Predicting nitrogen fertilizer needs for sugarcane and reactions of dissolved organic and mineral nitrogen in straw and soil from sugarcane fields

Abstract

Recommendation of nitrogen (N) rates for sugarcane (Saccharum spp.) based upon soil testing is essential for increasing the sustainability of this agroecosystem. In addition, little is known about the dynamics of dissolved organic nitrogen (DON) in the straw-soil transition zone in sugarcane fields. The influence of mineral N forms (ammonium-N and nitrate-N) in the carbon (C) mineralization in this transitional region is also unknown. Thus, the objectives of this study will be: i) correlate several methods to predict the soil mineralizable N with sugarcane N response parameters; ii) quantifying the C mineralization and sorption processes of DON compounds applied to straw and soil (both sampled in the transition zone); and iii) determine the C mineralization and mineral N content in straw and soil (from the transition zone) by addition of 14C-glucose with or without mineral sources of N. Twenty one sugarcane ratoons N response trials will be setup in the State of São Paulo, Brazil, to correlate biological, chemical, microbiological and isotopic (15N) methods with stalks yield and total N uptake in the control treatment, relative stalks yield and the N rate required to achieve 90% of the RSY. Soil sampling will be performed at 0-20 and 20-40 cm depths. To study the reactions of DON and mineral N, straw and soil samples will be collected in the transition region between the two materials, in two experimental sites located in the State of São Paulo. The C mineralization and sorption of 14C-labelled amino acids mix, peptides, urea and protein will be performed by addition of the compounds in straw and soil. The C mineralization and mineral N content by the addition of 14C-glucose with or without mineral N forms (ammonium-N and nitrate-N) will be also checked.