Acidifying peat as an aid to the reconstruction of lowland heath on arable soil: lysimeter experiments

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1. Soil acidification is prerequisite for the reconstruction of lowland heath communities dominated by Calluna vulgaris on land that has been cultivated intensively. Naturally occurring sulphidic peat from a coastal basin deposit was investigated as a potential soil modifier. Its total S and Fe contents confirmed the presence of pyrite (FeS2). 2. In a lysimeter experiment, the surface pH of peat dropped rapidly from 6·5 to 2·5 as pyrite oxidized to produce sulphuric acid. It increased only slightly in the following 5 years. The pH of leachate from the lysimeters dropped to c. 2·0, rising to c. 3·0 over 5 years. Large pulses of soluble iron and sulphate were released from the lysimeters, peaking in the first winter; the Fe discharge fell to background levels after 2 years but SO42− concentrations were still elevated after 5 years. Calluna vulgaris seeds were not able to germinate and survive on the raw peat, even with irrigation. Some seedlings became established on the control agricultural soil but succumbed rapidly to competition from arable weeds. 3. In a second experiment, we amended the surface 15 cm of agricultural soil with peat and compared its properties with bracken litter. The peat sample acidified less, but both peat and bracken incorporation resulted in surface acidification. Neither acidity nor Fe was released in leachates, although the expected pulse of SO42− was discharged from treatments involving peat. Calluna vulgaris seedlings established well on peat mixtures and responded to initial weed control and irrigation treatments. However, after 1 year, competition from Trifolium repens, Agrostis gigantea and Agrostis capillaris had extinguished the C. vulgaris populations, except on the bracken litter mixture where some seedlings persisted for a further 2 years. Weed roots had grown through all the surface layers and exploited the underlying arable soil. 4. The weed seed bank is a major obstacle to heathland reconstruction on arable soil. Pyritic peat has potential as a soil amendment to lower pH and fertility and improve water retention, but it would be needed in greater proportions and to greater depths than used here in order to reduce weed competitiveness. Its use would release large amounts of mobile sulphate into the soil, potentially an issue for groundwater protection; the evidence suggests that the soluble iron and acidity released would not penetrate to a great depth in the profile.