Land use and nitrogen export in the Piracicaba River basin, Southeast Brazil

Anthropogenic N inputs and riverine export were determined for a meso-scale river basin in one of the most developed and economically important regions of South America. The Piracicaba River basin is located in southeastern Brazil and drains into a tributary of the Paraná River. The basin supports over 3 million people (about 2% of the population of Brazil) with intensive agricultural and industrial activities. During two years from 1995 to 1997, biweekly samples were collected at 10 stations along the Piracicaba River and its tributaries for analyses of dissolved and particulate N. The average annual flux of dissolved inorganic N and total N increased by a factor of 15 and 20 times, respectively, from the headwaters to the lower reaches of the main channel, whereas discharge increased by only 7 times. On a per area basis, the export of TN varied according to land use and was significantly correlated to the net input of anthropogenic N. Among 10 sub-catchments composing the basin, areas mostly covered by pasture and forest had the lowest export, whereas more agricultural and urban areas had higher export. The amount of N exported from each sub-catchment varied widely, but inputs were consistently higher than fluvial outputs. Losses and retention of N occurred throughout the basin but were especially high in the sub-catchment with a main-stem reservoir, suggesting that aquatic processing plays an important role in controlling riverine N export. Total net anthropogenic input to the Piracicaba River basin was 4,500 (± 900) kg N km^–2 yr^–1 of which about 40% was exported via fluvial outputs.     

流域人类活动净氮输入量的估算、不确定性及影响因素

人类活动使得大量的氮素进入流域生态系统,大量氮的盈余导致了一系列生态环境问题的出现。为了评估人类活动对流域生态系统的影响,Howarth等于1996年提出了人类活动净氮输入(NANI)的概念。综述了当前人类活动净氮输入的估算方法、不确定性及影响因素,并得到以下结论:导致NANI估算结果的不确定性原因主要有:内涵分歧、数据来源、尺度转换、估算方法的分歧。影响NANI的主要因素包括:各输入项、人口密度、土地利用组成;对于各输入项而言,化肥施用是最主要的氮素输入来源,占人类活动净氮输入总量的79.0%,其次为作物固氮,占17.6%,食品/饲料氮净输入量占-14.5%,大气沉降占15.7%;对于人口密度,NANI随着人口密度的增大而增大,当人口密度高于100人/km2,人口密度对NANI的影响趋于稳定,其他因素起主导作用。对于土地利用组成:NANI与森林面积比例成负相关,而与耕地面积比例成正相关。     

Soil system budgets of N,Si and P in an agricultural irrigated watershed: surplus,differential export and underlying mechanisms

Ongoing socio-economic and climatic changes can differentially affect the biogeochemistry of the key nutrients nitrogen (N), silica (Si), and phosphorus (P) by altering their soil budgets, their transfer to aquatic environments and their ecological stoichiometry. This may lead to cascade consequences for aquatic communities and biogeochemical processes. Soil budgets, river export, and N, Si, and P ecological stoichiometry were assessed in a heavy impacted basin (Mincio River, Italy) in two decades (1991–2000; 2001–2010). The main aim was to analyse element-specific mechanisms of terrestrial-aquatic transport and retention within aquatic habitats. Budget results suggest a net accumulation (inputs exceeding outputs) of all nutrients in agricultural lands, mainly due to livestock manure, with a reduction for N (196 kg N ha^?1 year^?1 in 2000, and 132 kg N ha^?1 year^?1 in 2010), and constant values for Si (up to 3 kg Si ha^?1 year^?1) and P (43 kg P ha^?1 year^?1) along the study period. River export of N and P accounted for 3–27% and?~?2% of N and P soil net accumulation, respectively, while Si export was significantly greater (25 kg Si ha^?1 year^?1) than Si net accumulation on farmlands. The stoichiometry of net nutrient accumulation in soils was not reflected by the stoichiometry of nutrient riverine export, due to element-specific mechanisms. We speculate that N and Si vertical and horizontal mobilization is increased by the irrigation loop, while P retention is favored by limited erosion due to limited slopes in the Mincio River basin. The simultaneous analysis of N, Si and P allows us to better understand the different paths, transformation and retention mechanisms at the watershed scale.     

Influences of climate,hydrology, and land use on input and export of nitrogen in California watersheds

Human activities have greatly increased the input of biologically available nitrogen (N) from land-based sources to aquatic ecosystems; yet few studies have examined how human actions influence N export in regions with a strong seasonality in water availability. In this study, we quantified N inputs and outputs for 23 California watersheds and examined how climate, hydrology, and land use practices influenced watershed N export. N inputs ranged from 581 to 11,234 kg N km⁻² year⁻¹ among watersheds, with 80% of total input for the region originating from agriculture (inorganic fertilizer, manure, and legumes). Of the potential N sources examined, mean annual concentrations of dissolved organic N and dissolved inorganic N in study rivers correlated most strongly with manure N input (r ² = 0.54 and 0.53, respectively). Seasonal N export varied by basin and was correlated with climate, anthropogenic N inputs, and reservoir releases. Fractional export of watershed N inputs by study rivers annually was small (median of 8%) and scaled exponentially with runoff (r = 0.66). Collectively, our results show that anthropogenic activities have altered both the magnitude and timing of watershed N export in California and suggest that targeted management in specific locations and times of the year could reduce N export to downstream systems in the region.     

Anthropogenic nitrogen sources and relationships to riverine nitrogen export in the northeastern U.S.A.

Human activities have greatly altered the nitrogen (N) cycle, accelerating the rate of N fixation in landscapes and delivery of N to water bodies. To examine relationships between anthropogenic N inputs and riverine N export, we constructed budgets describing N inputs and losses for 16 catchments, which encompass a range of climatic variability and are major drainages to the coast of the North Atlantic Ocean along a latitudinal profile from Maine to Virginia. Using data from the early 1990's, we quantified inputs of N to each catchment from atmospheric deposition, application of nitrogenous fertilizers, biological nitrogen fixation, and import of N in agricultural products (food and feed). We compared these inputs with N losses from the system in riverine export.The importance of the relative sources varies widely by catchment and is related to land use. Net atmospheric deposition was the largest N source (>60%) to the forested basins of northern New England (e.g. Penobscot and Kennebec); net import of N in food was the largest source of N to the more populated regions of southern New England (e.g. Charles & Blackstone); and agricultural inputs were the dominant N sources in the Mid-Atlantic region (e.g. Schuylkill & Potomac). Over the combined area of the catchments, net atmospheric deposition was the largest single source input (31%), followed by net imports of N in food and feed (25%), fixation in agricultural lands (24%), fertilizer use (15%), and fixation in forests (5%). The combined effect of fertilizer use, fixation in crop lands, and animal feed imports makes agriculture the largest overall source of N. Riverine export of N is well correlated with N inputs, but it accounts for only a fraction (25%) of the total N inputs. This work provides an understanding of the sources of N in landscapes, and highlights how human activities impact N cycling in the northeast region.     

Uneven rise in N inputs to the Lake Michigan Basin over the 20th century corresponds to agricultural and societal transitions

By constructing nitrogen (N) budgets from 1880 to 2002 for watersheds that have undergone urbanization, intensive agricultural specialization or experienced minimal change, we document an uneven timeline of increase in anthropogenic N inputs. N loading to the watersheds of the Lake Michigan Basin grew six-fold from 1880 to 2002, peaking in 1987. Human activities influenced N inputs as early as 1880, and the magnitude and timing of increase differed markedly across regions in accord with population growth, land use, and type of agriculture. The greatest increase occurred from 1950 to 1980, corresponding with rapidly accelerating use of artificial fertilizers, but increases in atmospheric deposition and shifting patterns in crop and livestock production also affected trends. Net anthropogenic N inputs have changed little since about 1980, showing a modest decline due to a leveling out of fertilizer use and greater export of animal feed and products. Using a model that predicts riverine N export from watershed N loadings and river discharge, we found that river TN fluxes from all tributaries increased approximately threefold from 1900 to 2000 but have stabilized or declined over the past two decades, consistent with national surveys that show near-constant or declining riverine TN concentrations. For the LMB, the past two decades has been a period of relative stasis in N inputs to its terrestrial systems and N export from watersheds. This retrospective analysis also points to the challenge of forecasting future trends in N budget terms, which can both increase and decline in response to policy and societal transitions.     

Critical source area controls on water quality in an agricultural watershed located in the Chesapeake Basin

The importance of agricultural land use activities for supplying nutrients (N, P) to the Chesapeake Bay is examined and nutrient sources for a typical agricultural hill-land watershed within the Chesapeake Basin are identified and assessed. Based on up to 30 years of experimental and monitoring data, the outflow, N, and P exported from this Pennsylvania watershed is examined in terms of critical source areas. Most of the surface runoff and P export occurs from areas near the stream. About 90% of the algal-available P exported in outflow was generated during the largest 7 storms/year. In contrast, nearly all the nitrate (NO₃) exported originated as subsurface flow entering the soil or ground water some distance from the stream, and mostly occurred during nonstorm flow periods. The NO₃ export observed over the long term corresponds to the N excess computed by N balance obtained by farmer survey for agricultural land. By combining land use, hydrologic processes, watershed position, soil P status, and N balance information for agricultural land, the major source areas for P and N are predictable and identifiable. We apply these ideas and techniques to our research watershed and present the results as an example of this approach.     

Estimated historical and current nitrogen balances for Illinois

The Midwest has large riverine exports of nitrogen (N), with the largest flux per unit area to the Mississippi River system coming from Iowa and Illinois. We used historic and current data to estimate N inputs, outputs, and transformations for Illinois where human activity (principally agriculture and associated landscape drainage) have had a dominant impact. Presently, approximately 800,000 Mg of N is added each year as fertilizer and another 420,000 Mg is biologically fixed, primarily by soybean (Glycine max L. Merr.). These annual inputs are greater than exports in grain, which results in surplus N throughout the landscape. Rivers within the state export approximately 50% of this surplus N, mostly as nitrate, and the remainder appears to be denitrified or temporarily incorporated into the soil organic matter pool. The magnitude of N losses for 1880, 1910, 1950, and 1990 are compared. Initial cultivation of the prairies released large quantities of N (approximately 500,000 Mg N year(-1)), and resulted in riverine N transport during the late 19th century that appears to have been on the same order of magnitude as contemporary N losses. Riverine flux was estimated to have been at a minimum in about 1950, due to diminished net mineralization and low fertilizer inputs. Residual fertilizer N from corn (Zea mays L.), biological N fixed by soybean, short-circuiting of soil water through artificial drainage, and decreased cropping-system diversity appear to be the primary sources for current N export.     

Temporal and spatial trends in nitrogen and phosphorus inputs to the watershed of the Altamaha River,Georgia, USA

The watershed of the Altamaha River, Georgia, is one of the largest in the southeastern U.S., draining 36,718 km² (including parts of metro Atlanta). We calculated both nitrogen (fertilizer, net food and feed import, atmospheric deposition, and biological N fixation in agricultural and forest lands) and phosphorus (fertilizer and net food and feed import) inputs to the watershed for 6 time points between 1954 and 2002. Total nitrogen inputs rose from 1,952 kg N km⁻² yr⁻¹ in 1954 to a peak of 3,593 kg N km⁻² yr⁻¹ in 1982 and then declined to 2,582 kg N km⁻² yr⁻¹ by 2002. Phosphorus inputs rose from 409 kg P km⁻² yr⁻¹ in 1954 to 532 kg P km⁻² yr⁻¹ in 1974 before declining to 412 kg P km⁻² yr⁻¹ in 2002. Fertilizer tended to be the most important input of both N and P to the watershed, although net food and feed import increased in importance over time and was the dominant source of N input by 2002. When considered on an individual basis, fertilizer input tended to be highest in the middle portions of the watershed (Little and Lower Ocmulgee and Lower Oconee sub-watersheds) whereas net food and feed imports were highest in the upper reaches (Upper Oconee and Upper Ocmulgee sub-watersheds). Although the overall trend in recent years has been towards decreases in both N and P inputs, these trends may be offset due to continuing increases in animal and human populations.     

A Phosphorus Budget for the Lake Mendota Watershed

A phosphorus (P) budget was calculated for the agriculture-dominated Lake Mendota watershed located in Dane and Columbia Counties, Wisconsin, USA. P inputs included fertilizer for agricultural crops and lawns, dietary supplements for dairy cattle, and natural inputs such as dry and wet deposition. Outputs included agricultural crops, livestock and livestock products, and hydrologic export to Lake Mendota. The total P input to the watershed (1,307,000 kg year^{− 1}) and total output (732,000 kg year^{− 1}) are large relative to the average of 34,000 kg P washing into the lake each year, indicating that the P flux that eutrophies Lake Mendota is a veryminor component of the total watershed P budget. Using the formula inputs − outputs = change in storage, we found that 575,000 kg P accumulated in the watershed in 1995. This estimate was corroborated by long-term soil P concentration data, which showed an average annual increase in soil P of over 450,000 kg year^{− 1}. Future management programs designed to reduce P inputs to Lake Mendota will be compelled to cope with the large amount of P being stored in the watershed. Received 31 August 1998; accepted 21 October 1998.     

Environmental, Social, and Management Drivers of Soil Nutrient Mass Balances in an Extensive Andean Cropping System

Sustainable nutrient cycling in agroecosystems combining grazing and crops has global ramifications for protecting these ecosystems and for the livelihoods they support. We sought to understand environmental, management, and social drivers of nutrient management and sustainability in Andean grazing/crop systems. We assessed the impact of farmer wealth, fields’ proximity to villages, topography, and rangeland net primary productivity (NPP) on mass balances for nitrogen (N), phosphorus (P), and potassium (K) of 43 fields. Wealthier farmers applied greater total amounts (kg) of manure nutrients. However, higher manure application rates (kg ha^?1) were associated with field proximity and NPP rather than wealth. Manure P inputs in far fields (> 500-m distant) were half those in near fields. Harvest exports increased with manure inputs (P < 0.001) so that balances varied less than either of these flows. Erosion nutrient losses in steeper far fields matched crop exports, and yields declined with increasing field slope (P < 0.001), suggesting that erosion reduces productivity. Balances for P were slightly positive in near and far fields (+2.2 kg P ha^?1 y^?1, combined mean) when calculated without erosion, but zero in near fields and negative in far fields with erosion included (?6.1 kg P ha^?1 y^?1 in far fields). Near/far differences in both inputs and erosion thus drove P limitation. Crop K exports dominated K balances, which were negative even without accounting for erosion. Modeled intensification scenarios showed that remediating far field deficits would require P addition and erosion reduction. Management nested within environmental constraints (NPP, erosion) rather than socioeconomic status drives soil nutrient sustainability in these agroecosystems. Time-lags between management and long-term degradation are a principal sustainability challenge to farming in these montane grazing/crop agroecosystems.     

Nitrogen budget and riverine nitrogen output in a rice paddy dominated agricultural watershed in eastern China

The nitrogen (N) budget calculation approach is a useful means of evaluating the impact of human activity on the N cycle. Field scale N budget calculations may ignore the interactions between landscapes, and regional scale calculations rely on statistical data and indirect parameters. Watershed scale budget calculations allow for a more direct quantification of N inputs and outputs. We conducted N budget calculations for a rice paddy-dominated agricultural watershed in eastern China for 2007?C2009, based on intensive monitoring of stream N dynamics, atmospheric deposition, ammonia (NH₃) volatilization and household interviews about N-related agricultural activities. The results showed that although total N input to the watershed was up to 280 kg N ha^?1 year^?1, riverine discharge was only 4.2 kg N ha^?1 year^?1, accounting for 1.5% of the total N input, and was further reduced to 2.0 kg N ha^?1 year^?1 after reservoir storage and/or denitrification removal. The low riverine N output was because of the characteristics of the rice paddy-dominated landscape, which intercepts run-off and enhances soil denitrification. The watershed actually purified the N in rainwater, as N concentrations in river discharge were much lower than those in rain water. Major N outputs included food/feed export, NH₃ volatilization from chemical fertilizer and manure, and emissions from crop residue burning. Net reactive gaseous emissions (emissions minus deposition) accounted for 5.5% of the total N input, much higher than riverine discharge. Therefore, the agricultural N cycle in such paddy-dominated watersheds impacts the environment mainly through gas exchange rather than water discharge.     

Production,nutrient availability,and elemental balances of two meadows affected by different fertilization and water table regimes in The Netherlands

The restoration of degraded peat-grasslands is an important nature conservation goal in The Netherlands. We investigated the effects of ceased fertilization (15 years) combined with a groundwater-raised water table (6 years) on the production of the peat-grassland vegetation and soil nutrient availability in a meadow. Furthermore, we evaluated whether and how this difference between meadows affected the balances between nutrient inputs and outputs in the ecosystem. We used an adjacent fertilized meadow in which the water table followed agricultural practice as a control. Yield of the grassland vegetation was significantly lower in the wet than in the control meadow. The tissue concentrations of N, P, and K in the harvested vegetation were significantly lower, but those of Ca higher in the wet than in the control meadow. The difference between both meadows significantly affected the annual nitrification rate, but not the annual C and N mineralization rates and the annual net P and K release rates. The difference between both meadows also significantly affected the seasonal nitrification and K release rates. Season exerted a significant effect on the seasonal C and N mineralization and nitrification rates. The elemental balances and relative contributions of the balance terms to elemental inputs and outputs varied considerably with element. Annually, the wet meadow lost N, P and K, while the control meadow gained these elements. The elemental demand of the grassland vegetation in the wet meadow was met for N for a large part by mineralization and for the remainder by atmospheric deposition, for P it was in the same order of magnitude as the net soil-P release, as it was for K. It is to be expected that the soil resources of N, P and K will continue to decrease under a continued regime of ceased fertilization and a raised water table, with those of N decreasing with the same rate, of P morerapidly and of K more slowly than estimated from regressions.     

Where did all the nitrogen go? Fate of nitrogen inputs to large watersheds in the northeastern U.S.A.

To assess the fate of the large amounts of nitrogen (N) brought into the environment by human activities, we constructed N budgets for sixteen large watersheds (475 to 70,189 km²) in the northeastern U.S.A. These watersheds are mainly forested (48–87%), but vary widely with respect to land use and population density. We combined published data and empirical and process models to set up a complete N budget for these sixteen watersheds. Atmospheric deposition, fertilizer application, net feed and food inputs, biological fixation, river discharge, wood accumulation and export, changes in soil N, and denitrification losses in the landscape and in rivers were considered for the period 1988 to 1992. For the whole area, on average 3420 kg of N is imported annually per km² of land. Atmospheric N deposition, N₂ fixation by plants, and N imported in commercial products (fertilizers, food and feed) contributed to the input in roughly equal contributions. We quantified the fate of these inputs by independent estimates of storage and loss terms, except for denitrification from land, which was estimated from the difference between all inputs and all other storage and loss terms. Of the total storage and losses in the watersheds, about half of the N is lost in gaseous form (51%, largely by denitrification). Additional N is lost in riverine export (20%), in food exports (6%), and in wood exports (5%). Change in storage of N in the watersheds in soil organic matter (9%) and wood (9%) accounts for the remainder of the sinks. The presence of appreciable changes in total N storage on land, which we probably under-rather than overestimated, shows that the N budget is not in steady state, so that drainage and denitrification exports of N may well increase further in the future.     

Nutrient budgets in a dry heathland watershed in northeastern Spain

Dissolved nutrient inputs in bulk precipitation and outputs in streamwater were measured during 3 years of contrasting hydrological conditions in a 6.3-ha, grazed heathland watershed on schists in the Montseny mountains (NE Spain), drained by an intermittent stream. On average, 39% of the precipitation became streamflow. Bulk precipitation delivered positive net alkalinity (mean 0.22 keq/ha/yr), sulphate input was moderate (9.0 kg SO₄-S/ha/yr), and the mean input of inorganic N was not exceptionally high (6.6 kg/ha/yr). Ion concentrations were relatively low in streamwater; SO₄ ^2- was the dominant anion. Most concentrations in streamwater varied seasonally, with maxima in late summer or early autumn and minima in spring. This pattern probably resulted from increased availability of ions for leaching due to decomposition of organic matter and chemical weathering during the warm period. Nitrate concentrations were relatively high in winter and dropped sharply in early spring, probably because of biological uptake. Annual element outputs in streamwater varied between years and seemed to be controlled by both the amount of annual streamflow and its seasonal distribution. Annual inputs exceeded outputs for dissolved inorganic N. The watershed accumulated H⁺ and Ca²⁺, had net losses of Na⁺ and Mg²⁺, and was close to steady state for K⁺, SO₄ ^2-, Cl^- and alkalinity. The chloride budgets gave no evidence of substantial dry deposition in this system. The cationic denudation rate was negative (-0.14 keq/ha/yr) because Ca²⁺ retention was higher than net exports of Na⁺ and Mg²⁺ from silicate weathering. Low nutrient export and little production of alkalinity suggest that this watershed has a low buffering capacity.     

Prorocentrum minimum tracks anthropogenic nitrogen and phosphorus inputs on a global basis: Application of spatially explicit nutrient export models

Nutrient over-enrichment from land-based sources has degraded estuarine and coastal marine waters worldwide. Linking nutrient loading, in magnitude and form, to specific ecosystem effects, however, has been a challenge on the global scale. The harmful algal species Prorocentrum minimum has long been thought to be associated with eutrophication based on several site-specific long-term databases and a previous review of its global spreading. Using recently developed spatially explicit models that quantify global river nitrogen (N) and phosphorus (P) export to the coastal zone and the contribution of natural and anthropogenic sources, as well as a review of the global distribution of P. minimum, we show that this HAB species is associated with regions of high dissolved inorganic nitrogen (DIN) and phosphorus (DIP) exports that are strongly influenced by anthropogenic sources (such as fertilizers and manures for DIN). Blooms of this species were also linked to regions with relatively high anthropogenic contributions to dissolved organic N and P export. The global distribution of this species is expected to expand, given that nutrient inputs to watersheds from agriculture, sewage and fossil fuel combustion are projected to more than double by 2050 unless technological advances and policy changes are implemented.     

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.     

Spatial and temporal variation in phosphorus budgets for 24 watersheds in the Lake Erie and Lake Michigan basins

We estimated net anthropogenic phosphorus inputs (NAPI) to 18 Lake Michigan (LM) and 6 Lake Erie (LE) watersheds for 1974, 1978, 1982, 1987, and 1992. NAPI quantifies all anthropogenic inputs of P (fertilizer use, atmospheric deposition, and detergents) as well as trade of P in food and feed, which can be a net input or output. Fertilizer was the dominant input overall, varying by three orders of magnitude among the 24 watersheds, but detergent was the largest input in the most urbanized watershed. NAPI increased in relation to area of disturbed land (R² = 0.90) and decreased with forested and wetland area (R² = 0.90). Export of P by rivers varied with NAPI, especially for the 18 watersheds of LM (R² = 0.93), whereas the relationship was more variable among the six LE watersheds (R² = 0.59). On average, rivers of the LE watersheds exported about 10% of NAPI, whereas LM watersheds exported 5% of estimated NAPI. A comparison of our results with others as well as nitrogen (N) budgets suggests that fractional export of P may vary regionally, as has been reported for N, and the proportion of P inputs exported by rivers appears lower than comparable findings with N.     

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.