Methodology for studying fluxes of soil mineral-N in situ

Soil Bid. Biochem. Vol. 19, No. 5, pp. 521-530, 1987 Printed in Great Britain. All rights reserved

0038-0717/87 53.00 + 0.00 Copyright Q 1987 Pcrlpmoa Journals Ltd

METHODOLOGY FOR STUDYING FLUXES SOIL MINERAL-N IN SITU R. J. RAISON,M. J. COhwLL

OF

and P. K. KHANNA

Division of Forest Research, C.S.I.R.O., P.O. Box 4008, Canberra, ACT 2600, Australia (Accepted 20 December 1986) Summary-Improved methods are needed to measure the mineralization and uptake of soil N under field conditions. Because both soil disturbance (e.g. sieving, drying and rewetting) and environmental conditions markedly affect rates of soil N mineralization, rates measured in disturbed soils in the laboratory are unlikely to be quantitatively similar to, and may not be a reliable index of those in the field. It is generally impossible to assess the usefulness of most estimates of N mineraliition as predictors of field rates because of the unknown effects of assay conditions (especially soil mixing). We show that depending on soil type, sieving can either increase net N mineralization, cause immobilization of N, alter the proportion of nitrate nitrogen produced during incubation, or induce or obscure the effects of previous fertilization on rates of N mineralization. A methodology for studying fluxes of mineral-N in soils, based on sequential soil coring and in situ exposure of largely undisturbed soil columns confined within metal or PVC tubes is described aad evaluated. The sequential measurement of changes in the quantity of soil mineral-N ia confined (no N uptake by roots) and unconfined soil allows rates of net N mineralization (or immobilization). plant uptake of N, and maximum N leaching to be calculated. It is argued that the method causes less disturbance to natural processes than others, and that it provides reliable quantitative estimates of fluxes of mineral-N in field soils. The severing of roots appeared to have no effect on the pattern of accumulation of mineral-N in several undisturbed forest soils exposed in situ for up to 130 days.

remain the best measure of the N-supplying capacity of the soil, this approach will only be useful for studying short-term N dynamics in grasslands or agronomic crops, being too insensitive and difficult to measure in forest communities which have substantial biomass and temporally varying production of litter both above and below ground. Measurements of N mineralization under controlled laboratory conditions provide an estimate of the pools of mineralizable N present at the time of sample collection. These pools are replenishable under field conditions as a result of inputs of organic N from root and above-ground litter, and the slow transformation of the more resistant pools of soil organic N. Laboratory measures of mineralization, especially on disturbed soils, can be an unreliable index of field rates (Lamb, 1980; Hart and Binkley, 1985). Methods used to measure or estimate patterns of N mineralization under field conditions include the following. (i) Exposure (equivalent to incubation of soil, except that temperatures are varying) of disturbed soil in plastic bags buried in the field (e.g. Eno, 1960; Runge, 1974; Westetmann and Crothers, 1980; Vitousek and Matson, 1985). (ii) Exposure of relatively undisturbed soil columns enclosed in plastic bags under field conditions (Vine De Santo et al., 1982; Nadelhoffer et al., 1983, 1985; Matson and Boone, 1984). Rapp et al. (1979) isolated soil columns within thin metal cans pushed into the solurn, and Adams and Attiwill (1986) used capped PVC tubes for this purpose. (iii) Measurement of mineral-N collected by ion exchange resins placed in the field for extended periods (e.g. Binkley and Matson, 1983; Hart and Binkley, 1985). (iv) Determining the effects

INTRODUCTION

Most plant-available N in soils is derived from the microbiological mineralization of N bound in soil organic matter or detritus. Mycorrhizal fungi and plants are also capable of utilizing simple organic N compounds (Lundeberg. 1970; Stribley and Read, 1980; Read, 1983). but the significance of these for total N uptake by plants under field conditions is unknown. Slow rates of N mineralization often limit plant growth resulting in large growth responses to N fertilization. Pastor et al. (1984) and Nadelhoffer et al. (1985) have shown that rates of N mineralization can regulate the productivity of forest communities. Quantification of the relationships between soil N mineralization and other ecosystem processes has been restricted by the lack of suitable methods to measure N mineralization under field conditions. A number of techniques have been used to measure field rates of N mineralization or to obtain an index of them, but it has been impossible to assess the reliability of results because of the unknown effects of assay conditions (especially soil disturbance) on rates of mineralization in differing soils. There is no reference method which is known to measure mineralization rates accurately under field conditions. Some would argue that it is impossible to precisely measure these because all methods rely on measuring accumulation of mineral-N in the absence of active roots, and that to achieve this there will be some alteration to soil physical structure, moisture content and rhizosphere processes. The objective then must be to tind methods which minimize these effects, and to devise tests establishing the likely importance of any artifacts induced. Whilst N uptake by the plant must 521

R. J.

522

hISON

of varying temperature and moisture on N mineralization in disturbed soils, and then assuming that these relationships can be linked to field fluctuations in these variables during the year-i.e. modelling approach (Cameron and Kowalenko, 1976; Marion et al., 1981; Macduff and White, 1985). In addition to possible changes induced by severing of roots, the following potential difficulties may apply. (a) Soil disturbance markedly affects mineralization (Runge, 1974; Nordmeyer and Richter, 1985). This effect will be large for method (i) and may be quite small for method (ii). (b) Rapid fluctuations in soil water content significantly affect mineralization in many environments. With methods (i) and (ii) the soil moisture content at the time of sampling is maintained throughout the period of exposure. Rapp et al. (1979) attempted to minimize differences in soil moisture by using cans with perforated walls. (c) Method (iii) is unlikely to be quantitative because capture of mineral-N by resin is very dependent on transport by percolating water (Binkley, 1984) and any N either absorbed by roots or exchanged as NH: on soil surfaces is not measured. (d) For modelling approaches to be useful, temperature and moisture response surfaces must be determined for undisturbed soils and account needs to be taken of seasonal variations (Popovic, 1971; Ellis, 1974; Theodorou and Bowen, 1983; Richards et al., 1985; Nordmeyer and Richter, 1985) in the pools of mineralizable organic N in soils-i.e. response surfaces are not constant in time. Of the above methods, (ii) appears most suitable and Nadelhoffer et al. (1985) provided evidence that this technique gives reliable estimates of net N mineralization in soils supporting widely varying types of forest. Use of relatively undisturbed cores (soil-litter systems) also enables measurements of the significant contributions which macro- and microfauna may make to N mineralization from litter (Anderson et al., 1985). microbial biomass (Ingham et al., 1985) or the important pools of labile N held in the light fraction of soil organic matter (Sollins et al., 1984). We describe the justification for, procedure, testing and application of a sequential coring and in situ exposure technique which can be used to study N fluxes (mineralization, immobilization, plant uptake, maximum leaching) in soils. The method is particularly applicable to plant communities subjected to a low frequency of soil disturbance (e.g. forests) in which there is a marked development of organic matter gradients above and within the surface soil horizons. The methodology overcomes most of the problems discussed above, and we also attempt to assess the effects of severing of plant roots on the pattern of accumulation of mineral-N in undisturbed soils. METHODS

Field sites Studies were carried out at the following sites or on soils collected from them. (1) Pinus radiata plantation, 10 yrs of age, growing on yellow podzolic soil (State er al., 1968) at Pierces Creek about 15 km southwest of Canberra in the Australian Capital Territory. Unfertilized, fertilized (with 200 kg N ha-’ as (NH,)rSO, in both September

er al.

and November 1983) and sewage sludge-treated (with 13.3 t ha-’ of municipal sludge containing 176 kg N ha-’ in October 1983) 0.25 ha areas were studied. T’his site and treatments are more fully described by Linder et al. (1987). (2) Natural sub-alpine eucalypt forests growing on red earth soils (State et al., 1968) and located at N 1200m elevation in the Brindabella Range about 5Okm west of Canberra in the Australian Capital Territory. Communities dominated by an overstorey of (a) Eucalyptus aklegatensis, (b) E. dives-E. dalrympleana and (c) E. pauctflora were studied. These sites are described by Woods and Raison (1983). Table 1 lists relevant properties of the soils from the P. radiata and E. paucipora (other eucalypt soils are similar) sites. Laboratory processing chemical properties

and measurement

of soil

Field moist soils were stored at 4°C. usually for a few days, until processed. Soil cores were sectioned by depth (e.g. O-2.5, 2.5-5. 5-10, 10-15, 15-20 and 20-40 cm) and the sample sieved (< 5 mm). The fine and coarse earth fractions were weighed. A log subsample of the fine earth was extracted by shaking with 50 ml 2 M KCI for 1 h, followed by filtering. Filtrates were analyzed by automated calorimetry after distillation of NH.,+-N. Total mineral-N was measured following reduction of NO; to NH:-N using Ti(SO,)* (Heffeman, 1985). NO;-N was determined as the difference between total mineral-N and NH:-N. pH of the KC1 extract was measured using a glass electrode-calomel electrode assembly, and soil moisture content determined by drying at 105°C to constant weight. The mass of fine earth was used to convert soil mineral-N concentrations to the quantity of N ha-‘. Organic C was determined following Walkley and Black wet digestion and total N after Kjeldahl digestion. Soil pH was measured on a 1:5 (soil:water) slurry using a glass electrode-calomel electrode assembly. Effects of disturbance (sieving) on rates of N mineralization by contrasting soils under field conditions Two studies were made: at the control, fertilized and sewage-treated plots in the P. radiata forest in autumn 1984; and at the E. paucipora forest during the summer of 1983 on two plots that had been burnt with a low-intensity prescribed tire either 1 or 1Oyr previously. The effects of soil disturbance on soil N mineralization was assessed by comparing rates under the same field temperatures for soils which had either been sieved ( < 5 mm) at field moisture content and placed in sealed plastic bags, or which were intact columns inside steel cores (5 cm i.d.). Tubes were driven into the soil at 32 random locations on each 0.25 ha plot. Half of these cores were immediately removed, divided into O-2.5.2.5-5, 5-10 and IO-2Ocm depth segments, sieved and bulked to produce 4 samples (each of 4 cores) for each depth. These were subsampled for determination of initial mineral-N content, and the remainder placed in thin polyethylene bags which were buried in the field on the same day and at the depth from which they were collected. Plastic bags were

In

situ study of soil N fluxes

523

Table I. Selatcd soil propcrtin C

C:N

(%)

fUi0

(1:s.

PH wil:H,O)

Bulk density h3cm-1)

O-2.5 2.5-5 5-10 IO-15 15-25 25AO

16.1 11.5 9.6 8.5 7.7 5.8

Red earth(E. pa~c~@wa site) 3.8 0.522 30.8 4.1 0.419 21.4 4.3 0.373 25.1 4.4 0.309 27.5 4.4 0.275 28.0 4.5 0.187 31.0

0.65 0.80 0.90 0.85 0.83 I.11

O-2.5 2.5-S 5-10 IO-IS 15-25 25-40

2.9 1.4 0.8 0.5 0.4 0.3

Yellow podzolic(P. diarrr rite) 5.3 0.123 23.3 5.4 0.055 24.5 5.4 0.034 23.2 5.1 27.0 0.020 4.8 29.2 0.012 4.6 0.017 15.9

1.09 1.40 1.48 1.70 1.60 1.60

either covered with soil or litter. Soil in bags or isolated in covered cores (the remaining 16 per plot) was left (exposed) in the field for 82 days (P. rudiuta site) or 28 days (E. pauc~Q?orasite). Soils were then removed and processed in the same way as the initial samples and their mineral-N content measured. Net mineralization rates (mg N kg soil-’ 30 day-‘) were calculated for the P. rudiutu plots, but since mineralization was either low or negative (i.e. immobilization occurred) at the E. puuc~~oru site, results are expressed as soil mineral-N content (mg kg-‘) at the end of the exposure. Effect of sieving and higher assay temperatures on N mineralizution in control andfertilized P. radiata soils Mineralization of N was measured when soil (O-5 cm layer) was exposed in situ (as undisturbed covered cores during spring when soil temperatures were increasing from 10°C initially to 16°C) or incubated in the laboratory (after sieving, 30°C) for 70 days. Soils incubated in the laboratory were COIlected when the field study commenced. Twenty four cores were bulked (n = 3) to 8 samples before laboratory incubation or analysis. Soil pH and mineral-N content were measured before and after exposure or incubation, and net mineralization of N calculated. Effect of galvanized (Zn coated) cores on N mineralization by two soils in the laboratory Heavy metals including Zn can inhibit mineralization of soil N (Wilson, 1977; Smith and Young, 1984). Because galvanized (Zn coated) steel tubing is an inexpensive and convenient material from which to construct cores for use in stony or dense soils, it was necessary to determine if rates of mineralization were similar in these and cores made of inert poly vinyl chloride (PVC). Moist surface (O-5 cm) soils from the P. rudiutu (control) and E. pauciporu sites were sieved ( < 5 mm) and about 200 g of each soil was loosely packed into either PVC or new (with less ZnO coating) steel tubes (both 5 cm i.d.). Twenty four samples were prepared for each soil-core type combination, and 4 of these were analyzed for mineral-N content after 0,15,35,50,70 and 120 days of incubation at 30°C. Cores were sealed with plastic film during incubation to minimise loss of moisture.

Effect of steel tube on surface soil temperature The possible effect of heating up of the sunexposed section (cu. 2 cm in length) of steel cores on the temperature of surface soils inside the cores was studied. Replicate cores, with and without covers of clear plastic film were inserted in the soil at the P. rudiutu site under sparse (giving periods of direct exposure to the sun) or dense pine canopy. Air temperatures and those at 2.5cm depth for soils inside and outside cores were measured using probe thermometers at various times during 1 yr. Effect of period of exposure on the pattern of uccumulution of mineral-N in undisturbed field soils The following factors need to be considered when selecting an appropriate period for field exposure. (a) It needs to be sufficiently prolonged to produce a statistically significant change in the pools of soil mineral-N. (b) The effects of root severing and subsequent root decomposition need to be minimized. Examination of the temporal pattern of mineral-N accumulation in soils inside the cores during exposure in situ is a guide as to the likely effect of root cutting: any marked deviations from a linear pattern of N mineralization probably indicates artifacts resulting from decomposition of cut roots. (c) There is a need to vary the length of exposure periods to coincide with major changes in environmental variables which affect mineralization rates e.g. temperature rise in spring, soil wetting and drying cycles. (d) Exposure should not be so prolonged that accumulation of NH:-N may induce nitrification in systems which do not otherwise nitrify significantly. Accumulation of NO;-N may also increase denitrification (Knowles, 1981; Firestone, 1982). As a basis for selecting appropriate periods of field exposure, the accumulation of NH: and NO;-N under field conditions was measured at the E. puucipora and E. dives sites during a 70-day period in autumn 1981 and at ‘the P. rudiata (control) and E. delegatensis sites during a 13 l-day period in spring and summer 1983-84. Sixty four steel cores were inserted at each site at the beginning of the study and 16 were removed and individually analyzed for mineral-N on four occasions during the experiment.

R. J. mu

et al.

field cxposute 7

1

for n dafi

NW)

3

Nb(t+l)

(W

soiimineral-N 16 F

1

NMtl

Maximum

leaching

NbiZ+ll

t

start(4

- Ne(t+tJc

-

Nc(t+l),,

- 16-1.4- 2

end (a) excmsure (days)

Fig. 1. Scheme used to estimate N mineralization Ov,,,) and uptake (N,,) using the sequential coring and in situ exposure technique. (A) Sampling scheme: each core represents 24 spatial replicates: figures

represent typical soil mineral-N content (kg ha-‘). (B) Depiction of changes and catculations. See text for further details.

THE USE OF SEQUENTIAL CORING AND IN SfTfJ EXPOSURE OF SOILS TO STUDY FLUXES OF MINERAL-N

Rationale Net ~~era~ization or mobilization of soil N (see Fig. 1) Net mineralization (or immobilization) of N in field soils is calculated from measured changes in the mineral-N content of largely undisturbed soil isolated inside tubes in situ. Tubes prevent uptake of mineralN by roots, and the soil columns can either be covered to prevent leaching, or left open and thus be subjected to a more varying moisture regime. The moisture content of soils in cores can also be manipulated either by excluding rainfall or by adding water. Net mineralization (or immobilization) is calculated (net as the sum of changes in NH:-N ammonification) and NO;-N (net nitrification). All

values represent means based on intensive spatial sampling (see Experimental section below) and can be expressed either as concentrations (mg N kg soil-‘) or as total amounts on an area basis (kg N ha-’ soil layer-‘). The following equations represent the above transfo~ations: ANH:-N

= net ammonification = NH:-N,,.

ANO;-N

during exposure

,)c- NH:-N,,,

= net nitrification

during exposure

= NO; -NtiI + ,)c- NO,-N, Nndn= net ~ne~li~tion = ANH$-N

during

exposure

+ ANO;-N

where: NH:-N,,, and NO;-NM,) are the NH:-N and NO;-N content of bulk (b, uncodned) soil at the start (time t) of the exposure period, and

In situ

study of soil N fluxes

NH:-N,,+,, and NO;-N,,+,k are the NH;-N and NO;-N content of exposed and covered (c) soil at the end (time t + 1) of the exposure period. Immobilization during the field exposure produces a negative N,; this can occur in unamended soil, but is more common following either addition of fertilizer-N or soluble carbon. Uptake of N by vegetation (see Fig. 1)

If rates of net ammonification and nitrification are the same for soils inside and outside tubes, the amount of mineral-N taken up (N,) by vegetation during an exposure period is given by: N, = (Nmio) - (Asoil mineral-N = &.I).

pool) - (losses)

- N,,,l - [NW+ I) - NM,,1- (losses)

= N,, + I)r- N,, + I) - (losses) where: N,, + ,)c is the mineral-N content of covered explosed soil at the end of the period, and NW,, and NW,+,) are the mineral-N content of bulk soil at the beginning and end, respectively, of the exposure period. Losses are the changes in soil mineral-N content resulting from either leaching or denitrification. Denitrification is assumed to be zero unless independent measurements indicate otherwise, and leaching is also zero in covered (c) cores but maximal (see below) in open (0) cores. Thus leaching loss = N,, +l)c- N,, + I).9 and N, can be simplified from the above N, = N,, + ,). - N,, + ,) - (losses). to N u=N
are

+ NOT-N,.

Uptake for each exposure period can be summed to provide a measure of seasonal and annual uptake of N. Leaching of N (see Fig. I)

Leaching of mineral-N can be assessed in the following ways: (a) by comparing the quantity of mineral-N present in covered (no leaching) and open cores at the end of each exposure. The deficit in mineral-N in open cores represents an upper limit (because of the absence of uptake of water and N by roots inside cores) for leaching losses. In addition, if the incubation period is so prolonged that it results in greater nitrification in soil within tubes than in the surrounding soil, then leaching may also be overestimated; (b) by determining the volume and mineral-N concentration of soil percolate passing below the rooting zone. Percolate is usually sampled using some type of lysimeter plate or cup, and the volume of percolate estimated from independent measurements of soil water flux; (c) by periodically taking deep soil cores to enable measurement of the temporal and vertical redistribution of mineral-N. This method is less useful on highly permeable soils where nitrate may be rapidly “flushed” from the profile following heavy rain events.

525 Experimental

The methods described above were used to esti-

mate fluxa of soil mineral-N during a 2-yr period in the control, fertilized and sewage-treated plots in the P. radiata plantation (Raison et al., 1987). Sharpened (cutting edge -O.Smm thick) galvanized iron pipe (50 mm i.d., 3 mm wall thickness) was used to take initial soil samples and to isolate soil cores during field exposure. The length of metal tubes varied, but usually enabled sampling to 40 cm depth. Tubes were hand driven and were sufficiently robust to be used in dry, dense profiles containing weathered stone. PVC tubing can be substituted in sandy or more friable moist soils. About I-2cm of the tube was left protruding to facilitate extraction and attachment of covers (e.g. thin plastic film) where required. Very little compaction of soil resulted from driving of the tubes, even in very wet soils. The soils within tubes can thus be considered to be largely undisturbed. Because spatial variation in N pools and fluxes was high, about 24 randomly located cores were needed to be taken from each 0.25 ha plot at the start and end of each exposure (incubation) period, in order to measure soil mineral-N content with a standard error of IO-15% of the mean. Soils from three cores were bulked by depth increment before sieving and extraction. At the end of each exposure period cores were collected, the bulk (unconfined) soil was sampled, and a further set of tubes were inserted for the next sequential exposure. Fluxes are calculated from differences in means, and pooled variances were used to calculate the standard error for these fluxes. In general, variances were higher for mineral-N contents of incubated soils, so that these dominated the pooled variance. Quantities of N mineralized or taken up during incubation periods were summed to obtain seasonal and annual values, and variances were added for each period to calculate standard errors for these estimates. Examples of changes in soil mineralN pools and fluxes at the P. radiata site are given in Table 4. RESULTS

Efects

of soil disturbance on N mineralization

Sieving and incubation of soil in plastic bags in situ markedly increased net soil N mineralization rates in the surface (O-2.5 cm) horizon of the sandy yellow podzolic soil in the pine plantation (Fig. 2). The increases ranged from 2-fold on the fertilized plot to more than IO-fold in the soil treated with sewagesludge. On the unfertilized and sewage-treated plots mineralization was stimulated to a depth of 1Ocm. Mineralization rates of undisturbed soils were low throughout the study (autumn following a wet summer, with soil temperatures declining from 13” to 6°C). Disturbance also induced a large positive effect of sewage addition on N mineralimtion in the 0-2.5cm soil (Fig. 2). by incorporating organic matter (sewage) and increasing soil pH (sewage has a high CaCO, content). Disturbance significantly increased nitrification rates in all three soils (Fig. 3), especially in the upper (O-1Ocm) soil layer where greatest mineralization of N also occurred. Accumu-

526

R. 1.

hISoN

et al.

IS

10

3

.% .f 8

0

rQb -

loo

P. rod&a, fertilizer

z

2!.

s

d

1 0

P. radhua, fertilizer

\

.1OL 0

dt\

P. rodiota,

Control

5

g

01J,

1.3

3.8

I.3 3.8

7.5

soil Depth (cm)

Fig. 2. Rate of net mineralization of soil N at the P. rudiota site during field exposure in autumn: undisturbed soil cores (a), sieved soil in plastic bags (0). SE _ 10% of mean (n = 4).

tation of NH,+-N, whether this results from mineralization or fertilization, induces rapid nitrification in this soil which in the undisturbed state nitrifies only slowly (unpublished data). With the organic-rich red earth, disturbance decreased the mineral-N content after incubation (Fig. 4). Immobilization occurred on both burnt sites, and was greatest in the surface (0-5cm) soil where most mobilization of microbially_availabIe carbon would be expected from disturbance. This experiment demonstrates that disturbance (sieving) can have a major effect on rates of N mineralization in surface soils, and that the effect may be either positive or negative depending on soil type. Disturbance can alter nitrification rates and induce treatment (e.g. sewage) effects. Effect of assay conditions on N mineralization Assay conditions determined the interpretation of the effects of prior fertilization of N mineralization rates (Table 2). When mineralization of N was measured in situ on undisturbed soils, fertilization was found to increase it 4-fold, with production of NO;-N dominant in comparison to the unfertilized soil where most N accumulated as NH:-N. In contrast, following soil sieving and incubation at 30°C. mineralization was greater in unfertilized soils and both soils nitrified strongly (Table 2). N fertilization increased the small pools of labile organic N and the mineralization of this could be sensitively measured in situ on undisturbed soil, but in the laboratory where other larger pools of N were mineralized (rates

15

7.5

Soil Depth (cm)

Fig. 3. Proportion of mineral-N present as NO;-N following exposure of soil as undisturbed cores (a) or sieved in plastic bags (0) at the P. r&uru site. Initial concentrations of NOT-N (mg kg’“‘) were: control ~0.2, sewage treated 0.1-1.4, fertilized 2-7.

were increased by more than 5-fold) such effects were obscured. The lower mineralization by soil from the fertilized plot under laboratory conditions may have been due to a lower total N content (660 vs 89Omg N kg-’ in the O-5 cm horizon) and greater acidity (Table 2).

20

o-

1.3 3.8

7.5

12.5

Soil Depth (cm) Fig. 4. Concentrations of soil mineral-N at the E. puucipora site following field exposure in summer: undisturbed soil cores (O), sieved soil in plastic bags (0). SE (5 10% of mean

(n = 4).

In s&u study of soil N fhxes

527

Table 2. Compuison of net N mitmrahzatian (m9 N kg soil-‘) atIer 70 days under field (uadisturbed soil at 10” to 16%) and laboratory (sieved soil at 3O’C) conditions. Yellow podzolicsoil from P. radmto forest. SE of mean(n - 8) in prentbeses Fkld Control NW-N NH:-N NO,- + NH;-N OH (I :5. KCW

Fatifiral

0.2 (0.2) 2.2 (I .2) 2.4(1.3) 5.2 (0.1)

9.1(1.2) 0.6 (0.6) 9.7 (I .4) 4.9 fO.0)

Laboratory Control FWiliZd 78.1 (1.6) -l.S(O,l) 76.6 (1.6) 5.1 (0.0)

X4(1.3) -2.3(0.1) 64.l(l.3) 4.7 (0.0)

‘Final pH after field exposure or labomtory incubation.

Efect of galoanized cores on N mineraiization Galvanized steel tubes had little effect on mineralization of N in either soil (Fig. 5). There was no significant effect of tube type with the yellow podzolic soil, but the amount of N mineralized in the red earth was slightly less (up to 8%) in steel tubes after prolonged inactions during which > 80 mg N kg-r had mineralized. The low pH (cu. 4.0 in KCI) of this soil may have dissolved some Zn from the new tubes used under the high incubation temperatures. The small effects recorded are expected to be the maximum likely to occur under field conditions where both acidity and rates of biological activity would usually be less. Effect of metal tubes on soil temperature The presence of metal tubes, with or without covers, had little effect on the temperature of surface soils (Table 3). The difference between temperatures inside and outside tubes was never more than a few degrees C, weli within the range of natural spatial variation encountered. If metal tubes are used in the open in hot climates; high temperatures may be encountered in the thin zone of soil next to the metal surface-careful measurement would IX needed to ascertain this.

0 0

E. pauciflora

z

OL.

Effert of duration ox exposure on pattern of N mineralization The rate of net N mineralization in situ was approximately linear at the P. radiuta (control), fi. puuc@ora and E. dives sites for exposure periods of 70 and 131 days (Fig. 6). Higher rates were measured during the later stages of exposure (between days 74 and 131) at the sub-alpine E. akfegatensissite, possibly because soil temperatures had been increasing from 8” to 16°C at 1Ocm depth between September and January. The relatively smooth pattern of mineral-N accumulation suggests that severing of roots did not have any marked effect on the process, as a consequence of rapid decomposition of soluble C (Hendrickson and Robinson, 1984) or easily mineralizable root N. The pattern of N mineralization was also similar at all soil depths (although rates declined with depth), which would be unexpected if root decay was significantly affecting mineralization, because fine roots are concentrated in the surface horizons. Mine~li~tion of N was slow in undisturbed soil at ail sites, particularly at > 10 cm depth, so that use of exposure periods of <30 days would make detection of changes in soil mineral-N concentration difficult. The results indicate that incubation periods of 30-90 days would be suitable for the sites studied. During summer when rates of ~ne~li~tion are higher and the frequency and magnitude of wetting and drying cycles is greater, use of shorter exposures is desirable. During cool, moist winters longer incubations can be used. We have found with the yellow podzolic soil that if NH:-N is allowed to accumulate to concentrations exceeding about IS mg N kg-‘, nitrification is accelerated and nitrate may be leached from uncovered cores. A flexible duration of field exposure needs to be used and varied according to soil type, site treatment and changing seasonal weather conditions. Knowledge of site nsponse should bc used to modify sampling regimes. Tabk 3. Temperature (“C) of air and surface soil inside and outside metal cores under a range of conditions in P. mfiatu forest. Vahtes are means of sPot measures selected to show the gmeral pattern ObSXVCd

Season

C=OPY

Air

Winter

Dense sparse Dense sparse Dense Sparse

13 14 13 13 22 22

Autumn

Fig. 5. Mineralization of N in sieved soils maintained at 30°C in PVC (0) or galvanixed steel (@) tubes. SE ~5% of mean (n = 4).

Summer

Soil at 2.5 cm depth Covered @en No core core core 12 II 16 19 ::

11 II IS 18 19 21

12 11 17 17 I::

R. 3. RhlsoNet

528

af.

E. diver

!E!EEEc

74

43

12

131

68

Days of Field Exposun

Fig. 6. Pattern of accumulation of mineral-N during in situ exposure of soil as undisturhcd cores. Exposure from spring to summer for the P. ru&uta and E. deregatensis sites, and during autumn for the L &es and E. puuciporu sites. SE - 10% of mean (n = 16).

Pools and fluxes of soil mineral-N for two contrasting periods after fertilization of the P. rudiutu plantation are shown in Table 4. Pools of mineral-N could be measured with a standard error of -5 (unfertilized) to -20% (sewage-sludge treated) of the mean value. Within a few months of fertilization, a SE of about 10% of the mean was typical. The sampling procedure adopted for the present studies yielded acceptable estimates (SE typically 20% of the value) of N mineralization and uptake except for the initial 10 weeks after fertilization when SE’s were 3O-98% of the mean (Table 4). The method can be used to follow the general pattern of ~mobili~tion (negative N,,) and uptake of N after fertilization (Raison er al., 1987; and unpublished data). Mineralization and uptake of N 12-14 months after fertil-

ization could be ~tisfacto~ly estimated, and significant differences demonstrated between treatments (Table 4). We believe the fluxes estimated using this methodology are quantitative and evidence for this is briefly summarized below. DISCUSSION

The sequential coring and in situ exposure method described and evaluated above is similar in theory to that used by Nadelhoffer et al. (1983, 1984, 1985) and Adams and Attiwill (1986). Nadelhoffer ez al. however extracted cores, sealed them in thin pIastic bags with minimum disturbance and replaced them inside the same hole. Our method in which cores are not extracted before exposure (incubation) would cause less physical soil disturbance (associated with extrac-

Table4. Pools (kg ha-‘, O-40&

and fluxes (kg ha-‘Xdays-‘, 0-4Ocm) of mineral-N for two sctectcd periods following fcrGzat.ioo of a stand of P. rodiafu. Data obtained using sequential cnring and In situ exposure tahniquc-se text for details of nomeaciatumand mcthnds. SE of meana (II = 8) in narcnthescs Period of 14 November to 9 December 1983 147-72 davr after fertiiization~

Control

NW 5.8

N

“*lb

W) Sewage

Fertilizer

Fertilizer

(it:,

234.&

265.7 (22.8)

(21.1)

Kill

N”

(Z) -215 (13.4) -38.1

(Z) 28.1 (8.3) 37. I

(37.3)

8.2. (0.6) 28.3’ (8.3)

Control *w

N W+lk

(35.4)

Period of 24 September to 3 December 19&d (362-432 davs after fcrtiiization~

Soil mineral-N content N NW u ” (ii) 7.6 (0.3) ie6.4. 12.1 (20.6) (1.6)

Soil miner&N fluxes N leached @ax) 0.0

N W+lb 6.3 (0.4) 10.6 (0.6) 14.6 (0.8)

N et+

lb

&

[:;I, 12:4 (0.9) 24.9 (2.5)

11.4 (1.0)

NfUi,,

N.”

N icacbed”

0.4

0.9

‘z) (0.3) 0.8 (0.:) . .

y

WJ)

N W+ll

(0:2) 5.5_.

(1.3)

(;;) _.

(1.2)

(1.1)

‘Abamcc of leaching confIrmed by iysimetry (Khanna et crl.. 1987). Wptake calculated assuming no leaching. because with both active tree growth and slow rates of N mineralizationit would have hen negiigibie during this pzriod. NO;-N was lost from open exposed soils baause of the abxncc of plant uptake.

In situ study of soil N tluxes Tabk 5. Contribution (% of total in O-80 cm soil layer) of individual soil depths to N minertiition in yellow podzolic and red earth soik atIer CXDOIU~~ for 131 days in situ durinn sminn and summcl Yellow mdzolic &Ph

(W

O-2.3 2.5-S S-10 IO-IS IS40 N mineralized llre ha-‘, 0-4ocm)

(P.

rahara) 26 17 17 IS 25 14.5

Red exth

(E. &legarensir) 15 14 38 19 14 18.8

tion and replacement of cores), and is simpler and faster to use. In addition, it allows easy study of processes in the subsoil. Lower soil horizons can contribute significantly to N mineralization in the rooting zone (Table 5) because of the large mass of soil involved even if its rate of mineralization is low. It would be impossible to replace soil cores to a depth of 40+ cm without causing major physical disturbance. Nadelhoffer et 01. confined their studies to the surface horizons (maximum depth 20 cm). Further, when soil is sealed in plastic bags the initial moisture content is maintained throughout the exposure period, and this would make interpretation of results difficult in environments having widely fluctuating soil moisture regimes. With cores, use of open and covered systems (combined with addition of varying amounts of water) enables assessment of the effects of moisture. Nonetheless, the core system is easiest to use in a continuously moist environment where moisture content remains high and similar for soils inside and outside cores. Inputs of chemicals to the soil in litter leachates can markedly affect seasonal patterns of mineralization (e.g. Gosz and White, 1986) and such effects are more difficult to detect using closed systems. In cultivated soils, in situ cores may be easier to use because soil columns may break up during their transfer to plastic bags before being re-buried. Exposed cores should not contain living plants, unless account is taken of plant N uptake. In most situations, cores can be placed between plants, but this may be difficult in dense grass swards. Any continued uptake of mineral-N during the incubation period by severed but still active roots (particularly woody roots which still contain reserves of carbohydrate) will result in an underestimate of net N mineralization rates. The errors from such effects will be greatest in slowly mineralizing soils. Use of small cores will minimize the opportunity for connection of 6ne roots to larger woody roots, but further study of such possible effects is warranted. Where simple organic forms of N are utilized by mycorrhizal root systems, plant uptake will be underestimated using the methods described here. When estimating plant uptake of mineral-N, other sources of available N (precipitation inputs and release from decomposing titter) can be included and “apparent N uptake” calculated (Nadelhoffer et al., 1985). The coring technique can also be used to study net mineralization of N in the forest floor; large (-30 cm dia) steel rings 5 cm deep can be pushed 2cm into the soil and covered with thin plastic film

s29

to prevent rainfall leaching any N mineralized. This method will not work in moist enviromncnts where large numbers of roots invade the forest floor from the underlying soil. The sequential soil coring and in situ exposure technique has proved satisfactory for studying fluxes of soil mineral-N in the unfertilized and N-fertilized P. radium forest during a 3-yr period(Raison et al., 1987; and unpublished data). The following findings, considered together, suggest to us that it provides quantitative estimates of N fluxes: (a) in each year a strong seasonal pattern of N mineralization was measured, with the annual amount of N mineralized being consistent with other fluxes (e.g. increment into above ground biomass, N input in litterfall) of N; (b) net mineralization and plant uptake of N during study periods were equivalent when the soil was moist. This is expected on the highly Ndeficient site; (c) estimated uptake of N followed the same pattern as the accumulation of N in tree foliage; this was especially evident during the first growing season after fertilization. The increment into foliage was about two-thirds of the estimated uptake; (d) the pattern and degree of N leaching was similar when estimated by coring, or by the product of the volume and mineral-N concentration in soil percolate passing below the main rooting zone (4Ocm depth); (e) the increment in tree basal area annual was positively linearly correlated with esimated annual N uptake. (f) I yr after fertilization, 95% of the applied N could be accounted for by fluxes estimated using this technique. In conclusion, the sequential coring and in situ exposure technique described minimizes the effects of the assay procedure on mineralization rates and appears to give a quantitative measure of N mineralization and uptake in surrounding undisturbed soil.

Acknowledgements-We thank S. J. Smith, P. Piotrowski, J. Holtxapffel, R. A. Falkiner and P. V. Woods for providing invaluable technical support; and P. J. Smethhunt and Dr E. K. S. Nambiar for useful comments on an earlier draft. Mr B. Hefferman developed the automated distillation-calorimetric method for determining mineral-N in soil extracts. These studies have been carried out in forests managed by the Forests Branch, Department of Territories, Canberra and the co-operation by officers of that organization is gratefully acknowledged.

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