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Effects of fertilization and previous land use on carbon and nutrient storage and dynamics in Eucalyptus globulus Labill. plantations in south-western Australia

Aggangan, Romulo T. (1998) Effects of fertilization and previous land use on carbon and nutrient storage and dynamics in Eucalyptus globulus Labill. plantations in south-western Australia. PhD thesis, Murdoch University.

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In Western Australia, Eucalyptus globulus Labill. plantations are being established on land previously used for pasture, as well as on land recently cleared of the native forest vegetation. Little is known about the effects of previous land use and fertilizer treatments on soil fertility and nutrient cycling processes in E. globulus plantations established on these two contrasting sites. This thesis investigates the effects of fertilizer treatments (N, P or N-and-P) and previous land use on: (i) stores of carbon (C), nitrogen (N), phosphorus (P) and exchangeable cations in E. globulus plantations (Chapter 2); (ii) rates and amount of C and N mineralization (Chapter 3), (iii) changes in soil microbial biomass C and N (Chapter 5), and (iv) soil carbon fractions (Chapter 6). In addition, the influence of E. globulus leaf litter on C and N mineralization in soils from pasture and native forest is examined (Chapter 4) and the Carbon, Energy, Nutrients and Water (CenW) model is used to assess the effect of fertilizer and E. globulus leaf litter addition on soil N mineralization (Chapter 7).

Application of N-and-P fertilizer significantly increased the pools of C, N, P, and C:N ratio in soil under the E. globulus plantation established on an ex-native forest site. The pools of N and P were also enhanced by N-and-P fertilization in soil from the ex-pasture plantation site, but fertilization had no effect on the amount of total soil organic C or the C:N ratio. For the ex-native forest plantation soil, addition of N fertilizer decreased the pools of exchangeable Ca and Mg, whereas addition of P fertilizer reduced the pool of exchangeable Na. For the ex-pasture plantation soil, the pool of exchangeable K was significantly increased by P fertilization, whereas the pools of exchangeable Ca, Mg, and Na were not affected by N, P or N and P fertilization. The pools of C, N, P and exchangeable cations also varied among land uses suggesting that changes in these pools were related to previous land use.

Rates of microbial respiration and net N mineralization were measured using a long-term laboratory aerobic incubation (226 days at 20°C) in leaching microlysimeters. Potentially mineralizable N (N0) and net mineralization rate constant (k) were determined using a first-order kinetic model. For both ex-native forest and ex-pasture plantation soils, addition of N fertilizer markedly increased (about twofold) k, and reduced N0. Addition of P in the absence of N to the ex-native forest plantation soil reduced cumulative net N mineralization. The relative amounts of C02-C respired per unit of net N mineralized differed among the land use units. Amounts of net N mineralized (kg ha-1 in 0-200 mm soil) during the initial 28-day aerobic incubation period declined in the order: pasture (29), ex-pasture (18), ex-native forest (14), native forest (3), reflecting the low net N mineralization in native forest soil compared to managed systems.

The effects of E. globulus leaf litter on C and net N mineralization in pasture and native forest soils were evaluated using leaching microlysimeters. Cumulative net N mineralization declined in all treatments when litter was added, and was less when leaf litter was mixed with soil. Cumulative C02-C respired increased with the increasing rate of leaf litter addition, both when leaf litter was mixed through the soil and placed on the soil surface. These increases were associated with increases in microbial biomass C content of soil.

Changes in microbial biomass C and N were evaluated in soils under the E. globulus plantations that were fertilized with either N, P or N-and-P and at different seasons using the chloroform fumigation-extraction method. In the two plantation soils, microbial biomass C and N were at maximum levels in spring and minimum levels in summer. Addition of P or N-and-P fertilizer reduced the microbial biomass N concentration in the ex-native forest plantation soil, and addition of N and N-and-P fertilizer reduced the microbial biomass N in the ex-pasture plantation soil. Microbial biomass C concentration (kg ha-1 in 0-200 mm soil depth) declined in the order: native forest soil (1018), pasture soil (944), ex-native forest plantation soil (600), and ex-pasture plantation soil (591), whereas microbial biomass N concentration declined in the order: pasture soil (152), native forest (109), ex-native forest soil (84), and ex-pasture soil (83).

The soil C fractions were measured using a fractionation procedure based on ease of oxidation with 333 mM potassium permanganate. Soils from adjacent native forest and pasture were sampled and analysed for C fractions. In the ex-native forest plantation soil, the amount of total soil organic carbon (CT), labile carbon (CL) and non-labile carbon (CNL) were not affected by addition of either N or P, but were significantly increased by application of N-and-P fertilizer. The carbon pool index (CPI) which reflects changes in the amount of total soil 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 combined application of N-and-P. By contrast, in the ex-pasture plantation soil, fertilizer amendment had no effect on CT, CL CNL, CPI or CMI. Further, the soil C fractions show that the conversion of native forest or pasture to E. globulus plantations has resulted in losses of C relative to the reference soils. This is also reflected in the CPI and CMI, suggesting that eucalypt plantations are likely to result in relative losses of soil C.

Rates of N mineralization were predicted using the CenW model and outputs were validated using measured net N mineralized in aerobic laboratory incubations. The CENW model adequately predicted the rates of N mineralization in both the ex-native forest and ex-pasture plantation soils either using long or short-term aerobic laboratory incubations. There was also good agreement between observed and simulated rates of N mineralization when E. globulus leaf litter was added to soil at different rates.

In conclusion, the results of these studies show that previous land use and fertilizer treatments had significant influence on the organic matter status of the soil. Moreover, the rate of net N mineralization in both plantation soils differed markedly and these were associated with previous land use. Hence, management strategies to maintain soil fertility and sustainability of the E. globulus plantations must consider past land use and fertilizer history.

Item Type: Thesis (PhD)
Murdoch Affiliation: Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): O'Connell, A.M. and Dell, Bernard
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