Fluxes of dissolved organic carbon and nitrogen in cropland and adjacent forests in a clay-rich Ultisol of Thailand and a sandy Ultisol of Indonesia

In the humid tropics, conversion of forests into croplands has the potential to alter the flux of dissolved organic matter (DOM) and influence C balance and N availability in soil. Sites located on a clay-rich Ultisol in Thailand and a highly acidic and sandy Ultisol in Indonesia were used to quantitatively evaluate effects of land use change on fluxes of dissolved organic C (DOC) and N (DON) in cropland and adjacent forest. The DOC flux from surface soil horizons was lower in cropland (poultry manure-amended) than in the forest on the sandy Ultisol (Indonesia), while DON flux increased in cropland due to release of DOM enriched in N from the decomposed SOM and manure. In contrast, DOC and DON fluxes in cropland (non-fertilized) and forest on the clay-rich Ultisol (Thailand) were consistently low due to high adsorption of DOM and rapid DON mineralization. Total dissolved N (TDN) in soil solution was dominated by nitrate for both cropland soils; DON represented a major fraction of TDN in the sandy cropland soil due to lower DOM adsorption capacity and slower DON mineralization. The conversion of forests to croplands could enhance SOM decomposition and produce DOM enriched in N, but leaching losses of DOC and DON were small under low-input agriculture in tropical region. The effects of agricultural use on DOC and DON fluxes could vary depending on application rates of manure and adsorption and mineralization capacities of soils.

► Continuous cropping enhanced decomposition of SOM and produce DOM enriched in N.
► The DOC and DON fluxes from surface soil varied between clay-rich and sandy Ultisols.
► The DOC flux from surface soil was lower in cropland than in forest on sandy Ultisol.
► The DON represented 32–42% of total dissolved N in sandy cropland soil.
► Leaching loss of DOM was small in tropical soils under low-input agriculture.