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 determine the effect of liming on soil organic matter. The amount of lime as calcium hydroxide [Ca(OH)2] required to completely neutralise exchangeable Al was found to be five times the standard lime requirement. This large amount of lime had a limited overall effect on the short-term 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 matter and organic P, as shown by a marked reduction in the extractability 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 calcium hydroxide and precipitation of exchangeable Al on the nature and solubility of soil organic constituents and organomineral complexes. The addition of lime increased the degradation of added oak leaf litter by 50%, from 3.2 to 4.7 mg g-1, as determined by CO2 evolution. The enhanced litter degradation indicated increased microbial activity in limed soil, but this improvement had only minor effects on the stability of native organic matter. This study highlights the need for further research 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.