The effect of soil compaction, profile disturbance and fertilizer application on the growth of eucalypt seedlings in two glasshouse studies

Soil & Tillage Research 71 (2003) 95–107

The effect of soil compaction, profile disturbance and fertilizer application on the growth of eucalypt seedlings in two glasshouse studies J.R. Williamson a , W.A. Neilsen b,∗ a

Department of Natural Resources and Environment, P.O. Box 3100, Bendigo, Vic. 3554, Australia b Forestry Tasmania, 79 Melville St., Hobart, Tasmania 7000, Australia Received 13 June 2001; received in revised form 8 January 2003; accepted 12 January 2003

Abstract Soil damage, compaction and displacement, during logging or clearing and cultivation affects both soil physical and chemical properties and reduces growth of regenerated or planted tree seedlings. Understanding the factors involved will aid management and set limits for indicators of sustainable management in eucalypt forests. In the first of two glasshouse studies, three Eucalyptus species were grown for 110 days in soils from six forest sites in Tasmania, Australia. Sites sampled ranged from low rainfall dry forest to very high rainfall wet forest. Soil was collected from three soil depths, in 10 cm increments to 30 cm, each packed in pots to four different bulk densities, ranging from that present in undisturbed field sites to that plus 0.17 g cm−3 . In the second study Eucalyptus globulus Labill. seedlings were grown in soil collected from disturbed and undisturbed sites, packed to two bulk densities, and fertilized with combinations of N and P. Increasing soil compaction, in Study 1, caused a proportional decrease in final mass of seedlings of up to 25%. Growth on soil from lower horizons (10–30 cm) averaged only 41% of that on topsoil, a significantly greater restriction of growth than that achieved through compaction. It was concluded that topsoil displacement and profile disturbance was a more significant form of soil damage than compaction. Above-ground dry weight of seedlings was most strongly correlated with soil total N but poorly correlated with other macronutrients. Growth of E. globulus seedlings grown on disturbed soils, in Study 2, averaged 30% of that on undisturbed sites. With added P and N on undisturbed sites growth averaged seven times that of the unfertilized seedlings indicating a general deficit of available P and N on the three soils tested. On soils from disturbed areas, there was also a response to fertilizing with N and P together but the response varied on the three soils. The effects of profile disturbance were ameliorated with fertilizer applications on only one of the soils. The results highlighted the importance of retaining topsoil in situ during forest operations. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Soil compaction; Fertilizer; Eucalyptus seedling growth; Soil disturbance; Glasshouse; Skid trails; Australia

1. Introduction

∗ Corresponding author. Tel.: +3-62-338-215; fax: +3-62-338-292. E-mail address: [email protected] (W.A. Neilsen).

The Montreal Process Working Group, a group of 12 countries, including Australia, manages 90% of the world’s temperate and boreal forest (National Forest Inventory, 1998). The group has prepared criteria and indicators of sustainable forest management, and a

0167-1987/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-1987(03)00022-9

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framework of regional indicators for reporting on performance. One series of indicators covers soil health, and baseline standards are currently being developed. Information on the relative importance of soil physical and chemical factors in limiting growth, and the extent to which fertilizers can be used to restore productivity to degraded soils, would help set baseline standards for indicators (Lyke, 1996). It would also help determine the importance of care in managing soils for sustainability and provide tools to assist in management. A reduction in the growth and stocking rates of forest trees is often associated with skid trails and log landings following mechanized harvesting (Lockaby and Vidrine, 1984). This growth reduction has been associated with soil physical property degradation that includes compaction (Foil and Ralston, 1967), soil horizon mixing and topsoil removal (Calais and Kirkpatrick, 1983). Soil compaction, induced through forest harvesting traffic along skid trails, increases the soil bulk density (BD) and strength of the soil (Froehlich, 1972; Williamson and Neilsen, 2000). Compaction, displacement of topsoil and soil mixing during site preparation for plantation development, when heavy machinery is used for clearing and cultivation, have also been investigated (Neilsen, 1990; Smith et al., 1997). Increased soil strength inhibits root growth (Foil and Ralston, 1967; Sands and Bowen, 1978; Heilman, 1981). Soil horizon mixing and removal redistribute fertile O and A horizons which may lead to decreased growth through decreased nutrient availability (Van Der Weert, 1974; Braunack, 1986). Soil structure decline may also exacerbate the problem. In extreme cases, plant death may occur (Wert and Thomas, 1981; Lockaby and Vidrine, 1984). Eucalypt logging in Australia has involved heavy machinery moving large logs (Kerruish, 1978; Rab, 1998). Compaction and soil displacement result from this traffic and have been described and studied (Jakobsen, 1983; Wronski, 1984). In Tasmania, increased compaction was measured following up to 21 passes by laden logging machines was found to be 0.17 g cm−3 across a wide range of soil types (Williamson and Neilsen, 2000). Displacement of topsoil in wet conditions was also considered an important type of damage. Two glasshouse studies were undertaken to help elucidate the relative importance of physical and chemical changes to productivity. The first examined the effect

of increasing BD on the growth of eucalypt seedlings following different levels of soil horizon removal and horizon mixing, for a range of soils. The second examined the growth of Eucalyptus globulus seedlings on soils subjected to profile disturbance at two BD levels and monitored the response of the seedlings to fertilization.

2. Methods 2.1. Study 1 Six locations in Tasmania were selected for collection of soil, based on previous studies of soil damage (Williamson and Neilsen, 2000). The sites covered a range of vegetation and soil types (Table 1). Soils varied markedly with texture ranging from light clay to sand (Table 2). Soils were selected based on three site types with differing rainfall, each with two experimental sites (Table 1). The site types were, dry forest with low rainfall (900–1100 mm), wet forest with high rainfall (1200–1600 mm) and wet forest with very high rainfall (over 1700 mm). One of the soils under very high rainfall was a deep medium clay loam, while the other was an organic soil over sandy loam. Initial BDs, in unlogged forests, were highest under the dry forest and lowest under the wet forest with very high rainfall. Cores for BD determination were collected using the method described by Williamson and Neilsen (2000) and processed using the method of Blake (1965). In order to obtain uniform conditions in pots, the research was carried out using repacked soil cores rather than intact field cores. Soil from each site was collected from three depths, 0–10, 10–20 and 20–30 cm. The three depths corresponding to the A1 horizon, B1 horizon and the top of the B2 horizon, respectively. The 30 cm depth was also the layer most severely affected during logging. A mixed soil consisting of equal parts by volume (air-dried) of the 0–10, 10–20 and 20–30 cm depth soils was also prepared for each site. This soil mix was included to simulate the effect of soil horizon mixing and minor puddling. Surface leaf litter (O1 horizon) was discarded and depth measurements were taken from the top of the decomposing organic layer (O2 horizon) or the A horizon. In the laboratory, soils were air dried before grinding to pass through a 2 mm sieve. Each soil was moistened using

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Table 1 Site type, vegetation, geology and soils of the six study areas in Tasmania, Australia, used in Studies 1 and 2 Site type and study sites

Geology

USDA soil type (FAO-UNESCO)

Dry forest with low rainfall Forester (Study 1) and Golconda (Study 2) Goulds Country (Studies 1 and 2)

Devonian–Silurian siliceous sediments Granite

Hapludult (Orthic Podzol) Hapludult (Eutric Podzoluvisol)

Wet forest with high rainfall Oldina (Study 1) Gad’s Hill (Study 1)

Permian sandstone Basalt

Haplohumult (Plinthic Luvisol) Eutrudox (Dystric nitosol)

Dolerite Precambrian Orthoquartzite and Mudstone sequences

Hapludalf (Plinthic ferralsol) Endoaquept (Cambric arenosol)

Wet forest with very high rainfall Picton (Studies 1 and 2) Sumac (Study 1)

a water spray and concrete mixer to a water content below the plastic limit and steam-sterilized at 80 ◦ C for 45 min. Soils were then bagged and stored for 3 days to allow moisture equilibration to occur. The BD of the lowest compaction was set at that present at the undisturbed field-sampling site. The

highest compaction force was set to obtain an increase of 0.17 g cm−3 in BD, this being the increase caused by standard logging machinery in field studies in Tasmania (Williamson and Neilsen, 2000). The BD range for each soil type and depth was then determined within that range. For each soil group, four compaction levels

Table 2 Soil texture (particle size distribution) and soil OM, estimated as loss on ignition, for the six soils used in Studies 1 and 2 Sand (g kg−1 )

Silt (g kg−1 )

Clay (g kg−1 )

OM (g kg−1 )

Forester and Golconda 0–10 Black silty loam 10–20 Brown silty clay 20–30 Brown light clay

490 270 350

420 320 240

90 410 410

190 80 50

Goulds Country 0–10 10–20 20–30

Very dark gray sandy loam Dark brown sandy clay loam Dark brown sandy clay loam

760 700 640

110 90 80

130 210 280

190 110 110

Oldina 0–10 10–20 20–30

Dark gray loamy sand Dark gray sandy loam Dark gray sandy loam

830 810 820

130 90 80

40 100 100

120 60 30

Gads Hill 0–10 10–20 20–30

Dark reddish brown loam Very dark brown clay loam Dark yellowish brown light clay

620 540 410

240 200 210

140 260 380

270 180 130

Picton 0–10 10–20 20–30

Dark reddish brown sandy loam Dark grayish brown silty clay loam Yellowish brown silty clay

750 300 250

100 390 330

150 310 420

380 180 130

Sumac 0–10 10–20 20–30

Black fine organic sandy loam Very dark gray sandy clay loam Gravelly silty loam

680 700 470

210 80 330

110 220 200

220 110 110

Depth (cm)

Texture (McDonald et al., 1990)

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were created using standard forces applied using a hydraulic ram with a time of compression of approximately 15 s achieving compaction (Dexter and Tanner, 1974). Pots were cut from 112 mm internal diameter PVC pipe to a length of 125 mm. Calculated weights of soil were packed into the core using a single piston powered by the hand operated hydraulic ram. The soil was pressed to a height of 110 mm (volume 916 cm−3 ) allowing a lip of 15 mm to facilitate watering. Particle size analysis was determined (Loveday, 1974) and organic matter (OM) content estimated by loss on ignition (Williamson and Neilsen, 2000). Total porosity (TP) and air filled porosity (AFP) of the filled cores at field capacity were estimated gravimetrically using water as a filling medium (Vomocil, 1965). Field capacity was calculated based on soil water content after 2 days free drainage of the cores on a suction plate with low suction. Soil chemical analysis was determined by digestion in nitric acid followed by optical emission spectrometry to determine P, K, Ca and Mg (Zarcinas and Cartwright, 1983) and total N by the method of Keay and Menage (1969). Cores were set out in the glasshouse in a four randomized block design, one replicate to each block. Three Eucalyptus species, Eucalyptus amygdalina Labill., E. globulus and Eucalyptus obliqua L’Herit., typical of the forest types growing in Tasmania on the soils studied, were grown on each soil treatment, giving 1152 pots in total. A 3-week-old seedling of the appropriate species was pricked into each pot, being careful to minimize disturbance and to press the soil back to the original surface. Deaths occurring in the first week were considered to be associated with pricking out and were replaced with a seedling of the same age and species. No fertilizer was applied and soil moisture was kept at a pressure of between 100 and 500 kPa. The glasshouse was not heated but was cooled using an evaporative cooler when temperatures exceeded 25 ◦ C. The study ran through the late summer and autumn, a total of 110 days. Pots were randomized within blocks and re-randomized every 4 weeks. Seedling height was measured at approximately 12-day intervals. At the completion of the study, each plant was destructively measured for leaf area (LA) and stem, leaf and root dry weight (RDW) (70 ◦ C). LA was measured using a Paton planimeter. Roots were extracted manually following sieving and washed to remove traces of soil.

2.2. Study 2 Two forms of soil damage during logging can be visually identified: rutting and puddling. Rutted soils are highlighted by the removal of the O and parts or all of the A horizon exposing a compacted B horizon. Puddled soils are highlighted by the inversion and mixing of the O, A and parts of the B horizon. Three locations were selected for collection of soil. Soils covering three parent material types were chosen, granite and silicious sediments under low rainfall and dolerite under very high rainfall (Table 1). Each site was stratified into three damage classes for collection of soil: rutted, puddled and control. From each damage class, only the 0–10 cm depth soil was collected. Two compaction levels were used, one emulating field BD’s and one of lower BD (set at 90% of undisturbed field BD), approximating the tillage effect. Five fertilizer treatments were used in the study: control, N, P, N and P, and a solution with all macronutrients (Table 3). Potassium and Ca salts were used and pH balanced to that of the soil. Potassium and Ca were not considered limiting in these soils while Mg has been found to be limiting in some soils (Grant et al., 1995). Fertilizer solutions treatments were renewed at weekly intervals and the soils were maintained at a soil moisture pressure of between 100 and 500 kPa. There were four replicates of each treatment giving a total of 360 pots. Soils were collected and treated in the same way as Study 1 and the experiment was conducted in the same type of pots. The experiment was run for 68 days with weekly measurement as in Study 1. In addition (to the analyses conducted in Study 1), soil OM was determined by the Walkley–Black method (fraction under 0.5 mm) (Allison, 1965). 2.3. Data analysis Comparison of growth of seedlings on various depths and the mixture of soils, were carried out. Data were analyzed using GENSTAT® (Genstat 5 Committee, 1989) for analysis of variance (ANOVA), correlation analysis, regression analysis and group regression analysis. ANOVA was calculated for main and first order interaction effects and least significance difference (LSD) levels determined. Significance levels of <0.05 were considered significant and <0.01 highly significant. In a similar growth

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Table 3 Fertilizer solutions used (mg l−1 of element) in this study based on Hoagland’s Solution (Hoagland and Arnon, 1938) and A–Z micronutrient solution (Hoagland and Synder, 1933)a Treatment

Element

Salt used

Control (no fertilizer)

KNO3

Ca(NO3 )2

KH2 PO4

CaCl2

MgSO4

–

–

–

–

–

–

–

–

N (−P, −Mg) N K Ca

2.5 6.8

4.1 5.8

P (−N, −Mg)

–

–

–

P K Ca

1.1 1.5 5.0

N + P (−Mg)

– N P K Ca

2.5

N P K Ca Mg

2.5

1.1 1.5

6.8 5.8

Complete

a

–

4.1

–

Micronutrients—B, Mn, Zn, S, Cu:

4.1 1.1 1.5

6.8 5.8

1.2 1 ml l−1

and Fe (as EDTA) added to all fertilizer treatments.

study, Sands and Bowen (1978), reported a constant root/shoot ratio for Pinus radiata D.Don grown on soils of varying compaction levels. However, Conlin and Van den Driessche (1996) observed reduction in root/shoot ratios with increasing compaction in studies of Pinus contorta Dougl. ex Loud. var. latifolia Engelm. The root/shoot ratio was used to determine if compaction or nutrition affected root growth more than above-ground dry mass in these studies. 3. Results 3.1. Relationships between measured growth parameters Highly significant positive correlations (P < 0.01) were recorded between leaf, stem, and root dry mass in the two studies (Table 4). No significant variations were found in the effect of relative BD on the root/shoot ratio, this ratio varying from 0.53 at the

Table 4 Correlation of measurement parameters for Studies 1 and 2 (seedling height (Ht), LA, leaf dry weight (LDW), stem dry weight (SDW), AGDW, RDW)

Study 1 Ht LA LDW SDW AGDW RDW Study 2 Ht LA LDW SDW AGDW RDW

Ht

LA

LDW

SDW

AGDW

RDW

–

0.839 –

0.775 0.876 –

0.835 0.929 0.847 –

0.812 0.914 0.992 0.908 –

0.603 0.695 0.722 0.653 0.726 –

–

0.911 –

0.915 0.975 –

0.835 0.893 0.903 –

0.912 0.973 0.995 0.943 –

0.848 0.856 0.895 0.817 0.892 –

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lowest BD to 0.49 for the highest BD in Study 1, and averaging 0.40 and 0.46 for the two levels in Study 2. This indicated that in these studies compaction did not alter growth allocation between seedling components. Above-ground dry weight (AGDW) was therefore used for comparisons of factors affecting growth. 3.2. Study 1: effects of site, depth of soil sample, species and BD on seedling growth There were significant differences in seedling growth for different sites, soil from different depths, species and relative BD (Fig. 1). For ANOVA analysis of soils from different depths, results of the B1 and B2 horizon, 10–20 cm depth and 20–30 cm depth, were averaged for comparison with the A1 horizon, 0–10 cm depth. Growth of seedlings on soils from different depths showed highly significantly (P < 0.01) reduced growth on the 10–30 cm depth averaging only 41% of that on the 0–10 cm depth (data not shown). Increased BD also highly significantly (P < 0.01) restricted growth, but to a lesser extent, with mean growth at the highest compaction averaging 75% of that at the lowest compaction. Second order interactions were significant (P < 0.05), except for BD × depth and BD × site. BD × species was significant (P < 0.05) with growth of E. obliqua

declining comparatively more than E. globulus or E. amygdalina with increasing BD (Fig. 1). E. globulus growth overall was greater than that of E. obliqua or E. amygdalina (Fig. 1). While E. obliqua grew nearly as well as E. globulus on three of the soils (98% of the E. globulus growth), it did not grow as well on the three soils where growth was generally poor (62% of E. globulus growth). Variation between sites was marked, with the Sumac soil producing far more growth than soil from other sites (Fig. 2). Growth on soil from the 10–20 and 20–30 cm depths was less on all sites than growth on soil from the 0–10 cm depth (Fig. 2). On three of the sites growth was very poor on the 10–30 cm depth. At Sumac the growth on the soil from the 10–30 cm depth was only 20% less than the growth on the 0–10 cm depth and was better than growth on the surface soil (0–10 cm depth) of the other sites (Fig. 2). Group regression analysis of AGDW versus BD for the 0–10 cm soil layer, the 10–30 cm depth and the mixture of those soils showed that the three regressions were on highly significantly (P < 0.01) different projection (intercept) (Fig. 3). The slope of the regressions was not significantly different (P > 0.05) showing that increasing compaction had a similar effect in the different layers. The soil mixture of the three soil layers resulted in growth close to an average of the growth of the A and B horizons.

Fig. 1. AGDW of E. globulus, E. obliqua and E. amygdalina seedlings grown on a range of compacted BD levels in Study 1. Data are averaged over six sites and soil from two depths, 0–10 and 10–30 cm.

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Fig. 2. AGDW of eucalypt seedlings growing on a range of soils from six sites and from two depths, in Study 1. Data are averaged over three Eucalyptus species and four relative DB classes.

A regression of Relative AGDW against soil depth and BD produced the equation (P < 0.01): relative AGDW = 1.1498 − 0.062 depth − 0.2599 BD (R2 = 0.42)

(1)

where relative AGDW = AGDW − (mean AGDW for each site and Eucalyptus species), depth is the mean depth in cm and BD is in g cm−3 .

3.3. Study 1: relationship of growth with soil factors OM levels decreased with soil depth for all sites (Table 2). The decreasing OM content with depth enabled higher BD’s to be attained with the same compactive force and a resultant strong negative correlation between BD and OM (Table 5). Soil total N

Fig. 3. Regressions for AGDW of eucalypt seedlings against BDs of repacked soil cores in Study 1. Data is presented for soils from two depths and a mixture of those soils. Points shown are means of three Eucalyptus species and six sites.

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Table 5 Correlation coefficients for AGDW (mean value for species and BD) against initial soil BD, soil texture (sand (SA), silt (SI) and clay (CL)), soil chemical factors (soil OM, total nitrogen (N), phosphorus (with potassium) (P)) and soil porosity (TP) and AFP at field capacity for the six soils and three soil depths tested in Study 1

AGDW BD OM SA SI CL N P TP AFP ∗

AGDW

BD

–

−0.12∗ –

OM 0.16∗

−0.87∗ –

SA 0.34∗

0.52∗ −0.40∗ –

SI

CL

−0.37∗

−0.52∗

0.53∗ 0.40∗ −0.91∗ –

−0.36∗ 0.19∗ −0.64∗ 0.74∗ –

N 0.54∗

−0.64∗ 0.76∗ −0.15∗ 0.07 −0.15∗ –

P

TP

AFP

0.08 −0.20∗ 0.51∗ −0.35∗ 0.51∗ 0.32∗ 0.39∗ –

0.08 −0.91∗ 0.82∗ −0.50∗ 0.49∗ 0.39∗ 0.62∗ 0.51∗ –

0.02 −0.87∗ 0.82∗ −0.53∗ 0.51∗ 0.43∗ 0.58∗ 0.53∗ 0.96∗ –

Significant at P < 0.05.

and P were highly correlated with soil OM (Table 5). Across all soils, OM content was strongly negatively correlated with BD, but was weakly correlated with AGDW (Table 5). Within soil types correlations of OM with AGDW were much higher (R = 0.37–0.77). Total N was positively correlated with AGDW across the soils, while total P was poorly correlated (Table 5). Within the soil types, the correlations between AGDW and N were much stronger on the majority of sites and approximated the correlations with OM (R = 0.32–0.80). Silt and clay showed weak to moderate positive correlations with P but no correlation with N (Table 5). However, the clay percentage was negatively correlated with AGDW on the majority of sites. Apart from Oldina, the percentage of sand in the soil was related to soil depth, with a higher sand content closer to the surface (Table 2). The effect of sand was therefore confounded by superior growth on topsoils due to lower BD levels and higher OM contents. It was also confounded by the superior growth on the more sandy Sumac soil. Across the soil types TP and AFP showed strong negative correlations with BD (Table 5), and compaction further reduced both TP and AFP, both by an average of 2%. TP decreased with soil depth, decreasing from an average of 65% in the surface 0–10 cm depth to 50% for the 20–30 cm depth. Air filled pores did not decrease with depth and averaged 7.8% v/v. However, AFP fell below 12% v/v for all depths and compaction levels, for all sites, and the majority fell below 10% v/v. Neither TP nor AFP was correlated with growth (Table 5).

3.4. Study 2: effects of soil damage class and fertilizer regime on seedling growth Soil damage class was associated with reduced OM, Walkley–Black organic C levels, averaging 3.24, 3.16 and 1.22%, and increased BD, averaging 1.09, 1.17, and 1.29 g cm3 , for the undisturbed, puddled and rutted soils, respectively. Seedling growth parameters such as AGDW and height growth were significantly affected by soil type, soil damage class, BD and fertilizer treatment (Fig. 4, Table 6). Significant interactions were also recorded between soil type, soil damage class and fertilizer treatments (P < 0.05). Disturbed soils had significantly poorer seedling growth on the granite and dolerite soils but not on the siliceous sedimentary soil (Fig. 4). Soils collected from rutted sites showed the poorest growth (Table 6). The lower BD treatment, simulating tillage, produced an average of 50% more growth than the soils at field BD. However, this reduction in compaction alone, without P or P plus N fertilizer, did not improve growth on either the rutted or puddled soils (Table 6). Fertilizer responses were large on all soils. The response was variable to P and/or N alone or in combination. There was no additional response to the addition of Mg. Fertilizer response varied with soil type (Fig. 4). On the undisturbed sites, there was no significant response to either N or P on their own for the granite and siliceous sedimentary soils but a significant (P < 0.05) interaction response to P plus N. On the dolerite soil, there was a significant

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Fig. 4. Total AGDW of E. globulus seedlings grown on undisturbed and on rutted soil from six sites treated with various fertilizers, nil (O), nitrogen (N), phosphorus (P) and phosphorus and nitrogen combined (PN). Means sharing a common letter do not differ significantly at P = 0.05 based on LSD calculated from ANOVA.

(P < 0.05) P response with an additional response to P plus N. On the rutted soils, growth was significantly (P < 0.05) improved with added N plus P but not with P alone. On the granite and siliceous

sedimentary soils, the P plus N growth was better than the unfertilized undisturbed site while it was still significantly (P < 0.05) poorer on the dolerite soil (Fig. 4). The puddled soils did not respond as well

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Table 6 AGDW and seedling height (mm) giving fertilizer response for three soil damage classes and two relative BD classes in Study 2 (mean of the three soils studied) Damage class

BDa

Fertilizer treatmentb Unfertilized

AGDW (g) Control

P

N

PN

Reduced Field

0.258 fgh 0.170 fgh

0.460 cdef 0.295 efgh

0.681 cd 0.577 cde

1.887 a 1.323 b

Puddled

Reduced Field

0.064 gh 0.053 h

0.172 fgh 0.096 gh

0.054 h 0.045 h

0.762 c 0.372 defg

Rutted

Reduced Field

0.035 h 0.044 h

0.163 fgh 0.129 gh

0.029 h 0.024 h

1.202 b 0.682 cd

Height (mm) Control

Reduced Field

84 gh 80 gh

148 def 115 fg

158 cde 128 ef

Puddled

Reduced Field

53 ij 45 ij

96 gh 75 hi

46 ij 45 ij

180 bc 107 fgh

Rutted

Reduced Field

38 j 39 j

90 gh 82 gh

33 j 25 j

213 b 169 cd

a b

253 a 203 b

Reduced BD is 90% of undisturbed field BD to simulate tillage. For AGDW and height, means sharing a common letter do not differ significantly at P = 0.05 based on LSD calculated from ANOVA.

as the rutted soils to additions of P plus N fertilizer (Table 6).

4. Discussion 4.1. BD and soil depth on seedling growth Mechanical impedance of root growth has been cited as the reason for decreased growth of Pinus spp. following soil compaction (Sands and Bowen, 1978; Wasterlund, 1985). Root growth is preferential through zones of weakness in the soil, the principal zones being soil pores. Where no zones exist or are limited, root growth is impeded. However, barely significant correlations between BD and AGDW and the lack of overall significance (P > 0.05) between porosity and AGDW in this research indicate that other factors in addition to BD are limiting growth. In this research, compaction caused a gradual and continuous reduction in growth, rather than reaching a threshold BD value which seriously reduced or prevented growth. The rate of reduction was irrespective of the initial BD, or depth from which the soil was collected. The depth from which the soil

was collected, whether surface soil (A horizon) or soil from the 10–30 cm depths (B horizon), produced differences in growth greater than that expected from compaction alone, with substantially greater growth on the surface soil. The effect of soil from different horizons on seedling growth can be related to a visual damage classification system (Williamson and Neilsen, 2000). The growth of seedlings on the 0–10 cm depth should indicate growth response on sites with the least severe damage where the surface soil is left in place, while 10–20 cm depth and 20–30 cm depths indicate the response on sites where surface soil is displaced. Using the growth comparison in relation to visual assessment, seedling growth decreases as visual damage increases. At lower soil damage levels, seedling growth is good and the effect of compaction plays a relatively small role in decreasing seedling growth. At higher soil damage levels, overall growth is poor and the proportional effect of compaction on growth is greater. The reduction in AFP with increasing BD agreed with the findings of others (Van Der Weert, 1974). A porosity of 10% v/v has often been quoted as being the minimum level for adequate plant growth (Brady,

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1984). Eavis (1972) reported that critical AFP levels varied with the level of compaction and concluded that AFP levels of 30, 22 and 11% v/v were required for low, medium and high bulk densities. Simmons and Pope (1987) concluded that poor aeration was the primary cause of reduced root growth in compacted soils. Restricted aeration alone caused reduced root growth in pecan seedlings (Smith et al., 1989). The range of bulk densities used in Study 1 fell into the low porosity range, bringing aeration into question. However, eucalypt seedlings attained high growth rates in soils with AFP levels of less than 12%. 4.2. Seedling growth and species effects Performance of the Eucalyptus species used in the research reflects their ecology. E. amygdalina is usually associated with drier forest types and E. obliqua with wetter, more productive, sites. E. globulus grows naturally on a range of sites in Tasmania and in plantations grows well under a range of cultivation treatments (Madeira et al., 1989). With increasing BD growth of E. obliqua declines substantially more than growth of the other species, indicating a greater need to avoid compaction in managing the wetter E. obliqua forests (Fig. 1). Although E. amygdalina growth does not decline substantially with increasing BD, its growth at all levels is inferior to that of E. obliqua reflecting the general slower growth of Eucalyptus species from the drier forests. E. globulus seedlings show good growth and tolerance of compaction. 4.3. Nutrition and response to fertilizers A growth response to added P and N together has been evident for many soils in Tasmania where large responses in plantation growth have been attained with fertilizer additions (Neilsen et al., 1981). Tree root growth has also been found to be strongly related to soil OM (Davis et al., 1983). The strong response of the undisturbed dolerite soil to P application may be due to the P in that soil being poorly available. These soils are noted for their high phosphorus adsorption capacity (2000–3000 ␮g g−1 , compared with 450 ␮g g−1 for the siliceous sedimentary soil and 1500 ␮g g−1 for the granite soil) (W. Neilsen and R. King, pers. comm.). The lack of any response to P alone on disturbed soils indicates that N is the most limiting nu-

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trient, with low N availability correlated with the reduced soil OM in disturbed soils. The further response to P when added with N is in accord with field trials (Neilsen et al., 1981). The very poor growth of seedlings on soil from ruts on dolerite site is possibly due to fixing of applied P on the mineral soil. The research shows the effects of profile disturbance can be ameliorated with fertilizer applications. A combination of fertilizer application and tillage and restoration of displaced surface soil should alleviate the majority of the chemical and physical problems. However, a reduction in soil OM levels, which provides a source of nutrients on unfertilized soils, as well as temporary binding sites for applied nutrients, will reduce the sites resilience. The role of topsoil, with adequate OM levels, is evident in the superior growth on undisturbed soils under N and P regimes as compared to disturbed soils. Low levels of soil OM will only build up slowly following afforestation and growth problems on disturbed soils will develop unless fertilizer is applied at regular intervals.

5. Conclusions This research highlights the effect of profile disturbance on regeneration as being a problem of nutrient supply, as much as of physical soil damage. In many soils the surface horizon (A1 horizon) is relatively shallow. Displacement of approximately 10 cm exposes sub-soil (B horizon), meaning that on severely disturbed sites forest regeneration often establishes on sub-soil of low nutrient status. This research demonstrates that topsoil displacement is the factor of most importance in poor regeneration and growth on disturbed sites. In forest operations retention of topsoil in situ is of critical importance for sustaining growth at pre-harvest potential. This is not only in logging operations, but also in any soil disturbance during clearing or cultivation for the establishment of forest regeneration or plantations. Although fertilizers can ameliorate detrimental effects of topsoil removal and profile disturbance on many soils, productivity will only be maintained and sustainability objectives achieved where soil disturbance has been minimized. On many sites, topsoil retention will be essential to meet these productivity requirements. Baseline

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standards for indicating soil health should incorporate chemical as well as physical parameters (Montreal Process Implementation Group, 1998). Strategies that conserve topsoil and the integrity of the soil profile will be required to achieve levels of sustainability adequate to meet forest certification standards (Lyke, 1996; Wenbaun-Smith, 1997). The research shows that fertilizer programs should be considered for afforestation projects, even on soils with minimal disturbance, as they can shorten the time to establish forest cover and maintain or increase productivity.

Acknowledgements Financial support for this project was provided by the National Soil Conservation Program, the Tasmanian Forest Research Council and the Forestry Tasmania. We wish to thank Ron King and Gordon Davis for advice and assistance in developing the research. Tom Lynch provided field and laboratory assistance with the research.

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