| dc.description.abstracteng | Rice (Oryza sativa L.) is one of the predominant food sources and is mainly cultivated on flooded paddy soils. However, finite and dwindling phosphate rock, the main source of phosphorus (P) fertilizer, becomes increasingly a threat to rice yield. Moreover, sustainable rice cultivation requires techniques that consume less irrigation water, i.e., alternate wetting and drying (AWD) regime, to cope with increasing water scarcity due to population growth, climate change, and water pollution. Soil flooding or drying leads to the rapid onset of reducing or oxidizing conditions, respectively. Changing redox conditions significantly affect the characteristics of redox-sensitive iron (Fe) minerals, which in turn influence the cycling of important elements such as P and carbon (C) in the soils by a multitude of redox, sorption/desorption, and (co)precipitation/dissolution processes. Biotic processes of the hydrolysis of organic P (Po) and C by enzymes play a key role in P and C dynamics. Although the ‘enzyme latch’ and ‘iron gate’ theories for regulating soil organic matter (SOM) transformation under changing redox conditions were proposed, it remains unclear whether these mechanisms require time (weeks to months) to have clear effects on enzymatic reactions, or whether a short-term oxygen (O2) exposure of samples adapted to anoxic conditions to O2 can elicit a rapid specific response in enzyme assays. This rapid response is expected to be negative due to the O2 toxicity for active anoxic microbial communities. Therefore, this thesis aims to exploring the biogeochemical mechanisms of the P and C cycling in rice-paddy soil ecosystem under fluctuating redox conditions. We hypothesized that (i) short-term (minutes to hours) aeration suppresses the activities of hydrolytic enzymes that catalyze organic matter mineralization, while medium-term (days to weeks) aeration has contrasting effects due to the adaptation of anoxic microbial communities to oxic conditions; (ii) the reductive dissolution of ferric Fe (Fe(III))-bound inorganic P (Fe–P) can contribute to the P nutrition of plants and microorganisms; and (iii) the AWD regime increases the contribution of Po from organic residues to P nutrition of rice plants and microorganisms.
Flooded paddy rice soil mesocosms were established to investigate (i) aeration effects on the activities of three hydrolytic enzymes contributing to P (phosphomonoesterase), C (β-glucosidase), and nitrogen (N; leucine aminopeptidase) turnover (Study 1 and 2) and two oxidative enzymes mainly involved in C cycling (phenol oxidases and peroxidases) (Study 1), (ii) aeration effects on the rhizosphere extent of growing rice and the percentage of hotspot area of three hydrolytic enzyme activities (Study 3), (iii) the quantitative contribution of the Fe–P dissolution to the demands of rice plants and microorganisms (Study 4 and 5), and (iv) the effects of the Fe(III) reduction on rice root growth and the allocation dynamics of P released from Fe-P with root growth (Study 6). Additionally, a mesocosm experiment with rice plants grown either in continuous flooding (CF) or AWD paddy soils was conducted to explore the effects of these water regimes on the dynamics of P release from the Fe-P and wheat straw (Study 7). For the latter, the quantitative contributions of the Fe-P dissolution and Po mineralization to the P nutrition of rice plants and microorganisms were estimated.
Short-term aeration (for ca. 2–2.5 h) suppressed the maximal reaction rate (Vmax) of hydrolytic enzymes by 5–43% but increased oxidative enzyme activities by 2–14 times in the Study 1. Short-term aeration (for 35 min) in the Study 3 decreased the in-situ activity of three hydrolytic enzymes by 4–61% and increased the percentage of hotspot area of enzyme activities by 3–158%. In contrast, medium-term aeration (after 10-day oxic vs. anoxic pre-incubation) increased Vmax of hydrolytic enzymes by 12–253% in the Study 2. Thus, the activation of oxidative enzymes by short-term aeration was uncoupled from the hydrolytic enzymes. This put to a question both the “iron gate” and “enzyme latch” mechanisms. The effect of aeration on anoxic enzyme activities ultimately depends on the aeration duration and controls sequential mechanisms starting from suppression of microbial activity and enzyme production followed by adaptation of microbial communities to newly established conditions.
To quantify relevance of the Fe-P dissolution for plant and microbial P uptake in the Studies 4 and 5, 32P-labeled ferrihydrite (31 mg P kg−1) was supplied either (1) in polyamide mesh bags (30 μm mesh size) to prevent roots but not microorganisms from direct Fe-P mobilization, or (2) directly mixed with soil to enable roots and microorganisms unrestricted access to the Fe-P. The contribution of Fe–P to microbial biomass P (MBP) remarkably decreased from 4.5% to almost zero from day 10 to 33 after rice transplantation. Nearly 2% of Fe–P compensated up to 16% of the plant P uptake 33 days after rice transplantation. Microbial biomass C (MBC) and dissolved organic C contents decreased from day 10 to 33 by 8–54% and 68–77%, respectively, suggesting that the microbially-mediated Fe(III) reduction was C-limited. A novel in-situ 32P phosphor-imaging approach under flooding was developed in the Study 6 to estimate P uptake by rice roots released from Fe-P dissolution. Direct root access to Fe-P raised both the number and mean diameter of crown roots and root tips, and increased P uptake by 149–231%. Therefore, rice varieties with extended crown root densities will increase P uptake when P is limiting in paddy soils.
To estimate the contributions of the Fe-P dissolution and Po mineralization to the P nutrition of plants and microorganisms under different water regimes in the Study 7, pre-germinated rice plants were grown for 40 days in rhizoboxes under either CF or AWD. 32P-labeled ferrihydrite (30 mg P kg–1) and 33P-labeled wheat straw (10 g straw kg–1) were added. Continuous flooding stimulated P release to the soil solution by increasing Fe(III) reduction and wheat straw mineralization compared to AWD. The proportions of P from Fe-P and straw in rice plants and MBP were 5–64% higher under CF vs. AWD regime. The contributions of Fe-P and straw to MBP were 72–78% lower and 16–42% higher in rooted vs. bulk soil, respectively, suggesting a strong competition between plants and microorganisms for P. The Fe-P and straw compensated up to 16% and 20% of the plant P uptake and 15% and 35% microbial P uptake 40 days after rice transplantation, respectively. Relatively high contribution of Po to the P nutrition of plants and microorganisms calls for the adaptation of the P fertilization strategies to avoid excess chemical P fertilizer inputs.
In summary, short-term and medium-term aeration had contrasting effects on hydrolytic enzyme activities, thus supporting the first hypothesis. The underestimation of hydrolytic enzyme activities due to short-term aeration bias may lead to a strongly skewed mechanistic understanding of SOM transformations in anoxic environments with follow-up complications for process-based modeling. The sensitivity of anoxic enzymes to the O2 adaptation mechanisms requires strong consideration for understanding the SOM dynamics in fluctuating O2 environments. Fe-P and wheat straw compensated up to 16% and 20% of plant P demand, respectively, thereby supporting the second hypothesis that Fe-P can serve as a P source. In reality higher than expected hydrolytic enzyme activities lead to considerable decomposition of fresh organic matter – one of the underlying mechanisms making straw as a P source even in anoxic systems. An increase in C availability for microorganisms intensifies P mobilization, which is especially critical at early stages of rice growth. Continuous flooding increased the contributions of P released from Fe-P and wheat straw to P nutrition compared to AWD, thus is against the third hypothesis that AWD increases the contribution of Po to P nutrition. This can be attributed to (i) decreased P availability caused by increasing P sorption capacity of soils after drying and (ii) increased competition between plants and microorganisms for P. Microorganisms appear to be more competitive for Po than rice plants, and rice plants are more competitive than microorganisms for P released from the Fe-P dissolution. Therefore, the P fertilization strategies should consider the P mobilization from Fe (oxyhydr)oxides and straw under different water regimes in paddy soils during rice growth.
Finally, this thesis developed new approaches dedicated to accuracy of enzyme activity measurements and to quantitative assessment of the dynamics of P uptake by plants under low redox conditions. Our studies also open further perspectives for the investigation of (i) the mechanisms of aeration effects on enzyme activities and (ii) the contribution of inorganic P and organic P to the nutrition balance and competition between microorganisms and plants under changing redox conditions. | de |