To overcome phosphorus (P) deficiency, about 80% of plant species establish symbiosis with arbuscular mycorrhizal fungi (AMF), which in return constitute a major sink of photosynthates. Information ...on whether plant carbon (C) allocation towards AMF increases with declining availability of the P source is limited. We offered orthophosphate (OP), apatite (AP), or phytic acid (PA) as the only P source available to arbuscular mycorrhiza (AM) (
Solanum lycopersicum x Rhizophagus irregularis
) in a mesocosm experiment, where the fungi had exclusive access to each P source. After exposure, we determined P contents in the plant, related these to the overall C budget of the system, including the organic C (OC) contents, the respired CO
2
, the phospholipid fatty acid (PLFA) 16:1ω5c (extraradical mycelium), and the neutral fatty acid (NLFA) 16:1ω5c (energy storage) at the fungal compartment. Arbuscular mycorrhizal (AM) plants incorporated P derived from the three P sources through the mycorrhizal pathway, but did this with differing C-P trading costs. The mobilization of PA and AP by the AM plant entailed larger mycelium infrastructure and significantly larger respiratory losses of CO
2
, in comparison with the utilization of the readily soluble OP. Our study thus suggests that AM plants invest larger C amounts into their fungal partners at lower P availability. This larger C flux to the AM fungi might also lead to larger soil organic C contents, in the course of forming larger AM biomass under P-limiting conditions.
The Tibetan Plateau's Kobresia pastures store 2.5% of the world's soil organic carbon (SOC). Climate change and overgrazing render their topsoils vulnerable to degradation, with SOC stocks declining ...by 42% and nitrogen (N) by 33% at severely degraded sites. We resolved these losses into erosion accounting for two-thirds, and decreased carbon (C) input and increased SOC mineralization accounting for the other third, and confirmed these results by comparison with a meta-analysis of 594 observations. The microbial community responded to the degradation through altered taxonomic composition and enzymatic activities. Hydrolytic enzyme activities were reduced, while degradation of the remaining recalcitrant soil organic matter by oxidative enzymes was accelerated, demonstrating a severe shift in microbial functioning. This may irreversibly alter the world´s largest alpine pastoral ecosystem by diminishing its C sink function and nutrient cycling dynamics, negatively impacting local food security, regional water quality and climate.
Microaggregates in soils Totsche, Kai Uwe; Amelung, Wulf; Gerzabek, Martin H. ...
Journal of plant nutrition and soil science,
February 2018, Letnik:
181, Številka:
1
Journal Article
Recenzirano
Odprti dostop
All soils harbor microaggregates, i.e., compound soil structures smaller than 250 µm. These microaggregates are composed of diverse mineral, organic and biotic materials that are bound together ...during pedogenesis by various physical, chemical and biological processes. Consequently, microaggregates can withstand strong mechanical and physicochemical stresses and survive slaking in water, allowing them to persist in soils for several decades. Together with the physiochemical heterogeneity of their surfaces, the three‐dimensional structure of microaggregates provides a large variety of ecological niches that contribute to the vast biological diversity found in soils. As reported for larger aggregate units, microaggregates are composed of smaller building units that become more complex with increasing size. In this context, organo‐mineral associations can be considered structural units of soil aggregates and as nanoparticulate fractions of the microaggregates themselves. The mineral phases considered to be the most important as microaggregate forming materials are the clay minerals and Fe‐ and Al‐(hydr)oxides. Within microaggregates, minerals are bound together primarily by physicochemical and chemical interactions involving cementing and gluing agents. The former comprise, among others, carbonates and the short‐range ordered phases of Fe, Mn, and Al. The latter comprise organic materials of diverse origin and probably involve macromolecules and macromolecular mixtures. Work on microaggregate structure and development has largely focused on organic matter stability and turnover. However, little is known concerning the role microaggregates play in the fate of elements like Si, Fe, Al, P, and S. More recently, the role of microaggregates in the formation of microhabitats and the biogeography and diversity of microbial communities has been investigated. Little is known regarding how microaggregates and their properties change in time, which strongly limits our understanding of micro‐scale soil structure dynamics. Similarly, only limited information is available on the mechanical stability of microaggregates, while essentially nothing is known about the flow and transport of fluids and solutes within the micro‐ and nanoporous microaggregate systems. Any quantitative approaches being developed for the modeling of formation, structure and properties of microaggregates are, therefore, in their infancy. We respond to the growing awareness of the importance of microaggregates for the structure, properties and functions of soils by reviewing what is currently known about the formation, composition and turnover of microaggregates. We aim to provide a better understanding of their role in soil function, and to present the major unknowns in current microaggregate research. We propose a harmonized concept for aggregates in soils that explicitly considers the structure and build‐up of microaggregates and the role of organo‐mineral associations. We call for experiments, studies and modeling endeavors that will link information on aggregate forming materials with their functional properties across a range of scales in order to better understand microaggregate formation and turnover. Finally, we hope to inspire a novel cohort of soil scientists that they might focus their research on improving our understanding of the role of microaggregates within the system of aggregates and so help to develop a unified and quantitative concept of aggregation processes in soils.
Paddy soils make up the largest anthropogenic wetlands on earth, and are characterized by a prominent potential for organic carbon (C) sequestration. By quantifying the plant‐ and microbial‐derived C ...in soils across four climate zones, we identified that organic C accrual is achieved via contrasting pathways in paddy and upland soils. Paddies are 39%–127% more efficient in soil organic C (SOC) sequestration than their adjacent upland counterparts, with greater differences in warmer than cooler climates. Upland soils are more replenished by microbial‐derived C, whereas paddy soils are enriched with a greater proportion of plant‐derived C, because of the retarded microbial decomposition under anaerobic conditions induced by the flooding of paddies. Under both land‐use types, the maximal contribution of plant residues to SOC is at intermediate mean annual temperature (15–20°C), neutral soil (pH~7.3), and low clay/sand ratio. By contrast, high temperature (~24°C), low soil pH (~5), and large clay/sand ratio are favorable for strengthening the contribution of microbial necromass. The greater contribution of microbial necromass to SOC in waterlogged paddies in warmer climates is likely due to the fast anabolism from bacteria, whereas fungi are unlikely to be involved as they are aerobic. In the scenario of land‐use conversion from paddy to upland, a total of 504 Tg C may be lost as CO2 from paddy soils (0–15 cm) solely in eastern China, with 90% released from the less protected plant‐derived C. Hence, preserving paddy systems and other anthropogenic wetlands and increasing their C storage through sustainable management are critical for maintaining global soil C stock and mitigating climate change.
We provide a framework to illustrate the pathways of soil organic carbon (C) formation in waterlogged paddy and well‐drained upland. Paddy soils are enriched with greater proportion of plant‐derived C, whereas upland soils are more replenished by microbial‐derived C. Although the pool size of soil organic C in paddies is larger than their adjacent upland counterparts, the stored C in paddies is less stable than that in uplands and can be prone to loss under changing land use
Abstract
Rice paddies account for ~9% or the world’s cropland area and are characterized by environmental conditions promoting soil organic carbon storage, methane emissions and to a lesser extent ...nitrous oxide emissions. Here, we synthesize data from 612 sites across 51 countries to estimate global carbon stocks in paddy soils and determine the main factors affecting paddy soil carbon storage. Paddy soils (0–100 cm) contain 18 Pg carbon worldwide. Paddy soil carbon stocks decrease with increasing mean annual temperature and soil pH, whereas mean annual precipitation and clay content had minor impacts. Meta-analysis shows that paddy soil carbon stocks can be increased through several management practices. However, greenhouse gas mitigation through paddy soil carbon storage is generally outweighed by increases in methane and nitrous oxide emissions. Our results emphasize the key role of paddies in the global carbon cycle, and the importance of paddy management in minimizing anthropogenic greenhouse gas emissions.
Background
Atmospheric sulfur (S) and nitrogen (N) deposition has impacted many regions across the Northern Hemisphere inducing acidification and eutrophication of terrestrial ecosystems. However, ...acidification and eutrophication processes may differently impact litter decomposition and thus soil carbon (C) dynamics.
Methods
We performed a field soil chemistry manipulation in two mountainous temperate forest stands (
Picea abies
and
Fagus sylvatica
) historically affected by acid (S and N) deposition. In each stand, four treatments were established: control, acid addition (H
2
SO
4
– 50 kg S·ha
− 1
·year
− 1
), N addition (NH
4
NO
3
– 50 kg N·ha
− 1
·year
− 1
) and their combination. In fourth year of manipulation, we established litter decomposition experiment. Litter bags of contrasting quality and origin (green tea, rooibos tea, spruce needles and beech leaves), in total 1536 samples, were buried below the organic layer and left to decompose up to 24 months. Retrieved samples were analysed for mass loss, C/N, and concentration of CuO oxidation lignin. Data were complemented by monitoring soil water pH and soil CO
2
efflux.
Results
Acid additions decreased soil water pH, soil respiration and suppressed decomposition of the high-quality litter (green tea) in both stands, whereas mass loss of remaining litter was reduced only in the spruce stand. Nitrogen treatments, when coupled with decreasing soil water pH, constrained needle decomposition in the naturally more acidic spruce stand.
Conclusions
Our study demonstrates a suppressing effect of soil acidity on decomposition processes and soil C dynamics. The effect of N addition, as a nutrient, was insignificant, likely because of previous ecosystem adaptation to historical N loadings.
Globally, soil organic matter (SOM) contains more than three times as much carbon as either the atmosphere or terrestrial vegetation. Yet it remains largely unknown why some SOM persists for ...millennia whereas other SOM decomposes readily--and this limits our ability to predict how soils will respond to climate change. Recent analytical and experimental advances have demonstrated that molecular structure alone does not control SOM stability: in fact, environmental and biological controls predominate. Here we propose ways to include this understanding in a new generation of experiments and soil carbon models, thereby improving predictions of the SOM response to global warming.
Lignin is a main component of plant litter. Its degradation is thought to be critical for litter decomposition rates and the build-up of soil organic matter. We studied the relationships between ...lignin degradation and the production of dissolved organic carbon (DOC) and of CO
2
during litter decomposition. Needle or leaf litter of five species (Norway spruce, Scots pine, mountain ash, European beech, sycamore maple) and of different decomposition stage (freshly fallen and up to 27 months of field exposure) was incubated in the laboratory for two years. Lignin degradation was followed with the CuO method. Strong lignin degradation occurred during the first 200 incubation days, as revealed by decreasing yields of lignin-derived phenols. Thereafter lignin degradation leveled off. This pattern was similar for fresh and decomposed litter, and it stands in contrast to the common view of limited lignin degradation in fresh litter. Dissolved organic carbon and CO
2
also peaked in the first period of the incubation but were not interrelated. In the later phase of incubation, CO
2
production was positively correlated with DOC amounts, suggesting that bioavailable, soluble compounds became a limiting factor for CO
2
production. Lignin degradation occurred only when CO
2
production was high, and not limited by bioavailable carbon. Thus carbon availability was the most important control on lignin degradation. In turn, lignin degradation could not explain differences in DOC and CO
2
production over the study period. Our results challenge the traditional view regarding the fate and role of lignin during litter decomposition. Lignin degradation is controlled by the availability of easily decomposable carbon sources. Consequently, it occurs particularly in the initial phase of litter decomposition and is hampered at later stages if easily decomposable resources decline.
Rice is a Si‐accumulator plant, whereby Si has physio‐chemical functions for plant growth. Its straw contains high shares of plant silica bodies, so‐called phytoliths, and can, when returned to the ...soil, be an important Si fertilizer. Release of Si from phytoliths into soil solution depends on many factors. In order to improve prognosis of availability and management of Si located in phytoliths, in this study we analyzed the effect of pretreatment of rice straw by dry and wet ashing and the soil‐solution composition on Si release. Dry ashing of rice straw was performed at 400°C, 600°C, and 800°C and wet ashing of the original straw and the sample from 400°C treatment with H2O2. To identify the impact of soil‐solution chemistry, Si release was measured on separated phytoliths in batch experiments at pH 2–10 and in presence of different cations (Na+, K+, Mg2+, Ca2+, Al3+) and anions (Cl–, NO$ _3^- $, SO$ _4^{2-} $, acetate, oxalate, citrate) in the concentration range from 0.1 to 10 mmolc L–1. After burning rice straw at 400°C, phytoliths and biochar were major compounds in the ash. At an electrolyte background of 0.01 molc L–1, Si released at pH 6.5 was one order of magnitude higher than at pH 3, where the zeta potential (ζ) was close to zero. Higher ionic strength tended to suppress Si release. The presence of cations increased ζ, indicating the neutralization of deprotonated Si‐O– sites. Monovalent cations suppressed Si release more strongly than bivalent ones. Neutralization of deprotonated Si‐O– sites by cations might accelerate polymerization, leading to smaller Si release in comparison with absences of electrolytes. Addition of Al3+ resulted in charge reversal, indicating a very strong adsorption of Al3+, and it is likely that Si‐O‐Al‐O‐Si bonds are formed which decrease Si release. The negative effect of anions on Si release in comparison with deionized H2O might be due to an increase in ionic strength. The effect was more pronounced for organic anions than for inorganic ones. Burning of rice straw at low temperatures (e.g., 400°C) appears suitable to provide silicon for rice in short term for the next growing season. High inputs of electrolytes with irrigation water and low pH with concomitant increase of Al3+ in soil solution should be avoided in order to keep dissolution rate of phytoliths at an appropriate level.
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