Effect of organic wastes on soil biological and enzyme activity involved in nitrogen mineralisation

Abstract

Land application of organic waste needs to be sustainable. In order to maximise the benefits to plant production whilst minimising adverse impacts on the environment, there is an urgent need to improve our knowledge of nitrogen (N) mineralisation rates in soil amended with organic waste. The aim of this study was, therefore, to improve our understanding of the effects of land application of organic wastes on soil biological and extracellular enzyme activity involved in N-mineralisation. Nitrogen mineralisation rates were measured in soil amended with dairy shed effluent (DSE), dairy pond sludge and nitrogen fertiliser (NH₄Cl). Two laboratory incubation studies (open and closed) were conducted to determine N mineralisation rates under controlled moisture and temperature regimes and to examine the relationships of N mineralisation with key soil microbiological and extracellular enzyme activities (protease, deaminase and urease). Two field experiments were conducted to determine N mineralisation rates under natural conditions and to compare the results with the laboratory derived relationships.

In the open incubation study, mineralisation rates were higher at the higher sludge rates while the percent of N that mineralised was lower at the higher sludge rate. The application of organic waste in the closed laboratory incubation significantly increased soil microbial biomass and extracellular enzyme activities, which intum increased the rates of gross N-mineralisation. Gross N mineralisation rate in the closed incubation study was 5.4 µg g⁻¹ soil day⁻¹ in the DSE treatment compared to 2.2 µg g⁻¹ soil day⁻¹ in the NH₄Cl and 3.4 µg g⁻¹ soil day⁻¹ in the control treatments. The increased gross mineralisation rate in the DSE treatment was attributed to the presence of readily available carbon and nitrogen substrates in the organic wastes. The increases in soil microbial and extracellular enzyme activity in the organic waste treatment were not maintained for long and decreased after the exhaustion of the readily mineralisable organic substrates.

Results from the closed incubation study also showed that the gross N mineralisation rate in the organic waste amended soil was significantly correlated with soil microbial biomass and extracellular enzyme activities, confirming the hypothesis that mineralisation of organic wastes in soil involves a sequence of different microbial and extracellular enzyme activities. Stepwise regression analysis showed that protease was the variable that was most frequently included in the equation accounted for the variations of gross N mineralisation rate and has the potential to be used as one of the predictors for N mineralisation. Extracellular enzyme activities were not only correlated with each other but also with the soil microbial biomass, indicating their close association with active microbial biomass during N-mineralisation.

Though natural variations in soil and environmental conditions and the difficulty of making accurate measurements in the field lowered the correlation coefficients for the individual treatment, the key relationships derived in the laboratory were still apparent in the field. The effect of DSE on N mineralisation, nitrification, microbial biomass, and extracellular enzyme activity was more pronounced in the surface soil than in the sub-surface soil. This was attributed to the naturally higher microbial biomass and extracellular enzyme activity in the surface layers.

The field experiment also showed that more N0₃⁻ was leached from the autumn application than from the summer application. This was attributed to the higher rainfall and lower plant growth rate and N uptake during winter compared with summer. The practical implications for sustainable production and environmental protection are that organic waste application rates may need to be modified depending on the time of year. The mineral N (NH₄⁺ and N0₃⁻) that was present in the 0-40 cm soil depth as a proportion of N applied after autumn and summer application was higher in the NH₄Cl treated soil than the DSE treated soil, which indicates that N availability and thus potential losses depend on the source of N. Therefore regulatory authorities should differentiate between different sources of N when establishing rules for land application of organic wastes.

The high concentration of ammonium produced by the application of NH₄Cl fertiliser had no effect on the gross mineralisation rate. However it produced the highest gross rate of nitrification (3.7 µg g⁻¹ soil day⁻¹) in the NH₄Cl treatment compared to (1.5 µg g⁻¹ soil day⁻¹) in the DSE treatment and (0.8 µg g⁻¹ soil day⁻¹) in the control. Gross mineralisation and nitrification rates, microbial and extracellular enzymatic activities were found to be optimum at a soil water potential of -10 kPa, while at 0 or at -80 kPa, these activities were adversely affected due to anaerobic or dry conditions respectively.