Quantifying the Agronomic Value of Biosolids and Biowastes

Abstract

Predicting N release in agricultural soils amended with organic residuals is essential to ensure there are sufficient nutrients for crops, and minimal losses to the environment. Factors such as moisture, temperature, soil type and organic residuals type may affect the amount of mineralised nutrients. Information is required to describe microbial N immobilisation in biosolids-amended soil, which may vary between soil types influencing extent and rate of nutrient release. Industrial biowastes are increasingly being used as soil amendments as a diversion from landfill disposal; these materials result from a diverse range of processes and vary greatly in physical and chemical characteristics. There is little published or advisory information regarding their agronomic benefit, therefore research is required to quantify their fertiliser value. The aims of this research were to quantify mineralisation of N, and investigate other agronomic benefits of biosolids and biowastes, and to investigate microbial biomass N (MBN) dynamics, with the aim of improving fertiliser guidelines. A field trial was established at the Imperial College field station in Wye, Kent, in spring 2005, to investigate N transformations in contrasting soil types amended with conventional and enhanced treated biosolids. Analysis of MBN in biosolids-amended soil indicated that there were differences in N immobilisation processes in soils of differing fertility status, with greater immobilisation observed in the lower fertility soil in some cases. Despite initial differences in rate of mineralisation and nitrification, overall extent of N release was similar in both soils. A programme of field trials was established at Imperial College Silwood Park campus in 2006-2007 to quantify the agronomic value of a range of industrial biowastes. Biowastes from the vegetable, meat and dairy industries and from aerobic and anaerobic digestion plants were investigated. Yield response and N offtake of perennial ryegrass to biowastes, at five rates of application, in comparison to mineral N fertiliser, was used to calculate the replacement N fertiliser value. The results of a laboratory incubation experiment, to investigate MBN and mineral N in biowastes-amended soil, demonstrated lower recoveries of N in fine textured soil with low stability waste. Denitrification was suspected as the potential mechanism for this observation. Interactions between soil type and biowaste type on N availability, and the implications for gaseous N losses to the environment from biowastes-amended soils require further research.