Current wheat (Triticum aestivum L.) N fertility diagnosis models do not take into account organic N provided by mineralization. Anaerobically incubated N (Nan) could contribute to assess this N pool for crops. The aim of this research was to assess the Nan contribution to spring wheat yield without added N (GY0N), to grain N export (GNE), and to N fertilization response. A total of 28 N fertilization experiments were conducted in 2006, 2008, 2009, 2010, and 2011 in southeastern Buenos Aires Province (Argentina). At sowing, soil organic matter (SOM) content, Nan, and NO3−–N availability varied between 44 and 68 g SOM kg−1, 34 and 94 mg NH4+–N kg−1, and 39 and 130 kg NO3−–N ha−1, respectively. Average grain yield and protein content without N added were 3450, 4330, 5020, 5288 and 6262 kg ha−1, and 116, 97, 95, 91, and 90 g kg−1, for 2006, 2008, 2009, 2010, and 2011, respectively. Initial NO3−–N availability explained only 24% of GY0N variation, but R2 increased to 66% when Nan was integrated into the model. Soil NO3−–N content and Nan explained 58% of GNE variation, with a higher partial contribution of Nan to GNE than to GY0N (51 and 41%, respectively). A model was developed to predict the response to N (RN = –625.7 + 7.2Pp – 31.6Nan + 0.28GY0NPp, where Pp is total precipitation from July to December; R2 = 0.58). Soil Nan determination and initial NO3−–N content should be taken into account together when assessing spring wheat N needs.

Th e objective of this work was to evaluate if soil mineralizable N under anaerobic conditions (Nan) improves the reliability of preplant soil nitrate test (PPNT) and presidedress soil nitrate test (PSNT). Th e study was conducted under no-tillage (NT) in the southeastern Buenos Aires province, Argentina, in 26 site-years. In some site-years treatments were fi xed N rates (0, 70, 140, and 210 kg ha -1 or 0, 60, 120, 180, and 240 kg ha -1 ) applied at planting, whereas in others site-years N rates were determined by adding N to lead targets of N supply up to 120, 160, and 200 kg N ha -1 . When sites were not separated by Nan, relative yield (RY) correlated with PPNT (r 2 = 0.37) and reliability of PPNT did not increase when sites were separated by low (<48 mg kg -1 ) and high Nan (>48 mg kg -1 ). A high relationship between RY and PSNT (r 2 = 0.56) was determined. However, when sites were separated by Nan, reliability of PSNT increased, mainly for sites with low Nan (r 2 = 0.68 and 0.44 for sites with low and high Nan, respectively). Critical PSNT values for 94% of RY were 75 and 90 kg N ha -1 for sites with high and low Nan, respectively. Th e results show that PSNT was a better diagnostic method for maize (Zea mays L.) than PPNT. However, the separation of sites by its N mineralization capacity increased reliability of PSNT by indicating a lower N requirement to reach 94% of RY, decreasing the risk of overfertilization, an important feature from economical and environmental standpoints.

. 2008. Time of nitrogen application affects nitrogen use efficiency of wheat in the humid pampas of Argentina. Can. J. Plant Sci. 88: 849Á857. Nitrogen (N) fertilization is an important management practice to increased grain yield; however, it is imperative to increase nitrogen use efficiency (NUE) in order to diminish risks of environmental pollution. The objective of this study was to determine the effect of fertilization times on wheat grain yield, grain N accumulation and grain N recovery efficiency (RE) in different sites and years at the south-eastern wheat belt of the Pampas. The experiments were a factorial combination of N rates and fertilization times (sowing and tillering). Grain yield ranged from 1600 to 7900 kg ha(1 and fertilization at tillering increased grain yield compared with fertilization at sowing (5465 vs. 5110 kg ha(1 ), similar behavior was observed for grain N accumulation (95 vs. 86 kg ha(1 ) and RE (0.41 vs. 0.32). Predicted grain yield by CERES-Wheat model for different N rates and fertilization times was correlated with observed grain yield (r 2 00.71). While fertilization at tillering significantly increased grain yield, CERES-Wheat model estimated nitrate leaching losses that ranged from 12 to 62 kg N ha(1 and from 7 to 16 kg N ha(1 for fertilization at sowing and tillering, respectively. However, denitrification losses ranged from 1.2 to 3.9 and from 0.5 to 2.4 kg N ha (1 for fertilization at sowing and tillering, respectively. Leaching losses for fertilization at sowing are a consequence of water excess early in the growing season and would be the main N loss factor. Therefore, N application at tillering is an appropriate strategy to improve NUE in the south-eastern wheat belt of the Pampas.

2 A key aspect of any sulfur (S) availability indicator is its stability throughout the crop cycle. Nine experiments were conducted with the objective to evaluate the stability of the nitrogen (N):S ratio in aerial biomass (W) in spring wheat, and to define the critical curve of S dilution. As the crop cycle progressed, a decrease (P < 0.05) in total N:S was determined. This lack of stability was explained by a lower S dilution in relation to N (P < 0.05). A decrease (P < 0.05) in N accumulation rate in relation to S was obtained as the crop cycle progressed, which points out that S accumulation in relation to N is later. A first approach was determined to the definition of the S dilution critical curve (Sc) from tillering beginning to stem elongation end (Sc = 0.37 W −0.169 ; r 2 = 0.71 and n = 24).