Nitrogen fluxes in a cropped sandy and a loamy soil measured by sequential coring

Eur. J. Agron., 1994, 3(4), 311-316

Nitrogen fluxes in a cropped sandy and a loamy soil measured by sequential coring F. P. Vinther The Danish Institute for Plant and Soil Science, Department of Soil Science, Research Centre Foulum, P.O. Box 23, DK-8830 Tjele, Denmark.

Received 15 May 1993; accepted 4 March 1994

Abstract

During a three year period N fluxes in the plough layer (0-20 em) were measured in a coarse sandy soil and a sandy loam soil using in situ core incubations. N mineralization, nitrification and plant uptake plus leaching from the plough layer were calculated for treatments with barley and a catch crop. Due to high variability no statistically significant differences were observed between treatments, but temporal variations in N fluxes were reflected by the technique. Rates of N mineralization in the sandy loam soil were 184, 146, and 198 kg N ha-I yr- 1, respectively. In the coarse sandy soil the corresponding values were 65, 60, and 70 kg N ha- 1 yr- 1 in treatments fertilized with 60 kg N ha- 1 yr- 1, and 93, 73, and 87 kg N ha- 1 in treatments receiving 120 kg N ha- 1 yr- 1• Key-words : in situ incubation, N balance, nitrification, N mineralization.

INTRODUCTION Nitrogen availability in agricultural soils is determined by fertilization and by N mineralization. It has proved difficult to measure mineralization rates in the field because of the lack of suitable techniques. However, a sequential in situ coring technique has been developed (Adams and Attiwill, 1986; Raison et al., 1987) for measuring N mineralization, as well as other N fluxes such as nitrification, leaching, and plant uptake. The technique has been used to study N fluxes in forest soils (Adams and Attiwill, 1986 ; Raison et al., 1987; Whynot and Weetman, 1991), peat soil (Williams and Wheatley, 1992), grassland soils (Hatch et al., 1990, 1991 ), and fertilized agricultural soils (Debosz and Vinther, 1989; Boone, 1990; Mazzarino et al., 1991 ; Debosz et al., 1991 ; Debosz, 1994), and has shown promising results concerning mineralization measurements in these ecosystems. The main objectives of this study were to : (i) estimate N fluxes in an agricultural cropping system and (ii) further evaluate the technique for use in agricultural systems.

MATERIALS AND METHODS Measurements of the N fluxes were carried out during the years 1988-1991 on two soil types in DenISSN 1161-0301194104/$ 4.001© Gauthier-Villars - ESAg

mark, a coarse sandy soil in Jyndevad and a sandy loam soil in 0dum. The following treatments were included : (i) spring barley without a catch crop, (ii) spring barley with a catch crop (ryegrass, Lolium perenne L.), ploughed under in the autumn and (iii) spring barley with a catch crop, ploughed under in the following spring just before sowing the next crop. In Jyndevad two fertilization levels were included, corresponding to 60 and 120 kg N ha- 1 yr- 1 • The fertilizer was calcium-ammonium-nitrate containing ammonium and nitrate in the ratio 1 : 1. In 0dum there was only one level of fertilization (130 kg N ha- 1 yr- 1), and the fertilizer was applied as NPK (21-4-10) also containing ammonium and nitrate in the ratio 1 : 1. The field experiments were arranged in split-plot designs with four replications. The technique as described by Raison et al. ( 1987) was used. At the start of each incubation period, soil samples were collected and PVC-tubes with a diameter of 35 mm were inserted 20 em into the soil and equipped with a lid to prevent leaching of nitrogen during the incubation period. At the end of the incubation period, which lasted from one to two months depending of the time of the year, the soil from the tubes was collected, and a second incubation was started. The number of replicates and analyses per treatment are shown in Table 1.

F. P. Vinther

312

Table 1. Number of samples, treatment.

No. of samples per plot

replications and analyses per

No. of replicates (blocks)

No. of analyses per plot*

No. of analyses per treatment

Bulk soil:

10 10

Jyndevad 0dum

4 4

4 4

4 4

8 16

Incubated soil :

6 10

Jyndevad 0dum

2

4

* Soil in tubes (incubated soil) were pooled and divided into 2 and 4 samples, respectively.

Inorganic nitrogen in the soil samples was extracted by shaking for 1 h with 2 M KCl in a soil :liquid ratio of 1 : 2. Concentrations of extracted ammonium and nitrate were determined by colorimetric methods adapted to an automatic flow injection system (PIAstar®, Tecator, Hoganas, Sweden). The content of nitrogen was calculated on dry soil basis. Moisture content was determined gravimetrically on subsamples dried to constant weight at 105 oc. Statistical calculations were performed using SASprocedures (SAS Institute Inc., 1989). The General Linear Model (GLM) procedure was used for analysis of variance to determine statistically significant differences between treatments. The number of analyses (samples) required to estimate inorganic N pools to within a 10 or 20 per cent standard error of the mean were calculated for each treatment based on the formula of Whynot and Weetman ( 1991) :

The difference in the concentration of inorganic nitrogen in the bulk soil before incubation (Nb) and at the end of incubation (Nb
= t2

S2 I E 2 , where N = no. of analyses per treatment, t t for a given probability, N

= Student's

S = variance, and E = precision desired. The number of analyses (N) was then correlated with the months in which the samples were collected in order to evaluate whether fertilization and plant growth affected the number of analyses necessary to obtain a given accuracy rate.

30 25

z

Den itrif icati on( Nden) 20

0 'Q)

-~ 15

Mineraliza!" n(N~;n) Nitrifica . n(Nn;1,J

E

~ 10

ll1

Nb(o)= N H4(b(OJJ + N 03(b(O))

N,<•l=NH•
Plant uptake plus leaching ( Nplupt+leoch)

Nb(tl= NH•(b(tJJ+ N03(b(tJJ

5

Exposure period starf(O)

end( I)

Figure 1. Scheme used to estimate N fluxes during one incubation period.

Net mineralization

= Nmin

Net nitrification Plant uptake plus

= Nnitc

= (Ncttl - NbtOJ) + Ncten = (N03c(t) - N03b(OJ) + Nden

= Nplupt+leach

= Nc(t) - Nb(t)

leaching

The denitrification loss (Ncten) has been measured (Vinther, 1992), and was in the coarse sandy soil less than 1 kg N ha- 1 yr- 1 , and in the sandy loam soil 14, 9 and 14 kg N ha- 1 yr- 1 , respectively, during the three years of investigation.

RESULTS AND DISCUSSION No significant differences between treatments were found for either mineralization (Table 2), nitrification or 'loss' (plant uptake + leaching), when testing individual soils. Only the effects of major components were significant, e.g. soil type (p < 0.05), level of fertilization (p < 0.001) and the time of the year at which incubations took place (p < 0.001). The interactions between treatments and incubation period were also non-significant, indicating that there were no periods during the year where significant differences between treatments could be found. Any real treatment effects that may have occurred were obscured by high variability. An analysis of the number of samples required to estimate inorganic N pools with 10 per cent precision showed that up to 150 samples were needed, as compared to the 8 or 16 used in the two soils, respectively (Table 1). The number of samples required depended on the period of the year where incubations took place. In Figure 2, the number of samples needed to obtain 20 per cent precision is presented. This shows a need to sample more intensively after fertilization and in the growing period, than in the autumn and winter periods. Similarly, Whynot and Weetman ( 1991) concluded that due to high spatial variability in inorganic pools following fertilization, significant differences could not be found between the calculated Eur. J. Agron.

Nitrogen fluxes in soil as measured by sequential coring

flux rates. Due to the high spatial variability, and consequently the lack of significant differences between treatments, the N fluxes presented below were calculated as means of treatments.

313

c-30 +-o 0

!....

(I)

Q)

Table

2. Summary

of analysis

of variance

on

calculated

N-mineralization.

soil N level residual total Jyndevad: N level residual total

I 2

u

u

>-

0 20

0

c

~

Sandy loam soil Coarse sandy soil

25

(I)

0

Degrees of freedom

::J

eoeee a:xxy::)

15

F-value 7.8* 16.2***

654 657

15.6**"' 429 430

J FMAMJ JASOND Figure 2. Number of samples required to estimate pools of inorganic N with 20 per cent precision.

60-N: incub. period (ip) block treatment ip * treatment residual total

17 3 2 34

2.6** L5n' O.ln' 1.2n'

159 215

120-N: incub. period (ip) block treatment ip * treatment residual total

17 3 2 34 !59 215

12.2*** 0.5n' 1.4n'

1.3ns

0dum: incub. period (ip) block treatment ip * treatment residual total

18

4.56***

3 2

0.4Jn'

36 168 227

1.45"' 0.80n'

* p < 0.05 ** p < 0.001 *** p < 0.001 ns

non significant

Monthly fluxes of nitrogen showed similar trends for mineralization, nitrification and plant uptake + leaching ; increasing rates during the spring and summer followed by decreasing rates during the autumn and winter periods. Figure 3 shows the monthly N-mineralization rates. Highest mineralization rates were measured in the periods after fertilizer application, at the same time as the content of inorganic Vol. 3, ll 0 4- 1994

nitrogen dropped to background level, and during the growing season. The mineralization rates measured during the growing periods in the sandy loam soil ranged from 0.3 to 1.5 kg N ha- 1 d- 1 , whereas the rates during the winter periods were in the range 0.1 to 0.5 kg N ha- 1 d- 1 • The corresponding values in the coarse sandy soil were 0.15-0.5 kg N ha- 1 d- 1 and 0.03-0.17kg N ha- 1 d- 1 , respectively. Debosz et al. (1991 ), also using the in situ core technique, measured N mineralization rates from 0.02 to 1.9 kg N ha- 1 d- 1 in a sandy loam soil in Denmark to which pig slurry had been applied. At the same location as in the present investigation (coarse sandy soil in Jyndevad) Debosz et al. (1991) measured rates from 0.1 to 1.3 kg N ha- 1 d- 1, which corresponds well with the present results. In grass and grass/clover swards in S.W. England, Hatch et al. (1990) found daily mineralization rates ranging from 0.02 to 1.90 kg N ha- 1 • This is slightly lower than those found by Hatch et al. (1991), who measured overall mean daily rates from 1.7-2.3 kg N ha- 1 d- 1 in grass swards. The higher N mineralization rates during the growing periods can be attributed to different factors such as soil temperature, the in situ core technique itself, and re-mineralization of microbially immobilized fertilizer nitrogen, or most likely to a combination of these factors. Firstly, soil temperatures in Denmark during the months from March to July increase from 3-5 oc to 15-20 °C, which partly explains the increasing N mineralization rates during this period. The relationship between temperature and N mineralization is generally

314

F. P. Vinther

40

~

"'

120 ..c:

=

100 ~ r:

80

g"' '1'

. 0

T:

Ol

i5

£

1988

I~

1989 Mineralization

1990

1991

lnorg. nitrogen

=0>1

.>£

-~ -~

~ c

6

~

2

1988

1989

1990

1991

Figure 3. Mineralization rates 011 a monthly basis in the sandy loam soil (A) and the coarse sandy soil (B). Arrows i11dicate application time and these values of i11organic N were 1101 measured.

described with a Q 10 of approximately 2 (Stanford et al., 1973; Addiscott, 1983; Kladivko and Keeney, 1987). Secondly, the method itself may contribute to higher mineralization rates during the growing season by disturbing the natural water balance in the incubated soil and by severing roots when inserting the tubes into the soil or by isolating roots from the incubated soil. Mineralization rates are known to be sensitive to the content of soil water (Cassman and Munns, 1980; Myers et al., 1982). By protecting soil in the covered tubes from rain and by preventing plants from taking up soil water from the incubated soil, the moisture content is likely to be different from that of the bulk soil. In the present investigation, the difference between soil moisture content inside and outside the tubes on most occasions were statistically nonsignificant. However, moisture contents in soil incubated in tubes were during the growing season consis-

tently higher than the moisture contents in the bulk soils, with a few exceptions where sampling was done shortly after rain showers. Maximum difference was observed once where samples were collected while it was raining following a dry period. This resulted in a 6 per cent higher soil moisture content in the bulk soil than in the incubated soil. During the winter period (October to March) the difference was never above 1 per cent. Similarly, Whynot and Weetman (1991 ), who used the coring technique, and Redman et al. (1989) incubating their soils in plastic bags, observed that the soil moisture contents were higher in the incubated soils than in the bulk soils during the growing period. The slightly higher moisture content inside the tubes may also contribute to higher mineralization rates during these periods. The problems arising as a consequence of isolating roots from the incubated soil or root severing have been discussed with somewhat contradictory conclusions (e.g. Raison et al. , 1987 ; Rees, 1989 ; Hatch et al., 1990). Incubating soil in isolation from plant roots often results in a rapid increase in mineral N, and immobilization of mineralized N, especially in soils with high microbial activity (Rees, 1989). In that case an underestimation of the mineralization rates by the in situ core method can be the consequence. Mineralization rates obtained by the method may also be overestimated through decomposition of excised roots and the release of inorganic nitrogen (Hatch et al., 1990). On the other hand, Raison et al. ( 1987) noted that the inclusion of excised roots might result in increased N immobilization and therefore underestimate mineralization. During the first two months after fertilizer application a considerable increase in the mineralization rates was observed (Figure 3), which partly can be explained by the factors described above. But also re-mineralization of microbially immobilized fertilizer nitrogen may contribute to the higher rates in this period. Whynot and Weetman (1991) found that even during the period immediately following fertilization, net mineralization occurred in all fertilized plots. The accumulated N fluxes expressed as annual fluxes are shown in Table 3. During the three years of investigation, the annual N mineralization was 184, 146 and 198 kg N ha- 1, respectively, in the sandy loam soil. The corresponding values for nitrification were 179, 178 and 195 kg N ha- 1, showing that all mineralized nitrogen was also nitrified. In the coarse sandy soil the corresponding values for N mineralization were 65, 60, and 70 kg N ha- 1 yr- 1 in treatments fertilized with 60 kg N ha- 1 yr- 1, and 93, 73, and 87 kg N ha- 1 yr- 1 in treatments receiving 120 kg N ha- 1 yr- 1• In the coarse sandy soil nitrification accounted for 97-146 per cent of the mineralization. The amount of nitrogen which was nitrified was generally greater than the amount mineralized. This was probably due to the fact that 50 per cent of the fertilizer was in ammonium form, and could therefore also contribute Eur. J. Agron.

Nitrogen fluxes in soil as measured by sequential coring

315

Table 3. Annual fluxes of inorganic nitrogen (kg N ha- 1 ) in the plough layer (0-20 em) of a sandy loam soil and a coarse sandy soil. Bracketed values represent standard error of mean of the three treatments.

Period

N input/output

1988-89:

Input Deposition* Fertilization Mineralization Output Uptake+ leaching Denitrification** Input - output

1989-90:

1990-91 :

Input Deposition* Fertilization Mineralization Output Uptake+ leaching Denitrification** Input - output Input Deposition* Fertilization Mineralization Output Uptake+ leaching Denitrification** Input - output

Soil type Sandy loam

Coarse sand

21 130 184(18)

21 60 65(7)

21 120 93(4)

317(30) 14 4

139(11)

269(48)

6

-36

21 130 146(6)

21 60 60(3)

21 120 73(9)

305(10) 9

154(4)

250(8)

-17

-14

-37

21 130 198(23)

21 60 70(18)

21 120 87(19)

302(10) 14 33

107(10)

198(49)

43

29

I

I

*from Asman and Runge (1991), **from Vinther(l992).

to the nitrification. At both locations, but especially with the sandy loam soil, the total mineralization was less in the second year than in the first and third year. This was most likely due to the climatic conditions. The second year of investigation (1989) was very dry. The precipitation was at the sandy loam location only 75 per cent of normal, whereas the precipitation at the coarse sand location was 91 per cent of normal, resulting in only a minor reduction in mineralization (Table 3). To estimate in situ mineralization, which is highly dependent upon environmental conditions, it is important that measurements are carried out under conditions which are as close as possible to the natural conditions. By using the in situ core method, soil disturbance and compaction are minimal, and the method provides a relatively rapid and convenient means of estimating the N fluxes. Results showed that seasonal and annual fluctuations were positively correlated to temperature and precipitation, respectively. The major disadvantages of the method are related to the different conditions which exist inside and outside the incubated cores. Vol. 3, n° 4- 1994

ACKNOWLEDGEMENTS I wish to thank Ingrid Hoy and Bodil Molnitz for technical assistance, and the Danish Agricultural Research Council for financial support. REFERENCES Adams M.A. and Attiwill P.M. (1986). Nutrient cycling and nitrogen mineralization in eucalypt forests of South-eastern Australia. Plant Soil 92, 341-362. Addiscott T. M. (1983). Kinetics and temperature relationships of mineralization and nitrification in Rothamsted soils with differing histories. J. Soil Sci. 34, 343-353. Asman W. H. and Runge E. H. (1991). Atmospheric deposition of nitrogen compounds in Denmark. In : Anonymous (Ed.) Nitrogen and phosphorus in soil and air. Copenhagen : Danish Ministry of the Environment, pp. 287-312 Boone R. D. (1990). Soil organic matter as a potential net nitrogen sink in a fertilized cornfield, South Deerfield, Massachusetts, USA. Plant Soil 128, 191-198. Cassman K. G. and Munns D. N. (1980). Nitrogen mineralization as affected by soil moisture, temperature, and depth. Soil Sci. Soc. Am. J. 44, 1233-1237.

316

Debosz K. and Vinther F. P. (1989). An in situ technique for simultaneous measurements of mineralization, leaching and plant uptake of nitrogen applied to agricultural soils. In : Hansen, J. Aa. and Henriksen, K., (Eds.) Nitrogen in organic wastes applied to soils. London: Academic Press, pp. 3-10. Debosz K., Djuurhus J., Maag M. and Lind A.M. (1991). Studies of N-transformation in arable soils. N-mineralization, denitrification and N-leaching. In : Anonymous, (Ed.) Nitrogen and phosphorus in soil and air. Copenhagen, Danish Ministry of the Environment, pp. 149-165. Debosz K. (1994). Evaluation of soil nitrogen mineralization in two spring barley fields. Acta Agric. scand. (in press). Hatch D. J., Jarvis S.C. and Phillips L. (1990). Field measurements of nitrogen mineralization using soil core incubation and acetylene inhibition of nitrification. Plant Soil 124, 97-108. Hatch D. J., Jarvis S.C. and Reynolds S. E. (1991). An assessment of the contribution of net mineralization to N-cycling in grass swards using a field incubation method. Plant Soil 138, 23-32. Kladivko E. J. and Keeney D. R. (1987). Soil nitrogen mineralization by water and temperature interactions. Bioi. Fertil. Soils 5, 248-252. Mazzarino M. J., Oliva L., Nunez A., Numez G. and Buffa E. (1991). Nitrogen mineralization and soil fertility in the Dry Chaco ecosystem (Argentina). Soil Sci. Soc. Am. J. 55, 515522. Myers R. J. K., Campbell C. A. and Weier K. L. (1 982). Quantitative relationship between net nitrogen mineralization and moisture contents of soils. Can. J. Soil Sci., 62, 111-124.

F. P. Vinther

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