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Impact of short-rotation Acacia hybrid plantations on soil properties of degraded lands in Central Vietnam.


Mismanagement of forests in tropical environments has led to large areas of degraded land in several countries (Lamb 2011). The loss of forest cover due to widespread herbicide use during the Vietnam war, and unsustainable logging and land-use practices between the 1960s and 1980s (Sunderlin and Ba 2005), were the major factors resulting in soil degradation in Vietnam (MARD 2005). Water and wind erosion following loss of tree cover removes topsoil, leading to reduced biological activity, poor soil structure and depiction of nutrient capital (Lai et al. 1989; Lai 19966). Such changes also result in soil compaction, which in turn accelerates erosion by the feedback process of increased runoff (Lai 1997). In Vietnam, 9.4 Mha of land is at risk, with monsoonal rainfall causing topsoil erosion losses of up to 10 t [ha.sup.-1] [year.sup.-1]. Soil in runoff can contain 1% total organic carbon (C), 0.1% total nitrogen (N), 0.035% extractable phosphorus (P) and 0.042% exchangeable potassium (K) (Sam et al. 2006). The losses of these natural capital stocks and soil function take a long time to recover through pedogenesis.

To halt erosion and return degraded areas to productive use, appropriate successional processes based on the level of degradation and the goal of restoration must be artificially applied (Hencghan et al. 2008). A common approach uses vegetation adapted to degraded soils. While various species are used for soil conservation and restoration, tropical acacias are widely planted in South East Asia and South China (Turnbull et al. 1998; Yang et al. 2009; Wang et al. 2010a). Their tolerance of very poor soil conditions and ability to produce a marketable product in a rotation of <10 years make them a preferred choice (Cole et al. 1996; Turnbull et al. 1998; McNamara et al. 2006). Their high productivity and leguminous properties are assumed to be associated with recovery of soil nutrients and an acceleration of nutrient cycling in these degraded soils. For example, total C, total N, extractable P and some exchangeable cations in soils under acacia plantations were significantly increased by acacias and were higher than under other planted species (Bcmhard-Reversat 1996; Yamashita et al. 2008; Kasongo et al. 2009; Schiavo et al. 2009; Yang et al. 2009; Wang et al. 2010a; Sang et al. 2013). These outcomes have been linked to the large amounts of litter deposited by acacia plantations, which, in principle, can lead to increased capacity of the soil to store and supply nutrients, and improve site condition. Their fast canopy closure can protect soil from heavy rainfall and create buffered microclimates, which facilitate soil biological processes (Binkley and Giardina 1997; Norisada et al. 2005; Tsukamoto and Sabang 2005; Schiavo et al. 2009). However, these findings have been associated with long rotations of 8-25 years.

In Thua Thien Hue province in Central Vietnam where this study was done, large areas had become extensively deforested and the landscape seriously degraded by the 1980s; the vegetation then became dominated by Imperata grasslands and scrub species (McNamara et al. 2006). Since then, the Vietnamese Government and international aid organisations have funded countrywide restoration programs, primarily by introducing exotics including Eucalyptus and Acacia species (Binh et al. 2004). Acacia hybrid, a naturally occurring hybrid of A. mangium x A. auriculiformis, has become the dominant species for commercial planting since the late 1990s due to its fast early growth, wide adaptation to degraded soils, and available product markets, particularly pulpwood in a prevailing cutting rotation of about 5 years (Kha 2001; Bueren 2004; Amat et al. 2010), leading to a mean annual increment (MAI) of 22-30 [m.sup.3] [ha.sup.-1] [year.sup.-1] (Bueren 2004; Kha et al. 2006). As a result of these reforestation programs in Thua Thien Hue province, in 2009, plantations of A. hybrid covered 14 884 ha, A. mangium, A. auriculiformis and A. crassicarpa 7692 ha, and mixtures of Acacia and native or Pinus species 11 658 ha (Forestry Subdepartment, Thua Thien Hue province, unpublished data).

In this paper, we test the hypothesis that consecutive plantings of short-rotation Acacia hybrid on degraded land will lead to a cumulative change in some soil properties. This hypothesis was tested by comparing soil properties after one to two short rotations established on degraded lands with those found in adjacent areas having the same land-use history but not planted and abandoned for at least 15 years. The change in these soil properties in a single rotation was also examined by testing their dependency on age of these same Acacia hybrid plantations. As the soils are gravelly, element stock was calculated by correcting for gravel content. The implications of differences between stock and concentration in C and nutrient accounting are discussed.

Materials and methods

Location, climate and soil

The study sites are in the hilly lowland areas of Thua Thien Hue province in Central Vietnam at latitude 16.5[degrees]N (Fig. la). The climate is monsoonal with distinct rainy and dry seasons. The average annual rainfall is >3500 mm, concentrated between September and December, and associated with high-frequency tropical typhoons (Fig. 16). Mean annual temperature is 24.9[degrees]C, with lowest and highest mean monthly temperatures in January (19.4[degrees]C) and June (29.3[degrees]C); mean air humidity is 86.8% (Fig. 16) (Thua Thien Hue Statistical Office 2010). The main soils are siliceous and sandy, and are classified as Acrisols (Sang et al. 2013), with high proportions (~25%) of weathered coarse fragments (>2 mm) derived from siliceous and parent rocks such as granite, sandstone and gritstone (Que et al. 2010).

Site selection

Thirty forested sites in six locations were selected from second-or third-rotation Acacia hybrid plantations re-established between 2006 and 2010. They were of five age classes: 0.5, 1.5, 2.5, 3.5 and 4.5-5.5 years old (the oldest class will be referred to as 5 years old). Each age class was represented at the six locations (Fig. la). This estate was part of a forest-sector development project supported by the World Bank (FSDPWB3) (The World Bank 2013) and used uniform seedling stock and silvicultural practices (Table 1).

At each location, one adjacent 'abandoned' site was selected for collecting comparative samples. These sites had the same land-use history as the forested sites, in that they had been afforested with eucalypt, pine or acacia plantations for a brief period before 1995, but had then been harvested and abandoned as they were under national high-voltage power lines constructed at that time. Plantations were not allowed under the power lines, and the distance between the pylons was ~500 m. The areas selected for sampling showed no sign of disturbance caused by construction. The common indicator for these areas was the dominance of sparse Rhodomyrtus tomentosa and Melastoma candidum, scrub species that are also indicators of degraded and acidic lands.

Sampling and sample analyses

Sampling was carried out in the dry season from May to August 2010. In each plantation, 15 m by 15m (225 [m.sup.2]) sampling plots were used, the number of plots depending on the size of the plantation: 2-3 ha, three plots; 4-5 ha, four plots; and >5 ha, five plots. The plots were used for soil sampling and measurements of tree growth. For plot selection, a transect was first drawn randomly through the longer dimension of the plantation, and plots were located 30 m from the plantation border at each end of the transect. Other plots were evenly spaced between these two end plots. In each plot, a composite mineral soil sample was aggregated from five soil cores randomly collected by a 100-mm-diameter auger at 0-20 cm depth. The sampling points were located in the centre of the inter-row area to maximise the distance from trees and stumps from the previous rotations in the same rows and any disturbance caused by silvicultural operations associated with pit planting. The litter layer was carefully removed before sampling. In addition, three cores to 20 cm depth were randomly collected from the inter-row area in each plot for bulk density (BD) analysis using a 53-mm-diameter ring. Planting stock, diameter at breast height (DBH), total height, crown length and crown diameter of all trees were measured in each forested plot.

In each of six abandoned-land sites, a plot area similar to that used in the plantations (15 m by 15 m) was selected for sampling. Each site met the following criteria: no sign of disturbance in the past; area at least 100 m by 100 m; plot located in the middle of that area. A composite sample and three BD cores were collected as controls using the same methods as for plantations.

Geographic location and elevation of each sampling plot were determined by GPS (Garmin 60CSx; Garmin Ltd, Olathe, KS, USA). The mean of three randomly selected measures of percentage change in elevation per horizontal distance using a straight pole of 4 m length attached to a levelling tool was used to determine the slope of the plot.

Bulk density was determined after the cores were dried at 105[degrees]C to constant weight. Gravel (>2 mm) in each BD core was separated using a 2-mm sieve and weighed. Preparation and analysis of the composite soil samples followed the Australian Laboratory Handbook of Soil and Water Chemical Methods (Rayment and Higginson 1992). The samples were first air-dried and put through a 2-mm sieve. Soil [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and electrical conductivity (EC) were measured by a handheld Laboratory Navigator (Forston Labs, Fort Collins, CO, USA) in a 1:5 mixture of soil and distilled water. Soil [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] was measured in a 1: 5 mixture of soil and 0.01 m calcium chloride. Total C (TC) and total N (TN) were determined using a CHNS/O Element Analyser (PerkinElmer Inc., Waltham, MA, USA). Extractable P was determined using the Olsen manual colour method and measured by a spectrophotometer (UNICO 1100RS; UNICO, Dayton, NJ, USA) at a wavelength of 882 nm. Exchangeable cations (Ex-K, Ex-Ca, Ex-Mg and Ex-Na) were extracted with 0.01 m silver-thiourea [(AgTU).sup.+]; exchangeable [K.sup.+] and [Na.sup.+] were determined by flame photometry (Jenway; Bibby Scientific Ltd, Stone, UK) and [Ca.sup.2+] and [Mg.sup.2+] by atomic absorption spectroscopy (Avanta; GBC Scientific Equipment Pty Ltd, Melbourne, Vic.). Particle sizes were determined on subsamples dispersed by heating in water followed by 16 h end-over-end shaking in the presence of NaOH and Calgon. Fractions were determined by settling with a Bouyoucos hydrometer. All data are reported as unit per oven-dried weight.


Carbon and nutrient stock per ha were calculated as the product of their concentration in soil fractions <2 mm with BD of the fine fraction <2 mm ([BD.sub.<2mm]) and the thickness of the soil layer (20 cm), in which:

[BD.sub.<2 mm] = {[W.sub.t] - [W.sub.g])/[V.sub.c]

where [W.sub.t] is total weight of BD core (g), [W.sub.g] is weight of gravel >2 mm in BD core (g), and [V.sub.c] is volume of BD core ([cm.sup.3]).

Basal area ([m.sup.2] [ha.sup.-1]) was calculated as the sum of the cross-sectional area over bark at breast height of all individual trees per ha. Standing volume ([m.sup.3] [ha.sup.-1]) was the sum of the standing volume of all individuals per ha, in which tree standing volume was a product of basal area with tree height and stem form factor (f) = 0.495 (Binh 2003).

Statistical analyses

Differences in soil properties between plantations and abandoned lands were examined by analysis of variance using the PROC GLM procedure in SAS version 9.2. Dunnett's procedure was used to adjust the 2-sided values to compare the age classes with the abandoned lands (Dunnett 1955). The dependencies of soil properties and stock of elements on plantation age were modelled after adjustment for effects of gravel content, clay content, slope angle and elevation by a regression approach using the PROC MIXED procedure in SAS version 9.2. A random effect corresponding to site was assumed. To ensure that the assumption of homogeneity was met, the analyses were weighted by the reciprocal variances per site. Simple linear regression in IBM SPSS Statistics 21 was used to test the relationship between clay contents and TC and TN of individual plots of all ages, and between growth of individual plots and soil properties, but only for plots aged 2.5-5 years.



The mean altitude of the sampling sites at each location ranged from 24 to 61m and the mean slope from 11.5 to 23.5% (Table 2). The dominant particle fraction <2 mm at all sites was sand (60-82%), whereas silt and clay ranges were 4-19% and 14-22%, respectively (Table 2). The gravel content >2 mm was low at location 4 (6% by mass) but otherwise in the range 26-48% of the total soil mass (Table 2). According to the Australian Soil and Land Survey Handbook (McDonald et al. 1998), all locations fell within the same classes for elevation, slope and gravel content, except for location 4, which was in a lower gravel-content class.

Soil properties--effects of consecutive, short-rotation plantations

Compared with abandoned land, the stock of TC was significantly higher in plantations at ages 0.5, 1.5, 2.5 and 5 years (21.8, 20.7, 18.4 and 19.5 v. 13.0 Mg [ha.sup.-1], respectively), and TN in plantations at ages 0.5 and 1.5 years (1.68 and 1.54 v. 1.04 Mg [ha.sup.-1]). The C/N (= TC/TN) ratio was significantly higher in plantations at ages 2.5 and 5 years. Although all absolute cation stocks were very low, there were large and significant differences in Ex-Ca, Ex-Mg and Ex-Na, which were, respectively, ~5, 2 and 10 times higher in the plantations than abandoned land, but there were no significant differences in extractable P and Ex-K (Table 3, Fig. 2).

Whereas [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] was significantly lower by ~0.1-0.2 of a unit in plantations at ages 0.5 and 5 years compared with abandoned land, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] was significantly lower only in plantations at age 5 years (Table 3). In all age classes, EC in plantations was significantly higher and BD was significantly lower than in abandoned land (Table 3). There were no significant differences in percentage clay, silt or sand contents, or percentage of gravel to total soil mass (Table 3).

The content of gravel >2 mm in the 0-20 cm topsoil layer ranged from 30 to 42% of total soil mass among plantation age classes and was 47% in the abandoned land (Table 3). Because of this high gravel content, concentrations exhibited a pattern with plantation age different from that for stocks. Compared with abandoned land, concentration of TC was significantly higher only in soils of 5-year-old plantations, and there were no significant differences for TN (Fig. 2). While statistical differences in the other element concentrations remained similar to those of stocks, Ex-Ca and Ex-Mg concentration were significantly higher at all ages of plantations (Fig. 2).

Effects of site and rotation age

For nutrient concentration, only the C/N ratio increased significantly with age; concentrations of TC, TN and exchangeable cations did not change significantly (Table 4). Although non-significant, the levels of TC and TN appeared to follow an observed trend that indicated a decline until age 2.5-3.5 years and then an increase at age 5 years (Fig. 2). Phosphorus and the exchangeable cations also appeared to decline with age (data not shown); EC decreased until age 2.5 years but returned to the initial level at 5 years (Table 3).

For nutrient stock, TN (kg [ha.sup.-1]) changed significantly with plantation age; changes for other nutrients were not significant (Table 4). The decline with age noted for concentration was greater for nutrient stock and recovery with later age was delayed, or absent (Fig. 2).

Clay content was significantly related to concentration of extractable P; gravel content to TC and TN; slope to [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]; and altitude to C/N, exchangeable cations (except Ex-Na), pH [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and EC (Table 4). Similarly, clay was significantly related to stock of extractable P, and altitude to all exchangeable cations (Table 4). Examination of the relationships between TC or TN and clay content show that both were significantly related to clay at age 0.5 year (P = 0.009 and <0.001; [r.sup.2] = 0.34 and 0.56, respectively) and at age 2.5 years and older (P = 0.009-0.048 and [r.sup.2] = 0.24 - 0.31 between clay and TC, and P = 0.001-0.017 and [r.sup.2] = 0.32 - 0.44 between clay and TN). There was no relationship at age 1.5 years for either TC or TN (P = 0.86 and 0.69; [r.sup.2] = 0.001 and 0.008, respectively).

Relationship between tree growth and soil properties

All measured variables indicated that growth rates were relatively high throughout the rotation; mean MAI of 5-year-old plantations was 28.7 [+ or -] 5.9 [m.sup.3] [ha.sup.-1] [year.sup.-1] (Table 5). There were weak, though significantly positive, linear relationships between both [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] ([r.sup.2] = 0.14, P=0.003) and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] ([r.sup.2] = 0.10, P=0.012) and mean annual DBH increment; between elevation and mean annual DBH increment ([r.sup.2] = 0.09, P=0.016); and between elevation and mean annual height increment ([r.sup.2] = 0.16, P = 0.001). A negative linear relationships was found between C/N ratio and mean annual DBH increment ([r.sup.2] = 0.12, P= 0.005). There were no significant relationships between other soil properties and tree growth.


This study has shown that soils after planting with one to two short rotations of Acacia hybrid compared with soils in abandoned land are associated with increases in soil C, although these are more apparent in stock than concentration. There were also significant increases in stock of TN but not in concentration of TN, extractable P or Ex-K. Although significantly higher Ex-Ca, Ex-Mg, Ex-Na and sustained increases in EC were generally observed, these were accompanied by slightly lower pH in some age classes. Within the measured rotations, age had little effect on the soil variables measured, but clay, gravel content and altitude did. The soils used in this study were degraded, strongly acidic, leached, and low in TC, so the absolute changes in soil nutrients and TC were small. Nevertheless, MAIs approached 30 [m.sup.3] [ha.sup.-1] [year.sup.-1] at age 5 years, showing that Acacia hybrid can be an economic crop on soils with meagre nutrient levels. These results are now discussed in the context of the changes in soil properties between soils planted with one to two short rotations of Acacia hybrid and those of abandoned land, soil change within a single rotation, and the importance of considering nutrient stock in such an analysis.

Soil properties change after one to two short rotations

Vegetation cover is crucial to the recovery of soil organic matter (SOM) on degraded sites, and SOM is the reservoir of soil fertility (Craswell and Lefroy 2001). In A. mangium plantations, there is a significant relationship between biomass production and TC in soil (Sang et al. 2013). In this study, the stock of TC at age 0.5, 1.5, 2.5 and 5 years in second- and third-rotation Acacia hybrid plantations became significantly higher than in adjacent, abandoned sites that had only supported scrub vegetation for the previous 15 years (21.8,20.7, 18.4 and 19.5 v. 13.0 Mg [ha.sup.-1], respectively). A significant difference may have emerged in the earlier rotation if the current conventional land preparation practice in Thua-Thien-Hue province of burning the accumulated litter and slash from the previous rotation had been avoided (Table 1). Previous studies have also shown that TC (%) in longer rotations of A. mangium and A. auriculiformis planted on degraded soils is significantly higher than in adjacent scrublands and grasslands (Kasongo et al. 2009; Yang et al. 2009; Wang et al. 2010a). Thus, tropical Acacia species appear to have the ability to contribute to the recovery of SOM. These tropical acacias are fast-growing and quickly provide high litter deposition rates of 9.4-11.1 t [ha.sup.-1] [year.sup.-1] in A. mangium (Li et al. 2000; Hardiyanto and Wicaksono 2008) and 4.8-6.7 t [ha.sup.-1] [year.sup.-1] in A. auriculiformis (Li et al. 2000; Huong et al. 2008) plantations. Acacia hybrid, the natural hybrid between these species, is reported to have higher growth rates than its parents (Kha 2001; Bueren 2004), so even higher rates of litter production are expected.

Total nitrogen stock (TN) was significantly higher only in the first and second year of the rotation (1.68 and 1.54 Mg [ha.sup.-1] v. 1.04 Mg [ha.sup.-1]) compared with abandoned land. Acacias fix atmospheric N, and deposition rates of tropical Acacia species can exceed 100 kg N [ha.sup.-1] [year.sup.-1] (Bemhard-Reversat 1996; Galiana et al. 2002; Yang et al. 2009). This N is mainly released through the litter and root exudates (Brockwell et al. 2005; Forrester et al. 2006). As tropical acacias have relatively low decomposition rates compared with some other fast-growing species (Li et al. 2001), and low N mineralisation and nitrification rates (Wang et al. 20106), N can remain trapped in the litter fraction for longer periods. This may be associated with the significant increase in the soil C/N ratio during the rotation in this study. Burning of accumulated litter during land preparation (Table 1) resulting in losses of sources of N and large amounts of N uptake by Acacia hybrid associated with its high rate of growth may be other reasons for the low TN concentration in soils. Where tropical acacias have been managed over longer rotations, TN was found to be significantly higher than in adjacent scrublands and grasslands (Macedo et al. 2008; Kasongo et al. 2009; Yang et al. 2009).

Phosphorus is often the most limiting nutrient for the growth of legumes (Munns and Franco 1982), and in natural systems, P is replenished from mineral weathering (Walker and Syers 1976). Little or no enhancement of extractable P stock in the topsoil layer is therefore expected, and after 5 years of short-rotation Acacia hybrid forestry in this study, extractable P was not significantly different from that in the abandoned lands. There is an accepted belief that legumes have a high demand for P for their [N.sub.2] fixation, hence the high P concentration in their nodules (Sun et al. 1992; Sprent 1999). At these sites in Central Vietnam, the soils were highly siliceous and coarse-textured with low natural P levels. The primary source of P for the trees was probably from the fertiliser applied early in the rotation; P may also have been depleted in harvested products. Longer rotations may be necessary for recovery of extractable P levels (Yang et al. 2009; Wang et al. 2010a) via plant uptake from deeper, less weathered soil layers followed by litter decomposition at the surface.

The low vegetation coverage of the abandoned lands and the sandy soil in this very high- and intense-rainfall environment in Central Vietnam are probably the major factors contributing to the loss of exchangeable cations by erosion and leaching, and this may explain the significantly higher stock and concentrations of Ex-Ca, Ex-Mg and Ex-Na in planted Acacia hybrid. In addition, clay content and SOM together provide the capacity for retaining exchangeable cations (Astera 2010). As the clay content was similar between abandoned lands and plantations, the higher TC of plantations may have contributed to the higher concentrations of exchangeable cations. This may also in part be related to cations being added to the soil by burning litter and debris when preparing land for planting. Similar enhancement has also been reported for Ex-Ca and Ex-Mg in a longer rotation A. auriculiformis plantation on acidic sandy soil in the Congo (Kasongo et al. 2009), and for Ex-Ca and Ex-Na in longer rotation A mangium and A. auriculiformis plantations on degraded soils in South China (Wang et al. 2010a). The stock of Ex-K was not significantly different between plantations and abandoned lands, but the values were extremely low in both treatments, 10.5-17.3 v. 14.8 Mg [ha.sup.-1]. Ex-K is easily leached in sandy soil (Kasongo et al. 2009) and absorbed by plants in larger amounts than other cations (Verheye 2006).

In this study, the plantations had been fertilised at planting and in the second year with a total of ~40 kg N, 17 kg P and 17 kg K [ha.sup.-1] (Table 1). However, wood harvesting and burning associated with previous rotations may have removed a greater amount of nutrients from the sites. Similarly managed A. mangium plantations in Indonesia lost 264-371 kg N, 8-12 kg P and 73-91 kg K [ha.sup.-1] through harvested stems (Hardiyanto and Wicaksono 2008). Thus, recovery of soil nutrients under acacia plantations is potentially higher.

Although reductions of pH were not always significant, there was some indication that Acacia hybrid plantations had ~0.1-0.2 lower [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.78-3.87 v. 3.98 and 4.30-4.43 v. 4.52, respectively). Nitrogen-fixing plants can produce acid (Binkley and Giardina 1997; Tang 1997; Tang et al. 1997, 1999) and acacia plantations are associated with increasing soil acidity (Yamashita et al. 2008; Kasongo et al. 2009; Schiavo et al. 2009; Sang et al. 2013). If the dominant form of N in soil is N[H.sub.4.sup.+], as is typical under acacia, cation uptake exceeds anion uptake (Tang 1997; Tang et al. 1999). This excess uptake of cations leads to excretion of [H.sup.+] by plant roots, the mechanism by which plants regulate their charge balance (Haynes 1990), and a decrease in rhizospherc pH (Tang 1997; Tang et al. 1997). Any nitrification in the soil would also enhance acidification (Tang 1997). Soil pH in an A. auriculiformis plantation on sandy soils in the Congo decreased from age 1 to 17 years and was significantly lower than in nearby savannah (Kasongo et al. 2009). That this is a real effect is demonstrated by a reversal of this decline after conversion of Acacia plantations to non-legume cropping systems (El Tahir et al. 2009). The significantly higher EC in plantations than in abandoned land (58.5-69.4 v. 32.7 [micro]S [cm.sup.-1]) is most likely related to greater water use and hence drier soils and lower leaching in plantations.

Bulk density was significantly higher in abandoned lands than plantations (1.55 v. 1.36-1.42 g [cm.sup.-3]). Bulk density commonly increases after deforestation (Lai 1996a; Yuksek et al. 2010), but decreases with time after reforestation, especially in the upper layers (Lai 1996a). The increase comes from soil compaction and erosion induced by raindrop impact when vegetation cover is removed. Fine soils are lost, leaving gravels, which have higher specific gravity. The decrease in BD is associated with the role of vegetation in protecting the soil from raindrop impact and thereby mitigating against soil compaction, as well as facilitating enhancement of soil SOM, soil organism activity and root penetration, all of which loosen soil.

Soil properties change within a single short rotation

Soil nutrient dynamics in short-rotation forestry are affected by management practices, and in these Acacia hybrid plantations there was an indication, although non-significant, of depletion of TC stock after planting. This is often associated with afforestation but normally followed by recovery (Paul et al. 2002), as also occurred by age 5 years in this study. Disturbance caused by land preparation and susceptibility of bare land to erosion when trees are small are the major reasons for the initial decline (Paul el al. 2002). This was illustrated in the examination of the relationships between TC (%) or TN (%) and clay content. The lack of correlations at age 1.5 years (P>0.05; [r.sup.2] = 0.001 and 0.008, respectively) is possibly related to the effects of erosion being at a maximum following a period of little or no tree coverage. After canopy closure, which occurs at around age 2 years, the relationships became significant.

An observed trend of nutrient recovery in later years of the rotation, although only TN stock (Mg [ha.sup.-1]) was significant, indicates that some enhancement of nutrient availability may occur in a longer rotation. In a previous study, TC (%) and TN (%) in soils of an A. auriculiformis plantation increased significantly after 4 years, and doubled after 8 years; exchangeable cations increased significantly after 10 years (Kasongo et al. 2009). In 23- and 24-year-old A. mangium plantations in South China, concentrations of TC and TN at 0-20 cm soil depth were substantially higher than in this study, 2.11-5.58% and 0.103-0.104%, respectively (Yang et al. 2009; Wang et al. 2010a). Thus, while some elements are just maintained in a short rotation of tropical Acacia species, marked increases may happen over a longer rotation.

Nutrient stocks in gravelly soils

In this study, gravels >2 mm represented 30-42% and 47% of the total soil mass for plantations and abandoned land, respectively. Correcting for gravel content resulted in significant differences in some clement stocks between plantations and abandoned land, in particular TC at ages 0.5, 1.5, 2.5 and 5 years and TN at age 0.5 year, whereas they were not significant for element concentration, except for TC at age 5 years. This also resulted in the different trends of changes with plantation age between element stocks and concentrations. Which is the better measure of soil nutrients in plantations: stock or concentration? Whereas nutrient concentration provides a direct measure of soil nutrient availability for plant uptake, this is only the case if there is no between-root competition for, and free access to, available nutrients. In dense, fast-growing species plantations, where plant roots have limited space and must compete for nutrients, nutrient stock is a better measure of available soil nutrients (Gersani et al. 2001; O'Brien and Brown 2008). Therefore in gravelly soils, nutrient accounting should be represented by nutrient stock after correcting for gravel content.

Tree growth and soil fertility

Although nutrient stocks and concentrations in the soils were extremely low, tree growth rates were high, with MAI of 28.7 [m.sup.3] [ha.sup.-1] [year.sup.-1] by the end of a 5-year rotation. However, correlations between soil properties and tree growth were largely absent. This suggests that the initial condition of the soils used in this study, particularly with respect to the key plant nutrients, was relatively similar among sites except for the factor of elevation, which did appear to slightly effect DBH and height growth (P<0.05, [r.sup.2] = 0.094 and 0.16, respectively). Thus, the differences in current soil properties between plantations and abandoned lands were clearly related to the acacia plantations rather than local variations in the initial soil properties.


We conclude that short-rotation Acacia hybrid plantations on degraded lands can either increase or conserve some key soil chemical and physical properties. However, levels of soil acidification of Acrisols dominated by sand remain high. Most soil properties were not significantly changed within a 5-year rotation despite consistently observed trends of depletion in the first 2-3 years and later recovery after that in some key nutrients. This suggests potential for further improvements in some soil properties over a longer rotation. Soil nutrients are clearly related to some initial soil and site factors such as gravel, clay content and elevation. In soil with a high proportion of gravel content, element stock per unit volume should take this factor into account, and provide a better measure of soil element levels than concentration.


This research forms part of a PhD research program by the senior author, who is funded by an Australian Centre for International Agricultural Research (ACIAR) John Allwright Fellowship. Support by ACIAR Project FST/2006/087 is also acknowledged. We thank Dr Dao Cong Khanh and fieldwork team of Vietnamese Academy of Forest Sciences and WB3 project in Hue province for assistance with fieldwork, and Mr Garth Oliver for assistant in soil sample analysis. Thanks to Drs David Forrester and Leigh Sparrow and two anonymous reviewers for their valuable comments on an earlier draft of the manuscript.

Received 29 May 2013, accepted 24 November 2013, published online 31 March 2014


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Tran Lam Dong (A,C,E), Richard Doyle (A), Chris L. Beadle (A,B), Ross Corkrey (A), and Nguyen Xuan Quat (D)

(A) Tasmanian Institute of Agriculture (TIA) and School of Land and Food, University of Tasmania, Private Bag 98, Hobart, Tas. 7001, Australia.

(B) CSIRO Ecosystem Sciences, Private Bag 12, Hobart, Tas. 7001, Australia.

(C) Silviculture Research Institute, Vietnamese Academy of Forest Sciences, Dong Ngac, Tu Liem, Hanoi, Vietnam.

(D) Vietnam Association of Forest Sciences and Technology, 114 Hoang Quoc Viet, Hanoi, Vietnam.

(E) Corresponding author. Email:;

Table 1. Silviculture applied to the plantations of A. hybrid used in
this study in Thua Thien Hue, Vietnam

Year   Month        Activities                      Fertiliser

1      Mar.-Aug.    Harvesting previous rotation
       July-Aug.    Bum litter and slash from
                      previous rotation
       Aug.-Sept.   Dig 40 by 40 by 40 cm
                      planting holes manually or
                      by excavator at 3 by 2 m or
                      2 by 2 m spacing (1660-2500
                      trees [ha.sup.-1]
       Oct.-Dec.    Mix soil in hole with 0.05kg    13.3 kg N + 5.8 kg
                      16:16:8N:P:K fertiliser;        P + 5.5 kg K
                      plant clonal cuttings           [ha.sup.-1]
2       Mar.-May    Weeding and pruning; hoe-up     26.6kg N + 11.6 kg
                      topsoil around each             P + 11.0 kg K
                      seedling and incorporate
                      0.1 kg 16: 16: 8 N : P : K
                      in ring at
                      0.7-1.0-m-diameter from
3      Apr.-June    Weeding                           [ha.sup.-1]
5-6    Mar.-Aug.    Harvesting and next rotation

Table 2. Means and standard deviations of site and soil factors at
each location in Thua Thien Hue, Vietnam Number of sites at each
location was six, i.e. five plantation age classes and one control
(abandoned land)

Factor                                     Location

                                   1                    2

Altitude (m)               35.3 [+ or -] 2.1    26.6 [+ or -] 4.5
Slope (%)                  14.9 [+ or -] 3.5    11.5 [+ or -] 6.3
Clay (%fraction <2 mm)     17.5 [+ or -] 4.1    14.4 [+ or -] 5.8
Silt (%fraction <2 mm)      9.4 [+ or -] 2.7     3.9 [+ or -] 2.7
Sand (%fraction <2 mm)     73.1 [+ or -] 5.3    81.7 [+ or -] 8.1
Gravel >2 mm (% of total   44.1 [+ or -] 19.4   26.0 [+ or -] 23.9
  soil mass)

Factor                                     Location

                                    3                    4

Altitude (m)                64.9 [+ or -] 18.4    60.7 [+ or -] 20.4
Slope (%)                   23.5 [+ or -] 6.2     14.2 [+ or -] 4.5
Clay (%fraction <2 mm)      21.9 [+ or -] 4.7     15.9 [+ or -] 3.7
Silt (%fraction <2 mm)       7.0 [+ or -] 2.6      5.9 [+ or -] 2.4
Sand (%fraction <2 mm)      71.1 [+ or -] 6.3     78.2 [+ or -] 5.3
Gravel >2 mm (% of total    36.0 [+ or -] 11.4     5.8 [+ or -] 5.3
  soil mass)

Factor                                     Location

                                    5                     6

Altitude (m)                24.6 [+ or -] 7.3    31.3 [+ or -] 13.2
Slope (%)                   13.7 [+ or -] 8.6    17.6 [+ or -] 9.5
Clay (%fraction <2 mm)      18.7 [+ or -] 3.9    21.4 [+ or -] 3.8
Silt (%fraction <2 mm)       9.7 [+ or -] 3.7    18.7 [+ or -] 5.6
Sand (%fraction <2 mm)      71.6 [+ or -] 6.4    59.9 [+ or -] 8.7
Gravel >2 mm (% of total    47.6 [+ or -] 10.2   45.0 [+ or -] 14.9
  soil mass)

Table 3. Means, standard errors and significant differences of soil
properties in 0-20 cm topsoil of second-or third-rotation Acacia
hybrid plantations and nearby abandoned lands in Thua Thien Hue,

TC, Total Carbon; TN, total nitrogen; Ext-P, extractable phosphorus;
Ex-K, Ex-Ca, Ex-Mg, Ex-Na: exchangeable potassium, calcium,
magnesium, sodium; EC, electrical conductivity; BD, bulk density.
Significant differences for each plantation age compared with
abandoned lands: * P < 0.05, ** P < 0.01; P-values are adjusted using
Dunnett's procedure for comparison with the abandoned land

                                              Age of Acacia hybrid
                                              plantations (years)
Properties                    lands                   0.5

TC (Mg [ha.sup.-1])    12.99 [+ or -] 1.75   21.76 [+ or -] 0.72 *
TN (Mg [ha.sup.-1])     1.04 [+ or -] 0.08    1.68 [+ or -] 0.07 *
C:N ratio               12.3 [+ or -] 0.8     13.1 [+ or -] 0.4
Ext-P (Mg               4.21 [+ or -] 1.29    4.56 [+ or -] 0.46
Ex-K (Mg               14.81 [+ or -] 3.49   17.25 [+ or -] 2.75
Ex-Ca (Mg               41.5 [+ or -] 4.6    293.6 [+ or -] 50.7 **
Ex-Mg (Mg              19.48 [+ or -] 3.76   48.40 [+ or -] 6.42 **
Ex-Na (Mg               5.32 [+ or -] 1.62   58.49 [+ or -] 4.69 **
[pH.sub.CaC12]          3.98 [+ or -] 0.04    3.84 [+ or -] 0.04 *
  (1 : 5)
[pH.sub.[H.sub.2]O]     4.52 [+ or -] 0.05    4.35 [+ or -] 0.05
  (1 : 5)
EC (1 : 5) ([micro]S    32.7 [+ or -] 2.2     69.4 [+ or -] 5.4 **
BD (g [cm.sup.-3])      1.55 [+ or -] 0.04    1.37 [+ or -] 0.02 **
Clay (% of fraction     19.1 [+ or -] 2.0     17.3 [+ or -] 0.8
  <2 mm)
Sand (% of fraction     71.0 [+ or -] 4.0     74.1 [+ or -] 1.9
  <2 mm)
Gravel >2 mm (% of      46.7 [+ or -] 10.0    30.2 [+ or -] 4.5
  total soil mass)

                          Age of Acacia hybrid plantations (years)

Properties                      1.5                      2.5

TC (Mg [ha.sup.-1])    20.72 [+ or -] 0.97 *    18.44 [+ or -] 1.06 *
TN (Mg [ha.sup.-1])     1.54 [+ or -] 0.08 *     1.29 [+ or -] 0.07
C:N ratio               13.7 [+ or -] 0.4        14.5 [+ or -] 0.5 *
Ext-P (Mg               3.24 [+ or -] 0.28       3.75 [+ or -] 0.70
Ex-K (Mg               14.39 [+ or -] 1.95      11.93 [+ or -] 1.23
Ex-Ca (Mg              248.0 [+ or -] 29.4 *    214.5 [+ or -] 35.2 *
Ex-Mg (Mg              40.70 [+ or -] 3.78 *    40.14 [+ or -] 3.89 *
Ex-Na (Mg              59.06 [+ or -] 7.15 **   58.05 [+ or -] 5.55 **
[pH.sub.CaC12]          3.86 [+ or -] 0.03       3.87 [+ or -] 0.03
  (1 : 5)
[pH.sub.[H.sub.2]O]     4.40 [+ or -] 0.03       4.43 [+ or -] 0.04
  (1 : 5)
EC (1 : 5) ([micro]S    60.5 [+ or -] 3.6 **     58.5 [+ or -] 3.2 **
BD (g [cm.sup.-3])      1.42 [+ or -] 0.02 **    1.37 [+ or -] 0.02 **
Clay (% of fraction     19.6 [+ or -] 1.1        16.8 [+ or -] 1.4
  <2 mm)
Sand (% of fraction     71.9 [+ or -] 1.4        74.9 [+ or -] 2.6
  <2 mm)
Gravel >2 mm (% of      32.3 [+ or -] 3.7        33.4 [+ or -] 4.7
  total soil mass)

                          Age of Acacia hybrid plantations (years)

Properties                      3.5                       5

TC (Mg [ha.sup.-1])    17.64 [+ or -] 1.14      19.48 [+ or -] 1.34 *
TN (Mg [ha.sup.-1])     1.28 [+ or -] 0.09       1.31 [+ or -] 0.08
C:N ratio               14.0 [+ or -] 0.5        14.8 [+ or -] 0.3 *
Ext-P (Mg               3.54 [+ or -] 0.45       3.23 [+ or -] 0.55
Ex-K (Mg               11.89 [+ or -] 1.60      10.49 [+ or -] 0.97
Ex-Ca (Mg              190.7 [+ or -] 31.7      182.5 [+ or -] 28.2
Ex-Mg (Mg              38.91 [+ or -] 3.61 *    34.34 [+ or -] 2.24
Ex-Na (Mg              52.65 [+ or -] 7.27 **   46.23 [+ or -] 6.99 *
[pH.sub.CaC12]          3.85 [+ or -] 0.02       3.78 [+ or -] 0.02 **
  (1 : 5)
[pH.sub.[H.sub.2]O]     4.40 [+ or -] 0.04       4.30 [+ or -] 0.03 *
  (1 : 5)
EC (1 : 5) ([micro]S    60.5 [+ or -] 3.9 **     69.4 [+ or -] 4.3 **
BD (g [cm.sup.-3])      1.38 [+ or -] 0.03 **    1.36 [+ or -] 0.02 **
Clay (% of fraction     18.6 [+ or -] 1.4        19.2 [+ or -] 1.2
  <2 mm)
Sand (% of fraction     71.5 [+ or -] 2.9        70.2 [+ or -] 2.1
  <2 mm)
Gravel >2 mm (% of      37.6 [+ or -] 4.5        42.2 [+ or -] 4.3
  total soil mass)

Table 4. Models of soil clement concentration or soil and nutrient
stocks and some key properties of second-or third-rotation Acacia
hybrid plantations in Thua Thien Hue, Vietnam

TC, Total Carbon; TN, total nitrogen; Ext/P, extractable phosphorus;
Ex/K, Ex/Ca, Ex/Mg, Ex/Na: exchangeable potassium, calcium,
magnesium, sodium; EC, electrical conductivity. Shown are the model
coefficients and their significant P values (P < 0.05). n.s., not
significant (P [greater than or equal to] 0.05)

Independent      Effect
                               TC       TN      C/N        Ext-P
                                   (%)                  [kg.sup.-1])

                 Intercept   0.975    0.078    15.19       2.758
Clay (%)         Estimate                                  -0.037
                 P            n.s.     n.s.     n.s.       0.032
Gravel content   Estimate     0.47    0.029
  (% of BD)
                 P           0.001    0.016     n.s.        n.s.
Slope            Estimate
  ([degrees])    P            n.s.     n.s.     n.s.        n.s.
Altitude (m)     Estimate                      -0.049
                 P            n.s.     n.s.    <0.001       n.s.
Age              Estimate                      0.319
                 P            n.s.     n.s.    0.021        n.s.

                             (Mg [ha.sup.-1])               (kg

                 Intercept   21.47    1.325                5.262
Clay (%)         Estimate                                  -0.07
                 P            n.s.     n.s.                 0.01
Slope            Estimate
  ([degrees])    P            n.s.     n.s.                 n.s.
Altitude (m)     Estimate             0.008
                 P            n.s.    <0.001                n.s.
Age              Estimate             -0.089
                 P            n.s.    0.0032                n.s.

Independent      Effect          Dependent variable
                             Ex-K     Ex-Ca     Ex-Mg     Ex-Na

                                      (cmol [kg.sup.-1])

                 Intercept   0.007     0.41      0.112     0.133
Clay (%)         Estimate
                 P            n.s.     n.s.      n.s.      n.s.
Gravel content   Estimate
  (% of BD)
                 P            n.s.     n.s.      n.s.      n.s.
Slope            Estimate
  ([degrees])    P            n.s.     n.s.      n.s.      n.s.
Altitude (m)     Estimate    0.0003    0.006     0.002
                 P           <0.001    0.011    <0.001     n.s.
Age              Estimate
                 P            n.s.     n.s.      n.s.      n.s.

                                       (kg [ha.sup.-1])

                 Intercept   6.0839   132.27    26.8626   53.2934
Clay (%)         Estimate
                 P            n.s.     n.s.      n.s.      n.s.
Slope            Estimate
  ([degrees])    P            n.s.     n.s.      n.s.      n.s.
Altitude (m)     Estimate    0.2829    2.939    0.5461    0.4329
                 P           <0.001    0.004    <0.001    0.0227
Age              Estimate
                 P            n.s.     n.s.      n.s.      n.s.

Independent      Effect
                             [pH.sub.Ca     [pH.sub.          EC
                             [Cl.sub.2]]   [H.sub.2]]O    ([micro]S

                 Intercept      3.794         4.268         35.78
Clay (%)         Estimate
                 P              n.s.          n.s.           n.s.
Gravel content   Estimate
  (% of BD)
                 P              n.s.          n.s.           n.s.
Slope            Estimate      -0.009
  ([degrees])    P              0.027         n.s.           n.s.
Altitude (m)     Estimate       0.004         0.003         0.639
                 P              0.005         0.047         <0.001
Age              Estimate
                 P              n.s.          n.s.           n.s.
Clay (%)         Estimate
Slope            Estimate
  ([degrees])    P
Altitude (m)     Estimate
Age              Estimate

Table 5. Means and standard deviations of tree density, diameter at
breast height, total height, basal area, standing volume and mean
annual increment of 0.5-, 1.5-, 2.5-, 3.5-and 5-year-old second-or
third-rotation Acacia hybrid plantations in Thua Thien Hue, Vietnam

DBH, Diameter at breast height; MAI, mean annual increment

Stand description                                   Age of Acacia
                                                  hybrid plantations

                                    0.5                  1.5

Density (tree [ha.sup.-1])   2015 [+ or -] 215    2036 [+ or -] 433
DBH (cm)                     0.54 [+ or -] 0.31   4.40 [+ or -] 0.85
Height (m)                   1.62 [+ or -] 0.41   5.39 [+ or -] 0.97
Crown diameter (m)           1.12 [+ or -] 0.29   2.27 [+ or -] 0.41
Basal area ([m.sup.2]                --           3.59 [+ or -] 1.37
Standing volume ([m.sup.3]           --           10.5 [+ or -] 5.2
MAI ([m.sup.3] [ha.sup.-1]           --            7.0 [+ or -] 3.5

Stand description             Age of Acacia hybrid plantations (years)

                                     2.5                   3.5

Density (tree [ha.sup.-1])    2137 [+ or -] 495     1846 [+ or -] 331
DBH (cm)                      7.56 [+ or -] 0.89    9.23 [+ or -] 0.69
Height (m)                    8.71 [+ or -] 0.64   11.55 [+ or -] 1.46
Crown diameter (m)            2.57 [+ or -] 0.33    2.75 [+ or -] 0.29
Basal area ([m.sup.2]        10.58 [+ or -] 2.40   13.85 [+ or -] 3.30
Standing volume ([m.sup.3]    47.7 [+ or -] 12.7    84.5 [+ or -] 26.6
MAI ([m.sup.3] [ha.sup.-1]    19.1 [+ or -] 5.1     24.1 [+ or -] 7.6

Stand description


Density (tree [ha.sup.-1])    1456 [+ or -] 542
DBH (cm)                     11.86 [+ or -] 1.36
Height (m)                   15.41 [+ or -] 1.31
Crown diameter (m)            3.19 [+ or -] 0.54
Basal area ([m.sup.2]        17.40 [+ or -] 3.41
Standing volume ([m.sup.3]   139.3 [+ or -] 34.5
MAI ([m.sup.3] [ha.sup.-1]    28.7 [+ or -] 5.9
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Article Details
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Author:Dong, Tran Lam; Doyle, Richard; Beadle, Chris L.; Corkrey, Ross; Quat, Nguyen Xuan
Publication:Soil Research
Article Type:Report
Geographic Code:9INDO
Date:May 1, 2014
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