Better estimation of soil nitrogen use efficiency by cereals and oilseed rape

Making appropriate allowance for the contribution of soil N supply to crop requirement is of environmental and economic importance, and the assumed efficiency with which soil mineral nitrogen (SMN) is used is fundamental to most fertiliser recommendation systems. Estimates of efficiency with which SMN is used are normally obtained from unfertilised (zero N) crops or crops treated with 15N-labelled fertiliser. The uptake of soil N depends on the amount of SMN present initially and also the amounts that are subsequently lost (mainly by leaching) or added through mineralisation of crop residues or soil organic matter. Data analyses and computer simulations using the SUNDIAL model were used to re-examine the efficiency with which SMN is used, and identify factors and management practices that might influence this.

Analysis of 400 winter cereal trials with zero N treatments revealed that crop N uptake varied widely in relation to the amount of SMN present. Apparent efficiencies of use for autumn- or spring-harvest ranged above and below 100% of the SMN, but average efficiency decreased with increasing SMN. Efficiencies were often below 100% for SMNs above 100 kg N/ha. Previous crops or soil types that gave the lowest SMN levels gave the highest apparent efficiencies. There were fewer datasets for winter oilseed rape and spring barley, but efficiencies of use again declined as the amount increased. These trends can be explained by higher N losses where the SMN amount was high, and also uptake being limited by crop demand. However, the data analyses and model simulations provided evidence that the actual efficiency of SMN use is likely to be less than 100%, with net increases in amount of available N between measurement and harvest accounting for higher apparent efficiencies. Better estimation of N losses and N mineralisation are therefore vital to improving estimates of soil N use efficiency.

Model simulations for winter wheat indicated that, with the same amount of SMN present, actual efficiencies of SMN use are lower on sandy soils and in high rainfall situations. SMN present at below topsoil depth in the autumn was found to be used less efficiently. This might also be the case where a high amount is present at depth on sandy soils in wet springs. Early sowing of winter cereals or oilseed rape improved N uptake and SMN use efficiency between autumn and spring, and early sowing of spring barley was also beneficial due to a longer growing season. Experiments on winter wheat, oilseed rape and spring barley revealed apparent increases in SMN use efficiency when fertiliser N was applied, but reductions where the N supply exceeded crop demand. Further research is needed to determine whether amount and timing of fertiliser N affects the efficiency with which crops recover SMN from different depths.