Chinese cropping systems are a net source of greenhouse gases despite soil carbon sequestration

Abstract

Soil carbon sequestration is being considered as a potential pathway to mitigate climate change. Cropland soils could provide a sink for carbon that can be modified by farming practices; however, they can also act as a source of greenhouse gases (GHG), including not only nitrous oxide (N O) and methane (CH ), but also the upstream carbon dioxide (CO ) emissions associated with agronomic management. These latter emissions are also sometimes termed "hidden" or "embedded" CO . In this paper, we estimated the net GHG balance for Chinese cropping systems by considering the balance of soil carbon sequestration, N O and CH emissions, and the upstream CO emissions of agronomic management from a life cycle perspective during 2000-2017. Results showed that although soil organic carbon (SOC) increased by 23.2 ± 8.6 Tg C per year, the soil N O and CH emissions plus upstream CO emissions arising from agronomic management added 269.5 ± 21.1 Tg C-eq per year to the atmosphere. These findings demonstrate that Chinese cropping systems are a net source of GHG emissions and that total GHG emissions are about 12 times larger than carbon uptake by soil sequestration. There were large variations between different cropping systems in the net GHG balance ranging from 328 to 7,567 kg C-eq ha year , but all systems act as a net GHG source to the atmosphere. The main sources of total GHG emissions are nitrogen fertilization (emissions during production and application), power use for irrigation, and soil N O and CH emissions. Optimizing agronomic management practices, especially fertilization, irrigation, plastic mulching, and crop residues to reduce total GHG emissions from the whole chain is urgently required in order to develop a low-carbon future for Chinese crop production.

Weakened growth of cropland‐N2O emissions in China associated with nationwide policy interventions

“…Increased SOC offset 19% and 51% of the negative effects from N rate for P1 and P2, respectively. Thus, while C sequestration can help offset some of the cropland emissions of CO 2 , a recent study suggests that carbon emission equivalents of non‐CO 2 GHG emissions are currently ~12 times greater than carbon uptake by Chinese croplands over 100 year time horizon (Gao, Huang, et al, ). SOC also played a role in increasing N 2 O emissions with a positive correlation between N 2 O emissions and SOC reported in field (Figueiredo, Enrich‐Prast, & Rütting, ), laboratory studies (Jäger, Stange, Ludwig, & Flessa, ), meta‐analyses (Bouwman, Boumans, & Batjes, ; Charles et al, ), and data mining analysis (Perlman et al, ).…”

Assessing the Climate Regulation Potential of Agricultural Soils Using a Decision Support Tool Adapted to Stakeholders' Needs and Possibilities

“…Although generally no effect is observed (e.g., Nardi et al, 2004;Triberti et al, 2008;Poeplau et al, 2017), some authors report a small increase in SOC stocks after mineral N fertilization (Dersch and Böhm, 2001;Ladha et al, 2011). However, a potential increase in SOC stocks can be offset by the greenhouse gases produced during the manufacturing of mineral N fertilizer (Gao et al, 2018). Therefore, this has been omitted from the model.…”

“…Although increasing the OC content of soils can lead to a net removal of CO 2 from the atmosphere, trade-offs with N 2 O emissions should be taken into account, as these can reduce or completely offset the climate mitigation effect of certain management practices (Gao et al, 2018). For example, while the application of farmyard manure (FYM) can significantly increase topsoil OC stocks (Bai et al, 2018;Sandén et al, 2018Sandén et al, , 2019b, an accompanying increase in N 2 O emissions can offset this benefit (Zhou et al, 2017).…”

Optimizing Nitrogen and Residue Management to Reduce GHG Emissions while Maintaining Crop Yield: A Case Study in a Mono-Cropping System of Northeast China

“…In China, cropping systems for grain production are a net source of GHG emissions from a lifecycle perspective [14], and it is of significance to assess the mitigation potential of different management practices in agriculture. N fertilizer is being overused at a national scale, and thus optimizing N management is a priority for sustainable agriculture [15].…”

“…N fertilizer is being overused at a national scale, and thus optimizing N management is a priority for sustainable agriculture [15]. Incorporating crop straw into soil could mitigate GHG emissions; however, when the straw used as household fuel is returned to the field, farmers may have to use fossil fuels instead and emit fossil CO 2 to the atmosphere, which is rarely considered when promoting straw retention [7,14,16,17]. Additionally, previous studies have included CO 2 emissions from fossil fuel consumed by machines on cropland in China [9,14,18,19], and a detailed accounting of machinery emissions from manufacture, transportation, and repair and maintenance is still lacking because of the complexity of farmers' practices and limited data availability.…”