Nitrogen mineralization capacity of coastal sandy soils of the Thua Thien Hue Province, Central Vietnam. (oral communication)

Coastal sandy soils of Thua Thien Hue Province in Central Vietnam represent an important soil order that increasingly contributes to regional economic growth. However these soils have generally low productivity because of chemical and physical constraints associated with low pH values and coarse texture; sand contents exceeding 70%, are common for those soils. Obviously, organic matter management represents a key factor for crop productivity improvement on these soils. However, before considering the possible contributions of various organic amendments, it is important to evaluate the actual contribution of the initial soil organic matter, through its N-mineralization, considered as a prime source of N for plants. Therefore, 14 soil samples (0-20 cm) representing different cropping patterns on coastal sandy soils were collected before the spring season to determine their N-mineralization capacity. After air drying and grinding to pass 2 mm sieve, the samples were incubated under waterlogged conditions for 7, 14, 28, and 42 days; in addition, these soil samples were analysed for chemical and physical characteristics. Because of significant differences between major physical and chemical characteristics, the soils were grouped in two classes: soils under rice cultivation and soils with other crops. The rice soils had, on average, lower sand content, higher clay content, higher organic carbon content, higher cation exchange capacity and lower pHwater. The release of NH4+ was, on average, higher in rice soils, but no statistically significant differences were found between the two groups of soils. Fitting the results with a first order kinetic equation led to the calculation of potentially mineralizable nitrogen. As expected, the values were much smaller than the total soil-N content, which indicates different soil-N pools. The N-pool identified in this study can be considered as very labile N which might be available to crops within few weeks. Therefore, the total N-content of soils cannot be considered as a reliable indicator of short term N-availability, though some limited correlation was observed between these characteristics.