Effects of polyacrylamide, biopolymer and biochar on the decomposition of ¹⁴C-labelled maize residues and on their stabilization in soil aggregates

The efficacy of applying plant residues to agricultural soils as a carbon (C) source for microorganisms and C sequestration is dependent on soil physiochemical properties, which can be improved by aggregation using soil conditioners. However, no attempt has been made to assess the effects of soil conditioners such as biochar (BC), biopolymer (BP) or polyacrylamide (PAM) on plant residue decomposition. We assessed the effects of BC, synthesized BP and anionic PAM on the decomposition of ¹⁴C-labelled maize residues and on their stabilization in aggregate fractions in sandy and sandy loam soils. Polyacrylamide and BP were applied at 400kgha^-1 and BC was applied at 5000kgha^-1, and the soils were incubated for 80days at 22°C. The conditioners improved the physical and biological properties of both soils, as shown by a 24% increase in the 1-2mm aggregates. Biochar and BP accelerated the decomposition of plant residues as indicated by ¹⁴CO₂ efflux, and resulted in reduced stabilization of residues in both soils relative to that observed in the control and PAM treatments. The reduction in ¹⁴C incorporation and C stabilization in the BC- and BP-treated soils was observed mainly in the <0.25-mm aggregates. This was confirmed by reduction of activity of hydrolytic enzymes (β-cellobiosidase and β-glucosidase). Decomposition of plant residues in sandy soil was more sensitive to BP and PAM application than that in sandy loam soil. Improved soil structure after applying BC and BP increased aeration and decreased the contact between plant residues and mineral soil particles and consequently accelerated plant residue decomposition and reduced C sequestration.