Humus on karst rock outcrops and their adsorption of ammonia nitrogen：A case from three ecosystems of Shilin, Yunnan

A lot of researches had paid attention on canopy humus. Nevertheless, little information is available about the rock outcrop canopy humus in karst ecosystem. Humus offers the growth matrix and nutrients for lithobiontic communities. Humus were collected on the carbonate rocks located in three karst ecosystems with different degree of rocky desertification and quantified; and their adsorption capability on ammonia were investigated in this study by means of simulation experiments. The results showed that the humic substanceson per square meter are 40.45±25.38 g, 38.89±9.92 g and 397.66±142.71 g in stony desertification, manmade forest and secondary forest ecosystems, respectively. They were significantly different in three ecosystems. When different ammonia solutions were added, humus would absorb 65%~75% within 2 min and adsorption equilibrium time was approximately 20 min. The dynamic adsorption data of NH4+-N were well described by pseudo second order reaction rate model. Adsorption isotherm of NH4+-N on three kinds of humus could be well fitted by Boxlucas 1. The equilibrium adsorption capacity of humus followed an increasing order from secondary forest ecosystems (4.73 g/kg), to manmade forest ecosystems (5.29 g/kg), and then stony desertification ecosystems (7.79 g/kg). The retention rates of humus for added NH4+-N was 20%~50%. It revealed that carbonate rock in southern China function as a nutrient pool, even in oligotrophic stony desertification ecosystem.