Nitrogen turnover in bare soil planted subsequently with grass as investigated by electro‐ultrafiltration (EUF)

Changes of EUF‐extractable nitrogen (N) (nitrate, ammonium, organic N) in 20 arable bare soils, subsequently planted with ryegrass ( Lolium multiflorum L.) and cutting three times were investigated in pot experiments. All 20 soils responded qualitatively in the same way. During the period of bare soil, there was a significant increase of EUF‐extractable nitrate (EUF NO $ _3^- $ ), while extractable ammonium (EUF NH $ _4^+ $ ) remained on the same level and organic N (EUF N org ) decreased. This decrease, however, was not significant. From sowing until the first cutting of the grass, EUF‐NO $ _3^- $ concentration decreased to almost zero. This low EUF‐NO $ _3^- $ level was maintained throughout the subsequent experimental period (three cuttings of grass). During the growth of the first cutting, EUF N org decreased while EUF NH $ _4^+ $ remained constant, however, on a low level. EUF NH $ _4^+ $ fell during the growth of the second and third cutting. In this period, however, the N supply of the grass was insufficient. EUF N org decreased during the growth of the second cutting, but increased during the growth of the third cutting. This shows that the EUF‐N org fraction represents a transient pool, which gains and loses N. EUF NO $ _3^- $ , EUF NH $ _4^+ $ , and EUF N org correlated with the N uptake of the grass. Strongest correlation for EUF NO $ _3^- $ was found for the first cutting ( p < 0.001), and for EUF NH $ _4^+ $ and EUF N org for the second and third cutting ( p < 0.001). Total soil N was not correlated with the N uptake of the grass. EUF N org was only about 2% of the total N. This relatively small EUF‐N org fraction, however, is relevant for the mineralization of organic soil N, and the N quantity indicated by EUF N org is in the range of the N amount mineralized in arable soils within a growing season.