Mineral nitrogen forms alter ¹⁴C-glucose mineralisation and nitrogen transformations in litter and soil from two sugarcane fields

In Brazil, N fertilisers are applied onto the surface of the sugarcane (Saccharum spp.) litter layer, which represents a large portion of the C stored in the agroecosystem. However, little is known about the influence of mineral N on decomposition of organic C compounds and mineral N transformations in the litter-soil transition zone. This investigation determined the effect of mineral N forms (NH₄⁺ and NO₃^-) on glucose mineralisation and N dynamics in litter and topsoil from two sugarcane fields located in the state of São Paulo, Brazil. Carbon mineralisation and N availability were measured using ¹⁴C-labelled glucose, applied alone (glucose) or combined with NH₄⁺ [as (NH₄)₂SO₄] (glucose + ammonium) or NO₃^- (as KNO₃) (glucose + nitrate) in litter and soil from the litter-soil transition zone. The ¹⁴C immobilised after 168 h had the following trend for the litter of both sites: glucose > glucose + ammonium > glucose + nitrate, while no differences among treatments were found for the soils. Higher immobilisation (increased by 165%, on average) occurred for NH₄⁺ (glucose + ammonium) compared to NO₃^- (glucose + nitrate), primarily in the sugarcane litter. Carbon mineralisation in litter was N-limited, and the addition of mineral N accelerated the return of C to the atmosphere. Lower immobilisation of NO₃^- may increase N availability to plants relative to NH₄⁺ in the short term, but also may lead to reduced C storage and larger greenhouse gas emissions due to faster mineralisation of labile carbohydrate through the NO₃^- reduction process.