The influence of biochar and digestate on short term nitrogen transformations in three contrasting soils.

Student thesis: Doctoral ThesisDoctor of Philosophy


In recent times agricultural production has placed an emphasis the maximisation of yield rather than on effects on the wider environment which has caused a shift in the nitrogen cycle leading to deleterious consequences in agricultural systems and beyond. Agronomic consequences include declining soil organic matter content, pH and increasing soil erosion which has led to decreasing yields. Environmental consequences include increased greenhouse gas emission. Two potential solutions to these challenges are the use of pyrolyzed or digested organic materials, including waste organic material. Pyrolyzed organic material results in a carbonaceous product called biochar and anaerobic digestion results in a product called digestate. Biochar offers the advantage of storing carbon in soil to mitigate climate change, and digestate is a nutrient rich fertiliser. However, they both have their disadvantages. Biochar is low in fertilising value which limits its use to crops, and digestate is low in carbon which limits its use in improving soil carbon stocks. This thesis explores whether combining these amendments in different soils results in a shift in the nitrogen cycle such that more nitrogen compounds are retained in the soil for crop use, and less nitrogenous gases are emitted to cause environmental pollution.

Incubation experiments were conducted on three different soils, predominantly silt, sand or clay, to investigate the effect of the addition of a hardwood biochar or maize digestate applied as single amendments or together on a range of parameters of agronomic and environmental concern. Those parameters included soil mineral nitrogen accumulation and mineralisation rates, total and organic nitrogen and carbon concentration, pH, cation exchange capacity, electrical conductivity, ammonia, nitrous oxide and carbon dioxide emission.

Whether or not combining biochar with digestate improved agronomic factors and reduce negative environmental ones depended, in large part, on soil type. In the silt soil pH and cation exchange capacity was not influenced by either biochar or digestate, also when co-applied. The addition of digestate generally increased mineralisation of ammonium, however, there was some evidence that, when combined with biochar, this process was reduced. Given the lack of change in other soil parameters, it was concluded that biochar brought about a shift in the micro-organism population such that mineralisation processes were suppressed. The emission of carbon dioxide and nitrous oxide were all increased with digestate amendments, but not with biochar alone. When in combination with biochar, there was no significant statistical difference between digestate alone treatments. Hence, in the silt soil, combining biochar with digestate did not reduce greenhouse gas emission.

In the sand soil, both pH and electrical conductivity increased as a result of digestate application, alone and co-applied with biochar but not with biochar alone. Due to the low carbon and nitrogen concentrations in the sand soil it was possible to detect a greater shift in total nitrogen and carbon levels, with a significant change in the C:N ratio. Again, digestate application promoted mineralisation rates, but the addition of biochar appeared to reduce them such that there was a higher accumulation of ammonium after one week. There were significant differences in carbon dioxide emission from the sand soil treated with digestate compared to control, and this was not mitigated by biochar emission. However, there was some evidence that the co-application of biochar did reduce nitrous oxide emission from digestate amended soils.

Unsurprisingly, in the fertile clay soil biochar did not influence pH, cation exchange capacity or electrical conductivity, however, digestate did, resulting in an increase in pH. There was also some evidence that biochar decreased nitrous oxide emission from digestate amended soils.

This research has contributed new insights into the use of digestate and biochar in agricultural production to meet agronomic and environmental requirements. It has provided an important perspective on the potential and limitations of combining these two amendments in different soil types.

Overall, in these incubation experiments, biochar did not quite prove to be the useful ‘sponge’ for the nitrogen supplied by digestate that was anticipated. However, this thesis provides some important indicators which may guide the co-application of a biochar and digestate to land, not only for field scale experimentation but also for future application in food production. Firstly, different soil textures respond differently to the co-applied amendment, and it may be that farmers on sandy or clay soils may reap more direct agronomic benefits and, secondly, be able to show evidence of environmental benefits (and therefore earn more subsidy) as co-applied biochar and digestate saw a decrease in nitrous oxide emissions over digestate alone.

Date of Award2023
Original languageEnglish
Awarding Institution
  • Coventry University
SupervisorSM Charlesworth (Supervisor), Francis Rayns (Supervisor) & Rob Lillywhite (Supervisor)

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