The methods used for estimating below-ground carbon (C) translocation by plants, and the results obtained for different plant species are reviewed. Three tracer techniques using C isotopes to quantify root-derived C are discussed: pulse labeling, continuous labeling, and a method based on the difference in 13C natural abundance in C3 and C4 plants. It is shown, that only the tracer methods provided adequate results for the whole below-ground C translocation. This included roots, exudates and other organic substances, quickly decomposable by soil microorganisms, and CO2 produced by root respiration. Advantages due to coupling of two different tracer techniques are shown. The differences in the below-ground C translocation pattern between plant species (cereals, grasses, and trees) are discussed. Cereals (wheat and barley) transfer 20%—30% of total assimilated C into the soil. Half of this amount is subsequently found in the roots and about one-third in CO2 evolved from the soil by root respiration and microbial utilization of rootborne organic substances. The remaining part of below-ground translocated C is incorporated into the soil microorganisms and soil organic matter. The portion of assimilated C allocated below the ground by cereals decreases during growth and by increasing N fertilization. Pasture plants translocated about 30%—50% of assimilates below-ground, and their translocation patterns were similar to those of crop plants. On average, the total C amounts translocated into the soil by cereals and pasture plants are approximately the same (1500 kg C ha—1), when the same growth period is considered. However, during one vegetation period the cereals and grasses allocated beneath the ground about 1500 and 2200 kg C ha—1, respectively. Finally, a simple approach is suggested for a rough calculation of C input into the soil and for root-derived CO2 efflux from the soil. Kohlenstoffeintrag in Böden durch Pflanzen In diesem Review werden Methoden zur Einschätzung der Translokation von Kohlenstoff (C) in Böden durch Pflanzen und entsprechende Ergebnisse für unterschiedliche Pflanzengruppen diskutiert. Drei Methoden unter Verwendung von C-Isotopen als Tracer für wurzelbürtigen C werden vorgestellt: Pulsmarkierung, kontinuierliche Markierung und die Methode, die auf dem Unterschied in der natürlichen 13C-Abundanz von C3- und C4-Pflanzen basiert. Es wird gezeigt, dass nur die Tracer-Methoden adäquate Ergebnisse zur unterirdischen Translokation des gesamten wurzelbürtigen Kohlenstoffs liefern. Die Vorteile der Kopplung von zwei unterschiedlichen Tracer-Methoden werden gezeigt. Die Unterschiede im Muster der unterirdischen C-Translokation zwischen den Pflanzengruppen (Getreide, Weidegräser und Bäume) werden untersucht und in Tabellen zusammengefasst. Getreide (Weizen und Gerste) bringen 20%—30% des gesamten assimilierten C unter die Bodenoberfläche. Die Hälfte dieser Menge wird anschließend in den Wurzeln wiedergefunden. Ein Drittel verlässt den Boden als CO2, das durch Wurzelatmung und mikrobielle Veratmung der wurzelbürtigen organischen Substanzen entsteht. Der Rest des durch die Wurzeln in den Boden eingebrachten C wird in die mikrobielle Biomasse und in die organische Bodensubstanz eingebaut. Der Anteil des assimilierten C, der in den Boden durch Getreide eingebracht wird, verringert sich im Laufe der Pflanzenentwicklung und mit steigender N-Düngung. Die Weidegräser transportieren sogar 30%—50% des assimilierten C in den Boden, aber die Relation zwischen den einzelnen Pools bzw. Flüssen ist ähnlich wie beim Getreide. Sowohl Getreide als auch Weidegräser transportieren durchschnittlich ca. 1500 kg C ha—1 in den Boden bei Voraussetzung gleich langer Wachstumsperioden. Bei Berücksichtigung einer vollen Vegetationsperiode transportieren die Weidegräser ca. 600 kg C ha—1 mehr in den Boden als Getreidekulturen. Eine einfache Methode zur groben Kalkulation der durch Pflanzen in den Boden eingebrachten C-Mengen und des wurzelbürtigen CO2-Effluxes aus dem Boden wird vorgeschlagen.
The temperature sensitivity of enzymes responsible for organic matter decomposition in soil is crucial for predicting the effects of global warming on the carbon cycle and sequestration. We tested the hypothesis that differences in temperature sensitivity of enzyme kinetic parameters V max and K m will lead to a canceling effect: strong reduction of temperature response of catalytic reactions. Short-term temperature response of V max and K m of three hydrolytic enzymes responsible for decomposition of cellulose (β-glucosidase, cellobiohydrolase) and hemicelluloses (xylanase) were analyzed in situ from 0 to 40°C. The apparent activation energy varied between enzymes from 20.7 to 35.2 kJ mol(-1) corresponding to the Q 10 values of the enzyme activities of 1.4-1.9 (with V max - Q 10 1.0-2.5 and K m - Q 10 0.94-2.3). Temperature response of all tested enzymes fitted well to the Arrhenius equation. Despite that, the fitting of Arrhenius model revealed the non-linear increase of two cellulolytic enzymes activities with two distinct thresholds at 10-15°C and 25-30°C, which were less pronounced for xylanase. The nonlinearity between 10 and 15°C was explained by 30-80% increase in V max . At 25-30°C, however, the abrupt decrease of enzyme-substrate affinity was responsible for non-linear increase of enzyme activities. Our study is the first demonstrating nonlinear response of V max and K m to temperature causing canceling effect, which was most strongly pronounced at low substrate concentrations and at temperatures above 15°C. Under cold climate, however, the regulation of hydrolytic activity by canceling in response to warming is negligible because canceling was never observed below 10°C. The canceling, therefore, can be considered as natural mechanism reducing the effects of global warming on decomposition of soil organics at moderate temperatures. The non-linearity of enzyme responses to warming and the respective thresholds should therefore be investigated for other enzymes, and incorporated into Earth system models to improve the predictions at regional and global levels.
The trait-based strategies of microorganisms appear to be phylogenetically conserved, but acclimation to climate change may complicate the scenario. To study the roles of phylogeny and environment on bacterial responses to sudden moisture increases, we determine bacterial population-specific growth rates by 18 O-DNA quantitative stable isotope probing ( 18 O-qSIP) in soils subjected to a free-air CO 2 enrichment (FACE) combined with warming. We find that three growth strategies of bacterial taxa – rapid, intermediate and slow responders, defined by the timing of the peak growth rates – are phylogenetically conserved, even at the sub-phylum level. For example, members of class Bacilli and Sphingobacteriia are mainly rapid responders. Climate regimes, however, modify the growth strategies of over 90% of species, partly confounding the initial phylogenetic pattern. The growth of rapid bacterial responders is more influenced by phylogeny, whereas the variance for slow responders is primarily explained by environmental conditions. Overall, these results highlight the role of phylogenetic and environmental constraints in understanding and predicting the growth strategies of soil microorganisms under global change scenarios.
No abstract is provided for this article.
No abstract is provided for this article.
The capacity of some additives to soil originated from sewage sludge to fix Cd, Zn, and Cu, was evaluated using a pot experiment. Following substances were applied: two natural clay minerals (Na-bentonite and zeolite) at 1% and 2%, and iron oxide (goethite) at 1%. Metal content of wheat shoots and heavy metals available in the soil were determined after 45 days. The largest effects to reduce the mobility and phytoavailability of heavy metals occurred after addition of Na-bentonite at 2%. Compared to the untreated soil, the mobility of all heavy metals was significandy reduced by 36% and 16% for Cd, by 25% and 9% for Zn, and by 33% and 14% for Cu using Na-bentonite and zeolite at 2%, respectively. However, the addition of goethite reduced significandy the mobility of Zn and Cu by 9% and 7%, respectively. The highest reduction in shoot uptake of all heavy metals was occurred after addition of Na-bentonite at 2%. Among tested additives, Na-bentonite exhibited a most promising potential to reduce bioavailability of Cd, Zn and Cu to wheat plant.
The strong affinity of phosphorus (P) to iron (Fe) oxides and hydroxides in highly weathered tropical soils limits P availability and therefore plant productivity in tropics. In flooded soils, however, P fixed by Fe oxides and hydroxides can be released into more available forms because of Fe3+ reduction to Fe2+. These P dynamics in flooded soils are well documented for rice paddies. Such effects are much less studied in other land-use types influenced by seasonal flooding, especially in the tropics during heavy monsoon rains. The aim of this study was to investigate the P mobilization during flooding leading to anaerobic conditions in topsoil and subsoil depending on land-use type. Samples were collected in highly weathered Acrisols from four replicate sites under natural rainforest, jungle rubber, rubber and oil palm plantations in Sumatra, Indonesia. Topsoil and subsoil were taken to ensure a wide range of soil organic matter (SOM) and P contents. Soils were incubated under anaerobic, flooded conditions at 30±1°C for 60days. Our results confirmed the hypothesis that soil flooding mobilizes P and increases P availability. Two distinct and opposite periods were observed during the flooding. During the first three weeks of flooding, the dissolved P (DP) concentration peaked, simultaneously with the peak of dissolved Fe2+ (DFe2+) and dissolved organic carbon (DOC). After three weeks, P availability in soils decreased, although Fe-P (PNaOH) and available P (PNaHCO3) did not reach the initial, pre-flooding levels. The impacts of flooding on P and Fe forms was strong in the topsoil, where P dissolution and availability were generally higher under forest and, to a lesser extent, under jungle rubber. A positive correlation between DOC and DFe2+ (R2 =0.42) in topsoil indicates that the intensity of microbially-mediated Fe3+ reduction is limited by the amount of available carbon (C) as an energy source for microorganisms and as electron donor. Microbial mineralization of organic P from SOM also increases P availability, and this process requires available C. This interpretation was supported by the strong correlation (R2 =0.58) between available P and DOC, as well as between DP and DOC (R2 =0.56) in topsoil. The increasing pH in topsoil and subsoil after flooding of all land-use types may also influence the P release over time. In summary, the increase of available P and DP during flooding is due to three main mechanisms: (1) P release via the microbially-mediated reductive dissolution of Fe3+ oxides; (2) P release during SOM mineralization and (3) solubility of Fe phosphate due to increasing pH. These mechanisms are relevant not only in riparian areas, where flooding occurs, but also in soils waterlogged after regular heavy rainfalls during the wet season. Therefore, we speculate that the P turnover is faster in compacted soils under plantations because of regular changes of oxic and anoxic conditions. Consequently, more P is pumped by the vegetation and then removed from plantations due to yield export.
