Nitrogen Cycling in Freshwaters V
Presiding: E Strauss, U.S. Geological Survey; R Alexander, U.S. Geological Survey
NB31D-01 INVITED 08:30h
Hydrologic Control of Nitrate Loading and Transformation in Backwater Lakes of the Upper Mississippi River
Floodplain backwater lakes (BWL) are biogeochemically active with potential to remove large quantities of transported nitrate (NO3-) from the Upper Mississippi River. We measured nitrate transformations in BWL receiving high NO3- water under natural flooding and controlled inflow conditions to determine: 1) patterns of NO3- loss; 2) biogeochemical processes affecting NO3- transformation; and 3) effect of loading rate on removal capacity. In a large (300 ha) BWL, floodwater NO3- concentrations dropped from 6.5 to < 0.5 mg-N L-1 in 12 d, with a loss of >18 tons-N. Under controlled inflow another BWL (Third Lake, 15 ha) exhibited high rates denitrification (22 Μg-N cm-2 d-1), limited by NO3- loading and tightly coupled with nitrification. Nitrate retention was linear with load (r2=0.95), with greatest retention occurring in late June. An average of 48 kg - N d-1 NO3- was removed from Third Lake (43 % of total inflow load, 32 % via denitrification). These results show NO3- removal from backwater lakes is directly related to river-flood plain connectivity, river discharge, and NO3- loading rate. Engineered reconnection of backwaters to main channels could reduce downstream flux of NO3- while also restoring other ecologic functions and meeting multiple management goals.
NB31D-02 08:45h
Transport Distance Effects in Regional Predictions of Nitrate Discharge: Implications for Nitrogen Transformation
Recent publications suggest that lotic uptake plays a significant role in basin-scale nitrogen transformation. If transport through terrestrial or aquatic systems has significant, generalizable impacts on nitrogen discharge at the scale of whole watersheds, then predictions of nitrogen losses should depend on both amounts of source area and transport distances from source areas to watershed outlets. We calculated terrestrial and stream transport distances from croplands along surface flow pathways to explore effects of transport and stream size on nitrate concentrations in streams draining 420 rural watersheds in 4 physiographic provinces of the Chesapeake Basin. Transport-distance distributions were statistically related to average nitrate concentrations using non-linear regression models with fitted coefficients representing transport-distance effects. Results were compared to models predicting nitrate solely from percent cropland. In three of four provinces, accounting for transport distance improved nitrate predictions and coefficients representing transport terms were statistically significant. In Coastal Plain watersheds, predictive improvements were solely attributable to terrestrial transport, whereas channel transport in larger order streams accounted for predictive improvements within the Piedmont and Appalachian Plateau. Our results provide broad-scale support for the general significance of both aquatic and terrestrial transformations, yet suggest the relative importance of these processes varies among physiographic settings.
NB31D-03 09:00h
Will the unexpected and unexplained trend towards declining nitrate concentrations in New Hampshire streams soon reverse?
A recent report describes unexpected and unexplained declines in stream nitrate concentrations in forested watersheds of New Hampshire during recent decades. These declines are unexpected because continuing elevated atmospheric nitrogen (N) deposition is believed to exceed plant nutritional requirements which should result in increasing (rather than decreasing) stream nitrate concentrations. These declines are unexplained because several hypotheses have either been ruled out or remain untested. Increased rates of tree uptake are unlikely because region-wide assessments indicate stagnant or decreasing forest growth rates. Additionally, rates of atmospheric N deposition have remained fairly constant in recent years. Forest maturation has also been rejected as a potential explanation. Increasing tree N uptake during forest recovery from disturbances like insect defoliations and severe drought were provisionally ruled out because they do not explain the regional nature of the decline. Atmospheric CO2 fertilization effects are thought to be too small, and have not been detected as increases in aboveground growth. Interannual climatic variability, including soil frost dynamics, could explain part of the declines, but these hypotheses remain untested. Regional trends towards warmer, wetter, and longer growing seasons are consistent with increasing N uptake, but there is no evidence for a corresponding increase in forest growth. Nitrogen sequestration in soil organic matter could explain the observed nitrate decline, and may have increased in recent years through stimulation of belowground carbon cycling due to CO2 fertilization. In addition, climate changes have favored an intensification of belowground N cycling that could lead to a narrowing of the C:N ratio and increased N sequestration. If increasing atmospheric CO2 and a warmer and wetter climate are at least partially responsible for increasing N sequestration, it is not likely that the current trends in stream water nitrate will soon reverse. The rate of tree N uptake is also likely to increase if current trends towards increasing atmospheric CO2, temperature, and precipitation continue as expected.
NB31D-04 INVITED 09:15h
Importance of Stream Denitrification in the Nitrogen Mass Balance of a Midwestern Agricultural Region
Agricultural regions of the Midwestern US have large N fluxes as a result of inputs from fertilizer and biological fixation, and outputs through rivers and grain harvest. These inputs and outputs are not balanced, however, and denitrification has been suggested to be an important loss mechanism. We examined the role of in-stream denitrification in the N mass balance of Illinois, a predominantly agricultural region. Nitrate concentrations in streams were often >10 mg nitrate-N L-1, suggesting denitrification was not N-limited throughout most of the year. Denitrification rates were measured at many headwater stream sites throughout the year, in both sediments and primary producer habitat, under different geomorphic conditions. Although in-stream denitrification rates were generally high, hydraulic retention time limited the importance of denitrification in terms of export on an annual basis. Geomorphology was important in explaining rates, but extensive channelization has eliminated most in-stream structures, which could have more effectively reduced stream export of N. Therefore, stream denitrification was only minor sink for N and most nitrate in these headwater sites was exported downstream. In the overall mass balance of N, reservoir and in-field denitrification are thought to be much more important than in-stream denitrification.
NB31D-05 INVITED 09:30h
Apportioning Sources of Riverine Nitrogen at Multiple Watershed Scales
Loadings of reactive nitrogen (N) entering terrestrial landscapes have increased in recent decades due to anthropogenic activities associated with food and energy production. In the northeastern USA, this enhanced supply of N has been linked to many environmental concerns in both terrestrial and aquatic ecosystems, such as forest decline, lake and stream acidification, human respiratory problems, and coastal eutrophication. Thus N is a priority pollutant with regard to a whole host of air, land, and water quality issues, highlighting the need for methods to identify and quantify various N sources. Further, understanding precursor sources of N is critical to current and proposed public policies targeted at the reduction of N inputs to the terrestrial landscape and receiving waters. We present results from published and ongoing studies using multiple approaches to fingerprint sources of N in the northeastern USA, at watershed scales ranging from the headwaters to the coastal zone. The approaches include: 1) a mass balance model with a nitrogen-budgeting approach for analyses of large watersheds; 2) a spatially-referenced regression model with an empirical modeling approach for analyses of water quality at regional scales; and 3) a meta-analysis of monitoring data with a chemical tracer approach, utilizing concentrations of multiple elements and isotopic composition of N from water samples collected in the streams and rivers. We discuss the successes and limitations of these various approaches for apportioning contributions of N from multiple sources to receiving waters at regional scales.
NB31D-06 09:45h
Temporal Patterns in Sediment Denitrification Rates in an Agricultural Stream
Denitrification is an important component of the nitrogen cycle in streams, representing a true nitrogen sink. Few long-term studies of stream denitrification have been made, making it difficult to identify factors responsible for temporal patterns in denitrification rates. We measured denitrification, water chemistry, and sediment and hydrological characteristics in an Illinois stream for two years during all seasons. Denitrification rates spanned nearly 3 orders of magnitude, from 0.05 to 25 micrograms N/g AFDM/hr. The mean (n=16) was 7.0 micrograms N/g AFDM/hr, with a coefficient of variation of 115%. Sediment characteristics (total C, N, C:N) varied little during the study, and did not explain temporal variation in denitrification rates. Nitrate-N (range = 0.6 to 14 mg/L), water temperature, and discharge varied substantially during the study, but were not correlated with denitrification rates. DOC explained 39% of the variation in denitrification rates, if both variables were log-transformed. These results suggest that sediment denitrification was not directly controlled by characteristics of the water column or standard measures of sediment quality, or these variables were not measured at a relevant spatial scale. Variation in denitrification rates may reflect microbial population changes or community interactions that are not accounted for with typical environmental variables.