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Indices of soil nitrogen availability in five Tasmanian Eucalyptus nitens plantations.

Introduction

About 20% of Australia's hardwood plantations are in Tasmania, including 120 000 ha of Eucalyptus species (Wood et al. 2001; National Plantation Inventory 2002), of which most are E. nitens. The area planted to E. nitens in Tasmania is expanding rapidly, especially at 300-700m elevation on basalt-derived soils, i.e. Ferrosols (Isbell 1996). In Tasmania, nitrogen (N) is applied widely to E. nitens plantations, as profitable responses at planting and later ages are possible (Smethurst et al. 2004). However, rates of net N mineralisation (NNM) in these soils cover a wide range (12-188 kg N/ha.year, 0-10 cm depth; Moroni et al. 2002), and sites in the upper end of this range did not respond to N fertilisation at planting, and not all sites responded to N applied at later ages (Moroni 2001 ; Smethurst et al. 2004).

The in situ soil-core technique (Raison et al. 1987) is favoured for measuring field rates of NNM, but this method is usually restricted to surface soils (e.g. 0-10cm depth) and is labour- and time-intensive and therefore not suitable as a management tool. A simpler soil analysis that is well correlated with rates of NNM in the field would aid in more judicious fertilisation of these plantations. Roots of E. nitens also explore beyond the surface 0-10 cm soil, e.g. beyond 50 and 80 cm depths by ages 10 and 34 months, respectively (Misra et al. 1998). Hence, the contribution of available N from soils below the 00-10 cm depth needs to be considered when managing N in E. nitens plantations.

Only a small proportion of total soil N is mineralised per year (Keeney 1982). It is this portion of labile soil N that soil analyses attempt to identify when assessing potentially available N. Procedures include (i) physical separation, (ii) chemical extraction (usually via extraction with acids, bases, or salts), and (iii) incubation, which utilises the native microbiota responsible for mineralisation and can be aerobic or anaerobic. A combination of biological and chemical indices is generally recommended (Keeney 1982; Binkley and Hart 1989). Useful relationships have been found between NNM and the following analyses (either alone or in combination) of crop N uptake, yield, or response to N additions: total soil N and C (Keeney 1982; Connell et al. 1995), total soil P (Nielson et al. 1984; Falkiner et al. 1993), anaerobically mineralisable N (AMN) (Ryan et al. 1971; Shumway and Atkinson 1978), hot KCl-extractable N (Selmer-Olsen et al. 1981), and concentrations of mineral N (N[H.sub.4.sup.+] and N[O.sub.3.sup.-]) (Hill and Shackleton 1989; Knoepp and Swank 1995). Hence, these soil analyses warranted testing on a range of Tasmanian forest soils.

There is a need to estimate NNM and N supply of E. nitens plantations after plantation establishment, when variations in temperature, moisture, and organic matter turnover are most pronounced. To have confidence in an indicator of NNM during this period, it will need to be temporally stable for predictable periods of time. Hence, of concern is the possibility of temporal fluctuations in concentrations of indicators of N NM. For example, Adams and Attiwili (1986) found that the highest concentrations of AMN occurred during summer.

When establishing forest plantations, herbicides are commonly used to control competition between weeds and tree seedlings. The cessation of N uptake by the previous crop and weeds, combined with flushes of mineralisation after cultivation and little N demand by the young plantation, may result in increased concentrations of mineral N in surface soils, which are subsequently leached down the soil profile. Large amounts of mineral N have been observed in subsoils of Tasmanian Ferrosols under other crops (Sparrow and Chapman 2003) and are anticipated under fertile forest plantation sites, especially those previously managed intensively as pasture. Mineral N in subsoils may become available to plantations as their roots reach these depths or some may be lost through off-site leaching or denitrification.

Objectives of the research reported here were to (i) determine the temporal stability of KCl-extractable N[O.sub.3.sup.-] and N[H.sub.4.sup.+], soil solution N[O.sub.3.sup.-] and N[H.sub.4.sup.+], AMN, and hot KCl-N and their usefulness as indicators of in situ rates of NNM in soils supporting young E. nitens plantations; and (ii) determine the concentrations of mineral N and potentially mineralisable N in subsoils of these plantations.

Materials and methods

Site description and sampling

Sites studied were 1 ex-pine site (Boulder), 3 ex-native forest sites (Basils, Nunamara, Tim Shea), and 1 ex-pasture site (Potters). In situ measurements had been reported 1-4 years after planting at all sites except the Potters site, which was examined 1-3 years after planting (Morom et al. 2002). Soil and site characteristics are summarised in Table 1. The sites had been cleared of trees and shrubs then strip-cultivated (about 2.5 m ploughed and 1.5 m uncultivated) prior to planting with E. nitens. These sites typify a broad range of sites in Tasmania where E. nitens plantations are being established. Herbicides were applied to the total area of each site pre- and post-planting.

Soils were sampled from the 0-10 cm depth of the uncultivated zone (between tree rows) every 5-8 weeks over a 2-year period. Samples were collected from 3 plots per site, except at Potters where 5 plots were sampled. Each plot was a randomly distributed control plot (c. 250 [m.sup.2] area) that had not received N fertiliser during establishment. Soils were sampled using a stainless steel tube of 50mm internal diameter (i.e. initial soil samples described in Moroni et al. 2002), and 8 cores bulked per plot. Sampling began in November 1995 at Basils, Boulder, Nunamara, and Tim Shea, and in January 1996 at Potters. In September 1997, samples were also taken from walls of one soil pit per replicate at Potters, Basils, and Boulder. Four PVC cores of 50 mm internal diameter were used to sample soil from 0-10cm depths and horizontally into the vertical wall of each soil pit at depths 20, 45, 75, and 105 cm.

Analytical methods

After collection, soil samples were refrigerated for up to 7 days prior to sieving (<5 mm fraction retained) and then analysed fresh for KCl-extractable N, soil solution N, total C, and AMN by the following methods. Subsamples of sieved fresh soil were analysed for soil water content. Another subsample was air-dried prior to analysis of total N, total P, and hot KCl-N. For N and P, extracts or digests were analysed by colourimetric flow injection methods.

KCl-extractable N was that mineral N (N[H.sub.4.sup.+] and N[O.sub.3.sup.-]) extracted from soils using 2 M KCl in a 1 : 5 soil : solution mixture at room temperature. Concentrations of soil solution N[H.sub.4.sup.+] and N[O.sub.3.sup.-] were estimated using the paste method (Smethurst et al. 1997). Parameters required to describe phase partitioning were taken from Smethurst et al. (2001). Hot KCl-N and AMN were determined using the methods of Wang et al. (1996a). Total carbon was calculated from loss-on-ignition data using regression relationships determined by Wang et al. (1996b). Loss-on-ignition was determined by measuring weight loss of oven-dried (105[degrees]C) soil samples following heating at 375[degrees]C for 17 h. Total N was determined at 6-monthly intervals using Rayment and Higginson (1992) method 7A2. Total P was calculated using the same method and soil samples as for total N, except that during the heating to 360[degrees]C for 2 h, glass teardrop stoppers were placed on each digestion tube to prevent losses of P.

Bulk density (BD) was estimated from 4 soil cores (420[cm.sup.3]) taken from each replicate (0-10cm depth). Soils from each replicate were bulked and separated into coarse and fine fractions using a 5-mm sieve. Soil fractions were dried at 105[degrees]C for 24h and their weights recorded. Only the fraction < 5 mm was used to calculate B D, which was 83-98% of the total soil weight. The soil fraction >5 mm was assumed to have negligible contribution to N availability (Raison et al. 1987; Wang et al. 1998). The total amount of soil in a 1-ha area to 10 cm depth was estimated by BD (kg/ha) x (1 - [alpha]), where [alpha] is the proportion of rocks (0-6%) estimated visually for each replicate using the method described by McDonald et al. (1990).

Statistical analysis

For each soil property, differences with time within sites were determined using a repeated measures analysis of variance (ANOVA) using SAS version 8.02 with Proc Mixed and the Bonferroni correction to prevent inflating the Type 1 error. Comparison of sites for each type of soil property was by I-way ANOVA using the SAS version 8.02 Proc Mixed program. Correlations and regressions were determined by standard statistical methods; adjusted [r.sup.2] is used throughout.

Results

The ex-pine site (Boulder) had low values and the ex-pasture site (Potters) had high values for all soil analyses (Table 2). The only soil analysis that was significantly correlated with NNM was total P (P < 0.05) (Fig. 1). Annual rates of NNM corresponded to 4.7, 2.0, 1.0, 0.9, and 0.7% of total N (0-10cm) at Potters, Basils, Nunamara, Boulder, and Tim Shea, respectively, suggesting that these sites are listed in order of decreasing organic matter quality.

Concentrations of hot KCl-N were not significantly different between measurement periods within sites, except at Tim Shea, where a slight, unexplained increase occurred in May 1997 and was sustained until the end of the measurement period (Fig. 2). As expected, no differences in total N or total P were observed with time. Only at Boulder were there changes in total C with time, where total C decreased by c. 8 Mg C/ha between October 1994 (Wang et al. 1996b) and November 1997 (end of this study). There were large temporal variations in AMN and in soil solution and KCl-extractable concentrations of N[O.sub.3.sup.-] and N[H.sub.4.sup.+] due to site and sampling date effects. For example, AMN varied by a factor of 2 at all sites with time, with no strong seasonal pattern. Also, at Potters between April 1997 and June 1997, soil solution N[O.sub.3.sup.-] dropped from 10.2 to 1.5mM, and KCl-N[O.sub.3.sup.-] from 83 to 15 [micro]g N/g soil. However, values of solution and KClextractable N[O.sub.3.sup.-] dropped below 0.1 mM N and 1 [micro]g N/g soil, respectively, at sites with NNM [less than or equal to] 24 kg N/ha.year (Boulder, Nunamara, Tim Shea) c. 30 months after planting, which is beyond the maximum age at which Potters was last sampled.

Concentrations of AMN, hot KCl-N, total N, and total C decreased exponentially with increasing soil depth at Potters, Basils, and Boulder. Total P decreased exponentially with depth at Potters, increased with depth at Basils, and remained constant with depth at Boulder. However, when profile soil bulk density was assumed to be equivalent to surface bulk density and soil analyses values from the 20, 45, 75, and 105 cm depths were assumed to be representative of depth intervals 10-30, 30-60, 60-90, and 90-120 cm, respectively, contributions of subsoil to these analyses ranged from 1.9 to 12.1 times that of the top 10cm (Table 3). Ferrosols under forests generally have a bulk density that is low at the surface (Grant et al. 1995, i.e. similar to those indicated in Table 1) and increases by about 20-30% with depth as clay content increases (Sparrow and Chapman 2003). Hence, the assumption that profile bulk density is equivalent to surface bulk density would have underestimated the contribution of subsurface soils to profile N availability. There were some minor differences in soil parameters from 0-10cm depth within sites between Tables 2 and 3 as a result of bulked 0-10 cm soil being taken from across all replicates over time in Table 2 and soil samples being taken only from the face of one pit per replicate on one occasion in Table 3.

At Basils and Boulder, concentrations of KCl-extractable and soil solution N decreased with depth (Fig. 3), but at Potters, high concentrations of mineral N were found at 75-105 cm depth (KCl-extractable N, 289.3 [micro]g N/g soil; soil solution N, 2.8 mM N in soil solution). The proportions of mineral N extracted as N[O.sub.3.sup.-] in soil solution were 98-99% at Potters, 93% decreasing to 45% with increasing depth at Basils, and 35% at 0-45 cm depths, decreasing to 17% at 75 cm depth at Boulder.

[FIGURE 3 OMITTED]

Discussion

In surface soils (0-10 cm), the Potters ex-pasture site had the highest rates of NNM, proportions of total N mineralised, and concentrations of total P, and the lowest C : N ratio. Basils (ex-native forest) and Potters had similar rainfall and temperature; however, Basils had 24% more C/ha (0-10cm depth) than Potters yet less than half the rate of NNM (Table 1). The high fertility of Potters is indicative of changes in soil following conversion of native forests to pasture that are well recognised (Skinner and Attiwill 1981), i.e. phosphate fertilisers are added and N supply is increased by biological N fixation, which probably resulted in improved organic matter quality at Potters. Hence, indicators of NNM might be more useful if separate relationships are sought for various combinations of previous vegetation and management, as suggested by Gonzales-Prieto et al. (1994) and others. For example in forests, correlations between total soil N and NNM were improved when sites were grouped by primary profile form (Connell et al. 1995) or into strongly and weakly nitrifying soils (Carlyle et al. 1990). Alternatively, these soil analyses may be used in combination with other site attributes in models to predict rates of NNM, e.g. AMN (O'Connell and Rance 1999) and aerobically mineralisable N (Carlyle et al. 1998; Paul et al. 2002).

With high variability in rates of NNM (Moroni etal. 2002) from just 5 sites, where regressions were strongly influenced by the Basils and Potters sites, little confidence can be placed on relationships between NNM and measured soil analyses. However, sites with low rates of NNM ([less than or equal to] 24 kg N/ha.year) (Boulder, Nunamara, Tim Shea; Table 1) also had total P of [less than or equal to] 0.8 MgP/ha (Table 2) and KCl-extractable and soil solution N[O.sub.3.sup.-] concentrations that dropped below those of sites with high rates of NNM (Basils and Potters, 70-188 kgN/ha.year) 27-34 months after planting. Tree growth within these low NNM sites responded to N additions within 3 years of planting. Responses to N additions occurred approximately when rates of potential N uptake of E. nitens plantation exceeded the supply of mineral N through NNM. Hence, it is not surprising that concentrations of total P (and total N) indicated the age of onset of N deficiency at the study sites and concentrations of soil solution N[H.sub.4.sup.+], soil solution N[O.sub.3.sup.-], and KCl-extractable N[O.sub.3.sup.-] indicated N sufficiency at the time of sampling (Smethurst et al. 2004).

Concentrations of AMN varied by more than a factor of 2 at all sites with sampling date, probably due to varying environmental conditions. This finding is in contrast to Polglase et al. (1992), who found little seasonal or annual variation in AMN, but it is in agreement with Adams and Attiwill (1986), who reported higher concentrations of AMN during summer. The source of AMN is thought to be mainly microbial biomass, which fluctuates with season and soil temperature (Boone 1992). Harvesting will result in changes to soil temperature, and water and nutrient regimes (Londo et al. 1999). Hence, indices involving incubation, such as AMN, are likely to be more variable after harvest and during early growth until the stand matures and attains more stable nutrient and litter cycles. Although there is some variability in AMN in mature forests (Adams and Attiwill 1986), AMN may have application in mature stands prior to harvesting for estimating N supply during the next rotation, as suggested for aerobically mineralisable N (Carlyle et al. 1998). The temporal stability of hot KCl-N will render it suitable for sampling under post harvest conditions to predict NNM rates, if future studies find a relationship between hot KCl-N and NNM.

At our study sites, canopies closed at age 3-4 years, after which there was significant litterfall from the lower portion of the canopy. Prior to canopy closure, little substrate is returned to the soil, and continued microbial respiration will cause a decrease in soil organic matter (Smith and Paul 1990). At Boulder, a significant (P < 0.05) decreasing trend in total C was observed, equivalent to a loss of c. 8 Mg C/ha over 37 months. This trend in soil C was similar in timing and magnitude to that predicted for a Tasmanian E. nitens plantation after afforestation or reafforestation (Paul et al. 2003), but it is unclear why such a trend was not evident at the other 4 sites. Longer term predictions were that total soil C content would be replenished in the middle and later portion of the rotation.

Concentrations of mineral N (comprising 98% N[O.sub.3.sup.-]) were very high at 75 cm depth at Potters (KCl-N was 521 kgN/ha in the 60-90cm depth). The source of this mineral N was probably leaching from higher in the profile, as rates of NNM generally decrease with depth, and the amount of N leached from 0-10 cm could account for that found at depth at Potters (Moroni 2001; Moroni et al. 2002). Large amounts of mineral N have also been recorded at depth in Ferrosols used for agriculture in Tasmania; 100-200 kg N/ha was found at depths 40-100 cm, in some cases representing higher amounts of mineral N than those found in surface soils (Sparrow and Chapman 2003). In Ferrosols, N[O.sub.3.sup.-] leached from the top 10 cm may be held on anion exchange sites at depth, thereby reducing leaching losses (Black and Waring 1976). As the plantation's root system develops and N demand increases, the rate of N leaching from surface soils will decrease; however, it is unknown to what extent mineral N accumulated at depth is later taken up.

Rates of NNM in subsoils (10-120 cm) will be limited by available substrate, i.e. total C and total N. Although present in lower concentrations than in surface soil, when aggregated over the 10-120cm depth range, total C and total N were present in subsoils in quantities 3.1-4.9 times greater than the contents of 0-10 cm soils (Table 3). However, mineralisation is affected by organic matter quality as well as quantity, which is probably better represented by indices of available N. These indices, AMN and hot KCl-N, were present in the subsoil in quantities 1.9-2.6 times greater than the contents of surface soils (Table 3), and probably provide the best estimate of the contribution of subsoils to profile NNM. However, due to the poor relationship between these indices and rates of NNM in surface soils of our study, the contribution of subsoils to total profile NNM requires substantiation by other methods, e.g. using the buried bag method for measuring in situ rates of NNM (e.g. Westermann and Crothers 1980).

In summary, NNM was a function of soil total P across 5 sites. Three of the sites that had NNM rates [less than or equal] 24kgN/ha.year (n = 3) also had [less than or equal to] 0.8 Mg/ha of total P and <0.1 mMN and 1 [micro]g N/g soil in soil solution and KCl-extractable N[O.sub.3.sup.-], respectively, after about 30 months following planting. Concentrations of AMN, KCl-extractable N, and soil solution N varied considerably with time, whereas hot KCl-N was temporally stable under post-establishment conditions. At one site, a gradual decrease in total C of 8 MgC/ha was observed over 37 months. Large amounts of mineral N were found at depth under the ex-pasture site, which also had very high rates of NNM. Based on concentrations of AMN and hot KCl-N, the contribution of the 10-120 cm depth to total profile NNM was at least double the contribution of the 0-10 cm depth.
Table 1. Site characteristics of 5 E. nitens plantations in
Tasmania where indicators of N mineralisation were studied

Within rows, values followed by the same letter
are not significantly different at P=0.05

Characteristic Boulder Nunamara

Previous vegetation P. radiata E. viminalis
Planting date June 1993 Oct. 1993
Age examined (years) 2-4 (A) 2-4 (A)
Latitude 41[degrees]12' 41[degrees]21'
Longitude 145[degrees]50' 147[degrees]15'
Elevation (m) 390 400
Rainfall (mm/year) (B) 1400 1000
Daily soil temperature (C)
 [Jan.sub.min]-[Jan.sub.max]
 ([degrees]C) 15-19 15-19
 [July.sub.min]-[July.sub.max]
 ([degrees]C) 3-7 4-7
Soil type (D) Ferrosol Ferrosol
Parent material Basalt Basalt
Surface texture Clay loam Clay loam
Bulk density (E) (g/cm) 0.77 0.99
Rock fraction (%) 0 3
pH (F) 5.1 5.8
NNM 95/96 (kgN/ha.year) (E)(G) 24c 23c
NNM 96/97 (kg N/ha.year) (E)(G) 13a 20a

Characteristic Tim Shea Potters

Previous vegetation E. regnans Pasture
Planting date Oct. 1993 Oct. 1995
Age examined (years) 2-4 (A) 1-3
Latitude 42[degrees]40' 41[degrees]9'
Longitude 146[degrees]29' 145[degrees]45'
Elevation (m) 420 510
Rainfall (mm/year) (B) 1500 1570
Daily soil temperature (C)
 [Jan.sub.min]-[Jan.sub.max]
 ([degrees]C) 12-15 13-15
 [July.sub.min]-[July.sub.max]
 ([degrees]C) 3-6 2-6
Soil type (D) Kurosol Ferrosol
Parent material Siltstone Basalt
Surface texture Clay loam Clay loam
Bulk density (E) (g/cm) 0.76 0.60
Rock fraction (%) 0 0
pH (F) 4.6 4.3
NNM 95/96 (kgN/ha.year) (E)(G) 12c 188c
NNM 96/97 (kg N/ha.year) (E)(G) 23a 175ab

Characteristic Basils

Previous vegetation Euc.-myrtle
Planting date July 1993
Age examined (years) 2-4 (A)
Latitude 41[degrees]19'
Longitude 145[degrees]39'
Elevation (m) 550
Rainfall (mm/year) (B) 1800
Daily soil temperature (C)
 [Jan.sub.min]-[Jan.sub.max]
 ([degrees]C) 13-15
 [July.sub.min]-[July.sub.max]
 ([degrees]C) 2-6
Soil type (D) Ferrosol
Parent material Basalt
Surface texture Clay loam
Bulk density (E) (g/cm) 0.51
Rock fraction (%) 6
pH (F) 5.0
NNM 95/96 (kgN/ha.year) (E)(G) 70b
NNM 96/97 (kg N/ha.year) (E)(G) 77b

(A) Wang et al. (1998) measured N fluxes
at these sites during 1-2 years of age.

(B) Approximate long-term mean.

(C) Approximate average of the daily
maxima and minima (10 cm).

(D) Isbell (1996).

(E) Fraction <5 mm; 0-10 cm depth.

(F) 1:5 soil : water.

(G) Data from Moroni et al. (2002).

Table 2. Concentrations and contents of potential indicators
of NNM at the study sites 0-10 cm depth; n = 3, except Potters
where n = 5; averaged across all collection periods. Within rows,
values followed by the same letter are not significantly different
at P = 0.05

Soil parameter Boulder Nunamara Tim Shea Potters Basils

 Mass

Total N (Mg/ha) 2.0bc 1.6c 3.2ab 3.9a 3.5a
Total P (Mg/ha) 0.36d 0.39cd 0.77c 2.44a 1.51b
Total C (Mg/ha) 52.9a 26.5b 60.4a 56.5a 65.9a
Hot KCl-N (kg/ha) 9.2c 7.2c 61.6b 98.4a 88.7a
AMN (kg/ha) 46.9c 36.6c 99.7a 68.0b 74.2ab

 Concentration

Total N (g/ 100 g soil) 0.27b 0.22b 0.33b 0.65a 0.74a
Total P (g/100g soil) 0.05b 0.08b 0.08b 0.41a 0.32a
Total C (g/100 g soil) 7.0c 3.6d 6.1c 9.4b 13.7a
Hot KC1-N (pg/g soil) 11bc 10c 59b 164a 185a
C : N 26a 16bc 18bc 14c 19b

Table 3. Contents of total N, P, and C and hot KCL-N and AMN
in surface soils (0-10cm depth) and subsoils (10-120cm depth)
at Potters, Basils, and Boulder

Standard errors are shown in parentheses (n=3). Soils
were sampled in September 1997 from the faces of soil pits

Depth Total N Total P Total C
(cm) (Mg/ha)

 Potters

0-10 (A) 4.1 (0.4) 2.7 (0.1) 60.9 (2.7)
10-120 (B) 12.7 (2.1) 18.6 (0.8) 298.4 (42.1)
B/A 3.1 6.9 4.9

 Basils

0-10 (A) 3.4 (0.9) 1.4 (0.1) 74.8 (9.1)
10-120 (B) 12.2 (3.6) 16.9 (l.9) 362.8 (48.5)
B/A 3.7 12.1 4.9

 Boulder

0-10 (A) 2.4 (0.4) 0.4 (0.0) 57.4 (13.9)
10-90 (B) 7.3 (0.9) 2.9 (0.1) 186.2 (22.7)
B/A 3.1 6.7 3.2

Depth Hot KC1-N AMN
(cm) (kg/ha)

 Potters

0-10 (A) 108 (9) 120 (21)
10-120 (B) 273 (58) 262 (38)
B/A 2.5 2.2

 Basils

0-10 (A) 129 (52) 146 (29)
10-120 (B) 271 (135) 300 (142)
B/A 2.1 2.1

 Boulder

0-10 (A) 39 (8) 50 (10)
10-90 (B) 75 (23) 128 (36)
B/A 1.9 2.6


Acknowledgments

The authors would like to thank the University of Tasmania, the Federal Government, and Gunns Ltd for funding; Gunns Ltd and Norske Skog, Fletcher Challenge, and Boral for access to experimental sites; and R. Hand (deceased), A. Wilkinson and L. Ballard for technical help. The authors would also like to thank D. Mendham, C. Carlyle, and S. Nambiar, and 2 anonymous referees for comments on earlier drafts of the manuscript.

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Author:Moroni, M.T.; Smethurst, P.J.; Holz, G.K.
Publication:Australian Journal of Soil Research
Geographic Code:1U6TN
Date:Dec 1, 2004
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