Short-term effects of incubated legume and grass materials on soil acidity and C and N mineralisation in a soil of north-east Australia

Perennial grass growth forms the basis of beef production systems in northern Australia. To improve pasture productivity the woody legume Stylosanthes has been introduced into these native pastures. However, the growth of legumes has been recognised to be a major factor in soil acidification, thereby reducing soil fertility. In order to determine impacts of Stylosanthes scabra (stylo) or Urochloa mosambicensis (urochloa) residues on soil pH, acid neutralising capacity (ANC), and C and N mineralisation, their tops and roots were incubated at a rate equivalent to 10 t dry matter/ha at 25°C for 25 days in topsoil samples of a Mottled-Subnatric Yellow Sodosol from a long-term field experiment under urochloa or under stylo cover. The amount of CO2-C released during the first 2 days of incubation was correlated with the decrease in dissolved organic C. Plant material addition immediately raised the pH and ANC relative to the control. This was related to the amount of ash alkalinity of the plant residues added to the soil. Since the ash alkalinity is a measure for the organic anion content of plant material, it was concluded that the pH buffering was due to protonation of organic anions. During incubation, net N mineralisation was only observed in the urochloa soil amended with stylo leaves. In all other treatments, N added in the residues was immobilised by microorganisms due to the high availability of easily degradable C-sources. Consequently, there was no further change in pH or ANC during incubation, since no significant amounts of H+ were produced or consumed during N conversion processes