No abstract

, has an area of 544 km2 and a wide range of rain forest formations on several soil types and at altitudes of 5G2376 m. It has an annual rainfall of about 5000 mm. (2) Sites of 1 ha were established in each of four contrasting types of primary lowland rain forest: alluvial forest (AF); dipterocarp forest (DF); heath forest (HF) and forest over limestone (LF). All trees (210 cm dbh) were measured for dbh and height, except for the LF where height was calculated using a regression equation based on height and diameter measurements of a sample of thirty-four trees. Estimates of numbers or biomass or both of small trees (< 10 cm dbh), lianes, ground herbs and ferns, and epiphytes (including ferns) < 3 m from the ground were also made. Epiphytes > 3 m from the ground and bryophytes were not ennumerated. The total above-ground forest biomass (t ha-' dry weight) was calculated as: AF, 250; DF, 650; HF, 470; LF, 380. (3) Leaf and branch material were collected from each tree and, as far as possible, identified to species. There were at least 223 species ha-' in the AF; 214 in the DF; 123 in the HF; and seventy-three in the LF. (4) Soil analyses were carried out using standard methods on twenty-five samples from each site. The A F soil was heterogenous, with gley soils of high base status in the lower, occasionally flooded, part and podzols and peats in the higher part. The D F soil was acid and very low in calcium. It was lower in total exchangeable bases than the very acid podzolic soils of the HF. The LF soils were shallow, highly organic, of high base status, and neutral to mildly acid pH. (5) It was demonstrated that the species-rich D F occurred on very poor soils but there was no simple relationship between soil nutrient element concentrations and biomass or species richness. Many factors are probably involved in controlling these attributes. (6) The causes of the distinctive sclerophyllous leaves of the heath forest are discussed. It is suggested that extreme soil acidity (in the absence of a buffering effect of Alt++) in the organic soils limits nitrogen mineralization and that low levels of biologically active nitrogen favour sclerophylly. Those features of heath forest which reduce transpiration may be important in reducing the mass flow of soil toxins to the root surfaces.

Current models of climate change predict a reduction of area covered by northern coniferous forests and tundra, and an increase in grasslands. These scenarios also indicate a northerly shift in agricultural regions, bringing virgin soils under cultivation. The direct effects of man on tundra, boreal forest, and temperate grassland ecosystems are likely to result in less carbon mobilization from soils and vegetation than from tropical forests. However, as a consequence of climate change, carbon mineralization rates from arctic and sub-arctic soils could be very rapid under warmer and drier conditions because of low stabilization of soil organic matter (SOM) and enhanced microbial responses to small changes in soil moisture and temperature. Predicting the response of these systems to climate change is complicated where the edaphic environment regulating SOM dynamics is not a direct function of macroclimatic conditions. Grasslands contain a greater proportion of highly stabilized SOM than coniferous forests, distributed over greater depth in the soil profile, which is less susceptible to changes in mineralization rates. It is concluded that short-term responses of soil processes to climate change are more predictable in well-drained grassland and forest soils than in waterlogged soils of the tundra and boreal region. Over longer periods of time, however, plant species and soil types will alter in response to new temperature and moisture regimes above- and belowground interacting with the effects of carbon enrichment and changes in nutrient availability. The dynamics of these plant-soil interactions and the future status of soils in different life zones as sources or sinks of carbon is poorly understood. More data are also needed on the distribution of waterlogged forest soils in the boreal zone and responses to warming, which include the production of methane as well as CO . The primary recommendation for future research is for integrated studies on plant and soil processes.

Rice systems in Asia have intensified rapidly in the past 30 years, and significant areas of irrigated lowland rice are now supporting two or three rice crops per year. Our objective was to compare the chemical composition of soil organic matter (SOM) from four fields with different histories of rice cropping intensity and soil submergence: (i) a single-crop rainfed, dryland rice system without soil submergence, (ii) an irrigated rice and soybean rotation, and irrigated (iii) double-or (iv) triplecrop rice systems in which soil remains submerged during much of the year. In all four soils, extracted mobile humic acid (MHA) and calcium humate (CaHA) fractions were of modem age by I4C-dating, and represented about 20% of total N and organic C. The MHA was enriched in N and hydrolysable amino acids (AA) compared with CaHA in all soils. With increased frequency of irrigated rice cropping, however, there was a large increase in phenolic content of SOM. We speculate that slower lignin decomposition caused by deficiency of 0 2 in submerged soil leads to incorporation of phenolic moieties into young SOM fractions. The increased phenolic character of these fractions may influence N cycling and the N supplying capacity of lowland soils supporting two or three annual crops of irrigated rice.