Nutrient behaviour accompanying afforestation in Iceland

Biogeochemical Cycles

IAP2-20-102

Overview

Forests play a key role in the carbon cycle, acting as both sources and sinks of carbon. Forests sequester carbon by capturing atmospheric carbon dioxide and transforming it into biomass through photosynthesis. Sequestered carbon then accumulates in the form of biomass, litter and as carbon in forest soils. Afforestation, the transformation of non-forested lands to forest plantations, has the potential to enhance carbon sequestration and increase carbon stocks, and is seen as a viable way of mitigating greenhouse gas emissions. But trees take up considerable amounts of nutrients from soils; so repeated harvesting of this biomass or planting in low nutrient soils may impair long-term soil productivity.

It has been estimated that some 30% of Iceland was covered with forests before the time of settlement (c. AD 874), but by the 1890’s this had fallen to less than 1% (Sigurdsson et al., 2005). With the decline of forests, soil erosion increased, causing the loss of valuable fertile land. Soil protection and carbon sequestration are therefore the main aims of afforestation in Iceland. Formal afforestation started in 1899, and more recently the Regional Afforestation Project Act (no 56/1999) aimed to afforest 5% of the area in Iceland below 400 m above sea-level by 2040 (no 56/1999).

Planting of Siberian larch, the most substantial non-native species in Iceland, began in 1938 (Sigurdsson et al., 2005). Few long-term studies have been conducted on changes in soil nutrients after afforestation in Iceland, but the little work that has been done indicates that some macronutrients show an increase in bioavailability with forest maturity (Sigurdsson et al., 2005, Ritter, 2007) but the sources and pathways of nutrient delivery remain poorly understood.

This project aims to understand and quantify the behaviour of the micro-nutrients Zn, Fe, Cu and Ni in soils and trees accompanying afforestation. These essential trace elements are integral to forest growth and reproduction, and where their concentrations are limited this leads to physiological stress and external symptoms such as stunting. The sources of these micro-nutrients and how they are used by the trees can be traced using their stable isotope composition.

For example, Zn in soils is largely controlled by bedrock mineralogy, litter recycling and aeolian deposition (Alloway, 2008), each with a distinct isotope composition (Viers et al., 2007; Moynier et al., 2009). Zinc uptake by plants also involves isotope fractionation as does translocation within the plant itself. Typically, the heavy isotopes of Zn are preferentially absorbed to root cell walls (so, roots are enriched in heavy Zn isotopes). Zinc isotopes also show a preferential aerial migration of light isotopes during transport by the xylem from root to shoot, attributed to cross-membrane diffusion and Zn binding to cell walls (e.g. Caldelas & Weiss, 2016). Our preliminary results for Siberian larch (Larix sibirica) from forest stands planted up to 63 years ago at Hallormstadur in east Iceland, show systematic Zn stable isotope variations between soils, pore waters and trees that evolve with stand age, indicating substantial changes in the source and utilisation of the nutrients over time.

This project aims to use Fe, Zn, Cu and Ni isotope and elemental concentrations to determine: (1) the main sources of micronutrient supply and removal to soils and trees; (2) the mechanisms that control uptake and translocation of these elements in trees and other vegetation, and how these impact the nutrient concentrations in the different parts of the forest environment (e.g. leaves, bark, soils); (3) How these micro-nutrient isotope and elemental compositions evolve with time, following afforestation.

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Image Captions

Fig. 1 Pore water sampling at Hallormstadur

Fig. 2 Zn isotope composition against age of forest stand (years) showing temporal variations following afforestation.

Methodology

The project will involve fieldwork in Iceland, collecting soil and vegetation samples from afforestation sites at Hallormstadur, east Iceland (Birch and Larch 1905 to 1992) and Skorradalur, west Iceland (Birch, Spruce and pine, 1958 to 2004). Fieldwork will involve multiple visits to these sites to capture seasonal variations in soil and plant chemistry. Measurement of transition metal stable isotopes by MC-IC-MS, in addition to other trace elements and macronutrients. Interpretation of elemental and isotope data to achieve the project aims outlined.

Project Timeline

Year 1

Fieldwork at the forest sites in Iceland. Training in the measurement of metal stable isotopes and elemental abundances. Year 1 Research Proposal and review.

Year 2

Continued isotope and elemental analysis of soil and vegetation sample; Further seasonal field sampling. Prepare research for presentation / publication; attend International geochemistry conference.

Year 3

Completion of isotope work and interpretation and modelling of data. Presentation at national / international conferences.

Year 3.5

Complete and submit thesis; finalise manuscripts for publication.

Training
& Skills

Fieldwork in Iceland, involving the sampling of soils (and pore waters) trees and other vegetation, and field measurements (e.g. pH, temperature, alkalinity)

Training in the measurement of metal stable isotopes using high precision MC-ICP-MS and TIMs techniques at Durham, as well as elemental analysis and sample characterisation.

Interpretation and modeling of soil and vegetation data data to place new constraints on the sources of micronutrients, and their utilisation and pathways through trees over time

Presentation of research at both national and international geochemistry conferences.

References & further reading

Sigurdsson, B.D. et al. Biomass and composition of understory vegetation and the forest floor carbon stock across Siberian larch and mountain birch chronosequences in Iceland. Ann. For. Sci. 62, 881-888 (2005).

Ritter, E. Development of bioavailable pools of base cations and P after afforestation of volcanic soils in Iceland. Forest Ecology and Management 257, 1129-1135 (2009).

Alloway, B.J. Zinc in soils and Crop nutrition. 2nd Edition, IZA & IFA, Brussels, Belgium, Paris, France (2008).

Viers J. et al., Evidence of Zn isotopic fractionation in a soil-plant system of a pristine tropical watershed (Nsimi, Cameroon). Chem. Geol., 239, 124-137 (2007).

Moynier F. et al. Isotopic fractionation and transport mechanisms of Zn in plants, Chem. Geol., 267, 125-130 (2009).

Caldelas, C & Weiss, D.K. Zinc Homeostasis and isotopic fractionation in plants: a review, Plant and Soil, 411, 17-46 (2016).

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