Soil erodibility,microbial biomass,and physical–chemical property changes during long-term natural vegetation restoration: a case study in the Loess Plateau,China

Soil erodibility (K factor) is an important index for measuring soil susceptibility to water erosion, and an essential parameter that is needed for the prediction of soil erosion. Field investigation and laboratory analysis were conducted to study the changes of soil characteristics during long-term vegetation restoration in the hilly gullied loess area. The soil erodibility K values were calculated using the EPIC model and the physico-chemical properties as well as microbial characteristics were evaluated along a chronosequence of natural vegetation recovery (0–50 years) in abandoned land in the Zhifanggou Watershed of Ansai County, northwestern Shaanxi Province, China. The results showed that natural vegetation recovery following abandonment resulted in improvement of the soil properties and structure and these improving effects were closely related to the date of abandonment. Specifically, the K value of the surface layer (0–20 cm) was significantly reduced with time, while the total organic carbon, total nitrogen and soil microbial biomass C, microbial N and microbial P and the water-stable aggregate increased quickly. During the first 10 years of abandonment, these changes occurred relatively quickly due to a significant increase in soil organic matter, after which they gradually fluctuated for approximately 20 years, reaching their uttermost or minimum levels finally. However, these values differed greatly under Platycladus orientalis forest, which suggests that soil rehabilitation is a long-term task that requires several generations to complete.     

Vegetation recovery alters soil N status in subtropical karst plateau area: Evidence from natural abundance δ15N and δ18O

Plant and Soil - Forest recovery from disturbance can alter soil nitrogen (N) status as a result of complex interactions in plant-soil system. The δ15N and δ18O are indicators that...     

Molecular-level changes in soil organic matter composition after 10 years of litter,root and nitrogen manipulation in a temperate forest

With climate change, forests are expected to receive increased inputs of carbon (C) and nitrogen (N) but it is unclear how this will modify forest C cycling and storage at the molecular-level. To investigate the response of forest soil organic matter (SOM) to changes in soil inputs, a study area was established in a Michigan hardwood forest as part of the Detrital Input and Removal Treatments (DIRT) network. Experimental treatments were comprised of both exclusions of detrital inputs (No Litter, No Roots, No Inputs) and additions of C and N (Double Litter, N-Addition, Double Litter?+?N, Wood). After 10 years of treatment, the soils were characterized using elemental analysis, molecular biomarker techniques, nuclear magnetic resonance spectroscopy, and microbial biomass C measurements. Although manipulation of detrital inputs did not significantly change the soil C and N content after 10 years, alterations in the cycling and distribution of SOM components were observed. Root exclusion enhanced SOM degradation, while doubling litter favoured the degradation of more labile forms of soil C such as unsaturated n-alkanoic acids and simple sugars. N-Addition and Double Litter?+?N increased the concentrations of extractable biomarkers, including aliphatic and cyclic lipids and compounds derived from cutin, suberin, and lignin. Microbial biomass C also varied with experimental litter input manipulations and N addition, and these data were consistent with the observed changes in SOM composition. Overall, the observed shifts in SOM chemistry after 10 years of manipulating ecosystem inputs highlight the sensitivity of natural systems to changes in amounts of C and N inputs from roots and litter, and N inputs from external sources.     

Seasonal variation of vegetation productivity over an alpine meadow in the Qinghai–Tibet Plateau in China: modeling the interactions of vegetation productivity, phenology, and the soil freeze–thaw process

Phenology controls the seasonal activities of vegetation on land surfaces and thus plays a fundamental role in regulating photosynthesis and other ecosystem processes. Therefore, accurately simulating phenology and soil processes is critical to ecosystem and climate modeling. In this study, we present an integrated ecosystem model of plant productivity, plant phenology, and the soil freeze–thaw process to (1) improve the quality of simulations of soil thermal regimes and (2) estimate the seasonal variability of plant phenology and its effects on plant productivity in high-altitude seasonal frozen regions. We tested different model configurations and parameterizations, including a refined soil stratification scheme that included unfrozen water in frozen soil, a remotely sensed diagnostic phenology scheme, and a modified prognostic phenology scheme, to describe the seasonal variation in vegetation. After refined soil layering resolution and the inclusion of unfrozen water in frozen soil, the results show that the model adequately reproduced the soil thermal regimes and their interactions observed at the site. The inclusion of unfrozen water in frozen soil was found to have a significant effect on soil moisture simulation during the spring but only a small effect on soil temperature simulation at this site. Moreover, the performance of improved phenology schemes was good. The phenology model accurately predicted the start and end of phenology, and its precise prediction of phenology variation allows an improved simulation of vegetation production.     

Plant phenology and life span influence soil pool dynamics: Bromus tectorum invasion of perennial C3–C4 grass communities

In water-limited ecosystems, small rainfall events can have dramatic impacts on microbial activity and soil nutrient pools. Plant community phenology and life span also affect soil resources by determining the timing and quantity of plant nutrient uptake, storage, and release. Using the replacement of C₃–C₄ perennial grasses by the invasive annual grass Bromus tectorum as a case study, we investigated the influence of phenology and life span on pulse responses and sizes of soil carbon (C) and nitrogen (N) pools. We hypothesized that available and microbial C and N would respond to small rainfall events and that B. tectorum invasion would increase soil C and N pools by reducing inter-annual plant C and N storage and alter seasonal pool dynamics by changing the timing of plant uptake and litter inputs. We tested our hypotheses by simulating small rainfall events in B. tectorum and perennial grass communities three times during the growing season. Microbial pools responded strongly to soil moisture and simulated rainfall events, but labile C and N pools were affected weakly or not at all. All pools were larger beneath B. tectorum than perennial grasses. Soil C and N pools increased after senescence in both communities. Our results suggest that transforming a perennial into a B. tectorum dominated community increases the overall size of soil C and N pools by decreasing plant C and N storage and changes seasonal pool dynamics by altering dominant plant phenology. Our results indicate strong roles for water, life span and phenology in controlling soil C and N pools and begin to elucidate the biogeochemical effects of altering plant community phenology and life span.     

Selectively 13C-enriched DNA: Evidence from 13C1′ relaxation rate measurements of an internal dynamics sequence effect in the lac operator

Summary In order to study some internal dynamic processes of the lac operator sequence, the ¹³C-labeled duplex 5d(C₀G₁C₂T₃C₄A₅C₆A₇A₈T₉T₁₀) · d(A₁₀A₉T₈T₇G₆T₅G₄A₃G₂C₁G₀)3 was used. The spreading of both the H1 and C1 resonances brought about an excellent dispersion of the ¹H1-¹³C1 correlations. The spinlattice relaxation parameters R(C_z), R(C_x,y) and R(H_zC_z) were measured for each residue of the two complementary strands, except for the 3-terminal residues which were not labeled. Variation of the relaxation rates was found along the sequence. These data were analyzed in the context of the model-free formalism proposed by Lipari and Szabo [(1982) J. Am. Chem. Soc., 104, 4546–4570] and extended to three parameters by Clore et al. [(1990) Biochemistry, 29, 7387–7401; and (1990) J. Am. Chem. Soc., 112, 4989–4991]. A careful analysis using a least-squares program showed that our data must be interpreted in terms of a three-parameter spectral density function. With this approach, the global correlation time was found to be the same for each residue. All the C1-H1 fragments exhibited both slow (_s = 1.5) and fast (_f = 20 ps) restricted libration motions (S _inf2 ^sups =0.74 to 1.0 and S _inf2 ^supf =0.52 to 0.96). Relaxation processes were described as governed by the motion of the sugar relative to the base and in terms of bending of the whole duplex. The possible role played by the special structure of the AATT sequence is discussed. No evident correlation was found between the amplitude motions of the complementary residues. The 5-terminal residues showed large internal motions (S²=0.5), which describe the fraying of the double helix. Global examination of the microdynamical parameters S _inf2 ^supf and S _inf2 ^sups along the nucleotide sequence showed that the adenine residues exhibit more restricted fast internal motions (S _inf2 ^supf =0.88 to 0.96) than the others, whereas the measured relaxation rates of the four nucleosides in solution were mainly of dipolar origin. Moreover, the fit of both R(C_z) and R(HzC_z) experimental relaxation rates using an only global correlation time for all the residues, gave evidence of a supplementary relaxation pathway affecting R(C_x,y) for the purine residues in the (53) G₄A₃ and A₁₀A₉T₈T₇ sequences. This relaxation process was analyzed in terms of exchange stemming from motions of the sugar around the glycosidic bond on the millisecond time scale. It should be pointed out that these residues gave evidence of close contacts with the protein in the complex with the lac operator [Boelens et al. (1987) J. Mol. Biol., 193, 213–216] and that these motions could be implied in the lac-operator-lac-repressor recognition process.     

Order parameters of unsaturated phospholipids in membranes and the effect of cholesterol: a 1H–13C solid-state NMR study at natural abundance

Most biological phospholipids contain at least one unsaturated alkyl chain. However, few order parameters of unsaturated lipids have been determined because of the difficulty associated with isotopic labeling of a double bond. Dipolar recoupling on axis with scaling and shape preservation (DROSS) is a solid-state nuclear magnetic resonance technique optimized for measuring ¹H–¹³C dipolar couplings and order parameters in lipid membranes in the fluid phase. It has been used to determine the order profile of 1,2-dimyristoyl-sn-glycero-3-phosphocholine hydrated membranes. Here, we show an application for the measurement of local order parameters in multilamellar vesicles containing unsaturated lipids. Taking advantage of the very good ¹³C chemical shift dispersion, one can easily follow the segmental order along the acyl chains and, particularly, around the double bonds where we have been able to determine the previously misassigned order parameters of each acyl chain of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). We have followed the variation of such order profiles with temperature, unsaturation content and cholesterol addition. We have found that the phase formed by DOPC with 30% cholesterol is analogous to the liquid-ordered (l_o) phase. Because these experiments do not require isotopic enrichment, this technique can, in principle, be applied to natural lipids and biomembranes.Electronic Supplementary Material Supplementary material is available for this article at .     

The exposure,sensitivity and vulnerability of natural vegetation in China to climate thermal variability (1901–2013): An indicator-based approach

Vulnerability assessments can be helpful in assessing the impact of climate change on natural ecosystems and are expected to support adaptation and/or mitigation strategies in the 21st century. A challenge when conducting such assessments is the integration of the multi-level properties and processes of ecosystems into an assessment framework. Focusing on the primary stresses of climate thermal variability (at both upper and lower extremes), this study proposes a quantitative indicator system—following the IPCC framework of vulnerability assessment—that assesses the impact of historical climate change, during 1901–2013, on the natural terrestrial vegetation types in China. The final output of the vulnerability assessment was expressed as a composite index, composed of ecosystem exposure, sensitivity and resilience to climate thermal change, and including biological, ecological and spatial traits of vegetation types in the assessment. The exposure to temperature variability was generally higher in January than in July, and higher in non-arborous vegetation types than forests. In contrast, sensitivity was higher for forests, wetlands and alpine tundra regions, especially for small areas and areas with scattered patterns. Original forests—especially those distributed in the north—had lower resilience than other vegetation types. The vulnerability of natural vegetation types in China to the temperature variability of the past century was very low to moderate, with a few exceptions, including tropical mangroves and the semi-arid to arid vegetation types in northwestern China, which had high vulnerability. Vulnerability was stronger in winter than in summer. Our results are generally in accord with the scenario-based projections on the geographical pattern of vegetation vulnerability to climate change, and revealed the difference caused by not considering moisture. The risks for these fragmented and narrow-range ecosystems are highlighted, and the importance of natural resilience is stressed for the assessment of vegetation vulnerability to climate change. Given the inadequate coverage of the natural reserve network in China (after the large investment in recent decades) found in the high-vulnerability vegetation types (with a few exceptions), the assessment of natural resilience of ecosystems could be critical for the optimal design of socio-economic strategies in response to the impacts of future climate change.     

Reconstructing Mid-Pleistocene paleovegetation and paleoclimate in the Golan Heights using the δ(13)C values of modern vegetation and soil organic carbon of paleosols

The Golan Heights borders the Upper Jordan Valley on its eastern side and likely served as a prime foraging area for hominin groups that inhabited the Upper Jordan Valley during the Mid-Pleistocene. This study tests the hypothesis that Mid-Pleistocene climate in the Golan region was similar to that of the present day. Carbon isotope composition of present day plant communities and soil organic carbon from the Golan were compared to those of paleosols from Nahal Orvim to reconstruct Mid-Pleistocene paleoclimatic conditions. After correcting the paleosol values for recent changes in atmospheric carbon isotope values and potential biodegradation, the isotopic results show a strong similarity to those of present day local plants and soils. These results indicate that during the Mid-Pleistocene, the Golan was dominated by C₃ vegetation, shared similar climatic conditions with the present day, and displayed long-term environmental stability. The span of time of paleosol formation is unknown and might cover multiple climatic episodes; thus, although short climatic fluctuations may have occurred, their impact was not substantial enough to be detected in the Nahal Orvim paleosols. This study concludes that the Golan slopes provided hominins and large grazers with a reliable and highly nutritious foraging area that complemented the Jordan Valley riparian ecosystem.     

Palynology of a 250-m core from Lake Biwa: a 430,000-year record of glacial–interglacial vegetation change in Japan

Lake Biwa is situated in western Honshu, and is the largest and oldest freshwater lake in Japan. During 1982–1983, a long core was drilled to a bottom depth of 1422 m in order to investigate the palaeolimnological record, which contains 911 m of various sedimentary units overlying Palaeozoic–Mesozoic basements. A 249.5-m core of the uppermost bed (T Bed) was palynologically examined at intervals of ca. 2 m. The investigated part of the core covers approximately the last 430,000 years of the mid-Upper Pleistocene and Holocene, and five glacial–interglacial cycles can be recognized. Ten major vegetational zones could be recognized from the bottom to the surface. The zones bearing even numbers, BW-10, 8, 6, 4 and 2, matched glacial periods in which pollen of subarctic taxa (Pinaceae, Betula) and cool–temperate taxa (Fagus, Lepidobalanus) was dominant. In the glacial periods, high pollen values for temperate conifers such as Cryptomeria, Cupressaceae and Sciadopitys indicate interstadial periods. Zones bearing odd numbers, BW-9, 7, 5, 3 and 1, matched interglacial periods with high pollen values for the warm–temperate taxon Cyclobalanopsis, or showed the characteristic appearance of Lagerstroemia, and temperate coniferous taxa (Cryptomeria, Cupressaceae). There were two vegetational types in the interglacial periods. One was the type indicated by BW-9 and 1 zones, when warm–temperate evergreen broad-leaved trees such as Cyclobalanopsis and Castanopsis showed high pollen values, and the warm–temperate deciduous broad-leaved tree Lagerstroemia was lacking. The climate of these interglacial periods seems to have been cold and dry in winter, and warm and wet in summer. The other was the vegetational type recorded in BW-7, 5 and 3 zones, where Lagerstroemia showed low pollen values, and the warm–temperate evergreen broad-leaved trees mentioned above were poorly represented. The climate seems to have been mild and wet in winter, and cool and wet in summer. The dominant pollen values for cool–temperate deciduous broad-leaved taxa such as Fagus and Lepidobalanus are very important indicators of the initiation of both glacial and interglacial periods in western Japan.     

Mercury–organic matter relationships in pre-pollution sediments of thermokarst lakes from the Mackenzie River Delta, Canada: the role of depositional environment

This study investigates the influence of organic matter (OM) on the historical variations of Hg in sediments from two closely-situated Canadian Arctic lakes, prior to the advent of Hg pollution inputs. Because of variable landscape evolution in the Mackenzie River Delta over the past 1–3 millennia, the lakes provide distinctly different histories of OM sources, types, and degree of aquatic productivity (i.e., depositional environments). They also differ significantly in their pre-1900 Hg concentration profiles. When labile, kerogen-like carbon (“S2”) from aquatic sources (diatoms and other unicellular algae) increased between 750 and 1900 A.D. in the more productive lake (Nesbitt), Hg concentrations also increased by ca. 50%. In contrast, S2 carbon concentrations in the nearby organic-poor lake (Big Lake) were several-times lower than in Nesbitt and decreased over the past millennium, while Hg concentrations showed no trend probably reflecting the stable input of clastic material from tundra soils. The contrast between lakes suggests that OM derived from unicellular algae is more effective at scavenging Hg than OM from terrestrial plants or aquatic macrophytes, possibly because of a higher content of labile, sulphur-rich compounds, high particle surface area and its dispersion throughout the water column. The results indicate that, in the absence of anthropogenic Hg inputs, increasing phytoplankton productivity and Hg scavenging alone can lead to significant increases in the Hg content of lake sediments. This finding is consistent with the hypothesis that increasing lake productivity because of climate warming during the twentieth century has confounded the interpretation of recently increasing Hg levels in northern lake sediments as being unequivocally due to anthropogenic Hg deposition. This study also suggests that sedimentary TOC by itself is a poor and sometimes misleading indicator of possible changes in the source and quality of OM in aquatic systems, which can have a major impact on Hg concentrations in sediments.     

Changes in soil organic carbon and total nitrogen after 28 years grassland afforestation: effects of tree species, slope position, and soil order

The effect of conversion of grassland to woodland on organic carbon (OC) and total nitrogen (TN) has significance for global change, land resource use and ecosystem management. However, these effects are always variable. Here, we show results of a study in an arid area in China on profile distribution of OC and TN in soils covered by two different woody tree canopies and outer canopy space (grassland between woody plant canopies). The soils were at various slope positions (upper, middle and lower slopes) for Chinese pine (Pinus tabulaeformis) and Korshrinsk peashrub (Caragana korshinskii) lands, and of different soil orders (Castanozems, Skeletal, Loessial and Aeolian soils). The objectives were to relate the effects of land use change on OC and TN to slope position and soil order. Soil OC and TN were significantly larger at Korshrinsk peashrub slope locations than at Chinese pine slope locations. Soil OC and TN were small at the lower slope position for Korshrinsk peashrub, however, they were largest at the middle slope for Chinese pine. Korshrinsk peashrub always increased soil OC and TN under brush canopy at the three slope positions, while Chinese pine increased them at lower slopes and decreased them at upper slopes. For the soil types, OC and TN in Korshrinsk peashrub land were in the order of Castanozems > Skeletal > Loessial > Aeolian soils. Korshrinsk peashrub also increased OC and TN under brush canopy in the four soils. Our results indicated that soil OC and TN in canopy soils differed greatly from associated values in the outer canopy soils, and the effects of grassland afforestation varied significantly with tree species, slope position, and soil type. Therefore, we suggest that differentiating such factors can be an effective approach for explaining variances in OC and N changes caused by land use conversion.     

Origin of carbon in agricultural soil profiles deduced from depth gradients of C:N ratios,carbon fractions, δ13C and δ15N values

Plant and Soil - Agricultural soils in Germany store 2.54 Pg of organic carbon (C). However, information about how and when this C entered the soils is limited. This study illustrates how depth...     

Analysis of δ13C of CO2 distinguishes between microbial respiration of added C4-sucrose and other soil respiration in a C3-ecosystem

The main aim of this study was to test various hypotheses regarding the changes in ¹³C of emitted CO₂ that follow the addition of C₄-sucrose to the soil of a C₃-ecosystem. It forms part of an experimental series designed to assess whether or not the contributions from C₃-respiration (root and microbial) and C₄-respiration (microbial) to total soil respiration can be calculated from such changes. A series of five experiments, three on sieved (root-free) mor-layer material, and two in the field with intact mor-layer (and consequently with active roots), were performed. Both in the experiments on sieved mor-layer and the field experiments, we found a C₄-sucrose-induced increase in C₃-respiration that accounted for between 30% and 40% of the respiration increase 1 h after sucrose addition. When the course of C₃-, C₄- and total respiration was followed in sieved material over four days following addition of C₄-sucrose, the initially increased respiration of C₃-C was transient, passing within less than 24 h. In a separate pot experiment, neither ectomycorrhizal Pinus sylvestrisL. roots nor non-mycorrhizal roots of this species showed respiratory changes in response to exogenous sucrose. No shift in the ¹³C of the evolved CO₂ after adding C₃-sucrose to sieved mor-layer material was found, confirming that the sucrose-induced increase in respiration of endogenous C was not an artefact of discrimination against ¹³C during respiration. Furthermore, we conclude that the C₄-sucrose induced transient increase in C₃-respiration is most likely the result of accelerated turnover of C in the microbial biomass. Thus, neither respiration of mycorrhizal roots, nor processes discriminating against ¹³C were likely sources of error in the field. The estimated ¹³C of evolved soil CO₂ in three field experiments lay between –25.2 and –23.6. The study shows that we can distinguish between CO₂ evolved from microbial mineralisation of added C₄-sucrose, and CO₂ evolved from endogenous carbon sources (roots and microbial respiration).     

Climatic Control on Plant and Soil δ13C along an Altitudinal Transect of Lushan Mountain in Subtropical China: Characteristics and Interpretation of Soil Carbon Dynamics

Decreasing temperature and increasing precipitation along altitude gradients are typical mountain climate in subtropical China. In such a climate regime, identifying the patterns of the C stable isotope composition (δ¹³C) in plants and soils and their relations to the context of climate change is essential. In this study, the patterns of δ¹³C variation were investigated for tree leaves, litters, and soils in the natural secondary forests at four altitudes (219, 405, 780, and 1268 m a.s.l.) in Lushan Mountain, central subtropical China. For the dominant trees, both leaf and leaf-litter δ¹³C decreased as altitude increased from low to high altitude, whereas surface soil δ¹³C increased. The lower leaf δ¹³C at high altitudes was associated with the high moisture-related discrimination, while the high soil δ¹³C is attributed to the low temperature-induced decay. At each altitude, soil δ¹³C became enriched with soil depth. Soil δ¹³C increased with soil C concentrations and altitude, but decreased with soil depth. A negative relationship was also found between O-alkyl C and δ¹³C in litter and soil, whereas a positive relationship was observed between aromatic C and δ¹³C. Lower temperature and higher moisture at high altitudes are the predominant control factors of δ¹³C variation in plants and soils. These results help understand C dynamics in the context of global warming.     

Effects of atmospheric CO2 enrichment on δ 13C, δ 15N values and turnover times of soil organic matter pools isolated by thermal techniques

CO₂ applied for Free-Air CO₂ Enrichment (FACE) experiments is strongly depleted in ¹³C and thus provides an opportunity to study C turnover in soil organic matter (SOM) based on its δ ¹³C value. Simultaneous use of ¹⁵N labeled fertilizers allows N turnover to be studied. Various SOM fractionation approaches (fractionation by density, particle size, chemical extractability etc.) have been applied to estimate C and N turnover rates in SOM pools. The thermal stability of SOM coupled with C and N isotopic analyses has never been studied in experiments with FACE. We tested the hypothesis that the mean residence time (MRT) of SOM pools is inversely proportional to its thermal stability. Soil samples from FACE plots under ambient (380 ppm) and elevated CO₂ (540 ppm; for 3 years) treatments were analyzed by thermogravimetry coupled with differential scanning calorimetry (TG-DSC). Based on differential weight losses (TG) and energy release or consumption (DSC), five SOM pools were distinguished. Soil samples were heated up to the respective temperature and the remaining soil was analyzed for δ ¹³C and δ ¹⁵N by IRMS. Energy consumption and mass losses in the temperature range 20–200°C were mainly connected with water volatilization. The maximum weight losses occurred from 200–310°C. This pool contained the largest amount of carbon: 61% of the total soil organic carbon in soil under ambient treatment and 63% in soil under elevated CO₂, respectively. δ ¹³C values of SOM pools under elevated CO₂ treatment showed an increase from −34.3‰ of the pool decomposed between 20–200°C to −18.1‰ above 480°C. The incorporation of new C and N into SOM pools was not inversely proportional to its thermal stability. SOM pools that decomposed between 20–200 and 200–310°C contained 2 and 3% of the new C, with a MRT of 149 and 92 years, respectively. The pool decomposed between 310–400°C contained the largest proportion of new C (22%), with a MRT of 12 years. The amount of fertilizer-derived N after 2 years of application in ambient and elevated CO₂ treatments was not significantly different in SOM pools decomposed up to 480°C having MRT of about 60 years. In contrast, the pool decomposed above 480°C contained only 0.5% of new N, with a MRT of more than 400 years in soils under both treatments. Thus, the separation of SOM based on its thermal stability was not sufficient to reveal pools with contrasting turnover rates of C and N. Responsible Editor: Bernard Nicolardot.     

Purification and properties of glycolate oxidase from plants with different photosynthetic pathways: Distinctness of C4 enzyme from that of a C3 species and a C3–C4 intermediate

Glycolate oxidase (GO; EC 1.1.3.1) was purified from the leaves of three plant species:Amaranthus hypochondriacus L.(NAD-ME type C₄ dicot),Pisum sativum L. (C₃ species) andParthenium hysterophorus L. (C₃–C₄. intermediate). A flavin moiety was present in the enzyme from all the three species. The enzyme from the C₄ plant had a low specific activity, exhibited lower K_M for glycolate, and required a lower pH for maximal activity, compared to the C₃ enzyme. The enzyme from the C₄ species oxidized glyoxylate at <10% of the rate with glycolate, while the GO from the C₃ plant oxidized glyoxylate at a rate of about 35 to 40% of that with glycolate. The sensitivity of GO from C₄ plant to -hydroxypyridinemethane sulfonate, 2-hydroxy-3-butynoate and other inhibitors was less than that of the enzyme from C₃ source. The properties of GO fromParthenium hysterophorus, were similar to those of the enzyme fromPisum sativum. The characteristics of glycolate oxidase from leaves of a C₄ plant,Amaranthus hypochondriacus are different from those of the C₃ species or the C₃–C₄ intermediate.     

Effect of seasonal freeze–thaw cycle on net nitrogen mineralization of soil organic layer in the subalpine/alpine forests of western Sichuan,China

Nitrogen mineralization, a main way that soil organic nitrogen converts to mineral nitrogen, is one of the key processes in soil nitrogen cycle. The mineral nitrogen has an important role in plant growth in the growing season. It has been widely accepted that soil freezing in winter can kill a number of microorganisms, weakening soil nitrogen mineralization. However, more and more recent studies have documented that soil microorganisms still have high activity during the deep freezing period, and obvious nitrogen mineralization in winter. Seasonal freeze–thaw cycle is a common phenomenon in the subalpine/alpine forest region, which may have a strong effect on soil ecological processes. Furthermore, the changing pattern of seasonal freeze–thaw cycles might have a significant influence on soil nitrogen mineralization in this region in the scenarios of global warming. As yet, little attention has been given to nitrogen mineralization of soil organic layer as affected by changed seasonal freeze–thaw pattern, although the increasing studies have demonstrated that winter warming might give strong effects on the litter decomposition and microbial activity in the subalpine/alpine forest regions. Therefore, a method of intact soil core incubation in combination with natural environmental gradient was employed by transferring forest soils from 3582 m (A₁) of altitude to 3298 m (A₂) of altitude and 3023 m (A₃) of altitude in the subalpine/alpine forests of western Sichuan, respectively. The amounts and rates of net nitrogen mineralization in soil organic layer were measured. The incubation period included the growing season and the freeze–thaw season from May 24, 2010 to April 19, 2011. The results suggested that significant net nitrogen mineralization was only observed in soil organic layer at low altitude (A₃) during the whole incubation period. Forest soils at higher altitudes (A₁ and A₂) showed obvious soil nitrogen immobilization. In comparison with the growing season which showed remarkable nitrogen immobilization characteristic, the freeze–thaw season showed obvious nitrogen mineralization at lower altitudes (A₂ and A₃). In contrast, the nitrogen immobilization amounts at high altitude (A₁) in freeze–thaw period were less than those in the growing season. Besides, the maximum of net nitrogen mineralization amounts and rates at high altitude (A₁) in soil organic layer mainly occurred in the late stage of growing season and the onset of freezing, soil nitrogen mineralization at the middle altitude (A₂) mainly occurred in the onset of freezing and the deep freezing period, while the highest amount and rate of net nitrogen mineralization at low altitude (A₃) occurred in the early stage of thawing and the late stage of growing season. Furthermore, the amount and rate of soil net nitrogen mineralization during the freeze–thaw season were increasing with the decrease of altitude, which correlated with soil freeze–thaw cycle and freezing process at different altitudes. These results indicated that increasing soil temperature in the future could not only significantly enhance soil nitrogen mineralization in the freeze–thaw season, but also improve soil nitrogen mineralization by increasing freeze–thaw cycle times and shortening freeze–thaw period. However, the processes were significantly influenced by soil micro-environment of subalpine/alpine forest regions.