Contrasting nutrient exports from a forested and an agricultural catchment in south-eastern Australia

Dissolved organic carbon (DOC) and total and inorganic nitrogen and phosphorus concentrations were determined over 3 years in headwater streams draining two adjacent catchments. The catchments are currently under different land use; pasture/grazing vs plantation forestry. The objectives of the work were to quantify C and nutrient export from these landuses and elucidate the factors regulating export. In both catchments, stream water dissolved inorganic nutrient concentrations exhibited strong seasonal variations. Concentrations were highest during runoff events in late summer and autumn and rapidly declined as discharge increased during winter and spring. The annual variation of stream water N and P concentrations indicated that these nutrients accumulated in the catchments during dry summer periods and were flushed to the streams during autumn storm events. By contrast, stream water DOC concentrations did not exhibit seasonal variation. Higher DOC and NO₃ ⁻ concentrations were observed in the stream of the forest catchment, reflecting greater input and subsequent breakdown of leaf-litter in the forest catchment. Annual export of DOC was lower from the forested catchment due to the reduced discharge from this catchment. In contrast however, annual export of nitrate was higher from the forest catchment suggesting that there was an additional NO₃ ⁻ source or reduction of a NO₃ ⁻ sink. We hypothesize that the denitrification capacity of the forested catchment has been significantly reduced as a consequence of increased evapotranspiration and subsequent decrease in streamflow and associated reduction in the near stream saturated area.     

Biogeochemical Budgets in a Mediterranean Catchment with High Rates of Atmospheric N Deposition – Importance of Scale and Temporal Asynchrony

In this study biogeochemical export in a set of catchments that vary from 6 ha to almost 1500 ha is investigated. Studying catchments across this large range of scales enables us to investigate the scale dependence and fundamental processes controlling catchment biogeochemical export that would not have been possible with a more limited data set. The Devil Canyon catchment, in the San Bernardino Mountains, California, has some of the highest atmospheric N deposition rates in the world (40–90 kg ha⁻¹year⁻¹ at the crest of the catchment). These high rates of deposition have translated into consistently high levels of NO⁻in 3 some streams of the San Bernardino Mountains. However, the streams of the Devil Canyon catchment have widely varying dissolved inorganic nitrogen (DIN) concentrations and export. These differences are also, to a more limited extent, present for dissolved organic carbon (DOC) but not in other dissolved species (Cl⁻, SO²⁻₄,Ca²⁺ and other weathering products). As catchment size increases DIN and DOC concentrations first increase until catchment area is ∼150 ha but then decrease as catchment scale increases beyond that size. The scale dependence of DIN export implies that catchments at different spatial scales are at different degrees of N saturation. The reason for this scale effect appears to be the dominance of flushing of DIN out of soil at small scales due to the temporal asynchrony between nutrient availability and biological N demand, the groundwater exfiltration of this flushed DIN at intermediate scales and the removal of this DIN from streamflow through in-stream processes and groundwater–surface water interaction at larger scales. While the particular scale effect observed here may not occur over the same range in catchment area in other ecosystems, it is likely that other ecosystems have similar scale dependant export for DIN and DOC.     

Nitrogen exports at multiple-scales in a southern Chilean watershed (Patagonian Lakes district)

We evaluated nitrogen (N) export for various catchments in the San Pedro River watershed of South-central Chile (39°20′ to 40°12′S) during the dry season (February to March). We measured concentrations and export of the various N species at 16 points from the Andean headwaters to the lowland portion of the watershed: eight main nested points along the main watershed and eight secondary points on tributaries. We expected that, given a downstream increase in pastureland and decrease in native pristine forest cover, inorganic forms of N (DIN) would increase downstream, while conversely, dissolved organic nitrogen (DON) would decrease compared with concentrations in the forested headwaters. Nitrogen concentrations did not show statistically significant differences among the nested catchments. However, there were statistically significant differences in N concentrations associated with land cover among the tributaries. The results suggest that in the presence of base flow, natural landscape properties (barren land, lakes and rivers), explained most of the spatial variation in the N exports, while anthropogenic disturbance was not detectable. There was a negative relationship between DIN export and the coverage of lakes and rivers, suggesting that lakes might be acting as N traps. On the other hand, DIN, DON and total N exports were positively associated to barren land. Total nitrogen export during this 60-day dry season was less than 20 kg km⁻² and the annual export was not larger than 100 kg km⁻². This study documents the as yet pristine conditions of rivers in southern Chile.     

Dissolved organic nitrogen budgets for upland, forested ecosystems in New England

Relatively high deposition ofnitrogen (N) in the northeastern United States hascaused concern because sites could become N saturated.In the past, mass-balance studies have been used tomonitor the N status of sites and to investigate theimpact of increased N deposition. Typically, theseefforts have focused on dissolved inorganic forms ofN (DIN = NH₄-N + NO₃-N) and have largelyignored dissolved organic nitrogen (DON) due todifficulties in its analysis. Recent advances in themeasurement of total dissolved nitrogen (TDN) havefacilitated measurement of DON as the residual of TDN– DIN. We calculated DON and DIN budgets using data onprecipitation and streamwater chemistry collected from9 forested watersheds at 4 sites in New England. TDNin precipitation was composed primarily of DIN. Netretention of TDN ranged from 62 to 89% (4.7 to 10 kgha^{minus 1} yr^{minus 1}) of annual inputs. DON made up themajority of TDN in stream exports, suggesting thatinclusion of DON is critical to assessing N dynamicseven in areas with large anthropogenic inputs of DIN.Despite the dominance of DON in streamwater,precipitation inputs of DON were approximately equalto outputs. DON concentrations in streamwater did notappear significantly influenced by seasonal biologicalcontrols, but did increase with discharge on somewatersheds. Streamwater NO₃-N was the onlyfraction of N that exhibited a seasonal pattern, withconcentrations increasing during the winter months andpeaking during snowmelt runoff. Concentrations ofNO₃-N varied considerably among watersheds andare related to DOC:DON ratios in streamwater. AnnualDIN exports were negatively correlated withstreamwater DOC:DON ratios, indicating that theseratios might be a useful index of N status of uplandforests.     

The age of terrestrial carbon export and rainfall intensity in a temperate river headwater system

Riverine dissolved organic carbon (DOC) supports the production of estuaries and coastal ecosystems, constituting one of the most actively recycled pools of the global carbon cycle. A substantial proportion of DOC entering oceans is highly aged, but its origins remain unclear. Significant fluxes of old DOC have never been observed in temperate headwaters where terrestrial imports take place. Here, we studied the radiocarbon age of DOC in three streams draining forested headwater catchments of the river Mulde (Ore Mountains, Germany). In a 4 week summer precipitation event DOC aged at between 160 and 270 years was delivered into the watershed. In one stream, the DOC was modern but depleted in radiocarbon compared to other hydrological conditions. The yield was substantial and corresponded to 20–52 % of the annual DOC yields in wet and dry years, respectively. The analysis of long-term data suggested that the DOC export in extreme precipitation events added to the annual yield and was not compensated for by lower exports in remaining periods. We conclude that climate change, along with additional processes associated with human activities, channels old soil carbon into more rapidly cycled carbon pools of the hydrosphere.     

Deforestation of watersheds of Panama: nutrient retention and export to streams

A series of eight watersheds on the Pacific coast of Panama where conversion of mature lowland wet forest to pastures by artisanal burning provided watershed-scale experimental units with a wide range of forest cover (23, 29, 47, 56, 66, 73, 73, 91, and 92 %). We used these watersheds as a landscape-scale experiment to assess effects of degree of deforestation on within-watershed retention and hydrological export of atmospheric inputs of nutrients. Retention was estimated by comparing rainfall nutrient concentrations (volume-weighted to allow for evapotranspiration) to concentrations in freshwater reaches of receiving streams. Retention of rain-derived nutrients in these Panama watersheds averaged 77, 85, 80, and 62 % for nitrate, ammonium, dissolved organic N, and phosphate, respectively. Retention of rain-derived inorganic nitrogen, however, depended on watershed cover: retention of nitrate and ammonium in pasture-dominated watersheds was 95 and 98 %, while fully forested watersheds retained 65 and 80 % of atmospheric nitrate and ammonium inputs. Watershed forest cover did not affect retention of dissolved organic nitrogen and phosphate. Exports from more forested watersheds yielded DIN/P near 16, while pasture-dominated watersheds exported N/P near 2. The differences in magnitude of exports and ratios suggest that deforestation in these Panamanian forests results in exports that affect growth of plants and algae in the receiving stream and estuarine ecosystems. Watershed retention of dissolved inorganic nitrogen calculated from wet plus dry atmospheric deposition varied from 90 % in pasture- to 65 % in forest-dominated watersheds, respectively. Discharges of DIN to receiving waters from the watersheds therefore rose from 10 % of atmospheric inputs for pasture-dominated watersheds, to about 35 % of atmospheric inputs for fully forested watersheds. These results from watersheds with no agriculture or urbanization, but different conversion of forest to pasture by burning, show significant, deforestation-dependent retention within tropical watersheds, but also ecologically significant, and deforestation-dependent, exports that are biologically significant because of the paucity of nutrients in receiving tropical stream and coastal waters.     

N Retention in Urbanizing Headwater Catchments

Urbanization can potentially alter watershed nitrogen (N) retention via combined changes in N loading, water runoff, and N processing potential. We examined N export and retention for two headwater catchments (∼4 km²) of contrasting land use (16% vs. 79% urban) in the Plum Island Ecosystem (PIE-LTER) watershed, MA. The study period included a dry year (2001–2002 water year) and a wet year (2002–2003 water year). We generalized results by comparing dissolved inorganic nitrogen (DIN) concentrations from 16 additional headwater catchments (0.6–4.2 km²) across a range of urbanization (6–90%). Water runoff was 25–40% higher in the urban compared to the forested catchment, corresponding with an increased proportion of impervious surfaces (25% vs. 8%). Estimated N loading was 45% higher and N flux 6.5 times higher in the urban than in the forested catchment. N retention (1 − measured stream export / estimated loading) was 65–85% in the urban site and 93–97% in the forested site, with lower retention rates during the wetter year. The mechanisms by which N retention stays relatively high in urban systems are poorly known. We show that N retention is related to the amount of impervious surface in a catchment because of associated changes in N loading (maximized at moderate levels of imperviousness), runoff (which continues to increase with imperviousness), and biological processes that retain N. Continued declines in N retention due to urbanization have important negative implications for downstream aquatic systems including the coastal zone.     

Stream Nitrate Responds Rapidly to Decreasing Nitrate Deposition

Ecosystem acidification and eutrophication resulting from increased deposition of dissolved inorganic nitrogen (DIN) are issues of increasing global concern. Consequently, costly policy decisions are being implemented to decrease nitrogen oxide (NO _x ) emissions. Although declining DIN deposition along with rapid declines of DIN in surface waters have been reported in parts of Europe, the same observation is just emerging in North America. Here we find a significant decline in bulk deposition NO₃ ⁻ during the later part of a 28-year record in southcentral Ontario, Canada. Despite high N retention and substantial inter-annual variability in the long-term record due to periods of drought, we find significant declines in annual NO₃ ⁻ concentrations and export at six out of 11 streams that drain upland-dominated catchments. In contrast, five streams draining primarily wetland-dominated catchments with lower levels of NO₃ ⁻ show no decreasing trend in NO₃ ⁻ concentration or export. The rapid response in stream NO₃ ⁻ to declining atmospheric inputs was observed at sites with historically moderate inputs of DIN (~870 mg m⁻² y⁻¹) in bulk deposition. Topographic features such as slope, and related catchment features including wetland cover, appear to influence which catchments will respond positively to declining DIN deposition. These findings force us to revise our original conceptualization of the N saturation status of these catchments.     

Climate effects on sulphate flux from forested catchments in south-central Ontario

Net export of sulphate from watersheds may delay the response of surfacewaters to changes in acid deposition. Long-term (18-yr) sulphatebudgets were calculated for 8 headwater streams located in the acid-sensitiveregion of Muskoka-Haliburton, south central Ontario. Sulphate deposition inthisregion has decreased by almost 40% over the last 2 decades, and sulphate exportfrom catchments has also generally declined over time, but most catchments arestill a net source of sulphate to drainage streams. Net export of sulphateoccurred in the majority of catchments in most years of record, but wasparticularly large following dry, warmer than average summers, when stream flowceased for up to several weeks at a time. In years with warm dry summers, suchas occurred in 1983/84 and between 1987/88 and 1990/91, inclusive, streamexportfrom most catchments was between 1.5 and 2 times greater than was input viabulkdeposition. Annual average sulphate concentrations in streams were stronglycorrelated with stream dryness, and were greater in years in which streams weredry for longer periods of time. Temporal patterns of annual sulphateconcentrations and export were highly coherent among the 8 streams, and netsulphate export occurred in both wetland-draining and predominantly uplandstreams. Climate variables, specifically temperature and precipitation act on aregional scale and are likely responsible for similar temporal patterns ofsulphate retention among these 8 physiographically different catchments. Netsulphate export from catchments may delay the recovery of acid impacted surfacewaters, despite reductions in industrial SO₂ emissions.     

Nutrient stoichiometry of linked catchment‐lake systems along a gradient of land use

1. Catchments export nutrients to aquatic ecosystems at rates and ratios that are strongly influenced by land use practices, and within aquatic ecosystems nutrients can be processed, retained, lost to the atmosphere, or exported downstream. The stoichiometry of carbon and nutrients can influence ecosystem services such as water quality, nutrient limitation, biodiversity, eutrophication and the sequestration of nutrients and carbon in sediments. However, we know little about how nutrient stoichiometry varies along the pathway from terrestrial landscapes through aquatic systems. 2. We studied the stoichiometry of nitrogen and phosphorus exported by three catchments of contrasting land use (forest versus agriculture) and in the water column and sediments of downstream reservoirs. We also related stoichiometry to phytoplankton nutrient limitation and the abundance of heterocystous cyanobacteria. 3. The total N : P of stream exports varied greatly among catchments and was 18, 54 and 140 (molar) in the forested, mixed‐use and agricultural catchment, respectively. Total N : P in the mixed layers of the lakes was less variable but ordered similarly: 35, 52 132 in the forested, mixed‐use and agricultural lake, respectively. In contrast, there was little variation among systems in the C : N and C : P ratios of catchment exports or in reservoir seston. 4. Phytoplankton in the forested lake were consistently N limited, those in the agricultural lake were consistently P limited, and those in the mixed‐use lake shifted seasonally from P‐ to N limitation, reflecting N : P supply ratios. Total phytoplankton and cyanobacteria biomass were highest in the agricultural lake, but heterocystous (potentially N fixing) cyanobacteria were most abundant in the forested lake, corresponding to low N : P ratios. 5. Despite large differences in catchment export and water column N : P ratios, the N : P of sediment burial (integrated over several decades) was very low and remarkably similar (4.3–7.3) across reservoirs. N and P budgets constructed for the agricultural reservoir suggested that denitrification could be a major loss of N, and may help explain the relatively low N : P of buried sediment. 6. Our results show congruence between the catchment export N : P, reservoir N : P, phytoplankton N versus P limitation and the dominance of heterocystous cyanobacteria. However, the N : P stoichiometry of sediments retained in the lakes was relatively insensitive to catchment stoichiometry, suggesting that a common set of biogeochemical processes constrains sediment N : P across lakes of contrasting catchment land use.     

Long-Term Trends in Stream Nitrate Concentrations and Losses Across Watersheds Undergoing Recovery from Acidification in the Czech Republic

Stream nitrogen (N) export and nitrate concentration were measured at 14 forested watersheds (GEOMON network) in the Czech Republic between 1994 and 2005. In the last several decades, emissions of sulfur (S) and N compounds have decreased throughout much of Europe. In the Czech Republic, atmospheric deposition of S has decreased substantially since the beginning of 1990s, whereas N deposition remains largely unchanged at most sites. The mean dissolved inorganic nitrogen (DIN) streamwater export ranged from 0.2 to 12.2 kg ha⁻¹ y⁻¹ at the GEOMON sites. Despite decades of elevated N deposition, 44–98% of DIN inputs to these watersheds were retained or denitrified, and many watersheds showed seasonal variation in nitrate concentrations. Dissolved organic N export was quantified in 1 year only and ranged from 0.05 to 3.5 kg ha⁻¹ y⁻¹. Spatial variability in DIN export among watersheds was best explained by spatial variability in average acidic deposition, particularly S deposition (R ² = 0.81, P < 0.001); DIN input and forest floor carbon:nitrogen (C/N) also provided significant explanatory power. DIN export was strongly influenced by the forest floor C/N ratio and depth of the forest floor soils (R ² = 0.72, P < 0.001). The only variable that predicted variations in forest floor C/N (R ² = 0.32, P < 0.05) among watersheds was S deposition. Forest floor depth was also related to deposition variables, with S deposition providing the most explanatory power (R ² = 0.50, P < 0.01). Variation in forest floor depth was also associated with climatic factors (precipitation and temperature). Temporal variability in DIN export was primarily associated with changes in acidic deposition over time; S deposition explained 41% of variability in DIN exports among all watersheds and years. Extensive acidification of forested watersheds was associated with the extraordinarily high S inputs to much of the Czech Republic during earlier decades. We hypothesize that recovery from acidification has led to improved tree health as well as enhanced microbial activity in the forest floor. As these watersheds move into a new regime with dramatically lower sulfur inputs, we expect continued declines in nitrate output.     

Hydrological and biological processes modulate carbon,nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Quebec,Canada)

Changes in atmospheric deposition, stream water chemistry, and solute fluxes were assessed across 15 small forested catchments. Dramatic changes in atmospheric deposition have occurred over the last three decades, including a 70% reduction in sulphur (S) deposition. These changes in atmospheric inputs have been associated with expected changes in levels of acidity, sulphate and base cations in streams. Soil retention of S appeared to partially explain rates of chemical recovery. In addition to these changes in acid–base chemistry we also observed unexpected changes in nitrogen (N) biogeochemistry and nutrient stoichiometry of stream water, including decreased stream N concentrations. Among all catchments the average flux of dissolved inorganic nitrogen (DIN) was best predicted by average runoff, soil chemistry (forest floor C/N) and levels of acid deposition (both S and N). The rate of change in stream DIN flux, however, was much more closely correlated with reductions in rates of S deposition rather than those of DIN. Unlike DIN fluxes, the average concentrations as well as the rates of decline in streamwater nitrate (NO₃) concentration over time were tightly linked to stream dissolved organic carbon/dissolved organic nitrogen ratios DOC/DON and DON/TP rather than catchment characteristics. Declines in phosphorus adsorption with increasing soil pH appear to contribute to the relationship between C, N, and P in our study catchments. Our observations suggest that catchment P availability and its alteration due to environmental changes (e.g. acidification) might have profound effects on N cycling and catchment N retention that have been largely unrecognized.     

Evidence for a Regime Shift in Nitrogen Export from a Forested Watershed

In this study, we document a functional regime shift in stream inorganic nitrogen (N) processing indicated by a major change in N export from a forested watershed. Evidence from 36 years of data following experimental clearcut logging at Coweeta Hydrologic Laboratory, NC, suggests that forest disturbance in this area can cause elevation of dissolved inorganic N (DIN) loss lasting decades or perhaps longer. This elevation of N export was apparently caused by an initial pulse of organic matter input, reduced vegetation uptake, increased mineralization of soil organic N, and N fixation by black locust-associated bacteria following clearcut logging. In forested reference watersheds at Coweeta, maximum DIN concentration occurs in summer when base flow is low, but the clearcut watershed shifted to a pattern of maximum winter DIN concentration. The seasonal pattern of DIN concentration and export from reference watersheds can be explained by terrestrial and in-stream processes, but following clearcutting, elevated DIN availability saturated both terrestrial and in-stream uptake, and the N export regime became dominated by hydrologic transport. We suggest that the long-term elevation of stream DIN concentration and export along with the changes in seasonality of DIN export and the relationship between concentration and discharge represent a functional regime shift initiated by forest disturbance.     

Nutrient export to an Eastern Atlantic coastal zone: first modeling and nitrogen mass balance

We have studied 15 catchments supplying freshwater to a French Atlantic coastal lagoon, where increase in nitrogen loads due to agriculture is supposed to have destabilized the ecosystem in the last decades. The catchment is a lowland composed of Pleistocene sands with an average slope of 0.25%. To study the nutrient export in relation to land-use surface waters were sampled bi-weekly between October 2006 and January 2009 and land-use was established by plane photographs and Geographic Information System (GIS). Cultivated pine forests represent more than 80% of the total surface and 7% of the catchment area has been deforested recently. Significant areas of some catchments are used for maize crop. Housing is confined to the coastal zone. Maize and forest crop give a robust signature in terms of nitrate export. In view of modeling the nutrient fluxes, we have established the mean export rate for every land-use: forested parcels, deforested parcels, cultivated surfaces, and housing areas export 45, 93, 2850, and 61 kg N-nitrate km^?2 year^?1, respectively. Exports of ammonium, dissolved organic N (DON), and dissolved inorganic P (DIP) could not be related to land use. The mean export is 13, 100, and 0.57 kg km^?2 year^?1 for N-ammonium, DON, and DIP, respectively. The modeling of nitrogen flux is in good agreement with our measures for the largest catchment, which supplies about 90% of the total continental DIN flux. However, small catchments are more dynamic due to hydrological conditions and the model is less accurate. This work has permitted to complete and unify scattered studies about nutrient cycling in this area. Thus we have established and compared the nitrogen budget of cornfields and cultivated pine forest. We have emphasized that (i) fertilizer use should be reduced in cornfields because they stock between 200 and 6400 kg DIN km^?2 year^?1, and (ii) the nitrogen budget in pine forest mostly depends on tree harvesting and symbiotic N-fixation, which is poorly constrained. Export of N by rivers represents a small contribution to the N budget of soils.     

Large Carbon Dioxide Fluxes from Headwater Boreal and Sub-Boreal Streams

Half of the world''s forest is in boreal and sub-boreal ecozones, containing large carbon stores and fluxes. Carbon lost from headwater streams in these forests is underestimated. We apply a simple stable carbon isotope idea for quantifying the CO₂ loss from these small streams; it is based only on in-stream samples and integrates over a significant distance upstream. We demonstrate that conventional methods of determining CO₂ loss from streams necessarily underestimate the CO₂ loss with results from two catchments. Dissolved carbon export from headwater catchments is similar to CO₂ loss from stream surfaces. Most of the CO₂ originating in high CO₂ groundwaters has been lost before typical in-stream sampling occurs. In the Harp Lake catchment in Canada, headwater streams account for 10% of catchment net CO₂ uptake. In the Krycklan catchment in Sweden, this more than doubles the CO₂ loss from the catchment. Thus, even when corrected for aquatic CO₂ loss measured by conventional methods, boreal and sub-boreal forest carbon budgets currently overestimate carbon sequestration on the landscape.     

Biogeochemistry of unpolluted forested watersheds in the Oregon Cascades: temporal patterns of precipitation and stream nitrogen fluxes

We analyzed long-term organic and inorganic nitrogen inputs and outputs in precipitation and streamwater in six watersheds at the H.J. Andrews Experimental Forest in the central Cascade Mountains of Oregon. Total bulk N deposition, averaging 1.6 to 2.0 kg N ha^–1 yr^–1, is low compared to other sites in the United States and little influenced by anthropogenic N sources. Streamwater N export is also low, averaging <1 kg ha^–1 yr^–1. DON is the predominant form of N exported from all watersheds, followed by PON, NH₄-N, and NO₃-N. Total annual stream discharge was a positive predictor of annual DON output in all six watersheds, suggesting that DON export is related to regional precipitation. In contrast, annual discharge was a positive predictor of annual NO₃-N output in one watershed, annual NH₄-N output in three watersheds, and annual PON output in three watersheds. Of the four forms of N, only DON had consistent seasonal concentration patterns in all watersheds. Peak streamwater DON concentrations occurred in November-December after the onset of fall rains but before the peak in the hydrograph, probably due to flushing of products of decomposition that had built up during the dry summer. Multiple biotic controls on the more labile nitrate and ammonium concentrations in streams may obscure temporal DIN flux patterns from the terrestrial environment. Results from this study underscore the value of using several watersheds from a single climatic zone to make inferences about controls on stream N chemistry; analysis of a single watershed may preclude identification of geographically extensive mechanisms controlling N dynamics.     

Partial river flow recovery with forest age is rare in the decades following establishment

Forest regeneration and expansion are occurring in many countries, with 80 million ha established from 2000 to 2012 under the Bonn accord and 17.5 million ha established from 1990 to 2005 according to the Food and Agriculture Organisation. Multiple reviews have linked increasing forest cover with reduced river flow and potentially detrimental effects downstream. Previous reviews have investigated trends in river flow response over time, but the influence of forest age remains uncertain. Partial river flow recovery (towards non‐forested conditions) has been reported in decades following forest establishment, but the role of climate in driving these trends has not been explored. Here, we evaluate river flow trends in 43 studies following forest establishment, which provide sufficient information to distinguish the effects of ageing forests from variable climate. Our meta‐analysis supports previous findings showing that forestation reduces annual river flow (by 23% after 5 years and 38% after 25 years) with greater reductions in catchments with higher mean annual precipitation, larger increases in forest cover, and which were idle, rather than agricultural land, prior to forestation. The impact of forests on river flow is sensitive to annual precipitation and potential evapotranspiration, but responses are highly variable. Forests affect river flow less when annual precipitation is low, and sensitivity to precipitation decreases as catchment aridity increases. The majority of catchments demonstrated persistent river flow declines after forest establishment. However, nine catchments showed partial flow recovery after an initial decrease, with peak flow reductions at an average age of 15 and across a range of tree species. The mean rate of recovery was 34 mm/year over 5 years. Partial flow recovery with forest age cannot be commonly expected, however, and forestation programmes should take into account that changes to annual river flow are likely to persist for up to five decades.     

Freeze–thaw processes and intense rainfall: the one-two punch for high sediment and nutrient loads from mid-Atlantic watersheds

Large runoff, sediment, and nutrient exports from watersheds could occur due to individual extreme climate events or a combination of multiple hydrologic and meteorological conditions. Using high-frequency hydrologic, sediment, and turbidity data we show that freeze–thaw episodes followed by intense winter (February) rainstorms can export very high concentrations and loads of suspended sediment and particulate organic carbon (POC) and nitrogen (PN) from mid-Atlantic watersheds in the US. Peak suspended sediment (> 5000 mg L^?1), POC (> 250 mg L^?1) and PN (> 15 mg L^?1) concentrations at our 12 and 79 ha forested watersheds for the February rainfall-runoff events were highest on record and the fluxes were comparable to those measured for tropical storms. Similar responses were observed for turbidity values (> 400 FNU) at larger USGS-monitored watersheds. Much of the sediments and particulate nutrients likely originated from erosion of stream bank sediments and/or channel storage. Currently, there is considerable uncertainty about the contribution of these sources to nonpoint source pollution, particularly, in watersheds with large legacy sediment deposits. Future climate projections indicate increased intensification of storm events and increased variability of winter temperatures. Freeze–thaw cycles coupled with winter rain events could increase erosion and transport of streambank sediments with detrimental consequences for water quality and health of downstream aquatic ecosystems.     

Leaf breakdown in streams differing in catchment land use

1. The impact of changes in land use on stream ecosystem function is poorly understood. We studied leaf breakdown, a fundamental process of stream ecosystems, in streams that represent a range of catchment land use in the Piedmont physiographic province of the south‐eastern United States. 2. We placed bags of chalk maple (Acer barbatum) leaves in similar‐sized streams in 12 catchments of differing dominant land use: four forested, three agricultural, two suburban and three urban catchments. We measured leaf mass, invertebrate abundance and fungal biomass in leaf bags over time. 3. Leaves decayed significantly faster in agricultural (0.0465 day^?1) and urban (0.0474 day^?1) streams than in suburban (0.0173 day^?1) and forested (0.0100 day^?1) streams. Additionally, breakdown rates in the agricultural and urban streams were among the fastest reported for deciduous leaves in any stream. Nutrient concentrations in agricultural streams were significantly higher than in any other land‐use type. Fungal biomass associated with leaves was significantly lower in urban streams; while shredder abundance in leaf bags was significantly higher in forested and agricultural streams than in suburban and urban streams. Storm runoff was significantly higher in urban and suburban catchments that had higher impervious surface cover than forested or agricultural catchments. 4. We propose that processes accelerating leaf breakdown in agricultural and urban streams were not the same: faster breakdown in agricultural streams was due to increased biological activity as a result of nutrient enrichment, whereas faster breakdown in urban streams was a result of physical fragmentation resulting from higher storm runoff.     

Regional analysis of inorganic nitrogen yield and retention in high-elevation ecosystems of the Sierra Nevada and Rocky Mountains

Yields and retention of dissolved inorganic nitrogen (DIN: NO₃ ^– + NH₄ ⁺) and nitrate concentrations in surface runoff are summarized for 28 high elevation watersheds in the Sierra Nevada of California and Rocky Mountains of Wyoming and Colorado. Catchments ranged in elevation from 2475 to 3603 m and from 15 to 1908 ha in area. Soil cover varied from 5% to nearly 97% of total catchment area. Runoff from these snow-dominated catchments ranged from 315 to 1265 mm per year. In the Sierra Nevada, annual volume-weighted mean (AVWM) nitrate concentrations ranged from 0.5 to 13 M (overall average 5.4 M), and peak concentrations measured during snowmelt ranged from 1.0 to 38 M. Nitrate levels in the Rocky Mountain watersheds were about twice those in the Sierra Nevada; average AVWM NO₃ ^– was 9.4 M and snowmelt peaks ranged from 15 to 50 M. Mean DIN loading to Rocky Mountain watersheds, 3.6 kg ha^–1 yr^–1, was double the average measured for Sierra Nevada watersheds, 1.8 kg ha^–1 yr^–1. DIN yield in the Sierra Nevada, 0.69 kg ha^–1 yr^–1, was about 60% that measured in the Rocky Mountains, 1.1 kg ha^–1 yr^–1. Net inorganic N retention in Sierra Nevada catchments was 1.2 kg ha^–1 yr^–1 and represented about 55% of annual DIN loading. DIN retention in the Rocky Mountain catchments was greater in absolute terms, 2.5 kg ha^–1 yr^–1, and as a percentage of DIN loading, 72%.A correlation analysis using DIN yield, DIN retention and surface water nitrate concentrations as dependent variables and eight environmental features (catchment elevation, slope, aspect, roughness, area, runoff, soil cover and DIN loading) as independent variables was conducted. For the Sierra Nevada, elevation and soil cover had significant (p > 0.1) Pearson product moment correlations with catchment DIN yield, AVWM and peak snowmelt nitrate concentrations and DIN retention rates. Log-linear regression models using soil cover as the independent variable explained 82% of the variation in catchment DIN retention, 92% of the variability in AVWM nitrate and 85% of snowmelt peak NO₃ ^–. In the Rocky Mountains, soil cover was significantly (p < 0.05) correlated with DIN yield, AVWM NO₃ ^– and DIN retention expressed as a percentage of DIN loading (%DIN retention). Catchment mean slope and terrain roughness were positively correlated with steam nitrate concentrations and negatively related to %DIN retention. About 91% of the variation in DIN yield and 79% of the variability in AVWM NO₃ ^– were explained by log-linear models based on soil cover. A log-linear regression based on soil cover explained 90% of the variation of %DIN retention in the Rocky Mountains.