Aged biochar affects gross nitrogen mineralisation and nitrogen recovery: a 15N study in two contrasting soils

Shamim Mia1, Feike A. Dijkstra1 and Balwant Singh1

1Center for Carbon, Water and Food, Faculty of Agriculture and Environment, School of Life and Environmental Sciences, The University of Sydney, Camden, NSW, 2570, Australia.

Corresponding author: Shamim Mia, email:


Biochar is pyrolysed biomass and comparatively more resistant to biodegradation than to its original biomass. When applied to soils, it could increase agricultural productivity through increased nutrient retention. Here, we examined the effects of a biochar after 21 months of application (20 t/ha) in two soil types, i.e., Tenosol and Dermosol, on gross nitrogen (N) mineralisation (GNM) and 15N recovery in a grassland field experiment using a 15N-labelled ammonium sulphate. The experiment also included a phosphorus (P) addition treatment (1 kg ha-1). The Demosol is clayey (52% sand and 29% clay) while the Tenosol is sandy (82% sand and 8% clay). We only found an increased GNM in the Tenosol, when it received both biochar and P. Biochar along with P addition possibly enhanced microbial activity in the nutrient limited Tenosol. Biochar significantly increased total 15N recovery in the Tenosol (on average by 12%) and reduced leaching to sub-surface soil layers (on average by 52%). Overall 15N recovery was greater in the Dermosol, but was not affected by biochar or P treatment. The increased N retention with biochar addition in the sandy Tenosol may be due to NH4+-N retention at cation exchange sites on aged biochar in the soil. Our results suggest that aged biochar may increase N use efficiency through reduced leaching or gaseous losses in sandy soils.