nitrogen mineralization in soils related to initial extractable organic nitrogen: effect of...

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Nitrogen Mineralization in Soils Relatedto Initial Extractable Organic Nitrogen:Effect of Temperature and TimeMabel M. Bregliani a , Gerard H. Ros b , Erwin J. M. Temminghoff b &Willem H. van Riemsdijk ba Unidad Académica Río Gallegos, Universidad Nacional de laPatagonia Austral , Río Gallegos, Argentinab Department of Soil Quality , Wageningen University , Wageningen,the NetherlandsPublished online: 20 May 2010.

To cite this article: Mabel M. Bregliani , Gerard H. Ros , Erwin J. M. Temminghoff & Willem H. vanRiemsdijk (2010) Nitrogen Mineralization in Soils Related to Initial Extractable Organic Nitrogen:Effect of Temperature and Time, Communications in Soil Science and Plant Analysis, 41:11,1383-1398, DOI: 10.1080/00103621003759387

To link to this article: http://dx.doi.org/10.1080/00103621003759387

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Communications in Soil Science and Plant Analysis, 41:1383–1398, 2010Copyright © Taylor & Francis Group, LLCISSN: 0010-3624 print / 1532-2416 onlineDOI: 10.1080/00103621003759387

Nitrogen Mineralization in Soils Related to InitialExtractable Organic Nitrogen: Effect of

Temperature and Time

MABEL M. BREGLIANI,1 GERARD H. ROS,2

ERWIN J. M. TEMMINGHOFF,2 AND WILLEM H. VANRIEMSDIJK2

1Unidad Académica Río Gallegos, Universidad Nacional de la Patagonia Austral,Río Gallegos, Argentina2Department of Soil Quality, Wageningen University, Wageningen, theNetherlands

An important source of nitrogen (N) for crops is mineralization of soil organic mat-ter during the growing season. Awareness is growing that dissolved organic nitrogen(DON) plays an important role in mineralization and plant uptake. We studied the influ-ence of temperature and time on extractable organic nitrogen (EON) levels, which isa measure of DON, and their relationship with N mineralization. Aerobic incubationexperiments were conducted in the laboratory for five soils at different temperatures(4 20, and 30 ◦C) and different time intervals with optimal water content (60% of itswater-holding capacity). Net N mineralization ranged between 14 and 155 mg kg–1

within 84 days and was correlated with the initial amount of EON. Net N mineralizationamong the soils, time, and incubation temperatures was linearly related to the squareroot of time multiplied by temperature, with mineralization rate k being independent oftime and temperature. Because initial EON values were also related to these k values,we were able to describe the net N mineralization at different temperatures based on ananalysis of initial EON. Preliminary validation with results from pot experiments in theliterature suggests that the approach is promising, although the proposed model needsto be calibrated with more soils.

Keywords Dissolved organic nitrogen, incubation, mineralization, prediction, solu-ble organic nitrogen

Introduction

Nitrogen (N) is an essential element in crop production, and it is a limiting factor for thegrowth of almost all crops. More than 90% of the soil N occurs in organic forms, and onlya very small proportion is available to crops (Stevenson 1982; Matsumoto and Ae 2004).This proportion consists of mineral N (nitrate, NO3, and ammonium, NH4) present beforesowing, and the amount of N that is mineralized from soil organic matter during the grow-ing season. Fertilizer recommendations (e.g., in the Netherlands) are based mainly on the

Received 18 April 2008; accepted 22 April 2009.Address correspondence to E. J. M. Temminghoff, Department of Soil Quality, Wageningen

University, P. O. Box 8005, 6700 EC Wageningen, the Netherlands. E-mail: [email protected]

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1384 M. M. Bregliani et al.

mineral pool of N and are sometimes improved by rough predictions of the amounts of min-eralized N. Because strict legislation and growing concern for the environmental impact ofagriculture urge farmers to use N fertilizers more judiciously, there is an increasing needfor accurate prediction of the total amount of plant-available N. Accurate prediction of Nmineralization allows farmers to reduce their fertilizer N application without losses in cropyield (Olfs et al. 2005).

There are several methods for assessing soil N availability, most of which can beclassified as either biological or chemical methods. Biological methods depend on themeasurement of net N mineralization in field, pot, or incubation studies (Bhogal et al.1998; Wang et al. 2001; Curtin and McCallum 2004). Unfortunately, these methods aretime-consuming, labor intensive, and often strongly dependent on the experimental condi-tions used (Cabrera, Kissel, and Vigil 2005). Many soil scientists are therefore interestedin chemical assays that could be used as proxies for mineralizable N (Fox and Piekielek1978; Michrina, Fox, and Piekielek 1981; Dou, Alva, and Appel 2000; Nunan et al. 2001;Curtin et al. 2006). These assays are based mainly on extraction of soil by weak chemicalsolutions including water, calcium chloride (CaCl2), potassium chloride (KCl), or sodiumbicarbonate (NaHCO3). The amount of extractable organic nitrogen (EON) in weak extrac-tants often give a good correlation with net N mineralization in incubation studies and Nuptake by crops (Stanford and Smith 1976; Appel and Mengel 1990; Groot and Houba1995; Velthof, Oenema, and Nelemans 2000).

The EON can be considered as the sum of actually dissolved organic N (DON) plusextra organic compounds that solubilize during extraction (Ros et al. 2009). These extracompounds originate from soil biomass and solid organic N. The DON is assumed to playan intermediate role in N mineralization: depolymerization of N-containing polymers (for-mation of DON) rather than NH4 production seems to be the controlling mechanism ofN cycling (Schimel and Bennett 2004). Although the role of EON in N mineralization isless clear (for example, Appel and Xu 1995), EON pools are often used as a proxy of DON(Cookson et al. 2007; Embacher et al. 2007; Ros et al. 2009). The carbon (C) content of thesame organic compounds is denoted by dissolved organic C (DOC) or extractable organicC (EOC).

Nitrogen mineralization is a biological process where organic N is converted to inor-ganic N by soil biomass. As for many biological processes, mineralization is affected byenvironmental conditions, including soil water potential and temperature. Most chemi-cal assays of N mineralization are calibrated on N mineralization experiments performedunder controlled environmental conditions (pot and incubation experiments). Therefore,they estimate rather a certain potential pool of available N rather than a pool that actuallymineralizes. The realization of this potential under field conditions depends on the actualconditions for mineralization in the specific year (Olfs et al. 2005). In spite of the impor-tance of temperature and water potential, less is known of their influence on the relationshipbetween EON and the amount of mineralizable N.

Laboratory studies of N mineralization have been performed typically under opti-mal temperature and water conditions. Within the physiological range of 0 to 35◦C, arise in temperature stimulates microbial activity. As a consequence, the EON pool mayincrease at higher temperatures as a result of enhanced microbial decomposition of insol-uble organic matter (Kalbitz et al. 2000). On the other hand, increased microbial activityalso results in enhanced mineralization of EON. Almost all studies addressing the influ-ence of temperature on the amount of EON are conducted in a field situation looking forseasonal dependency of dissolved organic compounds. These results are often contradic-tory and probably related to different experimental methodologies, seasonal dependency of

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Nitrogen Mineralization 1385

agricultural management, and varying hydrological situations (Jensen et al. 1997; Murphyet al. 2000; Christou, Avramides, and Jones 2006; Vinther, Hansen, and Eriksen 2006;Ghani et al. 2007). Few studies address the influence of temperature on the amount ofEON and its relation with net N mineralization under controlled and optimal water con-ditions. Because temperature and water interact with regard to N mineralization (Cabrera,Kissel, and Vigil 2005), it is nowadays not clear how the amount and the role of EONin mineralization is related to temperature-induced changes under optimal soil waterconditions.

In this study we investigated (1) the influence of temperature on EON, EOC, and Nmineralization and (2) how net N mineralization can be described for different times andtemperatures by the use of the initial EON fraction, measured in 0.01 M CaCl2.

Materials and Methods

Soil samples from five soils with differing soil characteristics were collected from a 0- to30-cm depth in the Netherlands and Argentina (Table 1). The soil samples in Argentinawere sampled in 2002. The Dutch soils were sampled in 1994, used in previous experi-ments (Van Erp, Houba, and Van Beusichem 2001; Bregliani et al. 2006), and stored driedfor more than 8 years before use in our incubation experiment. One agricultural soil wasderived from a small farm in Río Gallegos, Patagonia, Argentina (soil A) where the pre-vious crop was lettuce; the second one was derived from a natural forest soil (soil B) ofAnthartic B (Nothofagus antarctica Oerst.) and lenga beech (N. Pumilio Krasser) near ElChalten, Santa Cruz Province, Argentina. The other three agricultural soils originated fromBatenburg (soil C), Dwingeloo (soil D), and Velp (soil E), locations in the Netherlands.Maize (Zea mays L.) had been grown on sites D and E, and the previous crop on site C waswheat. The experimental work was performed in the Netherlands.

After air-drying and sieving (<2 mm), 100 g of each soil were rewetted to 60% ofwater-holding capacity (WHC) and well mixed. The WHC was determined by adding dem-ineralized water to the soil until it became completely saturated with water. The mass ofwater added was recorded, and the maximal WHC was calculated. Soil samples were incu-bated in duplicate in polyethylene bags (Audiothene 0.10 mm, art. no. A15100, AudionElektro BV, the Netherlands), allowing gas exchange with the atmosphere. Polyethylene ispermeable to O2 and CO2 but impermeable to water (Gordon, Tallas, and Van Cleve 1987).Soils A, C, D, and E were incubated at 4, 20, and 30 ◦C in climate chambers, whereas soil B(forest soil) was incubated at 4 and 20 ◦C only due to lack of soil. Different extractable N(EON, NO3, NH4) and C (EOC) fractions were determined on oven-dried soils (40 ◦C,24 h) after 0, 7, 14, 28, 56, and 84 days of incubation according to the 0.01 M CaCl2method of Houba et al. (2000).

The concentrations of total dissolved N (Nts), NH4, and NO3 were measured spec-trophotometrically using a segmented flow analyzer (Houba et al. 2000). The EON wascalculated by subtracting NO3 and NH4 from Nts. The EOC was measured using an auto-matic C analyzer (model SK 12, Skalar, Breda, the Netherlands; Houba et al. 2000). Therelative reproducibilities of the measurement of Nts, NO3, NH4, and EOC were 2.4, 2.0,2.2, and 3.8%, respectively.

Statistical analysis of the data was performed by use of the SPSS 12.0.1 for Windows(2003) program. Soil data presented were the mean of two replicates. Where Stanford andSmith (1972) found that cumulative net N mineralization is linearly related to the squareroot of time at constant temperature, we propose here that the N mineralization rate (k) canbe determined by linear regression of the dependent variable cumulative N mineralization

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Tabl

e1

Som

ech

emic

alan

dph

ysic

alpr

oper

ties

ofus

edso

ils

Cor

gN

tota

lN

O3–

NH

4+D

ON

DO

C

Soil1

pH(C

aCl 2

)C

lay

(%)

(gkg

–1)

C/N

(mg

kg–1

)D

OC

/D

ON

A6.

36

181.

413

13.1

4.5

10.6

127.

112

B4.

610

381.

624

0.1

25.2

17.2

491.

623

C4.

715

131.

310

1.6

2.3

7.2

178.

025

D5.

88

372.

714

39.0

4.7

7.0

158

23E

5.5

119

1.0

945

.22.

25.

810

6.0

18

1So

ilA

(ara

ble)

and

B(f

ores

t)w

ere

sam

pled

inA

rgen

tina.

Soil

C,D

,and

Ew

ere

Dut

char

able

soils

.2A

bbre

viat

ions

:DO

N(d

isso

lved

Org

anic

Nitr

ogen

);D

OC

(Dis

solv

edO

rgan

icC

arbo

n);C

org

(tot

alor

gani

cC

arbo

nco

nten

t);N

tota

l(to

talN

cont

ent)

.

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(Nmin) with the square root of time × temperature as independent variable. The model usedto describe the data, therefore, was

Nmint T = k × √(t × T) + et T (1)

where NmintT is the cumulative N mineralization at time t (t = 7, 14, 28, 56, and 84 days)and temperature T (T = 4, 20, and 30 C), k is the N mineralization rate to be estimated, andetT are the random errors, assumed to be independent and N(0, σ e

2). Because k dependson the characteristics of the soil, we assumed a nonlinear relationship between k and initialEON concentrations (EONi). The model used to describe the data, therefore, was

ki = a × LN(EONi) + b + ei (2)

where k is the mineralization rate independent of time and temperature for individual soils(i = 1–5), EONi is the initial EON concentration for each soil, a and b are the regressioncoefficients to be estimated, and ei are the random errors, assumed to be independent andN(0, σ e

2).The relation between EON levels and k values was validated by use of available data in

literature. These data included several pot experiments and incubation experiments whereEON concentrations and N mineralization were measured. Initial EON values and min-eralization rates were collected from these studies. The N mineralization in the sameexperiments was predicted by using Eqs. (1) and (2). Observed and predicted N miner-alizations were evaluated using simple linear regressions. Results of this evaluation werediscussed using the mean difference between predicted and observed value (MEP), thestandard error of prediction (SEP), and the correlation coefficient.

Results and Discussion

Major soil properties are summarized in Table 1. The characteristics of the five soils var-ied, with clay contents ranging from 60 to 150 g kg–1, pH from 4.6 to 6.3, organic Cfrom 9 to 38 g kg–1, and C/N ratios from 9 to 24. NO3 and NH4 concentrations variedstrongly, from < 0.1 mg kg–1 in soil B up to 45 mg kg–1 in soil E. Net N mineraliza-tion after 84 days, calculated as the increase in inorganic N, accounted for about 15 to155 mg kg–1 (Table 2; Figure 1) and are in agreement with values commonly reported(Stanford and Smith 1972; Michrina, Fox, and Piekielek 1981; Appel and Mengel 1990;Velthof, Oenema, and Nelemans 2000). Soil B, the forest soil, showed lower nitrifica-tion rates than the other soils: the amount of NH4 did not decrease during the incubation(Figure 1). Low rates of nitrification may be attributed to its low pH, because net N nitri-fication is strongly reduced when soil pH becomes less than 4.5 (Bardgett 2005). Thisinfluence of pH is also related to different microbial populations of nitrifiers (Nugrohoet al. 2007), changing P availability, or differences in organic matter quality (Carlyle et al.1990).

Roughly all soils show the expected tendency that greater incubation temperature leadsto more N mineralization (Table 2; Figure 1). Nitrification seems to be more limited atlower temperatures than ammonification, resulting in increasing amounts of NH4 at lowertemperatures (except soil A). We observed only small differences between the 20 and 30 ◦Ctreatment (Table 2), although De Neve, Pannier, and Hofman (1996) indicated that theoptimal temperatures for microbial activity was greater than 35 ◦C. After 84 days, net

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Table 2Net N mineralization for soils A to D after 84 days, incubated at 4, 20, and 30 ◦C

Net N mineralization (mg kg−1) at t = 84 days

Soil1 4 ◦C 20 ◦C 30 ◦C

A 45.0 108.2 154.5B 93.2 130.9 –C 23.8 67.7 74.6D 41.9 79.7 91.4E 14.7 36.7 47.4

1Soil A (arable) and B (forest) were sampled in Argentina. Soil C, D, and E were Dutcharable soils.

N mineralization was increased by factors of 1.4 to 2.9 and 1.1 to 1.4 when temperatureincreased from 4 to 20 ◦C and from 20 to 30 ◦C, respectively.

Concentrations of EOC and EON changed at different rates depending on soil char-acteristics and temperature (Figure 2). In the first 30 days, EOC concentration decreasedby 50 to 300 mg kg–1, with the greatest decrease in soils C and D. This change in EOCoccurred simultaneously with increasing amounts of mineral N. It is generally assumed thatwhere microbial growth is C limited, microorganisms will use DOC to support their energyneeds (Schimel and Bennett 2004; Bardgett 2005). Almost all soils showed a slightlyincreasing release of EOC after 30 days in correspondence with decreasing mineralizationrates, suggesting the realization of a new mineralization–immobilization equilibrium. TheEON concentrations are low (5–35 mg kg–1) compared to mineral N (50–150 mg kg–1).Changes in EON do not correspond to subsequent changes in mineral N. Appel and Mengel(1998) also observed no general trend of decreasing EON levels corresponding to net Nmineralization. This result suggests that the size of the EON pool does not necessarilyreflect the flux through it (Haynes 2005).

Appel and Mengel (1998), Murphy et al. (2000), and Groot and Houba (1995), how-ever, indicated that the amount of EON can provide an index of easily mineralizable soil N.In agreement with their indication, we observed a significant and positive correlationbetween initial EON concentrations and cumulative N mineralization (R2 = 0.76–0.94;P < 0.05). The significant relationship between EON and cumulative N mineralizationsuggests that differences in pool size of EON among soils can give an indication of theamount of mineralizable N. This relationship between EON and cumulative N mineral-ization, however, is not necessarily a causative relationship. This relationship may dependon the soil microbial biomass, because both EON and cumulative N mineralization areaffected by the size of the biomass (Carter et al. 1999; Haynes 2005).

The influence of temperature on the amount of EON is ambiguous, and no explanationis readily available for these different patterns. Different patterns of EOC and EON duringthe incubation indicated that they are not involved equally in the mineralization process.This agrees with results from others who indicated that factors driving DON productionare different from those driving DOC production (Michalzik and Matzner 1999; Kalbitzet al. 2000; McDowell 2003). It also implies that the quality of EON deviates during themineralization process, indicating the importance of techniques that quantify the size ofthe biologically active part of EON. These techniques could include the fractionation of

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Figure 1. Effects of temperature (4, 20, 30 oC) and time on NO3 and NH4 concentrations (mg kg–1)in soils A to E. Mean values and standard error (vertical lines).

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Figure 2. Effect of temperature (4, 20, 30 oC) and time on EOC and EON concentrations (mg kg–1)in soils A to E. Mean values and standard error (vertical lines).

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EON in high-molecular-weight or low-molecular-weight fractions (Jones et al. 2004) orthe fractionation of EON in humic, fulvic, or hydrophilic acids (Qualls 2005).

The response of mineralization to temperature is commonly represented by anArrhenius relationship or its derivative, the temperature coefficient Q10. Q10 values givean indication of the rate of change of a biological system as a consequence of increas-ing the temperature by 10 ◦C. Stanford and Smith (1972) reported that cumulative netN mineralization was related linearly to the square root of time at constant temperature.In our incubation experiment, net N mineralization was significantly linearly correlatedwith

√(t × T) with slope k (Figure 3, r2 values ranging between 0.71 and 0.95 for the

five soils). Therefore, we propose that temperature-induced changes in N mineralizationcan be described by this nonlinear relationship between N mineralization and time fordifferent temperatures [according to Eq. (1)]. The k values in our incubation experimentranged between 0.9 and 3.2 for the different soils. Recalculating k values from the results ofStanford and Smith (1972), who conducted an incubation experiment with 39 soils at 35 ◦Cfor 30 weeks, resulted in quite similar k values (ranging between 0.3 and 3.2). However,calculating Q10 values from Eq. (1) for temperature increases from 5 to 15, 15 to 25, and 25to 30 ◦C results in lower values (1.7, 1.3, and 1.2 respectively) than the widespread approx-imation of 2 given by Stanford, Frere, and Schwaninger (1973). Greater variation in Q10values was also observed by De Neve, Pannier, and Hofman (1996), Andersen and Jensen(2001), and Agehara and Warncke (2005). They indicated that the temperature coefficientheavily depends on the kind of organic matter, because soils incubated with crop residuesrevealed a much greater response.

A significant relationship (P = 0.02) between these k values and the initial EON con-centration [Eq. (2); Figure 4] suggests that the initial amount of EON is indicative for thesoil mineralization potential. The calibrated parameters a and b were 2.33 (±0.52; standarderror) and –2.83 (±1.13), respectively. So, Eq. (2) can be rewritten as follows:

k = 2.33 (±0.52) × LN(EONi) – 2.83 (±1.13) (3)

Based on this relationship, we are able to describe net N mineralization at different timesand at different temperature with relatively small deviations from the observed values (seeFigure 5A). The mean difference between predicted and measured values in this incubationexperiment is small, about 1 mg kg–1. The SEP is about 11 mg kg–1, indicating that 75%of the predicted values deviate with maximal 11 mg kg–1 and that 95% of the predictionsare within a deviation of 22 mg kg–1. Although the proposed relationship is based on arelatively small dataset (five soils in duplicate), the low SEP values and high R2 valuesranging between 0.80 and 0.98 suggest that this approach can be used for predicting Nmineralization with time. A preliminary validation of Eq. (2) with data from the literatureemphasizes that the relation between k and EON needs to be calibrated on a larger datasetwith soils varying in pH, organic matter, and EON content. Results of this preliminaryvalidation are summarized in Table 3, partly presented in Figure 5B.

Bregliani et al. (2006) conducted one incubation experiment and three pot experi-ments with maize on 18 agricultural soils for 2 months. We exclude one soil from theirdata because of its divergent organic matter content (24%) and unrealistic high mineraliza-tion rate for arable soils (10 mg kg–1 d–1). Initial extracted EON values were significantlyrelated to net N mineralization (R2 = 0.70–0.83, P < 0.001). Using Eqs. (1) and (2), we findk values ranging between –0.7 and 3.9 and a significant positive relationship between mea-sured and predicted cumulative N mineralization (R2 = 0.59–0.76, P < 0.001, Figure 5B).The SEP values vary between 26 and 36 mg kg–1, indicating that 95% of the predicted

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100

150

4º20º30º

4º20º30º

4º20º30º

4º20º30º

4º20º30º

4º20º30º

4º20º30º

4º20º30º

4º20º

4º20º

100

150

200Soil A Soil A

Soil B Soil B

Soil C Soil C

Soil D Soil D

Soil E Soil E

200

0

50

0 15 30 45 60 75 90

y = 2.91xR2

= 0.95

y = 2.18xR2

= 0.93

y = 0.90xR2

= 0.83

y = 1.52xR2

= 0.97

y = 3.52xR2

= 0.71

0

50

0 10 20 30 40 50 60

Cu

m. N

min

(m

g.k

g–

1)

Cu

m. N

min

(m

g.k

g–

1)

100

150

200

0

50

Cu

m.

Nm

in (

mg

.kg

–1)

100

150

200

0

50

Cu

m.

Nm

in (

mg

.kg

–1)

100

150

200

0

50

Cu

m.

Nm

in (

mg

.kg

–1)

100

150

200

0

50

Cu

m. N

min

(m

g.k

g–1)

100

150

200

0

50

Cu

m.

Nm

in (

mg

.kg

–1)

100

150

200

0

50

Cu

m.

Nm

in (

mg

.kg

–1)

100

150

200

0

50

Cu

m.

Nm

in (

mg

.kg

–1)

time (d)

0 15 30 45 60 75 90time (d)

0 15 30 45 60 75 90time (d)

0 15 30 45 60 75 90time (d)

0 15 30 45 60 75 90time (d)

√(t × T) (√d × °C)

0 10 20 30 40 50 60√(t × T) (√d × °C)

0 10 20 30 40 50 60√(t × T) (√d × °C)

0 10 20 30 40 50 60√(t × T) (√d × °C)

0 10 20 30 40 50 60√(t × T) (√d × °C)

100

150

200

0

50

Cu

m. N

min

(m

g.k

g–1)

Figure 3. Net N mineralization (mg kg–1) at different times and temperatures (4, 20, 30 oC) duringincubation of soil A to E (the left side), and the relation between net N mineralization (mg kg–1) and√(t x T), with slope k as a temperature and time independent N mineralization rate (the right side).Mean values and standard error (vertical lines).

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4

2

3

y = 2.33x – 2.83R2

= 0.87

0

1

1.5 2.5

Min

era

lizati

on

rate

, k

2 3

LN (EONi)

Figure 4. Mineralization rate (k) as a function of the log-transformed EONi concentrations(mg kg–1).

150

200

250

5A

0

50

100

1:1 lineSoil A - r2 = 0.94Soil B - r2 = 0.80Soil C - r2 = 0.98Soil D - r2 = 0.89Soil E - r2 = 0.84C

um

. N

min

pre

dic

ted

(m

g.k

g–1)

–50–50 0 50 100 150 200 250

Cum. Nmin observed (mg.kg–1)

Figure 5A. Comparison of the measured and predicted cumulative N mineralized (mg kg–1) for soilA to E at different temperatures and times.

values are within a deviation of 52 to 72 mg kg–1. The best predictions were given forthe incubated soils and for pot experiment 1 (performed in February–March) with a meandifference of about 1 mg kg–1. This validation results indicated that our approach couldbe applied for a situation with fluctuating moisture contents. The prediction of the netN mineralization in the fertilized pot experiment is overestimated by about 27 mg kg–1,suggesting either lower mineralization rates after fertilization or greater losses due to deni-trification. Prediction of the net N mineralization in the experiments of Curtin et al. (2006)and Nunan et al. (2001) results also in significant positive correlations (P < 0.01) betweenmeasured and predicted values, with SEP values ranging between 14 and 17 mg kg–1

(Table 3). In both situations, however, the mineralization is overestimated by about 55to 70 mg kg–1, probably due to different experimental characteristics. Curtin et al. (2006)measured EON in hot water extracts instead of 0.01 M CaCl2, thereby obtaining greateramounts of EON. In addition, the EON concentrations measured by Nunan et al. (2001)were greater due to the lower soil–solution ratio used, and their cumulative amounts of N

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Cu

m. N

min

pre

dic

ted

(m

g.k

g–1)

200

250

5B

50

100

150

–50

0

–50 0 50 100 150 200 250

line 1:1incubation exp.pot exp. 1 unfertilizedpot exp. 2 unfertilizedpot exp. 3 fertilized

Cum. Nmin observed (mg.kg–1)

Figure 5B. Measured and predicted values of cumulative N mineralized (mg kg–1) for oneincubation and three pot experiments derived from Bregliani et al. (2006).

mineralized were less through the use of field-moist and unsieved soils in their pot exper-iment. The N mineralization rates observed by Mengel, Schneider, and Kosegarten (1999)were low because of high immobilization in their grass-growing pot experiment: grassroots release generally high amounts of labile DOC compounds, resulting in N immobi-lization. The N mineralization in their experiment was also underestimated because theyharvested only the aboveground plant material, although root organic matter can containhigh amounts of N (Whitehead, Bristow, and Lockyer 1990). The poor prediction of theirresults questions whether the relation between k and EON depends on the agriculturalland use (grassland vs. arable land). In addition, it seems that the prediction is influ-enced negatively by the length of the experiment. Predictions in short-term experiments(<3–5 months) were generally better than in long-term experiments (Table 3).

Based on our results, the observations of Groot and Houba (1995) and Appel andMengel (1998), we conclude that the initial EON values often are correlated significantlywith net N mineralization. Net N mineralization can be predicted with our approach withrelatively low SEP values, although the approach seems to be valid only within periodsshorter than 3 to 5 months. Before application into field-based fertilizer recommendations,it is necessary (1) to calibrate the relation between k and EON for more grassland andarable soils and (2) to investigate the length of time for which the obtained relationship isapplicable.

Conclusions

Net N mineralization increased with temperature. Amounts of EOC decreased with timeand temperature, indicating enhanced microbial uptake of C. Temperature-induced changesshowed contradictory effects on the amount of EON, suggesting different quality of theinvolved fraction. Changes in EON do not correspond to subsequent changes in mineral N,indicating that the flux through the EON pool was greater than the change in the size ofthe EON pool. Nonetheless, the size may well be indicative of the flux through it, and as aresult, EON could be a valuable indicator of N mineralization. This indication is supportedby our finding that the cumulative N mineralization at different times and temperatures

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Tabl

e3

Val

idat

ion

mod

elw

ithlit

erat

ure

data

;sta

tistic

alre

sults

and

gene

rali

nfor

mat

ion

Res

ults

valid

atio

nb

Met

hod

desc

ript

ion

Exp

DO

N-N

min

ak

valu

esr2

�SE

P

Sour

ce:

Nun

anet

al.(

2001

)Y

=1.

46X

+14.

0;1.

6to

4.1

0.39

5517

Pote

xp.b

arle

yat

8–15

◦ Cfo

r60

days

;16

agr.

soils

,fie

ld-m

oist

;0.0

1M

CaC

l 2ex

trac

t.on

drie

dsa

mpl

esfo

r2

h,1:

5w

:v;I

nitia

lDO

Nra

nge:

6.5–

19.9

mg

kg−1

r2=

0.42

Sour

ce:

Men

gele

tal

.(19

99)

IY

=1.

73X

−0.0

71.

3to

3.7

0.51

108

332

pote

xp.f

allo

w(I

)/gr

ass

(II)

at20

–25

◦ Cfo

r20

8da

ys;2

0(m

ainl

y)ag

r.so

ils,d

ried

and

rew

ette

d;0.

01M

CaC

l 2r2

=0.

61

extr

act.

2h

ondr

ied

sam

ples

,1:1

0w

:v;

IIY

=3.

70X

+34.

41.

3to

3.7

0.36

5630

Initi

alD

ON

rang

e:5.

6–16

.5m

gkg

−1r2

=0.

44

Sour

ce:

App

elet

al.(

1990

)Y

=5.

26X

−21.

92−0

.4to

1.5

0.11

1720

Pote

xp.r

ape

at18

.5◦ C

for

62da

ys;1

2ag

r.so

ils,d

ried

and

rew

ette

d;0.

01M

CaC

l 2ex

trac

t.on

drie

dsa

mpl

esfo

r2

h,1:

10w

:v;I

nitia

lDO

Nra

nge:

2.7–

6.1

mg

kg−1

r2=

0.36

Sour

ce:

Cur

tin

etal

.(20

06)

Y=

0.83

X+1

4.0

2.8

to6.

30.

4868

14In

cuba

tion

exp.

at20

◦ Cfo

r28

d;30

agr.

soils

;fiel

dm

oist

stat

us;w

ater

extr

act.

ondr

ied

sam

ples

,1:7

.5w

:v;I

nitia

lDO

Nra

nge:

11–5

5m

gkg

−1

r2=

0.41

a Rel

atio

nst

atis

tics

betw

een

initi

alD

ON

(X)

and

netN

min

eral

izat

ion

(Y).

bk

valu

esar

epr

edic

ted

with

Eq.

2.;

r2is

corr

elat

ion

coef

ficie

ntof

linea

rre

gres

sion

ofm

easu

red

and

pred

icte

dN

min

eral

izat

ion;

�gi

ves

the

mea

ndi

ffer

ence

betw

een

mea

sure

dan

dpr

edic

ted

Nm

iner

aliz

atio

n(m

gkg

−1)

and

SEP

give

sth

est

anda

rder

ror

ofth

epr

edic

tion

(mg

kg−1

).

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could be predicted by the initial amount of EON measured in a 0.01 M CaCl2 extract. NetN mineralization rates at different times and temperatures were determined assuming alinear relationship between N mineralization and the square root of time × temperature.Initial concentrations of EON were correlated positively to these soil-dependent mineral-ization rates. This result seems promising, although preliminary validation with results ofpot experiments emphasizes the need for additional calibration of the proposed approach.

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nitrogen mineralization in soils related to initial extractable organic nitrogen: effect of...

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