Nitrogen use efficiency of plant species matters: CO2 emission from soil inorganic carbon and its temperature dependence in a calcareous soil

Terrestrial ecosystems play a significant role in global warming by regulating CO₂ concentration in the atmosphere. A comprehensive understanding of carbon (C) sources and stocks in soils, as well as the driving mechanisms, are critical to reducing CO₂ emission from soil and thus mitigating climate change. To date, most studies have solely focused on processes involving soil organic C (SOC), but few studies have addressed the potential contribution of soil inorganic C (SIC) mostly CaCO₃ pool to ecosystem C fluxes. SIC can potentially be a regulator of atmospheric CO₂. However, so far the effects of plant species (i.e. variations in nitrogen (N) demand and N use efficiency (NUE)) as well as soil temperature on SIC-derived CO₂ are unclear. We hypothesized that 1) relatively less SIC-derived CO₂ is expected from soils covered under plant species with lower N demand and higher NUE. We conducted a 4-month field experiment from June to October 2021 at the research station of the University of Göttingen in Deppoldshausen (51.58^oN, 9.97^oE) with ca. 6% CaCO₃ equivalent in the topsoil. We analyzed the effects of two plant species 1) wheat (high N demand and low NUE), 2) legume (low N demand and high NUE) and two N fertilization (urea) levels, 1) low (50 kg N ha^-1), 2) high (200 kg N ha^-1) on CO₂ emission out of SIC. Each treatment had four replicate plots (1×1 m²), and at least a 0.5 m gap was established between plots. We measured CO₂ fluxes weekly by using the static chamber method. The δ¹³C natural abundance was used to determine the contribution of SIC and SOC in the emitted CO₂. The total CO₂ emission and its δ¹³C signature increased with soil temperature, indicating that the portion (%) of SIC-derived CO₂ was stimulated by temperature (^oC) (slope = 0.33). The portion of SIC-derived CO₂ stimulated by temperature increased faster under wheat than under legume (slope = 0.36 vs. 0.26), especially under high N treatment (slope = 0.65 vs. 0.54). The portion of SIC-derived CO₂ under wheat (13.0%) was higher than that under legume (11.3%). Moreover, the portion of SIC-derived CO₂ was 1.2% higher under wheat than under legume at high N fertilization level, whereas it was increased to 2.2% under low N fertilization. This indicates a significant role of plant species with different N demand and NUE on dynamics of SIC pool and its contribution in CO₂ emission from soil. The rate of SIC-derived CO₂ was comparable between wheat and legume under high N fertilization, but it was 1.6 times higher under wheat than that under legume at low N fertilization. The contribution of SIC-derived C to the atmosphere was ~63.7 g C m^-2 yr^-1 under legume with low N demand vs. ~82.1 g C m^-2 yr^-1 under wheat with high N demand. In this regard, the impacts of plant species and their N demand and NUE are important controlling factors determining the dynamics of the SIC pool in agroecosystems.