Anthony's Science Blog

Soil nitrogen cycling field day on Niwot Ridge

July 6, 2008 – 9:25 am

If all goes well this week, a small army of field assistants and I will undertake what I believe to be the most comprehensive spatially explicit examination of soil nitrogen cycling rates in any ecosystem to date. We will simultaneously measure gross N mineralization, gross nitrification, and gross microbial N immobilization (thick arrows in the Niwot Ridge N cycle figure below) at 120 points across my alpine-subalpine study site on Niwot Ridge. This data set will help us to understand spatial variation in a key set of N cycling reactions and provide some insight on how this ecosystem is responding to anthropogenic nitrogen deposition.

The fastest and most important nitrogen cycling reactions in terrestrial ecosystems are the release of soil nitrogen from the large soil organic matter pool (catalyzed by microbial enzymes) and the subsequent assimilation of that nitrogen by the soil microbes (again, the thick arrows in the diagram). The constant cycling between the soil organic matter, the soil inorganic pool and the microbes is the underlying engine of terrestrial nitrogen cycling and occurs at rates that are on the order of 10 times higher than plant-soil exchange rates.

Variation in these rates with temperature, moisture, organic matter quality and other factors are essential in understanding how ecosystems processes nitrogen and how they will respond to the increased rates of available N stemming from human activities. While there have been many studies examining how different factors and conditions affect these processes, I know of only one small-scale study that has put these results in a landscape context to understand how the different parts of a landscape contribute to the overall ecosystem-level cycling of nitrogen. The data we will collect this week should help to fill this gap in our knowledge.

The measurement procedure we will be using is called the pool dilution method. At each sampling point, we will inject 6 ml of ¹⁵N-labeled solution into a patch of soil and then allow the microbes to process the nitrogen in that solution for four hours. After that four-hour incubation time, we will extract all of the inorganic nitrogen in the soil core and measure how much of the ¹⁵N has been processed. We will be able to tell how much processing has occured because the ¹⁵N-labeled soil inorganic pool will have been diluted by new nitrogen mineralized from the soil organic matter pool.

The field procedure is difficult to organize, but we’ve already done most of that and the implementation is actually pretty easy, so it should be a fun field day. Look forward to seeing some preliminary results posted here.