Effects of Gross N Mineralization Rates On Sugarcane Response to N Fertilization.

Most of the N assimilated by plants is derived from inorganic-N pools, NH₄⁺ and NO₃^-, and these forms of N are produced via the mineralization and nitrification processes. The net rate of inorganic N production can be estimated by measuring the change in the soil inorganic-N pool size over time, as a result of the mineralization-immobilization processes. Gross rates of N mineralization (GM) can only be estimated using ¹⁵N-techniques. The GM drives the N supply power of the soil, possibly affecting the soil responsiveness to N fertilization. This study aimed to measure GM in soil samples obtained in N-response field trials, in which sugarcane is grown. The main purpose was to clarify the factors resulting in a wide range of N fertilizer response at the sites. Soil samples (10 g) were moistened to 60% water-holding capacity (WHC) and pre-incubated at 25°C for 2 wk, followed by drying at 25ºC using a dehumidifier. The samples were then treated with 30 µg of N (25.34 atom % ¹⁵N), and incubated (25°C, 60% WHC) in triplicate for 0, 1, 2, 3 and 7 d. Quantitative and isotopic analyses were performed on exchangeable NH₄⁺-N recovered by 2 M KCl extraction. Variation on the size of N-NH₄⁺ pool was minimal over the incubation. Incubations longer than 3 d showed evidences of 15N re-mineralization during the course of incubation. The GM varied from 0.6 to 3.63 mg N kg^-1 soil d^-1 and showed an inverse relationship to N-fertilizer response. GM rates accounted for 71% of the variation in N-fertilizer response obtained in the trials.