CHANGES IN NITROGEN MINERALIZATION, TISSUE NUTRIENT CONCENTRATIONS AND BIOMASS COMPARTMENTATION AFTER CESSATION OF FERTILIZER APPLICATION TO MOWN GRASSLAND

1 Nitrogen mineralization was studied in four grasslands (fields A-D), which had not been fertilized for 2, 6, 19 and 45 years, respectively , thereby forming a chronosequence. Fertilizer application was stopped in these fields in order to restore former species-rich communities c haracteristic of nutrient-poor conditions. 2 The annual nitrogen miner alization rate was unexpectedly low in field A (124 kg ha(-1) year(-1) ) because of the absence of net mineralization during the winter perio d. This may have been due to high nitrification and subsequent denitri fication rates. Winter mineralization was observed in fields B-D where annual mineralization rates were 176, 140 and 61 kg ha(-1) year(-1). The soil total N and C pool sizes (0-10cm) decreased over the chronose quence (from 5110 to 2460 kg ha(-1) and from 71 800 to 29 400 kg ha(-1 ) for N and C, respectively). The relative nitrification rate (nitrate as a fraction of total mineral N) strongly decreased during the chron osequence (88, 75 54 and 51%, respectively). 3 The seasonal dynamics o f the compartmentation of biomass and of tissue nitrogen, phosphorus a nd potassium concentrations were studied in order to obtain insight in to some of the causes and consequences of these changes in nitrogen mi neralization. Field B had a high shoot and root production, with high turnover of both fractions. Field C had a low shoot production, but a high root production with high turnover. Field D had a high shoot and rhizome biomass, but within-season turnover was probably low. The peak standing crop in field A was unexpectedly low, but was consistent wit h the low annual mineralization rate measured in this field. 4 Fine ro ot turnover is assumed to be the main source of the organic material w hich plants add to the soil in these grasslands. The N concentration o f the fine roots generally decreased over the chronosequence, suggesti ng a decreasing quality of the dead organic matter which the plants ad ded to the soil. There were also clear decreases in shoot phosphorus a nd potassium concentration during the chronosequence. 5 The amount of regrowth immediately after cutting showed a good correspondence with t he nitrogen mineralization rate over the same period, with highest reg rowth and mineralization rates observed in the most recently fertilize d fields. However, peak standing crop prior to cutting did not corresp ond to the annual nitrogen mineralization rate. It is suggested that t he dominant plant species in field D had more below-ground storage and remobilization and had longer-lived tissues (especially below-ground) . This enables the species dominating this field to form a high biomas s, despite a very low N mineralization rate.