Provision of nitrogen as ammonium rather than nitrate increases silicon uptake in sugarcane

Silicon (Si) is important in mitigating abiotic and biotic plant stresses, yet many agricultural soils, such as those of the rainfed production areas of the South African sugar industry, are deficient in plant-available Si, making Si supplementation necessary. However, Si uptake by sugarcane (Saccharum spp. hybrids) is limited even where silicate amendments improve soil Si status. Rhizosphere pH, which can affect Si uptake, can be manipulated using different N-form fertilizers. We tested whether (i) fertilization with NH4+ (rhizosphere acidification) increased Si uptake compared with NO3− (rhizosphere alkalinization); and (ii) uptake differed between an N-efficient, more acid-tolerant cultivar (N12) and an N-inefficient, less acid-tolerant cultivar (N14). Two pot trials with low-Si soil were fertilized with calcium silicate (Ca2SiO4) slag, plus N from ammonium sulphate [(NH4)2SO4], ammonium thiosulphate [(NH4)2S2O3] and calcium nitrate [Ca(NO3)2] (Trial 1) or N from (NH4)2S2O3 and Ca(NO3)2 only (Trial 2). Trial 2 included cultivars N12 and N14. Nitrate treatments significantly increased soil pH and soil Si compared with NH4+. However, NH4+ treatments significantly increased leaf and stalk Si content compared with NO3−, reflected in a significant negative relationship between soil pH and leaf Si. Acid-extracted soil Si was negatively related to leaf and stalk Si, likely due to adsorption of silicic acid to soil surfaces under higher pH of the NO3− treatment and its reduced availability for plant uptake. We conclude that NH4+ increased Si uptake into leaf and stalk, and propose that reduced rhizosphere pH solubilized Si from Ca2SiO4 and increased silicic acid availability for plant uptake. By contrast, NO3− may have reduced Si uptake due to adsorption of Si to soil surfaces at higher pH. Our results indicate that ammoniacal fertilizers, such as (NH4)2SO4 and urea, have potential for promoting dissolution of applied Ca2SiO4 and subsequent uptake of Si by sugarcane.