Changes in village-scale nitrogen storage in China's Tai Lake Region

Long-term change in nitrogen storage in densely populated rural landscapes of China's Tai Lake Region was investigated by comparing soil and sediment N storage within an entire village ecosystem under traditional vs. contemporary management. Contemporary data were gathered on site from 1993 to 1996 by field surveying and sampling. Traditional period data, similar to 1930, were obtained from historical sources, interviews, and back estimation. N storage in the top 40 cm of soil and in low-density sediments (depth to density >1.3 g/cm(3)) was estimated within 35 village landscape components that were then aggregated into village-scale estimates and compared using Monte Carlo uncertainty analysis and a data quality index. Our results demonstrate with 76% probability that village soil and sediment N storage has increased from 1930 to 1994, most likely by 25% over the 1930s level, or similar to 1.4 Mg N/ha on average. A 20% increase in agricultural soil N concentrations caused more than half of the increase, potentially improving soil fertility. Sediment: accumulation in village canals, ponds, and marshes caused the remaining N storage increase, after sediment use for fertilizer ended in 1982, increasing the risk of flooding and impeding irrigation. Sedimentation at current rates will fill most canals within 25 years, and N concentration in agricultural soils may now be declining. Compounding these problems, village food security is threatened by a 30% decline in agricultural soil N per person since 1930 and a doubling in the proportion of village soil N under buildings and infrastructure, from 5% in 1930 to 11% in 1994. Village landscapes in the Tai Lake Region sequestered 1.7 Tg N and 16 Tg C between 1930 and 1994, forming a significant regional sink that may become a source of atmospheric C and N emissions, if organic N use continues to decline.