Phosphorus (P) loss, associated with excess nutrient addition to cropland and livestock production, has increasingly been recognized as a major contributing factor to water eutrophication. Also, knowledge of headwater influences on the water-quality and flow conditions of downstream waters is essential to water-resource management. Scientists are very curious about what role the small headwater watershed played in water eutrophication.

Although the impacts of cropland and livestock production on water quality, particularly on P loss at the watershed scale, are not well known to date, a preliminary study found that annual P loss associated with animal production was above 60% in a subtropical headwater watershed in southern China.

Using long-term observation data of total phosphorus (TP) concentrations in the water and runoff volume, a team of researchers from China Agricultural University, the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA), spent 3 years to investigate the monthly dynamics and annual P loadings and to determine the main contributing factors for these P dynamics in 10 subtropical agricultural headwater watersheds in southern China.

According to Prof. LI Yuyuan, the first author on the paper, livestock density (LD) was the dominant factor in P loadings levels. The annual P loadings (APL) in the 10 watersheds ranged from 22.8 to 247.8 kg P/km², These values, especially in watersheds with intensive agriculture (113.0 kg P/km²) and livestock production (197.2 kg P/km²), were greater than those determined in other studies for watersheds dominated by crops. Annual P loadings was linearly related to LD (R=0.92, p<0.01), whereas the eutrophication ratio of stream water was significantly (p<0.05) correlated with LD (R=0.61). Therefore, the control of livestock production has the greatest potential for reducing watershed P loadings in watersheds in this subtropical area.

The researchers also found that stream systems play a substantial role in reducing P loadings. TP and SP concentrations (0.204 - 0.364 mg P/L) and TP loadings (151.4 - 198.8 kg P/km²) in most upper-reach tributaries were all significantly greater than data (0.081 - 0.164 mg P/L and 74.5 kg P/km²) in the outlet. This trend could be explained by the P reduction effects of sediment adsorption, deposition, or consumption by aquatic systems as water flows along the river. Thus, the streams themselves can play an important role in reducing both P loadings of surface water, at least temporarily.

This study was financially supported by the National Science Fund of China (41171396), the National Science & Technology Pillar Program (2012BAD14B17), and the Creative Research Teams Program of the Chinese Academy of Sciences (KZCX2- YW-T07).

The study entitled “Study on phosphorus loadings in ten natural and agricultural watersheds in subtropical region of China” has been published in Volume 186, Issue 5, May 2014 of Environmental Monitoring and Assessment. Details could be found at http://link.springer.com/article/10.1007/s10661-013-3573-9