Hourly and daily rainfall intensification causes opposing effects on C and N emissions, storage, and leaching in dry and wet grasslands

Abstract

Climate change is expected to alter hourly and daily rainfall regimes and, in turn, the dynamics of ecosystem processes controlling greenhouse gas emissions that affect climate. Here, we investigate the effects of expected twenty-first century changes in hourly and daily rainfall on soil carbon and nitrogen emissions, soil organic matter (SOM) stocks, and leaching using a coupled mechanistic carbon and nitrogen soil biogeochemical model (BAMS2). The model represents various abiotic and biotic processes involving 11 SOM pools. These processes include fungal depolymerization, heterotrophic bacterial mineralization, nitrification, denitrification, microbial mortality, necromass decomposition, microbial response to water stress, protection, aqueous advection and diffusion, aqueous complexation, and gaseous dissolution. Multi-decadal modeling with varying rainfall patterns was conducted on nine Australian grasslands in tropical, temperate, and semi-arid regions. Our results show that annual CO 2 emissions in the semiarid grasslands increase by more than 20% with a 20% increase in annual rainfall (with no changes in the rainfall timing), but the tropical grasslands have opposite trends. A 20% increase in annual rainfall also increases annual N 2 O and NO emissions in the semi-arid grasslands by more than 10% but decreases emissions by at least 25% in the temperate grasslands. When subjected to low frequency and high magnitude daily rainfall events with unchanged annual totals, the semi-arid grasslands are the most sensitive, but changes in annual CO 2 emissions and SOM stocks are less than 5%. Intensification of hourly rainfall did not significantly alter CO 2 emissions and SOM stocks but changed annual NH 3 emissions in the tropical grasslands by more than 300%.

Abiotic and Biotic Controls on Soil Organo–Mineral Interactions: Developing Model Structures to Analyze Why Soil Organic Matter Persists

“…Moreover, organo-mineral interactions may also show contrasting behaviors; for example, a group of compounds falling into comparable intrinsic decomposition rates may have different affinities to soil minerals or responses to environmental conditions (Gordon and Millero 1985;Gu et al 1994;Mikutta et al 2007;Kleber et al 2011). Overall, we argue that there is a need to represent SOM compounds using their molecular structures in reactive transport models along with physical protection mechanisms (e.g., MAOM) and different functional groups of microbes (e.g., Riley et al 2014;Dwivedi et al 2017;Tang et al 2019. ) As discussed in the Introduction and the sction `Current Use of Reactive Transport Modeling of SOM Dynamic', SOM decomposition is governed by environmental conditions such as physical heterogeneity, physical disconnection, plant inputs, microbial diversity, and microbial activity, as well as the molecular structure of organic matter (Schmidt et al 2011;Riley et al 2014;Dwivedi et al 2017a).…”

“…There is a suite of SOM dynamics models incorporated into land models ranging from simple pool-based CENTURY-like representations (Parton et al 1987(Parton et al , 1998(Parton et al , 2010 to more complex models such as the Biotic and Abiotic Model of SOM (BAMS; Riley et al 2014;Dwivedi et al 2017;Tang et al 2019) and the COntinuous representation of SOC in the organic layer and the mineral soil, Microbial Interactions and Sorptive StabilizatION (COMISSION; Ahrens et al 2015), which explicitly represents physicochemical processes and microbially mediated reactions. Here we briefly describe the traditional and some of the emerging modeling approaches.…”

A Mechanistic Analysis of Wetland Biogeochemistry in Response to Temperature, Vegetation, and Nutrient Input Changes

“…The proposed BAMS3 reaction network (Figure ) is developed based on the C‐N coupled BAMS2 model (Tang et al, ), which integrates the C cycle (BAMS1) reaction network developed by Riley et al () and the N cycle in Maggi et al (). Here, BAMS2 was extended to include the anaerobic degradation of organic matter, the anaerobic ammonia oxidation (ANAMMOX), and was coupled to a newly developed sulfur biogeochemical cycle.…”

“…BAMS3 consists of four organic polymer pools (lignin, cellulose, hemicellulose, and peptidoglican), eight organic monomer pools (monosaccharide, fatty acids, organic acids, phenols, nucleotides, amino acids, amino sugars, and organic sulfur), two inorganic carbon species (CO 2 and CH 4 ), seven inorganic nitrogen species (NH 3 , NH , NO , NO , NO, N 2 O, and N 2 ), five inorganic sulfur species (SO , SO , S 2 O , S 0 , and HS − ), and 13 microbial functional groups including fungi ( F DEP ), heterotrophic bacteria ( B AER ), methanogenic bacteria ( B MGB ), methanotrophic oxidizing bacteria ( B MOB ), ammonia‐oxidizing bacteria ( B AOB ), nitrite‐oxidizing bacteria ( B NOB ), denitrifying bacteria ( B DEN ), anaerobic ammonia oxidation bacteria ( B ANMX ), sulfur reducing bacteria ( B SrRB ), thiosulfate‐ and sulfide‐reducing bacteria ( B ThSRB ), sulfate‐reducing bacteria ( B SRB ), thiosulfate‐ and sulfide‐disproportioning bacteria ( B SDB ), and photolithoautotroph‐oxidizing bacteria ( B SOB ). The C‐N coupled cycle is described in Tang et al () and includes depolymerization, mineralization, nitrification, denitrification, nitrogen fixation, and microbial immobilization, while detailed description of the C and N cycles can be found in Riley et al () and Maggi et al (). In addition to those processes, BAMS3 includes the anaerobic monosaccharide degradation mediated by methanogenic bacteria ( B MGB ) that results in CH 4 production (R13, in Figure ), CH 4 oxidation by methanotrophs ( B MOB ) (R14 in Figure ), and the ANAMMOX by anaerobic ammonnia oxidation bacteria ( B ANMX ) (R25 in Figure ), following reaction from Strous et al () as …”

“…This study aims to conduct a comprehensive mechanistic modeling analysis of the soil organic matter (SOM) dynamics coupled with N and S cycles to explore the biogeochemistry of a wetland and investigate the extent to which GHG emissions are affected by multiple environmental and anthropocentric changes. We improved the BAMS2 C‐N coupled model developed in Tang et al () by including the anaerobic ammonia oxidation (ANAMMOX) and CH 4 production, transport, and oxidation reactions, and by coupling these to the S cycle to understand how essential is its role in mitigating CH 4 emissions in wetlands. The C‐N‐S reaction network presented here is called BAMS3 (Biotic and Abiotic Model for SOM version 3) and it involves 12 organic pools (SOM), 13 microbial functional groups, 15 inorganic species of C, N, and S, and plant nitrogen, sulfur, and CH 4 uptake.…”