title: Paddy soil C turnover affected by fertilization and water management
Creators: name: Liu, Qiong
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Paddy soil
subject: wet-drying
subject: fertilization
subject: microbial community composition
subject: straw addition
subject: soil depth
subject: organic manure
subject: priming effect
subject: 550

description: Rice feeds more than half of the world’s population; in addition, artificial rice paddies are an important contributor to global carbon emissions. Carbon turnover (production and oxidation) primarily comprises microbially mediated processes, and microorganisms (abundances and communities) are strongly affected by anthropogenic activities in paddy soils, such as fertilization and water management. In turn, microbial abundance and community shifts can strongly influence soil C turnover by affecting soil organic matter (SOM) decomposition through priming effects (PEs). This thesis aimed to gain a more comprehensive understanding of C turnover (CO2 and CH4) in paddy soils by determining the temporal dynamics of CO2 and CH4 and disentangling the underlying mechanisms. Uncertainty still exists about the contributions of the two main pathways (acetoclastic methanogenesis and hydrogenotrophic methanogenesis) to CH4 production throughout the rice growing season, especially under field conditions. To fill this knowledge gap, in Study 1 and 2, we took soil gas samples using passive diffusion gas samplers to determine the temporal dynamics of CO2 and CH4. Since labile C input affects microbial processes in soil, the C allocation from above- to belowground pools was quantified by 13CO2 pulse labelling of rice plants. Of particular interest was the result of the natural abundance 13C enrichment of CO2 over time under flooded conditions, indicating that hydrogenotrophic methanogenesis continuously contributed to CH4 production. Alternating wet-dry cycles (AWD) resulted in a significant decrease in CH4 concentrations, along with conspicuous isotopic signals indicative of CH4 oxidation. However, the time and magnitude of AWD should be carefully considered as it can reduce rice yield. Sulfate fertilizer had positive effects on rice plant biomass and grain yield, though it showed no effect on lowering CH4 concentrations. Especially under AWD conditions, sulfate fertilizer increased shoot biomass and stabilized grain production. Additionally, based on 13CO2 pulse labelling results, more than 50% of newly assimilated C was retained aboveground at grain-filling stage (14 days after pulse labelling), which is likely explained by a high C demand for fruit production. We did not detect any effect of sulfate addition and AWD on 13C allocation in the plant-soil system. In addition to these short-term effects of fertilizer addition, in Study 3, soil samples were collected from a long-term experimental trial and analyzed for microbial biomass and community composition. Both mineral and organic fertilization can prevent microbial biomass from decreasing vertically. In addition, Gram-positive (G+) bacteria benefited the most from mineral fertilizers, and the partial replacement of mineral fertilizer with manure primarily enhanced the abundance of G+ bacteria at 0−30 cm soil depth. In contrast, replacement with straw particularly enhanced the abundances of fungi at 10−20 cm soil depth, which is explained by the key role of fungi in straw decomposition. Study 4 was designed to investigate how the changes of microbial activity and communities, shown in Study 3, affect SOM decomposition through PE. 13C-glucose was added to incubated soils to mimic C input by rhizodeposition. Following glucose addition, SOM-derived microbial biomass C decreased at 0–10 cm in all soils (apparent PE). It was suggested that in upper soil depths with frequent C input through rhizodeposition and organic fertilizers, microorganisms focused on renewing their C rather than investing in growth after substrate addition. N mining mechanism suggests that primed CO2 is higher under nutrient-limited conditions as microorganisms produce extracellular enzymes to mine SOM for limiting nutrients. Our results fit well with this mechanism, as lower percentage primed CO2 (PECO2) was observed in soil with balanced nutrient conditions (NPK). In contrast, in low nutrient (unfertilized) or extra C (organic fertilized) conditions, PECO2 was higher because of the demand for nutrients. However, the higher positive primed-DOC together with weak PECO2 in topsoil than subsoil cannot be explained by N mining. Therefore, we proposed a mineral-related mechanism: glucose addition increased DOC by release of mineral-bound C biotically and/or abiotically. Apart from accelerating SOM decomposition, positive PECO2 can also be achieved by direct utilization of those released C. However, after 20 days of incubation, the organic C concentration was reduced by rebinding or co-precipitating to mineral surfaces, which can explain the negative PE at later incubation phases commonly observed in paddy soils. In summary, this thesis extends our understanding of plant-microbe interactions on CO2 and CH4 turnover in paddy soils. In addition, we drew attention to complex mechanisms of C turnover in submerged paddy soils: 1) apparent PE, 2) biotic and/or abiotic release of mineral-bound OC, 3) negative PE at later incubation phases. Considering the mineral-associated mechanisms (substrate desorption and resorption with soil matrix) in further investigations is crucial, as they alter substrate availability to microorganisms and thus affect soil C stocks. Moreover, the studied mechanisms are vital for maintaining food security and mitigating global warming through adaptations in management practices in rice cropping systems.