Lateral movement of organic matter (OM) due to erosion is now considered an
important flux term in terrestrial carbon (C) and nitrogen (N) budgets, yet
most published studies on the role of erosion focus on agricultural or
grassland ecosystems. To date, little information is available on the rate
and nature of OM eroded from forest ecosystems. We present annual sediment
composition and yield, for water years 2005–2011, from eight catchments in
the southern part of the Sierra Nevada, California. Sediment was compared to
soil at three different landform positions from the source slopes to
determine if there is selective transport of organic matter or different
mineral particle size classes. Sediment export varied from 0.4 to 177 kg ha
The processes of soil erosion and terrestrial sedimentation have been a
focus of a growing number of studies because of their potential to induce a
net terrestrial sink for atmospheric carbon dioxide (CO
Recent studies have identified major implications of erosion on soil organic matter (SOM) stabilization, changes in composition, and input to the soil system. Identified stabilization mechanisms for this eroded organic matter (OM) deposited in low-lying landform positions include burial, aggregation, and sorption of OM on the surfaces of reactive soil minerals (Berhe et al., 2012a, 2015; Vandenbygaart et al., 2012), and changes in the biomolecular composition of OM during transport (Rumpel and Kogel-Knabner, 2011; Vandenbygaart et al., 2015). Removal of organic- and nutrient-rich topsoil material from eroding positions and its concomitant accumulation in depositional landform positions also has impacts for net primary productivity (NPP) in both locations (Yoo et al., 2005; Berhe et al., 2008; Parfitt et al., 2013). These factors – the balance of organic matter production, stabilization and loss across the landscape – are ecosystem-specific. Several studies have assessed the impact of erosion on C balances in agricultural lands (Van Oost et al., 2007; Quinton et al., 2010; Chappell et al., 2012; Vanderbygaart et al., 2012; Rumpel et al., 2014). Some ecosystems with less human influence have also been studied in this context (Yoo et al., 2006; Berhe et al., 2008; Boix-Fayos et al., 2009; Hancock et al., 2010; Nadeu et al., 2012), but there are currently few published data from minimally disturbed temperate forests.
Erosion processes in forested ecosystems, especially upland or steep catchments, have notable differences from agroecosystems. For instance, average sediment erosion rates are orders of magnitude higher for agricultural lands compared to forested lands (Pimentel and Kounang, 1998). Forest land erosion rates are lower in part due to greater live plant and litter cover of the mineral soil than in agroecosystems, as the vegetation cover reduces the energy of incoming precipitation. In landscapes that have experienced little anthropogenic disturbance, overland erosion transports material from the uppermost soil horizons, which often have a high proportion of undecomposed OM and high C concentrations. Such C enrichment in the transported material relative to the residual soil has been observed in croplands and rangelands, but increased incision into the landscape – through gullies, mass wasting or other processes – also erodes material from deeper layers with lower C concentrations in these managed ecosystems, resulting in relatively low C enrichments (Nadeu et al., 2011). The intensive cultural practices used frequently in agricultural, but less often in forestry, such as tilling or vegetation removal, disrupt soil stability and can increase erosion by orders of magnitude (e.g., Pimentel and Kounang, 1998; Van Oost et al., 2006).
Sediment exported from small, minimally disturbed low-order catchments can experience C oxidation during transport (Berhe, 2012) through the disruption of aggregates (Nadeu et al., 2011; Boix-Fayos et al., 2015), exposure to oxygen and new microbial decomposers, or other means. The oxidative C loss during erosion is typically assumed to be less than 20 % in agroecosystems partly owing to the relatively low OM concentrations in these soils (Berhe et al., 2007). This same assumption may not be valid in forested ecosystems because upland forest soils typically have much higher concentrations of OM in surficial soils (as organic horizons or OM-rich mineral topsoil). Furthermore, C in forested soils or undisturbed grasslands is likely to have a larger unprotected (free, light) fraction compared to agricultural soils, where most of the C is typically associated with the soil mineral fraction (Berhe et al., 2012a; Wang et al., 2014; Wiaux et al., 2013; Stacy, 2012). Hence, forested sites are likely to have a substantially higher proportion of their eroded OM transported as unprotected, carbon-rich sediments that are free from any physical (aggregation) or chemical (bonding, complexation) association with soil minerals when compared to the better-studied agricultural soils.
Furthermore, determining the role of erosion on forested ecosystems is timely since even forested systems that previously did not experience much anthropogenic modification are expected to experience considerable changes in precipitation amount, timing, and nature with anticipated changes in climate. Anticipated changes in climate are expected to have important implications for sediment and OM erosion from forest ecosystems. In the Sierra Nevada, large tracts of relatively undisturbed forest still exist. Even though some land has experienced intensive management for timber production (especially in historical periods), most is only minimally influenced by human activities, including fire management, roads, and the water reservoir system. In these ecosystems, increasing temperatures associated with climate change are expected to alter the erosional process due to the anticipated shift in the nature of precipitation. A shift in the type of precipitation from snow to rain, and a higher number of rain-on-snow events, compared to even the last few decades (Bales et al., 2006; IPCC, 2007; Klos et al., 2014), are expected to provide greater force to detach, scour, and transport material from the soil overall (Boix-Fayos et al., 2009; Nadeu et al., 2011), with subsequent implications for amount of C transported. Higher erosive forces will also provide more energy to disrupt aggregates, exposing OM previously protected from decomposition to loss (Nadeu et al., 2011). The dearth of data on the effect of climate change on soil C erosion is complicated by the inherent variability of erosion events, such as episodic, large storm events or an extreme weather season, that make it challenging to create conceptual or numerical models that can easily scale up across time and space (Kirkby, 2010).
Here, we focus on determining the nature and magnitude of the sediment and
associated OM exported out of forested upland catchments at mid-range scales
(spatially and temporally) to further our understanding of how climate
affects soil erosion processes in such ecosystems. We quantified the mass
and composition of sediments exported from eight low-order catchments to
determine the effect of soil erosion on C and N dynamics in these upland
forest ecosystems. Our study catchments are located in the southern Sierra
Nevada, at two contrasting elevation zones with differences in the
proportion of precipitation falling as rain or snow. This work builds on
previous publications on the sediment transport and composition from the
same site (Eagan et al., 2007; Hunsaker and Neary, 2012), covering sediment
transport for all water years (2005–2011) after the construction of all
sediment basins and prior to planned forest management treatments (fire and
thinning); implementation of those treatments began in 2012. In addition, we
expand on the characterization of sediment composition with additional
measurements and a comparison to soil samples from potential source
locations. This work is part of a larger investigation at this site on
changes in OM stabilization mechanisms due to erosion. Specifically, we
addressed two critical questions:
in forested catchments with minimal disturbance, how are rates of
sediment yield related to interannual and elevational differences in precipitation? Is the chemical composition of eroded sediments better correlated to
catchment characteristics (e.g., soil properties and slope geometry) or climate (e.g., precipitation form, water yield timing)?
We hypothesized that variation in sediment yield is directly related to stream discharge (as a proxy for precipitation), based on results from previous years, and that the precipitation form would impact sediment yield due to the higher energy of rain events compared to snow, and the greater potential for rain-on-snow events at lower elevations in the Sierras. We also hypothesized that sediment chemical composition (in contrast to total yield) is better correlated with watershed characteristics than with precipitation amount or water yield timing.
This study was conducted within the US Forest Service Kings River
Experimental Watersheds (KREW), located in the Sierra National Forest
(37.012
Map of the Kings River Experimental Watershed and Southern Sierra Critical Zone Observatory showing soil sampling points (green circles, at depositional, backslope, and crest hillslope positions from left to right along transects) and sediment sampling basins (black triangles).
Elevation range and size of the catchments (left) and annual precipitation from four meteorological stations (right) during the years of study. Roughly half of the precipitation at the lower-elevation Providence catchments falls as rain, while the Bull catchments (high elevation) receive > 75 % of precipitation as snow.
Seven of the catchments have experienced common forest management practices such as timber harvest, tree planting, grazing, and road construction and maintenance. However, no activities other than occasional road grading and grazing have occurred in the past 15 years since KREW was established. One catchment (T003) is undisturbed and has never had timber harvest or road construction. No fire has been recorded in these catchments for 110 years.
Both the lower- and higher-elevation sites are characterized as Sierra
mixed-conifer forests, with a more open canopy at Bull than Providence
(Fig. 3). Dominant tree species at Providence Creek site include sugar
pine (
Forests at Providence (left) and Bull (right) catchments. At both sites, vegetation cover is variable, with occasional clearings, meadows, and exposed bedrock.
Soil in the study area is derived from granite and granodiorite bedrock. Dominant soil series include Shaver, Cagwin, and Gerle–Cagwin. The Shaver series is most prominent (48–66 % coverage) in the Providence catchments, while the higher-elevation Bull catchments are dominated by the Cagwin series (67–98 % coverage; Johnson et al., 2011). The Shaver series is in the US Department of Agriculture Soil Taxonomic family of coarse-loamy, mixed mesic Pachic Xerumbrepts. The Cagwin series is in the loamy coarse sand, mixed, frigid Dystric Xeropsamments family. The Gerle series is in the coarse-loamy, mixed, frigid Typic Xerumbrepts family. Johnson et al. (2011) give detailed information on chemical and physical variation of soil in the study catchments. The dominant aspect of these catchments is southwest (Bales et al., 2011).
Stream discharge was quantified using a pair of flumes on each stream (Hunsaker et al., 2007). Annual stream discharge presented here was integrated from average daily flow rates based on continuous 15 min interval sampling. We characterized newly collected sediment samples from the catchments for water years 2009–2011 (Table 1) and sediment samples from water years 2005, 2007, and 2008 (Eagan et al., 2007; Hunsaker and Neary, 2012) that were collected and archived by the US Forest Service Pacific Southwest Research Station in Fresno, CA (stored air-dried, at room temperature in the dark). There were no archived sediments preserved from water year 2006.
Annual sediment yield per hectare for water years 2005–2011,
including mineral material, and coarse and fine organic matter (coarse,
> 2 mm, organics are comprised of material pinecones and conifer
needles, and accounts for
Sediment from each catchment was captured in basins that allow sediment
particles to settle as stream water slows passing through the basin
(Eagan et al., 2007). Constructed to fit the topography,
basin dimensions vary in size but are about 2–3 m wide by 8–15 m long.
Annual sediment loads were quantified at the end of the water year (WY;
1 October of the previous year through 30 September) in August and
September, when water flows were lowest. Streams were diverted underneath
the basin lining for collection. Material in the sediment basins was emptied
using buckets and shovels and weighed in the field using a hanging spring
scale (capacity of 50
Sediment samples were compared to soil samples considered as potential
sources, collected from 18 sampling points along representative transects
for each elevation group of catchments (see Fig. 1). Sites were selected
to be as comparable as possible; however, transect P2 had a non-representative,
highly saturated meadow as the depositional location. Transect P2 was not
evaluated in further analyses because other depositional locations were in
the forest. Each transect was laid out along a hillslope toposequence and
sampled at crest, backslope, and foot-/toe-slope (hereafter characterized as
“depositional”) landform positions. Crest samples were taken at the top of
the ridgeline, where the slope was < 5
Soil and sediment (air-dried, < 2 mm sieved samples) pH was measured
in 1 : 2 (
Total C and N were measured at the < 2 mm fraction following
grinding (8000M Spex Mill, SPEX Sample Prep, Metuchen, NJ, USA) with a
Costech ECS 4010 CHNSO Analyzer (Valencia, CA, USA). All values have been
moisture-corrected and reported here on an oven-dried (105
Data are presented as mean
Area-normalized sediment yield (hereafter referred to as sediment yield) in
the eight catchments varied over several orders of magnitude. There were
large differences among years and catchments (Fig. 4, Table 1). Mean annual sediment yield across all
catchments and years was 26.0
Top: annual sediment yield is directly correlated with annual water yield. Middle: annual sediment carbon (C) and nitrogen (N; not shown) concentrations have an inverse relationship to water yield. Bottom: the C-to-N mass ratio is weakly correlated with water yield. Data presented for WY 2005, and 2007–2011 (sediment basins constructed over the period 2002–2004, samples were not preserved for testing from WY 2006).
The P304 catchment had very high export rates relative to the other
catchments; excluding this catchment improved the
In contrast to the sediment yield, C (Fig. 4) and N (not shown)
concentrations in the sediment were both negatively correlated with annual
water yield (
Sediment yield among both catchments and years was more variable (higher
coefficients of variation) than the sediment C and N concentrations
(Table 4). While sediment composition was less variable than sediment yield
overall, C and N concentrations still showed statistically significant
interannual and interbasin variation (Fig. 5). Catchment size, catchment
elevation group, and mean elevations were eliminated as significantly
contributing variables in a stepwise regression model run simultaneously in
both directions. In the sediment samples, C concentrations ranged from 15.5
to 190 g kg
Physical and chemical characterization of the sediment material
(< 2 mm), including pH
Carbon (C) and nitrogen (N) concentrations and C : N mass ratios of < 2 mm material collected in sediment
basins within the Providence (low-elevation) and Bull (high elevation)
catchments between water years 2005 and 2011. Left panels (
Mineral soils had similar C and N concentrations and C : N ratios at both
sampling sites (Table 3). The low-elevation
Providence catchment had a wider range in C concentrations (9.0 to 98 g kg
Enrichment ratios for carbon (ER
Mineral soil physical and chemical characterizations (air-dried
< 2 mm) for a subset of the soil transects (the two sent out for
physical analysis), including pH
Coefficients of variation (standard deviation relative to the mean, expressed in %) for sediment yield, carbon (C) and nitrogen (N) concentrations, and C : N mass ratios averaged across years for each catchment, and averaged across catchments for each water year within the Kings River Experimental Watershed. Archive samples from 2006 were not available for sampling (indicated by no data or nd).
Sediments exported from all of the study catchments had higher sand
concentrations and lower clay concentrations compared to surface mineral
soils in the source hillslope (
Soil pH declined with elevation, with higher pH values in the low-elevation
Providence catchments than the Bull catchments (
Enrichment ratios of C and N (ER, the ratio of C or N concentration in the eroded sediment divided by their concentration in source soil in hillslopes) were highest during years with low precipitation and lowest during high precipitation years (Fig. 6) for both the upper- and lower-elevation watersheds. During years of low precipitation, we observed selective transport of fine material that is high in OM concentration, characteristic of the organic and A horizons. Furthermore, calculated ERs for the crest, backslope or the depositional positions differed substantially in the high-elevation Bull catchments, but not in lower-elevation Providence catchments. The depositional positions in these catchments were highly varied and had points with very high C and N concentrations. For high water years 2010 and 211, Bull ER values were more similar between slope positions than in low WY 2007 and 2008. In the low-elevation Providence catchments, ERs were similar across hillslope positions for both C and N.
Our analyses of sediment transport rates and their composition from the KREW
catchments showed a positive relationship between water yield and erosion
exports for these catchments that have had experienced minimal disturbance
for the past 15 years. In agreement with our hypothesis that sediment yield
is closely related to interannual differences in precipitation, we found
that total area-normalized annual sediment yield was strongly and positively
correlated to annual stream discharge (a proxy for precipitation amount)
more than watershed size, slope or soil characteristics. The range and
magnitude of exported sediment was comparable to total sediment transport
rates in water years 2001–2009 from a subset of these catchments (installed
2002–2004, with the first full set of archived sediments from 2005;
Eagan et al., 2007; Hunsaker and Neary, 2012). The
range of sediment yield was as much as an order of magnitude greater than
the difference in water yield for any given year, supporting a nonlinear
response for this ecosystem (Fig. 4). Though small, the sediment yield in
low-flow years is not negligible. We observed sediment yield rates on par
with a range of other ecosystems. Annual sediment export rates observed in
our catchments are more variable than but comparable to average reported
rates for “stable forest” ecosystems (4–50 kg ha
We hypothesized that the higher-elevation Bull watersheds would have lower erosion rates than the low-elevation Providence watersheds because of the greater proportion of the precipitation falling as snow at higher elevations, and the greater potential for rain-on-snow events at lower elevations in the Sierras (Bales et al., 2006; Hunsaker et al., 2012). However, we found no significant difference between elevation groups, suggesting that these differences in elevation are not significant drivers of sediment yield for the years we observed. These results suggest that higher elevations, where the rain–snow transition zone is predicted to occur as the climate warms (Klos et al., 2014) in the Sierra, will likely not lead to increased short-term sediment erosion rates from these catchments. However, any associated changes in the intensity or amount of precipitation that will alter water yield will likely lead to changes in erosion rates (cf. Fig. 4).
We hypothesized that sediment chemical composition is correlated more with
catchment characteristics such as soil composition and slope geometry, which
could influence detachment and transport mechanisms, than with precipitation
or water yield. However, we found sediment chemical composition was not well
correlated with the source catchment, or catchment elevation or size. The
one catchment (B201) with an exceptionally low sediment C : N ratio could be
attributed to the meadow bordering the stream. Sediment chemical composition
was far more consistent than sediment yield across catchments as well as
years. Sediment chemical composition was most closely correlated with annual
water yield. Hence, we reject our hypothesis that sediment chemical
composition is dependent on catchment differences more than water yield.
Sediment C and N concentrations, and the C : N ratio were weakly correlated
with water yield, but the correlations had low predictive values, suggesting
other factors may be more important. With relatively consistent C and N
concentrations, these results suggest that the total amount of OM exported
from the Sierra Nevada depends largely on total sediment yield. The average
annual sediment yield resulted in the export of 0.2–4.4 kg C ha
The soils in the two elevation watershed groups (i.e., Providence and Bull watersheds) were consistent, and perhaps too consistent to expect differences in sediment chemical composition between the elevation groups based on lithology or soil composition. Few soil characteristics show an elevational pattern (Johnson et al., 2011); however, there were differences between the hillslope locations, particularly the depositional locations compared to the other locations. Given the differences among hillslope locations, contributions from upland sediment sources may lead to more variation in sediment composition than elevational differences in these and similar regions of the western Sierra Nevada.
Hillslope gradient, especially in areas adjacent to streams, plays a role in
sediment yield (Litschert and MacDonald, 2009). The three catchments with
the highest sediment yields (T003, P304 and D102) had steep (frequently
greater than 25
Slopes in the eight catchments are moderately steep as shown by a
weighted scale (< 1
Two catchments, T003 and P304, had exceptionally high sediment yield. High sediment yield from the T003 catchment was especially surprising because this catchment has never been impacted by logging or roads (Hunsaker and Neary, 2012). Compared to companion catchments, T003 and P304 have long, narrow geometries and eroded soil travels shorter distance to travel to streams (Hunsaker and Neary, 2012). Several other factors, including low rock fraction in topsoil, and low proportion of exposed granite, and ongoing down-cutting of channels in P304 have previously been suggested to explain the P304 sediment response (for more in-depth discussion on these factors see Hunsaker and Neary, 2012; Eagan et al., 2007; Martin 2009).
Multiple reasons may explain the inverse relationship between C and N concentrations and sediment yield, including preferential transport, differences in the source of the material, or sampling basin capture efficiency. Water-based surface erosion processes (for example sheet erosion) preferentially mobilize fine particles with their associated OM over mineral soils from deeper in the soil profile, resulting in C and/or N enrichment in eroded sediments (Nadeu et al., 2012). We found enrichment of OM in sediment compared to soils in years with low precipitation for both elevation groups (cf. Fig. 6), which could support preferential transport of surficial organic material to streams during these periods.
Another possible reason for the inverse relationship between C and N
concentrations and sediment yield is that erosive processes detach and
transport OM-poor material from different sources or deeper in the soil
profile than in low precipitation years. Erosion processes that impact
deeper layers (including gullies, mass wasting or bank erosion) mobilize
material with lower OM concentrations as well as water-stable aggregates
(Nadeu et al., 2012). However, geomorphic features which increase
connectivity in the catchments (e.g., gullies or convex hillslopes) are
present but not common in our study catchments (Stafford, 2011).
Stafford (2011) reported that water-driven surface erosion from or near
roads (OM-poor sources) in these catchments to be orders of magnitude higher
than erosion on vegetated hillslopes. In two of the five years, hillslope
sediment fences captured no measureable sediment; however in other years
(2005, 2006 and 2008), mean hillslope sediment erosion rates ranged from
6 to 32.9 kg ha
Changes in the trapping efficiency of the sediment basins with changes in water yield is another possibility for the inverse relationship between C and N concentrations and sediment yield. For instance, lower efficiency of capture of low density, high C and N concentration material (e.g., free organics) during high discharges would have lead to low C and N concentrations in captured sediment in these high water yield years. In a review of several studies, Verstraeten and Poesen (2000) found trapping efficiency rates of sediment mass in individual events can be as low as 50 %, especially in high discharge events. The trapping efficiency of the sediment basins was not measured in this project due to labor and budget constraints. However, considering the nature of soils and SOM in our study catchments, and the discharge events recorded, we can assume that most of the C laterally distributed from the hillslopes is likely trapped in the basins. It is likely that some C existing as free organic particles and C associated with very small mineral particles (that remain in suspension the longest) could be transported further and at least partially contribute to the inverse relationship discussed above. However, the loss of C as dissolved and suspended particulate OM is likely to be, at least partially, compensated by input of C from vegetation growing above the sediment basins.
The process of soil OM erosion in upland forest ecosystems, and its contribution to the erosion-induced C sink is fundamentally different than those in cultivated and grassland ecosystems. These montane Sierra Nevada catchments have higher surficial concentrations of C and N (Dahlgren et al., 1997; Johnson et al., 1997) and steeper slopes (cf. Fig. 7) than agroecosystems (Quine and Van Oost, 2007; Van Oost et al., 2007; Berhe et al., 2007), which could contribute to export of OM-rich material without allowing for significant decomposition during transport. If deposited within the source or adjacent catchments, the OM can be protected through various mechanisms with burial (Berhe and Kleber, 2013) or through chemical associations that OM forms with soil minerals during or after transport, leading to stabilization of the eroded OM (VandenBygaart et al., 2012, 2015). In the KREW catchments, there is potential for C loss during transport as well as stabilization through various mechanisms compared to other non-montane ecosystems (Stacy, 2012). These forest ecosystems had low erosion rates, with only a small fraction of the total C pool subject to erosion. Furthermore, the OM-rich nature of eroded sediment raises important questions about the fate of the eroded OM during and after erosional transport. If a large fraction of the SOM eroded from forest ecosystems is lost during transport or after deposition, the eroded organic matter would not be preserved. At least under contemporary rates of erosion, we did not find evidence that erosion in these forest ecosystems can constitute a significant C sink. Changing climate could potentially alter this balance through changes to water yield, through vegetative shifts to shrubland or grassland, or through the increased risk of fire. The ultimate fate of this eroded C and N and its contribution towards erosion-induced C sequestration will depend on how far the material is transported and rates of OM decomposition after deposition (Berhe and Kleber, 2013; Berhe et al., 2012b).
Overall, our findings show that there was no consistent, statistically
significant difference in erosion rates of sediment, C or N from rain-
versus snow-dominated headwater catchments in the southern Sierra Nevada.
Water yield does not strongly moderate sediment C and N concentrations, but
it is a major driver of total C and N export from these catchments because
of the correlation with sediment yield. Enrichment in OM supports the
contribution of surficial sources and the dominance of sheet erosion over
other erosional processes. Differences in enrichment ratios of C and N in
captured sediments may be driven by higher rates of eroded sediment during
wetter years or preferential loss from the sediment basins during high
stream discharge. Including precipitation-event-based sampling and
quantification of trap efficiency in each catchment would help improve
quantification of sediment and associated OM export rates for such upland
forest catchments. Based on our results, we conclude that changes in the
amount of precipitation but not the timing or precipitation form will have
important implications for both the nature and amount of OM that is eroded
from forested ecosystems, and for whether erosion in forested catchments can
induce a significant sink for atmospheric CO
This research was conducted in the Kings River Experimental Watersheds (KREW), which was established and is managed for US Forest Service Pacific Research Station. The KREW study was implemented using funds from the National Fire Plan of the USDA Forest Service. Additional funding for this work was provided from the US Forest Service, the Southern Sierra Critical Zone Observatory Project from the National Science Foundation (EAR-0725097). NSF award to A. A. Berhe, S. C. Hart and D. W. Johnson (EAR-1147977), and a Graduate Research Council grant from the University of California, Merced. We thank Matthew McClintock, members of the Berhe and Hart labs at UC Merced, researchers within the Southern Sierra Critical Zone Observatory (SSCZO), and the US Forest Service crews from the Pacific Southwest Research Station for assistance in the field and in lab portions of this work. Edited by: D. Obrist