Early Growing Season Immobilisation Affects Post-Tillering Wheat Nitrogen Uptake From Crop Stubble And N-15 Fertiliser In A Sandy Soil (Nov, 10.1071/Sr20158, 2020)

In semiarid sandy soil environments there is a dual challenge of carbon and nitrogen (N) limitation that needs to be managed to ensure timely supply of N to crops. Management of N inputs to soil using combinations of legume stubble addition and fertiliser N in cereal systems is essential to meet crop demand and maintain N in soil organic matter. The aim of this study was to assess soil mineral and biological N pools that influence N supply and N uptake of wheat at early growth stages. The recovery of N-15-labelled fertiliser by wheat was evaluated using a factorial combination of either wheat, lupin or no stubble incorporated with or without N-15 fertiliser in a sandy soil system. Soil and plant samples were collected at sowing, tillering, first node and booting to monitor changes in N pools and N-15 uptake by the wheat. Crop stubble incorporation one week before sowing increased biological N pools in the surface soil (0-10 cm). Early N immobilisation (sowing-tillering) in all the treatments without N-15 fertiliser may have limited N availability for wheat uptake in the subsequent period (tillering-first node), when fertiliser N appeared critical to maximise N supply for plant requirements. Up to 38% of the N-15 fertiliser applied at sowing was incorporated into the soil microbial biomass pool, so that fertiliser N was critical to relieve short-term inherent N limitations for both plant and microbial growth, and to supply the longer-term biological pools (microbial biomass) to support subsequent mineralisation potential. Reducing the energy limitation to the microbial pool through inputs of carbon from stubble was also critical to ensure fertiliser N supplied sufficient N to satisfy plant demand later in the growing period. These results have implications for management decisions on semiarid sandy soil systems that aim to synchronise N from inputs of legume stubbles and fertiliser with crop N demand during early growth stages of wheat.