The anaerobic oxidation of methane (AOM) in marine ecosystems is ubiquitous and largely coupled to sulfate reduction. In contrast, the role of AOM in terrestrial environments and the dominant electron acceptors driving terrestrial AOM needs deeper understanding. Submerged rice paddies with intensive CH4 production have a high potential for AOM, which can be important for greenhouse gas mitigation strategies. Here, we used 13CH4 to quantify the AOM rates in paddy soils under organic (Pig manure, Biochar) and mineral (NPK) fertilization. Alternative-to-oxygen electron acceptors for CH4 oxidation, including Fe3+, NO3 −, SO4 2−, and humic acids, were examined and their potential for CH4 mitigation from rice paddies was assessed by 13CH4 oxidation to 13CO2 under anoxic conditions. During 84 days of anaerobic incubation, the cumulative AOM (13CH4-derived CO2) reached 0.15–1.3 μg C g-1 dry soil depending on fertilization. NO3- was the most effective electron acceptor, yielding an AOM rate of 0.80 ng C g-1 dry soil h-1 under Pig manure. The role of Fe3+ in AOM remained unclear, whereas SO42- inhibited AOM but strongly stimulated the production of unlabeled CO2, indicating intensive sulfate-induced decomposition of organic matter. Humic acids were the second most effective electron acceptor for AOM, but increased methanogenesis by 5–6 times in all fertilization treatments. We demonstrated for the first time that organic electron acceptors (humic acids) are among the key AOM drivers and are crucial in paddy soils. The most pronounced AOM in paddy soils occurred under Pig manure, followed by Control and NPK, while AOM was the lowest under Biochar. We estimate that nitrate (nitrite)-dependent AOM in paddy fields globally consumes ~3.9 Tg C–CH4 yr-1, thereby offsetting the global CH4 emissions by ~10–20%. Thus, from a broader agroecological perspective, the organic and mineral fertilizers control an important CH4 sink under anaerobic conditions in submerged ecosystems. Appropriate adjustments of soil fertilization management strategies would therefore help to decrease the net CH4 flux to the atmosphere and hence the global warming.
Tropical mountain ecosystems cover a broad variety of climatic and vegetation zones and are global hotspots of biodiversity. These ecosystems are severely threatened by climate and land-use change, which also strongly affect soil properties. Mt. Kilimanjaro, with its large elevation gradient and relatively homogeneous geology of volcanic rocks and ashes, provides a unique opportunity to study and interpret soil organic matter (SOM) responses to climatic changes. Our objectives were to identify key SOM compounds in six elevation zones (covering ecosystems from tropical dry-lowland, through montane forest, up to alpine heathlands) that are affected by the climatic changes along a 3400 m transect, and to relate these SOM changes to ecosystem specific characteristics. The SOM composition in four topsoils (0–10 cm) samples per ecosystem was characterized by analytical double-shot pyrolysis-gas chromatography–mass spectrometry (Py-GC/MS). Evolving-gas analysis-mass spectrometry (EGA-MS) was used to quantitatively compare the thermal desorption (first shot: <280 °C) and pyrolysis step (second shot: 280–600 °C). The percentage of thermally desorbed compounds increased 10 fold from lowland (<900 m) to mountain forest soils (>2000 m), followed by a 40% decrease in alpine ecosystems (>4000 m). Alkanes/-enes/-ols contributed between 4% and 30% to the identified SOM composition, with a maximum at mid elevation (2120 m). Fatty acids and fatty acid esters contributed with <3% to SOM composition and decreased to a minimum of <1% at Podocarpus forest soils (2900 m), followed by a re-increase at higher elevation. The percentage of lignin-derived compounds followed a similar pattern but also responded to reduced woody inputs above the tree line. Two main factors that seem to affect SOM quality and composition at Mt. Kilimanjaro were: 1) the rate and composition of organic matter inputs that in turn are controlled by climatic characteristics and the vegetation type and 2) the decomposition rate and efficiency, mainly controlled by soil pH, temperature and water availability. High forest productivity at mid elevations (2200 m) leads to high amounts of volatile compounds and increases stable SOM pools. The overall carbon accumulation in Andosols of Mt. Kilimanjaro is linked to the percentage of bound lipids (mainly alkanes, alkenes and alcohols), while site specific input patterns (e.g. vegetation or wild fires) are strongly reflected by sterols, lignin derived compounds or polycyclic aromatic compounds in SOM.
Psychrotolerant microbes are crucial for carbon cycling and biotechnological applications. Nonetheless, the mechanisms enabling their survival and function
&lt;p&gt;Biogeochemical cycles of phosphorus (P) and iron (Fe) are tightly intertwined, especially in highly weathered and acidic subtropical and tropical soils rich in ferric Fe (Fe(III))oxides. In low-redox and P-deficient paddy soils, the quantitative contribution of the reductive dissolution of Fe(III)-bound P (Fe-P) to the demands of rice plants (&lt;em&gt;Oryza sativa&lt;/em&gt;&amp;#160;L.) and microorganisms remains unclear.&amp;#160;We hypothesized that Fe(III) reductive dissolution can cover the P demand of microorganisms but not of rice plants during the initial growth stages, when P demand is high but the root system is still limited.&amp;#160;We grew pre-germinated rice plants for 33 days in flooded rhizoboxes filled with a paddy soil&amp;#160;of poor P availability. &lt;sup&gt;32&lt;/sup&gt;P-labeled orthophosphate sorbed to ferrihydrite (80 kg ha&lt;sup&gt;-1&lt;/sup&gt;) was supplied either&amp;#160;(1) in&amp;#160;polyamide mesh bags (30 &amp;#956;m mesh size) to prevent roots from directly mobilizing Fe-P&amp;#160;(Pellets-mesh bag treatment), or&amp;#160;(2)&amp;#160;in the form of pellets directly to the soil without mesh bags to enable roots&amp;#8217; accessing the&amp;#160;Fe-P (Pellets-no-mesh bag&amp;#160;treatment). With&amp;#160;the application of Fe-P directly to the soil,&amp;#160;P was more available resulting in the increases in microbial biomass carbon (MBC) by 18&amp;#8211;55% and&amp;#160;nitrogen (MBN) by 4&amp;#8211;108% in rooted soil as&amp;#160;compared to the pellet not available to roots directly.&amp;#160;The maximum enzyme activities (V&lt;sub&gt;max&lt;/sub&gt;) of phosphomonoesterase and &amp;#946;-glucosidase followed this pattern.&amp;#160;During rice root growth, MBC&amp;#160;and&amp;#160;microbial biomass phosphorus (MBP) in both rooted and bottom bulk soil&amp;#160;gradually decreased by 28&amp;#8211;56% and&amp;#160;47&amp;#8211;49%, respectively. In contrast to our hypothesis, the contribution of Fe-P to MBP strongly decreased from 4.5% to almost zero during 10&amp;#8211;33 days after rice transplantation, while&amp;#160;Fe-P compensated up to 16% of the plant P&amp;#160;uptake 33 days after rice transplantation, thus outcompeting microorganisms.&lt;/p&gt;
&lt;p&gt;Microbial transformation of organic substances is a key process of soil organic matter (SOM) formation. Carbon (C) entering the soil can be transformed in three main directions: i) stabilization over long period without relevant microbial utilization, ii) recycling by microorganisms for production of new and reparation of old cells, and iii) microbial utilization for energy production leading to C losses from soil as CO&lt;sub&gt;2&lt;/sub&gt;. So, individual compounds within huge diversity of the organic substances entering the soil will follow predominantly one of these directions, depending on the substance chemistry, soil properties, microbial activities and environmental conditions. Therefore, organic substances can have two general trends: i) they converge from any initially distinct compounds (e.g. in litter or rhizodeposition) to completely mixed, so that it is impossible to trace back their origin; or ii) divergence: the substances maintain their differences despite microbial transformations by SOM formation.&lt;/p&gt;&lt;p&gt;We proved two opposite hypotheses that convergence and divergence of the fate of organic substances depends on microbial utilization at two levels: 1) intermolecular: high recycling intensity leads to convergence, whereas stabilization leads to divergence of the C originated from various organic compounds, and 2) incorporation of C from various molecule positions into microbial metabolic cycles define the C fate at intramolecular level. We tested the first hypothesis based on own and literature data to the fate of polymeric substances: sugars, proteins, lipids and lignin. The second hypothesis was tested by the C atoms from various positions of pentoses and hexoses by position-specific &lt;sup&gt;13&lt;/sup&gt;C and &lt;sup&gt;14&lt;/sup&gt;C labeling.&lt;/p&gt;&lt;p&gt;The polymeric substances as well as monomers from the same chemical group clearly converge to three groups stabilization, recycling and losses. Carboxylic acids will be nearly completely mineralized and are lost from soil. The fate and functions organic compounds depend mainly on microbial recycling. Proteins, amino acids and sugars - key components of microbial biomass - are intensively recycled and e.g. proteins remain relatively long in soil.&lt;/p&gt;&lt;p&gt;For the intramolecular differences, we traced the fate of position-specific &lt;sup&gt;13&lt;/sup&gt;C labeled glucose and ribose under field conditions for 800 days. Both sugars were simultaneously metabolized via glycolysis and pentose phosphate pathway. The similarity between position-specific &lt;sup&gt;13&lt;/sup&gt;C recovery in microbial biomass and soil reflected high contribution of microbial necromass to SOM. The mean residence time of uniformly labeled &lt;sup&gt;13&lt;/sup&gt;C ribose in the soil was 3 times longer than that of glucose. Consequently, ribose and glucose were incorporated into different cellular components, defining their long-term fate in soil. The convergence of glucose C positions in soil and microbial biomass revealed that recycling dominated glucose transformation. In contrast, divergence of ribose C positions in soil revealed that intact ribose-derived cell components are reused or preserved in SOM.&lt;/p&gt;&lt;p&gt;Thus, convergence vs. divergence distinguished the two general trends explaining the long persistence of C at inter- and intra-molecular levels: microbial recycling leads to convergence, whereas slow decomposition and preservation define the divergence of C pathways in soil.&lt;/p&gt;
