Artificial ponds as hotspots of nitrogen removal in agricultural watershed

Small waters, like ponds, are the most abundant freshwater environments, and are increasingly recognized for their function in ecosystem service delivery. In agricultural watershed, artificial ponds play an essential role in reducing nitrogen pollution. However, until now artificial ponds remain the least investigated part of water environments. The importance of microbial activities has seldom been discussed, which makes the microbial pathways and processes rates in nitrogen removal poorly understood. To illustrate the role of artificial ponds in microbial nitrogen removal in agricultural watersheds, 21 pond sediments and 11 soils are collected in an agricultural watershed of China. Results show that surface sediments in ponds carry significantly higher dissolved inorganic nitrogen (9.1-21.9 mg/kg) and total organic matter (64.8-113.0 g/kg) compared to the surrounding agricultural soils. High rates of microbial nitrogen removal in ponds (12.4-25.5 nmol N g(-1) h(-1)) are observed, which are 2-9 times higher than those in dryland soils. In pond sediments, denitrification dominates (> 90% N-loss) the microbial nitrogen removal process with only a minor contribution of anaerobic ammonium oxidation. A high potential of N2O production (up to 9.4 nmol N g(-1) h(-1)) occurs in ponds along with the rapid nitrogen removal. For denitrifier genes, nir gene are always more abundant than nosZ gene. Additionally, the nirS gene is more abundant under flooded conditions, while nirK gene prefers higher dissolved oxygen and NO3- in drylands. These findings highlight the ecosystem function of ponds in agricultural watersheds, and provide new ideas on pollution control and global nitrogen cycling.

Automated summary

Small waters, such as ponds, are important freshwater environments that have been increasingly recognized for their role in ecosystem service delivery. In agricultural watersheds, artificial ponds play a vital role in reducing nitrogen pollution. However, artificial ponds have been poorly investigated compared to other water environments. The importance of microbial activities in these ponds and their role in nitrogen removal are not well understood. To study the microbial nitrogen removal in agricultural watersheds, sediment samples from 21 ponds and 11 soil samples were collected in China. The results revealed that pond sediments had significantly higher nitrogen and organic matter content compared to surrounding soils. The microbial nitrogen removal rates were also much higher in ponds than in dryland soils. Denitrification was found to be the dominant process in nitrogen removal in ponds, while anaerobic ammonium oxidation contributed only minimally. These findings highlight the ecosystem function of ponds in agricultural watersheds and provide new insights into pollution control and global nitrogen cycling.