Dissolved organic nitrogen dynamics in sugarcane cropping soils

Abstract

The input of sugarcane straw on the soil surface can increase the N immobilization due to the high C/N ratio of this residue. However, the application of N fertilizer on sugarcane ratoons can stimulate the soil microbial population, promoting the net N mineralization. Besides mineral N from the mineralization process, plants can uptake organic compounds with low molecular weight, such as amino acids, peptides and proteins. Thus, the goals of this study are to evaluate the addition of N fertilizer and sugarcane residue on the soil surface in the dynamics of soluble C and N, and to clarify the mineralization dynamics of dissolved organic N (DON) in two soils. In laboratory, 100 g of soil (Typic Hapludox) will be placed in polypropylene tubes, and the treatments (doses of NH4NO3-N and sugarcane residue) will be applied on the soil surface. Soil samples will be incubate at 25°C during 60 d. Soil solution will be analyzed for determination of DON, dissolved organic C, total dissolved N, dissolved inorganic N, total free amino acids, concentration of humic substances and microbial biomass (C and N). To quantify the DON mineralization dynamics, 10 g samples of two soils (Typic Hapludox and Rhodic Eutrustox) will be transferred to polypropylene tubes and pre-incubated at 25°C for 14 d. After this period, 1 mL of DON solution (containing protein, peptides, amino acid and amino sugar) will be added to the soil. The tubes will be incubated at 25°C for 60 d. At time periods, individual tubes will be removed and soluble and exchangeable mineral N will be extracted. Amino acid mineralization will be determined by adding a mixture of 15 uniformly 14C-labeled amino acids to soil and measuring the subsequent evolution as 14CO2. The experiments are designed to identify the critical control points in the soil N cycle under sugarcane management regimes. (AU)