Effect of reforestation on nitrogen and phosphorus dynamics in the catchment ecosystems of subtropical China: the example of the Hanjiang River basin.

Background: To enable effective management and decision making for the sustainable use of water resources, we successfully integrated factors such as dams, land use and soil properties as well as management factors in the Hanjiang River basin, a subtropical catchment of China, into the SWAT model to simulate water cycles as well as the distribution, movement, and transformations of nutrients.

Results: The accuracy of the model was validated by monitoring data over the Hanjiang River. The validated model was then used to evaluate the effects of the Reforestation of Cultivated Land (RFCL) initiative. The simulation results showed that RFCL would cause an obvious decrease in surface runoff (-23.6%, P < 0.01) but an increase in groundwater (71.8%, P < 0.01) and percolation out of the soil (24.7%, P < 0.01). The total water yield does not change significantly (-4.4%), but the decrease in total sediment loading is substantial (-56.2%, P < 0.01). The simulation results also show that RFCL would greatly decrease the organic N (-42.6%, P < 0.01), NO(3) yield in surface flow (-37.1%, P < 0.01), and the NO(3) yield in subsurface flow (-25.5%, P < 0.01), whereas the NO(3) yield in groundwater flow would increase (107%, P < 0.01). In terms of phosphorus, RFCL would cause both organic phosphorus (-38.2%, P < 0.01) and the phosphorus yield from the soil (-33.3%, P < 0.01) to decrease.

Conclusion: The results suggest that RFCL is an effective policy for watershed environment management, which might have a relatively small effect on river discharge but that the purification effects on water quality in the river would be remarkable.