Lignite amendment of livestock manure: Mechanisms for nitrogen retention and effects on composting and nutrient release dynamics

© 2020 Robert ImpraimIntensive livestock production systems, such as cattle feedlots, play an important role in meeting the increasing global demand for animal products, driven by a growing population and increasing affluence. These intensive systems account for a significant proportion (~39%) of global atmospheric ammonia (NH3), which is released from the large amounts of manure generated. In cattle feedlots, the loss of NH3 represents about 75% of the excreted nitrogen (N) in the manure and this has important implications for environmental pollution. Consideration of management strategies to reduce this loss provides potential to obtain N enriched manures that can be used as fertilizer. A number of mitigation techniques have been shown to reduce NH3 loss from manure. These include: acidifying agents and urease inhibitors (both requiring frequent reapplication), dietary manipulation (can affect animal productivity) and manure compaction and covering (only applicable to stockpiled manure). Lignite (brown coal) has a demonstrated capacity to suppress NH3 emission from manure by 30-66% and is seen as a more effective, practical and potentially long lasting option. Lignite’s ability to effectively reduce the loss of NH3 from manure has been credited to its pH, cation exchange capacity (CEC), pH buffer capacity and labile carbon (C) content. However, we do not fully understand what the actual capacity for N retention by lignite is, or the main mechanisms by which lignite retains N, and how these are influenced by the properties of the lignite and changes in the environmental conditions when lignite is mixed with other materials (such as manure). Also, the impact of lignite on the quality of the manure during deposition, storage, processing (such as composting) and land application, has not been studied. The objectives of this study were to: (i) characterize different lignite materials for their capacity for nitrogen retention, and to understand the mechanisms of N retention; (ii) determine how lignite amendment of manure affects the composting process under both field and laboratory conditions by monitoring changes in biochemical parameters and gaseous emissions that occur during composting, and (iii) determine the C and N mineralization from non composted and composted manure applied to soil and how this is influenced by lignite. The capacity for N retention by lignite, and the mechanisms of N retention were determined by characterizing five lignites sourced from Victorian brown coal deposits. The chemical properties that were examined for each lignite (e.g. pH, total and labile C, forms of C, pH buffering capacity, CEC, etc.) were related to its ammonium (NH4+) adsorption capacity as a function of pH and also to the biological immobilization of N in the lignite. The properties of the lignites, their capacity for NH4+ adsorption and the immobilization of N, were determined through a series of laboratory experiments involving SEM, 13C NMR spectroscopy, sample digestion and extraction, batch adsorption isotherms techniques, and controlled environment incubations. Lignite’s impact on composting of manure was assessed under field (windrow) conditions to determine the benefits under typical industry scale management processes. The mechanisms by which lignite affected composting of manure was determined under laboratory (in-vessel) conditions. Changes in biochemical parameters (e.g. N forms, organic matter and pH), emissions of NH3 and greenhouse gases (GHG), as well as compost maturity indices, were monitored. Lignite’s effect on C and N mineralization from manure (non-composted and composted) when applied to soil, was determined through a laboratory incubation experiment which monitored CO2 evolution (as a measure of C mineralized) and changes in NH4+ and NO3- concentration in soil (as a measure of N mineralized) over a period of 40 days. The mechanism by which lignite retains N was found to be mainly through pH dependent adsorption of NH4+ on exchange sites of deprotonated carboxyl groups. The maximum NH4+ adsorption capacity (Qmax) increased up to 3 fold when pH was increased from 3.6 to ~7. Biological immobilization of N was insignificant (e.g. maximum of 0.1 mg N g-1 lignite) compared to NH4+ adsorption on exchange sites (e.g. highest Qmax of 25.6 mg NH4+-N g-1 lignite at pH ~7). These findings suggest lignite’s ability to suppresses NH3 from manure occurs by i) initially favouring the formation of NH4+ over NH3 due to the acidic pH of the lignite, and ii) deprotonation of more carboxyl groups with continuous deposition of alkaline manure leading to increased CEC of the lignite and increased retention of NH4+. Amending manure with lignite did not inhibit the composting of manure. The addition of lignite suppressed the emission of NH3 from manure during both field windrow (by 45%) and laboratory in-vessel (by 35-54%) composting. Lignite addition reduced the emissions of GHGs (12-23% for CO2, 58-72% for N2O and 52-59% for CH4) during in-vessel composting. However, during windrow composting, addition of lignite caused the emissions of GHGs to increase (41% for CO2, 136% for N2O, and from -5.0 mg kg-1 initial dry matter (DMi) day-1 in the manure only to 9.8 mg kg-1 DMi day-1 for CH4). The higher emissions of CH4 and N2O were presumed to be due to anaerobic pockets that developed within the lignite treated windrow resulting from the small particle size of the lignite, and facilitated by the largely passive aeration method (unlike the forced aeration for in-vessel composting) used during the windrow composting. For the in-vessel composting, lignite addition to manure increased the organic matter (OM) and N contents of the final compost by 10-19 and 28-38% respectively, and also the germination index, a measure of compost maturity, (from 71% to 90-113%). These changes were not observed under windrow composting conditions likely due to the passive aeration method used. In addition, the effect of lignite in the windrow compost may have differed to that in the in-vessel systems because i) the mode of manure collection using large scale field equipment meant that the lignite content of the manure was estimated based upon that applied but could have been less due to some being left on the pen surface, and ii) the source of soil used in the cattle pens was quite alkaline (pH 8.8) which may have reduced the lignite pH effect. Lignite addition to the manure suppressed microbial activity (soil respiration) which reduced the mineralization of C from the manure when applied to soil, more noticeably when the manure was composted. Over a 40-day incubation period, with non-composted manure application rates of 30 and 60 t ha-1 soil, the C mineralized was 26.4 to 27.8% for manure only, and with lignite amendment it was 16.3 to 21.4%. The corresponding C mineralized in the composted manure was 12.4 to 14.1% and 3.5 to 6.5%. The addition of lignite had a mixed effect on N mineralized from manure when applied to soil. The N mineralized was significantly higher in the non-composted manure amended with Loy Yang lignite (10.4 to 13.5% for 30 and 60 t ha-1) than with Bacchus Marsh lignite (4.1 to 9.8%) and non-composted manure only (3.2 to 8.7%). For composted manure, there was no significant difference between N mineralized in the manure only (4.8 to 6.7%) and the lignite treatments (2.5 to 7.8%). Results presented in this study, for the first time, show the dominant N retention mechanism by lignite in manure, its influence on biochemical changes and gaseous emissions during manure composting under both laboratory and field conditions, and finally how the lignite impacts nutrient mineralization from non-composted and composted manure when applied to soil. In conclusion, this study shows lignite’s capacity to mitigate the emissions of both NH3 and GHGs from manure under optimum composting conditions. Hence, in intensive livestock production systems, lignite has the ability to improve the agronomic value of the large volumes of manure generated and reduce the environmental pollution associated with livestock production and manure management. Lignite addition to manure, especially when composted, has the potential to increase soil OM and improve long term soil health due to the inhibitory effect of lignite on C mineralization from manure. The findings of this study provide the option for the use of lignite as a tool for sustainable livestock production in intensive livestock industry