Nitrogen economy of early and late-sown maize crops

Abstract Late sowing dates of maize are widely adopted in the Pampas region of Argentina, stabilising grain yields due to a more favourable water balance around flowering. However, late-sown crops are exposed to high soil N availabilities (N av ), high temperatures during the pre-flowering period and declining photo-thermal conditions during grain filling, which may affect nitrogen use efficiency (NUE, kg of grain per kg of N av ). These effects could be exerted through nitrogen uptake efficiency (N up E, kg of N uptake per kg of N av ) and/or nitrogen utilisation efficiency (N ut E, kg of grain per kg of N uptake). Environmental conditions could affect i) pre (N upt pre) and/or post-flowering N uptake (N upt post) and, consequently, N up E and ii) the determinants of N ut E, such as N harvest index (NHI) and N source per grain. Early- and late-sown maize were cropped in order to analyse i) grain yield, N av and NUE and ii) relationships among NUE and related-N efficiencies. The experiments were carried out in Parana (31°48′ S 60°32′ W), Argentina, during 2014–2015 and 2015–2016. Treatments were combinations of two sowing dates (early and late), three N rates (0, 90, and 270 kg N ha −1 ) and two genotypes (DK 70-10 VT3P and DK 73-10 VT3P). NUE decreased in late-sown crops ( ca . 32 to 26 kg grain kg N av −1 ), mediated by lower grain yields ( ca . 8564 kg ha −1 and 7832 kg ha −1 in early- and late-sown crops, respectively) and higher N av ( ca . 267–312 kg N av  ha −1 ). DK 73-10 VT3P exhibited the highest NUE ( ca . 31 kg grain kg N av −1 ) and N ut E ( ca . 63 kg grain kg N upt −1 ). N rate affected more strongly N av than grain yield; and there was a greater association between NUE and N up E ( P 2  = 0.72) relative to N ut E ( P 2  = 0.65). In both sowing dates, N upt pre had a positive impact on N up E, which strongly declined with N rate especially in late-sown crops. The lower N ut E of late-sown crops (66 vs. 52 kg grain kg N upt −1 in early and late sowing dates, respectively) was related to the highest post-flowering N source per grain (2.5 vs. 3.5 mg N grain −1 ). Thus, our study highlights the components of N economy of late-sown crops with the highest impact on NUE, i.e., N upt pre and N ut E. Therefore, nutritional management of late-sown maize crops should be focused on these NUE components. High plant densities could be useful to increase N upt pre. Finally, the choice of a genotype with high N ut E appears as a valid strategy to mitigate NUE reductions, promoted by the high N av typical of late sowing dates.