15N Riparian - Stream 12

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15N Riparian - Stream 25

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Below Ground Edge

Stream Size Influences Morphology and Biogeochemistry of Pasture Streams

Deforestation in the Amazon has the potential to alter the biogeochemistry of nitrogen over large regions and to alter nitrogen transport among adjoining ecosystems. Small streams dominate the total length of stream channels in the landscape. Human-induced changes in land use, such as the clearing of forest for pasture, can substantially alter the physical structure and chemical environment of small streams and the amount, chemical form and timing of materials delivered to streams from adjacent terrestrial ecosystems. The alterations to stream characteristics and nutrient regimes that follow forest conversion to agriculture are also likely to have important influences on primary production and inputs of organic material that control stream trophic dynamics. These changes may in turn influence the function of streams as habitat for a variety of aquatic organisms. An understanding of how forest conversion to agriculture influences stream biogeochemistry and functioning is required to assess how changes to large areas of the terrestrial biosphere are linked to changes in the chemical and biological functioning of aquatic ecosystems. We examined the influence stream size in pasture streams has on morphology and nitrogen biogeochemistry.

Christie L. Haupert1, Christopher Neill1, Linda A. Deegan1, Alex V. Krusche2, Reynaldo L. Victoria2, Victoria R. Ballester2

 1The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543, USA, chaupert@mbl.edu

2Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, Avenida Centenário 303, CEP 13416-000, Piracicaba, SP, Brasil

Methodology

Results

By Day 21 (the final day of 15N addition) all of the organic matter pools sampled in both streams: Fine Benthic Organic Matter, Coarse Benthic Organic Matter, Algae, and Epilithon were enriched with 15N.

The study was conducted in a 500 m reach of a small second order pasture stream (Stream 12) and 760 m of a larger third order pasture stream (Stream 25) located at Fazenda Nova Vida, a 20,000 ha cattle ranch near the center of the Amazonian state of Rondônia, Brazil.

The climate is humid tropical, with a mean annual temperature of 26 C and mean annual precipitation of 2.2 m. The region has a distinct dry season from June to October and rainy season from November to May.

MORPHOLOGYStream 12 was on average deeper (60-cm) and wider (600-cm) than the larger stream 25 (40-cm deep, 350-cm wide). It had an average discharge of 36 L/s, low levels of dissolved oxygen, organic rich sediments, 30 times more NH4 than NO3, and the N:P ratio was 1.9. Paspalum grass grew on the stream banks and in the stream channel.

Stream 25 has an average discharge of 91 L/s, high dissolved oxygen and sandy sediments. Nitrate was 9 times more abundant than NH4 and the N:P ratio was 17.0 The stream was primarily bordered by high soil/sand banks with few grasses growing in the stream channel.

AbstractSmall streams act as receptors for nutrients arriving from adjacent upland areas, and subsequently transform, retain or release these nutrients to larger rivers. To understand how deforestation and conversion of Amazon forest into pasture alters stream function, we evaluated geomorphology, organic matter stocks and biogeochemical processes of a small (30 L/s) and large (100 L/s) pasture stream in central Rondônia. We conducted two 21-day 15N-ammonium additions to trace transformations and downstream transport of nitrogen and organic matter turnover. The small stream was on average deeper (60 cm) and wider (600 cm) than the large stream (40 cm deep, 350 cm wide). The small stream had low dissolved oxygen, organic-rich sediments and had Paspalum grass on the stream banks and in the stream channel. The large stream had higher dissolved oxygen, sandy sediments and was primarily bordered by high soil/sand banks with few grasses growing in the stream channel. The small stream took up NH4 faster (0.038 g N min-1 m-2) and had a shorter NH4 uptake length (416 m) than the large stream (uptake rate: 0.008 g N min-1 m-2, length: 2000 m). Low NO3 concentrations and the absence of downstream 15NO3 enrichment in the small stream indicated that nitrification in the stream channel was negligible. The large stream had low rates of nitrification in the channel.Benthic organic matter in the small stream acquired 3 x as much 15N as the large stream. Grasses growing within 1 m of the edge of both stream channels were highly enriched in 15N (up to 180 o/oo). The small stream retained more N than the large stream, suggesting that deforestation in the adjacent landscape can have different effects on stream channel biogeochemical processes depending on stream size.

The study consisted of a 3 wk 15N-ammonium chloride addition to each stream followed by a 2 wk post drip analysis. Before the start of the 15N-addition the stream was marked at 20-m intervals and stations were chosen relative to the location of the 15N-drip point (0 m) at -90, 30, 50, 80, 140, 232, 500 m for stream 12 and –90, 40, 60, 100, 195, 460, 760 m for stream 25. Stream depth, width and benthic habitat type (Riparian, Leaf, Fine, Sand, Wood, Clay, Gravel) were recorded across the stream every 5-20 m along the stream reaches.

Nitrogen-15 samples for water chemistry, suspended particulate organic matter, coarse benthic organic matter, fine benthic organic matter, riparian grasses, algae, and epilithon were first collected from the reference station located upstream of the dripper, and then from the bottom of the reach to the site closest to the 15N source. Full sampling (all compartments at all stations) or truncated sampling (all compartments at selected stations) was conducted once before the start of the 15N-addition, on days 1, 3, 7, 14, 21 and post addition days 1, 3, 9, 14.

Study Site

Conclusions

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Sampling Stations

Stream flow

7

Dripper

15N

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STEP 1: Diffuse NH4 onto acidified filter.

STEP 2: Analyze filter for 15N and N mass.

(Sample + NaCl + MgO)

15NH4 (Holmes et al.

1998)

(Beaker w/ Sample + NaCl + MgO)

STEP 1: Concentrate sample and remove NH4

STEP 2: Convert NO3 to NH4 and diffuse onto acidified filter.

(Sample + Devarda’s alloy + MgO)

STEP 3: Analyze filter for 15N and N mass.

15NO3 (Sigman et al.

1997)

While both streams had riparian grasses enriched with 15N, due to the extensive coverage of riparian grasses in stream 12, 19% of the total 15N added was taken up by Day 21, in contrast to < 1% of 15N added to stream 25.

SH 37.5-P

Pasture Stream 12 Pasture Stream 25

Stream 1236

51854

24.56.1

22.61.4

0.083.000.109.182.191.92

Stream 2591

35834

25.06.1

17.67.5

0.031.459.238.620.87

17.03

Stream CharacteristicsDischarge (L/s)Width (cm)Depth (cm)Water temperature (oC)pHAlkalinity (mg/L)Dissolved oxygen (mg/L)Conductivity (mS/cm)Ammonia concentration (M)Nitrate concentration (M)Dissolved organic N concentration (M)Orthophosphate concentration (M)Dissolved Inorganic N:Dissolved Inorganic PStream averages from August – September 2003

Introduction

Stream Habitat Coverage (%)

Riparian GrassesLeaf PackFine Organic MaterialSandWoodClayGravel

Stream 12

551712115--

Stream 25

6231442573

15NO3 - Stream 12

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15NO3 - Stream 25

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15N Organic Matter - Stream 12

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Fine Benthic Organic MatterCoarse Benthic Organic MatterAlgaeEpilithon

Nitrate in stream 12 was not enriched suggesting that in- stream nitrification did not occur.

Enrichment of NO3 in stream 25 indicated low levels of in stream nitrification.

NH4 uptake length for stream 12 (345m) was approximately half of the NH4 uptake length of stream 25 (770m).

NH4 uptake rate in stream 12 (0.05 g min-1 m-2) was greater than uptake in stream 25 (0.02 g min-1 m-2).

BIOCHEMISTRY

NH4

The smaller stream 12, had extensive riparian grass habitat, anoxic waters, low nitrate concentrations, low N:P ratio. It appeared to be strongly nitrogen-limited.

The larger stream 25, had a sandy bottom, oxic waters, higher concentrations of nitrate, and is likely to be phosphorus-limited or co-limited by nitrogen and phosphorus.

The two streams processed nitrogen differently: stream 12 had a greater NH4 uptake rate, shorter NH4 uptake distance and showed no evidence of nitrification; whereas stream 25 had an NH4 uptake length twice that of the smaller stream, slower NH4 uptake rate and showed clear evidence of nitrification.

Benthic organic matter, algae and epilithon were enriched with 15N in both streams. The larger pool of benthic organic matter in stream 12 took up more 15N in relation to stream 25.

The extensive riparian zone of stream 12 was the largest sink for 15N, accounting for nearly 20% of the added 15N.

Small streams appear to be the portions of stream networks in deforested lands that most strongly retain N.