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WATERLINE - June, 2025

Impacts of stand age on nitrogen retention in restored riparian buffers

By Cara Gutenberg, MS candidate, Western Washington University Biology Department (College of Science and Engineering) and David U. Hooper, Professor, WWU Biology Dept.

Riparian buffers can reduce the amount of nitrogen that reaches streams from upland sources, which mitigates the effects of nitrogen pollution on drinking water quality and habitats in lakes, streams, and connected nearshore marine ecosystems. Excess nutrients from human activities can reduce water quality in streams and downstream areas through eutrophication, harmful algal blooms (HABs), and pollution of drinking water.1 Eutrophication causes an overgrowth in algae, which creates a hypoxic (low oxygen) environment unfit to support diverse life.2 In lakes, HABs result from overgrowths of cyanobacteria, some of which produce toxins dangerous to humans and animals, both from contact and in drinking water.3 In addition, nitrate in drinking water above the Environmental Protection Agency limit of 10 mg N/L increases the risk of illnesses such as blue-baby syndrome and multiple types of cancer.4

Riparian (i.e., streamside) restoration can help reduce nutrient pollution and improve water quality. As nitrogen (N) is transported to riparian zones via ground and surface water, buffers retain and store N through plant and microbial processes.5 Plants take up inorganic N and store it in their tissues. Soil microbes (prokaryotes and fungi) take up N from soil solution as they decompose dead plant organic matter, storing N in the process. The microbial process of denitrification permanently removes N from buffer systems, returning it to the atmosphere as N2O or N2.6 Some N may also end up in soil organic matter (SOM) that is difficult to decompose. Long-term storage of N in SOM and perennial plant biomass is the main pathway of N accumulation in riparian buffers. Determining the rate of accumulation in the long-term pools of buffers will allow for a better understanding of the abilities of buffers to reduce nitrogen loading to aquatic ecosystems.

However, N retention can vary greatly across different buffers. For example, buffers may retain anywhere from 30-90% of the N they receive.7 The amount of N retained depends on buffer characteristics such as age, N input level, dominant plant type, and plant species present, as well as hydrologic and topographic characteristics.8 In early primary succession, nutrient pool sizes increase steadily during initial establishment.9 As a stand reaches maturity in late succession, N pool size begins to level off and reach a steady state10 because plants have reached near-maximum or maximum size. In contrast, restored areas typically start at secondary succession since the soil pool already has a significant amount of organic matter that contains N.11 Plant production by native species in temperate ecosystems is often N-limited, so buffers receiving high amounts of N will likely retain more N than those with lower N supply.12 At high levels of N input, however, plants and microbes can reach saturation if more N is transported into the system than they are capable of storing. Any N above this limit can be lost to streams via leaching or to the atmosphere via denitrification.12 

Figure 1: Sample sites aged 2 years old (top left), 23 years old (top right), 40 years old (bottom left), and ~120 years old (bottom right).

Limits of retention, and levels of saturation, of restored riparian buffers in the Pacific Northwest are relatively understudied. Determining absolute (total) values of N retention accounts for the potential of N saturation in buffers, which is not considered when using the commonly reported percent N retention. Measurements of absolute N accumulation and rates of retention in buffers over time will help to better parameterize restoration models for use in the PNW. We measured the sizes of, and rates of change in, plant biomass and soil nitrogen pools in 11 buffers ranging from two years old to ~120 years old to assess growth in N pools over time (Figure 1). The sampling sites, in lowland Whatcom County, Washington, are in watersheds with mixed land uses, but without urban or agricultural land use directly upgradient. Using a nested plot design13, we measured plant biomass (trees, shrubs, herbaceous matter, and below-ground biomass), soil (leaf litter and soil organic nitrogen), and total (plant + soil) N pool sizes in each site across the age gradient. To determine the size of the plant biomass N pool, we directly measured biomass and N content of herbaceous species and fine roots using CHN (carbon, hydrogen and nitrogen) analyses and measured amounts of nitrogen in trees and shrubs using allometric equations for each species. We also directly measured amounts of nitrogen in the litter and soil to a depth of 60cm.

Preliminary results show that sites have a large range of nitrogen retention (Figure 2). Nitrogen storage in plant biomass increased over time, with a rate of nitrogen retention of 103.4 kg N/ha/yr. Similar to secondary succession, soil N pools initially predominated total ecosystem N, but increased about three-fold more slowly than did plant N, accumulating just 37.6 kg N/ha/yr. Total ecosystem (plant + soil) N retention also increased over time, at 141.0 kg N/ha/yr. Throughout time, the soil pool’s contribution to total N pool size decreased, from 87.0% soil and 13.0% plant biomass at 2 years, to 63.3% soil and 36.7% plant biomass at 40 years, to 46.7% soil and 53.3% plant biomass at 120 years. Pool sizes were particularly low in the 40-year-old site, which may be due to the semi-frequent flooding that occurs on approximately half of the buffer.14, 15 These quantitative values of N accumulation in restored riparian buffers in the Pacific Northwest will help refine watershed models for restoration prioritization. 

Figure 2: Changes in the soil (green), plant biomass (pink), and total (soil + plant biomass, blue) nitrogen pool sizes (kg N/ha) over time (years). Linear regressions proved significant for each pool.

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