Publications

Rice paddies supply half the global population with staple food, but also account for ~48% of greenhouse gas (GHG) emissions from croplands. In this Review, we outline the characteristics of GHG emissions (CH4 and N2O) from paddy soils, focusing on climate change effects and mitigation strategies. Global mean annual area-scaled and yield-scaled GHG emissions are ~7,870 kg CO2e ha−1 and 0.9 kg CO2e kg−1, respectively, with 94% from CH4. However, emissions vary markedly, primarily reflecting the impact of management practices. In particular, organic matter additions and continuous flooding of paddies both stimulate CH4 emissions, whereas fertilizer N application rate is the most important driver of N2O emissions. Although contemporary changes in emissions are uncertain, future elevated [CO2] and warming are projected to increase CH4 emissions by 4–40% and 15–23%, respectively. Yet, integrated agronomic management strategies — including cultivar, organic matter, water, tillage and nitrogen management — offer GHG mitigation potential. In particular, new rice variety selection, non-continuous flooding and straw removal strategies reduce GHG emissions by 24%, 44% and 46% on average, respectively. However, approaches need to be optimized on the basis of seasonal CH4 emission patterns, necessitating improved quantification and reduced uncertainty in regional and global GHG estimates, especially in low latitudes. Rice paddies account for a large proportion of total agricultural methane and nitrous oxide emissions. This Review outlines the characteristics, changes and mitigation options for these emissions, highlighting the benefits of water and organic matter management.

Conservation tillage has been adopted worldwide as an attractive alternative to conventional tillage. However, suitable conservation tillage for increasing soil organic carbon (SOC) and crop yield simultaneously is still limited. Two conservation tillage methods, subsoiling to the 40 cm depth (ST) and no-tillage (NT), were combined with three straw return treatments (i.e., no return [−0], return of whole wheat (<i>Triticum aestivum</i> L.) straw and 1 m high maize (<i>Zea mays</i> L.) stubble [−1], and return of whole wheat and maize straw [-a]) to study their impacts on SOC content, labile C fractions, and crop yields, with conventional tillage (CT) used as a control in a 15-year field experiment. Subsoiling with 1 m high maize stubble return (ST–1) increased the mean annual grain yields by 18% and the mean SOC content by 39% at the 0 to 100 cm depth compared with conventional tillage with no maize straw return (CT–0) in 2016 and 2017. The mean SOC at the 0 to 100 cm depth of the NT treatment was lower than those of ST and CT because of the reduced transformation from straw to SOC and labile C fractions. One meter high maize stubble return can maintain high SOC content, C fractions, and crop yield compared with whole maize straw return. Thus, subsoiling combined with 1 m high maize stubble return was an effective conservation tillage to increase the SOC content and crop yield.

. Tidal wetlands, such as tidal marshes and mangroves, are hotspots for carbon sequestration. The preservation of organic matter (OM) is a critical process by which tidal wetlands exert influence over the global carbon cycle and at the same time gain elevation to keep pace with sea-level rise (SLR). The present study assessed the effects of temperature and relative sea level on the decomposition rate and stabilization of OM in tidal wetlands worldwide, utilizing commercially available standardized litter. While effects on decomposition rate per se were minor, we show strong negative effects of temperature and relative sea level on stabilization, as based on the fraction of labile, rapidly hydrolyzable OM that becomes stabilized during deployment. Across study sites, OM stabilization was 29 % lower in low, more frequently flooded vs. high, less frequently flooded zones. Stabilization declined by ∼ 75 % over the studied temperature gradient from 10.9 to 28.5 ∘C. Additionally, data from the Plum Island long-term ecological research site in Massachusetts, USA, show a pronounced reduction in OM stabilization by &gt; 70 % in response to simulated coastal eutrophication, confirming the potentially high sensitivity of OM stabilization to global change. We therefore provide evidence that rising temperature, accelerated SLR, and coastal eutrophication may decrease the future capacity of tidal wetlands to sequester carbon by affecting the initial transformations of recent OM inputs to soil OM.

The Tibetan Plateau hosts the world’s largest alpine pastoral ecosystems, dominated by the endemic sedges Kobresia pygmaea and Kobresia humilis. Owing to the very harsh environment and also to soil nitrogen (N) and phosphorus (P) limitations, these pastoral ecosystems are very sensitive to disturbances (e.g. anthropogenic activities and climate change) and recover extremely slowly. Overgrazing on the Tibetan Plateau has caused severe degradation of vegetation and soils in the last 30–50years. For the first time, for Kobresia pastures in Tibetan Plateau, we have summarized and generalized the consequences of pasture degradation for soil organic carbon (SOC) and nutrient (N, P) stocks, and evaluated the main biotic and abiotic mechanisms of their loss. Based on 44 literature studies as well as own data, we demonstrated that 42% of SOC stocks were lost, relative to non-degraded pastures. These SOC losses are similar to the decreases in N stocks (-33%), and aboveground (-42%) and belowground (-45%) plant biomass. Although P losses are lower (-17%), its precipitation reduces its availability for plants. These losses are in fact underestimates, since undisturbed natural sites no longer exist on the Tibetan Plateau. The losses are much higher in the upper 10cm and in some areas extend to complete removal of soil cover. This has dramatic repercussions for local livestock, human populations and river pollution. While some rehabilitation projects have shown positive outcomes, the complete recovery of degraded pastures (e.g. soil fertility, ecosystem stability) is infeasible, because of very slow pedogenic processes, slow vegetation restoration, as well as continuously increasing anthropogenic pressure and climate change. Considering the rapid losses of SOC and nutrients, and the very slow recovery potential, Tibetan pastures in some regions may disappear in the next few decades without proper and effective recovery strategies.

Aggregate stability is important for soil architecture; however, the specific response to long-term fertilization in upland and paddy fields are unknown. H

No abstract is provided for this article.

Mikrobieller Abbau ist ein wesentlicher Transformationsprozess der organischen Bodensubstanz. Sorption von niedermolekularen organischen Substanzen wie Alanin an Mineraloberflachen kann diese vor dem mikrobiellen Abbau und Mineralisierung schutzen. Somit ware Sorption ein entscheidender Stabilisierungsprozess fur die organische Bodensubstanz. Dennoch ist es fur Mikroorganismen moglich, niedermolekulare organische Substanzen von Mineraloberflachen zu desorbieren. Das exakte Prozessgefuge dieser Desorption und mikrobiellen Verwertung, aber auch die involvierten mikrobiellen Gruppen sind jedoch weitestgehend unbekannt. Wir verwendeten positionsspezifisch C-13-markiertes Alanin, dessen Mineralisation zu CO2 und dessen Einbau in Phospholipid-Fettsauren (PLFAs) quantifiziert wurde, um die mikrobielle Verwendung von sorbiertem und nicht-sorbiertem Alanin im Boden zu unterscheiden. Um die durch die Sorption bedingten Veranderungen der Verfugbarkeit und Metabolisierung des Alanins zu erfassen, wurden einheitlich- und positionsspezifisch C-13- und C-14-markiertes Alanin in einem Ap-Horizont einer lehmigen Parabraunerde uber 10 Tage inkubiert. Das CO2 aus der Respiration wurde in NaOH-Fallen erfasst und dessen C-14-Aktivitat bestimmt. Die Verwendung der funktionellen Gruppen des Alanins durch die verschiedenen mikrobiellen Gruppen wurde mittels C-13-PLFA Analyse ermittelt. Keine der mikrobiellen Gruppen bevorzugte das unsorbierte Alanin gegenuber dem sorbierten – der Grosteil der Gruppen inkorporierte die gleiche Menge in ihre PLFAs. So zeigten beispielsweise Gram negative Bakterien eine hohe Wettbewerbsfahigkeit hinsichtlich des Alanins, allerdings keine Praferenz bezuglich des Unsorbierten. Nur die Gruppe der Pilze inkorporierten signifikant mehr sorbiertes Alanin: Sie sind pradestiniert dafur, Mineraloberflache zu umwachsen und daran sorbierte Substanzen mit ihren Hyphen aufzunehmen. Diese Praferenz ist mit einem veranderten Abbauweg, der uber Glukoneogenese und den Pentose-Phosphat-Weg fuhrt, verbunden. Sorption kann zu einer kurzfristigen Verzogerung des Abbaus kleiner geladener Molekule fuhren, jedoch ist kein langfristiger Stabilisierungseffekt erkennbar. Die meisten mikrobiellen Gruppen nehmen sowohl sorbiertes und nicht-sorbiertes Alanin im gleichen Mase auf und nur einzelne Gruppen, wie Pilze, haben sich auf die praferentielle Aufnahme sorbierter Substanzen spezialisiert.

Iron oxides affect the stability of soil organic matter (SOM), which in turn affects greenhouse gas emissions in paddy soils. They also regulate the direction and magnitude of the rhizosphere priming effect (RPE) by restricting SOM accessibility and microbial activity. However, the controlling steps and key factors that regulate the RPE magnitude under anoxic conditions are unknown. In this study, we investigated the mechanisms through which Fe(III) reduction affects the RPE using humic acid as an electron shuttle in paddy soils and conducting continuous 13CO2 labeling of rice plants. The RPE, measured via CO2 emission, was approximately 25 % greater in soils with humic acid than in soils without. A rapid increase in the RPE of CH4 emissions after 41 days was attenuated in soils containing humic acid. Root growth and Fe(III) reduction stimulated the total primed CO2 emissions from the rhizosphere independent of the microbial biomass and enzyme activities. Humic acid accelerated Fe(III) reduction, leading to a decrease in Fe-bound organic carbon and an increase in RPE (CO2 emissions). The rhizosphere-primed CO2 emissions decreased with increasing amounts of reactive Fe(III) (oxyhydr)oxides, which protected the SOM from microbial and enzymatic attacks. Biochemical Fe(III) reduction and physical aggregate destruction controlled the abiotic transformation of inaccessible SOM into bioavailable organic carbon, thereby regulating the RPE. The results suggest that the reduction of reactive Fe(III) minerals is the rate-limiting step in the release of the physicochemically protected SOM, which in turn determines the magnitude of rhizosphere priming in paddy soils.

Summary The most modern molecular approaches fail to link structure of soil microbial community with its functions. We used classical physiological approaches based on (i) microbial growth kinetics and (ii) microbial affinity to the substrate to show the shift in functional properties of the soil microbial community after amendments with substrates of contrasting availability. Kinetic parameters of substrate‐induced respiration and substrate availability in soil were monitored during the growth of maize, and decomposition of glucose or maize straw. Input of small amounts of easily available substrates (i.e. glucose or root exudates) significantly increased the specific growth rates of soil microorganisms by up to 13 and 20%, respectively. This increase, showing the shift from K to r strategies, was confirmed by a 50% decrease in the affinity of microorganisms to the easily available substrates. In contrast, maize straw lowered specific growth rates by 16–30% and increased the affinity of microorganisms to the substrate by 23–131% compared with untreated soil. After maize straw addition, the shift of the microbial community to K strategy was accompanied by an increase in the fraction of microbial biomass responding to glucose addition by immediate growth. The generation time of this fraction was 1.8 to 2.8 hours, which was 100 to 1000 times faster than that of the whole microbial community. Easily available carbon (C) in soil amended with maize straw comprised only half of that extracted by 0.05 m K 2 SO 4 . Therefore, we conclude that C extracted by 0.05 m K 2 SO 4 from the soil with maize straw residues had a lower availability for microorganisms as compared with glucose. Adding N to the soil prolonged the intensive mineralization period of plant residues, decreased specific microbial growth rates and increased the amount of easily available C in the soil. This indicates a more efficient use of maize residues by K‐selected microorganisms after the removal of N limitation. Combination of two complementary physiological approaches based on microbial growth kinetics and substrate affinity showed contrasting effects of easily and less available substrates on the shift of growth strategies (r vs. K) of the whole microbial community. These approaches are also suitable for estimation of microbial availability of indigenous C in the soil.

Summary The complexity of soil food webs and the cryptic habitat hamper the analyses of pools, fluxes and turnover rates of carbon (C) in organisms and the insight into their interactions. Stable isotope analysis has been increasingly used to disentangle soil food web structure, yet it has not been applied to quantitatively characterize C dynamics at the level of the entire soil food web. The present study employed 13 CO 2 pulse labelling to investigate the incorporation of maize root‐derived C into major soil compartments and food web players in an arable field for 25 days. Bulk tissue and compound‐specific (lipids) C isotope ratios were used to quantify pool sizes and 13 C incorporation in bacteria and fungi as primary decomposers, nematodes as key drivers of the microfood web and decomposers and predators among the meso‐ and macrofauna. About 20% of the C assimilated by maize was transferred to below‐ground pools. 13 C was predominantly incorporated into rhizosphere micro‐organisms rather than in those of the bulk soil. 13 C in phospholipid fatty acid biomarkers revealed that root‐derived C was incorporated into the soil food web mainly via saprotrophic fungi rather than via bacteria. Only small amounts of 13 C were transferred to higher trophic levels, predominantly into fungal‐feeding nematodes and macrofauna decomposers. Most importantly, C pool size and 13 C incorporation did not match closely. Although the fungal C stock was less than half that of bacteria, C transfers from fungi into higher trophic levels of the fungal energy pathway, that is fungal‐feeding nematodes and meso‐ and macrofauna decomposers, by far exceed that of bacterial C. This challenges previous views on the dominance of bacteria in root C dynamics and suggests saprotrophic fungi to function as major agents channelling recent photoassimilates into the soil food web.

No abstract is provided for this article.