Spatial and temporal variation in autochthonous and allochthonous contributors to increased organic carbon and nitrogen burial in a plateau lake.

Increased organic carbon and nitrogen accumulation rates (OCAR and ONAR) in lake sediment significantly regulate the global carbon cycle. However, the reasons for and contributors to the increased OCAR and ONAR are unclear. Seven sediment cores, collected in July 2014 from Dianchi Lake, China, were used to evaluate the effects of autochthonous and allochthonous sources on OCAR and ONAR. The results indicate that OCAR and ONAR increased by factors of 4.33 and 7.34 over the past hundred years (1900-2000), particularly after algal blooms began to occur frequently (beginning in the 1980s). Dianchi stored 0.467±0.0055Tg (mean value±standard deviation) organic carbon (OC) and 0.033±0.0004Tg organic nitrogen (ON) after 1986, which is almost equal to the total storage of OC and ON from 1900 to 1985 (OC, 0.468±0.0022Tg; ON, 0.032±0.0002Tg). OCAR and ONAR increases were due to increasing autochthonous production and allochthonous loading. Examination of the increased OCAR, which was estimated from a newly developed multi-source mixing model, suggests that >90% of increased OCAR was caused by allochthonous sources (such as intensified cultivation, land-use cover change, etc.) in southern and eastern Dianchi and that >70% of the increased OCAR was due to autochthonous sources in western and northern Dianchi Lake. The significant spatial and temporal variation in the contributors to increased OCAR indicates complicated migration and transformation of OC in inland lakes. Land use cover change around Dianchi and the occurrence of algal blooms regulate the contributions of allochthonous and autochthonous sources to the increased OC and ON.