RESEARCH ARTICLE

Laboratory assessment of the effect of cattle slurrypre-treatment on organic N degradation after soilapplication and N2O and N2 emissions

David Fangueiro Æ Jose Pereira Æ David Chadwick ÆJoao Coutinho Æ Nuno Moreira Æ Henrique Trindade

Received: 19 December 2006 / Accepted: 9 July 2007 / Published online: 27 July 2007

� Springer Science+Business Media B.V. 2007

Abstract Slurry separation using mechanical and

chemical methods is one of the options considered to

solve problems of slurry management at the farm

scale. The fractions obtained with such treatments

have distinct compositions, which allow different

options for their utilization (composting, direct

application, and fertigation). In this study, four

fractions of slurry were obtained using a combined

treatment system including slurry treatment with a

screw press separator (solid and liquid fractions)

followed by sedimentation with the addition of

Polyacrylamide (PAM) (PAM-Supernatant and

PAM-Sediment) to the LF. These fractions were then

incorporated into arable soil under controlled labora-

tory conditions and the organic N degradation from

each treatment was followed for 94 days. Total N

emissions (N2O + N2) as well as the sources of the N

emissions (nitrification or denitrification) were also

studied during this period.

Results showed that the slurry fractions (SFs) had

distinct behavior relative to the whole slurry (WS),

namely in terms of N degradation in soil, where

N mineralization was observed only in the WS

treatment whereas N immobilization occurred in the

other treatments. In terms of N2O emissions, higher

losses, expressed as a percentage of the total N

added, occurred from the LF treatments (liquid,

PAM-Supernatant and PAM-Sediment).

This work indicates that the slurry treatment by

mechanical and chemical separation may be a good

option for slurry management at the farm scale since

it allows greater utilization of the different fractions

with a small effect on N2O emissions after SFs’

application to soil.

Keywords Dairy-cattle slurry � N immobilization �N mineralization � N2O emissions � Polyacrylamide �Slurry treatment � Slurry separation

Introduction

In intensive dairy farming systems large quantities of

dairy-cattle slurry are produced representing a major

challenge for their effective management and protec-

tion of the environment. At the farm scale, slurry

storage is required to optimize spreading timings, but

D. Fangueiro (&) � J. Coutinho � N. Moreira �H. Trindade

CECEA, Department of Plant Science and Agricultural

Engineering, Universidade de Tras-os-Montes e Alto

Douro, Apartado 1013, 5001-801 Vila Real, Portugal

e-mail: dfangueiro@utad.pt

J. Pereira

Department of Animal Sciences and Agricultural

Engineering, Escola Superior Agraria de Viseu,

Quinta da Alagoa, 3500-606 Viseu, Portugal

D. Chadwick

Institute of Grassland and Environmental Research,

North Wyke Research Station, Okehampton,

Devon EX20 2SB, UK

123

Nutr Cycl Agroecosyst (2008) 80:107–120

DOI 10.1007/s10705-007-9124-4

this requires significant investment. Effective slurry

spreading can also be expensive and time consuming.

Although slurry applied to soil is a valuable source of

nutrients to crops, it may also lead to water and air

pollution due to high N losses by nitrate leaching or

ammonia (NH3) and nitrous oxide (N2O) emissions.

Nitrous oxide is one of the major greenhouse gases

contributing to global warming and is also considered

as a stratospheric ozone destroyer (Davidson 1991).

As a consequence, management practices have been

developed for the treatment of slurry (prior to land

application) in order to improve plant use efficiency

of slurry nutrients and reduce losses. High emissions

of N2O were observed in the few days following the

application of animal slurries to the soil (Clemens

and Huschka 2001). These N emissions are originated

mainly by the nitrification and denitrification pro-

cesses, which are stimulated after slurry application

to soils due to high concentrations of NH4+ and readily

available organic C (Granli and Bockman 1994; Ellis

et al. 1998). Large amounts of N are also lost via

dinitrogen (N2) emissions to the atmosphere after

slurry application to soil. However, N2 is a neutral

gas with no effect for the environment.

Strategies to reduce N losses after slurry applica-

tion include slurry treatment by physical and chem-

ical methods that result in fractions with different

physical and chemical characteristics that increase

the options for dairy slurry management. A single

treatment using a screw press separator results in a

solid fraction (SF) and a liquid fraction (LF) and

removes a considerable part of solids and nutrients

from the LF and, consequently, a reduction of ca.

20% of the slurry storage requirement (MAFF 1991;

Burton and Turner 2003). However, the nutrient

concentrations as well as the amounts of solids in the

LF obtained by screw press separation are still

considerable, making the direct utilization of this

fraction for fertigation difficult to manage.

Four different fractions may be obtained when the

mechanical separation is combined with a sediment

settling treatment. Indeed, the LF obtained by sepa-

ration of solids using a mechanical separator can be

treated with a flocculating agent such as polyacryl-

amide (PAM) or bentonite in order to produce two

fractions by chemically enhanced settling. The low

concentration of solids and nutrients in the superna-

tant fraction obtained presents advantages in terms of

handling, odors, required storage volume, partial or

total waste treatment, land application and fertigation

(Burton and Turner 2003).

In contrast to the mineral N content, which is

immediately available to plants, the release of N from

organic forms contained in slurry depends on the

mineralization process (Beauchamp 1986). Quantifi-

cation of organic N mineralization from slurry in

intensive dairy farms is an important step to improve

the N use efficiency and to reduce losses to the

environment. It may be hypothesized that the differ-

ent fractions of slurry obtained after separation have

distinct C:N ratios, considering that large particles are

dominated by relatively intact plant material whereas

smaller particles sizes consist of partially digested

plant and microbially processed material. Since the

C:N ratio is commonly used as a parameter to predict

the mineralization rate of organic materials, it is

expected that each fraction may display a specific

mineralization rate (Haynes 1986).

The aim of the present work was to evaluate the

organic N degradation kinetics after addition of

different fractions of dairy-cattle slurry applied to

arable soils and the N emissions (N2O and N2) arising

from nitrification and denitrification.

Materials and methods

Soil and dairy slurry treatment

The soil used was collected in NW Portugal (Vila do

Conde) from the upper 20 cm arable layer during the

period of maize cropping. The selected soil was

characteristic of the main soil type of the region with

a sandy loam texture (27% coarse sand, 45% fine

sand, 21% silt and 6% clay) and classified as a

Dystric Cambisol. The soil was sieved through a

2 mm sieve, homogenized and then stored at 4�C

until required. The main physico-chemical properties

of the soil were then determined.

Cattle-slurry was obtained from a concrete tank on

an intensive dairy farm where cows were fed mainly

with maize silage and concentrate feeds. In order to

obtain the different SFs, the whole slurry (WS) was

first subjected to a mechanical separation with a

screw press separator (FAN model S650,

PAC2505009) generating a solid and a LF. Then

the LF was subjected to chemically enhanced settling

for 20 h after addition of 200 mg l�1 (0.02%) of a

108 Nutr Cycl Agroecosyst (2008) 80:107–120

123

flocculating agent, the cationic polyacrylamide

(PAM) (VTA F 941) (Vanotti et al. 2002a). This

chemical treatment resulted in two more fractions: a

supernatant and a sediment fraction. Representative

samples of the WS and the different fractions were

collected and sub-samples were frozen before anal-

ysis. The cattle slurry used in the present work was

representative of the slurry produced in the intensive

dairy farms of North West Portugal since it has

characteristics very close to those observed in this

region (Trindade 1997; Trindade et al. 2002).

The efficiency of the separation processes was

estimated using the percentage of removal calculated

as followed: percentage removal = [(inlet concentra-

tion�outlet concentration)/inlet concentration] · 100

(Vanotti and Hunt 1999; Vanotti et al. 2002a).

To estimate the influence of the slurry treatment by

separation on the N losses by gas emissions, N losses

from the WS were compared with the sum of losses

observed from each SF. Assuming that the N losses

from each slurry faction are not C limited, the sum of

losses was calculated taking into account the propor-

tion of each fraction relative to the WS and using the

following formulae:

NLT ¼ ðNL1 � Q1Þ þ ðNL2 � Q2Þ þ � � � þ ðNLn � QnÞð1Þ

with NLT, sum of N losses from the individual

fractions; NLn, N losses observed from fraction n; Qn,

proportion of fraction n relatively to the WS (%).

Aerobic incubation experiment

An aerobic laboratory incubation of soil/slurry

(treated and untreated) mixtures was performed over

94 days in PVC boxes ([ = 24 cm, h = 20 cm)

allowing a 18 cm depth soil, at controlled tempera-

ture (20�C). This temperature was chosen because it

offered optimal conditions for potential N minerali-

zation and N emissions (Bateman and Baggs 2005).

There were five replicates of each of the following

treatments: control (no slurry), WS, SF, LF, sediment

fraction with PAM (PAM-Sed) and supernatant

fraction with PAM (PAM-Sup). The volume of

effluent applied in each treatment was the equivalent

rate of 32 t ha�1 of effluent. The soil/slurry mixture

was homogenized and the water-filled pore space

(WFPS) was adjusted to 60% in order to promote

N-mineralization as well as N2O emissions originat-

ing from nitrification and denitrification processes

(Merino et al. 2001). A WFPS around 60% favours

nitrification because the diffusion of substrates and

O2 is not restricted (Parton et al. 1996). The mixture

soil/slurry was packed at a bulk density close to 1.0

similar to field conditions and the PVC boxes were

daily aerated during the incubation period. In addi-

tion, the soil water content and the temperature used

here were similar to the conditions observed when

slurry is applied to soil in the intensive dairy systems

of the northwest region of Portugal.

Total N2O measurements and emissions source

Independent incubations were also performed in

order to determine total N2O emissions and identify

the source of these emissions (nitrification or

denitrification). Three sub samples (200 g each) of

each replicate from each treatment were trans-

ferred to three hermetically sealable PVC boxes

(14 · 19 · 8 cm) with two septa fitted to the lid and

each soil/effluent mixture was exposed to 0, 0.01 or

2% acetylene (C2H2) atmospheres (injected thought

the septa). Incubation under the three acetylene

concentrations leads to the identification of the

source of the N2O losses (Mosier and Klemedtsson

1994; Merino et al. 2001).

The first step in autotrophic nitrification, mediated

by ammonium oxidase, can be inhibited by using

acetylene at low pressure (0.01–0.1%) (Mosier and

Klemedtsson 1994); thus, N2O produced by incubation

with acetylene at low pressure originates from deni-

trification and heterotrophic nitrification. The produc-

tion of N2O by autotrophic nitrification can be

estimated by the difference between N2O production

in the incubations without acetylene and with low-

pressure acetylene. Thereafter, in this work autotroph-

ic nitrification will be referred using the term

nitrification. On the other hand, N2O reduction to N2,

last step in the denitrification process, can be blocked

by acetylene at high pressure (1–10%) (Merino et al.

2001). Rates of total denitrification (N2O + N2) can be

measured as the amount of N2O produced in soil

incubated under high-pressure acetylene (Merino et al.

2001). The production of N2 by denitrification can be

estimated by the difference between N2O production

in the incubations with high-pressure acetylene and

with low-pressure acetylene. The incubation with no

Nutr Cycl Agroecosyst (2008) 80:107–120 109

123

acetylene allows determining the N2O production from

nitrification and denitrification processes. Previous

incubation studies (unpublished data) were performed

with the same soil and slurry with similar composition

using different acetylene concentrations in order to

check if the acetylene levels used are sufficient to

completely inhibit nitrification (0.01%) and reduction

of N2O to N2 (2%).

All boxes were incubated under the same con-

trolled environmental conditions used for the main

incubation experiment and 10 ml gas samples were

taken from each box after 30 min (T0) and 20 h (T1)

of C2H2 addition and stored in 10 ml vials (Vacu-

tainer1) prior to analysis. Nitrous oxide fluxes were

measured on days 1, 2, 4, 5, 9, 13, 16, 22, 35, 49, 73,

and 94. Nitrous oxide concentrations were measured

using a gas chromatograph (DANI1 86.10) equipped

with an electron capture 63Ni detector (ECD). Emis-

sions rates were calculated from the difference

between N2O concentrations at time T1 and T0.

Cumulative N2O losses were calculated assuming a

mean flux rate between sampling dates as described

by Mosier and Klemedtsson (1994).

It was assumed that any ammonia emission was

prevented by mixing the slurry or the fractions with

the soil. Therefore, ammonia emissions were not

measured.

Soil mineral N measurements

On the same days as N2O measurements, the N

mineral content was measured for each treatment in

order to assess the mineralization/immobilization

rates. For this, 15 g of the soil/slurry mixture were

removed from boxes containing no acetylene and

shaken with 30 ml 2 M KCl for 1 h. The suspension

was then centrifuged for 10 min at 3,000 rpm and the

supernatant analyzed for NH4+ and NO3

� content by

automated segmented-flow spectrophotometric meth-

ods (Houba et al. 1994). The remaining soil was used

to determine soil moisture content after drying at

105�C for 24 h.

Statistical analysis

Results were analyzed by analysis of variance

(ANOVA) to test the effects of slurry treatment and

time. To determine the statistical significance of the

mean differences, least significant difference (LSD)

tests were carried out based on a t-test. The statistical

software package used was Statistix 7.0 (Analytical

Software—User Manual 2000).

Results

Slurry fraction characteristics and efficiency of

separation technologies

The slurry pre-treatments using a screw press sepa-

rator resulted in a solid and a LF in the proportions of

20% and 80% by mass, respectively. The subsequent

treatment of the LF by sedimentation after addition of

the PAM resulted in two more fractions, PAM-Sup

(42.4% by mass) and PAM-Sed (57.6% by mass).

Figure 1 illustrates the fractionation scheme and

gives the relative proportions of each fraction relative

to an initial slurry quantity of 100 kg.

The four SFs were distinct in terms of nutrients

and solids contents. As can be seen in Table 1, the

solid fraction was very different from the others and

characterized by a high dry matter content as well as

a high total C, N, and P content. The liquid and PAM-

Sed fractions had similar compositions whereas the

PAM-Sup fraction had a very low dry matter and a

low total C, N, and P content. The PAM-Sed fraction

Screw press separator

Addition of PAM

Whole slurry 100 kg

Solid Fraction 20 kg Liquid Fraction 80 kg

PAM-Sup 33.9 kg PAM-Sed 46.1 kg

Fig. 1 Yields of the slurry

fractionation scheme used

in the present work

110 Nutr Cycl Agroecosyst (2008) 80:107–120

123

had lower values of total C and dry matter than was

expected since this fraction should be more concen-

trated than the original LF. The water soluble C

represents more than 50% of the total carbon in the

liquid and PAM-Sed fraction with a maximum value

of 98% in the PAM-Sup fraction whereas only 6%

and 22% of total carbon is water soluble in the solid

fraction and untreated slurry, respectively. It is worth

noting that the total K content did not vary very much

between fractions and that the lowest K concentration

was found in the solid fraction.

The results of nutrients and solids separation

efficiency obtained with the treatment technologies

are shown in Table 2. The screw press separator was

efficient at separating solid matter but also organic N

as well as total and water soluble P, while the

addition of PAM allowed the efficient separation of

all the nutrients except K and water soluble C. The

screw press separator removed 50% of the dry matter

from the WS, whereas the application of PAM to the

LF resulted in an additional removal of 42% of dry

matter. The PAM treatment removed a significant

amount of water soluble P.

In the case of total K, it appears that its concen-

tration in the LF was higher than in the WS but this

difference was lower than 5% so it can be assumed

that there is no K removal from the LF by slurry

screw press separation. For water soluble C, the

separated fractions obtained after treatment (liquid

and PAM-Sup) had higher concentrations than the

Table 1 Characteristics of the effluent used in the present work (data expressed on a fresh weight basis)

WS SF� LF� PAM-Sed fraction PAM-Sup fraction

Total C (g kg�1) 26.84 81.94 12.37 11.30 7.00

Total N (g kg�1) 4.00 4.86 3.75 4.01 2.24

C:N 7 17 3 3 3

Total P (g kg�1) 0.40 0.58 0.34 0.45 0.19

Total K (g kg�1) 2.39 2.33 2.50 2.29 2.56

Organic N (g kg�1) 2.78 3.85 2.48 2.73 1.10

NH4+–N (g kg�1) 1.22 1.01 1.27 1.28 1.14

NO3�–N (g kg�1) <1 <1 <1 <1 <1

Water-soluble P (mg kg�1) 63 78 51 55 15

Water-soluble C (g kg�1) 5.94 5.13 6.72 6.69 6.90

Water-soluble C:total C 0.22 0.06 0.54 0.59 0.98

Dry matter (g kg�1) 86.0 248.1 43.3 37.8 25.2

PH 8.1 8.1 7.8 7.3 8.2

� Fractions of the whole slurry obtained after mechanical separation

Table 2 Separation efficiency percentage of nutrients and solids by the mechanical and chemical treatments applied

Parameters Screw press

(LF/WS) (%)

PAM treatment

(PAM-Sup/LF) (%)

Screw press + PAM treatment

(PAM-Sup/WS) (%)

Total N �6 �40 �44

Organic N �11 �56 �60

Total C �54 �43 �74

Water soluble C +13 +3 +16

Total P �15 �44 �52

Water soluble P �19 �71 �76

Total K +5 �2 �7

NH4+–N +4 �10 �6

Dry matter (%) �50 �42 �71

Nutr Cycl Agroecosyst (2008) 80:107–120 111

123

non separated material due to the lower dry matter

content of these fractions (less than 50% in the LF

and less than 42 % in the PAM-Sup fraction).

Soil mineral N concentration

As can be seen on Table 3, the soil used in this study

has an high carbon content and consequently an high

C:N ratio resulting from the long term maize

cropping. The amounts of N and C applied in each

treatment considering a soil depth of 10 cm are

indicated in Table 4. The rate of liquid effluents

applied corresponded to an addition of about 40 kg

NH4+–N ha�1 except in the case of the solid and

PAM-Sup fractions where due to the low ammonium

content of these fractions only 32 and 36 kg NH4+–

N ha�1 were applied, respectively.

The values of the soil concentration of NO3�–N

and NH4+–N measured in each treatment during the

experiment are shown in Fig. 2. The initial concen-

tration of NH4+–N in the control (soil only) is very low

(<2 mg NH4+–N kg�1) and remained unchanged up to

day 73 when it started to increase slowly to values

close to 4.3 mg N–NH4+ kg�1. In all other treatments,

the N–NH4+ concentration on day 0 was close to

60 mg NH4+–N kg�1 and decreased slowly with time

to reach values <10 mg NH4+–N in the second half of

the experiment (after 35 days). In the WS and solid

fraction treatments, the NH4+–N concentration

reached a minimum and constant value on day 35,

whereas for the other treatments the lowest concen-

tration did not occur until day 49. It also appears that

the NO3�–N concentration of the control had a high

initial value of 62 mg NO3�–N kg�1, which slowly

decreased up to day 22 where it reached a constant

value of about 30 mg NO3�–N kg�1. For all other

treatments except PAM-Sed, the NO3�–N concentra-

tion strongly decreased in the first 5 days to reach a

minimum value on day 5. Then, in the WS and PAM-

Sup treatments, the NO3�–N concentration increased

gradually up to the end of the experiment but in the

solid and LF treatment, the NO3�–N concentration

varied during the rest of the experiment with a

general trend to increase. In the case of the PAM-Sed

treatment, the NO3�–N concentration decreased

slowly up to day 35 when it reached a minimum

value and then start to increase till the end of the

experiment.

Mineralization-immobilization of the slurry N

Figure 3 shows the mean values of the total N mineral

concentration in soil–slurry treatments mixtures dur-

ing the experiment time. The mineral N content

decreased for all treatments (including the control)

during the first 22 days, and continued up to the 35th

day for treatments LF, PAM-Sed and PAM-Sup, and

up to the 49th day for treatments WS and SF. After

these periods the mineral N content tended to increase.

The mineral N content was significantly lower in the

SF treatment than in the other treatments (except the

Table 3 Main characteristics of the soil used in the present

work

Total C (g kg�1 dry soil)) 28.0

Total N (g kg�1 dry soil) 2.3

C:N 12

OM content (%) 2.5

pH (H2O) 5.8

pH (KCl) 5.1

P2O5 (mg kg�1 dry soil)� 117

K2O (mg kg�1 dry soil)� 157

� Egner-Riehm method

Table 4 Amounts of N and C applied to soil in each treatment (mg kg�1 dry soil)

Treatments WS SF� LF� PAM-Sed fraction PAM-Sup fraction

Total C 848 2,588 392 358 221

Total N 126 154 119 127 71

Organic N 88 122 79 86 35

NH4+–N 39 32 40 41 36

Water-soluble C 188 162 212 211 218

� Fractions of the whole slurry obtained after mechanical separation

112 Nutr Cycl Agroecosyst (2008) 80:107–120

123

control). Furthermore, the PAM-Sup and PAM-Sed

treatments had, in most of days, the highest values of N

mineral content. The ANOVA for both treatments and

concentration of mineral N showed significant effects

from treatment, incubation time (P < 0.001), as well

as the interaction (treatment · incubation time)

(P < 0.05) during the experiment.

Table 5 shows the values of mineral N (NH4+–

N + NO3�–N) at the beginning and at the end of the

experiment as well as the total mineral N released by

the SFs during the experiment (including the total N

gaseous losses N2O + N2) expressed in absolute

value and percentage of N supplied by slurry in each

treatment. Values of net mineralization or net immo-

bilization for the 94 days incubation were calculated

considering the immobilization value that occurred in

the control as the contribution of soil organic N and

subtracting it from each slurry treatment. As can be

0

10

20

30

40

50

60

70

80

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95Days

HN lioS

4+gk

N gm( tnetnoc

N- 1-

lios yrd)

0

10

20

30

40

50

60

70

80

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

Days

ON lio

S3

-gk

N gm( tnetnoc

N-1-

lios yrd )

Control WS SF LF PAM-SED PAM-SUPa)

b)

Fig. 2 Evolution of the

NH4+–N (a) and NO3

––N

(b) concentrations in the six

treatments studied during

the whole experiment.

Vertical bars represent

SEM (N = 5)

0

10

20

30

40

50

60

70

80

90

100

110

120

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92Days

gkNlareni

mg

m1-

Control WS SF LF PAM-SED PAM-SUP

Fig. 3 Mean values of the

N mineral concentration

(NO3�–N + NH4

+–N) in

soil–SFs mixtures during

the experiment. Vertical

bars represent SEM (N = 5)

Nutr Cycl Agroecosyst (2008) 80:107–120 113

123

seen, after 94 days of incubation, liquid mineraliza-

tion was observed only on treatment WS whereas

liquid immobilization took place in all others treat-

ments. The percentage of slurry N mineralized was

similar for the SF and LF treatments with a mean

value of 23.5 %, but much lower in the PAM-Sup and

PAM-Sed treatments with a mean value of 5.7%.

N gaseous emissions

The cumulative amounts of N lost during the

experiment by gaseous emissions in the N2O or N2

form are shown in Table 6. As can be seen, slurry

application induced an increase of total N losses

(N2O + N2), relative to the control, of about 240%

on treatments WS and PAM-Sed, 220% on treat-

ments SF and LF, and 167% on treatment PAM-Sup.

Furthermore, the amounts of total N (N2O and N2)

in treatments with slurry were all similar except for

the PAM-Sup treatment, which had a lower mean

value. However, there were no significant differ-

ences (P > 0.05) between treatments in terms of

total (N2O + N2). Nevertheless, significant differ-

ences in total N2O emission were found among

treatments. The highest N2O emissions were

observed on treatments LF and PAM-Sup whereas

the highest N2 emissions were observed on treat-

ments WS and SF.

Incubations with three different acetylene concen-

trations allowed assessing the sources of the N2O

emissions. As can be seen on Table 6, the amount of

N2O coming from nitrification is statistically nul in

all treatments. Therefore, it can be concluded that

nitrous oxide emissions came exclusively from

denitrification in all treatments.

Total N emissions (N2O + N2) observed were

considerably higher during the first 5 days after

effluent application. During this period, 34% of total

N emissions were measured on the control, 40% on

treatment WS, 47% on treatment SF, 72% on

treatment LF, 52% on treatment PAM-Sed and 43%

on treatment PAM-Sup. As for total N emissions, the

application to soil of WS or the individual SFs

increased the total N2O losses (Fig. 4). Moreover

N2O emissions were considerably higher in the first

4 days and an important decrease in N2O emissions

was observed the second day after application on

treatments WS (78%) and SF (65%) whereas

decreases in a range of 29–45% were observed for

the other treatments. Values of daily total N2O

emissions were significantly affected by treatments

(P < 0.05) and time (P < 0.001) with an interaction

effect between these two factors (P < 0.01).

Values of the mole fraction of N2O obtained in the

present study are also shown in Table 6. It appears

that the lower values of N2O/(N2O + N2) were

obtained in the WS and SF treatments, which

correspond to the fractions with the higher total C

content and/or lower water soluble C/total C ration.

As can be seen in Table 6, the percentage of total

N applied lost by N2O emissions was lower in the

solid fraction than in all other treatments. In contrast,

the higher value was observed in the PAM-Sup

treatment. Relative to the WS, all SFs except the solid

fraction led to an increase of the N2O emissions.

There was no significant difference (P > 0.05) in the

percentage of total N applied that was lost by N2 or

(N2 + N2O) emissions.

To estimate the effect of the slurry separation on

the N emissions after WS or SF application to soil,

Table 5 Initial and final mineral N content (mg kg�1 dry soil), total N gaseous losses and mineralization/immobilization (minera-

immobilized) rates expressed as absolute value and as percentage of N supplied by slurry in each treatment

Treatment Min. N

initial (1)

Min. N

final (2)

Total

N2O + N2

(3)

2�1 + 3 Minera-

immobilized

N

Organic N

applied

Percentage of slurry N

mineralized or immobilized

C:N

ratio

Control 64.0 33.4 3.7 �26.9

WS 95.0 63.7 8.6 �22.7 4.2 87.7 4.8 a* 7

SF 92.9 29.8 8.1 �55.0 �28.1 121.8 �23.1 b 17

LF 100.4 46.6 8.1 �45.7 �18.8 78.6 �23.9 b 3

PAM-Sed 111.7 71.9 8.9 �30.9 �4.0 86.4 �4.6 c 3

PAM-Sup 110.9 75.5 6.1 �29.3 �2.4 34.8 �6.9 d 3

*Values followed by the same letter are not significantly different (P < 0.05) by LSD test

114 Nutr Cycl Agroecosyst (2008) 80:107–120

123

the N emissions balance of the slurry separation

process has been established considering the relative

proportion of each fraction and assuming that the N

losses from each SF were not C-limited since all SFs

led to N-immobilization. As can be seen in Fig. 5,

relative to the WS, higher N2O emissions were

observed when the liquid and the solid fractions were

applied separately. However, in terms of N2 and total

N (N2O and N2) emissions, no significant differences

were observed. Considering the chemical separation

using PAM, no significant differences in terms of N2,

N2O and total N (N2O and N2) emissions were

observed between the LF s and the estimated sum of

the PAM-Sup and PAM-Sed fractions. When consid-

ering the combined separation processes (screw

press + PAM), higher N2O emissions were observed

when the solid, PAM-Sup and PAM-Sed fractions

were applied separately compared with the WS. But

there was no significant difference in terms of N2 and

total N emissions (P > 0.05).

Discussion

Slurry fractions characteristics and efficiency

of separation technologies

The separation efficiency of the screw press in this

study was higher than that obtained by other authors

with the same process (Moller et al. 2002) and in the

same range as those obtained by Jakobsen and Hjort-

Gregersen (unpublished data) with a decanter centri-

fuge. These differences were mainly due to the

original slurry composition, particularly the solids

content, which controls the separation efficiency. The

main practical advantage of this first separation is the

reduction in the amount of slurry that needs to be

stored prior to spreading in the fields, the lower cost

of slurry storage and the modification of the main LF

characteristics, which could improve its fertilizer

value. A larger storage capacity also provides farmers

with greater flexibility about the timings of slurry

applications. Furthermore, the solid fraction is gen-

erally composted and may be then exported out of the

farm representing a potential extra income. This

compost exportation may also be considered as a

solution for environmental problems in intensive

farms, which have generally high N and P surplusTa

ble

6C

um

ula

tiv

eN

lost

by

N2

and

N2O

emis

sio

ns

(lg

Nk

g�

1d

ryso

il)

du

rin

gth

eex

per

imen

t

To

tal

Ng

aseo

us

loss

esC

on

tro

lW

SS

FL

FP

AM

-Sed

PA

M-S

up

P

N2

2,0

31

a*(7

84

)5

,80

0a

(1,7

65

)5

,38

8a

(2,1

24

)3

,50

7a

(1,0

22

)5

,19

5a

(2,2

60

)1

,68

1a

(2,6

29

)>

0.0

5

To

tal

Nap

pli

edlo

stas

N2

(%)

4.6

3.5

2.9

4.1

2.4

To

tal

N2O

1,6

26

c(3

44

)2

,77

0b

(21

8)

2,6

96

bc

(23

4)

4,5

93

a(4

34

)3

,74

0ab

(44

3)

4,4

37

a(6

80

)<

0.0

01

To

tal

Nap

pli

edlo

stas

N2O

(%)

2.2

1.8

3.9

2.9

6.2

N2O

den

itri

fica

tio

n1

,24

8c

(17

0)

2,8

67

b(5

29

)2

,82

2b

(50

3)

4,5

95

a(1

,06

0)

4,3

85

a(6

74

)4

,61

8a

(77

1)

<0

.00

1

To

tal

N2O

emis

sio

n(%

)1

00

10

01

00

10

01

00

10

0

N2O

nit

rifi

cati

on

37

8a

(37

7)

�9

7a

(46

0)

12

7a

(52

1)

�1

a(1

,00

0)

�6

45

a(5

12

)�

18

1a

(60

2)

>0

.05

To

tal

N2O

emis

sio

n(%

)0

00

00

0

To

tal

N2O

+N

23

,65

7b

(2,0

88

)8

,57

0ab

(3,5

24

)8

,08

3ab

(4,2

75

)8

,10

0ab

(1,9

54

)8

,93

5a

(5,2

28

)6

,11

8ab

(5,4

66

)>

0.0

5

To

tal

Nap

pli

edlo

stas

(N2O

+N

2)

(%)

6.8

5.2

6.8

7.0

8.6

N2O

/(N

2O

+N

2)

rati

o0

.44

40

.32

30

.33

30

.56

70

.41

80

.72

5

Val

ues

bet

wee

np

aren

thes

esre

pre

sen

tS

EM

(N=

5)

*V

alu

esfo

llo

wed

by

the

sam

ele

tter

ina

sam

eli

ne

are

no

tsi

gn

ifica

ntl

yd

iffe

ren

t(P

<0

.05

)b

yL

SD

test

Nutr Cycl Agroecosyst (2008) 80:107–120 115

123

since it represents a nutrient exportation. The LF can

be applied directly to fields or be used for fertigation.

However, for fertigation, nutrients levels in the liquid

have to be reduced, so a second separation by

chemically enhanced sedimentation is of interest.

Indeed, the N removal efficiency with the screw press

Fig. 5 N emissions balance

(N2, N2O, and total) of: (a)

the mechanical separation

process (Screw press), (b)

the chemical separation

process (PAM), and (c) the

combined separation

processes (Screw

press + PAM)

Fig. 4 Total N2O

emissions (lg N2O–N

kg�1 day�1) by treatment

along the experiment. Bars

represent SEM (standard

error of mean) (N = 5)

116 Nutr Cycl Agroecosyst (2008) 80:107–120

123

separator was low compared to the PAM application.

This result may be related to the fact that most of the

organic nutrients of slurry are associated with small

particles and particles smaller than 0.25 mm have to

be removed to ensure an effective reduction of

nutrient contents and odor-generating compounds

contained in liquid manure (Vanotti and Hunt 1999).

Furthermore, the values of N removal efficiency

obtained in our study using PAM addition to the LF

were lower than those obtained in other works (on

average 85%), probably due to the fact that PAM had

no effect on the removal of inorganic N forms

(Vanotti and Hunt 1999; Vanotti et al. 2002a) since

about 30% of total N in the LF used in this study was

in an inorganic form. The results of P separation

efficiency by the screw press separator were similar

to those obtained for total N although a more efficient

removal was reached for total P than for total N with

the screw press separator.

Both separation processes have poor effect on total

K and NH4+–N separation. However, Chastain et al.

(2001) also obtained low percentage of K removal

(lower than 4.5%) from screened dairy slurry by

settling with PAM but they obtained higher percent-

age of NH4+–N removal using a screen separator

(46%). Similarly, Converse et al. (unpublished data)

using the screw press for dairy manure separation

obtained NH4+–N removal percentage lower than

3.5% and K removal percentage lower than 10%

with both techniques.

Separation processes, such as screw press and

PAM settling, result in effluents with low C:N ratios.

This characteristic may be useful for dairy effluent

management and application to fields assuming that

materials with low C:N ratio should lead to rapid N

mineralization and other nutrients release. Indeed, the

application of the solid and LFs at different periods of

the year according to the N plant requirement could

be a good tool to improve N fertilization efficiency.

Soil mineral N concentration and mineralization/

immobilization of slurry N

Slurry applications to soil increased its NH4+ content

of about 40 mg NH4+–N kg�1 and 35 days after

application this value was about 7 mg NH4+–N kg�1.

This decrease can be related to losses by NH3

volatilization, nitrification and denitrification pro-

cesses (Haynes 1986). But, in the present work, it

may be mainly due to the immobilization process that

occurred in this soil. A decrease of soil mineral N was

also observed by Merino et al. (2001) after cattle

slurry application to grassland; this was attributed to

NH4+–N immobilization. On another hand, in all

treatments with slurry applications, the NO3� concen-

trations strongly decreased during the first 13 days

and then increased except in the solid fraction where

NO3� content remained more or less constant after

day 13. It was evidence of a high immobilization of

NH4+–N during the first 13 days followed by the start

of the nitrification process. When mineral N immo-

bilization occurrs microorganisms generally have a

preference for NH4+ rather than NO3

� (Ragab et al.

1994). It may explain why N2O emissions are coming

exclusively from denitrification. Indeed, nitrous oxide

emissions were observed mainly during the first

5 days and during this period, N mineralization

resulted in NH4+–N immobilization and no nitrifica-

tion occurred.

In all treatments, the total soil mineral N content

declined during the first 36 days of incubation. This

decrease is due in part to the N2O and N2 emissions

even if these losses should have a small effect on total

soil mineral N considering that emissions occurred

mainly during the first 5 days of the experiment.

Therefore, the decrease of the total soil mineral N

content should be mainly due to N immobilization in

soil.

During the entire experiment, N mineralization

only occurred in the WS treatment whereas, in all

other treatments, N immobilization was observed.

Similarly, Clemens and Huschka (2001) obtained low

values of mineralization and verified immobilization

by the soil after application of different SFs. The C:N

ratio of organic residues is an important factor to

predict liquid mineralization or immobilization of

nitrogen. The effluents used in the present work had

low C:N ratios. Indeed, the WS and the solid fraction

had C:N ratios of 7 and 17, respectively; whereas the

other three fractions had a C:N ratios of 3. According

to Haynes (1986) and Tisdale et al. (1993), organic

material with C:N ratio values less than 20:1 or 30:1

lead to N mineralization. Indeed, Chadwick et al.

(2000) observed both N-mineralization and N-immo-

bilization in soil after application of manures with

different C:N ratios. In the present study, the

percentage of N immobilized relative to the organic

N incorporated by the slurry were very close on

Nutr Cycl Agroecosyst (2008) 80:107–120 117

123

treatments SF and LF as well as in treatments PAM-

Sed and PAM-Sup. However, if the effluents applied

in treatments with PAM have similar C:N ratio, the

same is not true for treatments SF and LF, and

consequently, a distinct behavior was expected

between the last ones.

The N immobilization occurred in the treatments

with slurry applications can not be explained through

the C:N ratio of the effluents applied. However, an

effect associated to the soil used should be considered

since the C:N ratio of the soil was much higher than

those of the SFs used. Moreover, in addition to the

C:N ratio, other parameters might have influenced the

balance between mineralization and immobilization.

In instance, the maintenance of the soil moisture

content at a constant value (60% WFPS) may have

limited the N mineralization process since soil

drying/wetting cycles stimulate the decomposition

of soil organic N (Haynes 1986). Furthermore, the N

immobilization observed from the solid fraction

treatment may be attributed to the high carbon

contents of this fraction and more specifically to the

readily available C supplied by the SF, which lead to

denitrification and immobilization of potentially

available N (Calderon et al. 2005).

The N immobilization that occurred in the control

may be due to some plant residues. Despite our best

efforts to remove them from the soil, the soil may

have contained roots that were not excluded during

sieving and which decomposed during the experiment

(Hatch et al. 1990). Roots and radicular exudates of

plants have a high C:N ratio and, therefore, their short

term decomposition can induce N immobilization

(Ross et al. 1985; Trindade 1997). Nevertheless, the

use of this type of soil allowed us estimating the

effect of utilizing the liquid and PAM-Sup fractions

for fertigation in maize production.

The N immobilization was more important in the

LF than in the PAM-Sup even if both fractions have a

C:N ratio equal to 3. Such difference in N immobi-

lization may be attributed to the N content of each

fraction, which was higher in the LF. However, the

differences in N immobilization observed between

the liquid and the PAM-Sed fraction may not be

attributed to the composition of each fraction since

they have similar contents of C and N (see Table 1).

The own difference between this two fractions is the

presence or absence of PAM. Even if no effect of this

flocculent on N mineralization was previously

reported, the results obtained here indicated that the

presence of PAM induced a higher nitrification and

consequently reduced the N immobilization in the

PAM-Sed fraction.

The differences observed between fractions in

terms of N degradation in soil could be used to

improve slurry management. Indeed, as the LFs

induced mainly N immobilization, this fraction could

be applied to plants when they need an immediate N

supply for a short period, whereas the WS could be

applied in another period of plant growth when there

is a requirement for a more continuous N supply.

N gaseous emissions

In the present work, nitrous oxide emissions were

restricted to the first 5 days after untreated slurry or

fractions application to soil. Similarly, Ellis et al.

(1998), in an experiment with application of cattle

slurry, observed high N losses by denitrification

during the first 5 days. The highest N2O emissions

were observed from the LF and PAM-Sup treatments,

whereas the highest N2 emissions were observed

from the WS and solid fraction. These differences

may be related to the (water-soluble C:total C) ratio,

which showed higher values in the liquid effluents.

Weier and MacRae (1993) report that an increase in

total C content may lead to an increase in denitrifi-

cation, but many authors argue that total C content is

less important than the readily available C in form of

volatile fatty acids, which have a strong influence on

N losses by denitrification (Beauchamp et al. 1989;

Paul and Beauchamp 1989; Misselbrook et al. 1998;

Petersen 1999).

We expected to see a reduction in emissions from

the PAM-Sup treatment. Indeed, the degree of solid

and nutrient removal normally induced by the PAM

(Vanotti et al. 2002b) should have more effect on

these emissions. However, in the present study, the

efficiency of nutrient and solid removal in the PAM-

Sup fraction was not as great as expected and this

fraction has an important readily available carbon

content coupled with higher values of mineral N,

which can help to explain the results obtained with

this fraction in terms of emissions

The fact that N2O emissions are coming quite

exclusively from denitrification may be explained

considering that a high NH4+–N and NO3

—N reduc-

tion occurred due to mineral N immobilization, in

118 Nutr Cycl Agroecosyst (2008) 80:107–120

123

particular during the first 13 days. However, Ellis

et al. (1998) showed in a similar study that N losses

generated by denitrifcation were much higher than

those arising from nitrification, and that N2 losses

represent an important portion of the total N emis-

sions following slurry application. Furthermore,

Clemens and Huschka (2001) verified that for WFPS

values of 35%, 54%, and 71%, the N2O emissions

were originated predominantly by denitrification and

were related, not with the input of N–NH4+, but with

the input of readily available carbon and the amount

of N–NO3� in the soil. The low N2O emissions

coming from nitrification can still be explained by the

fact that the N–NH4+ was immobilized instead of

being oxidized to form nitrates. During the first days

of incubation, the immobilization of mineral-N

applied in the treatments strongly reduced, or even

exhausted, the amount of ammonium available for the

nitrification process, which explains why N2O emis-

sions were found to be only originated by the

denitrification process. In contrast, the high denitri-

fication rates may have been due to the high amounts

of nitrate N and readily available C. In addition,

Tenuta et al. (2001) reported a positive correlation

between N2O emissions from manure and NO3�N

content, suggesting that denitrification is an important

source of N2O following manure applications to soil.

In the present work, the nitrate immobilization

together with the exhaustion of available carbon

explains the negligible emissions from denitrification

after 5 days of incubation.

The ratio N2O/(N2O + N2) defined as the mole

fraction of N2O, produced by denitrification varies

with the environmental conditions and on the micro-

bial populations present (Stevens and Laughlin

2001). A slurry application should tend to favor N2

production and lead to a low N2O/(N2O + N2) ratio

because it supplies organic C at high pH and low

NO3� concentration conditions (Stevens and Laughlin

2001). Values of the mole fraction of N2O reported in

previous studies were in a range <0.1–1 (Jarvis and

Pain, 1994; Stevens and Laughlin, 1997). Values

obtained in the present study were in the same range.

Conclusion

During the 94 days of incubation, N immobilization

was observed in all the separated SFs treatments

whereas N mineralization was observed in the WS

treatment. Our results showed that N2O emissions

observed after slurry incorporation to soils mainly

originated from the denitrification process and most

of total N (N2O and N2) lost occurred in 4 days

after slurry application. Furthermore, higher N2O/

(N2 + N2O) ratios were observed from the treated LF

than from solid fraction. The N emissions balance

following slurry separation showed that there was no

effect on N emissions from the separate applications

of the fractions relative to the WS. Therefore, it can

be concluded that the slurry treatment by mechanical

and chemical separation is potentially a good tool for

slurry management at the farm scale with no effect on

the N emissions after fractions are applied to soil.

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