By Sally Brown, University of Washington
Forest application of biosolids has been a signature beneficial end use in the Pacific Northwest. The Northwest has also been a leader in quantifying the benefits of biosolids for soil carbon storage with dryland wheat. We sampled forest soils outside of Seattle to see if forest application also results in carbon storage. This project, funded primarily by King County with additional funding provided by NW Biosolids, sampled two paired sets of soils. Both sets had one site with a long history of biosolids applications and one site within the same soil series that served as a control. One site, located on a Tokul soil is managed by WA DNR, is classified as a Site 2. Biosolids applications began at this site began in 1998 with a cumulative loading rate of 42 Mg ha-1. The other site, managed by Campbell Global is on a Klaus soil is classified as a Site 3. The first biosolids application to this soil was made in 1989 but we couldn’t find the application rate. Subsequent applications totaled 26 Mg ha-1.
We identified 4 random sites for control and treated for each site. At each site we dug 4 separate holes. Soils were collected to a 30 cm depth at 0-5, 5-10, 10-20 and 20-30 cm. We also measured bulk density. Total carbon and nitrogen were measured on the collected soils. This was not an easy or perfect process. There can be a lot of rocks in those forest soils. There is also a lot of variability.
We found a clear biosolids signature in both sites while sampling. The biosolids sites had richer understory vegetation and it was much easier to dig; biosolids seemed to melt away the rocks. That was reflected in total soil carbon and nitrogen storage in the Tokul soil but not the Klaus soil. Total carbon and nitrogen (soil concentration * bulk density) for both sites at each depth is shown below.
The Tokul soil, lower in carbon than the Klaus soil, showed a significant increase in both carbon and nitrogen storage with the biosolids. It is not clear why there was no change in the Klaus soil. In particular, we expected that the surface soils would have been enriched in nitrogen. We’ve gotten data from King County on total applications, carbon and nitrogen from a range of sites and will be seeing if there is a relationship. Hopefully the Klaus soil is an exception here.
With the data from the two sites we calculated a carbon balance for biosolids use in forestry. Even with no change in carbon storage on the Klaus soil, the biosolids come out ahead because of the fertilizer value of the material. On the Klaus soil, the biosolids came out way ahead, storing over 5 tons of CO2 for each dry ton of biosolids applied. We will finish the analysis on the additional King County sites, do a final edit and then submit this work to a peer review journal.