Nitrogen Mineralization Pattern of an Oxisol of Guadeloupe, French West Indies

It is generally assumed that Oxisols have a low level of soil organic matter (SOM) and a high rate of SOM turnover due to the high temperatures in the tropics. This work was carried out to test this assumption by analyzing the kinetics and the temperature–moisture response of N mineralization and nitrification in a neutral (pH 6.2, organic N 1.5 g kg⁻¹, bare for 10 yr) and an acid (pH 4.7, organic N 2.3 g kg⁻¹, bare for 1 yr) Oxisol. Samples of these soils were incubated using a factorial design of temperature (20, 30, 40, and 50°C) by soil water (30, 200, and 1500 kPa). The mineralization pattern was related to measurements of microbial biomass, SOM light-fraction, and the number of nitrifiers. Mineralization increased continuously with the increase of temperature and water content. No nitrification was observed at 50°C and at 1500 kPa. Although mineralization was well described by the double exponential model, nitrification showed a linear pattern with time. The size of the mineralizable N pool and N mineralization were different between the two soils, but microbial activity and microbial biomass were almost identical in both soils. Mineralization was very slow at 20 to 30°C (constant rate ≈ 10⁻⁴–10⁻⁵ d⁻¹) and was limited by a relatively small (light-fraction C = 8–10% of organic C) and low active mineralizable fraction. At >40°C, the constant rate of mineralization (≈10⁻³ d⁻¹) was equivalent to those found in temperate soils at 25 to 35°C. Our findings do not support the assumption of a fast SOM turnover at high temperatures. Nitrifiers constitute a small population (≈20 cells g⁻¹) well adapted to changes in moisture and substrate availability. However, NH⁺ ₄ was always present in soils, which was associated with the low number of nitrifiers. Microbial biomass decreased sharply with an increase in temperature and was not related to N mineralization. Thermal denaturation or changes in microbial population could cause this decrease.