Greenhouse gas emissions and soil microbial activity for selected land use systems in floodplains
Authors:   , Ansari Jamshid 
Published: September 2022

According to the Intergovernmental Panel on Climate Change, concentrations of atmospheric greenhouse gases (GHG) have increased since the 1950s caused by human activities which have likely increased the chance of extreme events including flooding and droughts (IPCC, 2021). An estimated 23 percent of total anthropogenic greenhouse gas emissions (2007-2016) derive from agriculture, forestry, and other types of land use. Land management activities affect the soil microbiome and thus greenhouse gas (GHG) emissions from soil. Therefore, investigating the effect of land management practices on soil microbiota as well as soil greenhouse gas emissions helps to optimize agronomic activities and climate change mitigation. The objectives of this study were: 1) to determine the effect of land management practices on soil enzyme activity in three selected land management systems agroforestry [pecan (Carya illinoinensis) orchard/ hay (AF)], a riparian forest area (RF), and row-crop agriculture {corn [Zea mays L.]/soybean [Glycine max (L.) Merr.]} rotation (AG), in the Missouri River floodplains (MRF); 2) to assess the influence of AF, RF, and AG land management systems located in MRF on the emission of soil GHG; and 3) to evaluate the effects of various soil moisture conditions, saturated (FD), field capacity (FC), and fluctuation (FL) on microbial diversity in soils from AF, RF, and AG land management in the MRF. The results from the first study revealed significantly higher levels of [beta]-glucosidase, [beta]- glucosaminidase, and dehydrogenase activity in the AF and RF treatments relative to AG management. Dehydrogenase activity was higher (p [less than] 0.0001) in RF relative to AF and AG sites. Due to the accumulation of soil organic matter from tree roots and litterfall in tree-based systems the greatest mycorrhizal fungi and total fungal biomass were observed in the RF management followed by AF and AG, respectively. This study concluded that efforts to incorporate perennial management systems in the MRF landscapes will help increase organic matter content, which stimulates microbial diversity and soil enzyme activity as well as improving the performance of conservation buffers. The results of the second study showed that fertilizer application and soil saturation conditions after flooding events were the key drivers of soil N2O and CH4 emissions. The RF area remained a CH4 sink during the study period while a peak in CH4 flux in the AF and AG treatments was observed after flooding events. Cumulative soil GHG emissions from three selected land management were in the order of AF [greater than] RF [greater than] AG Mg CO2-C ha-1 yr-1 carbon dioxide equivalent (CO2eq). A greater cumulative soil N2O flux was observed in the AF treatment which was due to the fertilizer application and soil saturation condition after flooding events. Land management and climatic events affect soil physicochemical and microbial properties and soil GHG emissions consequently. No fertilizer application practices in row-crop agriculture can substantially reduce the risk of non-CO2 emissions from flooded soils. The third study outcomes revealed that in all three soil water regimes the dominant bacterial phyla were Proteobacteria, Actinobacteriota, and Acidobacteriota whose abundance differed with land use and water content. The greatest Proteobacteria community was observed in AF system under FD conditions. Bacterial community size was the greatest in RF (Fall samples) under FC soil water conditions. The bacterial richness of RF was significantly higher than AF and AG systems in Fall samples. The Shannon indices were similar in the three soils, indicating that there was no significant difference in bacterial community diversity. Although different soil moisture regimes (saturated, field capacity, and fluctuation) did not affect soil bacterial richness in three selected land management substantially, the greater soil C content and extensive root systems in treed areas could increase the abundance of some microorganisms like Proteobacteria and Bacteroidetes.