Saturday, 15 July 2006

Influence of Land Use on Soil Nutrient Recovery in Previously Shifting Cultivation Areas in Lower Northern Thailand.

Jaruntorn Boonyanuphap, The United Graduate School of Agricultural Science, Ehime Univ, Matsuyama, 790-8566, Japan and Katsutoshi Sakurai, Faculty of Agriculture, Kochi Univ, Nankoku, Kochi, 783-8502, Japan.

Shifting cultivation is the predominant land use system in the forest area of Lower Northern Thailand and is the major cause of deforestation since the past decade. Shifting cultivation leads to the degradation of soil fertility during the successive seasons of cropping, whereas the fallow phase facilitates the regeneration of soil productivity. However, a long-term fallow land is currently impossible due to limited land availability. The focus of our study is to compare the influence of fallow period and land use type on soil nutrient recovery under infertile Ultisols with slightly acidic, moderate base saturation, low stocks of available phosphorus and total nitrogen. We selected three locations, which are all located in Thung Saleang Luang National Park, Phitsanulok, Thailand. The locations are selected based on such as slope, elevation, and parent material. Thirty study plots were sampled at six different land use types, namely, maize cultivation, longan orchard, short fallow (1-5 years fallow; grassland), medium fallow (6-10 years fallow; shrub land), long fallow (11-17 years fallow; secoundary forest), and natural forest (dry evergreen forest). Soil samples were collected from two soil layers, surface layer at the depth of 0-5 cm and subsurface layer at the depth of 20-25 cm in March 2004, November 2005 and April 2005. At the lowest point, soil shows higher clay content (Location A) showing more fertile value than the flatland soil derived from the sand stone (Location B) and upland soil derived from shale stone (Location C) with lower clay content in both soil layers. Compared with maize fields, long period of fallow sites had greater values of exchangeable bases (K, Mg, Ca, and Na), ammonium, cation exchange capacity, base saturation, total nitrogen, and total carbon. Ammonium and available phosphorus were highest in longan orchard and generally low in maize field. In addition, soil under consecutive maize cultivation and grassland had higher level of total acidity than other land use types. Among different fallow periods, total carbon, total nitrogen, and cation exchange capacity levels appeared to be improved with increasing years of fallow in both soil layers. Nutrient levels in the soil were especially high for medium fallow, followed by long fallow and natural forest. Soil pH was stable or had the tendency to slightly decrease in time of fallow. Total carbon and Total nitrogen stocks were highest in medium fallow for all locations. Among all, natural forest had the lowest bulk density. Grassland with short period of fallow result in decline in nutrient level especially in exchangeable bases (K, Mg, and Ca). The results indicate that inappropriate land use and management such as crop field without the maintenance of soil organic matter tends to reduce the soil fertility, whereas medium period of fallow fields contribute to recovery in term of soil nutrient status.

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