Biochar as an Additive for Poultry Litter Composting: A Strategy for Greenhouse Gas Mitigation and Soil Productivity Improvement

Abstract

<p>Poultry litter is rich in nitrogen (N) which is one of essential nutrients to support and enhance plant productivity. However, large quantities of the N are lost through volatilisation of ammonia (NH₃), and also through leaching of ammonium NH₄⁺) and nitrate (NO₃^-), when fresh poultry litter is applied to the soil. Also, during decomposition of poultry litter nitrous oxide (N₂O) and methane (CH₄), which are potent greenhouse gases (GHGs), are emitted. Composting of this nutrient rich product can still produce NH₃ which is a precursor for indirect N₂O emission, and also direct emissions of N₂O and CH₄ and possibly in greater quantity if the material is not kept well-aerated during composting. The loss of gaseous N decreases the fertilizer value of the final compost product.</p> <p>Biochar, which is a carbon rich material produced by heating biomass with little or no oxygen, has inherent and emerging sorption properties that may give it capacity to reduce losses of N when co-composted with other organic amendments such as poultry litter. This study cocomposted poultry litter and sugar cane straw with two different biochars: greenwaste biochar (GWB) and poultry litter biochar (PLB), in two separate experiments. The experiments were conducted in large compost tumblers, to investigate the influence of biochar on N₂O, NH₃ and CH₄ emissions during composting, and retention of N in the final compost. The greenwaste biochar compost (GWBC) and poultry litter biochar compost (PLBC) produced from these experiments were used in a subsequent laboratory trial: GWBC, PLBC, PLB and poultry litter compost (PLC, nil biochar) were applied to soil as organic amendments to assess their influence on N₂O emissions and leaching of nitrate (NO₃^-), ammonium NH₄⁺) and dissolved organic carbon (DOC) from two different soils (Tenosol and Ferrosol) in a 63-day incubation experiment across three wetting and drying cycles. The final experiment employed a stable isotope (10 % ¹⁵N urea) method in a field trail in Ghana to investigate whether biochar and biochar-amended compost could improve fertilizer N uptake, reduce fertilizer N loss and improve fertilizer N use efficiency (FNUE), to increase maize yield. The PLB, PLBC and PLC were co-applied with ¹⁵N labelled urea and compared with a urea-only control (Control+U) in two different soil types (Ferric Acrisol) and (Ferric Lixisol). </p> <p>In the first compost experiment, the GWBC decreased (<i>P</i> ≤ 0.05) cumulative NH₃ emissions per unit of initial dry mass by 56 % while PLBC decreased the NH₃ emissions by 38 % compared to the control (nil biochar). The GWBC and PLBC significantly decreased emissions of NH₃ per unit of initial total N in the amendments; by 65 % and 55 % respectively<i> cf.</i> control. The GWBC had the highest activities of beta-glucosidase and leucine aminopeptidase enzymes as well as the lowest DOC among the treatments. The concentration of NH4+ at the end of composting was higher in the GWBC than the PLBC. In the second composting experiment, The GWBC and PLBC emitted 65 - 75 % less N₂O cumulatively than the control expressed per unit initial total N. Furthermore, CH₄-C emission was significantly lower in the biochar treatment relative to the control only during a period of composting when oxygen was probably limited in the compost mixture (ie the period where there was no tumbling) More FeO and amine/NH₃ were found on surfaces and pores of the composted biochar compared to the uncomposted biochar, indicating a change in interaction of N functional groups with Fe-dominated minerals in biochar. When GWBC, PLBC, PLB and PLC were applied as organic amendments to soil in the incubation experiment, cumulatively N₂O emissions, which were highest when water filled pore space was between 0.70–0.85 across all the treatments, were significantly lower in the PLB and PLBC treatments compared to the PLC in both soils (Ferrosol and Tenosol). Compared to the control (unamended), all treatments had significantly lower N₂O emissions in the Ferrosol, whereas onlyPLB had lower N₂O emissions in the Tenosol. Cumulative emissions of CO2 were significantly higher in the GWBC and PLBC treatments than the PLB and the unamended control in both soil types. Leaching of NO₃^--N from the Ferrosol was greatest during the first wetting and drying cycle, while it was greatest in the third cycle in the Tenosol. Significantly more NH4+-N than NO₃^--N was leached from both amended soils. The total amount of NO₃^--N leached per unit of total N applied were 48 % and 71 % lower for PLB in the Ferrosol and Tenosol respectively compared to their controls. More DOC was leached from the PLC treatments in both soil types cf. PLB, PLBC and GWBC treatments (P < 0.05), thus suggesting that biochar, whether pure or co-composted, does stabilise labile organic C in the contrasting soils. In the field trial, the FNUE from the applied ¹⁵N-labelled urea was significantly higher in the PLBC+U (32 %) compared to 24 % and 20 % in the PLB+U and PLC+U in the Ferric Lixisol. The PLBC+U in the Ferric Lixisol had the higher (P > 0.05) N retained in soil (64 %) and lower N loss (4 %) compared with the PLC+U (31 % retained and 49 % loss). There was no significant difference in crop FNUE and fertilizer N recovery in soil between treatments in the Ferric Acrisol. The PLBC+U and PLB+U treatments significantly (<i>P</i> ≤ 0.05) increased maize yield compared to PLC+U in both soil types. Similarly, significant increase in the total C, total N, pH, and cation exchange capacity of the PLB+U, PLBC+U, compared to the PLC+U, at the end of the experiment was observed. </p> <p>Based on the findings of this study, it was hypothesized that the intrinsic structural and sorption properties of biochar and the sorption properties that develop through interaction between biochar and organic compounds, decreased NH₃, N₂O and CH₄ emissions. These inherent and emerging sorption properties may have enhanced adsorption of N precursors like NH4+ and NO₃^- and prevented them from being nitrified or denitrified during composting and in soils when applied as amendments. Similarly, these processes were hypothesized to reduce leaching of inorganic N and labile C from soil, possibly facilitated by the co-composted biochar interacting with other organic amendments. Furthermore, higher crop FNUE in the Ferric Lixisol (a lighter textured soil) compared to the Ferric Acrisol (silty loam), was a result of biochar and cocomposted biochar (in biochar-amended compost) that increased N uptake by ameliorating soil constraints, particularly soil acidity, low P and low CEC. The findings from this thesis have demonstrated that co-composting biochar with poultry litter and straw decreased GHG and NH₃ emissions and increased retention of N, thus increasing its fertilizer value. This biochar-amended compost when applied to soil would be effective in decreasing emissions of N₂O and leaching of inorganic N and labile forms of C from contrasting soils, while increasing N retention, N uptake and lowering soil N loss, with implications for increased crop FNUE and yield while minimising adverse environmental impacts.</p>