NITROGEN FRACTIONS IN SANDY SOILS IN RELATION TO PLANT NITROGEN UPTAKE AND ORGANIC-MATTER INCORPORATION

Pot experiments carried out with rye grass focused on two major aims: (1) to examine the importance of extractable organic soil N for net N mineralization and N uptake of plants and (2) to study decomposition and mineralization of organic matter incorporated into the soils, as related to extractable organic soil N fractions. Soil extraction was carried out by electro-ultrafiltration (EUF) and 10 mm CaCl2 solution. Ammonium, NO3-, and organic N was analysed in the extracts. The investigation was carried out with 13 sandy soils derived from fluvial and aeolian deposits, Hessia, Germany. They were sampled in June from the top layer of field trials grown with rye. CaCl2-extractable organic N was significantly related to net N mineralization and N uptake by the grass (r2 = 0.60*** and 0.66***, respectively). The relationship was less close for electro-ultrafiltration (EUF) extractable organic N (r2 = 0. 16* and 0.22**, respectively); coefficients of determination for total soil N were r2 = 0.23** and 0.33***, respectively. Decomposition and mineralization of sugar beet leaves incorporated into the soils enhanced available N for plants. This was clearly reflected by an increase of inorganic soil N as well as of extractable organic N fractions, followed by a decrease of organic and inorganic soil N as related to the N uptake of the grass. However, the amounts of extractable organic N determined at different times depended on the extraction method used. EUF extractable organic N was highest when the soils were sampled 2 weeks following amendment with organic matter. In contrast, the peak of CaCl2-extractable organic N compounds was observed 4 weeks after incorporation of organic matter. Net N mineralization of the added plant matter hardly differed between the various soils, although there was much variation in the initial amount of extractable organic soil N, which suggested differences in the microbial activities of soils. It was concluded by the results of the present investigation that the extractable soil organic N fractions reflect the soils microbial activities, and represent easily-mineralizable N pools in soils. CaCl2-extractable organic soil nitrogen appears to be a reliable indicator of the amount of organic soil N accessible for mineralization in sandy soils.