Organic matter fractions and N mineralization in vegetable cropped sandy soils

Soil organic nitrogen mineralization rates and possible predictors thereof were investigated for vegetable growing soils in Belgium. Soil organic matter (SOM) was fractionated into sand (>53µm) and silt+clay (<53µm) fractions. The latter fraction was further separated into 6%NaOCl-oxidation labile (6%NaOCl-ox) and resistant N and C and subsequently into 10%HF extractable (mineral bound) and resistant (recalcitrant) N and C. The N mineralization turnover rate (% of soil N year-1) correlated with several of the investigated N or C fractions and stepwise linear regression confirmed that the 6%NaOCl-ox N was the best predictor. However, the small R² (0.42) of the regression model suggests that soil parameters other than the soil fractions isolated here would be required to explain the significant residual variation in N mineralization rate. A next step could be to look for alternative SOM fractionations capable of isolating bio-available N. However, it would appear that the observed relationships between N fractions and N mineralization may not be causal but indirect. The number of vegetable crops per rotation did not influence N mineralization but it did influence 6%NaOCl-ox N, probably as an effect of differences in crop residues returned and organic manure supply. However, the nature of this relation between management, SOM quality and N mineralization is not clear. Explanation of correlations between N mineralization and presumed bio-available N fractions, like the 6%NaOCl-ox N, requires further mechanistic elucidation of the N mineralization process.