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Published online 5 April 2007
Published in Soil Sci Soc Am J 71:829-835 (2007)
DOI: 10.2136/sssaj2006.0020
© 2007 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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FOREST, RANGE & WILDLAND SOILS

Organic Matter Transformations through Arroyos and Alluvial Fan Soils within a Native American Agroecosystem

Jay B. Nortona,*, Jonathan A. Sandorb, Carleton S. Whitec and Vanissa Laahtyd

a Dep. of Renewable Resources, Univ. of Wyoming, Laramie, WY 82071-3354
b Dep. of Agronomy, Iowa State Univ., Ames, IA 50011-1010
c Dep. of Biology, Univ. of New Mexico, Albuquerque, NM 87131
d Zuni Conservation Program, Pueblo of Zuni, P.O. Box 339, Zuni, NM 87327

* Corresponding author (jnorton4{at}uwyo.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Linked biochemical and fluvial processes in discontinuous ephemeral streams may support sustained productivity of soils farmed by southwestern Native Americans for ≥3000 yr. Ephemeral stream channels transport forest floor litter and soil materials from upland hillslopes to alluvial fans. Improved understanding of how ephemeral streams transport and process forest-floor organic materials could improve conservation of ecologically important and productive headwater alluvial fans. We analyzed organic and mineral materials from source to mouth along two ephemeral streams and analyzed suspended sediments from four collection traps in each. Results suggest that decomposition processes differ by reach and frequent, low-energy flows preferentially transport organic detritus as it decomposes. Processing of organic-rich sediments in canyon reaches is dominated by microbial immobilization (low inorganic N and available P, high C/N ratio). Arroyo reaches receive organic materials chiefly from upstream so mineralization plays in increasing role as materials are transported and decomposed downstream without fresh inputs. The frequency of flow decreases in a downstream direction as water infiltrates sandy streambeds. In lower arroyo and fan reaches, inputs of organic-rich sediments are infrequent. Relatively frequent wetting and drying stimulates mineralization of organic materials so concentrations of inorganic N and available P in detritus are relatively high. Results suggest that organic-rich sediments processed through ephemeral streams and deposited on unincised alluvial fans are important in sustaining one of the most productive landscape positions in semiarid regions.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Traditionally farmed corn (Zea mays L.) fields on alluvial fans of the Zuni Indian Reservation in west-central New Mexico represent some of the oldest agricultural soils in North America (Damp et al., 2002; Homburg et al., 2005). In a southwestern North American version of runoff agriculture (Sandor et al., 2002; Sandor et al., 2007), Native American farmers rely on winter moisture stored in alluvium-derived soils for germination and early crop growth and then on summer rainfall for later growth and grain production (Muenchrath et al., 2002). Soil fertility is not maintained by applications of nutrient-supplying amendments, but is thought to be a function of watershed processes that transport and decompose forest-floor organic materials from uplands (Cushing, 1920; Norton et al., 1998, 2001).

Properly functioning ephemeral channel-fan systems store and transform runoff, sediments, and nutrients washed from uplands high in headwater watersheds (Bull, 1997). This process may underlie long-term sustainability of southwestern Native American agriculture but it also protects downstream aquatic systems (Peterson et al., 2001) and creates diverse and productive microclimates in arid and semiarid landscapes. Channel erosion—often called arroyo cutting—prominent across the Southwest during the 20th century, disconnects ephemeral channels from alluvial fans (Cooke and Reeves, 1976; Elliot et al., 1999). This eliminates storage and nutrient cycling functions, drying out and slowly depleting the fertility of existing alluvial soils as water and nutrients are taken up without being replenished (Bull, 1997). The Zuni and other southwestern Native American farmers actively manipulate ephemeral stream channels to maintain and enhance depositional processes (Cushing, 1920; Hack, 1942; Nabhan, 1979; Norton et al., 2002).

Norton et al. (2003) established that soil organic matter contents measured along summit to toeslope transects in the study watersheds follow parabolic trends that peak in soils of wooded backslopes, while plant-available N and P increase linearly and peak in soils of toeslopes. This suggests that organic materials are mixed and decomposed as they move downslope toward agricultural fields. Norton et al. (2007) showed that sediment and soil organic matter movement on the hillslopes reflects the distribution of soils and vegetation and that frequent, minor rainfall events may be important drivers of nutrient movement and transformation. This study investigated sediment and organic materials in ephemeral stream channels that link hillslopes to alluvial fans where traditional cornfields are located. Our objective was to describe how the composition of stream-deposited organic materials and suspended sediments change through ephemeral channels from their source to runoff agricultural fields. We hypothesized that, as in the hillslope system, ephemeral streams process organic materials as they transport them toward traditional agricultural fields with increasing proportions of available nutrients nearer the fields.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Description
We analyzed the properties of organic-rich deposits and suspended sediments along channels from watershed divides to distal alluvial fans in two watersheds above long-term traditional alluvial runoff agricultural fields on the Zuni Indian Reservation (Fig. 1 and 2 ). The Sanchez watershed drains 68 ha near the eastern edge of the reservation and the Weekoty watershed drains 125 ha about 10 km to the northwest. Each of the watersheds lies at about 2300-m elevation and is characterized by steep-walled canyons cut into sandstones and shales of the Gallup Sandstone formation (Anderson et al., 1989; Zschetzsche et al., 2005). Layered shale and sandstone members underlie shallow Entisols on mesa top caprocks; thickly bedded sandstone creates fall faces; thick, weathered shale underlies Alfisols on transportational backslope and colluvial footslope positions; and sandy and silty alluvium underlies Alfisols on toeslopes and alluvial fans (Fig. 3 ; Norton et al., 2003; Homburg et al., 2005). Mesas and hillslopes in the two study watersheds are dominated by pinyon pine (Pinus edulis Engelm.)–juniper (Juniperus spp.)–oak (Quercus gambelii Nutt.) woodlands with appreciable ponderosa pine (Pinus ponderosa P. Lawson & C. Lawson) on upper watershed slopes (Norton et al., 2003). The canyon-floor alluvium in each watershed is bisected by an arroyo ranging up to 6 m deep. In each case, the arroyo channel ends above a runoff agricultural field, near the canyon mouth. Each of the two ephemeral stream systems we studied are divided into three distinct reaches we called canyon, arroyo, and fan (Fig. 2). Canyon reaches are channels with bedrock beds at the foot of steep slopes and escarpments in upper watersheds. This corresponds to the "production zone" described by Schumm (1977). Arroyo reaches are deeply incised (>6 m) sections of upper alluvial fans near the outlets of the channel reaches and correspond to Schumm's (1977) "transfer zone." Fan reaches correspond to Schumm's (1977) deposition zone and are the active alluvial fans where traditional agricultural fields are located.


Figure 1
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  		Fig. 1. Study area location and physiographic provinces of the Southwest.

 

Figure 2
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  		Fig. 2. Study watersheds showing channel reaches and suspended sediment trap locations.

 

Figure 3
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  		Fig. 3. General topography, slope positions, and soils within the three watershed study sites on the Zuni Indian Reservation (from Norton et al., 2003).

 
Rainfall averages 300 mm annually but is highly variable and comes mostly during thunderstorms in July, August, and September (Tuan et al., 1973). Although rainfall at the core of the localized storms can be intense, most of the annual rainfall comes as low-intensity, but relatively frequent, events (Balling and Wells, 1990; Norton et al., 2003). The frequent, low-intensity rains often generate hillslope runoff that is absorbed in deeper soils of footslopes and toeslopes (Norton et al., 2003). Streamflow that just exceeds transmission losses through these systems occurs less than once per year on average, while larger flows that inundate alluvial fans are less frequent (Thomas et al., 1997; Norton, 2000; Norton et al., 2002).

Field and Laboratory Procedures
Stream-Deposited Organic Materials
We analyzed how fluvial transport affects forest litter by collecting samples of water-deposited forest floor materials in stream channels. Samples were collected each 100 m from channel source (defined as the upper end of the longest continuous channel) to distal alluvial fan (see Fig. 2) (samples from each clump of organic material 7.5 m upstream and 7.5 m downstream from the 100-m point were mixed and subsampled). Organic-rich deposits were somewhat scarce in the lower reaches of the alluvial fans, forcing us to collect organic matter samples opportunistically and note locations (as meters from the source) along channel trajectories.

Samples from organic-rich deposits were air dried in the field and then analyzed for concentrations of total C and N in subsamples oven dried at 105°C and ground to pass a 76-µm sieve using a Fissions EA1100 dry combustion CNSHO analyzer (Fissions Inst., Milan, Italy). Total P concentrations were determined in similar subsamples by alkaline oxidation (Dick and Tabatabai, 1977). Available P concentrations were measured by the Olsen extraction method (Olsen and Sommers, 1982). Ammonium- and NO3–N concentrations were determined in 2 M KCl extracts using the Berthelot reaction for NH4–N (Willis et al., 1993) and nitration of salicylate for NO3–N (Yang et al., 1998).

Suspended Sediments
Movement and transformation of fine organic particulate transported in ephemeral streamflows were analyzed by sampling suspended sediments in upper canyons and below the three reaches of each ephemeral stream system (Fig. 2). Samples were collected during the summers of 1997 and 1998 in the Weekoty and during 1998 in the Sanchez watershed. Samplers were based on USGS samplers designed to collect water samples during rising stages of runoff events. Each USGS sampler consists of four or more 1-L sample bottles stacked inside 10.16-cm (4-inch) i.d. polyvinyl chloride (PVC) pipe, each with an inlet tube piercing the upstream side of the PVC pipe and a vent tube extending out the top of the pipe. We modified the samplers to fit the scale of the small channels at our study sites by stacking two 1-L bottles and embedding the encasing PVC pipe vertically in the center of the channel bed, leaving about 15 cm exposed. Inlet tubes were 1 and 5 cm above the channel bed surface. Samples were collected after each runoff event, preserved with dilute phenyl mercuric acetate solution, and allowed to settle in refrigerators at 4°C. The total volume of each sample was recorded and clear supernatants were analyzed for cation concentration by atomic absorption spectrophotometry and anion concentration by ion chromatography. Sediment samples were air dried (oven temperature 60°C), weighed for calculation of sediment concentration, and analyzed for total C, N, and P by methods described above for samples of organic-rich deposits. Samples were generally too small for analysis of mineral N and P fractions or particle size distribution.

Precipitation volume, intensity, and duration were measured only at the Weekoty watershed at a tipping-bucket rain gauge and a CRX-20 data logger (Campbell Scientific, Logan, UT) located near sediment trap no. 3 in the Weekoty watershed (Fig. 2).

Concentrations of each measured constituent in stream-deposited organic material and suspended sediments were analyzed by regression analysis (SPSS 14.0, Chicago, IL) as functions of distance from the channel source for each watershed separately. Suspended sediment concentration as a function of rainfall intensity and the C/N ratio of organic matter in suspended sediments as a function of sediment concentration were also analyzed by regression analysis for the Weekoty watershed only (where the tipping-bucket rain gauge was located). The significance of each model was tested by ANOVA (SPSS 14.0, Chicago, IL) and reported P values are based on those analyses. Ratios, including C/N, available P/total P, NO3–N/total N, and NH4–N/NO3–N, were log10 transformed for analyses. Reported r2 and P values are based on the log10 transformations. Untransformed values are reported on Fig. 4 to 8 and equations are based on those values (calculated with the trendline function in Microsoft Excel).


Figure 4
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  		Fig. 4. Composition of organic-rich stream deposits as a function of distance from channel source. Sampling of the Sanchez watershed began approximately 500 m from its source. Weekoty watershed is reported in left column (a–e) and Sanchez watershed in right (f–j).

 

   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Organic-Rich Deposits
Regression analyses show that concentrations of total C, N, and P, as well as available N and P, in organic-rich deposits change along distinct parabolic and linear functions through Weekoty and Sanchez fluvial systems (Fig. 4). Available N concentration, both NH4– and NO3–N (only NO3–N is presented), increases distinctly with distance from the source at the Weekoty watershed but not at the Sanchez watershed (Fig. 4d and 4i). Phosphorus contents, both total (Fig. 4c and 4h) and available (Fig. 4e and 4j), follow especially strong parabolic relationships along the channels.

Relationships between the C, N, and P fractions are similar between the two study sites (Fig. 5 ). Carbon/N ratios are variable but decrease linearly (P < 0.05) in a downstream direction along the channels (Fig. 5a and 5e). Available P as a percentage of total P decreases slightly in the upper reach of the Weekoty watershed, but increases steadily (P < 0.01) through the arroyo and fan reaches of both watersheds (Fig. 5b and 5f). Nitrate-N as a proportion of total N increases along a highly significantly trend (P < 0.0001) with distance from the source of the Weekoty watershed (Fig. 5c) but does not follow a significant trend in the Sanchez watershed (Fig. 5g). The ratio of NH4– to NO3–N, an indicator of nitrification rates, is relatively high and variable in the canyon reaches of the Weekoty system, but very constant at about 1:1 in the arroyo and alluvial fan reaches (Fig. 5d and 5h).


Figure 5
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  		Fig. 5. Relationships between total and available nutrients and stream course position in organic-rich stream deposits. Weekoty watershed is reported in left column (a–d) and Sanchez watershed in right (e–h).

 
Suspended Sediments
Nine rainfall events at the Weekoty watershed in 1997 and 1998 and four events at the Sanchez watershed in 1998 produced samples in our sediment traps. While each trap received runoff during most of the events, it did not always appear to be the result of continuous flow from upper canyons to distal channel fans. Observations during sample collection suggest that flow intensity peaks where canyon reaches empty into arroyo reaches. There was considerable scour at these traps (trap no. 2 at each site; Fig. 2) after several of the events in each watershed. Transmission losses through the sandy arroyos below trap no. 2 appear to rapidly diminish flows. The most distal trap at each site did not receive runoff during several of the events, and during others runoff apparently came from local sheet flow. Some the events in each watershed did generate continuous flow past all the traps. Most of our samples came from bottles connected to the lower intakes, 1 cm above ground level, and probably represent the very beginning of flow at the traps. Samples from both intake depths were essentially identical. They are combined for the data reported here.

Suspended sediment concentrations are highly variable and range from 0.6 to 128 g L–1 without predictable differences between sediment trap locations. Both sediment concentration and variability increase with increasing rainfall intensity (P < 0.10; Fig. 6 ), but many more of the relatively high-intensity rainfall events we measured yielded higher concentrations of suspended sediments than did the lower intensity events. Also, C/N ratios of the suspended sediments increase with increasing sediment concentration (P < 0.0001; Fig. 7 ). Although sediment concentration in runoff varies greatly among events and trap locations, concentrations of total C, N, and P in the sediments remain relatively constant among events and appear to vary systematically among traps (Fig. 8 ). Carbon/N ratios in sediments decrease between the upper and lower traps at both sites, with the largest difference occurring between traps no. 2 and 3 at the Weekoty site and traps no. 3 and 4 at the Sanchez site (Fig. 8d and 8h).


Figure 6
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  		Fig. 6. Sediment concentration as a function of rainfall intensity as measured at suspended sediment traps and tipping bucket rain gauge in the Weekoty watershed.

 

Figure 7
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  		Fig. 7. Carbon/N ratio of sediment organic matter as a function of sediment concentration at suspended sediment traps in the Weekoty watershed.

 

Figure 8
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  		Fig. 8. Concentrations of organic C, total N, and total P in suspended sediment as a function of distance from channel source. Weekoty watershed is reported in left column (a–d) and Sanchez watershed in right (e–h).

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Organic Matter Movement and Transformation
The combination of rainfall intensity, sediment concentration, and organic matter and nutrient contents of the sediment suggest that relatively frequent, low-intensity runoff events preferentially move organic materials with more favorable C/N ratios and higher mineral N and P concentrations. This pattern (materials being moved downslope as they are broken down) was also noted in hillslope soils (Norton et al., 2003) and sediments (Norton et al., 2007). The same pattern is known to be important in organic detritus processing by perennial streams (Boling et al., 1975; Maltby, 1992; Wagener et al., 1998). Our data suggest that dominant processes transforming organic materials vary by reach depending on the amount and composition of materials entering the stream from surrounding slopes.

In upper canyon reaches on relatively level mesa tops, gentle slope processes contribute to conditions that stimulate nutrient immobilization in decomposing organic matter. Forest floor materials under these mesa-top woodlands decompose in situ, which, together with lower intensity runoff (because of shorter, flatter slopes) that minimizes disturbance, leads to relatively stable plant and microbial communities, rapid immobilization, and "tight" nutrient cycles (Stark and Hart, 1997; Norton et al., 2003). This is suggested by low available N and P concentrations in the organic-rich deposits of canyon reaches. Gentle runoff on the short, relatively level slopes preferentially moves partly decomposed materials toward channels (Norton et al., 2007). As this material is decomposed and moved downstream into lower canyon reaches, it mixes with large quantities of relatively fresh, high-C-content forest floor material from longer, steeper slopes forested by ponderosa pine, Gambel oak, and pinyon–juniper. This influx of C-rich forest litter probably stimulates immobilization in organic-rich deposits in lower canyon reaches (relatively high C/N ratios and low available P/total P). Steep gradients and the convergence of many tributaries through these reaches appear to create intense flows, however, even during relatively low-magnitude, high-frequency rainfall events. This frequent perturbation probably leads to a relatively "open" N cycle suggested by increasing inorganic N concentrations in the deposits.

The influx of fresh material from side slopes stops abruptly as channels emerge from canyons and enter arroyos cut through alluvial valley fills. Runoff intensity peaks at this transition from steep bedrock channels to more gently sloping alluvial channels. The relatively high stream power is reflected in low C, N, and P concentrations and high C/N ratios in suspended sediments (low-density, decomposed organic materials are preferentially removed by the flowing water). Essentially, flows emerge from the forested bedrock canyon rich with both suspended sediments and organic materials. Sediments are deposited as stream gradient decreases, but organic materials are carried farther downstream and deposited as transmission losses diminish flows. Each flow mixes, moves, and sorts the organic-rich materials, causing increasing mineralization rates through arroyo reaches. The predominance of low flows from minor rains preferentially moves fine, low C/N ratio particles downstream.

Alluvial fans receive influxes of coarse organic-rich materials only during low-frequency, intense runoff events that deposit loads of sandy and loamy sediments across valley floors. Some of the more frequent low-intensity runoff events also move across alluvial fans, but deposit only fine sediments and relatively highly decomposed fine-particulate organic materials. This combination of wetting by frequent low-intensity rains, occasional influx of organic particles with low C/N ratios, and lack of coarse forest detritus influx contributes to increasing mineralization rates in organic-rich deposits with distance through the alluvial fans. Sala and Lauenroth (1982) emphasized the importance of frequent small rainfall events in driving organic matter dynamics in semiarid soils.

Figure 5d suggests that nitrification in organic-rich deposits of the forested canyon reach may be inhibited. White (1994) described a mechanism for this type of inhibition in southwestern conifer forests by volatile organic compounds called monoterpenes. This effect appears to diminish as organic materials are moved farther from their source on the ponderosa pine forested slopes, possibly because the volatile compounds are lost as materials decompose.

Differences between mineral N concentrations in organic-rich deposits from the Weekoty and Sanchez watersheds (Fig. 4), along with similarities in organic C, total N, and total P concentrations seem to suggest greater N immobilization in all the Sanchez samples and may reflect differences in sampling times or inadvertent differences in handling of samples (i.e., the Sanchez samples were wetter at sampling and humid weather at the study site caused them to air dry slowly).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Our results emphasize the importance of hydrological connectivity that transports and transforms watershed soil and organic materials, which replenish alluvium-derived soils that have important cultural, ecological, and hydrological functions. Ephemeral storm-water flows that scour channels and transport and deposit fresh sediments are important drivers of hydrological functions of alluvial fans. These relatively high-impact events shape the fluvial system and have received a great deal of attention. Our results show that minor rainfall and runoff, which occur much more frequently, may drive biochemical processes that link forested watersheds to alluvial fans. In contrast to dramatic, bedload-transporting events that have multiyear average return intervals, minor low-energy flow events generally occur multiple times during the summer rainy season. These minor events repeatedly moisten, mix, and sort channel and hillslope materials as they move them downstream in a stepwise fashion. This results in mineralization rates that increase downslope through hillslope and fluvial systems and in accumulation of decomposed, nutrient-rich organic materials in lower reaches where traditional runoff fields are located.


   ACKNOWLEDGMENTS
 
This work was funded by National Science Foundation Grant no. DEB-9528458. We are indebted to the Zuni Sustainable Agriculture Project, the Zuni Conservation Project, and the Zuni Tribe. We thank Jeff Homburg, Todd Carlson, Marnie Criley, and Clara Wheeler for laboratory analysis and Troy Lucio, Lindsay Quam, and participants in Zuni's Job Training Partnerships Act program for field assistance. We are grateful to Tom DeLuca, Urszula Norton, and Stephen Siebert for technical advice and editorial review, and to the research team, including Deborah Muenchrath, Stephen Williams, Pete Stahl, and Mark Ankeny.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication January 13, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





This Article
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Right arrow Articles by Norton, J. B.
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