Estimating Nitrogen Availability of Heat-Dried Biosolids

As heat-dried biosolids become more widely produced and marketed, it is important to improve estimates of N availability from these materials. Objectives were to compare plant-available N among three different heat-dried biosolids and determine if current guidelines were adequate for estimating application rates. Heat-dried biosolids were surface applied to tall fescue (Festuca arundinacea Schreb.) in Washington State, USA, and forage yield and N uptake measured for two growing seasons following application. Three rates of urea and a zero-N control were used to calculate N fertilizer efficiency regressions. Application year plant-available N (estimated as urea N equivalent) for two biosolids exceeded 60% of total N applied, while urea N equivalent for the third biosolids was 45%. Residual (second-year) urea N equivalent ranged from 5 to 10%. Guidelines for the Pacific Northwest USA recommend mineralization estimates of 35 to 40% for heat-dried biosolids, but this research shows that some heat-dried materials fall well above that range. 1. Introduction Heat-dried biosolids are convenient to use in a variety of applications. The Class A heat-dried product is suitable as a fertilizer on lawns and gardens as well as for agricultural crops. Heat-dried biosolids are easy to transport and handle and are applied like inorganic fertilizers, except at higher rates. Because a large proportion of the nitrogen (N) in biosolids is in organic form, biosolids act as a slow-release N source, dependent on biological transformation of the organic N into available forms. Accurate estimates of the mineralization rate of biosolids N are critical to developing application rate recommendations that meet plant needs without compromising environmental quality. Smith and Durham [1] used laboratory incubation to compare five different biosolids sources with and without heat drying, and found that the mineralization rates of the heat-dried biosolids were more than double the undried (dewatered only) materials. This rapid mineralization more than compensated for the lower initial ammonium N in the heat-dried biosolids. Rigby et al. [2] observed similar results in a field incubation, estimating mineralizable N from heat-dried biosolids at twice that for dewatered biosolids. Matsuoka et al. [3] and Moritsuka et al. [4] produced heat-dried biosolids in an experimental scale vessel reaching final temperatures of 120 and 180°C. They found increased available N in the 120°C heat-dried biosolids compared with undried biosolids in laboratory incubation and pot studies. Heat drying to a