The impact of soil and moisture on nitrogen mineralisation rates in biosolids

Abstract

Nitrogen (N) based loading rates are commonly used to determine land application rates of biosolids, calculated to best target the agronomic N needs of the crop. The rate of N mineralisation following the amendment of soils with biosolids over a range of specific environmental conditions needs to be accurately quantified to prevent overloading the soil with N in excess of plant uptake. The N release characteristics of anaerobically digested dewatered biosolids cake (DBC), lime-amended biosolids (LAB) and alum sludge (AS), in comparison to urea as a source of readily available N, were investigated in a soil incubation study. The experimental design included two soil types and three moisture regimes (25%, 50% and 100% gravimetric water holding capacity (GWHC)). There was no significant effect of soil type on the proportion or rate of N mineralisation. Nitrogen mineralisation rate was greater for LAB and AS compared with DBC and lime amended biosolids which had been stockpiled (LABs) for 12 months.The rate of N mineralisation was also dependent on moisture and was generally greater at 50% GWHC compared to 25% GWHC, but at 100% GWHC losses of N were observed, especially from soil amended with LAB; this is attributed to denitrification. The proportion of mineralisable N (% organic N) at 50% GWHC was greater for LAB (72%) and AS (64%) in comparison with DBC (32%) and LABs (26%). These results are consistent with previous findings and demonstrate that the organic matter content of LAB and AS is of a lower stability than DBC and LABs. Plant available N in the first season following the land application of biosolids may be greater than current estimates of 20% and hence N mineralisation, volatilisation rate and denitrification losses for specific products under a range of environmental conditions needs further investigation.