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 characteristic of nutrient-poor conditions. 2 The annual nitrogen mineralization rate was unexpectedly low in field A (124 kg ha(-1) year(-1)) because of the absence of net mineralization during the winter period. This may have been due to high nitrification and subsequent denitrification 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 chronosequence (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 chronosequence (88, 75 54 and 51%, respectively). 3 The seasonal dynamics of the compartmentation of biomass and of tissue nitrogen, phosphorus and potassium concentrations were studied in order to obtain insight into some of the causes and consequences of these changes in nitrogen mineralization. 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 with the low annual mineralization rate measured in this field. 4 Fine root turnover is assumed to be the main source of the organic material which plants add to the soil in these grasslands. The N concentration of the fine roots generally decreased over the chronosequence, suggesting a decreasing quality of the dead organic matter which the plants added to the soil. There were also clear decreases in shoot phosphorus and potassium concentration during the chronosequence. 5 The amount of regrowth immediately after cutting showed a good correspondence with the nitrogen mineralization rate over the same period, with highest regrowth and mineralization rates observed in the most recently fertilized fields. However, peak standing crop prior to cutting did not correspond to the annual nitrogen mineralization rate. It is suggested that the 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 biomass, despite a very low N mineralization rate.