Effect of leaching and clay content on carbon and nitrogen mineralisation in maize and pasture soils

Abstract

We conducted a 7-week laboratory incubation experiment to evaluate the effect of leaching on net C and N mineralisation in soils. The soils were collected from adjacent fields of long-term pasture and maize, where each field contained an Inceptisol and an Andisol. The concentration of clay mineral was 200 g/kg halloysite in the Inceptisol and 120 g/kg allophane in the Andisol. Half the samples were leached weekly with 0.002 M CaCl₂ at a suction of 20 kPa to remove soluble products, and half were not leached. Carbon mineralisation was determined from CO_2-C evolved each week. Net N mineralisation was measured for the leached samples from the NH_4-N and NO_3-N in the CaCl₂ extracts, and for the batch of non-leached samples by extraction in 0.5 M K₂SO₄. Carbon and net N mineralisation were greater in the soils under pasture than in soils under maize. The proportion of total C mineralised as CO_2-C, and of total N mineralised as NH_4-N and NO_3-N, followed the order Inceptisol-pasture > Inceptisol-maize > Andisol-pasture > Andisol-maize, suggesting that allophane and Al ions reduced net mineralisation. Dissolved organic carbon (DOC) produced during incubation, as a proportion of total C, was greatest for the Inceptisol-maize sample and least for the Andisol-pasture sample. Non-leaching resulted in the accumulation of acids and solutes, and decreased C mineralisation for the Inceptisol samples.