Nitrogen fertilization raises CO₂ efflux from inorganic carbon: A global assessment

Abstract

Nitrogen (N) fertilization is an indispensable agricultural practice worldwide, serving the survival of half of the global population. Nitrogen transformation (e.g., nitrification) in soil as well as plant N uptake releases protons and increases soil acidification. Neutralizing this acidity in carbonate-containing soils (7.49 × 10 ha; ca. 54% of the global land surface area) leads to a CO release corresponding to 0.21 kg C per kg of applied N. We here for the first time raise this problem of acidification of carbonate-containing soils and assess the global CO release from pedogenic and geogenic carbonates in the upper 1 m soil depth. Based on a global N-fertilization map and the distribution of soils containing CaCO , we calculated the CO amount released annually from the acidification of such soils to be 7.48 × 10 g C/year. This level of continuous CO release will remain constant at least until soils are fertilized by N. Moreover, we estimated that about 273 × 10 g CO -C are released annually in the same process of CaCO neutralization but involving liming of acid soils. These two CO sources correspond to 3% of global CO emissions by fossil fuel combustion or 30% of CO by land-use changes. Importantly, the duration of CO release after land-use changes usually lasts only 1-3 decades before a new C equilibrium is reached in soil. In contrast, the CO released by CaCO acidification cannot reach equilibrium, as long as N fertilizer is applied until it becomes completely neutralized. As the CaCO amounts in soils, if present, are nearly unlimited, their complete dissolution and CO release will take centuries or even millennia. This emphasizes the necessity of preventing soil acidification in N-fertilized soils as an effective strategy to inhibit millennia of CO efflux to the atmosphere. Hence, N fertilization should be strictly calculated based on plant-demand, and overfertilization should be avoided not only because N is a source of local and regional eutrophication, but also because of the continuous CO release by global acidification.

Shrub encroachment decreases soil inorganic carbon stocks in Mongolian grasslands

“…To the best of our knowledge, our study is the first to provide evidence for a shrub encroachment‐induced decrease in SIC in grasslands at the regional scale, which has important implications for understanding the role of SIC in the terrestrial ecosystem C cycle. Traditionally, SIC is generally considered to play a minor role in the short‐term terrestrial ecosystem C cycle due to its long turnover time and relatively stable characteristics (Zamanian et al, ). However, our findings indicate that shrub encroachment resulted in a significant decrease in SIC reserves in Inner Mongolian grasslands, that is, 0.89 kg C m −2 of SIC was lost from the 1 m soil layer at the plot level, accounting for 18.8% of the SICD at 1 m in the control plots.…”

“…Traditionally, SIC is generally considered to play a minor role in the short-term terrestrial ecosystem C cycle due to its long turnover time and relatively stable characteristics (Zamanian et al, 2018).…”

“…Specifically, acid deposition induced by fossil fuel combustion has caused a significant decrease in the pH of grassland soil in northern China (Yang, Ji, et al, ) and has further resulted in a loss of 0.46 Pg C from the topsoil inorganic C stocks (Yang, Fang, et al, ). In addition, recent global analyses have shown that agricultural activities, particularly nitrogen fertilization and lime application, have resulted in significant SIC losses from farmlands that are approximately equivalent to 30% of the carbon dioxide (CO 2 ) emissions due to land‐use conversion (Zamanian et al, ). Overall, these studies clearly show that SIC dynamics can profoundly affect the C budgets of regional and global terrestrial ecosystems.…”

Managing for soil carbon sequestration: Let’s get realistic

“…Active management of agricultural soils may reduce the losses of soil organic matter, but full life cycle analyses for fertilized and irrigated soils seldom show net carbon sequestration (McGill et al, 2018), and manuring simply redistributes plant inputs from areas that are grazed to areas that receive amendments (Owen, Parton, & Silver, 2015). Production of nitrogen fertilizer is estimated to contribute 2%-3% of the anthropogenic additions of CO 2 to Earth's atmosphere, an amount matched by the acidification of carbonaterich agricultural soils by nitrifying bacteria (Zamanian, Zarebanadkouki, & Kuzyakov, 2018).…”

Responses of soil organic and inorganic carbon vary at different soil depths after long‐term agricultural cultivation in Northwest China

“…Soil pH in the top layers (0–60 cm) declined considerably with cultivation year (Figure a), which might be due to that the application of chemical fertilizers in the cropland causes soil acidification (Guo et al, ; Helyar, ). Declining soil pH would prompt the dissolution of soil carbonate and lead to declining SIC (Zamanian, Zarebanadkouki, & Kuzyakov, ). Meanwhile, irrigation in the arid region have positive impacts on the accumulation of SIC in two aspects.…”