CONVERSION FROM GRASSLAND TO CROPLAND AND LENGTH OF CROPPING HISTORY DRIVING SOIL METHANE UPTAKE IN CHINA

The change of land use from grassland to cropland in Northern China has raised serious concern about regional carbon (C) cycle and greenhouse gas balance. We measured soil methane (CH4) uptake using manual static chambers in grassland and cropland soils in the agro-pastoral ecotone of Inner Mongolia over three growing seasons (2010-2012). The primary aims were to assess the effect of undisturbed grassland and croplands from converted grassland with different land use histories on gas fluxes and systematically compare the site-specific CH4 uptake factor. We found a significant difference (P < 0.001) in CH4 uptake between grassland and croplands from reclaimed grassland for 5, 10 and 50 years old, and cropland soils in 5 and 10 years old were a significant sink of CH4. Compared with cropland soils, the grassland had the lowest cumulative CH4 uptake, with 141.4, 210.0 and 236.0 mg/m(2) during growing seasons of 2010, 2011 and 2012, respectively. Over the 3 growing seasons, the cumulative CH4 uptake of croplands aged 5, 10 and 50 was 544.5, 361.7 and 266.1 mg/m(2). With the increase of farming time, the methane accumulation and absorption of C5, C10 and C50 decreased. Differences in CH4 uptake of grassland and cropland with different length of cropping history can be explained by the amount of soil ammonium nitrogen (NH4+-N) and soil moisture. We conclude that (i) croplands for 5 and 10 years old from reclaimed grassland are the best approach considered here for optimizing the land use as a sink for atmospheric CH4, and (ii) the practice that croplands from grassland reclaimed for more than 10 years old should be planted into grasslands is recommended for managing CH4 uptake and soil carbon sink in the agro-pastoral ecotone of Inner Mongolia, China.