Physical and biological effects of kelp (seaweed) added to soil

The effects of chopped pieces of the fronds of the brown seaweed, Laminaria digitata (a kelp), on the pore volume and pore size distribution, aggregate stability, soil microbial biomass and biological activity (respiration and N mineralization) of a sandy soil have been investigated. Ninety days after addition of either 8.2 or 16.4 g kelp kg^-1 soil, the total pore volume of the soil had increased significantly. Most of the additional volume was present in pores ranging from 75 to 150 μm in diameter in the whole soil, and in pores of less than 60 μm diameter in aggregates of 4-7 mm diameter. The aggregate stability of both the whole soil and the 4-7 mm aggregates was also increased significantly by kelp addition compared with the unamended control. The soil microbial biomass content and the soil respiration rate were both significantly greater, relative to the control, following addition of either 8.2 or 16.4 g kelp kg^-1 soil. However, the soil respiration rate was lower in the soil amended with 16.4 g kelp kg^-1 soil compared with that amended with only 8.2 g kelp kg^-1 soil. The rate of potential N mineralization were also increased by amendment with the kelp at 8.2 g kg^-1 compared with the rates in the unamended soil.