Grasslands are one of the major biomes on earth and can serve as important soil carbon sinks. Nutrient enrichment of these grasslands can have a significant impact on carbon losses through the decomposition process. We investigated the effects of long‐term (12‐yr) experimentally increased N and/or P supply on litter production and on the chemistry and decomposition of bulk litter from two grasslands differing in soil nutrient status. Potential aboveground litter production in the controls of a P‐rich low‐productivity riparian grassland was lower than that in a N‐rich high‐productivity peat grassland and increased with enhanced N supply. Nutrient treatments did not enhance litter production in the high‐productivity peat grassland. The concentrations of phenolics in bulk litter from the peat grassland, dominated by sedges, were 2–3 times higher than those in the riparian grassland that was dominated by herbs and grasses. At both sites increased nutrient supply had no detectable effect on phenolics concentrations. All P‐related litter chemistry parameters reflected the higher soil P status of the riparian grassland. P fertilization had a greater effect on litter chemistry in the P‐deficient peat grassland than in the P‐rich riparian grassland. At both sites, there was no change in overall litter chemistry in response to N fertilization, except for higher lignin concentrations in the peat grassland litter. Both short‐term (8 wk) litter incubations in the laboratory and a 3‐yr litter bag study in the field showed that riparian grassland litter decomposed faster than litter from the peat grassland. The long‐term nutrient additions had no significant effects on the decomposition of the bulk litter of each grassland type. Regression analysis on the combined data of the two sites showed that phenolics, and to a lesser extent, P‐related litter chemistry parameters, exerted a strong control on both litter respiration and litter mass loss. Our study shows that long‐term experimental nutrient additions do not lead to increased decomposition rates in grasslands and that the initial plant community litter quality is the main determinant of carbon losses through the decomposition process. The results of this study suggest that nutrient enrichment will likely affect ecosystem carbon balance more by affecting litter production than by affecting litter decomposition rates.

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Correlative studies have shown a ‘hump‐backed’ relation between the vegetation N:P ratio and plant species diversity with the highest diversity at balanced N:P ratios (between 10 and 14). We tested the hypothesis that adding growth‐limiting nutrients to mesotrophic grasslands that were in shortage of either N (N:P ratio<10) or P (N:P ratio>14) would lead to an increase of plant diversity. Thereto, we studied the effects of long‐term (11 yr) experimentally increased N and/or P supply on soil nutrient pools, vegetation nutrient dynamics and biodiversity in a riverine grassland in the Netherlands with a low soil N:P ratio (N shortage) and a peat grassland with a high soil N:P ratio (P shortage), respectively. Eleven years of nutrient addition hardly had any effects on the total stocks of C, N and P in the soils of both sites, due to the large size of the soil nutrient pools already present and to the management at both sites (annual hay‐making and ‐removal). However, in the riverine grassland the treatments increased the cycling of the small pool of labile N and P compounds resulting in large increases in annual fluxes of especially N. In the unfertilised controls, species establishments balanced more or less species losses during an 11 year period, thus leading to a dynamic equilibrium of the species pool. However, contrary to our hypothesis, addition of the growth‐limiting nutrient led at both sites to a reduction of species diversity even when total biomass remained below critical levels. Species diversity and species evenness were strongly determined by N mineralisation and to a lesser extent by total soil N and extractable P, respectively. Total aboveground biomass of the vegetation was determined by total soil N. Our study shows that patterns found in correlative studies of the relation between plant diversity and soil and vegetation N:P ratio can not be translated into successful experimental manipulations to enhance biodiversity. The most likely explanation is that colonization limitation occurred in the fertilized plots and that not sufficient diaspores of potentially new species could reach and/or colonize the plots to compensate for the species extinctions as a result of increased nutrient supply.

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The effect of regional, subregional and local groundwater flow systems on mesotrophic fen ecosystems was studied in the polders of the Vecht River plain that borders the Pleistocene ice-pushed moraine of Het Gooi. Variation in the vegetation and in the habitat factors (groundwater and peat soil) of fens depends whether or not the fens are connected to the outflow of the regional groundwater system.Changes in the regional groundwater flow system, caused by changes in the water management of the polders, are probably responsible for the deterioration of mesotrophic fens. Drastic measures will have to be taken to restore the hydrology on a regional scale if the mesotrophic fens are to be saved from extinction.Hydrological research that integrates the results of regional and local studies is essential if the ecology of fen ecosystems is to be understood.