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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. Con- temporary data were gathered on site from 1993 to 1996 by field surveying and sampling. Traditional period data, - 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/cm3) was estimated within 35 village landscape components that were then aggre- gated 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 -1.4 Mg N/ha on average. A 20% increase in agricultural soil N concen- trations 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. |
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