Atmospheric deposition as an important nitrogen load to a typical agro-ecosystem in the Huang-Huai-Hai Plain. 2. Seasonal and inter-annual variations and their implications (2008-2012)

Atmospheric nitrogen (N) deposition, an important N source to agro-ecosystems, has increased intensively in China during recent decades. However, knowledge on temporal variations of total N deposition and their influencing factors is limited due to lack of systematic monitoring data. In this study, total N deposition, including dry and wet components, was monitored using the water surrogate surface method for a typical agro-ecosystem with a winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.) rotation system in the Huang-Huai-Hai Plain from May 2008 to April 2012. The results indicated that annual total N deposition ranged from 23.8 kg N ha−1 (2009-2010) to 40.3 kg N ha−1 (2008-2009) and averaged 31.8 kg N ha−1. Great inter-annual variations were observed during the sampling period, due to differences in annual rainfall and gaseous N losses from farmlands. Monthly total N deposition varied greatly, from less than 0.6 kg N ha−1 (January, 2010) to over 8.0 kg N ha−1 (August, 2008), with a mean value of 2.6 kg N ha−1. In contrast to wet deposition, dry portions generally contributed more to the total, except in the precipitation-intensive months, accounting for 65% in average. NH4+-N was the dominant species in N deposition and its contribution to total deposition varied from 6% (December, 2009) to 79% (July, 2008), averaging 53%. The role of organic N (O-N) in both dry and wet deposition was equal to or even greater than that of NO3−-N. Influencing factors such as precipitation and its seasonal distribution, reactive N sources, vegetation status, field management practices, and weather conditions were responsible for the temporal variations of atmospheric N deposition and its components. These results are helpful for reducing the knowledge gaps in the temporal variations of atmospheric N deposition and their influencing factors in different ecosystems, to improve the understandings on N budget in the typical agro-ecosystem, and to provide references and recommendations for field nutrient management in this region.