EFFECTS OF LIMING ON ORGANIC-MATTER DECOMPOSITION AND PHOSPHORUS EXTRACTABILITY IN AN ACID HUMIC RANKER SOIL FROM NORTHWEST SPAIN

A laboratory incubation experiment was carried out over 17 weeks to de termine the effect of liming on soil organic matter. The amount of lim e as calcium hydroxide [Ca(OH)2] required to completely neutralise exc hangeable Al was found to be five times the standard lime requirement. This large amount of lime had a limited overall effect on the short-t erm stability of soil organic matter, causing the release of 1300 mug g-1 of C (1.7% total soil C) above the control during the incubation. Liming may have altered the potential availability of soil organic mat ter and organic P, as shown by a marked reduction in the extractabilit y of soil organic P with sodium bicarbonate and sodium hydroxide. The latter was unlikely to be due to the formation of calcium-P artefacts, and may be attributed to the combined chemical effects of added calci um hydroxide and precipitation of exchangeable Al on the nature and so lubility of soil organic constituents and organomineral complexes. The addition of lime increased the degradation of added oak leaf litter b y 50%, from 3.2 to 4.7 mg g-1, as determined by CO2 evolution. The enh anced litter degradation indicated increased microbial activity in lim ed soil, but this improvement had only minor effects on the stability of native organic matter. This study highlights the need for further r esearch into the relationships between the chemical nature of organic P in soil and the physical, chemical, temporal, and agronomic factors that control its turnover and availability.