The influence of weed control on foliar δ15N, δ13C and tree growth in an 8 year-old exotic pine plantation of subtropical Australia
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Background and aims
The aim of weed control and fertilization in forest plantations was to increase tree growth by reducing competition for available nutrients and water. However, treatments that influence weed biomass can also have significant impacts on soil carbon (C) and nitrogen (N) cycling which can in turn lead to changes in the dynamics of stable C (δ13C) and N (δ15N) isotope compositions in soils and tree foliage.
We examined the key C and N cycling processes influenced by routine and luxury weed control and fertilization treatments as reflected by soil and foliar δ13C and δ15N and long-term tree growth in an 8-year old F1 hybrid pine (Pinus elliottii x P. caribaea) plantation in southeast Queensland, Australia. Weed control treatments varied by treatment frequency and intensity while fertilization treatments varied by the application of N, phosphorus (P), potassium (K) and micronutrients. Different soil and canopy sampling positions were assessed to determine if sampling position enhanced the relationships among soil N transformations and tree N use, water use efficiency and carbon gain under the early establishment silviculture.
Routine weed control was associated with increased weed biomass returned to the soil, compared with luxury weed control. Soil δ13C increased at the 0–5 cm soil sampling depth in both the inter-planting (IPR) and planting row (PR) as a result of the routine weed control treatments. In addition, soil δ13C was significantly higher as a result of fertilisation treatment in the 0–5 cm soil sampling depth in the PR. Soil δ13C was negatively correlated to soil δ15N at the 0–5 cm soil sampling depth in the IPR. Soil δ15N increased in the 0–5 and 5–10 cm soil sampling depths in the IPR, as a result of more frequent (luxury) weed control. Foliar δ15N and tree water use efficiency (WUE) (as indicated by foliar δ13C) were positively correlated with tree growth at age 8 years. While relationships between δ13C and δ15N in the soil and foliage varied depending on soil sampling depth and position, and with canopy sampling position where there were consistent relationships between soil δ13C (or δ15N) and foliar δ15N.
This study demonstrates how early establishment silviculture has important implications for soil C and N cycling and how soil δ13C and δ15N were consistent with changes in soil C cycling and N transformations as a result of weed control treatments, while foliar δ15N was linked to more rapid N cycling as reflected in the soil δ15N, which increased tree growth and tree WUE (as reflected by foliar δ13C).
KeywordsSoil δ13C and δ15N Foliar δ13C and δ15N Establishment silviculture
Respect and gratitude go to colleagues in the Centre for Forestry and Horticulture at Griffith University for their assistance with field work, guidance and persistence; and to Mr. Scott Byrne and Mr. and Mrs. Diocares of Griffith University for their technical assistance. We also acknowledge the Biometric advice given by Dr Carole Wright (Queensland Department Agriculture, Fisheries and Forestry), the operating funding, access to GYM 350 and technical support from Forestry Plantations Queensland viz. Dr. Ken Bubb, Mr. Paul Keay, Dr. Marks Nester, Mr Ian Last, and for the numerous staff who were responsible for the development and maintenance of the GYM 350 experimental site. Paula Ibell was supported by a research scholarship grant through the Australian Research Council and an extension scholarship from the Centre for Forestry and Horticulture Research, Griffith University.
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