By Sally Brown, University of Washington
Drawdown provides solutions that will help us reduce (drawdown) the concentrations of CO2 in the atmosphere. Biosolids didn’t make the list of solutions. If you look at that list, you can see a number of ways that biosolids could make the proposed solutions even better. Plantation forestry is one of those solutions that is better with biosolids. Drawdown estimates that there are currently 294 million hectares of forest plantations. Adding an additional 112-174 million hectares would store an additional 22-36 billion tons of CO2 by 2050. The question is how much better.
King County uses about 30% of their biosolids for forest fertilization. Other municipalities within NW Biosolids also use their biosolids to grow trees. In fact, forest fertilization with biosolids started with King County and research at the University of Washington. Many people will recognize the original tree ring that clearly showed the benefits of biosolids.
Nowadays biosolids are applied using side cast spreaders at rates that fall more into the agronomic category. Biosolids are typically applied from 2-6 tons per acre with 3-6 applications over a 40-year rotation. Partnering with King County, and with additional funding from NW Biosolids, we’ve been sampling soils and trees to quantify whether biosolids applications will increase soil carbon storage and grow bigger trees.
King County has been applying biosolids to WA DNR lands and commercial forest plantations near North Bend, WA since the mid 1980s. Emma Leonard, the graduate student working on the project, went into the old records at King County and created a map of applications over time. This map was overlaid with soil data to identify sites for sampling. Two sites were identified; one on a low productivity site located on the Campbell’s Global property and the second, a higher productivity site, located on the DNR Marckworth forest. Application histories for each site are shown here.
We collected soil samples from the sites starting last February and finishing last July. Each site had an applied and a control section. For each section we picked 4 random sites and at those sites dug 4 random pits. We collected soils from 0-5, 5-10, 10-20, and 20-30 cm depths. For each depth we also lined the pit with plastic and measured the volume of water required to fill it as a way to determine the volume of soil excavated. Samples were analyzed for total carbon and nitrogen. Using the soil weight and volume, we’ve also been able to calculate the bulk density. In addition to soils we collected data on tree volume. A random square was set up at each site and the diameter at breast height of all trees within the square was measured.
We are currently analyzing the data and writing up the report. Initial results do show that trees still grow better with agronomic rates of biosolids. Soil carbon storage is trickier as wildland soils in the Cascades have a lot of rocks and roots. Those rocks and roots make digging out even squares of soils difficult. Our data is currently being corrected to best compensate for those unique shapes.