Marshland conversion to cropland in northeast China from 1950 to 2000 reduced the greenhouse effect

Abstract

It has been well recognized that converting wetlands to cropland results in loss of soil organic carbon (SOC), while less attention was paid to concomitant changes in methane (CH 4 ) and nitrous oxide (N 2 O) emissions. Using datasets from the literature and field measurements, we investigated loss of SOC and emissions of CH 4 and N 2 O due to marshland conversion in northeast China. Analysis of the documented crop cultivation area indicated that 2.91 Mha of marshland were converted to cropland over the period 1950-2000. Marshland conversion resulted in SOC loss of $ 240 Tg and introduced $1.4 Tg CH 4 and $ 138 Gg N 2 O emissions in the cropland, while CH 4 emissions reduced greatly in the marshland, cumulatively $28 Tg over the 50 years. Taking into account the loss of SOC and emissions of CH 4 and N 2 O, the global warming potential (GWP) at a 20-year time horizon was estimated to be $ 180 Tg CO 2 _eq. yr À1 in the 1950s and $ 120 Tg CO 2 _eq. yr À1 in the 1990s, with a $ 33% reduction. When calculated at 100-year time horizon, the GWP was $ 73 Tg CO 2 _eq. yr À1 in the 1950s and $ 58 Tg CO 2 _eq. yr À1 in the 1990s, with a $ 21% reduction. It was concluded that marshland conversion to cropland in northeast China reduced the greenhouse effect as far as GWP is concerned. This reduction was attributed to a substantial decrease in CH 4 emissions from the marshland. An extended inference is that the declining growth rate of atmospheric CH 4 since the 1980s might be related to global loss of wetlands, but this connection needs to be confirmed.

Methane fluxes from waterlogged and drained Histosols of highland areas

“…In general, the results of CH 4 emission reduction followed by methane consumption fully agree with those of other studies that evaluated the effect of lowering the water-table level by draining in flooded soils in several parts of the world Nykanen et al, 1995;Maljanen et al, 2004;von Arnold et al, 2005;Furukawa et al, 2005;Bridgham et al, 2006;Elder & Lal, 2008;Jauhiainen et al, 2008;Jiang et al, 2009;Huang et al, 2010;Page & Dalal, 2011). All these authors showed that small water-table decreases cause drastic reductions in methane emissions, demonstrating the strong effect of drainage on CH 4 fluxes.…”

“…Huang et al (2010) estimated that soils of wetlands converted to cropland between 1950 and 2000 lost about 50 % C. Other authors also reported an increase in CO 2 emissions from Histosols (Lohila et al, 2003;Maljanen et al, 2002Maljanen et al, , 2004von Arnold et al, 2005;Furukawa et al, 2005;Yamulki et al, 2012), and Joosten & Couwenberg (2008) In view of the functions Histosols fulfill in the ecosystem, they should be preserved and not drained. Strategies for mitigating CH 4 emissions should be concentrated on the modification of human activities and on no account on interventions in the areas of occurrence of Histosols.…”

“…On the other hand, the drainage of these soils may affect its capability of fixing C and storing water. Huang et al (2010) estimated that the conversion of marshes into croplands reduced the soil organic C stock by approximately 50 %, with 70 % of this loss recorded to a depth of 20 cm. For gravimetric water content, Andrade (2010) reported a reduction from 6 to 2 kg kg -1 in the surface 10 cm, after the drainage of a Histosol.…”

“…These processes are regulated by oxygen (O 2 ) supply and availability of labile carbon (C), being methanogenesis predominant under anaerobic conditions. The CH 4 flux in waterlogged areas is controlled by soil properties and processes (Roulet & Moore, 1995;Whalen, 2005;Jauhiainen et al, 2008), by microbiological factors (Le Mer & Roger, 2001;Kögel-Knabner et al, 2010;Page & Dalal, 2011), climatic factors (von Arnold et al, 2005;Dalal & Allen, 2008;Jiang et al, 2009), vegetation Koh et al, 2009) and land management (Elder & Lal, 2008;Huang et al, 2010). Precipitation and air temperature are important climatic factors that can affect soil CH 4 emissions, although with great variability (Le Mer & Roger, 2001;Whalen, 2005;Dalal & Allen, 2008;Koh et al, 2009;Huang et al, 2010).…”

Engineering the global ecosystem

“…Expansion of arable agricultural lands has occurred by engineering the removal of wetlands, via drainage systems, levees, and other structures (Foote et al 1996;Lemly et al 2000;Huang et al 2010). The removal of wetlands has negative impacts on ecosystem services (i.e., economically quantifiable ecosystem benefits, such as recharge of drinking water aquifers, support of fisheries, recreational opportunities, etc.).…”