Land-use impact on soil carbon, nitrogen, neutral sugar composition and related chemical properties in a degraded Ultisol in Leyte, Philippines

Over the past several decades, the conversion of native forest to agricultural land use has accelerated and featured in the development of Philippine landscapes. This study evaluated the effect of land-use change on soil carbon, nitrogen, neutral sugar composition and other related soil chemical properties in a degraded soil (Typic Hapludult) in Leyte, Philippines. Using a space-for-time substitution (paired-area) approach, soil samples were collected and examined from secondary forest (SF), mahogany plantation (MP), rainforestation farming (RF), coffee plantation (CP) and grassland (GR) of comparable geology, parent material, soil type and climate. Soil pH, exchangeable Ca and Mg, and cation exchange capacity (CEC) tended to be higher after the conversion of deforested cultivated land into MP and RF land-use types. In contrast, land conversion decreased the soil carbon, nitrogen and carbohydrate-C in the order of SF, MP, CP, RF and GR, and the decrease was more marked in RF and GR sites that had been under intensive cultivation for a long period of time. Arabinose and xylose (mainly of plant origin) constituted the major non-cellulosic neutral sugar and represented 31-54% of the total soil carbohydrate-C. Soil carbohydrate-C content contributed 2-8% of the total soil organic carbon because of rapid decomposition of sugars. This suggests an adverse effect of land-use change leading to degradation of soil quality. The results of this study suggest that under the humid tropical climate of the Philippines, high temperature and favorable moisture enhanced the carbohydrate decomposition and, thus, affected the content and composition of neutral sugar in the soil.