Effect of land use types on decomposition of 14C-labelled maize residue (Zea mays L.)

The effect of three land use types on decomposition of ¹⁴C-labelled maize (Zea mays L.) residues and soil organic matter were investigated under laboratory conditions. Samples of three Dystric Cambisols under plow tillage (PT), reduced tillage (RT) and grassland (GL) collected from the upper 5 cm of the soil profile were incubated for 159 days at 20 °C with or without ¹⁴C-labelled maize residue. After 7 days cumulative CO₂ production was highest in GL and lowest in PT, reflecting differences in soil organic C (SOC) concentration among the three land use types and indicating that mineralized C is a sensitive indicator of the effects of land use regime on SOC. ¹⁴CO₂ efflux from maize residue decomposition was higher in GL than in PT, possibly due to higher SOC and microbial biomass C (MBC) in GL than in PT. ¹⁴CO₂ efflux dynamics from RT soil were different from those of PT and GL. RT had the lowest ¹⁴CO₂ efflux from days 2 to 14 and the highest from days 28 to 159. The lowest MBC in RT explained the delayed decomposition of residues at the beginning. A double exponential model gave a good fit to the mineralization of SOC and residue-¹⁴C (R² > 0.99) and allowed estimation of decomposition rates as dependent on land use. Land use affected the decomposition of labile fractions of SOC and of maize residue, but had no effect on the decomposition of recalcitrant fractions. We conclude that land use affected the decomposition dynamics within the first 1.5 months mainly because of differences in soil microbial biomass but had low effect on cumulative decomposition of maize residues within 5 months.     

Three‐source partitioning of CO2 efflux from maize field soil by 13C natural abundance

A natural‐¹³C‐labeling approach—formerly observed under controlled conditions—was tested in the field to partition total soil CO₂ efflux into root respiration, rhizomicrobial respiration, and soil organic matter (SOM) decomposition. Different results were expected in the field due to different climate, site, and microbial properties in contrast to the laboratory. Within this isotopic method, maize was planted on soil with C₃‐vegetation history and the total CO₂ efflux from soil was subdivided by isotopic mass balance. The C₄‐derived C in soil microbial biomass was also determined. Additionally, in a root‐exclusion approach, root‐ and SOM‐derived CO₂ were determined by the total CO₂ effluxes from maize (Zea mays L.) and bare‐fallow plots. In both approaches, maize‐derived CO₂ contributed 22% to 35% to the total CO₂ efflux during the growth period, which was comparable to other field studies. In our laboratory study, this CO₂ fraction was tripled due to different climate, soil, and sampling conditions. In the natural‐¹³C‐labeling approach, rhizomicrobial respiration was low compared to other studies, which was related to a low amount of C₄‐derived microbial biomass. At the end of the growth period, however, 64% root respiration and 36% rhizomicrobial respiration in relation to total root‐derived CO₂ were calculated when considering high isotopic fractionations between SOM, microbial biomass, and CO₂. This relationship was closer to the 50% : 50% partitioning described in the literature than without fractionation (23% root respiration, 77% rhizomicrobial respiration). Fractionation processes of ¹³C must be taken into account when calculating CO₂ partitioning in soil. Both methods—natural ¹³C labeling and root exclusion—showed the same partitioning results when ¹³C isotopic fractionation during microbial respiration was considered and may therefore be used to separate plant‐ and SOM‐derived CO₂ sources.     

Soil microbial biomass and its activity estimated by kinetic respiration analysis – Statistical guidelines

The use of kinetic respiration analysis to determine soil microbial biomass its active part and maximum specific growth rate has recently increased. With this method, the increase in soil respiration rate initiated by application of carbon growth substrate, e.g. glucose, and mineral nutrients is used to estimate parameters describing microbial growth in soil. This study refines the method by developing statistical guidelines for the data analysis and processing. The kinetic respiration analysis assumes that microbial growth is not limited by substrate and energy. That is why it is critically important to identify the time period corresponding to the unlimited growth. In this work, we studied how the unlimited growth phase can be identified in less subjective ways by examining 121 datasets of respiration time series of 44 different soil samples taken from field plots. Deflection of the respiratory curve from the exponential pattern indicates growth limitation. Subjective selection of the part of respiratory curve which fits to the exponential pattern resulted in a 30% bias in specific microbial growth rates. We propose rules that are based on inspecting the patterns in a series of plots of residuals of fitted respiration rate. By comparing those rules with a set of statistical criteria we find that the weighted-coefficient of determination (r²) can be used to objectively constrain the unlimited growth phase in those cases where double-limitation does not occur. Furthermore, we discuss how the uncertainty of estimated microbial parameters is influenced by a) measurement uncertainty, b) biased measurement at the beginning of the experiment, and c) the number and timing of respiration measurements. We recommend checking plots of fits and residuals as well as reporting uncertainty bounds together with the estimated microbial parameters. A free statistical package is provided to easily deal with those aspects.     

Nutrient limitation of alpine plants: Implications from leaf N : P stoichiometry and leaf δ15N

Nitrogen (N) deposition can affect grassland ecosystems by altering biomass production, plant species composition and abundance. Therefore, a better understanding of the response of dominant plant species to N input is a prerequisite for accurate prediction of future changes and interactions within plant communities. We evaluated the response of seven dominant plant species on the Tibetan Plateau to N input at two levels: individual species and plant functional group. This was achieved by assessing leaf N : P stoichiometry, leaf δ¹⁵N and biomass production for the plant functional groups. Seven dominant plant species—three legumes, two forbs, one grass, one sedge—were analyzed for N, P, and δ¹⁵N 2 years after fertilization with one of the three N forms: NO$ _3^- $ , NH$ _4^+ $ , or NH₄NO₃ at four application rates (0, 7.5, 30, and 150 kg N ha^–1 y^–1). On the basis of biomass production and leaf N : P ratios, we concluded that grasses were limited by available N or co‐limited by available P. Unlike for grasses, leaf N : P and biomass production were not suitable indicators of N limitation for legumes and forbs in alpine meadows. The poor performance of legumes under high N fertilization was mainly due to strong competition with grasses. The total above‐ground biomass was not increased by N fertilization. However, species composition shifted to more productive grasses. A significant negative correlation between leaf N : P and leaf δ¹⁵N indicated that the two forbs Gentiana straminea and Saussurea superba switched from N deficiency to P limitation (e.g., N excess) due to N fertilization. These findings imply that alpine meadows will be more dominated by grasses under increased atmospheric N deposition.     

Photoassimilate allocation and dynamics of hotspots in roots visualized by 14C phosphor imaging

Understanding photoassimilate allocation into the roots and the release of organic substances from the roots into the rhizosphere is an important prerequisite for characterizing the belowground C input, the spatial and temporal distribution of C, and the interactions between plants and soil microorganisms. Based on ¹⁴C phosphor imaging, we visualized the allocation of assimilates into Lolium perenne roots and estimated the life time of hotspots at the root tips. Lolium shoots were labeled in a ¹⁴CO₂ atmosphere, and herbariums of roots and shoots were prepared 6 h, 2 d, and 11 d after the ¹⁴C pulse. The ¹⁴C distribution in roots and leaves revealed that pulse labeling does not yield homogeneously labeled plant material. The spatial distribution of assimilate allocation was evaluated based on the ¹⁴C specific activity expressed as digital light units (DLU mm^–2) of the imaging plates. Areas with high relative ¹⁴C activity were classified as hotspots. Strong ¹⁴C hotspots were detected mainly at the root tips already 6 h after the ¹⁴C assimilation, and they remained active for at least 2 d. Eleven days after the ¹⁴C assimilation, the hotspots at the root tips disappeared and the ¹⁴C distribution was much more even than after 6 h or after 2 d. ¹⁴C phosphor imaging proved to be a promising tool to visualize the allocation of photoassimilates into the roots and the rhizosphere and can be used to identify hotspots and their dynamics.     

Identification of labile and stable pools of organic matter in an agrogray soil

The intensity of decomposition of the organic matter in the particle-size fractions from a agrogray soil sampled in a 5-year-long field experiment on the decomposition of corn residues was determined in the course of incubation for a year. The corn residues were placed into the soil in amounts equivalent to the amounts of plant litter in the agrocenosis and in the meadow ecosystem. A combination of three methods—the particle-size fractionation, the method of ¹³C natural abundance by C3–C4 transition, and the method of incubation—made it possible to subdivide the soil organic matter into the labile and stable pools. The labile pool reached 32% in the soil of the agrocenosis and 42% in the meadow soil. Owing to the negative priming effect, the addition of C4 (young) carbon favored the stabilization of the C3 (old) carbon in the soil. When the young carbon was absent, destabilization or intense decomposition of the old organic matter was observed. This process was found even in the most stable fine silt and clay fractions.     

Turnover and availability of soil organic carbon under different Mediterranean land‐uses as estimated by 13C natural abundance

Soil organic matter (SOM) is an important factor in ecosystem stability and productivity. This is especially the case for Mediterranean soils suffering from the impact of human degradation as well as harsh climatic conditions. We used the carbon (C) exchange resulting from C₃‐C₄ and C₄‐C₃ vegetation change under field conditions combined with incubations under controlled conditions to evaluate the turnover and availability of soil organic C under different land‐uses. The 40‐year succession of Hyparrenia hirta L. (C₄ photosynthesis) after more than 85 years of olive tree (Olea europaea L.; C₃ photosynthesis) growth led to the exchange of 54% of soil organic C from C₃ to C₄ forms. In contrast, 21 years of vine (Vitis vinifera L.) growing after H. hirta decreased the organic C content to 57%. Considering this exchange and decrease as well as the periods after the land‐use changes, we calculated the mean residence time (MRT) of soil C of different ages. The MRT of C under grassland dominated by H. hirta was about 19 years, but was 180 years under the vineyard. The rates of C accumulation under the H. hirta grassland were about 0.36 Mg C ha^?1 year^?1. In contrast, the rates of C losses after conversion from natural grassland to a vineyard were 1.8 times greater and amounted to 0.65 Mg C ha^?1 year^?1. We conclude that changes of land use from natural Mediterranean grassland to a vineyard lead to very large C losses that cannot be compensated for over the same periods.     

Land Use and Precipitation Affect Organic and Microbial Carbon Stocks and the Specific Metabolic Quotient in Soils of Eleven Ecosystems of Mt. Kilimanjaro,Tanzania

Tropical ecosystems are under increasing pressure of land‐use changes, strongly affecting the carbon cycle. Conversion from natural to agri‐cultural ecosystems is often accompanied by a decrease in the stocks of organic and microbial carbon (C_org, C_mic) as well as changes in microbial activity and litter decomposition. Eleven ecosystems along an elevation gradient on the slopes of Mt. Kilimanjaro were used to investigate impacts of land‐use changes on C_org and C_mic stocks as well as the specific metabolic respiration quotient (q_sCO₂) in surface soils. Six natural, two semi‐natural and three intensively used agricultural ecosystems were investigated on an elevation gradient from 950 to 3,880 m asl. To estimate the effects of precipitation, rainfall regimes of 3·6 and 20·0 mm were simulated. C_org stocks were controlled by water availability, temperature and net primary production. Agricultural management resulted in decreases of C_org and C_mic stocks by 38% and 76%, respectively. In addition, agricultural systems were characterized by low C_mic:C_org ratios, indicating a decline in available substrate. Enhanced land‐use intensity leads to increased q_sCO₂ (agricultural > semi‐natural > natural). The traditional homegardens stood out as a sustainable land‐use form with high substrate availability and microbial efficiency. Soil CO₂ efflux and q_sCO2 generally increased with precipitation level. We conclude that soils of Mt. Kilimanjaro's ecosystems are highly sensitive to land‐use changes and are vulnerable to changes in precipitation, especially at low elevations. Even though q_sCO₂ was measured under different water contents, it can be used as an indicator of ecosystem disturbances caused by land‐use and management practices. Copyright © 2015 John Wiley & Sons, Ltd.     

Signatures of electron fractionalization in ultraquantum bismuth

Because of the long Fermi wavelength of itinerant electrons, the quantum limit of elemental bismuth (unlike most metals) can be attained with a moderate magnetic field. The quantized orbits of electrons shrink with increasing magnetic field. Beyond the quantum limit, the circumference of these orbits becomes shorter than the Fermi wavelength. We studied transport coefficients of a single crystal of bismuth up to 33 tesla, which is deep in this ultraquantum limit. The Nernst coefficient presents three unexpected maxima that are concomitant with quasi-plateaus in the Hall coefficient. The results suggest that this bulk element may host an exotic quantum fluid reminiscent of the one associated with the fractional quantum Hall effect and raise the issue of electron fractionalization in a three-dimensional metal.     

Contribution of rhizomicrobial and root respiration to the CO2 emission from soil (A review)

Separate determination of root respiration and rhizomicrobial respiration is one of the most interesting, important, and methodologically complicated problems in the study of the carbon budget in soils and the subdivision of the CO₂ emission from soils into separate fluxes. In this review, we compare the main principles, the advantages and disadvantages, and the results obtained by the methods of component integration, substrate-induced respiration, respiratory capacity, girdling, isotope dilution, model rhizodeposition, modeling of the ¹⁴CO₂ efflux dynamics, exudates elution, and the δ¹³C measurements of the microbial biomass and CO₂. Summarizing the results of the determinations performed by these methods, we argue that about 40% of the rhizosphere CO₂ efflux is due to root respiration and about 60% of this efflux is due to the respiration of microorganisms decomposing root exudates.