Economics- and policy-driven organic carbon input enhancement dominates soil organic carbon accumulation in Chinese croplands

Abstract

China's croplands have experienced drastic changes in management practices, such as fertilization, tillage, and residue treatments, since the 1980s. There is an ongoing debate about the impact of these changes on soil organic carbon (SOC) and its implications. Here we report results from an extensive study that provided direct evidence of cropland SOC sequestration in China. Based on the soil sampling locations recorded by the Second National Soil Survey of China in 1980, we collected 4,060 soil samples in 2011 from 58 counties that represent the typical cropping systems across China. Our results showed that across the country, the average SOC stock in the topsoil (0-20 cm) increased from 28.6 Mg C ha in 1980 to 32.9 Mg C ha in 2011, representing a net increase of 140 kg C ha year However, the SOC change differed among the major agricultural regions: SOC increased in all major agronomic regions except in Northeast China. The SOC sequestration was largely attributed to increased organic inputs driven by economics and policy: while higher root biomass resulting from enhanced crop productivity by chemical fertilizers predominated before 2000, higher residue inputs following the large-scale implementation of crop straw/stover return policy took over thereafter. The SOC change was negatively related to N inputs in East China, suggesting that the excessive N inputs, plus the shallowness of plow layers, may constrain the future C sequestration in Chinese croplands. Our results indicate that cropland SOC sequestration can be achieved through effectively manipulating economic and policy incentives to farmers.

Temporal variability of soil organic carbon in paddies during 13‐year conservation tillage

“…The seasonal SOC variation was related with precipitation and temperature (Maillard, McConkey, Luce, Angers, & Fan, 2018;Melillo et al, 2017;Weissert et al, 2016 Table S6). During the rapid stage, C inputs resulting from residues incorporation might be the main reason for the increase of SOC in NTS, RTS, and CTS (Zhao et al, 2018). An irrigation-based system of traditional rice (both early and late) cropping could also increase SOC sequestration under CT. Additionally, the fluctuation or variation of SOC stock at the 0-30 cm depth was influenced by the attendant effect of C input from residues and C loss from mineralization.…”

“…When SOC sequestration approached an equilibrium level, the rate of additional sequestration was minimal (Zhao et al, 2018). Globally, several researchers have assessed the impacts of diverse agricultural practices (e.g., residue management, tillage) on SOC sequestration (Heitkamp, Wendland, Offenberger, & Gerold, 2012;Stewart, Paustian, Conant, Plante, & Six, 2007), and these studies suggest that (after a few years of increment) SOC stocks are saturated with respect to additional C input.…”

“…The C input from rice straw (Cs) was estimated according to Equation as follows: where Yield represents the economic yield of rice for a given year, WC is the water content of the economic yield, which is 0.06 according to Niu and Liu (), and 0.45 is the conversion factor required to the C content in crop biomass (Fang, Guo, Piao, & Chen, ). GSratio represents the grain/straw ratio of the rice of South Central China and has a value of 0.980 according to Zhao et al ().…”

Lower‐than‐expected CH4 emissions from rice paddies with rising CO2 concentrations

“…Our estimate of a 3.7% increase in global CH 4 emissions from rice agriculture through eCO 2 is likely an overestimate, because it does not take into account the increase in straw incorporation that has already occurred over the last decade (Zhao et al, ). Since straw incorporation tends to increase rice yields and reduces the need for fertilizer use (Huang, Zeng, Wu, Shi, & Pan, ; Jiang, Qian, Huang, et al, ), its popularity will likely continue to grow.…”

“…As such, eCO 2 and management practices are likely to interact in determining CH 4 emissions from rice paddies (Lou et al, 2008;Xie et al, 2012;Zheng et al, 2006). This is important, because management practices have changed drastically over the last few decades (Zhang, Yu, Huang, Li, & Wang, 2011;Zhang et al, 2019;Zhao et al, 2018). Nonetheless, previous assessments of eCO 2 effects did not consider variation in management practices (Dijkstra et al, 2012;van Groenigen et al, 2013;van Groenigen, Osenberg, & Hungate, 2011;Liu et al, 2018).…”