Impact of nitrogen input from biosolids application on carbon sequestration in a Pinus radiata forest

Forest management practices (e.g. choice of stand density, fertilisation) are just as important in carbon (C) forestry as in other types of forestry and will affect the level of C sequestration and profitability. Because C stored in wood is approximately proportional to the product of its volume and density, it is necessary to account for both volume growth and wood density when assessing the effects of fertilisation on C sequestration in pine forests.

The effects of nitrogen (N) input from biosolids application on forest C sequestration were quantified from an intensively monitored biosolids field trial in a Pinus radiata plantation on a sandy soil in New Zealand. The field trial tested the application of three biosolids rates: Control (no application), Standard (300 ​kg ​N·ha⁻¹ applied every three years), and High (600 ​kg ​N·ha⁻¹ applied every three years), across three levels of stand density: 300, 450, and 600 stems·ha⁻¹. Carbon sequestration was estimated using the C-Change model from annual plot measurements of stand density, stem height and diameter, and annual breast height wood densities obtained from increment cores.

By age 24 years, N-fertilised trees had sequestered 40 ​t ​C·ha⁻¹ more than unfertilised trees, an increase of 18%. Fertilisation increased stem volume by 23% but reduced stem wood density by 2.5%. Most of the increased C sequestration occurred between age 6 and age 17 years and the Standard rate gave the same increase in C sequestration as the High rate. On average, there was no significant difference in growth rate between fertilised and unfertilised trees after the 17th growth year, but the increased growth ceased earlier at higher stand densities, and later at lower stand densities.

This study indicates that 2–3 applications of the Standard rate would have been sufficient to achieve the increased C sequestration, with an applied N to C conversion ratio of 43–65 ​kg ​C·kg⁻¹ N. Our results highlight that N fertilisation will become more widespread under greenhouse gas emissions trading schemes which encourages forest management practices that improve C sequestration in young forests in New Zealand in particular and other countries in general.