Nitrogen is a limiting factor for the primary production of ecosystems, and has become a hot topic in climate change research over the past few decades (Galloway et al., 2008; Fowler et al., 2013). Lacustrine deposits play a vital role in overall nutrient cycling in lakes, especially regarding the nitrogen cycle (Elser et al., 2007; Conley et al., 2009). Lacustrine nitrogen has many forms with different biogeochemical characteristics. Sedimentary organic nitrogen (ON) is the main form of nitrogen. It accumulates in the sediment and is easily converted to nitrate and ammonia via mineralization. Most of the converted ammonium and nitrate are released into the overlying water, which may cause algal blooms (Galman et al., 2008; Yu et al., 2018). The effects of algal residues, which are considered important autochthonous sources, on organic matter burial have been widely studied (Anderson et al., 2014; Leithold et al., 2016). Overall, while carbon is a major element in organic matter and has been widely reported, research concerning nitrogen is limited.
Recent studies have proposed that the sources of sedimentary ON are very complex and include municipal sewage, agricultural fertilizer, bacteria, and algae, terrestrial and aquatic plants and so on (Leithold et al., 2016; Wang et al., 2020). In addition, most previous studies have found that the hydrological characteristics, including trophic state, climate, and vegetation, in lake watersheds affect organic matter accumulation in sediment (Downing et al., 2008; Anderson et al., 2013; Anderson et al., 2014). For instance, Heathcote et al. (2015) revealed that climate change could significantly increase organic carbon accumulation in northern lakes. However, most previous research has focused on boreal lakes, while some have focused on temperate-zone lakes to determine the burial efficiency and influencing factors of organic matter burial over different time scales (Anderson et al., 2013; Anderson et al., 2014; Heathcote et al., 2015; Fortino et al., 2016; Jiang et al., 2020). Since 1970, Dianchi Lake, which is the largest sub-tropic plateau lake in Southwest China, has not only been affected by a variety of climatic features (such as monsoons, high-altitude, and low-latitude climates) but also by eutrophication and intensive human activities (such as deforestation, urbanization, and fertilization) (Gao et al., 2015; Huang et al., 2017b; Chen et al., 2020). Eutrophication is considered to be an important driver of increasing organic carbon as a result of not only the enrichment of nutrients in lake water but also perennial climate change (Larsen et al., 2011; Chen et al., 2020). Therefore, the effect of human activities on the spatial and temporal sediment ON accumulation rate (ONAR) could provide a comprehensive understanding of the lake nitrogen cycle under climate change.
N-alkanes, widely present in bacteria, algae, aquatic, and terrestrial plants, are considered to be an ideal biomarker for tracing the source of organic matter under environmental and climatic changes because of their degradation resistance (Mead et al., 2005; Fang et al., 2014; Liu and Liu, 2016). The distribution of the n-alkanes carbon content in bacteria and algae ranges from n-C15 to n-C20. The n-alkanes distribution in aquatic macrophytes and crude oil is dominated by mid-chain alkanes (n-C20 to n-C25). Terrestrial higher plants are typically characterized by a high abundance of long-chain n-alkanes (n-C27 to n-C33) (Ficken et al., 2000; Liu and Liu, 2016). Complex relationships exist between climate change and human activities, which obscure the effect of individual factors on ON burial. The stochastic impacts by regression on population, affluence, and technology (STIRPAT) model can be used to statistically analyze the non-monotonic or non-proportional impacts of driving factors on the environment (Dietz, 1994; York et al., 2003). It has been successfully utilized to explore the influence of human factors and analyze the effects of driving forces on a variety of environments, including carbon dioxide emissions and environmental changes (Wang et al., 2013; Zhou and Liu, 2016).
Therefore, in this study, we combined chronology and n-alkanes indicators to explore the contributions of autochthonous and allochthonous to ON sources in various regions of Dianchi Lake over different periods. In addition, we quantitatively evaluated the effects of climate change and human activities on sedimentary ON burial in different regions of Dianchi Lake using the STIRPAT model.