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. contempo
rary 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 s
torage in the top 40 cm of soil and in low-density sediments (depth to dens
ity >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 demonstra
te with 76% probability that village soil and sediment N storage has increa
sed 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 concentrat
ions caused more than half of the increase, potentially improving soil fert
ility. Sediment: accumulation in village canals, ponds, and marshes caused
the remaining N storage increase, after sediment use for fertilizer ended i
n 1982, increasing the risk of flooding and impeding irrigation. Sedimentat
ion at current rates will fill most canals within 25 years, and N concentra
tion 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. Villag
e landscapes in the Tai Lake Region sequestered 1.7 Tg N and 16 Tg C betwee
n 1930 and 1994, forming a significant regional sink that may become a sour
ce of atmospheric C and N emissions, if organic N use continues to decline.