Soil Carbon Fractions in Eucalyptus Globulus Plantations in Western Australia

0183-B1

R. T. Aggangan^[1], A. M. O’Connell and B. Dell

Abstract

Soil carbon (C) from Eucalyptus globulus plantations established on ex-native forest and ex-pasture sites, which had been fertilized with either N, or P or N-and-P was fractionated using a method based on ease of oxidation by 333 mM potassium permanganate. Soils from adjacent native forest and pasture were also sampled and analyzed for C fractions. In the ex-native forest plantation soil, the amount of total soil organic carbon (C_T), labile carbon (C_L) and non-labile carbon (C_NL) were not affected by addition of either N or P, but were significantly increased by application of both N-and-P. The carbon pool index (CPI) which reflects changes in the amount of total soil organic C in fertilized plots relative to the control treatment, and carbon management index (CMI) which relates changes in CPI and the proportion of labile and non-labile soil C to fertilizer treatments also increased significantly with application of N-and-P. By contrast, in the ex-pasture plantation soil, fertilizer amendment had no effect on C_T, C_L C_NL, CPI or CMI. Further, the soil C fractions show that the conversion of native forest to E. globulus plantations has resulted in reduction of C relative to the reference soil. This was also reflected in the CPI and CMI, suggesting that where eucalypt plantations are established on ex-native forest sites there may be an initial reduction in soil C. The results suggest that at study site fertilization with N and P was important in maintaining the organic matter status of ex-native forest soils during the first rotation. However, the extent to which these trends will continue in the longer term is uncertain. Therefore, further study is necessary over more than one rotation, to determine the longer-term effects of silvicultural practices on soil C dynamics.

Introduction

Soil organic matter (SOM) is a major storage reservoir of plant nutrients and it is an important source and sink in the global carbon cycle (Ellert and Bettany, 1995). Soil organic matter has been notionally divided into three functional fractions based on their turnover rates: labile (active), slow and recalcitrant (passive) (Parton et al., 1987). The labile pools of SOM are assumed to have an important role in N availability because SOM dynamics and N cycling are closely related through the microbially mediated processes of N mineralization and immobilization (Biederbeck et al., 1994).

Little information exists on the amounts and forms of soil C in E. globulus plantations growing on sandy soils in Western Australia. Likewise, there have been no studies done to determine the effects of fertilizer addition on the storage and partitioning of soil C in these ecosystems. Hence, this study was conducted to quantify the effects of N, P, or N-and-P fertilization on the amounts and forms of soil C at sites that had been previously used for pasture or that had been cleared of native forest.

Materials and methods

Experimental sites and soil sampling

Briefly, the soil samples used in this study were collected (February 1994) at a depth of 0-200 mm from a 4.5 year old E. globulus plantation established in ex-native forest and ex-pasture sites. Fertilizer treatments were broadcast on both plantation sites in October 1992 (N as ammonium nitrate, P as double super phosphate, and K as muriate of potash). The four fertilizer treatments at each site were: (i) -N-P (0 kg N ha^-1, 0 kg P ha^-1), (ii) +N-P (400 kg N ha^-1, 0 kg P ha^-1), (iii) -N+P (0 kg N ha^-1, 200 kg P ha^-1), and (iv) +N+P (400 kg N ha^-1, 200 kg P ha^-1). All plots received 250 kg K ha^-1. Reference soils were also collected from adjacent native forest and pasture.

Carbon fractionation

The total soil organic C (C_T) was measured in finely ground air-dried soil using the method of Walkley & Black. Labile C (C_L) and non-labile C (C_NL) fractions in soil were determined by the procedure of Blair et al. (1995). The diluted extracts were measured on a UV/VIS spectrophotometer at 565 nm. The oxidation of organic matter is expressed in mg C g^-1, and assumes 1 mM KMnO₄ oxidizes 1 mg C. The C fraction oxidized by 333 mM KMnO₄ is the C_L, whereas the C fraction not oxidized by KMnO₄ is the C_NL.

Sustainability indices

Using the C_L and C_NL contents obtained in each sample, the following sustainability index was calculated as described by Blair et al. (1995).

Carbon management index (CMI) = CPI × LI × 100

Data analysis

Within each previous land use class (pasture and native forest), the effects of fertilizer treatments on C_T, C_L, C_NL and sustainability indices were analyzed by analysis of variance (ANOVA), and means were compared using Duncan’s new Multiple Range Test (DMRT) where the ANOVA showed that there was a significant difference between treatment means (p<0.05).

Results

Effects of N and P fertilizer on C fractions in the plantation soils

In the ex-native forest plantation soil, C_T, and C_NL were significantly affected by the addition of N and P fertilizer. There was a significant interaction between the effects of added N and P with highest concentrations of C_T and C_NL in soil from treatments receiving both nutrients (Table 1). The C_L was also significantly influenced by addition of N and there was a significant interaction between N and P with greater concentration of C_L (p<0.001) in the +N+P treatment than in the other fertilizer treatments (Table 1). The L and LI were significantly affected by -N+P with no interaction between the effects of added N and P. The CPI was significantly affected by the addition of N and P and there was a significant interaction between N and P. The CPI was greatest (p<0.01) in the +N+P treatment relative to the +N-P, -N+P, and -N-P treatments. The CMI was not affected by addition of N and P alone but there was a significant interaction between N and P (Table 1).

In the ex-pasture plantation soil, C_T, C_L and C_NL were generally lower than in ex-native forest plantation soil and none of the measures of soil C or the derived indices were significantly affected by the addition of N and P fertilizers. The +N+P treatment had higher concentrations of soil C_T, C_L and C_NL than the -N-P treatment but none of these changes were significant (Table 1). Likewise, none of the derived soil C status indicators were significantly affected by application of N and P fertilizers (Table 1).

Table 1. Effects of N and P fertilizer addition on soil carbon fractions and sustainability indices in soils (0-200 mm depth) from ex-native forest and ex-pasture plantation sites.

The -N-P treatment was used as the reference soils for the ex-native forest plantation soil and pasture for the ex-pasture plantation soil.

Means in a column (per site) with the same letter(s) are not significantly different at the 5% level by DMRT.

Carbon fractions in relation to previous land use

Concentration of soil C fractions in the ex-native forest plantation site tended to be lower than in adjacent native forest site (Table 2). As a result, the CPI, L and LI were also lower in the plantation than in the native forest soil except for 50-100 mm where there was a slight increase relative to the native forest (Table 2). The CMI in the ex-native forest plantation soil was lower than the native forest soil at all soil depths (Table 2). The C_T, C_L, and C_NL concentrations in the ex-native forest plantation and native forest soils all decreased with soil depth (Table 2). In ex-pasture plantation soil, soil C also tended to be lower than in pasture soil except at the 25-50 mm soil depth, where C_T, and C_NL were similar between pasture and ex-pasture plantation soils (Table 3). Also, the differences were reflected in the various indices of soil C status (Table 3).

Table 2. Carbon fractions in soils, with depths, from the ex-native forest (-N-P fertilizer treatment) plantation and native forest sites.

Values in parenthesis are standard deviation of the mean (n = 3).

Table 3. Soil carbon fractions in soils from the ex-pasture (-N-P fertilizer treatment) plantation and pasture sites at soil depths.

Values in parenthesis are standard deviation of the mean (n = 3).

Discussion

In the ex-native forest plantation soil, the application of +N+P fertilizers significantly increased both the total and labile C pools, presumably because of greater above and below ground litter production. Furthermore, the increased soil C with N and P addition could have been due to increased productivity. Some studies have shown that the addition of both N and P can significantly increase the early growth of E. globulus plantations (Judd et al. 1996; Bennet et al. 1996). It is recognized that the addition of fertilizer on a regular basis leads to an increase in SOM (Glendining and Powlson, 1995). In the ex-pasture plantation soil, fertilizer addition had little impact on the amounts of C_T, C_L and C_NL. This may be due to the residual effects of fertilizers applied annually to the pasture at a rate of about 18 kg P ha^-1 as superphosphate during the previous 21 years.

The results of this study suggest that the conversion of native forest or pasture to E. globulus plantations may lead to reductions in the levels of C_T, C_L, and C_NL. This may partly result from silvicultural operations conducted during plantation establishment such as residue burning, application of herbicides and ripping and mounding of planting lines which may accelerate organic C decomposition. The decrease in SOM is attributed to disruption of soil aggregates that protect SOM from decomposition, depletion of surface soil due to soil erosion, and reduction in plant residue input (Blair et al., 1995). The differences in concentration of soil carbon fractions between native forest and ex-native forest and between pasture and ex-pasture are reflected in the various soil C indices. In ex-pasture plantation soil, the C_T, C_L and C_NL concentrations were lower relative to the pasture soil. In undisturbed soil such as pasture and native forest, plant residues are continuously added to the soil, which can have a large pool of C of different qualities and ages, resulting in higher amounts of C_L and C_T (Blair et al., 1995).

The CMI has been suggested as a useful indicator for monitoring the organic matter status of soils because it incorporates the C_T, C_L and C_NL of the plantation and reference soils (Conteh et al. 1997). Changes in CMI reflect the reduction in total C stores in both plantation soils. However, if plantation soils are fertilized with both N and P, the C levels of the soils is increased suggesting that fertilization is an important management practices in order to maintain or improve the SOM status of the plantation forests.

In this study, the C_T, C_L and C_NL are highest in the near surface soil and decrease with increasing depth in both the ex-native forest and ex-pasture plantation sites. This is probably due to the greater organic inputs that are deposited on the surface soil. Biederbeck et al. (1994) reported that treatment effects on the concentrations of total organic C in surface soil layer (0-7.5 cm) are usually similar in the subsurface soil layer (7.5-15 cm), but effects in the surface layer are often more highly significant.

Conclusion

The results of this study indicate that application of N and P fertilizer increased the C pools in the ex-native forest plantation soil but had no significant effect in the ex-pasture plantation soil. This suggest that management practices such as N and P fertilization must consider the past land use. Also, this study shows that conversion of native forest or pasture to E. globulus plantations resulted in the reduction on the organic C fractions that is reflected in low CMI in the plantation soils. However, if the E. globulus plantations are fertilized with both N and P, the pools of C in the plantation soils are higher than the native forest or pasture soils. Knowledge of the magnitude of changes in the labile C in both plantation soils is important for understanding how land can be better managed to increase C and N sequestered into SOM in order to improve the long-term productivity of soil. Further study is required, especially in succeeding rotations, to determine the effects of forest management practices on soil C dynamics.

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