Short communicationWet deposition N and its runoff flow during wheat seasons in the Tai Lake Region, China
Highlights
► During wheat seasons wet deposition N ranges from 11 to 15 kg ha−1 (average, 13 kg ha−1), 61% of which is in the form of NH4+–N. ► NH4+–N in wet deposition is prone to immobilization by the soil–crop system, whereas NO3−–N is relatively easily lost to runoff. ► N loss through runoff accounts for 14% of wet deposition N. ► The contribution of wet deposition N should be adjusted in fertilizer recommendations for winter wheat cultivation.
Introduction
With the rapid development of the agricultural and industrial sectors in recent decades, the amount of anthropogenic reactive nitrogen (N) has sharply increased, and atmospheric N deposition has become an important component (having reached 70 Tg N yr−1) of the global N cycle as a consequence of increasing anthropogenic reactive N emissions (Zhu and Chen, 2002, Galloway et al., 2004). Elevated levels of wet deposition N, causing eutrophication and acidification, considerably burden various ecosystems, such as forests, grassland, and aquatic ecosystems (Goulding et al., 1998, Bouwman et al., 2002, Stevens et al., 2004).
Wet deposition N primarily exists in the form of ammonium (NH4+) and nitrate (NO3−) produced by the dissolution of atmospheric NH3 and HNO3. Most studies on wet deposition N have focused on its impacts and its flow in natural or semi-natural ecosystems (Providoli et al., 2006, Stevens et al., 2010). For example, an 15N-labeled experiment showed that 2–6% of added deposited NO3− and less than 1% of deposited NH4+ are present in runoff flow in N-limited forests and meadow ecosystems (Providoli et al., 2005). However, minimal information on the contribution of N deposition to intensive agricultural ecosystems is available (Fahey et al., 1999, He et al., 2010). Deposited N can supplement the supply of available N to agricultural crops—a feature that should be considered when calculating N fertilizer requirements for crop cultivation (He et al., 2007, Mei and Zhang, 2007, Xie et al., 2008, He et al., 2010). Therefore, an evaluation of the contribution of wet deposition N to agricultural fields is important in recommending N fertilizers, improving N use efficiency, and minimizing environmental hazards brought by N loss in croplands.
The Tai Lake Region (TLR) is one of the most important industrially and agriculturally developed regions in China. In this region, winter wheat is a prevalent crop after irrigated rice (Cao and Zhang, 2004, Xiong et al., 2006). The rapid development of the industry, excessive use of chemical N fertilizers, as well as the increasing amount of human and livestock excreta, enhance NOx and NH3 emissions; hence, HNO3 and NH4+ deposition may also be enhanced correspondingly (Richter and Roelcke, 2000, Xing and Zhu, 2002). The characteristics and sources of wet deposition N in this region have been documented (Wang et al., 2004, Xie et al., 2008, Zhao et al., 2009). Wang et al. (2004) reported annual wet depositions of up to 27 kg N ha−1 in the TLR. However, few studies have focused on the retention of wet deposition N by the soil–crop ecosystem, and which form of wet deposition N is likely to be retained or lost through water flow when precipitation occurs remains unelucidated. The field in the TLR is surrounded by a ridge (15–20 cm high) during the rice growing season and covered by a water layer (3–5 cm) during most of the rice season (Tian et al., 2007). Hence, runoff during the rice season will not occur unless rainfall exceeds 120 mm, which is a low probability event in this region. Therefore, the winter wheat season was chosen in this study to investigate the wet deposition of N and its rough flow by runoff. This study aims to (1) monitor wet deposition N input and the strength of NH4+ and NO3− during the winter wheat-growing season; (2) trace the flow of wet deposition N in runoff to preliminarily evaluate which forms of wet deposition N are easily absorbed by the soil-crop ecosystem and which form are prone to be lost to runoff; and (3) assess the contribution of wet deposition N to winter wheat cultivation and provide recommendations for the use of N fertilizers.
Section snippets
Study site
Experiments were conducted at Changshu Agro-Ecological Experiment Station, Chinese Academy of Sciences, Jiangsu Province (31̊32′45″N and 120̊41′57″E). Located in the TLR, the study site belongs to the northern subtropical humid climatic zone, with an annual mean temperature of 15.5 °C, rainfall of 1038 mm, and frost-free period of 224 days. The soil is developed from lacustrine deposits and is classified as Gleyi-stagnic Anthrosols, according to the FAO soil taxonomy system. The topsoil (0–15 cm;
Temporal variations in NH4+–N and NO3−–N concentrations in wet deposition
Inorganic N was the dominant component of wet deposition N, and the volume-weighted concentrations of NH4+–N and NO3––N in rainwater showed distinct temporal variations during the wheat season (seven months, from November to May of the following year). NH4+–N and NO3––N concentrations ranged from 0.3 to 8.5 (weighted average, 1.7) and 0.2 to 4 (weighted average, 0.8) mg N L−1, respectively (Fig. 1). NH4+ concentrations were higher and exhibited greater variability than did NO3− concentrations,
N wet deposition characteristics during wheat seasons
During the wheat-growing period, 45% of NH4+–N deposition was distributed in April and May. The primary source of NH4+ in wet deposition is NH3 volatilized from fertilizers, as well as human and animal excreta, which are influenced by climate and agricultural activities. The gradually rising temperature (monthly mean temperature in April and May is 17 and 21 °C, respectively, whereas in other months, temperature is lower than 15 °C) and intense biological activities in this period facilitated NH3
Conclusions
During winter wheat-growing seasons in the TLR, one of the most developed and populated regions in China, wet deposition N reaches 13 kg ha−1, mainly in the form NH4+–N. NH4+–N concentrations show large temporal variations and NH4+–N deposition is mainly distributed in April and May, resulting from heightened NH3 volatilization from human and livestock excreta. Fourteen percent of wet deposition N is lost to runoff, of which 75% is NO3−–N. The present study sheds light on the flow of wet
Acknowledgements
We thank the Knowledge Innovation Project of the Chinese Academy of Sciences (KZCX2-YW-440-1), National Basic Research Program (973) of China (2009CB118603), and CAS Knowledge Innovation Program (ISSASIP0701) for the financial support they provided.
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