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

Composting and storage of organic household waste with di�erentlitter amendments. II: nitrogen turnover and losses

Y. Eklind *, H. Kirchmann

Department of Soil Sciences, Swedish University of Agricultural Sciences, P.O. Box 7014, S-750 07 Uppsala, Sweden

Received 31 May 1999; received in revised form 8 December 1999; accepted 14 December 1999

Abstract

Composting of N-rich wastes can be associated with substantial gaseous N losses, which mean loss of an essential plant nutrient

but may also lead to environmental pollution. We investigated nitrogen dynamics and losses in household waste mixtures with

di�erent litter additives during composting, maturation and storage. Standardized, organic household waste was composted mixed

with six litter amendments; straw, leaves, hardwood, softwood, paper and sphagnum peat. Samples were analysed for total and

inorganic N and pH. Both the addition and the type of litter amendment greatly in¯uenced pH changes and formation of nitrate

during composting. Net N losses after 590 days were 43±62% in mixtures with litter additions, being lowest in the peat and the straw

mixtures and highest in the paper mixture, and 70% in the control without litter. A conclusion of the study was that there is no

obvious way to e�ciently decrease N losses during composting through addition of litter materials. Ó 2000 Elsevier Science Ltd.

All rights reserved.

Keywords: Straw; Leaves; Hardwood; Softwood; Paper; Sphagnum peat; Ammonia; Nitrate; pH; Temperature

1. Introduction

Composting of N-rich wastes, such as source-sepa-rated organic household wastes can be associated withsubstantial gaseous N losses, amounting up to 50±60%(Kirchmann and Wid�en, 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.

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 a�ecting 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,

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.

The aims of this study were to quantify gaseous Nlosses from waste mixtures during composting, matu-ration and storage, and investigate the in¯uence 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).

2. Methods

2.1. Materials used and performance of experiment

Standardized, organic household waste was com-posted mixed with six litter amendments; straw, leaves,

Bioresource Technology 74 (2000) 125±133

* Corresponding author.

0960-8524/00/$ - see front matter Ó 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

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).

2.2. Chemical analyses

Total N in the mixtures was measured by the Kjel-dahl method (Kjeltec, Tecator, Sweden), (Bremner andMulvaney, 1982). In samples containing signi®cantamounts of nitrate and/or nitrite, total N was deter-mined by a modi®ed 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 105°C for 24 h. The pHwas determined in water extracts (1:5 by volume) fromfresh samples after shaking for 1 h, by standard meth-ods.

2.3. Calculations and statistical analysis

Losses of N between di�erent sampling occasionswere calculated as the di�erence 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 Nñs, 1989) wereperformed with the software Unscramblerâ (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 in¯uence of each

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 >45°C,temperature sum (�P[compost temperature ) ambienttemp.] day 1±60), values on the 9 sampling occasions ofpH, C to N ratio, `àvailable C'' (total organic C ) ligninC) to N ratio, and concentrations of NH�4 ±N, NOÿ3 ±N,total N, lignin, cellulose and hemicellulose (Eklind andKirchmann, 2000). Full leave-one-out cross validationwas used to ®nd the optimal number of principal com-ponents (Martens and Nñs, 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 coe�cients of regression (Bw-coef-®cients) were successively excluded to ®nd the bestmodel.

3. Results

3.1. Temperature and pH

The mixtures containing leaves, hardwood, softwoodand paper reached maximum temperatures of 65±70°Cwithin 5 days, the peat mixture reached 58°C, but themaximum temperatures of straw mixture and controlwere only 40±50°C (Fig. 1). For the litter-amendedmixtures (control excluded), there was a signi®cantcorrelation (p < 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 ¯uctuation seen in Fig. 1 was caused by tem-perature decline in connection with weighing andsampling. The daily measurements showed thermophilictemperatures (>45°C) 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.

Initial pH values were low in the experimental mix-tures, 4.5 in the peat-amended one and 5.1±5.5 in theothers. In the mixtures with litter amendments, pHvalues rose initially, reaching 7 or more within 1±2weeks (Fig. 2). However, in the peat mixture, pH valueshad decreased again after 70 days to 5.5, and ®nally to4.8. Also in the straw-amended mixture, pH values

126 Y. Eklind, H. Kirchmann / Bioresource Technology 74 (2000) 125±133

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.

3.2. Total and inorganic N

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).

At the beginning of the composting period, there wasa peak in ammonium-N concentration in the mixtures,

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 signi®cant 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 gÿ1 dry matter).Small, and only temporary amounts of nitrite-Nwere present in leaf-, hardwood- and paper-amended

Fig. 1. Temperature development in the household waste mixtures.

Y. Eklind, H. Kirchmann / Bioresource Technology 74 (2000) 125±133 127

mixtures, with the highest levels of 0.14±0.5 mg N gÿ1

dry matter (0.8±2.8% of total N), on day 35.

3.3. Nitrogen losses

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 ®t to simple functions was possible.

None of the tested process-related variables or de-scriptors of initial organic matter quality (i.e. initial C toN ratio, `àvailable C'' [total C ) lignin C] to N ratio,lignin concentration, etc.) were signi®cantly correlated(p > 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).

In a PCA projection with all waste mixtures (8 ob-jects) and 50 ®nally selected variables, two signi®cantcomponents were extracted, explaining 76% in total ofthe variance in this matrix (Fig. 5). The variables were:

Fig. 2. pH development in the household waste mixtures.


net C loss; pH day 7±35, 590; NH�4 ±N day 106±590;NOÿ3 ±N day 0, 7, 106±590; total N day 0, 7, 70, 106, 590;lignin day 0±590, cellulose day 0±14; hemicellulose day0±106, C to N day 0, 7, 21±177; `àvailable C'' to N day0, 70±177. 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.

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 coe�cientsof regression: pH day 590, hemicellulose concentrationsday 0±106, and nitrate concentrations day 7 and 106±590.

4. Discussion

4.1. pH development

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 ®nal 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 di�ered depending on the litteramendment used. A decrease in pH consequently coin-cided with nitrate formation, and was probably causedby the H�-release during nitri®cation.

4.2. Nitrate and nitrite formation

Nitrate formation occurred at di�erent times in themixtures investigated. Nitri®cation started in the straw-and softwood-amended mixtures when the temperaturehad decreased and remained at 20°C for several weeks,whereas in the paper-, hardwood-and leaf-amendedmixtures nitri®cation started at somewhat higher tem-

peratures. However, at temperatures above 40°C nonitri®cation was observed; which is supported by theliterature (Alexander, 1977).

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 denitri®cation occurred. Furthermore, ammoniumconcentrations were low, indicating a de®ciency 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 e�ect of NH3 in inhibiting the second step of thenitri®cation process (Stevenson, 1986) may explain thetemporary nitrite accumulation.

4.3. Nitrogen losses

Initial C to N ratios (Eklind and Kirchmann, 2000)could not explain di�erences 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 di�erent C sources. However, anegative correlation between initial C to N ratio and Nloss during decomposition of manure plus straw hasbeen reported (Kirchmann, 1985; Kirchmann andWitter, 1989). Probably, it is when the same C source isused in increasing proportions that a strong negativecorrelation between initial C to N ratio and N losses is

Table 1

Changes in C to N ratios, contents of total N, organic N (total N ) inorganic N), ammonium-N and nitrate-N in the household waste mixtures after

590 days

Type of

compost

C to N ratio Total N

(% of dm)

Organic N

(% of dm)

NH�4 ±N

(% of dm)

NOÿ3 N

(% of dm)

pH (H2O)

Initial Final Initial Final Initial Final Initial Final Initial Final Initial Final

Straw 28 6 1.4 4.1 1.35 3.18 0.04 0.02 0.01 0.90 5.4 5.9

Leaf 22 9 1.7 2.8 1.65 2.34 0.05 0.00 0.00 0.46 5.3 7.8

Softwood 32 18 1.3 2.0 1.28 1.99 0.02 0.01 0.00 0.00 5.2 7.0

Hardwood 34 14 1.2 2.4 1.18 2.00 0.02 0.00 0.00 0.40 5.4 8.9

Paper 30 12 1.3 2.2 1.28 1.88 0.02 0.01 0.00 0.34 5.5 8.8

Peat 28 20 1.5 2.0 1.47 1.64 0.03 0.00 0.00 0.41 4.5 4.9

Control 13 5 2.8 3.8 2.75 3.52 0.04 0.03 0.01 0.25 5.1 8.3


found. In the present investigation, `àvailable C'',de®ned as [total organic C ) lignin C] to N ratio in thepreceding paper (Eklind and Kirchmann, 2000), wasalso found to be a weak predictor of N losses in the PCRanalysis. However, C quality measured as hemicelluloseconcentration was a strong variable in predicting Nlosses. Mote and Gri�s (1980) used three solubleC fractions, extracted with cold water, 0.1N or 6N HCl,respectively, to characterize the microbial availability ofC in di�erent waste mixes, but none of the fractions werefound to be suitable predictors for how well a materialwould compost. Thus, C quality seems to be one factordetermining N losses. However, the chemical methods

available that enable the analysis of the microbialavailability of a waste material are not su�cient to coverlonger periods of decomposition.

Nitrogen losses were lowest in the peat and straw-amended mixtures. Brink (1995) also reported lower Nlosses from food waste composted with straw as anadditional carbon source than from food waste com-posted with waste paper. In the peat-amended mixture,low pH values during most of the experimental period,and the high ammonium- and ammonia-adsorbing ca-pacity of peat (Witter and Kirchmann, 1989), may ex-plain the relatively lower N losses. The straw-amendedmixture had a lower maximum temperature, and inter-

Fig. 3. Ammonium- and nitrate-N (as a percentage of total N) during composting, maturation and storage.


mediate pH values (5.4±8.4). In addition, the lignincontent was low indicating relatively high C availability.The softwood-, hardwod-, paper- and leaf-amendedmixtures had higher maximum temperatures (64±71°C)and high pH values dominated during the experimentalperiod. It is most likely that these factors together withthe relatively low contents of available C due to highlignin contents, contributed to higher N losses in thesemixtures. The control without additional C source re-leased most gaseous N. The low ratio between availableC and N, which meant an excess of N compared to theavailable C was the most probable reason.

5. Conclusions

The conclusion from this study is that both the ad-dition, as well as the type of litter amendment used withorganic household waste greatly in¯uence the pHchanges and formation of nitrate during composting,maturation and storage. Nitrogen losses were lowest inthe peat- and straw-amended waste mixtures, and fromthis aspect can be recommended as litter amendmentsfor use in composting. However, since the losses were atleast 40% of initial N present, there is no obvious way toe�ciently decrease N losses during composting and

Fig. 4. Accumulated N losses (as a percentage of initial N) during composting, maturation and storage.


compost maturation through addition of litter amend-ments.

Acknowledgements

The Swedish Council for Forestry and AgriculturalResearch provided ®nancial support for this investiga-tion. We would like to thank Sta�an Bengtsson and BoStenberg for help with carrying out the multivariatestatistical analyses, and Mikael Pell and Ernst Witter forvaluable comments on the manuscript.

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