from surface-applied N fertilizers NH 3 loss favored by high ph Solution ph >7 or 7.5

12 Nitrogen Saturation N in excess of biotic demands Hypothesized biogeochemical response of temperate forest ecosystems to longterm nitrogen additions (modified from Aber et al 1998).

13 Experimental Studies Many experimental studies have demonstrated the potential negative impacts of N deposition. Common for N to be applied at a level far greater than current observed values Many studies show expected response to elevated N application. N fertilization plot at Harvard Forest

14 Rustad et al. are working on a meta-analysis of experimental N addition studies ( Summary of the response of Foliar N to N additions (56 manipulations in 8 countries)

15 Other Processes Regina et al (1998) demonstrated that net N mineralization, N 2 O and NO fluxes from a drained forested peatland may be affected by N additions (100 kg N ha -1 ), although response varies depending upon form of N application.

16 Results from experimental studies: Some things (commonly measured) to look for with respect to N-saturation. Increased mineralization Increased nitrification Increased foliar N (uptake) Decreasing soil C:N ratio (N-immobilized)

17 Monitoring and Gradient Studies Nitrogen budgets in central Ontario Forests Catchment scale Role of climate Forest plots Nitrogen budgets in eastern North America and European catchments

18 Nitrogen Budgets in Central Ontario 50 units: meq m 2 yr Nitrate Ammonium 5 Dorset Environmental Science Centre (45 07 N, W) Nitrogen bulk deposition is around 70 meq m -2 yr -1 (~10 kg N ha -1 yr -1 )

19

20 The region in summer

21 and in winter

22 Deposition Net forest uptake Soil nitrogen pool Soil leaching losses

23 Most nitrogen is retained % N retention % N exported as organic N Cumulative N accumulation PC CB RC HP3A HP HP HP6A Average And, most of N exported is organic and the catchments accumulated between 93 and 128 kg ha -1 over a 17-yr period ( )

24 Nitrate leaching varies considerably among catchments no increasing trend mean annual [NO3] µeq/l / / / /95 PC-1 CB-1 HP-3A RC-4 HP-4 HP-6 HP-6A So what is responsible for the spatial and temporal variability?

25 Expanded to 17 catchments 4.0 Two groups identified with respect to NO 3 leaching. Z-score RC2 RC3 RC4 CB-1 CB-2 PC-1 DE-5 DE-6 DE-11 Group / / / / / / HP-3A 2.0 HP-4 Z-score HP-5 HP-6 HP-6A RC1-1.0 BC Group / / / / / /98

26 The first group are seemingly more wetland influenced % Peat Grade (%) % deep till (>1 m) Upland Forest Annual NO 3 - N (mg/l) % N retention % NO 3 (of total stream N) Trend slope (mg/l) RC SM, YB, HE * RC SM, YB, RO n.s. RC SM, RO, RM n.s.. CB RM, RO, SM n.s. CB SM, RM, RO n.s PC WP, HE, RO n.s. DE RM,SM, YB n.s. DE RM, SM, YB n.s DE RM, SM, YB n.s. mean

27 The second group is less influenced by wetlands % Peat Grade (%) % deep till (>1 m) Upland Forest Annual NO 3 - N (mg/l) % N retention % NO 3 (of total stream N) HP3A SM, YB, RM n.s. HP SM, RM, YB n.s. HP SM, YB, RM n.s. HP SM, HE, YB n.s. HP6A SM, HE, YB n.s. RC SM, YB, HE n.s. BC SM, YB, BE n.s. mean Trend slope (mg/l)

28 Explanations Likely a number of climate-related factors. Stepwise multiple regression indicated that summer drought (both groups) and cumulative frost depth (sum of daily frost depths group 1) could explain around 50% of the variability in annual NO 3 concentrations.

29 Net NO 3 production also sensitive to changes in soil moisture: NO3 (mg N/l) NO3 SO SO4 (mg/l) NO3 (mg N/kg) wet dry 0.0 Jun-86 Dec-86 Jun-87 Dec-87 Jun-88 Dec-88 Jun-89 Dec-89 Jun-90 Dec-90 Jun Peat Decid LFH Conif LFH NO 3 concentrations increase following summer drought Drying and rewetting results in increase NO 3 availability in lab studies

30 Seasonal pattern of NO 3- export in upland catchments indicates that changes in winter temperature and precipitation could have implications for NO 3- leaching 100 % of annual NO 3 export %spring %winter

31 Rain-on-snow events Three factors indicate that much of the NO 3 -N and acidity associated with ROS events is transported relatively conservatively to drainage streams 1. High [NO 3 -N] in precipitation (also snow pack) Bulk dep: 280 to 2000 µg/l (annual avg streams: µg/l; max ROS event = 1480 µg/l) Snow: µg/l 2. Limited interaction with soil beneath snow pack 3. Relatively low potential for biological retention during dormant season

32 Contribution of ROS events to NO 3 -N export at an upland forested catchment: %winter export % annual export avg winter temp ROS NO3-N export (%) winter temp ( o C) -14.0

33 N budgets in Eastern North America and European Catchments N budgets (1990s average) calculated for 21 sites that span a deposition gradient. Trend analysis was conducted on monthly NO 3 and NH 4 concentrations when there were more than 9 years of data (14 regions; 17 sites).

34 The Sites

35 Nitrogen Deposition: N (kg/ha) * Canada US Europe ELA TL MH LL LC LT MP BB CP AB HB WF VL SH LH LG BK Ammonium Nitrate Site Annual N deposition (1990s) was between 2.8 kg/ha and 13.7 kg/ha (excluding WF). Nitrogen deposition was generally higher at the European sites where a greater proportion of the annual N deposition was as NH 4.

36 Trends in deposition/runoff Only a significant decline in NO 3 deposition in 1 region. NH 4 deposition response was varied increasing in 3 regions and decreasing in 2. NO 3 concentrations in runoff decreased at 4 sites and increased at 1 site. NH 4 concentrations increased in runoff at 5 sites (mineralization/climate?)

37 Currently - the majority of N is retained: N-NO3 export (kg/ha/yr) Inorganic N deposition (kg/ha/yr) Between 31 and 100% of inorganic N input in deposition was retained across the study sites; median retention of 94%

38 Source: Julian Aherne, Trent University Similar pattern found in eastern Canadian lakes

39 NO 3 -N decrease down the soil profile: LFH; NO 3 -N; 117 µg L -1 A; NO 3 -N; 142 µg L -1 B; NO 3 -N; 87 µg L -1 Stream; NO 3 -N; 37 µg L -1

40 A quick word on Critical Loads (Acidification) CL (S+N) = BC dep Cl dep + BC w BC u + N u + N de + N i + ALK le(crit) CL (S+N) = CL (S) + N u + N de + N i Nitrogen represents between 44 and 67% (median value 58%) of the bulk S and N deposition. But, NO 3 leaching only represents between 0 and 32 % (median value 5 %) of the total SO 4 and NO 3 export.

41 Current vs. long-term N behaviour Given the increasing relative importance of N deposition (with respect to S), exceedance of the critical load of acidity is greatly influenced by assumptions about the long-term N behaviour. Which NO 3 values are important - soils vs. streams vs. lakes

42 N in Ontario hardwood forest plots A number of sugar maple dominated plots are currently being monitored by the Ontario Ministry of Environment. These sites span a considerable range of N deposition.

43 nitrate wet deposition nitrate dry deposition ammonium wet deposition DORSET > 30.0 units: meq m 2 yr 1 ammonium dry deposition > 6.0 units: meq m 2 yr 1

44 No obvious relationship with N deposition

45 No Obvious Relationship with N deposition

46 No Obvious Relationship with N deposition

47 No Obvious Relationship with N deposition

48 Summary Basic processes affecting N cycling are understood. Much less certainty in how these process vary with respect to: biotic and abiotic factors (climate, forest type etc.) Changes in N deposition/time

49 Acknowledgements Work was supported by funding from NSERC, NERC, Ontario Ministry of Environment, Ontario Power Generation. Dozens of technicians/students.