Trees need nitrogen to grow. If soil nitrogen levels are low, then growth is limited. The ability to accurately monitor the amount of soil nitrogen in a working forest is helpful for predicting future growth and for knowing when to take corrective action (fertilizer, altered management practices) when soil nitrogen levels decline.

Efforts to monitor soil nitrogen levels have traditionally focus on a soil depth within one meter. For instance, the USDA Forest Inventory and Analysis Program monitors soils to a depth of 0.2 meters. Studies, however, show that a substantial amount (7% to 35%) of soil nitrogen exists in all soil types below one-meter depth. In addition, conifers in temperate forests can extend their roots well below one meter (old growth Douglas-fir stands can extend roots ten meters below ground) and can therefore affect nitrogen cycling at lower depths. The nitrogen present at levels below one meter is often not accounted for in ecosystem assessments, and the lack of accounting may affect the accuracy of nitrogen pool and cycling measurements.

In order to quantify the amount of deep-soil nitrogen located in Pacific Northwest working forests, researchers led by Rob Harrison at the University of Washington surveyed 22 intensively managed Douglas-fir sites in Oregon and Washington. Trees at each site were at similar age and located in a region bounded by the Pacific Ocean and the Cascade mountains. The nitrogen level was recorded at varying soil depths (maximum depth at 2.5 meters) and total nitrogen content in the soil was compared to total nitrogen estimates based on shallow sampling. Their research was partially funded by the USDA-NIFA through NARA and recently published in the Forest Ecology and Management journal.

Read Deep soil: Quantification, modeling, and significance of subsurface nitrogen here.

Total nitrogen distribution

The amount of total soil nitrogen varied extensively between sites. The site with the most soil nitrogen contained ten times more total nitrogen than the lowest rated site. The sites represented a wide range of soil types. On average, the greatest concentration of soil nitrogen occurred between 0.1 and 0.5 meters; however, 31% of the total available nitrogen was present at a depth below one meter.

Based on the total nitrogen amounts determined from the 2.5 meter sampling, the authors note that shallow soil sampling (<1.0 m depth) would have underestimated soil nitrogen stocks, which could bias both the magnitude and direction of nitrogen movement.

Equations to account for deep soil nitrogen

Determining the total nitrogen content from soil samples to a depth of 2.5 meters is often not practical due to time, budget, safety and logistical limitations. The authors therefore, tested various equations to project total nitrogen content based on nitrogen amounts from samples taken at a maximum depth of 1 meter and 1.5 meters. They then compared the projected results with total nitrogen based on samples collected up to 2.5 meters by applying the logarithmic function, Langmuir equation, log-log function and exponential function to extrapolate total nitrogen content based on sample data. Models using the data from 1 meter sampling had a mean error across all 22 sites that ranged from -10.9% to -27.7% with the logarithmic function providing the closest estimation to the total nitrogen content established with the 2.5 meter sampling. Models using the data from 1.5 meter sampling reduced the mean error to -7.6% using the Langmuir equation.

Conclusion

This paper does four things: 1) quantifies the amount and relative distribution of nitrogen in the soil, 2) determines the effect of sampling depth on soil nitrogen estimates, 3) evaluates the ability for mathematical models to accurately predict total nitrogen at 2.5 meters based on shallower sampling to 1 and 1.5 meters, 4) assesses which soils are most important to sample deeply for nitrogen.

The results from this paper suggest that measuring total nitrogen based on surface soil samples can bias estimates. Measures used to determine total soil nitrogen should incorporate sampling to at least 1-1.5 meters in depth or as deep as possible. A statistically significant difference in total nitrogen was recorded all the way down to 2 meters; however, modeling total nitrogen with samples taken within 1-1.5 meters can produce reasonably accurate results.

If researchers desire a greater number of samples in space, then designing a limited subset of deep soil samples at each site/stand could maximize their capability to model deep soil nitrogen from their more shallow samples.

This research contributes well to NARA’s efforts. Establishing accurate methods to monitor nitrogen levels in working forests is a critical step in evaluating the environmental sustainability of using forest residuals to produce biojet fuel and other co-products.
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