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 (Nav), 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 Nav). These effects could be exerted through nitrogen uptake efficiency (NupE, kg of N uptake per kg of Nav) and/or nitrogen utilisation efficiency (NutE, kg of grain per kg of N uptake). Environmental conditions could affect i) pre (Nuptpre) and/or post-flowering N uptake (Nuptpost) and, consequently, NupE and ii) the determinants of NutE, 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, Nav and NUE and ii) relationships among NUE and related-N efficiencies. The experiments were carried out in Paraná (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 Nav−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 Nav (ca. 267–312 kg Nav ha−1). DK 73-10 VT3P exhibited the highest NUE (ca. 31 kg grain kg Nav−1) and NutE (ca. 63 kg grain kg Nupt−1). N rate affected more strongly Nav than grain yield; and there was a greater association between NUE and NupE (P < 0.0001, R2 = 0.72) relative to NutE (P < 0.01, R2 = 0.65). In both sowing dates, Nuptpre had a positive impact on NupE, which strongly declined with N rate especially in late-sown crops. The lower NutE of late-sown crops (66 vs. 52 kg grain kg Nupt−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., Nuptpre and NutE. Therefore, nutritional management of late-sown maize crops should be focused on these NUE components. High plant densities could be useful to increase Nuptpre. Finally, the choice of a genotype with high NutE appears as a valid strategy to mitigate NUE reductions, promoted by the high Nav typical of late sowing dates.