Net mineralization of organic N in soils related to EUF (Electroultrafiltration) extractable organic N, organic C, nitrate and growth

Abstract

The objective of the investigation was to study the relationship between EUF (Electro-ultrafiltration) extractable N, EUF extractable carbon and interlayer NH₄⁺ on net N mineralization and N uptake of ryegrass (Lolium multiflorum cv. Aubade) cultivated in Mitscherlich pots on 20 different arable soils. Grass of the first harvest (first cut) exclusively fed from nitrate and N uptake of the first cut was significantly correlated with the EUF extracted nitrate in the soil at seeding (r=0.96***). In average from this nitrate about half was taken up by the grass and half was assimilated (immobilized) by soil microorganisms. This immobilized N was correlated with the organic C extracted by EUF at seeding (r=0.50*). The grass of the second and third harvest exclusively drew N from the organic soil N pool and N uptake of grass was correlated with EUF-Norg (r=0.66**, second cut and r=0.55*, third cut). Interlayer ammonium significantly decreased during the growth period, but this decrease was not correlated with the N uptake of the grass. The experiment shows that the N sources for grass differed during the experimental period with high nitrate concentrations at the beginning of the growth period causing N immobilization and later plants feeding from organic N. From the results it is concluded that the microbial assimilation (immobilization) of inorganic N depends on the presence of inorganic N, mainly nitrate, and on the supply of microbes with metabolizable organic carbon predominantly provided by plant roots. Therefore, it is suggested that the nitrate immobilization mainly occurs in the rhizosphere. EUF extractable soil N is an indicator for the mineralizable organic soil N; EUF Corg was correlated with the immobilization of inorganic soil N. Since, however, the correlation was only weakly significant further efforts are neccessary for elaborating a more precise extraction technique for the organic C which provides the energy for microbial mineral N assimilation.