Residual nitrogen pools in mature winter wheat straw as affected by nitrogen application

Reduction of the amount of nitrogen (N) remaining in mature wheat straw is an essential challenge in order to boost grain protein and N use efficiency. However, very limited information is available on the composition of the residual N pools and how they are affected by N fertilization. Winter wheat was grown under field conditions at five different N levels ranging from 60 to 280 kg N ha−1. The fraction of straw N present in amino acids, chlorophyll, DNA, nitrate and lignin-associated compounds were analyzed separately in leaf blades, sheaths and stems. Total straw N concentration increased from 0.29 to 0.69% with increasing level of N application. The largest N pool consisted of amino acids in hydrolysable proteins, which comprised 50 to 70% of the residual N. Lignin-associated, non-hydrolysable N compounds constituted 13 to 16% of the total N, while non-protein N pools (mainly chlorophyll breakdown products and DNA) amounted to 5–14%. Protein-derived amino acids constituted the most abundant N pool in mature wheat straw. Despite a 2.5-fold increase in total straw N content, the relative proportion of the different residual N pools to the total N content in the straw did not change with N fertilization.