SHORT COMMUNICATION
Carbon, ash and organic matter relationships for feedlot manures and composts Francis J. Larney, Benjamin H. Ellert, and Andrew F. Olson Agriculture and Agri-Food Canada, Lethbridge Research Centre, 5403 - 1st Ave. S., Lethbridge, Alberta, Canada T1J 4B1 (e-mail:
[email protected]). Lethbridge Research Centre contribution no. 38704049. Received 16 September 2004, accepted 30 December 2004. Larney, F. J., Ellert, B. H. and Olson, A. F. 2005. Carbon, ash and organic matter relationships for feedlot manures and composts. Can. J. Soil Sci. 85: 261–264. Composting is increasingly recognized as a means of handling livestock manure. Using relationships derived from a dataset of >3000 samples representing the decay spectrum from raw manure to mature compost (from unpaved feedlot pens bedded with straw or wood residuals) we propose that measurement of either total carbon or ash content is sufficient to estimate carbon, organic matter and dry matter mass changes during composting of beef feedlot manure. Key words: Compost, carbon, ash content, feedlot manure Larney, F. J., Ellert, B. H. et Olson, A. F. 2005. Liens entre le carbone, les cendres et la matière organique dans les fumiers et compostés. Can. J. Soil Sci. 85: 261–264. Le compostage est de plus en plus perçu comme une bonne méthode pour gérer le fumier des animaux d’élevage. Utilisant les liens issus d’un jeu de données qui regroupait plus de 3 000 échantillons couvrant toute l’éventail de stades de décomposition, du fumier brut au compost mature (d’enclos non pavés recouverts de paille ou de copeaux de bois), les auteurs estiment qu’il suffit de mesurer le carbone total ou la teneur en cendres pour estimer les variations de la masse de carbone, de matière organique et de matière sèche survenant lors du compostage du fumier de bovins. Mots clés: Compost, carbone, teneur en cendres, fumier
Composting has gained increased acceptance as a means of reducing transport requirements for the large amounts of manure generated by the beef feedlot industry in southern Alberta (Larney et al. 2000). Dry matter (DM) losses (up to 40% of initial DM) occur during composting (Eghball et al. 1997; Tiquia et al. 2002). These losses may be assessed by direct weighing and water content sampling of fresh manure and finished compost. This is often a logistical problem for the large mass of manure used in open-air windrow experiments. Dry matter losses may also be calculated from ash contents, assuming that mass losses are wholly attributed to organic matter (OM) and that the mass of ash is conserved during the composting process (Bernal et al. 1998). Accurate measurements of DM losses are essential in mass balance calculations of nutrient (e.g., N, P) and carbon changes during composting. Simply measuring the nutrient concentration change as fresh manure becomes compost, while useful for estimating nutrient application rates with the final compost, offers no indication of nutrient losses on a mass basis, because of the significant DM losses during composting. For example, an identical N concentration of 15 g kg–1 in initial manure and subsequent final compost does not mean zero N loss during composting. Assuming a 30% DM loss (1000 kg manure DM becomes 700 kg of compost DM), there is 15 kg of initial manure N (15 g N kg–1 × 1000 kg) and only 10.5 kg of final compost N (15 g N kg–1 × 700 kg), representing an N mass loss of 30%.
To understand nutrient dynamics during manure composting, it is essential to distinguish mineral constituents (ash content) which are likely to persist (soil particles, salts and recalcitrant plant compounds) from organic constituents (carbon or nitrogen, potentially susceptible to decomposition). Hence, both ash content and total carbon are generally measured in composting research experiments (Eghball et al. 1997). The objective of this study was to examine the relationships between carbon, ash (non-volatile solids) and organic matter (volatile solids) for typical beef feedlot manures and composts. We were especially interested in the use of total C data to predict ash contents for subsequent use in calculating DM and nutrient mass balance changes during composting. At Agriculture and Agri-Food Canada’s Lethbridge Research Centre, 3051 manure/compost samples were acquired during 5 yr (1997-2001) of beef feedlot manure composting trials. The samples included fresh manure cleaned from feedlot pens where cereal straw or wood chips had been used as bedding, and this same manure at various stages of composting. At pen cleaning, the bedding materials comprised ~20% of the DM mass of the manure. The composting protocol consisted of a 3-mo active thermophilic phase during which open-air windrows were turned ~7 times, followed by a 3-4 mo curing phase. Abbreviations: DM, dry matter; OM, organic matter; TC, total carbon 261
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Manure/compost samples were oven-dried at 60°C for 5 d and routine analyses included total carbon (TC) in finely ground (< 150 µm) sub-samples (8 mg), and ash in coarsely ground (< 2 mm) sub-samples (5 g). Total C content was determined by automated combustion-gas chromatography at 1000°C (Carlo Erba, Milan, Italy). Ash content was determined by manual combustion in a muffle furnace at 650°C for 24 h. Approximate OM or volatile solids was calculated as (% OM = 100 % DM – % ash content). We used regression analysis to establish relationships between TC, ash content and OM content. Total carbon values ranged from 69 to 501 g kg–1 (Table 1), which represented a broad decomposition scale from fresh manure at pen cleaning (high TC values) to finished compost 6-7 mo later (low TC values). The average TC value was 280 g kg–1. Similarly ash content and OM content, exhibited wide ranges: 78 to 860 g kg–1 for ash and 140 to 922 g kg–1 for OM (Table 1). A regression equation was developed using total C and ash content values for all 3051 samples. A total of 44 samples (or 1.4%) had predicted values for ash content that were >10 percentage points higher or lower than observed values. These were omitted and the equation was fitted for the remaining 3007 samples. The relationship between ash content and TC (Fig. 1) was defined by the equation: Ash content (g kg–1) = 968.9 – 1.797(TC, g kg–1) with an R2 value of 0.958 (P < 0.001***). The relationship between OM and TC was OM (g kg–1) = 31.1 + 1.797(TC, g kg–1). Since DM = OM + ash, the slope of this relationship is positive but identical to the absolute value for the slope of the relationship between ash and TC. We assumed that total C comprised entirely organic C. Corrections were not made for possible small amounts of inorganic C in samples (e.g., carbonates from underlying soil mixed with manure, either by cattle hoof action or by scraping too deep during pen cleaning). The intercept of 31.1 (± 2.0 SE) was significantly different from zero at P
