Nitrogen and organic matter losses during storage of

Journal of Agricultural Science, Cambridge (1998), 130, 69–79. Printed in the United Kingdom # 1998 Cambridge University Press

69

Nitrogen and organic matter losses during storage of cattle and pig manure S. O. P E T E R S EN*, A.-M. L I N D    S. G. S O M M E R Department of Soil Science, Danish Institute of Agricultural Sciences, Research Centre Foulum, DK-8830 Tjele, Denmark (Revised MS received 5 June 1997)

SUMMARY Solid pig manure (240 g kg−" DM) and solid cattle manure (150–180 g kg−" DM) were stored in an open storage facility during spring–summer and autumn conditions for periods of 9–14 weeks during 1994 and 1995. Concentrations of C, N, P and K were determined prior to and after storage, corrected for dry matter losses and distance from the surface. Temperature and, in experiments with pig manure, gas phase composition inside the manure heap were monitored during storage. Nitrogen losses as ammonia volatilization, nitrous oxide emission and leaching were measured, while total denitrification was estimated from mass balance calculations. For both cattle and pig manure there was little difference between seasons with respect to the pattern of decomposition, as reflected in temperature dynamics and C}N turnover. In contrast, there was a distinct difference between manure types. Pig manure was characterized by maximum temperatures of 60–70 °C, although the concentrations of oxygen and methane clearly demonstrated that anaerobic conditions dominated the interior parts of the heap for several weeks. Losses of C and N from pig manure both amounted to c. 50 %. In contrast, the temperature of cattle manure remained close to the air temperature throughout the storage period and cattle manure had lower, not significant losses of C and N. Leaching losses of N constituted 1–4 % with both manure types. Ammonia volatilization from cattle manure constituted 4–5 % of total N, and from pig manure 23–24 %. In pig manure a similar amount of N (23–33 %) could not be accounted for after storage, a loss that was attributed to denitrification. Nitrous oxide emissions amounted to ! 2 % of estimated denitrification losses. INTRODUCTION In many animal production systems, excreta are handled as solid manure, which is stock-piled for extended periods prior to field application. During storage the turnover of organic matter and nutrients may change the manure composition significantly (McCalla et al. 1970 ; Eghball et al. 1997). The loss of dry matter (DM) during storage may be considerable. Soluble lipids, carbohydrates, organic acids and proteins are degraded within a few days, while subsequent degradation will be limited by enzymatic or chemical hydrolysis of particulate organic material (Egli 1995). Temperature plays a key role for carbon turnover during storage ; temperatures of 60–70 °C may be reached depending on the intensity of mineralization. The physical structure of the manure will influence the magnitude and direction of biodegradation activity in stored manure, since the porosity affects both * Email : sop!pvf.sp.dk

the oxygen supply and the potential for removal of volatile compounds. Kirchmann (1985) distinguished between three types of storage conditions for solid manure, i.e. practically anaerobic fermentation, both aerobic and anaerobic decomposition, and mainly aerobic decomposition (composting). Nitrogen is the nutrient that is most susceptible to transformations affecting plant availability upon field application. These transformations include mineralization, immobilization, nitrification and denitrification, as well as leaching and ammonia volatilization. A nitrogen loss of 10–40 % is typical during the storage of animal manure (Kirchmann 1985 ; Eghball et al. 1997). Nitrogen losses to the atmosphere reduce the nutrient value of the manure, but they may also represent an environmental hazard. Ammonia can cause acidification or disturb natural ecosystems through deposition (Pearson & Stewart 1993), while nitrous oxide (N O) released from nitrification or # denitrification has a high global warming potential and is involved in the depletion of stratospheric ozone (Bouwman 1990).

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70

In this study, solid pig manure and solid cattle manure were stored under different climatic conditions. Nitrogen losses during storage were described and quantified, and C and N transformations in solid pig manure were investigated in some detail.

B1

E1

B2

MATERIALS AND METHODS Cattle manure was obtained from a 50-head dairy farm, while the pig manure came from a farm producing 450 pigs for slaughter per year. Both farms used straw as bedding material and stored the solid manure in top-loaded cone-shaped heaps. Care was taken to collect the manure as fresh as possible, and probably no part was more than a week old. Four individual storage experiments with a duration of 9–14 weeks were carried out between April 1994 and December 1995 (see Table 1). Hence, both cattle (C) and pig (P) manures were stored under spring–summer (S) and autumn (A) conditions, respectively. In the following, the four experiments will be referred to as CS, CA, PS and PA, respectively. The experimental storage facility was located in an open field with c. 200 m to the nearest landscape element. It consisted of a circular sealed area (diam. 5±1 m, height of edge 0±5 m) with an outlet to a 130litre reservoir for collection of leachate, which was subsampled and emptied when required. The manure heaps were flattened with an initial height of c. 125 cm at the centre and a diameter of 4–5 m. Problems with blockage of the outlet during the first experiment were met by installing a base of slatted boards, and by reducing the amount of manure. Ammonia volatilization Ammonia volatilization was measured with the mass balance method using passive flux samplers as described by Schjørring et al. (1992). A flux sampler

E2

B E

Fig. 1. Ammonia losses from manure heaps were determined with a mass balance technique using passive flux samplers. Each flux sampler (top), which consists of pairs of acidcoated glass tubes facing towards (E , E ) and away from the " # manure (B , B ), were mounted on masts (bottom, left) that " # were placed around the circular storage facility (bottom, right).

consisted of two parallel units, each composed of two 100 mm and one 23 mm glass tubes connected by silicone tubing (see Fig. 1, top). To the free end of the 23 mm tube, a stainless steel disc with a central hole of 1 mm diameter was attached, which reduced the wind speed and increased turbulence inside the tubes. The 100 mm glass tubes were coated with oxalic acid for absorption of NH . Flux samplers directed $ towards the centre of the sealed area were placed on

Table 1. Cattle manure (C ) and pig manure (P) was stored during spring–summer (S ) and autumn (A). The table presents storage period, climatic conditions, amount, dry matter (DM ) content, total N, carbon : nitrogen ratio (C : N ), total ammoniacal nitrogen (TAN ), P and K concentrations of the four storage experiments. The data on manure composition represent mean³S.E. (D.F. ¯ 5) Manure source (Expt) Cattle (CS) Cattle (CA) Pig (PS) Pig (PA)

Storage period 18 Apr. 94 –20 Jun. 94 10 Oct. 95 –13 Dec. 95 31 May 95 –04 Sep. 95 05 Oct. 94 –03 Jan. 95

Mean air temperature Precipitation (°C) (mm)

DM Amount (tonnes)

10±0

49

12±4

4±7

68

3±5

16±9

53

3±75

5±4

159

3±75

Total N

C:N

TAN

P

K

1±4 (0±03) 1±5 (0±08) 3±9 (0±14) 3±5 (0±30)

3±8 (0±15) 3±9 (0±26) 5±4 (0±37) 5±1 (0±10)

(kg t−" fresh weight) 151 (2±8) 178 (2±6) 246 (8±6) 241 (9±1)

5±3 (0±08) 6±7 (0±24) 11±5 (0±19) 11±9 (0±41)

12±0

1±4*

10±4

1±7 (0±13) 1±3 (0±05) 5±1 (0±25)

8±2 8±9

* Estimated.


71

Nitrogen and OM losses from stored manure II

(a)

I

III

IV

50 (b)

(c)

9

40 Methane (l1 1–1)

20 15 10

8

30 20

3 10

5

0

0

0 0

20

40

60

Time (min)

Nitrous oxide (l1 1–1)

25

Oxygen (%)

V

0

100

200

300

Time (min)

Fig. 2. (a) A closed, recirculated system for gas sampling inside a manure heap was tested (b) for oxygen diffusion with (D) and without (E) the gas permeable silicone tubing, and (c) for CH (_) and N O (^) diffusion. I, Silicone tubing ; II, % # perforated plastic support ; III, gas-tight nylon tubing ; IV, septum ; V, diaphragm pump.

masts at 25, 75, 150 and 250 cm height above the edge of the tank, respectively, and corresponding height intervals of 0–50, 50–112±5, 112±5–200 and 200–300 cm height were defined for the calculations of NH fluxes. $ Four masts were placed with 90° spacing around the tank (see Fig. 1, bottom). Accumulated NH losses within a measuring period $ were determined by stepwise summation of the net flux from each height interval (Schjørring et al. 1992) : 1 h=n ∆h (1) 3F x h= net, h " where ∆h is height interval (m), n is the number of heights, and x is the tank diameter (m). The horizontal net flux at height h ( µg NH ®N m−# s−") is defined as : $ m=% Fnet(h) ¯ 3 (Fhm, e®Fhm, b) (2) m=" where Fhm, e and Fhm, b is the NH flux through exposed $ and background tubes, respectively, at each mast (m), and : Fv ¯

Fhm ¯

(C C )V " # 2 π r# K ∆t

(3)

Here, C and C are the NH + concentrations ( µg l−") " # % in the two tubes facing in the same direction, V is the extraction volume (0±003 litre), r is the radius of the hole in the steel disc, ∆t is the time (s) of the measurement period, and K is a correction factor (K ¯ 0±77 ; see Schjørring et al. (1992)).

Flux samplers were mounted on the four masts immediately after a manure heap was established. The flux samplers were replaced after 8 h, and then at increasing intervals. Ammonia volatilization was measured continuously for 30–45 days, and then occasionally during 24–48 h periods in the remaining part of the four storage experiments. Volatilization rates were always very low during this latter phase. Temperature Temperature sensors (thermocouples Pt 100 ; Kontram A}S) were installed near the centre of the heap at c. 25, 50, 80 and 110 cm height. The sensors were connected to a datalogger (DataTaker DT600, Data Electronics Ltd, Australia) recording 4-hourly means of temperature measurements every 5 min. Climatic data were obtained from a meteorological station located ! 500 m from the storage facility. Gas phase composition The composition of the gas phase inside the manure heap was monitored in Expt PS to further characterize the decomposition processes in solid pig manure. Holter (1990) described a technique for gas sampling in dung pats using gas-permeable silicone tubes. In the present study this approach was modified to enable gas sampling from the centre of the manure pile at the depths indicated above for the temperature measurements (see Fig. 2 a). The two ends of a 1 m length of silicone tubing (i.d. 6 mm ; wall thickness, 1 mm) were connected to two 3 m lengths of gas-tight


72

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nylon tubing (i.d. 4±5 mm), which were in turn connected to a diaphragm pump (Model 5002, ASF GmbH, Germany). A septum for gas sampling was located immediately after the diaphragm pump. Connections between nylon tubes, pump and septum consisted of gas-tight Novopren (ASF GmbH, Germany). The silicone tubing was protected against compression by a flexible, but rigid, plastic tube containing four 1-mm diameter holes per cm. The air inside the closed system was equilibrated by recirculating at a rate of 2±5 l min−". Rates of gas exchange in the recirculated system were examined prior to the storage experiments. For O and CO measurements, gas samples were # # taken in 1 ml polypropylene syringes ; the needles were inserted into a butyl rubber stopper and transported to the laboratory for analysis within 2 h. For CH and N O, 1±2 ml gas samples were transferred % # to pressure-equilibrated 3±5 ml Venoject vacutainers and analysed within a week ; due to technical problems the CH analyses were interrupted after 1 month. % N O emissions # Nitrous oxide fluxes across the manure–air interface were examined in the storage experiments with pig manure (PS and PA) using an open chamber technique (Husted 1993). Metal frames (20¬70 cm, 20 cm high) for two chambers were driven into the manure to 15 cm depth at the start of the experiment, and perspex covers (height, 30 cm) were mounted onto the frames during measurements. Air intake for the chambers occurred just above ground level upwind relative to the tank ; a fan ensured complete mixing of the air volume inside the chambers. Air was drawn through the chambers at a rate of 1±5 l min−", and then through a water trap with Drierite, prior to sampling. Prior to the storage experiments the chambers were sampled at 10–20 min intervals for 3 h to determine the time required to obtain a steady state between N O flux rate and air flow ; the time required # was 2 h (data not shown). At steady state, gas samples (4±2 ml) were taken at the air intake and at the outlet from the chambers (before the pump) and transferred to 3±5 ml Venoject vacutainers which were analysed within 7 days. Background air was sampled at 2 m height. All samplings were done in triplicate. Nutrient composition During the establishment of a manure heap, 6–8 manure subsamples were withheld and frozen for later analysis. By the end of Expts CA, PS and PA, two samples were taken from the four heights where temperature sensors and gas sampling tubes were installed, by pooling several subsamples from an area of c. 1 m#. All manure samples were analysed for dry weight, ash content, total C, Kjeldahl N, TAN (total

h I II III IV

110 80 50 25

r

Fig. 3. The experimental manure heaps were divided into four zones (I–IV) for calculation of volume-adjusted nutrient recoveries. By the end of each experiment manure subsamples were removed from 25, 50, 80 and 110 cm height, respectively, for chemical analyses.

ammoniacal nitrogen), NO −, P and K. The ash $ content was considered to be conserved and was used for correction of C, N, P and K, thus providing a basis for estimating nutrient losses during storage (Dewes 1995). The leachate was analysed for Kjeldahl N, NH + and NO −. % $ Volume-adjusted recoveries of C, N, P and K in the manure were calculated by dividing the heap into four zones as indicated in Fig. 3. The volumes of the four zones were calculated on the basis of the equation : V ¯ π}3 h#(3r®h)

(4)

where V is the volume and h the height of the section of the sphere included, and r is the sphere radius (see Fig. 3, insert). The maximum radius was set to 3 m. Analytical techniques Kjeldahl N was determined using a Kjeltec 1030 Analyzer (Tecator, Ho$ gana$ s, Sweden). TAN and NO − in leachate and solid manure was extracted in $ 1  KCl for 30 min, filtered and frozen for later analysis, while NH trapped in the acid-coated $ sections of the flux samplers was dissolved in 3 ml distilled water. Inorganic N was analysed with a QuickChem 4200 flow injection analyser (Lachat Instr, Wisconsin, USA). Dry matter (DM) content was determined after drying at 105 °C for 24 h, and ash content after 4 h at 550 °C. Total C was determined after dry combustion (Leco model 521–275), K by flame photometry (FLM3, Radiometer) after dry ashing and solubilization in acid, and P was measured colorimetrically (Spectronic 1001, Bausch & Lomb) after reaction with ammonium molybdate vanadate. Oxygen and CO were measured on a Varian 3700 # gas chromatograph with thermal conductivity detector. It was equipped with a 2 m¬"§-column with ) molecular sieve for O and a 1 m¬"§ column with # ) Porapak N for CO . The carrier was He at a flow rate # of 40 ml min−", the temperatures of oven and detector were 30 and 190 °C, respectively. Nitrous oxide was measured on a Varian 3300 GC with electron capture detector. The 1 m¬"§ column contained Porapak T, ) Ar}CH (95 %}5 %) was used as carrier (50 ml min−"), %


73

Nitrogen and OM losses from stored manure

RESULTS Methodology The mass balance method for quantification of NH $ losses from the experimental storage facility has been validated in trials with (NH )HCO solutions % $ (Sommer et al. 1996). During periods with high fluxes, NH was in some cases carried over from the $ exposed tube to the background tube at the lower heights ; combining the NH in the background with $ the NH absorbed in the exposed tube yielded a good $ agreement between measured losses and those determined by mass balance calculations (Sommer et al. 1996). Carry-over was also observed in a few cases in the present study. The recirculated system for gas sampling in the manure heap (Fig. 2 a) was tested for leakages and for the time course of gas exchange. When the silicone tubing was left out and He recirculated in a closed circuit with nylon tubing, no air (i.e. O ) entered the # system (Fig. 2 b). In contrast, when the silicone tubing was included the O concentration in the recirculating # air reached equilibrium with atmospheric air within 40 min (Fig. 2 b). The diffusion rate of N O across the # silicone tubing was similar to that of O (Fig. 2 c ; note # different time scale), while the diffusion of CH was % relatively slow ; after 300 min the concentration inside the tubing was still higher than the background concentration of 1±8 µl l−" CH (Fig. 2 c). Diffusion of % CO across the silicone tubing was not examined ; # Holter (1990) found the rate constant for this gas to be higher than for O . To ensure near-equilibrium for # all gases it was decided to recirculate the air continuously. Manure composition Some characteristics of the fresh manure batches are given in Table 1. Nutrient composition of both cattle and pig manure was typical for Danish conditions (Kjellerup 1989). The C : N ratio of both pig and cattle manure was relatively low despite the differences in dry matter content (Table 1). The higher amounts of bedding material in the pig manure may have increased the absorption of urine in the stable. The two batches of pig manure had similar concentrations of total N, but differed with respect to the fraction accounted for by TAN which constituted 11 and 44 % in Expts PS and PA, respectively. Table 1 also specifies time periods and climatic conditions for the individual storage periods. The

3

NH3 loss (kg N t–1 manure)

and temperatures of oven and detector were 45 and 400 °C, respectively. Finally, CH was measured on a % Varian 3300 GC with flame ionization detector and a 2 m¬"§ column packed with Porapak T, N was used ) # as carrier (20 ml min−"), and temperatures of injection port, oven and detector were 120, 50 and 240 °C, respectively.

2

1

0 0

10

20

30

40

Time (days) Fig. 4. Accumulated N losses due to NH volatilization $ during the first 30–45 days of the four storage experiments with pig manure in autumn ( ) ; pig manure in spring (——), cattle manure in autumn (– – –), and cattle manure in spring (– – –).

year 1995 was extremely dry, and Expts CA and PS received ! 50 % of the normal precipitation for the given periods (Jensen & Sørensen 1995). Ammonia losses In Fig. 4, accumulated losses of NH during the $ continuously monitored periods are presented. The loss rates generally decreased after 1–2 weeks. The difference in NH volatilization between cattle manure $ (! 0±5 kg N t−") and pig manure (2±5–3 kg N t−") was far greater than the difference in total N (see Table 1). In contrast, there was little difference between seasons for either manure type. It is notable that accumulated NH losses in Expts PS and PA were very similar, $ although the initial TAN pool in Expt PS was only 25 % of the TAN pool in Expt PA (see Table 1). N leaching Figure 5 presents the leaching of total N and TAN as a function of accumulated leachate. Total N in the leachate was not determined in Expt CS where the leaching of TAN was approximately proportional to the amount of leachate. In the other three storage experiments N leaching apparently followed a twophased pattern, and the fraction of TAN in leachate N decreased with time (Fig. 5). The shift in N leaching rate occurred after about 1 week in Expt CA and after c. 2 weeks in Expts PS and PA, i.e. around the times where NH volatilization rates also dropped (see $ Fig. 4).


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0·3 (a)

(b)

0·2

N leached (kg N t–1 manure)

0·1

0·0 0

200

400

0

300

600

900

0·3 (c)

(d)

0·2

0·1

0·0 0

200

400

600

0

400

800

1200

Leachate (litres) Fig. 5. Accumulated leaching of total N ( ) and total ammoniacal nitrogen (——) as a function of leachate volume in the four manure storage experiments ; (a) Expt CS – cattle manure, spring ; (b) Expt CA – cattle manure, autumn ; (c) Expt PS – pig manure, spring ; (d ) Expt PA – pig manure, autumn.

Concentrations of NO − in the leachate generally $ accounted for ! 0±2 % of total N in the leachate (data not shown). The only exception was at the final sampling in Expt PS, where NO −-N constituted 40 % $ of the N leached, corresponding to 2±7 g N t−" manure, following a period of very dry weather. Table 2. Average temperature difference (∆ °C ), at four heights, between the manure heap and the surrounding air, based on recordings every 4 h. Cattle manure (C ) and pig manure (P) were stored during spring–summer (S ) and autumn (A) Height (cm) Manure source (Expt) Cattle (CS) Cattle (CA) Pig (PS) Pig (PA)  ¯ not determined.

25

50

80

110

3±2  24±0 17±6

2±3 6±2 27±0 24±7

1±2  25±4 20±8

0±0  19±6 14±9

Temperature The temperature inside the manure heaps was monitored at about 25, 50, 80 and 110 cm height except in Expt CA, where only one height (50 cm) was included. Heat derived from the decomposition processes will increase the temperature inside the manure heap, although the temperature at a given depth is also a function of the heat removal via air exchange. The accumulation of heat in the four experiments is illustrated in Table 2, which shows the average temperature difference at each height relative to the outside temperature as calculated on the basis of the 4-h recordings. Although the temperature difference decreased towards the surface, pig manure was able to maintain a considerable temperature elevation even at 10–15 cm distance from the manure surface. There was little heat production, i.e. biodegradation activity, in the cattle manure. The temporal patterns of temperature change inside the manure storages are presented in Fig. 6, which shows the recordings at 50 cm height in all four storage experiments. While the pig manure went


75

Nitrogen and OM losses from stored manure Dry matter and nutrient losses Temperature (°C)

60

40

20

0 0

10 20 30 Time (days)

40

Fig. 6. The temperature at 50 cm height inside the manure heaps during the first 6 weeks of storage. Pig manure was characterized by high temperatures compared to cattle manure. Pig manure in autumn ( ), pig manure in spring (——), cattle manure in autumn (– – –), cattle manure in spring (– – –).

through a temperature cycle with maximum temperatures of 60–70 °C, the cattle manure remained at temperatures of 5–25 °C, i.e. close to the air temperature.

The loss of nutrients from solid manure during storage cannot be determined on a DM basis as significant DM losses may occur simultaneously. To overcome this problem, it was assumed that the ash content of the manure was conserved during storage, and nutrient concentrations were corrected relative to this value. Profiles of C, N, P and K from Expt PS are shown in Table 3. Volume-adjusted recoveries of P and K in Expts CA, PS and PA were 96–112 and 78–86 %, respectively. The close agreement between P profiles before and after storage suggests that the loss of this nutrient was insignificant, or at least proportional to any loss of ash content. In contrast, the C and N content was reduced by 48–49 and 48–57 %, respectively, in pig manure. In cattle manure (Expt CA), C and N losses averaged 13 and 21 %, respectively, but the variability between manure subsamples was relatively high and the C and N losses were not statistically significant (P " 0±5). All data on N pools and losses are combined in Table 4. Leaching constituted 1–4 % of total N at the start of the four experiments, while NH losses from $

Table 3. Changes in nutrient composition of solid pig manure during 3 months of storage in spring–summer (Expt PS ), expressed with reference to the initial dry matter (DM ) content (numbers in parentheses represent S.E., D.F. ¯ 5 (initial ) or 1 (end of storage)) C

N

K

P

(kg t−" DMinitial) Initial concentration End of storage 110 cm height 80 cm height 50 cm height 25 cm height

407 (2±2)

48±1 (0±92)

22±0 (0±76)

15±8 (0±44)

165 (1±5) 200 (3±1) 299 (2±7) 317 (2±9)

19±9 (0±77) 23±9 (0±32) 34±0 (0±47) 37±8 (0±13)

17±7 (1±17) 19±8 (0±04) 24±4 (1±60) 19±9 (0±97)

16±7 (0±59) 17±3 (0±26) 16±0 (0±12) 17±1 (0±57)

Table 4. Mass balances for nitrogen in the four storage experiments (numbers in parentheses are pool sizes in % of total N at start) Total N at start

Total N after storage

Manure source (Expt) Cattle (CS) Cattle (CA) Pig (PS) Pig (PA)

Leaching loss

Ammonia volatilization

Denitrification*

0±2 (4) 0±3 (5) 2±6 (23) 2±8 (24)

— 0±8 (13) 2±6 (23) 4±0 (33)

(kg N t−" manure) 5±3 6±7 11±5 11±9

 5±3 (78) 6±2 (52) 5±0 (43)

0±12 (2)† 0±28 (4) 0±10 (1) 0±14 (1)

 ¯ not determined. * Calculated ; includes nitrous oxide. † Estimated from the ratio between total ammoniacal nitrogen (TAN) and total N in Expt CA (cattle manure stored in autumn).


. .       , .-.        . .      

76 60

by sampling the gas phase at 25, 50, 80 and 110 cm height (Fig. 7). The concentration of O was reduced # even at 10–15 cm distance from the surface during the initial 3–4 weeks. At 25 and 50 cm depth there was generally ! 1 % O for several weeks, followed by a # gradual increase towards atmospheric concentrations. Maximum CO concentrations of " 20 % were # initially observed at 80 and 110 cm height ; these concentrations probably reflected both convective transport from deeper layers and in situ production. At the centre of the manure heap CO accumulated to # a maximum of almost 60 % after one week. The CO # concentration subsequently dropped, although it stabilized at a level of 10–20 % at the lower heights throughout the experiment. Carbon dioxide was partly replaced by CH , the concentration of which % increased to a maximum of c. 45 % at the centre after 4 weeks, where measurements were interrupted due to analytical problems.

(a)

40 20 0

Concentration (%)

60

(b)

40 20 0 60

(c)

40 20 0 60

(d)

40 20 0 0

20

40 60 80 Time (days)

100

Fig. 7. The gas phase composition inside the manure heap was monitored in Expt PS (pig manure stored under spring–summer conditions) at (a) 110, (b) 80, (c) 50 and (d ) 25 cm height. O (– – –), CO ( ) and CH (——). # # % CH measurements were interrupted after one month. N2O concentration N2O flux ( ll l–1) (mg N m–2 d–1)

%

30

(a)

20 10 0 (b)

300 200 100 0

Nitrous oxide emissions The flux of N O from pig manure varied between 0 # and 25 mg N m−# d−" in Expt PS (Fig. 8 a), while it was only 0–1 mg N m−# d−" in Expt PA (data not shown). The two N O peaks recorded both coincided # with an 8–10 °C increase in air temperature, although a similar temperature increase after 3 weeks did not stimulate N O emission. Assuming linear rate changes # between the N O fluxes recorded, the accumulated # loss of N O-N constituted 0±5–2 % of the N loss # attributed to denitrification in Expt PS. Figure 8 b shows N O concentrations inside the manure heap at # the two upper gas sampling sites. The concentration maxima (day 57 and day 78) only partly coincided with maximum fluxes (day 42 and day 61). DISCUSSION

0

20

40 60 80 100 Time (days)

Fig. 8. The time course of nitrous oxide fluxes from the manure surface was followed in experiments with pig manure stored during autumn (not shown) and (a) spring using two open flow chambers ( , ——). The bottom panel (b) shows the corresponding nitrous oxide concentrations at 80 (– – –) and 110 cm (– – –) height inside the manure heap.

cattle manure constituted 4–5 %, and from pig manure 23–24 %. The N losses not accounted for constituted 13, 23 and 33 % of initial total N in Expts CA, PS and PA, respectively. Gas phase composition in stored pig manure Additional information about the decomposition process in stored pig manure (Expt PS) was obtained

Storage of manure normally precedes field application, and nutrient losses occurring during this period should be minimized or, alternatively, compensated for by proper adjustment of manure or mineral fertilizer application rates. In either case, more detailed recommendations to farmers about manure handling practices require knowledge of the influence of storage conditions and manure type on loss mechanisms. The difference in DM content between cattle manure (15–18 %) and pig manure (24 %) in this study was more pronounced than has typically been observed for Danish conditions, where the corresponding figures averaged 20 and 22 % (Kjellerup 1989), and this may have accentuated the differences between the two manure types with respect to the subsequent turnover during storage. The storage of pig manure was characterized by high temperatures (Fig. 6) and high C and N loss rates


Nitrogen and OM losses from stored manure (Table 3). The difference in temperature regime between cattle and pig manure agrees with a Swedish survey reporting that only four out of 83 dairy farms had solid manure storages with temperatures of 48 °C or more, while this was the case for 27 out of 34 pigletproducing farms (Forshell 1993). Heat production in stored cattle manure can be triggered by increasing the amount of straw in solid manure to 2±5–3 kg straw cow−" d−" (Iversen & Dorph-Petersen 1949 ; Forshell 1993). Methane concentrations gradually increased at the centre of the manure heap (Fig. 7) ; the initial CH % production was probably limited by the population density of methanogenic bacteria (Husted 1994). The accumulation of CH and the low concentrations of % O indicated that the interior of the heap was partly # anaerobic during a significant part of the storage period. Obviously temperature per se does not reveal whether composting, defined as aerobic biodegradation, predominates. Nitrogen was lost from all layers of solid pig manure during storage (see Table 3). This must be mainly due to atmospheric losses, since leaching could only account for a small proportion of the N loss. A similar distribution between atmospheric and leaching losses of N has been observed in other studies (Martins & Dewes 1992 ; Ule! n 1993). A shift in N leaching rate was suggested after 1–2 weeks (Fig. 5). This rate change may have been due to depletion of an initial pool of leachable N, while subsequent leaching losses reflected the accumulation of N released by mineralization activity. Nitrate was only found in the leachate on one occasion, after a period of dry weather ; this absence of NO − is in accordance $ with previous observations with storage of stockpiled and composting manure (Martins & Dewes 1992 ; Atallah et al. 1995 ; Eghball et al. 1997). The composition of N in the leachate did not necessarily reflect the occurrence of nitrification activity, since the coupling between oxidation and reduction of N may be very close (Petersen et al. 1992). Ammonia volatilization was highest during the first 2–3 weeks of storage (Fig. 4). The gaseous products of microbial activity, and the heat production associated with this activity, resulted in a pressure gradient that gave a strong convective transport of gases towards the surface of the manure heap. Since the vapour pressure of NH increases by 40–60 % for every 10 °C $ increase in temperatures between 0 and 70 °C (Schlessinger 1972), NH could potentially have been lost $ from all parts of the manure, and not just from the surface layer that was directly exposed to the atmosphere. In Expts PS and PA, TAN constituted 11 and 44 % of total N, respectively (Table 1). The higher initial temperature of Expt PA (Fig. 6) indicated that microbial activity was more intense in this batch of manure than in Expt PS, which implies that some N

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mineralization had already taken place at the time of collection. This is supported by the course of NH $ volatilization in the two experiments (Fig. 4), which was delayed in Expt PS but eventually reached the same relative N loss as in Expt PA. Hence, the manure used in Expts PS and PA mainly differed with respect to the course of N mineralization. The N mass balances in Table 4 suggested that denitrification and NH volatilization was of similar $ magnitude. The denitrification loss would include N O which in Expt PS constituted 0±5–2 % of deni# trification, in Expt PA much less, and hence N # appeared to be the main product of denitrification. Dewes (1996) stated, with reference to a previous study (Martins & Dewes 1992), that NH constitutes $ c. 95 % of gaseous N losses from storages with solid manure. However, Martins & Dewes (1992) apparently did not consider the emission of N and attributed # the 0–30 % discrepancy between measured losses and losses calculated from a mass balance to analytical errors. The accumulation of N O was highest near the # surface of the manure heap (Fig. 8 b). The distribution of N O does not necessarily reflect the activity of # nitrifiers and denitrifiers in the different layers ; N O is # an intermediate product of denitrification, and the extent of N O reduction to N will be influenced by # # the availability of electron donors and acceptors (Firestone & Davidson 1989). The very different N O # fluxes in Expts PS and PA (not shown) suggested that climatic conditions, such as air temperature, influenced the flux of N O from pig manure. Therefore the # N O emitted was probably produced near the surface # of the manure. The lack of correlation between N O # concentrations inside the heap and N O flux rates (cf. # Fig. 8 a, b) could have been caused by an increase in residence time for N O towards the end of the storage # period due to a decline in gas production from decomposer activity, i.e. high fluxes but low concentrations inside the heap around day 42, and vice versa around day 78. In a previous study N O flux rates of # 5–15 mg N m−# d−" from stored manure from a sow house were recorded in October (Sibbesen & Lind 1993). Similar rates were reached in this study, but only under spring–summer conditions. The disagreement is most probably related to differences in manure composition and storage prehistory, e.g. lower rates of NH volatilization would leave a larger pool of $ TAN for nitrification–denitrification. The N O data (Fig. 8) suggested that nitrification # and denitrification activity was small during the first several weeks of storage, and thus temporally separated from NH volatilization. This separation may $ have been strengthened by the development of high temperatures, which probably stimulated NH losses $ as discussed above, but probably also delayed the growth of nitrifying and most denitrifying bacteria


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. .       , .-.        . .      

(Alexander 1977 ; Mathur et al. 1993). The subsequent proliferation of these organisms would also be a function of TAN availability. The present study has demonstrated that the variability between manure types with respect to organic matter turnover and N losses can be extremely large, depending on manure composition and physical structure. There is little doubt that a temperature increase will promote extensive NH volatilization, $ provided that a pool of TAN is present in the manure, which calls for a careful separation of urine from dung and bedding material or, alternatively, the

addition of straw prior to storage to increase the N immobilization potential. Kirchmann (1985) stated that a C : N ratio " 30 was required for efficient retention of N during aerobic decomposition. Anaerobic storage will give the best retention of N, but part of this N may then be lost during the subsequent turnover in the field (Sommer & Hutchings 1995 ; Petersen et al. 1996). We thank A. Williams and E. Sibbesen for helpful comments. This study was funded by the Danish Ministry of Food, Agriculture and Fisheries.

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