3.3.4 Nitrogen Load/Cycling

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Goal: C. Protect and enhance landscape and habitats structure and processes to benefit ecosystem and watershed functions

Objective: 3. Protect and maintain natural variability and rates of primary production and nutrient cycling

WAF Attribute: Ecological Processes

What is it and why is it important?

Many macro and micro nutrients are essential to primary productivity, food-web dynamics, and ecological function. In particular, almost all organisms require nitrogen and phosphorus in some form for physiological processes (Ryther and Dunstan 1971, Vitousek et al. 1997b, Carpenter et al. 1998). The availability and forms of these elements play important roles in shaping communities, and organisms are frequently nitrogen or phosphorus limited (Galloway et al. 1995, Vitousek et al. 1997a). However, human alteration of nutrient cycles has resulted in many watersheds being highly enriched in certain elements, specifically nitrogen, phosphorus, and/or sulfur. Among other reasons, this is frequently a result of agricultural practices which use nitrogen and phosphorus enriched fertilizers to increase crop yield (Vitousek et al. 1997a, Vitousek et al. 1997b) or sulfur-based fungicides, which subsequently wash into the riverine systems. While artificial fertilization can increase ecosystem productivity, it can also decrease biological diversity (Tilman 1987). Currently, human activity adds as much fixed nitrogen to terrestrial ecosystems as do all natural sources combined (Vitousek et al. 1997a, Vitousek et al. 1997b). Nutrients, which are necessary for aquatic life, are only toxic at high concentrations, it’s the secondary impacts (e.g., low DO, disruption of nutrient cycling) that cause concern (SWRCB 2010). Thus, these natural elements can become a significant form of pollution in aquatic ecosystems by upsetting natural nutrient cycles and can result in eutrophication and fuel harmful algal blooms (Carpenter et al. 1998). Nitrogen is often of the greatest concern as a nutrient pollutant and has as a result more monitoring data. Future comprehensive assessments should strive to also evaluate phosphorus, sulfur, and other nutrients of concern specific to the region of interest.

Figure 1. Depiction of a nitrogen cycle and the ways in which human activities alter the natural nitrogen cycle

Reprinted from University of Waikato, New Zealand.

What is the target or desired condition?

Nutrients (nitrogen and phosphorus) were consistently one of the top pollutants on the Clean Water Act (CWA) Section 303(d) List to Congress Reports beginning in the early 1990’s, but “excess” concentrations of nutrients vary by waterbody type, climate, geologic areas, and other local risk cofactors (e.g., degraded riparian). Therefore, “nutrient criteria” cannot be developed as a single number for the country as a whole due to variability in background conditions and the role of other risk co-factors which affect nutrient processing within ecosystems. Total Maximum Daily Load (TMDL) levels for nutrients are currently being developed by the USEPA and the state of California, however currently specific values for criteria thresholds are not available for subwatersheds of the Feather River. In this Report Card, 0.1 mg/L Total Kjeldahl Nitrogen (TKN, ammonia + organic nitrogen) was used as the target for a good condition (score = 100) and 1 mg/L TKN for a target for poor condition (score = 0). TKN concentrations can be high as a result of human origina and natural organic material inputs into waterways, are indicative of ammonia concentrations, and can lead to increased ammonia and nitrate concentrations and biological oxygen demand. In the Sierra Nevada, they are also the dominant form of nitrogen, as compared to inorganic nitrogen (Coats and Goldman, 2001). (See Table 2). Once the responsible state and federal agencies set criteria for nutrients such as nitrogen for individual basins, then these scores can be recalculated.

What can influence or stress condition?

Nitrogen is one of the most essential elements for plant reproduction and growth, therefore the amount of available nitrogen can strongly limit plant productivity. At first glance, nitrogen limitation may be counterintuitive, since 78% of the atmosphere is composed of nitrogen gas (N2). Yet N2, and most forms of nitrogen found in terrestrial ecosystems, is not directly available to plants. Plants therefore rely on nitrogen fixing organisms to transform N2 into bioavailable forms. Consequently, although nitrogen is abundant, many natural systems are nitrogen limited. To get around this limitation, the Haber-Bosch process ws developed to artificially produce bioavailable nitrogen (ammonia) that can be used in fertilizers to increase productivity in cultivated crops. Fertilizers must be continuously reapplied because crops take up some the added nitrogen, and leaching causes the movement of nutrients out of the soil via irrigation or storm-water runoff. As this nitrogen moves into nearby waterways, it can accumulate and at higher than natural concentrations. High levels of nitrogen in waters can also produce harmful algal blooms. In turn, these blooms can produce “dead zones” in water bodies where DO levels are so low that most aquatic life cannot survive (USEPA 2010). Through these processes along with other sources such as urban effluent and atmospheric deposition, humans have roughly doubled the amount of fixed nitrogen, and this alteration may have drastic impacts on ecological systems (Fig. 1; B. Houlton, personal communication).

What did we find out/How are we doing?

For subwatersheds with data, the condition scores for nitrogen ranged from 0 (Deer Creek) to 100 (Lower Yuba; Table 1 and Figure 2). All subwatersheds except Deer Creek were evaluated using total Kjeldahl nitrogen concentrations. Friends of Deer Creek have measured concentrations of nitrate (a form of inorganic nitrogen) at >10 mg/L in Lower Deer Creek, the highest reported for the watershed and concentrations that are toxic to animals and contribute to excess periphyton growth.

Table 1. Nutrient condition scores, based on total Kjeldahl (organic nitrogen + ammonia) concentrations, for subwatersheds

Goal Measurable Objective Subwatershed Score
C. Protect and enhance landscape and habitats structure and processes to benefit ecosystem and watershed functions 3. Protect and maintain natural variability and rates of primary production and nutrient cycling NFF 92
EBNFF 89
MFF 38
LF 94
NY n/a
MY n/a
SY n/a
DC 0
LY 100
UB n/a
LB 98

Figure 2. Subwatershed scores for nitrogen conditions

Table 2. Basic statistics for monitored forms of nitrogen in each subwatershed during the latest year in which robust data was collected. All concentrations are in mg/L. “95% C.I.” refers to 95% confidence intervals. Total Kjeldahl Nitrogen is total organic nitrogen + ammonia.

    Dissolved Ammonia Dissolved Nitrate
+ Nitrite
Total Kjeldahl Nitrogen
Subwatershed Yr N Mean 95% C.I. N Mean 95% C.I. N Mean 95% C.I.
EBN Fork Feather 2007 2007 3 0.03 0.049 3 0.005   1 0.2  
Lower Bear 2009 2009 4 0.010 0.008 4 0.078 0.061 1 0.120  
Lower Feather 2009 2009 32 0.008 0.002 32 0.048 0.016 23 0.152 0.039
Lower Yuba 2009 2009 4 0.005   4 0.028   1 0.050  
Middle Fork Feather 2007 2007 29 0.098 0.088 29 0.004 0.032 9 0.657 0.280
North Fork Feather 2007 2007 38 0.018 0.016 38 0.046 0.014 9 0.171 0.092

As expected, the types of nitrogen vary across the watershed, primarily with elevation. Nitrate + nitrite concentrations are higher in the lower watershed and North Fork Feather than in other subwatersheds (Figure 3, Table 2) and highest in Deer Creek (data not shown). Ammonia and TKN occur in a different part of the nitrogen cycle and are higher in the upper watershed than the lower, especially in the Middle Fork Feather (Figures 4 and 5, Table 2). All 3 forms of nitrogen measured can vary widely among years (Figure 6) and within years, as is reflected in the sometimes large 95% confidence intervals (Table 2).

Figure 3. Mean + 95% C.I. concentrations for dissolved nitrate+nitrite (mg/L) in each subwatershed during the latest year in which sufficient data were available (all years between 2007-2009).

Figure 4. Mean + 95% C.I. concentrations for dissolved ammonia (mg/L) in each subwatershed during the latest year in which sufficient data were available (all years between 2007-2009).

Figure 5. Mean + 95% C.I. concentrations for total Kjeldahl nitrogen (mg/L) in each subwatershed during the latest year in which sufficient data were available (all years between 2007-2009).

Figure 6. Mean concentrations averaged across sites in the Lower Feather River for dissolved nitrate + nitrite, dissolved ammonia, and total Kjeldahl nitrogen (all mg/L) for all years in which data was available

Temporal and spatial resolution

Data were available for most subwatersheds at some point in the last several decades, but only a few waterways, such as the Lower Feather River, had long-term data sets, the longest of which are shown in Figure 6. The remainder of the subwatersheds had spatially and temporally sporadic sampling for various forms of nitrogen (and phosphorous). Data were gathered for monitoring sites in 7 of the subwatersheds, covering 1949 to 2010, with most measurements being in the last decade.

How sure are we about our findings (Things to keep in mind)

We focused our scoring on TKN concentrations — total organic nitrogen + ammonia. Nitrate concentrations are also available for waterways in the watershed, but nitrate concentrations may have less meaning ecologically when considering nutrient cycling, except at very high concentrations such as were found in Deer Creek. TKN includes ammonia, which is a fertilizer and at high enough concentrations is toxic to aquatic life. One waterway was scored based upon nitrate concentrations — Deer Creek, because it has had extremely high concentrations of nitrate in the past (>10 mg/L), concentrations that are toxic to humans in drinking water. In the upper watershed and in the absence of human activities (waste-water treatment, septic systems, livestock), higher concentrations of TKN may be indicative of healthy inputs of organic material into a river, such as the Middle Fork Feather (Figure 5). Because concentrations in this subwatershed were much higher than other subwatersheds, its lower score is a cautionary note rather than a conclusion that conditions are poor.

A large amount of nitrogen and other nutrient data has been collected over the past fifty years in the Feather River Watershed, but the methods, chemical species, and locations of data collection have varied greatly. This makes it difficult to apply statistical tools and determine what, if any longer term trends are occurring in each region. However, since we know nutrients play such an important role in ecosystem dynamics and health, the standardized collection of nutrient data would greatly aid in future watershed assessments. We recommend that both total and dissolved forms of nitrogen, sulfur and phosphorus, as well as flow data be collected across the watershed in a standardized method. These nutrient data coupled with flow data will allow analysis of nutrient flux, which will give a much better indication of nutrient content per area because high variability in flow rates can result in vastly different dilution factors for the concentrations observed. This may obscure trends between sites with different flows, and also may mask true trend within sites when flow rates vary widely between wet and dry seasons. Secondly, if both nitrogen and phosphorus data are collected, N:P ratios can be assessed. This ratio can be very important in determining which nutrients are limiting a system. Since these two nutrient cycles are often inextricably linked and many agricultural areas leach different forms of either or both of these nutrients from fertilizers and, it would be very advantageous to monitor both nutrients in concert with one another. Though some flow and phosphorus data are currently available to conduct these types of analyses in certain regions, due to time constraints of this pilot project we were unable to conduct these analyses.

Technical Information

Data Analysis, Transformations and Analysis

Data for 129 sites throughout the watershed was gathered, comprising >6,900 measurements taken by various agencies and non-profit groups of total Kjeldahl nitrogen, dissolved ammonia, and dissolved nitrate + nitrite. Total Kjeldahl nitrogen (TKN) is the sum of organic nitrogen, ammonia (NH3), and ammonium (NH4+). Though other forms of nitrogen data were available (total for each species, DON, etc.), we chose these three as representative of nitrogen condition because they were most consistent. All are reported in mg/L. We calculated basic statistics for each per year for each subwatershed; however, data collection and reporting methods were generally too variable over time to apply more advanced statistical methods with confidence. Concentration values and corresponding scores for each subwatershed were reported for the last year in which robust data is available (all between 2007-2009). Mean TKN concentrations were converted to scores for each subwatershed using 0.1 mg/L as a score of 0 and 1 mg/L as a score of 100, with a linear calculation of score between these concentrations and values.

Citations

Carpenter, S. R., N. F. Caraco, D. L. Correll, R. W. Howarth, A. N. Sharpley, and V. H. Smith. 1998. Nonpoint pollution of surface waters with phosphorus and nitrogen. Ecological Applications 8:559-568.

Coats, R.N. and C.R. Goldman. 2001. Patterns of nitrogen transport in streams of the Lake Tahoe basin, California-Nevada. Water Resources Bulletin, 37(2): 405-415.

USEPA (Environmental Protection Agency). 2010. Accessed April 2nd, 2010.

Galloway, J. N., W. H. Schlesinger, H. Levy, A. Michaels, and J. L. Schooner. 1995. Nitrogen fixation - anthropogenic enhancement - environmental response. Global Biogeochemical Cycles 9:235-252.

Galloway et al. 2003. The nitrogen cascade. Bioscience 53 (4): 341-356.

Ryther, J.H. and Dunstan, W.M. 1971. Nitrogen, phosphorus, and euthrophication in coastal marine environment. Science 171(3975): 1008-1013.

Tilman, D. 1987. Secondary succession and the pattern of plant dominance along experimental nitrogen gradients. Ecological Monographs 57:189-214.

Vitousek, P. M., J. D. Aber, R. W. Howarth, G. E. Likens, P. A. Matson, D. W. Schindler, W. H. Schlesinger, and G. D. Tilman. 1997a. Human alteration of the global nitrogen cycle: Sources and consequences. Ecological Applications 7:737-750.

Vitousek, P. M., H. A. Mooney, J. Lubchenco, and J. M. Melillo. 1997b. Human domination of Earth’s ecosystems. Science 277:494-499.