Sulfur and Nitrogen Deficiency Reduces Radiation Interception, Biomass Production and Grain Yield in Wheat.
Fernando Salvagiotti, Univ of Nebraska, 243 Keim Hall, 3350 Starr Street, Lincoln, NE 68583-0915 and Daniel Miralles Sr., Faculty of Agronomy, Univ of Buenos Aires, Ave San Martin 4453, Buenos Aires, Argentina.
Sulfur (S) and Nitrogen (N) are two of the essential nutrients required by crops. Although S is considered a secondary fertilizer nutrient, its deficiency became more important in the last years because of i) the use of S-free fertilizers, ii) reductions in gaseous S emissions from industry, and iii) depletion of S contents in soils due to lower organic matter contents, erosion and crop removal. In absence of water limitations, nutrient deficiency appear to be the main factor that might reduce the ability of the crop to intercept the incoming Radiation (IPAR) which is function of the Leaf Area Index (LAI) and canopy architecture (k) and/or reduce the Radiation Use Efficiency (RUE), diminishing biomass and thereby reducing yield. This study was designed to determine how different N and S supplies interact i) to modify aboveground biomass and physiological attributes that define dry matter production (LAI, IPAR, RUE) and ii) to assess the impact on grain yield. Two field experiments were conducted during 2000 and 2001 at INTA Research Station, Argentina (32º 33' S, 60º 51' W), on a silty loam soil. The study consisted of a factorial combination of four N (Nsoil+Nfertilizer) and two S (Ssoil +Sfertilizer) rates. Thus, four N levels: 46 (N1), 72 (N2), 98 (N3) and 124 (N4) kg N ha-1 in the soil at sowing (quantities indicate the Nsoil+Nfertilizer) were combined with two S levels: 5 and 35 kg S ha-1 (quantities indicate the total S available in the top 0-30cm at sowing) in a split-plot randomized complete blocks design, where N levels were the main plots and S the subplots. From the appearance of the fifth leaf to anthesis, incident and transmitted radiation to ground level were measured using a linear radiometer (LI 191 LI-COR Inc. Lincoln NE USA). The intercepted radiation (IPAR) was calculated as the ratio between transmitted radiation to incident radiation. To estimate daily canopy IPAR, values obtained in each plot were plotted against thermal time from emergence to anthesis and a logistic model was used for fitting the recorded points throughout the crop cycle. Radiation Use Efficiency (RUE) was calculated as the slope between accumulated biomass and cumulative IPAR up to anthesis. Leaf Area Index (LAI) was measured. The coefficient for light attenuation (k) was calculated as the slope of the exponential relation between IPAR and LAI. Above ground biomass (TDM) was measured after drying samples in an oven at 70 ºC during 72 hs and Grain Yield (GY) at harvest. Nitrogen and S increased GY in both growing seasons. Although the N x S interaction was slightly significative in statistical terms (P<0.10), there was not a significant interaction NxSxyear. Increases in the S fertilizer application rates produced increases GY 8%, while increases in N rates raised yield ca. 20%. Averaging over years, the increases in GY due to variations in the rate of N fertilizer varied from 217 to 529 kg ha-1 at lower S rate. However, when S was applied at a higher rate (i.e 35 kg S ha-1) the increases in GY ranged from 621 to 927 kg ha-1. GY and TDM were positively associated (r2=0.73, n=16; P<0.01). Averaging treatments, harvest index was 0.32 and was not altered by different S (P>0.38) or N (P>0.7) fertilizer rates. Increases in the rate of Nitrogen and S supplies raised TDM at maturity 28% and 8%, respectively, showing significant increases in biomass production (P<0.05) in all crop stages from terminal spikelet to anthesis. Before anthesis, N supplies higher than 98 kg N ha-1 increased 14% the Crop Growth Rate (CGR) (P<0.05), i.e ca 6.2 g N per degree day, but S did not show effect on this attribute. However, between anthesis and physiological maturity higher S fertilization increased CGR by 19% (averaging across N treatments) and N 32% (averaging S treatments) (P<0.05). Increases in CGR due to N fertilization stopped at N rates of ca 100 kg ha-1 at the lowest S level, but when the rate S supply was increased CGR continued increasing at the highest N supply. Nitrogen increased TDM at physiological maturity 28%. Both physiological components of biomass were also increases but at different proportion. While LAI was increased by 62% due to increases in nitrogen rates IPAR was only increased by 20% respect to the lowest N supply. S effect was four times smaller, increasing 8% biomass, 13% LAI and 7% IPAR.. The slope of the relationship between maximum IPAR (%) and N availability at sowing that represents the increase rate of IPAR per unit of available N was 40% and two fold higher during the 2000 than that was recorded in the 2001 growing season at the higher rate of sulfur. Variations in the S fertilizer application rate modified RUE. At low S rate, RUE was 1.4 g MJ-2 (r2=0.95, P<0.001), however, when S availability increased, RUE was 9% greater (r2=0.94, P<0.001). Sulfur and nitrogen increased grain yield and biomass in wheat mainly by a faster radiation interception more than changes in RUE. Sulfur impact was more important as N availability increased, showing the positive interaction between these two nutrients in crop growth.