Belowground Material Research Articles

Effects of source sample amount on biodiversity surveys of bacteria, fungi, and nematodes in soil ecosystems

Bacteria, fungi, and nematodes are major components of soil ecosystems, playing pivotal roles in belowground material cycles and biological community processes. A number of studies have recently uncovered the diversity and community structure of those organisms in various types of soil ecosystems based on DNA metabarcoding (amplicon sequencing). However, because most previous studies examined only one or two of the three organismal groups, it remains an important challenge to reveal the entire picture of soil community structure. We examined how we could standardize DNA extraction protocols for simultaneous DNA metabarcoding of bacteria, fungi, and nematodes. Specifically, in an Illumina sequencing analysis of forest and farmland soil samples, we performed DNA extraction at five levels of soil-amount (0.5, 2, 5, 10, and 20 g). We then found that DNA extraction with the 0.5 g soil setting, which had been applied as default in many commercial DNA extraction kits, could lead to underestimation of α-diversity in nematode community. We also found that dissimilarity (β-diversity) estimates of community structure among replicate samples could be affected by soil sample amount. Based on the assays, we conclude that DNA extraction from at least 20 g of soil is a standard for comparing biodiversity patterns among bacteria, fungi and nematodes.

Short phases of tropical forage legumes increase production of subsequent cereal crops in the seasonally dry tropics of eastern Indonesia

In temperate systems, it is well known that forage legumes can improve both nitrogen (N) supply and yields of subsequent cereal crops. While this is assumed to be true in tropical systems, it is less well tested, particularly in smallholder settings where forage is often cut and removed from the field. This paper confirms the potential of short phases of tropical forage legumes to provide N to subsequent crops in seasonally dry tropical farming systems. Across five experiments, maize and rice grain yields increased by up to 80 % after 4−8 month rotations of forage legumes, but the benefits were smaller when legume growth was reduced, biomass was removed for forage, or the yield potential of cereal crops was lower. We found that the additional N cycling from legumes can last for at least 2 years, although the benefit diminishes with time. When all legume material was retained as mulch, the estimated additional N provided to subsequent no-till maize crops was the equivalent of 9−15 kg urea-N per tonne of above-ground legume biomass produced but fell to 1.5−3 kg urea-N/t if forage was removed. After shoot removal, more legume N cycled to a subsequent rice crop (equivalent of 11–13 kg urea-N/t of legume biomass) than to a no-till maize crop, presumably because more below-ground material mineralised. Of the legumes tested, Clitoria ternatea grew best across a variety of environments and use patterns and provided the largest yield benefits to subsequent crops. This research demonstrates the potential to integrate short phases of tropical herbaceous forage legumes into smallholder crop-livestock systems in the seasonally dry tropics, and, in doing so, improve or maintain staple grain crop production and household food self-sufficiency.

Polysaccharides and phenolics of miscanthus belowground cell walls and their influence on polyethylene composites

Belowground materials from two miscanthus species were ground into fragments for preparing polyethylene composites. Both species show a lot of similarities in terms of polysaccharides, lignin and cell wall-linked p-coumaric and ferulic acids contents. The structures of polysaccharides and of lignins are markedly different in the miscanthus belowground and aboveground biomass. The non-cellulosic fraction of the samples comprises a high level of xylose, with the arabinose to xylose ratio about twice as high as that observed for analogous stem samples, suggesting that belowground arabinoxylans are more substituted than stem ones. The mechanical properties of the belowground miscanthus-polyethylene composites correlate with several of their compositional traits, with similar trends as for plant stem-polyethylene composites with positive correlations for lignin and p-coumaric acid contents and negative correlations for most non-cellulosic sugars.

First Report of Gaeumannomyces graminicola Causing Bermudagrass Decline of Ultradwarf Bermudagrass Putting Greens in North Carolina

HomePlant DiseaseVol. 104, No. 5First Report of Gaeumannomyces graminicola Causing Bermudagrass Decline of Ultradwarf Bermudagrass Putting Greens in North Carolina PreviousNext DISEASE NOTES OPENOpen Access licenseFirst Report of Gaeumannomyces graminicola Causing Bermudagrass Decline of Ultradwarf Bermudagrass Putting Greens in North CarolinaCameron M. Stephens and James P. KernsCameron M. Stephens†Corresponding author: C. M. Stephens; E-mail Address: cmsteph2@ncsu.eduhttp://orcid.org/0000-0002-4708-9669Varsity Research Building, Raleigh, NC 27695Search for more papers by this author and James P. KernsVarsity Research Building, Raleigh, NC 27695Search for more papers by this authorAffiliationsAuthors and Affiliations Cameron M. Stephens † James P. Kerns Varsity Research Building, Raleigh, NC 27695 Published Online:23 Feb 2020https://doi.org/10.1094/PDIS-10-19-2147-PDNAboutSectionsSupplemental ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat Bermudagrass decline is a detrimental disease of ultradwarf hybrid bermudagrass maintained at <5 mm for golf course putting greens. Stand symptoms include off-color to white, irregular patches, chlorosis of older leaf tissue, basal necrosis at plant crowns, and a decrease in stand density. Gaeumannomyces graminis is reported to be the causal agent of this disease (Elliott 1991); however, multiple closely related fungi within the Magnaporthaceae have recently been associated with take-all root rot and bermudagrass decline (Vines et al. 2015). When bermudagrass decline stand symptoms are present, darkly pigmented ectotrophic runner hyphae and lobed hyphopodia are ubiquitous on necrotic roots, the stoloniferous matrix, and the plant crown under severe disease pressure. Symptomatic turf samples from a golf course in Greensboro, NC, were collected using a golf course cup cutter (10.8-cm diameter) in October 2018. Pure cultures of Gaeumannomyces sp. were obtained through isolation from symptomatic root and stolon tissue. Symptomatic roots and stolons were surface disinfested for 1 min in 0.5% sodium hypochlorite solution, rinsed in three consecutive sterile water baths for 30 s, and air dried on sterile filter paper. Once dry, surface-disinfested samples were set on antibiotic-amended water agar containing 0.5 g/liter of streptomycin sulfate and penicillin-G potassium for 5 to 8 days and then subcultured onto potato dextrose agar. Gaeumannomyces sp. was successfully isolated from 80% of the symptomatic tissue evaluated following the aforementioned protocol. Colony morphology was consistent with Gaeumannomyces graminicola M. Hern.-Restr. & Crous sp. nov. (Hernández-Restrepo et al. 2016; Lin et al. 2019). The internal transcribed spacer (ITS) region was sequenced because this marker has the ability to distinguish between the majority of the species within the Gaeumannomyces genus (Hernández-Restrepo et al. 2016). The ITS region of Gaeumannomyces sp. isolates 31282-1A to 1D (accession nos. MN559724, MN559725, MN559726, and MN559727) had 506/507, 503/504, 495/495, and 502/504 similar nucleotides (99.6 to 100% homology) when compared with a known G. graminicola isolate, CPC 26025 (accession no. KX306495), originally isolated from Stenotaphrum secundatum (Walt.) Kuntze (Hernández-Restrepo et al. 2016). Koch’s postulates were performed on healthy ‘Champion’ ultradwarf hybrid bermudagrass plugs (4.4-cm diameter) that were processed by removing the root system and washing the remaining below-ground material with water. G. graminicola was grown on sterile oats for 3 weeks at 25 ± 2°C, and infested oats (n = 10) were placed in the center of 10-cm pots filled with 250 cm3 of sterile sand. A processed bermudagrass plug free of ectotrophic root-infecting fungi was placed directly on the inoculum and lightly top dressed (∼20 cm3 of sterile sand). Experimental units were maintained in the greenhouse at 28 ± 2°C and watered once daily. Basal necrosis, a decrease in plant density and turfgrass quality, and lobed hyphopodia (16.5 to 24 × 15.5 to 23.5 μm) were observed 4 weeks after inoculation. Symptoms were not present on noninoculated and noninfested oat inoculated control plants. G. graminicola was reisolated from symptomatic root and stolon material of inoculated plants and confirmed with ITS sequencing. To our knowledge, this is the first report of G. graminicola causing bermudagrass decline of ultradwarf hybrid bermudagrass in North Carolina, which may influence management of this important disease.The author(s) declare no conflict of interest.

The Distribution of Tree Roots in Douglas-fir Forests in the Pacific Northwest in Relation to Depth, Space, Coarse Organic Matter and Mineral Fragments

This study evaluated relationships between site or tree characteristics and below-ground materials in Douglas-fir forests of the Pacific Northwest. We core-sampled living roots, dead organic matter, and mineral fragments at three soil depths on a 300-sample grid at nine forested sites in western Washington and Oregon resulting in approximately 7200 samples. We explored relationships between materials across depths and at three scales—point, block and site—and examined the data to quantify the degree of spatial clustering. Mass of roots and organic matter declined with depth; there were no consistent patterns with depth for mineral fragments. Correlations between values at the same point but at different depths were low for roots and organic matter at all sites (mean r values < 0.3). Total soil carbon per site was negatively correlated (r = -0.85) with total root mass; correlations between other site or tree variables and below-ground materials were much lower (r = -0.21 to 0.34). There were no detectable spatial patterns in root mass. The number of samples needed to estimate mean root mass was calculated by site and for several desired precision levels; this power analysis will aid others working at similar sites in determining necessary sample sizes a priori, or conversely, in estimating level of precision given a particular sample size. Precision of ± 1.5 kg m-3 could be achieved with less than 100 samples in the upper layer at most sites.

Species and tissue type regulate long-term decomposition of brackish marsh plants grown under elevated CO2 conditions

Organic matter accumulation, the net effect of plant production and decomposition, contributes to vertical soil accretion in coastal wetlands, thereby playing a key role in whether they keep pace with sea-level rise. Any factor that affects decomposition may affect wetland accretion, including atmospheric CO2 concentrations. Higher CO2 can influence decomposition rates by altering plant tissue chemistry or by causing shifts in plant species composition or biomass partitioning. A combined greenhouse-field experiment examined how elevated CO2 affected plant tissue chemistry and subsequent decomposition of above- and belowground tissues of two common brackish marsh species, Schoenoplectus americanus (C3) and Spartina patens (C4). Both species were grown in monoculture and in mixture under ambient (350–385 μL L−1) or elevated (ambient + 300 μL L−1) atmospheric CO2 conditions, with all other growth conditions held constant, for one growing season. Above- and belowground tissues produced under these treatments were decomposed under ambient field conditions in a brackish marsh in the Mississippi River Delta, USA. Elevated CO2 significantly reduced nitrogen content of S. americanus, but not sufficiently to affect subsequent decomposition. Instead, long-term decomposition (percent mass remaining after 280 d) was controlled by species composition and tissue type. Shoots of S. patens had more mass remaining (41 ± 2%) than those of S. americanus (12 ± 2%). Belowground material decomposed more slowly than that placed aboveground (62 ± 1% vs. 23 ± 3% mass remaining), but rates belowground did not differ between species. Increases in atmospheric CO2 concentration will likely have a greater effect on overall decomposition in this brackish marsh community through shifts in species dominance or biomass allocation than through effects on tissue chemistry. Consequent changes in organic matter accumulation may alter marsh capacity to accommodate sea-level rise through vertical accretion.

The heavy metal content of soil and shoots of Vaccinium myrtillus L. in the Słowiński National Park

Abstract The research was carried out in the Słowiński National Park, in an area with 15 research stations in pine coniferous forests situated at locations (1) inaccessible to tourists, (2) most frequently visited by tourists as well as (3) in the vicinity of parking lots. The analysed samples comprised surface generic levels (Ol, Ofh, A), aboveground material (shoots; leaves and stems) and below-ground material (roots) of bilberry (Vaccinium myrtillus). The performed analyses showed statistically significant Spearman’s correlation coefficients for Zn content in the ‘soil - stems:’ (r = -0.44, p &lt; 0.05, n = 45) relationship and the ‘soil - roots’ relationship (r = -0.52, p &lt; 0.05, n = 45). Accordingly, there were significant statistical differences (U Mann-Whitney test) in zinc content in the ‘stems - roots’ relationship and the ‘leaves - roots’ relationship. Furthermore, the obtained results reveal an excessive accumulation of Mn in V. myrtillus. The content of the investigated heavy metals in V. myrtillus shoots decreased in the following order: Mn &gt; Fe &gt; Zn &gt; Cu

Effect of Storage Time and Conditions on the Diene Valepotriates Content of the Extract of Valeriana glechomifolia Obtained by Supercritical Carbon Dioxide

Valepotriates (epoxy iridoid esters) represent an important group of constituents that contribute to pharmacological effects for the genus Valeriana. Storage and extraction of valepotriates is a demanding task, as these compounds are thermolabile and unstable: even when decomposition products are not formed, isovaleric acid liberation from the iridoid nucleus originate compounds with less complex substituents. To study the influence of time and storage conditions on the diene valepotriates (valtrate, isovaltrate, acevaltrate, 1-β-acevaltrate, 1-β-aceacevaltrate) content of the Valeriana glechomifolia (native to southern Brazil), extract was obtained by supercritical fluid extraction using CO₂ as the fluid (SF-CO₂). Above-ground and below-ground material of V. glechomifolia was extracted by SF-CO₂ (40 °C, 90 bar). The extract was stored under nitrogen atmosphere or solubilised in methanol. Valepotriates stability was accessed during storage at -20 °C over 8 months through reverse-phase HPLC (mobile phase acetonitrile:water 50:50 (v/v); 254 nm). A gradual increase in valtrate levels and decrease in acevaltrate, 1-β-acevaltrate and 1-β-aceacevaltrate, concentration were observed from the first month of storage for the dry extract. However, for the methanol solubilised extract these changes occurred only after the third month and were accompanied by reduction in isovaltrate levels and formation of decomposition products. SF-CO₂ showed high selectivity for valepotriates extraction. This is the first report on valepotriates molecular conversion, which was less accelerated when the extract was stored in methanol, but under this condition degradation products are also present, probably baldrinals, that are not observed in the dry extract.

White lupine as a beneficial crop in Southern Europe. II. Nitrogen recovery in a legume–oat rotation and a continuous oat–oat

One experiment lasting for two years was carried out at Pegões (central Portugal) to estimate the impact of mature white lupine residue ( Lupinus albus L.) on yield of fodder oat ( Avena sativa L. cv. Sta. Eulalia) as the next crop in rotation, comparing with the continuous cultivation of cereal, under two tillage practices (conventional tillage and no-till) and fertilized with five mineral nitrogen (N) rates, with three replicates. Oat as a first crop in the rotation provided more N to the agro-ecosystem (63 kg N ha −1) than did lupine (30–59 kg N ha −1). This was at a cost of 100 kg of mineral N ha −1, whereas lupine was grown without addition of N. A positive response of oat as a second crop was obtained per kg of lupine-N added to the system when compared with the continuous oat–oat. The cereal also responded positively to mineral N in the legume amended soil in contrast with the oat–oat sequence where no response was observed, partly due to the fast mineralization rate of lupine residue and a greater soil N immobilization in the continuous oat system. Each kg N ha −1 added to the soil through the application of 73 kg DM ha −1 mature lupine residue (above- and belowground material) increased by 72 kg DM ha −1 the oat biomass produced as the second crop in rotation when 150 kg mineral N ha −1 were split in the season, independent of tillage practice. Mature legume residue conserved in the no-tilled soil depressed the yield of succeeding cereal but less than the continuous oat–oat for both tillage practices, where the application of mineral N did not improve the crop response.

Retention and partitioning of polycyclic aromatic hydrocarbons in Sarcocornia fruticosa from two Portuguese salt marshes

Sixteen PAH compounds (containing three to six rings) were quantified in sediments from the Rosário and Mitrena salt marshes in the Tagus and Sado estuaries, respectively, colonized by Sarcocornia fruticosa, and in belowground and aboveground plant tissues. Sediments from Rosário presented higher PAH concentrations (0.83–8.71 µg g–1) than those from Mitrena (0.22–1.63 µg g–1). Levels in belowground plant tissues showed a pronounced contrast between the two marshes: 84–165 and 0.15–0.43 µg g–1, respectively. Belowground material in contaminated layers contained 9–55 times more PAHs than sediments, while ratios lower than one were obtained in lower contaminated layers. Five- and six-ring compounds predominated (60–75%) in sediments and belowground plant material from both sites. Aboveground plant tissues presented no differences in PAH concentrations (0.17–0.18 ng g–1) and composition in both marshes, only three- and four-ring compounds being detected. This narrow concentration range appears to indicate that atmospheric deposition is the predominant pathway of PAH accumulation in aerial plant tissues.

Evidence for preferential depths of metal retention in roots of salt marsh plants

Depth variation (2-cm resolution) of Fe, Mn, Zn, Cr, Ni, Cu, As and Cd concentrations were determined in belowground biomass of Spartina maritima and Sarcocornia fruticosa and in sediments between roots from two marshes in Tagus (Rosário) and Guadiana (Castro Marim) estuaries in Portugal with different anthropogenic pressures. Levels of metals were also determined in aboveground plant parts. Metal concentrations in belowground material were 2–4 orders of magnitude greater than levels in aboveground plant parts providing evidence of weak upward translocation. Although both studied species showed poor extraction of Cr and Ni from sediments, S. fruticosa exhibited a large capability to remove Zn, Cu, As and Cd from contaminated sediments and stabilised them in belowground biomass. Accumulated metals showed a sub-surface concentration maximum or increase to basal roots. To evaluate whether these preferential layers of accumulation resulted from availability in sediments or controlled by plant activity, Enrichment Factors (EF = [Me] root/[Metal] sediment) were calculated for each sediment layer. Maximum values in Rosário plant species (Zn = 9.3, Cu = 18, As = 20, Cd = 46) exceeded those obtained in Castro Marim (Zn = 1.3, Cu = 4.3, As = 6.1, Cd = 18). Moreover, EFs varied with the depth indicating the presence of preferential layers of metal accumulation in roots of both plants, but depth zonation was not the same as in the sediments. These results suggested that levels in belowground biomass either integrated in time changes that occurred in solid sediments and pore water, or metal uptake by roots was not proportional to levels in sediments. The same sequence of metals transferred from sediment to belowground biomass for the two plants was obtained for the two marshes (Cd > As > Cu, Zn), although metals differed from mining ore to industrial/urban sources.

Nitrogen use by Pinus densiflora trees growing on a Mt. Fuji lava flow

To quantify the nitrogen (N) use by Pinus densiflora trees growing on an infertile lava surface, N pools, N requirement and N uptake through fine roots and N deposition from the atmosphere were estimated. The N requirement and the N uptake of fine roots were 55.5 kg N ha−1 year−1 and 39.7 kg N ha−1, respectively. Thus, the ratio of N uptake to N requirement of the fine roots was 71.5%. Including fine-root contribution, the total N requirement of the P. densiflora trees was 98.6 kg N ha−1 year−1, and the total N uptake was 64.2 kg N ha−1 year−1. Thus, the N uptake of the P. densiflora trees was 64.1% of the N requirement, indicating that P. densiflora trees growing on an infertile lava surface obtain some of their N from below-ground organic material layers every year and the contribution of N storage in trees for their growth is not any higher than indicated in previous reports that excluded fine-roots contribution. The wet N deposition of our research forest was only 5.8% of the N requirement of the P. densiflora trees and only 8.9% of the N uptake. Movement of the below-ground organic material layer N concentrations in the F- and L-layers coincides with needle development and fine-root growth, suggesting the possibility that P. densiflora trees extract N from the organic N of those layers for growth.

Decomposition of white clover ( Trifolium repens) and ryegrass ( Lolium perenne) components: C and N dynamics simulated with the DAISY soil organic matter submodel

Using data from a decomposition study, we aimed to test the parameterisation of the soil organic matter module of the DAISY model, and link measurable plant litter fractions (lignin, water-soluble) with the model defined plant litter pools. Shoot and root material from perennial ryegrass and white clover was incubated in a sandy loam soil at 9 °C for 94 days. Accumulated CO 2 evolution, soil mineral nitrogen (N) and soil microbial biomass-N were measured during the incubation. Marked differences in decomposition rates between above- and below-ground material as well as between the two plant species were observed. The DAISY model was used to interpret the incubation results. Decomposition rates and utilisation efficiencies were modified, under the constraint that rates of specific pools were independent of the type of material, to obtain good agreement between observed and simulated values. Measurable quality parameters were evaluated against the sizes of pools in the model and measured fluxes. The size of the slowest decomposing fraction of the DAISY model was proportional to the lignin content of the plant material, but twice as large. The easily decomposable fraction in the model was well correlated with the water-soluble fraction of the plant material ( r 2=0.84). The size of this pool in the model was larger than the water-soluble fraction of the plant material in three of the five plant materials. The initial carbon mineralisation was correlated with water-solubility of the plant material and total mineralisation with the lignin:N ratio. Net N mineralisation was well correlated with the C:N ratio and the N content of the added material. At the end of the experiment, the mineral N content was overestimated by the DAISY model for all treatments, except one. A soil microbial residual pool, consisting of undecomposed microbial tissue is suggested as a possible N-sink during the incubation. The study demonstrated a correlation between the model-defined pools and chemical plant fractions, but also that the pools in the model were larger than their measured counterparts.

Methanotrophic bacteria and their activity on submerged aquatic macrophytes

The presence and activity of methanotrophic bacteria on submerged freshwater macrophyte organs were examined for 13 species in 10 lake and river sites in New Zealand. Methanotrophic bacteria were first identified qualitatively using polymerase chain reaction (PCR) analysis. PCR successfully located the presence of the particulate methane monooxygenase gene associated with both shoot and root tissue of eight species found in habitats with organic, methane-rich sediments, but not on plants growing in oligotrophic habitats with low sediment methane concentrations. The rates of methane oxidation associated with the macrophyte organs were studied in vitro over a range of methane concentrations from 0 to 3000 ppm. Methanotrophic activity varied considerably between sites, with negligible activity in plants growing in oligotrophic sites, but maximum rates up to 16.2 μmol CH 4 g −1 (plant dry weight) h −1 in plants from eutrophic habitats. The response of methanotrophy to methane concentration was hyperbolic, and half-saturation constants were similar for all plant organs, ranging from 3.4 to 7.8 μmol CH 4 l −1. The submerged macrophyte-associated methanotrophic activity showed anoxia and acetylene sensitivity, similar to methanotrophs from other freshwater habitats. Activity also varied considerably between sites, and within species. In Potamogeton crispus L., the activity associated with the below-ground material was greater for stolons than for roots, whilst for shoots it declined from shoot base to apex. These patterns appeared to be related to differences in the availability of oxygen and methane between sediment, plant and water phases of the gas transport pathway.

Decomposition ofCarex andNuphar plants in a subalpine marsh

To assess the effect of water depth on the decomposition process, I measured the losses in dry mass of the above- and belowground materials ofCarex utriculata andNuphar luteum ssp.potysepalum as well as cellulose (Whatman filter paper) in the top 10 cm of sediment/soil in a subalpine marsh. Samples were examined by the litter bag technique at three flooding levels (0 to 5, 60, and 100 cm water depth). Over a 374-d period, the % mass losses of cellulose,Carex leaves and roots, andNuphar leaves and rhizomes ranged from 98.5 to 99.0, 74.8 to 81.8, 36.3 to 44.9, 95.8 to 97.7, and 78.4 to 91.5%, respectively. Rates for cellulose decay in this study were much higher than for samples from other wetlands; this difference resulted from the location of the litter bag (in the top 10 cm of soilvs in the water column). Water depth significantly affected the decomposition ofCarex roots andNuphar rhizomes. The rate of loss for K was highest in all tissues ofCarex andNuphar, followed by Na inCarex and P inNuphar. N and Ca loss rates generally were low. The C/N ratio tended to converge to a common value over the long term. This convergence has an important implication in the paleoecological interpretation of the C/N ratio change in sediment; i.e., this ratio shift in the sediment core results from a change in the environment, rather than the source material.

Production and decomposition of plant litter in an arid rangeland of Kenya

Data on litter production and decomposition in an arid rangeland in Kenya was collected over a two-year period. Utter sampling was carried out at monthly intervals using a rectangular 0.25m−2 quadrat frame. Utter within the quadrats was handpicked and washed with running water to get rid of soli particles, dried, and weighed. Weights were expressed on organic matter basis. Monthly litter production ranged from 31.4g m−2 to 130.0g m−2. Mean monthly yield was 92.5 ± 26g m−2, with a 28% coefficient of variation. There was no significant difference (p>0.01) in litter yield between 1992 and 1993. Rate of decomposition for aboveground material ranged from 0.005g g−1 day−1 to 0.084g g−1 day−1. The mean annual rate of decomposition was 0.026g g−1 day−1. Belowground plant material rates of decomposition spread from 0.009g g−1 day−1 to 0.062g g−1 day−1. with a mean annual rate of 0.041g g−1 day&minus1. Belowground material consistently decomposed faster than aboveground material. Peaks in both aboveground and belowground material decomposition rates coincided with rainfall peaks. Overall, in this arid environment, litter production and decomposition is pursed in nature, and trends are closely related to rainfall occurrence. Moisture is thus a limiting factor both to the production and decomposition of litter. Belowground litter plays a significant role in nutrient cycling.

Seagrass biomass and production: a reassessment

The biomass and production of seagrass populations were reassessed based on the compilation of a large data set comprising estimates for 30 species, derived from the literature. The mean (± SE) above- and below-ground biomass in the data set were very similar, 223.9 ± 17.5 and 237.4 ± 28 g DW m −2, respectively, indicating a general tendency for a balanced distribution of biomass between leaves and rhizomes + roots (mean ratio (± SE) = 1.11 ± 0.08). The biomass development and the ratio of above- to below-ground biomass varied significantly with latitude and was species-specific, with a significant tendency for large-sized seagrass species to develop high below-ground biomass. Maximum daily seagrass production differed significantly among species, but averaged 3.84 ± 0.34 and 1.21 ± 0.27 g DW m −2 per day for above- and below-ground organs respectively, with an average ratio of above- to below-ground production of 16.4 ± 8.5. The biomass turnover rates averaged 2.6 ± 0.3 and 0.77 ± 0.12% per day for the above- and below-ground material respectively, and tended to be faster for temperate species. The average annual seagrass production found here, 1012 g DW m −2 per year, exceeds previous estimates by 25%, because the average excedent carbon produced by seagrasses must be revised upwards to represent 15% of the total surplus carbon fixed in the global ocean.

Crop nitrogen utilization and soil nitrate loss in a lettuce field

Low N use efficiency and high nitrate (NO 3 - ) pollution potentials are problems in intensive vegetable production systems. The purpose of this study was to quantify N utilization by lettuce (Lactuca sativa L. cv Salinas), and identify periods of NO 3 - loss in an on-farm study in the Salinas Valley in coastal California. During autumn and winter, surface moisture remained low, and NO 3 - concentrations increased, reflecting high net mineralizable N, as determined by anaerobic incubation, and nitrification potential, as determined by the chlorate inhibition method. At the onset of a large winter storm, tracer levels of15NO 3 - were injected in the top 5 mm of soil in 30 cm-deep cylinders. After two weeks, most of the15N was present as15NO 3 - at 10–30 cm depth. By difference, losses to denitrification accounted for ~ 25% of the surface-applied15N. Leaching below 30 cm did not occur, since no15N enrichment of NO 3 - -N was measured in anion-exchange resin membranes placed at the base of each cylinder. During the crop period, NO 3 - losses were most pronounced after irrigation events. Uptake of N by two crops of lettuce (above- and belowground material) was approximately equal to fertilizer inputs, yet simulation of N fates by the Erosion/Productivity Impact Calculator (EPIC) model indicated losses of 14.6 g-N m−2 by leaching and 2.5 g-N m−2 by denitrification during the 6-month crop period. The large NO 3 - losses can be attributed to accumulation of soil NO 3 - during winter that was leached or denitrified during the irrigated crop period.

Seasonal variation of biomass and production dynamics for above- and belowground components of a Spartina alterniflora marsh in the euhaline sector of Paranaguá Bay (SE Brazil)

The seasonal variation of biomass and the annual net primary production for above- and belowground components of Spartina alterniflora Loisel intermediate form were evaluated by harvest methods over a 14-month period in Paranaguá Bay (SE Brazil). Belowground components were characterized by a larger biomass, ranging from 172±48 g m −2 at the end of summer to 569±211 g m −2 in late spring. The biomass of live aboveground tissue was usually higher than dead tissue, ranging from 51±21 g m −2 in winter to 116±36 g m −2 in summer. The overall mean root to shoot ratio was 4·8, ranging from 1·7 in the summer to 8·5 in late spring, reflecting storage and translocation events. Net belowground primary production was 358 g m −2 year −1, largely exceeding aboveground production, which ranged from 101 to 179 g m −2 year −1, depending on how data were handled. Turnover of above- and belowground material, expressed by a production to maximum biomass ratio, was fast, ranging from 0·66 to 1·52 years, respectively. In comparison with temperate Spartina marshes, the low productivity and low biomass of marshes of the high-energy sector of Paranaguá Bay are probably a result of different growth strategies, high salinities and fast sediment accretion, which usually lead to their ultimate replacement by mangroves.

Root dynamics of tallgrass prairie in wet and dry years

Root dynamics were studied using root windows at Konza Prairie, a tallgrass prairie in north central Kansas, during dry (1984) and wet (1985) years. Amounts, production, and disappearance of root length decreased during drought but increased when rains resumed; however, standing crop remained low. The 1985 root lengths increased throughout the growing season, while production and disappearance remained constant. Yearly summaries of amounts, productivity, and decomposition by 10-cm increments in soil depth show that the effect of drought on these variables decreased with increasing soil depth. Turnover rates of root length averaged 564 in the dry year versus 389% in the wet year, with the largest difference noted in the 0- to 10-cm depth (800 in 1984 versus 540% in 1985). Production and decay patterns observed using root windows were also noted in root biomass data (obtained from soil cores). The average total root biomass turnover rate was 31%. Failure to sort below-ground materials into tissue types (rhizomes, roots) and live versus dead status results in reduced estimates of biomass turnover rates. The greatest possible separation of plant components presents the most comprehensive picture of (belowground) growth dynamics.