Boron dynamics and availability in Pinus radiata plantation : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science, Institute of Natural Resources, College of Sciences, Massey University, Palmerston North, New Zealand
Pinus radiata is an important forest species in New Zealand. Over 89 % of the country’s plantation forestry area is planted with P. radiata. The species makes a major contribution to New Zealand’s $3.1 billion to GDP and the plantation area is projected to increase to 2.5 million hectares by 2025. Research work to date has recognised that soil B deficiency is a major issue in many forestry plantation areas. Edaphic and environmental conditions such as the coarse texture pumacious soils planted with radiata in the Central North Island, and summer drought condition in some areas, further aggravate B deficiency in New Zealand soils.
Boron deficiencies in P. radiata lead to growth defects in afflicted plants and a deterioration in wood quality and market value. The primary objective of this thesis was to assess the impact of ulexite, a slow-release B fertiliser, on the bioavailability of soil B, plant B uptake, and the relative effect of B application rate on plant growth and soil microbial activity. A further objective was to compare the rate of B adsorption to seven benchmark soils collected from the North Island of New Zealand. The purpose of the work was to propose a long-term slow-release fertiliser management solution for radiata pine forestry that may mitigate the economic damage caused by B deficiency in this important primary production sector.
Soil was collected from Taupo, the major P. radiata planting district in the Central North Island of New Zealand, and used to establish glasshouse studies with P. radiata at Massey University in Palmerston North. Plants for this research were obtained from the Forest Research Institute (SCION) in Rotorua, New Zealand. Two growth experiments were conducted. The second of these compared the B dynamics of a fast-growing and slow-growing clone of P. radiata.
The background concentration of B in this soil (less than 0.5 mg/kg calcium chloride extractable B) is low, and B fertiliser application induced a soil response. Results showed that the concentration of plant-available B (extracted using hot 0.02 M CaCl2) significantly increased with B application. Boron application at the highest level (32 kg/ha) led to a build-up of soil B to a critical toxicity level with the subsequent appearance of toxicity symptoms in plants.
Application of B resulted in rapid B uptake as shown by an increase in B concentration in all plant parts (needle, stem and roots), but with the greatest rate of increase in needles. The percentage distribution of B throughout the plant showed that B distribution was influenced by B application treatments. The root to needle B ratio is used in this work as an index of B transfer from source to sink parts of a plant. Results showed that under deficient and toxic soil B concentrations (defined through the CaCl2 extractable B concentration), B was restricted to source tissues. However, B application at the rate of 4 kg/ha enabled B to move to sink parts including the new emerging needles. Regardless of clone and B treatment, needles, particularly older needles, were the main site of B accumulation followed by roots and stem. The B concentration in needles of Clone 37 was higher than in Clone 18 and this result reflects a higher demand of B for the faster growing Clone 37 relative to Clone 18.
Application of B affected P. radiata growth in terms of height, diameter and plant dry weight. Plants responded positively to B application over a range of fertiliser treatments (8-16 kg/ha) leading to sufficiency in soil as quantified through increases in the plant growth parameters plant height and dry weight. Boron application improved plant physiology as quantified by photosynthesis in this study. Results showed that photosynthesis positively responded to B application up to 8 kg/ha, however a further increase in B application resulted in a decline in photosynthetic activity.
Results from a B fractionation study showed that the plant unavailable residual-B fraction was the major form of B in the Taupo soil. With B fertiliser application the concentration of readily-available B increased proportionally to the B application rate. This increase in readily-available B demonstrates the importance of using B fertiliser to provide for a long-term increase in plant-available soil B for P. radiata plantations on the Taupo soil.
Soil microbial and microbiological properties also responded to B application. Soil dehydrogenase activity, an index of microbiological activity in soil, showed a concentration gradient from the bulk to rhizosphere soil. Regardless of clone there was approximately a three-fold higher dehydrogenase activity in the rhizosphere soil compared to the bulk soil. Maximum dehydrogenase activity was recorded by a B application at 4-8 kg/ha in both clones with a decrease in activity at higher rates.
Regardless of the radiata clone used, mycorrhizal colonisation increased with B application. However, for both clones the maximum mycorrhizal infection on roots was recorded for a B application rate of 2-4 kg/ha.
A B adsorption study performed using seven benchmark soils collected from around the North Island showed that B adsorption increased in all soils with the concentration of B in equilibrium solution. Langmuir and Freundlich isotherms modelled B adsorption in all seven soils. Further studies showed that B adsorption corresponded to pH in solution and linearly increased up to pH 9 and reduced thereafter.
The results from this study demonstrate the importance of B fertiliser to P. radiata plantation forestry. Both plant and microbiological parameters are affected by both low and excess levels of soil B. Therefore, it is suggested that a B application rate in the range of 4-8 kg/ha is optimal for plant growth and will have no harmful effect on soil microbiological parameters. In contrast, B application at the rate of 16 kg/ha is toxic to both plants and soil microbes and will lead to inhibitory effects on activity and growth.