Quantifying soil nitrogen mineralization to improve fertilizer nitrogen management of sugarcane

Intensive use of synthetic nitrogen (N) fertilizers for sugarcane (Saccharum spp.) production presents environmental challenges for water and air quality as well as impacts profitability for producers. Central to these concerns is a widespread reliance on yield-based recommendations that invoke generic models of crop N response but lack any means to account for variations in soil N-supplying power, a critical determinant of fertilizer N need for cereal, fiber, and tuber crops. The work reported herein was designed to ascertain the impact of soil N mineralization on sugarcane response to N fertilization and was carried out in conjunction with eight N-response trials conducted between 2006 and 2010 at field sites in the largest sugarcane-cultivated area in Brazil. Soil samples were utilized in categorizing the sites as highly responsive, moderately responsive, or nonresponsive to fertilizer N, based on two chemical indices of soil N availability, the Illinois Soil Nitrogen Test (ISNT) and direct steam distillation (DSD), and assessments of (1) net mineralization during aerobic incubation for 12 weeks and (2) incubation-induced changes in soil N fractions obtained by acid (total hydrolyzable N, hydrolyzable NH4 (+)-N, amino sugar N, and amino acid N) or alkaline (ISNT-N) hydrolysis. Sugarcane varied widely in response to N fertilization, indicating that yield-based recommendations would often under- or overestimate N requirement and thus adversely impact sustainability of sugarcane-based ethanol production. In studies to evaluate feasibility of soil N testing to improve fertilizer N recommendations, mineral N production upon aerobic incubation was accompanied by significant decreases in hydrolyzable NH4 (+)-N and ISNT-N, indicating that both fractions were liberating mineralizable forms of soil N. Test values by the ISNT and DSD were highly correlated, and both showed promise for differentiating soil responsiveness to fertilizer N.