Short Communication

Priming effect of biuret addition on native soil N mineralisation

under laboratory conditions

J.M. Xuea,*, R. Sandsb, P.W. Clintona, T.W. Paync, M.F. Skinnerc

aNew Zealand Forest Research, P.O. Box 29237, Christchurch, New ZealandbSchool of Forestry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand

cNew Zealand Forest Research, Private Bag 3020, Rotorua, New Zealand

Received 11 October 2004; received in revised form 22 February 2005; accepted 24 February 2005

Abstract

This study was carried out to quantify the priming effect of biuret on native soil nitrogen (N) mineralisation during a 112-day incubation.

Addition of biuret (100 mg 15N-labelled biuret kgK1 soil) increased the turnover rate constant of soil organic matter and had a positive

priming effect on native soil N mineralisation in two soils. The additional mineralisation was 0.65% of the total soil N (equivalent to

47.1 kg N haK1) in a sandy loam soil and 0.62% of the soil N (equivalent to 46.5 kg N haK1) in a silt loam soil.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Net N mineralisation; Priming effect; 15N-biuret; Native soil N

Biuret (C2H5N3O2, C 23.3, N 40.8, O 31.0 and H 4.9%) is

a known contaminant of urea fertilisers that may act as a

plant growth regulator (Miller et al., 1988; Xue et al., 2004)

and may also be used as a slow release N fertiliser (Miller

et al., 1996; Xue et al., 2003) for forestry. It has been

demonstrated that the stimulation of the seedling growth in

the soil treated with lower concentrations of biuret is

partially associated with improved soil N availability

created by the biuret priming effect (Xue et al., 2004), and

that biuret has a positive priming effect on soil N

mineralisation when applied at a range of concentrations

(Xue et al., 2003). However, it is not possible to measure

precisely the priming effect of biuret addition on soil N

mineralisation and to state clearly the source of N released

in the priming effect without using 15N-labelled biuret.

Therefore, the objective of this study was, using 15N, to

quantify the priming effect of biuret on net mineralisation of

native soil N.

Soils were collected from the same sites as a previous

study and the soil pre-treatment was as described by Xue

et al. (2003). Selected soil properties were listed in Table 1.

0038-0717/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.soilbio.2005.02.034

* Corresponding author. Tel.: C64 3 364 2949; fax: C64 3 364 2812.

E-mail address: jianming.xue@forestresearch.co.nz (J.M. Xue).

Moist soil (equivalent to 90 g oven-dried soil) was weighed

into 500-ml polypropylene containers and treated with 0

(B0) and 100 (B100) mg 15N-labelled biuret kgK1 (oven-

dried) soil. A known volume of 15N-biuret (C2H515N3O2)

solution (24.409 at.% 15N excess) was added to each

container of the biuret treatment to give a rate of 100 mg

biuret kgK1 soil (B100). For the control treatment (B0), the

same volume of deionised water was added. The incubation,

sampling procedures and the measurements were as

described by Xue et al. (2003). The 15N content in the

mineral N extracts and the 15N content in the Kjeldahl

digests of unfumigated and fumigated samples were

recovered by the diffusion methods of Brooks et al.

(1989); Stark and Hart (1996), respectively. All the diffused

samples were packed into tin capsules for analysis of 15N

abundance by isotope ratio mass spectrometry (Europa

Scientific, UK). Air-dried soil samples from each sampling

time were milled in a Benchtop ring mill (Rocklabs Ltd,

Auckland) and then analysed for total N and 15N by isotope

ratio mass spectrometry. All the results were expressed on

the basis of oven-dried soil (105 8C).

Repeated measures analysis of variance was conducted

on data within each soil to test significant effects of biuret,

incubation time and their interactions. Least significant

difference (LSD) was calculated to separate the means when

differences were significant. The non-linear regression using

Soil Biology & Biochemistry 37 (2005) 1959–1961

www.elsevier.com/locate/soilbio

Table 1

Selected properties for the two forest soils used

Site Kinleith Forest Burnham Forest

Soil type Taupo sandy loam Lismore stony silt loam

NZ classification (Hewitt 98) Immature orthic pumice soil Pallic Orthic brown soil

US Taxonomy Vitrand (Andisol) Usiochrept (Inceptisol)

Sand (20–2000 mm) (g 100gK1) 40 45

Silt (2–20 mm) (g 100gK1) 48 30

Clay (!2 mm) (g 100gK1) 12 25

Total N (g 100gK1) 0.29 0.30

Total C (g 100gK1) 5.86 4.30

Total C/N 20 14

Bray-P (mg kgK1) 4.9 5.3

pH 5.0 4.8

Microbial biomass C (mg kgK1) 1071 805

J.M. Xue et al. / Soil Biology & Biochemistry 37 (2005) 1959–19611960

the Marquardt option was applied to solve the first order

kinetics equation for the potentially mineralisable N pool

(N0) and the turnover rate constant (k) (Thomsen et al.,

2001). All the analyses were performed using the SAS

software package (version 8.01, 2000).

In both soils, B100 increased (P!0.05) the net

mineralisation of native soil N (excluding biuret-N),

resulting in an additional mineralisation of native soil N

by 0.65% (equivalent to 47.1 kg N haK1 based on the

standard soil mass of 2500 t haK1 to 20 cm depth) in the

sandy loam soil and 0.62% (equivalent to 46.5 kg N haK1)

in the silt loam soil at the end of the incubation (data not

shown). It was found that B100 had no significant effect on

the N0 values in both soils, but significantly (P!0.05)

increased the k values, especially in the sandy loam soil

(Table 2). From the dynamics of PI, a positive priming

effect of biuret on mineralisation of native soil N was found

throughout the period of incubation in both soils (Table 3).

Table 2

Estimates of turnover rate constant (k) and potentially mineralisable pool of nativ

Soil Biuret treatment Estimated N0 (% of soil N)

MeanGSE 9

Sandy B0 19.2G6.5 5

loam B100 6.98G0.37 6

Silt B0 5.78G0.4 4

loam B100 5.54G0.32 4

Table 3

The priming index (PI) for soil N mineralisation in the two soils during the perio

Soil PI

(Days of incubation)

2 7 14

Sandy loam 1.68* 2.19* 3.05*

Silt loam 1.81* 1.89* 1.89*

The priming index (PI) proposed by Shen and Bartha (1996, 1997)) for C minerali

Kuzyakov et al. (2000). The PI was calculated as follows: PIZ[(TMNB100-TMNB

in B100 treatment; TMNB0Ztotal mineral N in B0 treatment; BNinputZbiuret-N15NinputZbiuret-15N applied in B100 treatment. PIZ1, no priming effect; PI!1,

values followed by this symbol are significantly different (P!0.001) from 1 (no

This study confirms our previous observation that

biuret applied at low concentrations had a positive

priming effect on soil N mineralisation (Xue et al.,

2003). The dynamics of PI indicates the positive priming

effect of biuret throughout the period of incubation.

However, the use of PI for assessing the priming effect

may not distinguish a real from an apparent priming

effect. In many studies with 15N-labelled fertiliser,

apparent positive priming effects resulted from the

displacement reactions and pool substitution of the

applied 15N with soil-derived N in different N pools

(Jenkinson et al., 1985). The key difference between a

real and apparent priming effect is that a change in the

turnover rate of C and N are observed in a real priming

effect, but not in an apparent priming effect (Kuzyakov

et al., 2000). The apparent priming effects must not be

larger than the amount of mineral 15N applied, and they

are usually less than 50% of the applied N (Stout, 1995).

e soil N (N0) in B0 and B100 of the two soils during a 112-day incubation

Estimated k (day K1)

5% confidence limit MeanGSE 95% confidence limit

.49–33.0 0.0026G0.0010 0.0005–0.0048

.20–7.76 0.0140G0.0013 0.0112–0.0168

.92–6.63 0.0123G0.0014 0.0092–0.0151

.85–6.22 0.0186G0.0021 0.0149–0.0232

d of 112-day incubation

28 56 112

3.38* 3.52* 2.36*

1.71* 1.73* 1.69*

sation was modified in this study for soil N mineralisation, as introduced by

0)/BNinput!100]/15NB100/15Ninput!100) where TMNB100Ztotal mineral N

applied in B100 treatment; 15NB100Z15N in total mineral N pool in B100;

a negative priming effect; PIO1, a positive priming effect. ’*’ means the

priming effect) by t-test.

J.M. Xue et al. / Soil Biology & Biochemistry 37 (2005) 1959–1961 1961

The difference between B100 and B0 in the total mineral

N produced during the incubation was significantly larger

than the biuret-N applied in the soils, providing clear-cut

evidence for a positive real priming effect. The increased

turnover rate constant of native soil N by biuret addition

in both soils further supported a positive real priming

effect observed in this study.

The stimulation of biuret addition on the decomposition

of soil non-biomass organic matter (stabilised N pool) only

occurred at the initial stages of incubation (data not

shown). So the acceleration of soil non-biomass organic

matter decomposition induced by biuret addition was only

partially responsible for the positive priming effect

observed. In addition, the death and decay of microbes at

the later stages of incubation also contributed to the

positive priming effect. The greatest priming effect

occurred during the decrease in the amount of microbial

biomass in both soils (data not shown), indicating that, to

some extent, it was caused by the release of N from the

death of microorganisms. This observation agrees with the

mechanisms of the priming effect explained by Kuzyakov

et al. (2000). Further investigation is required of microbial

death in the biuret-treated soil at the later stages of

incubation in both soils.

Acknowledgements

We thank the University of Canterbury and the New

Zealand Forest Research for funding this PhD research, Mr

Alan Leckie, Mr Bob Bullsmith and Ms Vicki Wilton for the

technical support, and Mr Murray Davis and Mr Graham

Coker for their help on the soil property data.

References

Brooks, P.D., Stark, J.M., McInteer, B.B., Preston, T., 1989. Diffusion

method to prepare soil extracts for automated N-15 analysis. Soil

Science Society of America Journal 53, 1707–1711.

Jenkinson, D.S., Fox, R.H., Rayner, J.H., 1985. Interaction between

fertilizer nitrogen and soil nitrogen-the so-called ‘priming’ effect.

Journal of Soil Science 36, 425–444.

Kuzyakov, Y., Friedel, J.K., Stahr, K., 2000. Review of mechanisms and

quantification of priming effects. Soil Biology & Biochemistry 32,

1485–1498.

Miller, R.E., Anderson, H.W., Young, D.C., 1988. Urea and biuret

stimulate growth of Douglas-fir and western hemlock seedlings. Soil

Science Society of America Journal 52, 256–260.

Miller, R.E., Reukema, D.L., Hazard, J.W., 1996. Ammonium nitrate, urea,

and biuret fertilizers increase volume growth of 57-year-old Douglas-fir

trees within a gradient of nitrogen deficiency. Research Paper PNW-

RP-490, March 1996, Pacific Northwest Research Station, USDA

Forest Service.

Shen, J., Bartha, R., 1996. The priming effect of substrate addition in soil-

based biodegradation tests. Applied and Environmental Microbiology

62, 1428–1430.

Shen, J., Bartha, R., 1997. Priming effect of glucose polymers in soil-based

biodegradation tests. Soil Biology & Biochemistry 29, 1195–1198.

Stark, J.M., Hart, S.C., 1996. Diffusion techniques for preparing salt

solution. Kjeldahl digests, and persulfate digests for nitrogen-15

analysis. Soil Science Society of America Journal 58, 1108–1116.

Stout, W.L., 1995. Evaluating the added nitrogen interaction effect in

forage grasses. Communications in Soil Science and Plant Analysis 26,

2829–2841.

Thomsen, I.K., Olesen, J.E., Schjønning, P., Jensen, B., Christensen, B.T.,

2001. Net mineralization of soil N and 15N-ryegrass residues in

differently textured soils of similar mineralogical composition. Soil

Biology & Biochemistry 33, 277–285.

Xue, J.M., Sands, R., Clinton, P.W., Payn, T.W., Skinner, M.F., 2003.

Carbon and net nitrogen mineralisation in two forest soils amended with

different concentrations of biuret. Soil Biology & Biochemistry 35 (6),

855–866.

Xue, J.M., Sands, R., Clinton, P.W., 2004. Effect of biuret on growth and

nutrition of Douglas-fir (Pseudotsuga menziesii (Mirb) Franco)

seedlings. Forest Ecology and Management 192, 335–338.