Abstract: Six soil samples were collected from Tieshan watershed, a typical mining small agricultural watershed in Liuyang City, which were used to analyze spatial variation of soil Cd content and its species. Effects of various nanohydroxyapatite addition doses on soil Cd availability and form transformation were examined using indoor flooding simulation experiment. Results showed that total Cd content in soils was 1.14, 1.37, 1.98, 7.44, 7.45 and 6.99 mg·kg^-1 from upstream to downstream of the watershed, with the percentage of available Cd being 51.75%, 40.15%, 39.39%, 16.40%, 28.86% and 29.76%, respectively. Spatial variation of soil total Cd and available Cd contents was highly significant in the watershed. The percentage of available Cd had significant negative correlation with soil pH. Flooding significantly decreased soil available Cd content. More available Cd transformed into the residual state with extension of flooding time. Soil available Cd content was decreased greatly with addition of nanohydroxyapatite, the effects of which were better with the increasing dose, being best at 5%. Compared with flooding alone, soil available Cd decreased by 77.97%, 83.64%, 82.05%, 77.87%, 74.88% and 73.56% in samples collected from sites S1-S6, respectively. The results can provide scientific reference for district management of safe agricultural production and control of heavy metal pollution in watershed scale of mining area in China.

Key words: Caragana korshinskii, Artemisia ordosica, stemflow, soil wetting front.