Plant-soil relations in forest, scrub and grassland on chalk in Southern England
The chief objective of the research was to establish the relative fertility of forest, scrub and grassland soils on chalk in SE England and to test hypotheses on the origin of fertility differences between the soils. A subsidiary objective was to relate the beech forests of the chalk in England to continental beech forests on calcareous parent materials and thus to utilize the results of continental studies on discussion of the nutrient status of the different forest types of Europe. Previous research by B.A. Key had concentrated on scrub and grassland, and my research concerned chiefly scrub and forest. The fertility of soils was assessed by bioassays with commonly occuring test species, i.e. Galium aparine, Deschampsia caespitosa and Arctium minus, by foliar analysis of component woody species and by soil incubation tests. The results of bioassays show that the scrub soils of the chalk in SE England are more fertile than either grassland or forest soils. Bioassay plants grown on forest soils showed considerable variation in yield and nutrient content indicating that there is much variation in the fertility of forest soils. Mercury beech forest soils as a class were more consistent and produced better growth of test species than grassland soils while beech forests without mercury, traditionally classed as sanicle beechwoods, were found to have soils of very wide ranging fertility. Beech foliar analysis did not distinguish between mercury and sanicle beech forests, although the trend was for higher N status in the beech leaves of mercury sites. No trend was observed in the P status of beech leaves. The potential for nitrogen mineralization was favourable in both scrub and forest soils but much lower in grassland soils. Soils underneath mercury beech forests were consistent in nitrification and yielded similar concentrations of nitrate during one-month incubation to soils beneath relict ash-hazel forests. No such consistency was found for the soils of beech forests without mercury, and contrary to expectation, their potential to supply nitrogen appeared to be higher than that of mercury beech forest soils. The fertility differences shown by bioassays were most probably due to phosphorus supply. Experiments with several species have shown that the primary limiting nutrient in chalk soils is phosphorus; especially when plants are grown in pots at low rooting density. Nitrogen limitation has been found but only after adding a phosphorus fertilizer solution or when plants are grown at high rooting density. In these experiments, except where the objective was to separate the effects of nitrogen from phosphorus, test plants were grown one per pot for a relatively short time. It was hypothesized that the differences in fertility level of soils supporting forest, scrub and grassland on chalk in SE England were chiefly the result of differences in the amounts of nutrients added each year in litter. Freshly fallen litter of several woody species of chalk beech forests was collected from nylon nets set out on the forest floor in autumn 1984 and 1985. In order to investigate the role of nutrient withdrawal, in autumn 1985 mesh bags were used to collect freshly fallen litter within the canopy of scrub and forest. In both types of experiments N and P content was analysed on a species-specific basis. In addition, fine litterfall in a typical mercury beech forest was estimated in 1984 and rates of litter decomposition of five key species were measured over a period of 1 1/2 yr. Beech litter has relatively low phosphorus concentrations and is slow to decompose. Phosphorus is retranslocated from falling leaves in greater amounts in beech than in Crataegus, and Cornus, the main constituents of scrub which would seem to explain the lower phosphorus status of beech forest soils than scrub soils.