Saying the words "Water eutrophication", many people get a mental picture of algal blooms in large lakes, such as Lakes Taihu, Caohu, and Dianchi. Phosphorus (P) runoff has increasingly been recognized as the essential cause of eutrophication of water bodies, posing serious threats to watershed environments and human health.

The mechanism of P loss is complex and multivariate, and the relationships between various factors, such as land use and livestock production remain unclear.

Currently, a team of researchers from Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA) and China Agricultural University, conducted a study to investigate the total P loads and contributing factors from one forested and nine agricultural watersheds with areas of 9-5212 ha in a hilly region of Hunan Province in a subtropical region of Southern China from 2010 to 2012.

More than 2 years of observations in subtropical China revealed that annual total P loads varied greatly, from 35.7 to 222.1 kg P km^-2 year^-1 among different types of watersheds, with 56.3–82.0 % occurring during the rainy season in spring and summer. By applying the load duration curve (LDC) method, the highest total maximum daily load (TMDL) was 0.5 kg P km^-2 day^-1. Daily reduction rates among the five flow conditions were relatively constant within each watershed, indicating that serious water quality issues occur even in the low flow periods, particularly for watersheds with intensive livestock production.

"Controlling P sources through watershed management, including reductions in livestock production and P fertilizer application rates, would be the most effective approaches for protect the local aquatic environments. In the meantime, controlling P losses would be effective, particularly in spring and summer when very high and high flows frequently occur in subtropical southern China.” said MENG Cen, one of the researchers.

The research was supported by the National Science Fund of China (41171396) and the National Science and Technology Pillar Program (2012BAD14B 17, 2014BAD14B02).

The study entitled “TMDL for phosphorus and contributing factors in subtropical watershed of southern China” was published in the August issue of Environmental Monitoring and Assessment, more details could be found at http://link.springer.com/article/10.1007/s10661-015-4737-6.

WU Jinshui

Institute of Subtropical Agriculture, Chinese Academy of Sciences

jswu@isa.ac.cn