Biochar in forest establishment: synergies between nutrient delivery, growth and carbon storage

Abstract

The increasing demand for sustainably produced timber and the expansion of land-based greenhouse gas removal technologies is driving the growth of the forestry sector in the U.K. This is creating increased competition for the finite land resource. Historic planting on nutrient limited upland soils, coupled with open nutrient cycles due to high intensity silvicultural systems, is creating a productivity crisis across upland plantation forestry. Climate change is expected to further impact the productivity of U.K forests and managing soil health is vital in ensuring crop resilience to our changing climate. Biochar is an established greenhouse gas removal technology (GGR) that can increase the carbon storage capacity of land and enhance the efficacy of other land based GGR technologies, such as woodland creation. The agronomic benefit of biochar is well understood, however, there has been minimal research thus far on the potential application of biochar in productive silvicultural systems as a novel method of forest fertilisation.

This thesis explores the potential utilisation of targeted biochar application to improve seedling growth on nutrient poor soils during the establishment phase of forest cycles. A selection of distinct biochar types, ranging from high to low available phosphorus concentration, were tested in a controlled environment growth experiment on Scots pine (Pinus sylvestris L.), Sitka spruce (Picea sitchensis (Bong.) Carr.) and Douglas fir (Pseudotsuga menziesii (Mirbel) Franco) along with conventional phosphorus fertilisation intervention. A specified biochar was also developed to address the primary soil limitations on seedling establishment in phosphorus limited soils. The vascular cambial zone of roundwood is relatively high in phosphorus and is a readily available sawmill co-product produced during ring debarking. This feedstock was used to create VCZ biochar. The physical and chemical properties of this specified biochar address the limitations of low phosphorus upland forest soils. Biochar amendment was further tested in a large-scale field experiment on a typical phosphorus limited restock establishment site. The effects of biochar on seedling growth and morphology were variable, according to biochar properties, dose and seedling species. The moderate native biochar phosphorus content of VCZ biochar, coupled with high porosity and internal surface area, improved above ground biomass growth, stimulated root proliferation and enhanced mycorrhizal abundance. Optical and SEM microscopy was used to investigate the importance of root and hyphal interaction with biochar structure and the accessibility of available nutrients within biochar particles in relation to seedling nutrient uptake strategies. High phosphorus fertilisers and high phosphorus biochar were found to limit the development of below ground biomass and suppress mycorrhizal fungi, in turn impacting above ground biomass development. The mechanisms which contribute to the effect of specified biochar on seedlings was explored. Biochar is a direct source of micro and macro-nutrients and can also improve localised soil nutrient availability and accessibility through sorption, pH and biofertilisation. Foliar analysis showed that biochar can be used to alleviate seedling phosphorus deficiency. The effect of biochar on soil water holding capacity (WHC) was also found to be a critical factor in improving establishment and increasing drought resilience.

This thesis details the impact of biochar on forest carbon. The implication of forest establishment practices, including biochar amendment, on carbon balance and year of net carbon storage was modelled to assess the optimal scenarios for carbon storage on nutrient poor soils. Early growth improvements from low dose targeted biochar amendment have the potential to reduce the time to net carbon storage by three to six years on low yield organomineral planting sites. It can be concluded that the targeted application of specified biochar can improve early-stage seedling growth and enhance the development of the rhizosphere, providing greater silvicultural benefit compared to conventional forest fertilisation. This in turn can create longer term benefits to forest health, crop stability and future rotation yields. Enhancing the localised soil conditions through specified biochar amendment should be considered as a viable management intervention to increase forest resilience, species suitability, carbon sequestration and close nutrient cycles.