Evaluation of the organic matter supplied to the soil by the decay of the roots of an intensively managedPanicum Maximum sward

Summary A simple three-compartment model was used to evaluate the supply of organic matter to the soil by the decay of the dead roots of an intensively managedPanicum maximum sward. The only known data were the weights of the total dry matter of the roots measured every two weeks for a whole year. By means of three simplifying hypotheses it was possible to evaluate the limits between which the supply of organic matter to the soil lay: between 9 and 16 t/ha/year for a sward, the mean root weight of which at any cutting data was about 3 t/ha. Thus, the root turnover was 2.5 to 4.6 per year.     

Soil Heterogeneity Effects on Tallgrass Prairie Community Heterogeneity: An Application of Ecological Theory to Restoration Ecology

Spatial heterogeneity of resources can influence plant community composition and diversity in natural communities. We manipulated soil depth (two levels) and nutrient availability (three levels) to create four heterogeneity treatments (no heterogeneity, depth heterogeneity, nutrient heterogeneity, and depth + nutrient heterogeneity) replicated in an agricultural field seeded to native prairie species. Our objective was to determine whether resource heterogeneity influences species diversity and the trajectory of community development during grassland restoration. The treatments significantly increased heterogeneity of available inorganic nitrogen (N), soil water content, and light penetration. Plant diversity was indirectly related to resource heterogeneity through positive relationships with variability in productivity and cover established by the belowground manipulations. Diversity was inversely correlated with the average cover of the dominant grass, Switchgrass (Panicum virgatum), which increased over time in all heterogeneity treatments and resulted in community convergence among the heterogeneity treatments over time. The success of this cultivar across the wide range of resource availability was attributed to net photosynthesis rates equivalent to or higher than those of the native prairie plants in the presence of lower foliar N content. Our results suggest that resource heterogeneity alone may not increase diversity in restorations where a dominant species can successfully establish across the range of resource availability. This is consistent with theory regarding the role of ecological filters on community assembly in that the establishment of one species best adapted for the physical and biological conditions can play an inordinately important role in determining community structure.     

Yield and species composition of a mesic grassland savanna in South Africa are influenced by long‐term nutrient addition

Species composition and productivity of natural grasslands are influenced by soil nutrient status. With high resource availability, productivity is expected to increase, and competition is assumed to gain prominence with predicted exclusion of species of lower competitive ability. During 2010 and 2011 we used the dry weight rank method to measure above‐ground phytomass production of herbage in 96 plots (9 m × 2.7 m) fertilized for 60 years with two forms of nitrogen (N as limestone ammonium nitrate or ammonium sulphate at four levels: 0, 7.1, 14.1, 21.2 g m^?2), phosphorus (P as superphosphate at two levels: 0, 33.6 g m^?2), and lime (two levels: 0, 225 g m^?2). Light attenuation was measured as the proportion of photosynthetically active radiation reaching the lower leaf layers of the grasses and the ground surface. Light conditions beneath the grass layer were reduced by nutrient addition to 30% of full sunlight but remained above 60% in non‐fertilized plots. Grass total above‐ground phytomass production increased with nutrient addition. The strongest yield responses were attained with N plus P addition. Species responses showed that Themeda triandra and Hyparrhenia hirta decreased in above‐ground phytomass production with nutrient addition while Panicum maximum, Eragrostis curvula and E. plana increased. These findings are discussed in terms of competitive interactions among species, their position in the grass canopy and their physiological tolerances to high nitrogen environments.     

Cadmium for all meals – plants with an unusual appetite

Since the beginning of the Bronze Age the acquisition of metals and the mastering of metal processing has been a driving force in human civilisation. It is difficult to imagine, however, how a plant could possibly profit from accumulating excessive amounts of a metal for which there is no metabolic need. Leaving behind an environmentally degraded wasteland, human mining and smeltering activities contributed to providing the niches for such plants, sometimes even accomplishing seed dispersal – through seeds attached to the boots of migrating miners. A stunning example of this is found in the healthy populations of Thlaspi caerulescens in the Cevennes (southern France), the leaves of which have been found to contain the toxic and non-essential metal cadmium (Cd) at concentrations between 1000 and 3000 mg kg⁻¹ dry biomass (R. D. Reeves, unpublished). In this issue, experiments under controlled experimental and glasshouse conditions are reported, confirming an extraordinary level of Cd accumulation and tolerance in these populations (Lombi et al ., pp. 11–20).     

Connecting carbon and nitrogen storage in rural wetland soil to groundwater abstraction for urban water supply

We investigated whether groundwater abstraction for urban water supply diminishes the storage of carbon (C), nitrogen (N), and organic matter in the soil of rural wetlands. Wetland soil organic matter (SOM) benefits air and water quality by sequestering large masses of C and N. Yet, the accumulation of wetland SOM depends on soil inundation, so we hypothesized that groundwater abstraction would diminish stocks of SOM, C, and N in wetland soils. Predictions of this hypothesis were tested in two types of subtropical, depressional‐basin wetland: forested swamps and herbaceous‐vegetation marshes. In west‐central Florida, >650 ML groundwater day^?1 are abstracted for use primarily in the Tampa Bay metropolis. At higher abstraction volumes, water tables were lower and wetlands had shorter hydroperiods (less time inundated). In turn, wetlands with shorter hydroperiods had 50–60% less SOM, C, and N per kg soil. In swamps, SOM loss caused soil bulk density to double, so areal soil C and N storage per m² through 30.5 cm depth was diminished by 25–30% in short‐hydroperiod swamps. In herbaceous‐vegetation marshes, short hydroperiods caused a sharper decline in N than in C. Soil organic matter, C, and N pools were not correlated with soil texture or with wetland draining‐reflooding frequency. Many years of shortened hydroperiod were probably required to diminish soil organic matter, C, and N pools by the magnitudes we observed. This diminution might have occurred decades ago, but could be maintained contemporarily by the failure each year of chronically drained soils to retain new organic matter inputs. In sum, our study attributes the contraction of hydroperiod and loss of soil organic matter, C, and N from rural wetlands to groundwater abstraction performed largely for urban water supply, revealing teleconnections between rural ecosystem change and urban resource demand.     

Direct emission of methane and nitrous oxide from switchgrass and corn stover: implications for large-scale biomass storage

Little is known about the contributions of biomass feedstock storage to the net greenhouse gas emissions from cellulosic biofuels. Direct emissions of methane and nitrous oxide during decomposition in storage may contribute substantially to the global warming potential of biofuels. In this study, laboratory-scale bales of switchgrass and corn stover were stored under a range of moisture (13.0–32.9%) and temperature (5–35 °C) conditions and monitored for O₂ consumption and CO₂, CH₄, and N₂O production over 8 weeks. Gas concentrations and emissions rates were highly variable within and between experimental groups. Stover bales produced higher CO₂ concentrations (P = 0.0002) and lower O₂ (P < 0.0001) during storage than switchgrass bales. Methane concentrations (1.8–2100 ppm) were inversely correlated with bale moisture (P < 0.05), with emissions rates ranging from 4.4–914.9 μg kg⁻¹ DM day⁻¹. Nitrous oxide concentrations ranged from 0 to 31 ppm, and emissions from switchgrass bales inversely correlated with temperature and moisture (P < 0.0001). Net global warming potential from each treatment (0–2.4 gCO₂e kg⁻¹ DM) suggests that direct emission of methane and nitrous oxide from aerobically stored feedstocks have a small effect on net global warming potential of cellulosic biofuels.     

Beta-amylase in germinating millet seeds

Beta-amylase (EC 3.2.1.2) was isolated from germinating millet (Panicum miliaceum L.) seeds by a procedure that included ammonium sulfate fractionation, chromatography on DEAE-cellulofine and CM-cellulofine, and preparative isoelectric focusing. The enzyme was homogeneous by SDS-PAGE. The M(r) of the enzyme was estimated to be 58,000 based on its mobility on SDS-PAGE and gel filtration with TSKgel G4000SW(XL), which showed that it is composed of a single unit. The isoelectric point of the enzyme was 4.62. The enzyme hydrolyzed malto-oligosaccharides more readily as their degree of polymerization increased, this being strongest for malto-oligosaccharides larger than 13 glucose residues and very weakly for maltotriose. Amylose, amylopectin and soluble starch were the most suitable substrates for the enzyme. While the enzyme showed some activity against native starch by itself, starch digestion was accelerated 2.5-fold using alpha-amylase, pullulanase and alpha-glucosidase. This enzyme appears to be very important for the germination of millet seeds.     

The Activity and Isoform Complement of Glutamine Synthetase in Panicum Species Differing in Photosynthetic Pathway

Anion exchange chromatography and immunoprecipitation have been used to demonstrate the presence of two forms (GS₁, and GS₂) of glutamine synthetase in the leaves of nine species of Panicum representative of C₃, C₄ and C₃-C₄ intermediate-type photosynthesis. GS₂ from the Panicum species, P. miliaceum and P. maximum was more thermostable than GS₁, GS₁, and GS₂ from P. laxum were equally thermostable but GS₂ from all the Panicum species examined was more sensitive to inhibition by N-ethylmaleimide than GS₁. GS₁, and GS₂ were characterised as being cytoplasmic and chloroplastic isoforms respectively by their reaction with N-ethylmaleimide and by immunoprecipitation with antibodies raised against the cytosolic isoform in barley and the chloroplastic form in tobacco. C₃ species were found to have higher activity of the chloroplastic isoform of glutamine synthetase than C₄ species. C₃-C₄ intermediate species had total leaf glutamine synthetase activities similar to those in C₃ species but were found to have a lower chloroplastic isoform content. The results are consistent with the reassimilation of photorespiratory ammonia by chloroplastic glutamine synthetase.