Agricultural acceleration of soil carbonate weathering

Abstract

Soil carbonates (i.e., soil inorganic carbon or SIC) represent more than a quarter of the terrestrial carbon pool and are often considered to be relatively stable, with fluxes significant only on geologic timescales. However, given the importance of climatic water balance on SIC accumulation, we tested the hypothesis that increased soil water storage and transport resulting from cultivation may enhance dissolution of SIC, altering their local stock at decadal timescales. We compared SIC storage to 7.3 m depth in eight sites, each having paired plots of native vegetation and rain‐fed croplands, and half the sites having additional irrigated cropland plots. Rain‐fed and irrigated croplands had 328 and 730 Mg C/ha less SIC storage, respectively, compared to their native vegetation (grassland or woodland) pairs, and irrigated croplands had 402 Mg C/ha less than their rain‐fed pairs (p < .0001). SIC contents were negatively correlated with estimated groundwater recharge, suggesting that dissolution and leaching may be responsible for SIC losses observed. Under croplands, the remaining SIC had more modern radiocarbon and a δ13C composition that was closer to crop inputs than under native vegetation, suggesting that cultivation has led to faster turnover and incorporation of recent crop carbon into the SIC pool (p < .0001). The losses occurred just 30–100 years after land‐use changes, indicating SIC stocks that were stable for millennia can rapidly adjust to increased soil water flows. Large SIC losses (194–242 Mg C/ha) also occurred below 4.9 m deep under irrigated croplands, with SIC losses lagging behind the downward‐advancing wetting front by ~30 years, suggesting that even deep SIC were affected. These observations suggest that the vertical distribution of SIC in dry ecosystems is dynamic on decadal timescales, highlighting its potential role as a carbon sink or source to be examined in the context of land use and climate change.

Significant loss of soil inorganic carbon at the continental scale

“…The exchangeable calcium (Exch. Ca) may increase due to the dissolution of carbonates [ 35 ], some of which may leach downward along with NO 3 – and HCO 3 – [ 36 ]. Here, Exch.…”

“…2 ). The dissolution, precipitation and leaching of carbonate may be jointly responsible for SIC losses [ 35 ]. Therefore, CO 2 emissions that are due to acidifying processes need to be accurately quantified [ 13 , 14 ].…”

Organic amendments facilitate soil carbon sequestration via organic carbon accumulation and mitigation of inorganic carbon loss

“…In addition, additional Ca 2+ and Mg 2+ input derived from manure application is also contribute to SIC accumulation by reprecipitation of carbonate (Dong et al, 2017; Wang et al, 2014). Furthermore, irrigation can leach these additional Ca 2+ and Mg 2+ into subsoil, resulting in a large amount of SIC reprecipitation in subsoil (Kim et al, 2020). Therefore, more studies are essential to focus on the impacts of agricultural management practices (i.e., fertilization, irrigation) on SIC accumulation in subsoil.…”

“…The stock of SIC in cropland is largely influenced by land management practices, such as irrigation and fertilization (Ferdush & Paul, 2021; Kim et al, 2020; Zamanian et al, 2016). Studies from long‐term fertilization fields demonstrate that applications of organic and mineral fertilizers can increase the SIC stock in North China (Bughio et al, 2016; Wang et al, 2015b; Wang et al, 2014), which may be a result of extra available Ca 2+ and Mg 2+ supplied by fertilizers.…”

Facts to acidification‐induced carbonate losses from Chinese croplands

“…Moreover, Ca 2+ is the main accompanying ion when nitrate is leached in calcareous soils (Figure 1), and its loss reduces the formation of pedogenic carbonates (Zamanian et al, 2016). Kim et al (2020) also reported that increased water flows and recharge during cultivation have accelerated carbonate losses, which were generally presumed to be stable over geological timescales, mainly through leaching and changing their stocks at decadal timescales.…”