Controls of rainfall patterns on C and N emissions and stocks in Australian grasslands

In a warmer climate, regional and global climate models have projected Australia to experience an increase in the intensification of rainfall extremes, which range from heavy monsoon rains to long droughts. Here, we use a coupled carbon and nitrogen cycles mechanistic model (BAMS2) to investigate how the projected rainfall patterns affect carbon and nitrogen emissions, as well as the soil organic stocks in Australian grasslands located in different climatic regions.

                     The BAMS2 model (Tang et al., 2019) considers the depolymerization and mineralization of 11 soil organic matter (SOM) pools (i.e., lignin, cellulose, hemicellulose, peptidoglycans, monosaccharides, amino acids, amino sugars, organic acids, lipids, nucleotides, and phenols) and the transformation of inorganic nitrogen through fixation, nitrification, and denitrification. We explicitly model the growth, mortality, necromass decomposition, and water stress response of five microbial functional groups that mediate the carbon and nitrogen cycles. We include a simplified plant dynamics model to describe plant nutrient uptake, SOM inputs through root exudations and aboveground litter, and plant response to water stress. The BAMS2 reaction network is solved using a general-purpose multi-phase and multi-component bio-reactive transport simulator (BRTSim-v3.1a). We model the water flow along a vertical soil column using the Richards equation and the Brooks-Corey model for the water saturation-tension-permeability relationships, while the transport of dissolved chemicals is modeled using Darcy’s advection velocity and Fick’s diffusion. Aqueous complexation and gas dissolution are described using the mass action law, and SOM protection to soil is modeled using Langmuir’s kinetics.

                     Our multi-decadal simulations suggest a 30% increase in annual CO­₂ emissions in tropical grasslands with a 20% decrease in annual rainfall amount, while temperate and semi-arid grasslands have opposite trends. A decreasing annual rainfall amount also results in a decrease in annual N­_­2O emissions in the semi-arid grasslands, and a decrease in soil organic stocks in all grasslands. In tropical grasslands, a 20% decrease in annual rainfall results in approximately a 10% decrease in soil organic carbon stock and less than 1% decrease in soil organic nitrogen stock. Less frequent and more intense events in the semi-arid grasslands lead to increased soil moisture at greater depths where evapotranspiration rates are lower, hence reducing water loss to atmosphere and allowing the storage of water for plant growth. Our results show that changes in rainfall regimes alter both the emissions and the total amount of SOM. This study provides a modeling framework suitable for investigating SOM dynamics under various climatic and anthropogenic forcing.

Acknowledgement: This work is supported by SREI2020 EnviroSphere program, the University of Sydney.

Reference:

Tang, F. H.M., Riley, W. J., & Maggi, F. (2019). Hourly and daily rainfall intensification causes opposing effects on C and N emissions, storage, and leaching in dry and wet grasslands. Biogeochemistry, 1-18, https://doi.org/10.1007/s10533-019-00580-7.