Carbon isotopes reveal soil organic matter dynamics following arid land shrub expansion

 Over the past century, overgrazing and drought in New Mexico’s Jornada Basin has promoted the replacement of native black grama (Bouteloua eriopoda Torr.) grass communities by shrubs, primarily mesquite (Prosopis glandulosa Torr.). We investigated the effects of shrub expansion on the distribution, origin, turnover, and quality of light (LFC) and heavy (HFC) soil organic matter (SOM) fractions using δ¹³C natural abundance to partition SOM into C₄ (grass) and C₃ (shrub) sources. Soil organic matter beneath grasses and mesquite was isotopically distinct from associated plant litter, providing evidence of both recent shrub expansion and Holocene plant community changes. Our δ¹³C analyses indicated that SOM derived from mesquite was greatest beneath shrub canopies, but extended at least 3 m beyond canopy margins, similar to the distribution of fine roots. Specific ¹⁴C activities of LFC indicated that root litter is an important source of SOM at depth. Comparison of turnover rates for surface LFC pools in grass (7 or 40 years) and mesquite (11 or 28 years) soils and for HFC pools by soil depth (∼150–280 years), suggest that mesquite may enhance soil C storage relative to grasses. We conclude that the replacement of semiarid grasslands by woody shrubs will effect changes in root biomass, litter production, and SOM cycling that influence nutrient availability and long-term soil C sequestration at the ecosystem level. Received: 17 May 1996 / Accepted: 12 November 1996     

Root biomass allocation in the world's upland forests

Because the world's forests play a major role in regulating nutrient and carbon cycles, there is much interest in estimating their biomass. Estimates of aboveground biomass based on well-established methods are relatively abundant; estimates of root biomass based on standard methods are much less common. The goal of this work was to determine if a reliable method to estimate root biomass density for forests could be developed based on existing data from the literature. The forestry literature containing root biomass measurements was reviewed and summarized and relationships between both root biomass density (Mg ha⁻¹) and root:shoot ratios (R/S) as dependent variables and various edaphic and climatic independent variables, singly and in combination, were statistically tested. None of the tested independent variables of aboveground biomass density, latitude, temperature, precipitation, temperature:precipitation ratios, tree type, soil texture, and age had important explanatory value for R/S. However, linear regression analysis showed that aboveground biomass density, age, and latitudinal category were the most important predictors of root biomass density, and together explained 84% of the variation. A comparison of root biomass density estimates based on our equations with those based on use of generalized R/S ratios for forests in the United States indicated that our method tended to produce estimates that were about 20% higher. Received: 3 July 1996 / Accepted: 23 January 1997     

Nitrogen mineralization and denitrification as influenced by crop residue particle size

Managing the crop residue particle size has the potential to affect N conservation in agricultural systems. We investigated the influence of barley (Hordeum vulgare) and pea (Pisum sativum) crop residue particle size on N mineralization and denitrification in two laboratory experiments. Experiment 1: ¹⁵N-labelled ground (3 mm) and cut (25 mm) barley residue, and microcrystalline cellulose+glucose were mixed into a sandy loam soil with additional inorganic N. Experiment 2: inorganic¹⁵ N and C₂H₂ were added to soils with barley and pea material after 3, 26, and 109 days for measuring gross N mineralization and denitrification.Net N immobilization over 60 days in Experiment 1 cumulated to 63 mg N kg^-1 soil (ground barley), 42 (cut barley), and 122 (cellulose+glucose). More N was seemingly net mineralized from ground barley (3.3 mg N kg^-1 soil) than from cut barley (2.7 mg N kg^-1 soil). Microbial biomass peaked at day 4 with the barley treatments and at day 14 with the cellulose+glucose whereafter the biomass leveled out at values 79 mg C kg^-1 (ground), 104 (cut), and 242 (cellulose+glucose) higher than for the control soil. Microbial growth yields were similar for the two barley treatments, ca. 60 mg C g^-1 substrate C added, which was lower than the 142 mg C g^-1 C added with cellulose+glucose. This suggests that the 75% (w/w) holocelluloses and sugars contained with the barley material remained physically protected despite grinding. In Experiment 2 gross mineralization on day 3 was 4.8 mg N kg^-1 d^-1 with ground pea, twice as much as for all other treatments. On day 26 the treatment with ground barley had the greatest gross N mineralization. In static cores ground barley denitrified 11-fold more than did cut barley, whereas denitrification was similar for the two pea treatments. In suspensions denitrification was similar for the two treatments both with barley and pea residue.We conclude that the higher microbial activity associated with the initial decomposition of ground plant material is due to a more intimate plant residue-soil contact. On the long term, grinding the plant residues has no significant effect on N dynamics.     

Influence of pastoral fallow on plant root growth and soil physical and chemical characteristics in a hill pasture

Pastoral fallowing over a growing season (October–May) has a profound effect on standing biomass and sward structure, and should have an impact on below ground plant growth and soil biological activities. Two field studies were conducted to compare the effects of pastoral fallow with rotational grazing on root growth and soil physical and chemical properties. Root growth and distribution was altered by pastoral fallowing and there was significantly (P < 0.01) less root biomass at 0–50 mm depth of soil in the fallowed sward than the grazed sward. Compared with the grazed treatment, pastoral fallow increased soil air permeability at 500 mm tension by 38%, saturated hydraulic conductivity by 26%, unsaturated hydraulic conductivity at 20 mm tension by 67% and soil moisture by 10–15%, and reduced soil bulk density by 11%. Fallowing had little effect on soil nutrients both at the end of fallowing, except for small reductions in K and Mineral N levels at 0–75 mm soil depth, and two to three years after fallowing.     

A mycorrhizal fungus changes microtubule orientation in tobacco root cells

Summary Cortical cells of mycorrhizal roots undergo drastic morphological changes, such as vacuole fragmentation, nucleus migration, and deposition of cell wall components at the plant-fungus interface. We hypothesized that the cytoskeleton is involved in these mechanisms leading to cell reorganization. We subjected longitudinal, meristem to basal zone, sections of uninfectedNicotiana tabacum roots to immunofluorescence methods to identify the microtubular (MT) structures associated with root cells. Similar sections were obtained from tobacco roots grown in the presence ofGigaspora margarita, an arbuscular mycorrhizal fungus which penetrates the root via the epidermal cells, but mostly develops in the inner cortical cells. While the usual MT structures were found in uninfected roots (e.g., MTs involved in mitosis in the meristem and cortical hoops in differentiated parenchyma cells), an increase in complexity of MT structures was observed in infected tissues. At least three new systems were identified: (i) MTs running along large intracellular hyphae, (ii) MTs linking hyphae, (iii) MTs binding the hyphae to the host nucleus. The experiments show that mycorrhizal infection causes reorganization of root MTs, suggesting their involvement in the drastic morphological changes shown by the cortical cells.     

Mapping genes controlling root morphology and root distribution in a doubled-haploid population of rice

 A deep thick root system has been demonstrated to have a positive effect on yield of upland rice under water stress conditions. Molecular-marker-aided selection could be helpful for the improvement of root morphological traits, which are otherwise difficult to score. We studied a doubled-haploid population of 105 lines derived from an indica×japonica cross and mapped the genes controlling root morphology and distribution (root thickness, maximum root length, total root weight, deep root weight, deep root weight per tiller, and deep root to shoot ratio). Most putative QTL activity was concentrated in fairly compact regions on chromosomes 1, 2, 3, 6, 7, 8 and 9, but was widely spread on chromosome 5 and largely absent on chromosomes 4, 10, 11 and 12. Between three and six QTLs were identified on different chromosomes for each trait. Individual QTLs accounted for between 4 and 22% of the variation in the traits. Multiple QTL models accounted for between 14 and 49%. The main QTLs were common between traits, showing that it should be possible to modify several aspects of root morphology simultaneously. There was evidence of interaction between marker locations in determining QTL expression. Interacting locations were mostly on different chromosomes and showed antagonistic effects with magnitudes large enough to mask QTL detection. The comparison of QTL locations with another population showed that one to three common QTLs per trait were recovered, among which the most significant was in one or other population. These results will allow the derivation of isogenic lines introgressed with these common segments, separately in the indica and japonica backgrounds. Received: 12 August 1996 / Accepted: 15 November 1996     

Production of lactic acid from wastepaper as a cellulosic feedstock

Lactic acid promises to be an important commodity chemical in the future as a monomer for the production of biodegradable polylactic acid (PLA). As the demand for lactic acid increases, the need to explore alternative feedstock sources and process options that are inexpensive and efficient is bound to gain importance. This paper reports the results of a study of the production of lactic acid from wastepaper as a representative cellulosic feedstock, using a batch, bench-scale simultaneous saccharification and fermentation (SSF) process. The effect on process performance of operating parameters such as pH, temperature, enzyme loading, solids concentration, and enzyme preparation has been examined. A lactic acid product yield of 84% of theoretical was achieved at a solids loading of 5%, using 25 filter paper units (FPU) of cellulase per gram of cellulose, at 45°C and pH 5.0. The pH and temperature of operation have been selected to achieve good performance of both the cellulase and the microoganism in the SSF process. Our studies show that a feedstock such as wastepaper offers considerable promise and opportunity in the future for development of a biomass-based process for lactic acid production. Received 09 January 1996/ Accepted in revised form 22 August 1996     

羊草种群地上部生物量与株高的分形关系

应用分形几何学（Ｆｒａｃｔａｌ ｇｅｏｍｅｔｒｙ）的原理和方法对羊草（Ａｎｅｕｒｏｌｅｐｉｄｉｕｍ ｃｈｉ－ ｎｅｎｓｅ）种群地上部生物量与株高的关系进行了研究．结果表明．羊草种群的地上部生物量 与株高存在很好的静态分形关系,所得分形维数是对地上部生物量在各器官中积累规律 （生物量配比）的表征;羊草种群地上部生物量与株高的动态分形关系表明在整个生长季 内该种群地上部生物量的增长具有自相似性．是一个分形生长过程,增长规律为分形维数 值Ｄ;在此基础上建立了羊草种群的分形生长模式．认为成熟植株可以看作是其幼苗经生 长放大过程而得到的．     

Growth of Fagus sylvatica saplings in an old-growth forest as affected by soil and light conditions

Abstract. Studies were conducted on 41 five yr-old common beech (Fagus sylvatica) saplings collected in an old-growth beech wood (Fontainebleau forest, biological reserve of La Tillaie, France), under varying humus and light conditions, following gypsy moth (Lymantria dispar) caterpillar injuries. Aerial and subterranean parts of each sapling were described by means of 34 parameters and environmental conditions at the microsite, where each sapling was excavated, were characterized by 23 parameters. The development of beech saplings is strongly affected by microsite conditions. An increase in sapling size was associated with darkness of the A-horizon, typical of zones with poor mineralization of organic matter. Light conditions were more important in influencing the development of the root system than that of the aerial parts. Rooting depth was shallower and rate of mycorrhiza development by the black ascomycete Cenococcum geophilum was lower in microsites receiving incident light during the morning than in those never receiving incident light during this period. Results are discussed in the frame of survival of young beech individuals in varying environmental conditions, when submitted to competition by other vegetation and adverse climate conditions.     

Effects of fire,mowing and nitrogen addition on root characteristics in tall-grass prairie

Abstract. Root harvests and root windows were used to study the influence of fire, mowing and nitrogen additions on root lengths, biomass, and nitrogen content in tall-grass prairie. Four years of nitrogen additions (10 g m² yr^?1) increased below-ground mass by 15 % and nitrogen concentration in that mass by 77 %. In general, live roots and rhizomes exhibited greater increases in nitrogen concentrations than detrital roots and rhizomes. After four years of treatment, live roots and rhizomes immobilized an additional 1.5 to 5 g/m² of nitrogen, depending upon specific treatment, while dead roots and rhizomes immobilized an additional 3 to 3.5 g/m². Average root growth parameters, as measured with root windows, were positively correlated with above-ground peak foliage biomass; however, the only significant correlation was between average new root growth and above-ground peak foliage biomass (r = 0.73, p ≤ 0.04). Root growth and decay, as measured by annual mean values for eight root windows over a four year interval, were insensitive to climatic and treatment effects.