Halina Lipińska
Evaluation of persistency of Poa pratensis L., Phleum pratense L. and Lolium perenne L. in sward on peat-muck soil according to groundwater level
The flora composition of permanent grasslands determines the quality and quantity of fodder obtained as well as the protective function of grassland ecosystems with regard to water and soil. Maintaining a valuable and stable species composition of these communities is difficult, particularly in post-bog habitats because the sward is often dominated by Poa pratensis within a short time after the reclamation of those habitats.The objective of the studies conducted was to assess the influence of the co-component and varying groundwater levels on the persistency of Poa pratensisPhleum pratense and Lolium perenne in sward on peat-muck soil, and the productivity and protective function of those communities with regard to water and soil. 
Material and methods. The studies were conducted in the years 2001–2005 in a lysimeter experiment and on microfields. Each species was sown in monoculture, with P. pratensis also sown in mixtures with Ph. pratense and L. perenne (50 : 50%). Three groundwater levels were used in the experiment: 50 cm, 90 cm and variable. The response of the species to the proximity of a co-component and to the groundwater level was assessed based on: the surface cover and proportion of the species sown in the surface cover, the species composition of sward, the amount of agricultural yield, water consumption and its production efficiency as well as the mineral uptake and the biogenic ingredient content in the groundwater for the particular study objects. 
Study results and discussion. Objects with the P. pratensis monoculture were characterised by the best surface cover and the highest proportion of the species sown in sward unlike objects with Ph. pratense. At objects with a high (50 cm) and variable groundwater level, the surface cover of the soil was better than in the case of objects with a low level (90 cm). Furthermore, the smallest changes in the species composition of sward occurred in conditions of a high groundwater level (50 cm). The stability of the species composition attests to the ability of the grass species sown to persist in sward [Grzegorczyk et al. 1995, Kozłowski et al. 2000, Stypiński et al. 2001]. Stable yields over the years are another indicator of the persistency of species in sward [Domański 1984]. Ph. pratense and L. perenne achieved the highest yields in the second and third year, while P. pratensis in the third and fourth year of utilisation. The persistency of species in sward and, in consequence, their yielding and uptake of minerals from soil is determined by the extent to which their demand for water is met [Łabędzki 1997]. All species achieved significantly higher yields in conditions of a higher groundwater level rather than a lower or variable level. Groundwater level had the strongest impact on the yielding of P. pratensis and L. perenne as well as their mixtures while the weakest impact was observed for Ph. pratense and mixtures of that species with P. pratensis. The sward of L. perenne and the P. pratensis  +  L. perenne mixture consumed more water for evapotranspiration and used it more efficiently than the P. pratensis and Ph. pratense monoculture and their mixture. The highest mineral uptake from soil was observed for objects with L. perenne while the lowest uptake occurred in objects with P. pratensis.The mineral uptake by P. pratensis and Ph. pratense was reduced to the greatest extent by a low groundwater level, while the uptake by L. perenne was reduced by a variable level.Significantly lower nitrogen content in water was found in objects with L. perenne while lower phosphorus content occurred in objects with P. pratensis. The N-NO3 and PO4-3 content in water from under the sward of mixtures was significantly higher than from under the sward of monocultures. The capacity for accumulating N-NO3 is a characteristic feature of species and varieties [Kozłowski and Zielewicz 2009], hence this property should be taken into account when composing mixtures for grasslands. Low N-NO3 and N-NH4 content occurred at a high groundwater level while low PO4-3 content was observed in conditions of a low groundwater level. 
Conclusions. In peat-muck soil conditions, the most constant and stable yields were observed for P. pratensis that had a lower groundwater uptake for real evapotranspiration and a lower macro-ingredient uptake than Ph. pratense and L. perenne. However, L. perenne ensured higher yields and had the most efficient and the highest water and mineral uptake, which led to reduced concentrations of biogenic ingredients, particularly nitrate and ammonium ions in groundwater. The sensitivity of species and mixtures to the deficit of water in the soil increased in the following order: P. pratensis < P. pratensis + P. pratense < Ph. pratense <  L. perenne + P. pratensis < L. perenne. 
Ph. pratense demonstrated a limited ability to compete against P. pratensis and did not have a negative impact on yielding and mineral uptake. Hence Ph. pratense did not prevent P. pratensis from dominating the sward. In groundwater under the rhizosphere of the P. pratensis + Ph. pratense mixture, the ammonium and phosphate ion content was similar to their content under the monocultures of those species, while the nitrate content was higher for mixtures. 
L. perenne exhibited a strong ability to compete against P. pratensis because it reduced the latter’s productivity and macro-ingredient intake, particularly in high groundwater level conditions.Therefore, it may limit the expansion of P. pratensis and prevent it from dominating the sward. However, the fact that P. pratensis + L. perenne objects exhibited a lesser degree of turfness and a higher nitrate and phosphate ion content in groundwater than the P. pratensis monoculture may indicate a weaker protective function of that mixture with regard to organic soil and groundwater.