Importance of organic nitrogen fractions in sandy soils, obtained by electro-ultrafiltration or CaCl2extraction, for nitrogen mineralization and nitrogen uptake of rape

摘要

Sandy soils have low reserves of mineral N in spring. Therefore organic-bound N is the most important pool available for crops. The objective of the present investigation was to study the importance of the organic-bound N extracted by electro-ultrafiltration and by a CaCl2solution for the supply of N to rape and for N mineralization. Mitscherlich-pot experiments carried out with 12 different sandy soils (Germany) showed a highly significant correlation between the organic N extracted (two fractions) and the N uptake by the rape (electroultrafiltration extract:r=0.76***; CaCl2extract:r=0.76***). Organic N extracted by both methods before the application of N fertilizer was also significantly correlated with N mineralization (electro-ultrafiltration extract:r=0.75***; CaCl2extract:r=0.79***). N uptake by the rape and the mineralization of organic N increased with soil pH and decreased with an increasing C:N ratio and an increasing proportion of sand in the soils. Ninety-eight percent of the variation in N uptake by the rape was determined by the differences in net mineralization of organic N. This show that in sandy soils with low mineral N reserves (NOinf3sup--N, NH4+-N) the organic soil N extracted by electro-ultrafiltration or CaCl2solutions indicates the variance in plant-available N. Total soil N was not related to the N uptake by plants nor to N mineralization.