Biochar improves brackish sediment structure and reduces saline–sodic constraints on plant growth: a case study from Te Roto o Wairewa/Lake Forsyth (Aotearoa New Zealand)

Abstract Purpose Sediment removal can reduce internal phosphorus loading in eutrophic lakes, yet extracted sediment is often treated as waste and its disposal represents a major remediation cost. Repurposing dredged material as plant growth medium provides a circular pathway, but the suitability of brackish sediments is poorly resolved. We tested the ability of biochar to alleviate compaction and salinity-sodicity of brackish dredged sediment for plant growth. Methods Sediment from Te Roto o Wairewa/Lake Forsyth (Aotearoa/New Zealand), a shallow brackish lake, was used to grow Italian ryegrass ( Lolium multiflorum ) under four treatments, a pure sediment control and three volumetric mixtures of biochar and sediment at ratios of 1:1, 1:3 and 1:4 (biochar:sediment). The sediment was nutrient-rich (total P ≈ 1150 mg kg⁻ 1 ; Olsen P ≈ 28 mg kg⁻ 1 ) but structurally challenging and saline–sodic (EC > 3000 µS cm⁻ 1 ; ESP ≈ 18.6%). Biomass, relative canopy cover, shoot concentrations and trace elements were monitored across harvests. Substrates bulk density was measured at the end of the experiment. Results Biochar improved substrate aeration and Na⁺ related constraints. Plant performance was most consistently enhanced by the 25% (v/v) biochar blending treatment (1:3). Treatment differences were greatest during early establishment: at the first harvest, P uptake under 1:3 reached ≈7 mg pot⁻ 1 , or ≈550 mg m⁻ 2 , approximately an order of magnitude higher than in the other substrates. Differences then narrowed across later harvests, with pure sediment improving over time. Across all treatments and harvests, shoot trace-element concentrations remained moderate, averaging ≈33 mg B kg⁻ 1 , 45 mg Zn kg⁻ 1 and 0.061 mg Cd kg⁻ 1 . Conclusions Brackish lake sediment retrieved from Te Roto o Wairewa functions as a productive plant growth substrate once saline-sodic and structural constraints are mitigated. Improvements were consistent with progressive ripening (aging, root penetration, and water-mediated Na⁺ leaching) and were greatest with 25% (v/v) biochar addition.