Carbon emissions induced by farmland expansion in China during the past 300 years

Scientific assessment of the accounting over carbon in the terrestrial ecosystem in the process land use/land cover changes caused by human activities will help reduce the uncertainty in estimating carbon emissions from the terrestrial ecosystem. This study employs a bookkeeping model to estimate the carbon emissions from farmland reclamation in China during the past 300 years based on the annual rate of land use changes (derived from historical natural vegetation, farmland data), preset carbon density and coefficients of disturbance curves. We find out that: (1) there was a net increase of 79.30×104 km2 in national farmland; about 65% of reclaimed farmland had been forest land and 26% of that had been grass land previously; (2) the total amount of carbon emissions from farmland expansion in China had been between 2.94 and 5.61 Pg with the median 3.78 Pg during the past 300 years; specifically, carbon emissions of vegetation were 1.58 Pg while those of soil ranged from 1.35 Pg to 4.03 Pg with the median 2.20 Pg; (3) carbon emissions vary greatly across various ecosystems: the emissions were most from forest land, and then grass land and swamps, and the least from shrubs; deserts functioned more likely to be carbon stock in the process of land reclamation; (4) along the time line, carbon emissions had decreased first and then increased while the peak emissions occurred in the first half of 20th century; and spatially, carbon emissions were most released in Northeast and Southwest China; Northwest China was of the minimum carbon emissions.