Composting and storage of organic household waste with different litter amendments. II: nitrogen turnover and losses

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<ul><li><p>Composting and storage of organic household waste with dierentlitter amendments. II: nitrogen turnover and losses</p><p>Y. Eklind *, H. Kirchmann</p><p>Department of Soil Sciences, Swedish University of Agricultural Sciences, P.O. Box 7014, S-750 07 Uppsala, Sweden</p><p>Received 31 May 1999; received in revised form 8 December 1999; accepted 14 December 1999</p><p>Abstract</p><p>Composting of N-rich wastes can be associated with substantial gaseous N losses, which mean loss of an essential plant nutrient</p><p>but may also lead to environmental pollution. We investigated nitrogen dynamics and losses in household waste mixtures with</p><p>dierent litter additives during composting, maturation and storage. Standardized, organic household waste was composted mixed</p><p>with six litter amendments; straw, leaves, hardwood, softwood, paper and sphagnum peat. Samples were analysed for total and</p><p>inorganic N and pH. Both the addition and the type of litter amendment greatly influenced pH changes and formation of nitrate</p><p>during composting. Net N losses after 590 days were 4362% in mixtures with litter additions, being lowest in the peat and the straw</p><p>mixtures and highest in the paper mixture, and 70% in the control without litter. A conclusion of the study was that there is no</p><p>obvious way to eciently decrease N losses during composting through addition of litter materials. 2000 Elsevier Science Ltd.All rights reserved.</p><p>Keywords: Straw; Leaves; Hardwood; Softwood; Paper; Sphagnum peat; Ammonia; Nitrate; pH; Temperature</p><p>1. Introduction</p><p>Composting of N-rich wastes, such as source-sepa-rated organic household wastes can be associated withsubstantial gaseous N losses, amounting up to 5060%(Kirchmann and Widen, 1994; Brink, 1995). Also duringcomposting of manure, gaseous N losses were reportedto be high, up to 77% (Martins and Dewes, 1992) andduring composting of sewage sludge up to 68% (Witterand Lopez-Real, 1988). Emissions of nitrogenous gasesmean loss of an essential plant nutrient but may alsolead to environmental pollution.</p><p>Gaseous N losses during composting occur mainly asNH3 (reviewed by Witter and Lopez-Real, 1987; Mar-tins and Dewes, 1992), but may also occur as N2(Mahimairaja et al., 1994) and NOx (Martins and De-wes, 1992), including N2O (Sibbesen and Lind, 1993).Factors aecting ammonia formation and losses duringcomposting (review by Witter and Lopez-Real, 1987),are pH, temperature, aeration level, ammonia and am-monium adsorbing capacity of added materials (Witterand Kirchmann, 1989), and presence of available ener-gy, that may lead to immobilization of N (Kirchmann,</p><p>1985; Kirchmann and Witter, 1989). One possible wayto reduce NH3 losses is to add a C-rich material, in-creasing the ratio between C and N. However, the totalC to N ratio provides no information about the micro-bial C availability and thus is only a rough measure ofthe quality of the compost raw material. One wouldexpect a litter material with a high proportion of avail-able C to give lower N losses, presupposing that thedecomposition of the C-rich litter takes place at a timewhen there is inorganic N to immobilize.</p><p>The aims of this study were to quantify gaseous Nlosses from waste mixtures during composting, matu-ration and storage, and investigate the influence of dif-ferent carbonaceous litter amendments on N turnoverand losses. Results of the C turnover of the same ex-periment were described in a preceding article by Eklindand Kirchmann (2000).</p><p>2. Methods</p><p>2.1. Materials used and performance of experiment</p><p>Standardized, organic household waste was com-posted mixed with six litter amendments; straw, leaves,</p><p>Bioresource Technology 74 (2000) 125133</p><p>* Corresponding author.</p><p>0960-8524/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.PII: S 0 9 6 0 - 8 5 2 4 ( 0 0 ) 0 0 0 0 5 - 5</p></li><li><p>hardwood, softwood, paper and sphagnum peat. Or-ganic household waste without any litter amendmentswas used for the control. Composting was conducted in125 l, insulated bins, rotatble around their horizontalaxes, octagonal in cross-section and with ventilationholes in the sidewalls. Temperature was measured dailyin the centre of the compost masses until the ambienttemperature was reached. Samples of the waste mixtureswere taken nine times during the experimental time(0, 7, 14, 21, 35, 70, 106, 177 and 590 days). On day 177,the compost masses were removed from the bins andstored until day 590. The relatively long experimentalperiod was chosen to check chemical changes in com-post material also during maturation and long-termstorage. For a complete description of the materials andmethods see the preceding paper (Eklind and Kirch-mann, 2000).</p><p>2.2. Chemical analyses</p><p>Total N in the mixtures was measured by the Kjel-dahl method (Kjeltec, Tecator, Sweden), (Bremner andMulvaney, 1982). In samples containing significantamounts of nitrate and/or nitrite, total N was deter-mined by a modified Kjeldahl method, where the samplewas pretreated with salicylic acid and thiosulphate(Bremner and Mulvaney, 1982). Inorganic N in thesamples was determined colorimetrically (autoanalyserTRAACS 800, Bran Luebbe) after extraction with 2MKCl. All N analyses were performed on wet, thawedmaterial, to avoid NH3 losses that might have occurredduring drying. Dry matter contents were analysed inparallel on replicate samples at 105C for 24 h. The pHwas determined in water extracts (1:5 by volume) fromfresh samples after shaking for 1 h, by standard meth-ods.</p><p>2.3. Calculations and statistical analysis</p><p>Losses of N between dierent sampling occasionswere calculated as the dierence in amounts of N pre-sent. Correlations between N loss and a number ofprocess-related variables such as pH and temperature, aswell as descriptors of initial organic matter quality (seeEklind and Kirchmann, 2000) were done with the soft-ware JMP (SAS Institute, 1989). Principal componentanalysis (PCA; Wold et al., 1987) and principal com-ponent regression (PCR; Martens and Ns, 1989) wereperformed with the software Unscrambler (CAMO A/S,Trondheim, Norway). All variables were centred andscaled to equal variance before further analysis withPCA or PCR. With PCA, a score-plot is obtained, in-dicating how the objects are related to each other in themultivariate space; and a loading-plot is also obtained,indicating how the variables are related. By studyingscore and loading plots interactively the influence of each</p><p>variable on each object can be estimated. With PCR, thescore vectors from a PCA are used as predictors in amultiple linear regression (MLR). In the PCR, the re-sponse parameter net N loss was used as the depen-dent variable. In the initial prediction the followingindependent variables were used: net C loss, maximumtemperature, number of days with temperature &gt;45C,temperature sum (P[compost temperature ) ambienttemp.] day 160), values on the 9 sampling occasions ofpH, C to N ratio, available C (total organic C ) ligninC) to N ratio, and concentrations of NH4 N, NO</p><p>3 N,</p><p>total N, lignin, cellulose and hemicellulose (Eklind andKirchmann, 2000). Full leave-one-out cross validationwas used to find the optimal number of principal com-ponents (Martens and Ns, 1989). With this method, allobjects are kept out of the model one at a time and thescores are predicted from the model made by the otherobjects. The models were evaluated with the residualmean squared error of prediction (RMSEP). Variableswith small weighted coecients of regression (Bw-coef-ficients) were successively excluded to find the bestmodel.</p><p>3. Results</p><p>3.1. Temperature and pH</p><p>The mixtures containing leaves, hardwood, softwoodand paper reached maximum temperatures of 6570Cwithin 5 days, the peat mixture reached 58C, but themaximum temperatures of straw mixture and controlwere only 4050C (Fig. 1). For the litter-amendedmixtures (control excluded), there was a significantcorrelation (p &lt; 0.05; R2 0:81) between maximumtemperature and the total amount of carbon in the bins(Eklind and Kirchmann, 2000). The short-term tem-perature fluctuation seen in Fig. 1 was caused by tem-perature decline in connection with weighing andsampling. The daily measurements showed thermophilictemperatures (&gt;45C) during 10 days in leaf-amendedmixture, 8 days in hardwood-, 7 days in paper-, 4 days insoftwood- and peat-amended mixtures, but only for 1day in straw-amended mixture and control. Tempera-ture fell in all mixtures to the ambient temperature onday 40 or earlier. However, in the control without litteramendments the temperature remained above the am-bient temperature for several more weeks.</p><p>Initial pH values were low in the experimental mix-tures, 4.5 in the peat-amended one and 5.15.5 in theothers. In the mixtures with litter amendments, pHvalues rose initially, reaching 7 or more within 12weeks (Fig. 2). However, in the peat mixture, pH valueshad decreased again after 70 days to 5.5, and finally to4.8. Also in the straw-amended mixture, pH values</p><p>126 Y. Eklind, H. Kirchmann / Bioresource Technology 74 (2000) 125133</p></li><li><p>declined during storage, from 7.7 on day 177 to 5.9 onday 590. In the control, pH values were below 6 untilday 35, and increased thereafter to 8.3.</p><p>3.2. Total and inorganic N</p><p>Nitrogen concentrations varied initially between 1.2%and 2.8% of dry matter. After 590 days of decomposi-tion, concentration of N was highest in the straw com-post, 4.1%, followed by the unamended control, 3.8% ofdry matter (Table 1).</p><p>At the beginning of the composting period, there wasa peak in ammonium-N concentration in the mixtures,</p><p>followed by a decrease to low concentrations (Fig. 3).Nitrate was formed after a minimum of 35 compostingdays in litter-amended mixtures and concentrations in-creased with time. However, the softwood mixture onlyshowed a significant nitrate-N concentration on day 70and the hardwood and paper mixtures showed a tem-porary decrease when sampled on day 177. The controlcontained nitrate only at the last sampling occasion, atthe end of the storage period. After 590 days, the pro-portion of nitrate-N of total N was highest in the strawcompost, amounting to 22% (9 mg N g1 dry matter).Small, and only temporary amounts of nitrite-Nwere present in leaf-, hardwood- and paper-amended</p><p>Fig. 1. Temperature development in the household waste mixtures.</p><p>Y. Eklind, H. Kirchmann / Bioresource Technology 74 (2000) 125133 127</p></li><li><p>mixtures, with the highest levels of 0.140.5 mg N g1</p><p>dry matter (0.82.8% of total N), on day 35.</p><p>3.3. Nitrogen losses</p><p>Substantial N losses occurred from all mixtures. Mostof the losses occurred during the initial 100 days ofcomposting and maturation (Fig. 4). Losses of N in themixtures after 590 days were: peat 43%, straw 44%,hardwood 49%, leaf 55%, softwood 58%, paper 62%,and control 70%. Due to the irregular shape of thecourse of N loss, no fit to simple functions was possible.</p><p>None of the tested process-related variables or de-scriptors of initial organic matter quality (i.e. initial C toN ratio, available C [total C ) lignin C] to N ratio,lignin concentration, etc.) were significantly correlated(p &gt; 0.05) with the total N losses. Highest correlationswere obtained with maximum temperature (R2 0:51)and number of sampling occasions with pH valuesabove 8 (R2 0:51).</p><p>In a PCA projection with all waste mixtures (8 ob-jects) and 50 finally selected variables, two significantcomponents were extracted, explaining 76% in total ofthe variance in this matrix (Fig. 5). The variables were:</p><p>Fig. 2. pH development in the household waste mixtures.</p><p>128 Y. Eklind, H. Kirchmann / Bioresource Technology 74 (2000) 125133</p></li><li><p>net C loss; pH day 735, 590; NH4 N day 106590;NO3 N day 0, 7, 106590; total N day 0, 7, 70, 106, 590;lignin day 0590, cellulose day 014; hemicellulose day0106, C to N day 0, 7, 21177; available C to N day0, 70177. The hardwood, softwood and paper mixtureswere located near each other, which indicated that theywere functionally similar. The control was far from thelitter-amended mixtures in the PCA projection.</p><p>Furthermore, the principal components from thePCA were used in a MLR model predicting net N lossesof the mixtures (PCR). From the data used, 89% of thevariation in N loss could be predicted (Fig. 6). Thefollowing variables had the highest weighted coecientsof regression: pH day 590, hemicellulose concentrationsday 0106, and nitrate concentrations day 7 and 106590.</p><p>4. Discussion</p><p>4.1. pH development</p><p>The pH rapidly increased to alkaline in all litter-amended mixtures, which could be explained by thepresence of ammonia formed during mineralization.Subsequently, however, pH values decreased again tovarying degrees; the final measurements were pH 5 inthe peat compost and pH 9 in the softwood compost.The changes in pH during maturation and storage werenot uniform, but diered depending on the litteramendment used. A decrease in pH consequently coin-cided with nitrate formation, and was probably causedby the H-release during nitrification.</p><p>4.2. Nitrate and nitrite formation</p><p>Nitrate formation occurred at dierent times in themixtures investigated. Nitrification started in the straw-and softwood-amended mixtures when the temperaturehad decreased and remained at 20C for several weeks,whereas in the paper-, hardwood-and leaf-amendedmixtures nitrification started at somewhat higher tem-</p><p>peratures. However, at temperatures above 40C nonitrification was observed; which is supported by theliterature (Alexander, 1977).</p><p>A decrease in nitrate concentrations in the softwood-,hardwood-and paper-amended composts between day106 and 177 was caused by immobilization. This wasconcluded because there were no net N losses and thusno denitrification occurred. Furthermore, ammoniumconcentrations were low, indicating a deficiency in am-monium-N. The possible C sources used by the immo-bilizing microorganisms could be cellulose andhemicellulose, initially protected against decompositionby lignin structures in the wood-based litter materials(Berg et al., 1984), but made available due to lignindegradation during the composting process. On day 106,about 33% of the initial lignin had been decomposed inthe softwood- and hardwood-amended mixtures and17% in the paper-amended mixture, as calculated fromdata presented earlier (Eklind and Kirchmann, 2000).Low concentrations of nitrite were present in the leaf-,hardwood- and paper-amended mixtures around day 35.The formation of nitrite coincided in all cases with highpH values. The presence of free NH3 at high pH valuesand the eect of NH3 in inhibiting the second step of thenitrification process (Stevenson, 1986) may explain thetemporary nitrite accumulation.</p><p>4.3. Nitrogen losses</p><p>Initial C to N ratios (Eklind and Kirchmann, 2000)could not explain dierences in N losses between mix-tures; both indicated by a linear correlation analysis aswell as the fact that the C to N ratio was a weak factorin the PCR analysis. Brink (1995) also found that Nlosses were independent of the C to N ratio in foodwaste composts with dierent C sources. However, anegative correlation between initial C to N ratio and Nloss du...</p></li></ul>

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