Quantifying benefits associated with land application of organic residuals in Washington State

This study was conducted to quantify soil C storage, N concentration, available P, and water holding capacity (WHC) across a range of sites in Washington State. Composts or biosolids had been applied to each site either annually at agronomic rates or at a one-time high rate. Site ages ranged from 2 to 18 years. For all but one site sampled, addition of organic amendments resulted in significant increases in soil carbon storage. Rates of carbon storage per dry Mg of amendment ranged from 0.014 (not significant) in a long-term study of turf grass to 0.54 in a commercial orchard. Soils with the lowest initial C levels had the highest rates of amendment carbon storage (r(2) = 0.37, p < 0.001). Excess C stored with use of amendments in comparison with control fields ranged from 8 to 72 Mg ha(-1). For sites with data over time, C content increased or stabilized. Increases in total N were observed at all sites, with increased WHC and available P observed at a majority of sites. Using a 50 Mg ha application rate, benefits of application of biosolids and compost ranged from 7 to 33 Mg C ha. This estimate does not account for yield increases or water conservation savings.