Human-Induced Changes in Holocene Nitrogen Cycling in North China: An Isotopic Perspective From Sedimentary Pyrogenic Material

Abstract

Reactive nitrogen (N) deposition from human activity significantly impacts temperate vegetation in areas where low natural N availability limits vegetative productivity. However, the ecosystem response to anthropogenic N deposition remains elusive owing to the scarcity of long-term empirical observations. Here a N isotope of a pyrogenic material was used to investigate long-term N availability evolution in a forest-steppe ecosystem in North China. The N availability was found to have significantly increased since circa 7,400 years BP when primitive agriculture began. Different N availability changes were observed under primitive agriculture (6,600-3,500 cal. years BP) and Sui-Tang traditional agriculture (AD 581-907), implying dependency on the agricultural mode. A 220-year N availability recovery period after Sui-Tang agriculture was observed. On this basis, a minimum of several hundred years is projected for the restoration of the temperate ecosystem if the modern N cascade were to be stopped.\nPlain Language Summary The manner in which a temperate ecosystem responds to continuous anthropogenic nitrogen (N) deposition remains an open question. The lack of long-term empirical observation has limited the understanding of this ecological issue. Agricultural practices over the past several thousand years in North China may be an ancient reference for the modern use of N fertilizer. In the present study, we used a geochemical method to reconstruct a 10,000-year history of N changes in the plant-soil system of the Daihai Lake region in North China. The reconstructed N history was then compared with archeological evidences of early agriculture in the region. We found that early agriculture significantly influenced N changes in the plant-soil system about 7,400 years ago and that 220 years were required for the plant-soil system to recover from the N disturbance caused by agricultural activity during the Sui-Tang dynasty (AD 581-907). On this basis, it is predicted that several hundred years would be required to reverse the effects of modern N disturbance on a temperate ecosystem.