THE OPTIMIZING OF N-FERTILIZATION BY SOIL ORGANIC MATTER
MINERALIZATION
Lončarić Z., Vukadinović V., Bertić B. and Teklić T.
Faculty of Agriculture in Osijek, University of J.J. Strossmayer in Osijek, Croatia
Keywords: soil respiration, soil organic matter, mineralization, soil water content, soil temperature, optimazin N-fertilization
ABSTRACT
Excess nitrogen (N) fertilization is both an unnecessary cost to producers and a polution risk to graundwater. Appropriate N fertilization requires an accurate estimate of the mineralized from soil organic matter and residue of the previous crop.
The objective of this study was to determine the effects of the soil respiration intensity (SRI) as the indicator of soil microbial activity and humus mineralization, respectively. Therefore, the soil samples of five soil types in eastern Croatia were analyzed: the eutric cambisol, pseudogley, rigosol, amphigley and hypogley. The soil samples were taken from soil depths
0-25 and 25-50 cm. The SRI was determined at several moisture content (SMC; 60, 70, 80 and 100 % of field water capacity FWC) during 24 h, on three temperature level (9, 21 and
30 o
C). Also, the humus content (%) was determined, and the amount of released CO
2
was converted into the amount of mineralized humus (%) and nitrogen (kg/ha weekly and for whole year).
All tested soil samples showed lower humus content in the soil layer 25-50 cm as well as the
SRI, compared to upper soil layer (0-25 cm). The increase of SMC resulted mostly by the increase of a difference between samples taken from upper and lower soil layer, measured at all temperature treatments. In general, the lowest SRI was at 9 o C, and the highest at 21 o C, excepting the eutric cambisol where the highest SRI was determined at 30 o C.
Based on the SRI, it could be appraised that the mineralization in eutric cambisol was 1.66 kg
N/ha weekly at 21 o
C and 60 % FWC, while it was 6.89 kg N/ha at 100 % FWC. If such conditions could be maintained whole year, that would be ranged 86.34-359.19 kg N/ha.
Under condition of 60 % FWC in whole year, 1.92 % of humus content would be mineralized and 7.99 % at 100 % FWC. On other localities, the estimated amount of mineralized nitrogen in kg N/ha weekly was ranged as follows: 0.74-5.43 (amphigley) , 7.49-9.19 (hipogley), 6.55-

9.03 (pseudogley) and 2.89-4.18 (rigosol), respectively. At higher temperatures and optimal humidity it increased to 58.34 kg N/ha weekly on amfigley, 57.40 kg N/ha on hipogley and
54.62 on pseudogley. Concerning that the soil was most of the year cold and wet in natural conditions, or warm and insufficient humid, such high mineralization rates could be expected only in short summer intervals.
The results showed doubtless influence of the soil moisture and temperature on the SRI but the relations between moisture and temperature were soil type specific.
INTRODUCTION
According to many reports, excess nitrogen (N) fertilization is both an unnecessary cost to producers and a polution risk to graundwater. Appropriate N fertilization requires an accurate estimate of the mineralized one from soil organic matter and residues of the previous crop.
Soil respiration is a process involving uptake of O
2
and release of CO
2
by living metabolizing entities in the soil (Anderson, 1982) and it can be measured in field or in laboratory. Rapid soil procedure for agricultural soil potential to mineralize C and N can be usefull do to avoiding overfertilization. The objective of this study was to determine the influence of soil mosture and temperature on the soil respiration intensity (SRI) and the correlations between soil respiration, total C and N content and C/N ratio. At the same time, the intention was to predict amount of potential mineralizing N depending on soil temperature and moisture.
MATERIALS AND METHODS
Five localities in eastern Croatia were chosen: Osijek, Krndija, Rakitovica, Našice and
Vukojevci and the agrochemical and pedological analyses were done. The soil layers 0-25 and 25-30 cm were analyzed for physical and agrochemical properties. Analysed physical properties were soil bulk density, soil water holding capacity (Škorić, 1982) and soil texture.
Conducted agrochemical analyses were pH (1 M KCl and H
2
O), organic matter content, total carbon and total nitrogen content (Vukadinović and Bertić, 1988) .
Acording to results of moisture contents, soil bulk density and soil water capacity, for all samples were made the next calculations: soil water content at 60, 70, 80 and 100 field water capacity (FWC) and amount of water (g) necessary to add to soil sample up to 60, 70, 80 or
100 FWC. Afterwards, the soil samples for soil respiration analyses were collected (shortly after wheat harvesting in July), field-moist samples were sieved to pass a 2-mm screen and

analyzed moisture content (gravimetry). The previous calculations are equated acording to soil moisture content and field-moist soil (100 g air-dried soil) samples are weted in 500 ml bottle to 60, 70, 80 and 100 FWC (gravimetry). In the same bottle containing soil are placed the open glass bottle with 10 ml 0,2 M KOH solution. The 500 ml bottles with triplicated samples weted to 60, 70, 80 and 100 FWC were hermetic closed and keeped on 9, 21 or 30°C durign 24 hours. Afterwards, in KOH solution was added 5 ml 20 % BaCl
2
solution, added few drops of the thymolphthalein indicator (1 % ethanol solution) and titrated the unneutralized alkali with standard 0,1 M HCl directly. As the blank sample a 100 g of weted quartz sand was used. For the calculation of releasaed CO
2
(mg) was used formula: mg CO
2
= (a – b) × F a = ml of acid to titrate the blank sample alkali, b = ml of acid to titrate the alkali in the CO
2
collectors from the treatment to the end point,
F = factor according to HCl strength (2,2 for 0,1 M HCl or 22 for 1 M HCl, as 22 is equivalent weight of carbon dioxide).
The statistic analyses for all data were done by analyses of variance and linear regression analysis using computer program (Ivezić and Vukadinović, 1985) created according to
Snedecor and Cochran, 1987.
RESULTS AND DISCUSSION
Soil types and soil analyses results
Soil types on analysed localities were eutric cambisol, amphigley, hypogley, pseudogley and rigosol. The water retention capacity of analyzed soils was medium, topsoil layer density was
1,55 (rigosol) – 1,63 kg dm -3 (amphigley and pseudogley) (Table 1).
Table 1. Soil physical properties locality and soil type depth
(cm)
Osijek - eutric cambisol 0-25
25-50
Krndija - amphigley 0-25
25-50
Rakitovica - hypogley 0-25
25-50
Našice - pseudogley
0-25
25-50
Vukojevci - rigosol 0-25
25-50 soil water holding capacity (cm
3 cm
-3
)
40,2
38,6
36,7
36,4
40,8
41,3
38,3
39,0
42,3
37,9 soil bulk density kg dm
-3
1,56
1,61
1,63
1,59
1,59
1,59
1,63
1,64
1,55
1,54 soil texture clay loam clay loam clay loam clay loam sandy loam sandy loam clay loam clay loam clay loam sandy clay loam

Soil texture was clay loam exept sandy loam on hypogley and sandy clay loam in the 25-50 cm soil layer on rigosol.
The lowest pH was on rigosol and indicated on liming requisite and the highest pH was on amphigley. The humus content of all analyzed topsoil layers were low or medium: 10,2
(Vukojevci) – 24,3 g kg -1 (Rakitovica) and C/N ratio of topsoil layer 7,7 -13,2 (Table 2).
Table 2. Soil agrochemical properties locality and soil type depth
(cm) pH
(H
2
O) pH
(KCl) humus g kg
-1 organic
C g kg
-1
Osijek - eutric cambisol 0-25 6,46 5,22 22,3 12,9
25-50 6,65 5,40 17,7 10,3
Krndija - amphigley 0-25 7,89
25-50 7,13
7,40
6,16
16,7
16,1
9,7
9,3
Rakitovica - hypogley
Našice - pseudogley
Vukojevci - rigosol
0-25 7,36 6,51 20,6 11,9
25-50 7,77 6,76 14,9
0-25 7,09 6,41 17,0
25-50 7,21 6,50 16,4
0-25 5,64 3,98 10,0
25-50 5,47 3,88 4,9
8,6
9,9
9,5
5,8
2,8
Soil respiration results total N g kg
-1
1,45
1,25
1,30
1,75
0,70
C/N ratio
8,9
8,2
1,05 9,2
0,80 11,6
9,2
0,80 10,8
0,75 13,2
0,65 14,6
7,7
4,0
The highest soil respiration intensity (SRI), depending on moisture and temperature, was determined on amphigley, pseudogley and hypogley and there were no significant differences between them. Significant lower SRI was on eutric cambisol inspite of the highest humus and total nitrogen contents. The lowest SRI was on rigosol (Table 3).
Table 3. Soil respiration intensity according to soil temperature and soil moisture
9°C 21°C 30°C
0-25 cm 60% 70% 80% 100% 60% 70% 80% 100% 60% 70% 80% 100%
Osijek 0,25 0,52 0,83 1,04 1,62 2,12 3,45 3,04 1,44 3,42 3,96 4,95
Krndija 0,11 0,16 0,81 0,29 3,74 5,54 7,27 8,7 3,63 4,82 5,99 7,43
Našice
1,02 0,98 1,14 1,35 4,34 5,56 6,56 8,17 3,33 4,13 4,79 6,07
Vukojevci 0,55 0,68 0,47 0,53 1,33 1,81 2,12 2,01 1,33 1,51 1,6 1,69
Rakitovica 1,17 1,35 1,13 1,1 6,12 6,93 7,26 8,43 4,96 5,88 6,53 7,53
25-50 cm
Osijek
Krndija
Našice
0,07 0,06 0,07 0,07 0,39 1,61 2,22 1,83 1,19 1,74 2,37 2,89
0 0 0 0,2 1,54 3,1 3,91 3,99 1,31 1,63 2,39 2,77
0,11 0,35 0,36 0,54 3,49 4,52 5,97 8,03 2,05 2,85 3,2 4,52
Vukojevci 0 0 0 0 0,02 0,03 0,01 0,06 0,02 0 0,06 0,05
Rakitovica 0,47 0,84 0,91 0,89 5,28 5,93 6 6,52 3,64 3,82 3,98 4,29

There were significant correlation between carbon content and SRI on moisture 100% FWC on all temperatures (9

C: r=0,737*, 21

C: r=0,860* i 30

C: r=0,937**).
Soil respiration intensity was much lower in soil layer 25-50 cm then in top soil layer on all localities, and decreasing was most intensive with moisture and temperature increasing.
Temperature increasing from 9

C to 21

C resulted in higher SRI regardless to soil type or soil moisture, but the increasing was higher with higher soil moisture (Figure 1).
Temperature increasing from 21

C to 30

C resulted in higher SRI only on cambisol on both depths, but on other localities the influence of temperature rising was opposite.
Soil moisture increasing from 60 to 100 % FWC resulted in higher SRI on all localities and temperautres but on localitie Osijek with highest SRI on 80% FWC.
Figure 1. Soil respiration intensity in top soil layer (mg CO
2
100 g
-1
soil 24h
-1
) mg CO
2
100 g
-1
day
-1
(Osijek 0-25 cm) mg CO
2
100 g
-1
day
-1
(Rakitovica 0-25 cm)
10
8
6
4
2
0
30oC
21oC
9oC
60%
100%
80%
70%
10
8
6
4
2
0
30oC
21oC
9oC
60%
100%
80%
70% mg CO
2
100 g
-1 day
-1
(Krndija 0-25 cm) mg CO
2
100 g
-1 day
-1 (Našice 0-25 cm)
10
8
6
4
2
0
30oC
10
8
6
4
2
0
30oC
21oC
9oC
60%
100%
80%
70% mg CO
2
100 g
-1
day
-1
(Vukojevci 0-25 cm)
21oC
9oC
60%
100%
80%
70%
10
8
6
4
2
0
30oC
21oC
9oC
60%
100%
80%
70%

Soil N mineralization prediction
Mesured soil respiration can be expressed as a function of soil moisture and soil temperature:
SRI
FWC, t
= (-0,027 t 2 + 1,3 t – 8,72 – 0,0085

FWC 2 + 1,475

FWC – 52)/2,35 in mg CO
2
100g -1 day -1 . Estimated differences between soil types should be included in formula (Soil Factor SF), as well as soil bulk density (SBD) and CN ratio. According to that, the predicted amount of daily mineralized N (min
N
) in top soil layer (0-25 cm) in kg N ha -1 is: min
N
= SRI
FWC, t
× SF × SBD × 6,82 / CN
Soil factor (SF) is calculated according to laboratory results of SRI and for hypogley and amphigley SF = 1, for pseudogley 0,82, for cambisol 0,48 and for rigosol 0,29. The formula listed above is calculated only for CN values between 7 and 15 as were the analyzed soil samples. Using this formula and different simulation models (Jeuffroy and Recous 1999,
Vukadinović et al. 1994, Greenwood et al. 1987) it can be approximate that the mineralization in eutric cambisol was 1.66 kg N/ha weekly at 21 o
C and 60 % FWC, while it was 6.89 kg N/ha at 100 % FWC. If such conditions could be maintained whole year, that would be ranged 86.34-359.19 kg N/ha. Under condition of 60 % FWC in whole year, 1.92 % of humus content would be mineralized and 7.99 % at 100 % FWC. On other localities, the estimated amount of mineralized nitrogen in kg N/ha weekly was ranged as follows: 0.74-
5.43 (amphigley), 7.49-9.19 (hipogley), 6.55-9.03 (pseudogley) and 2.89-4.18 (rigosol), respectively. At higher temperatures and optimal humidity it increased to 58.34 kg N/ha weekly on amfigley, 57.40 kg N/ha on hipogley and 54.62 on pseudogley. Concerning that the soil was most of the year cold and wet in natural conditions, or warm and insufficient humid, such high mineralization rates could be expected only in short summer intervals.
Agroecological spring conditions in Osijek using mentioned simulation models and SRI data resulted in mineralization prediction 0,30 – 1 kg N ha -1 daily, Krndija 0,37 – 1,08, Rakitovica
0,49 – 1,55, Našice 0,45 – 1,28, and Vukojevci 0,06 – 0,38 kg N ha daily -1 . Greenwood et al.
(1987) in their model calculated daily mineralization rate 0,22 – 0,88 kg N ha -1 .
CONCLUSION
There were clear correlation between soil respiration and soil moisture and tempearture. The soil respiration analyses show that the temperature increasing from 9 to 21ºC resulted in increasing of respiration of all analyzed soil types and moistures. However, this increasing

was higher in the top soil layer and at the higher soil moistures. The temperature increasing from 21 to 30ºC in most of analyzed soil types decreased the soil respiration. Soil moisture increasing from 60 to 100 % of field water capacity increased the soil respiration.
Soil respiration data can be used as a part of simulation models for mineralization prediction do to optimizing N fertilization recommendations. Results shows that the simulated N mineralization amount depends on soil type, temperature and moisture. In agroecological condition of spring in eastern Croatia under optimal temperature and moisture those amounts are from 0,38 kg N ha -1 daily on most acid rigosol with low humus content to 1,55 kg N ha -1 daily on hypogley with nearly neutral pH reaction and higher humus content.
Literature Cited
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(editors), Methods of Soil Analysis, Part 2: Chemical and Microbiological Properties, 2 nd
Edition. American Society of Agronomy, Madison, Wisconsin, pp. 831-871.
Greenwood, D. J., Verstraeten, L. M. J., Draycott, A. and Sutherland, R. A. 1987: Response of winter wheat to N-fertilizer: Dynamic model. Fertilizer Research 12: 139-156.
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129-144.
Snedecor, G.W. and Cochran, W.G. 1987. Statistical methods. The Iowa State Uni, Ames.
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