Uptake Of Grass Research Articles

Improving Iron and Copper Uptake by Changing the Ratios in Root of Vetiver Grass

Phytoremediation using Vetiver grass (VG) has been used in the past few decades all over the world, but the effectiveness of its uptake mechanism in water has less been discussed especially in mixed metal conditions. Hence, the focus of this research was to determine and evaluate the heavy metal removal effectiveness of two heavy metals, copper (Cu) and iron (Fe), based on vetiver grass uptake and toxicity. Three treatments of Fe and Cu mixture were chosen, and individual concentrations act as a control. The vetiver grass at a height of 10 cm and a root length of 25 cm for each treatment was used and harvested on days 0,1, 3, 6 and 7. The roots were randomly cut, oven-dried and used the digestion method to extract the Fe and Cu in the root. From the result, vetiver grass has better Fe and Cu uptake ability in the mixture treatment compared to the individual treatment. Fe uptake is increasing up to 25.3 % in the presence of Cu (mixture 1). Meanwhile, for Cu, mixture 3 is an ideal ratio to increase the Cu uptake by 14.5%. It shows that both Fe and Cu facilitated each other’s uptake in real wastewater conditions. There were also observed minimal toxicity signs such as necrosis throughout the experiment. The appearance of vetiver grass has changed in this experiment starting day 3 and remains yellowish. In conclusion, wastewater that contains a high concentration of Fe than Cu can promise the improvement of both Fe and Cu uptake by vetiver grass. This study benefits researchers that apply phytoremediation to remove heavy metals in wastewater by providing a guideline for Fe and Cu removal by vetiver grass. In future, the study will focus on the mechanism and interaction of Fe and Cu uptake by root

Measurement of grass uptake of the urease inhibitor NBPT and of the nitrification inhibitor dicyandiamide co-applied with granular urea.

Grass uptake and phytoaccumulation factors of N-(n-butyl) thiophosphoric triamide (NBPT) and dicyandiamide (DCD) were quantified. Following the application of urea fertilizer treated with the inhibitors in Irish grassland, grass samples were collected at 5, 10, 15, 20, and 30day time intervals following five application cycles. Uptake of NBPT by grass was below the limit of quantitation of the analytical method (0.010mg NBPT kg-1). Dicyandiamide concentrations in grass ranged from 0.004 to 28mgkg-1 with the highest concentrations measured on days 5 and 10. A reducing trend in concentration was found after day 15. The DCD phytoaccumulation factor was ranged from 0.004% to 1.1% showing that DCD can be taken up by grass at low levels when co-applied with granular urea. In contrast, NBPT was not detected indicating that grass uptake is unlikely when co-applied with granular urea fertilizer. The contrasting results are likely due to very different longevity of DCD and NBPT along with the much lower rate of NBPT, which is used compared with DCD.

Landspreading with co-digested cattle slurry, with or without pasteurisation, as a mitigation strategy against pathogen, nutrient and metal contamination associated with untreated slurry

North Atlantic European grassland systems have a low nutrient use efficiency and high rainfall. This grassland is typically amended with unprocessed slurry, which counteracts soil organic matter depletion and provides essential plant micronutrients but can be mobilised during rainfall events thereby contributing to pathogen, nutrient and metal incidental losses. Co-digesting slurry with waste from food processing mitigates agriculture-associated environmental impacts but may alter microbial, nutrient and metal profiles and their transmission to watercourses, and/or soil persistence, grass yield and uptake. The impact of EU and alternative pasteurisation regimes on transmission potential of these various pollutants is not clearly understood, particularly in pasture-based agricultural systems. This study utilized simulated rainfall (Amsterdam drip-type) at a high intensity indicative of a worst-case scenario of ~11 mm hr−1 applied to plots 1, 2, 15 and 30 days after grassland application of slurry, unpasteurised digestate, pasteurised digestate (two conditions) and untreated controls. Runoff and soil samples were collected and analysed for a suite of potential pollutants including bacteria, nutrients and metals following rainfall simulation. Grass samples were collected for three months following application to assess yield as well as nutrient and metal uptake. For each environmental parameter tested: microbial, nutrient and metal runoff losses; accumulation in soil and uptake in grass, digestate from anaerobic co-digestion of slurry with food processing waste resulted in lower pollution potential than traditional landspreading of slurry without treatment. Reduced microbial runoff from digestate was the most prominent advantage of digestate application. Pasteurisation of the digestate further augmented those environmental benefits, without impacting grass output. Anaerobic co-digestion of slurry is therefore a multi-beneficial circular approach to reducing impacts of livestock production on the environment.

Evaluation of Vetiver Grass Uptake Efficiency in Single and Mixed Heavy Metal Contaminated Soil

Most phyto-remediation studies have been conducted merely on a single type of contaminant element without consideration of the influence of other co-existent contaminants. In this study, Vetiveria zizanioides (Linn.) Nash was evaluated in both single and mixed heavy metal (Cd, Pb, Cu and Zn) spiked contaminated soil. The plant growth, metal accumulation and overall efficiency of metal uptake by different plant parts (lower root, upper root, lower tiller and upper tiller) were investigated in detail. The relative growth performance, metal tolerance and phyto-assessment of heavy metal in roots and tillers of Vetiver grass were assessed. Metals in plants were measured using the flame atomic absorption spectrometry (F-AAS) after acid digestion. The root-tiller (R/T) ratio, tolerance index (TI), translocation factor (TF), biological concentration factor (BCF), biological accumulation coefficient (BAC) and metal uptake efficacy were estimated to examine the ability of metal accumulation and translocation in Vetiver grass. No significant difference (p > 0.05) of plant height was observed among all single and mixed heavy metal spiked soils compared with the control. However, significantly higher (p > Cu > Pb > Cd. Moreover, lower roots and lower tillers of Vetiver grass revealed a strong tendency for greater uptake and accumulation of all four heavy metals in both single and/or mixed spiked contaminated soils.

Concentrations and uptake pathways of polychlorinated biphenyls from soil to grass

Field coupled samples in soil and grass were collected to determine the concentrations and identify the uptake pathways of PCBs into the grass at a pasture from Scotland, UK. Concentrations of indicator PCBs (∑7PCBs) in soils ranged from 0.20 to 0.88 ng g−1 dw (dry weight), with a mean of 0.33 ng g−1 dw, and in grass ranged from 0.20 to 2.14 ng g−1 dw, with a mean of 0.48 ng g−1 dw. The comprehensive factors of low concentrations and detection rate (PCB28: 18.8%; PCB52: 37.5%) of PCBs in soil, as well as continuously declined air concentrations of PCBs in the UK since the 1990s suggested that the secondary emission from the soil is becoming the supplied source of PCBs to air and grass. The significant correlations between bioconcentration factor (BCF) values and the log KOW (R = −0.850, p < 0.05) and log KOA (R = −0.860, p < 0.05) of indicator PCB congeners were found in the present study, indicating that these two parameters are likely to affect the bioaccumulation and uptake of grass. A generic one-compartment model was employed to identify uptake pathways of grass and evaluate the uptake amounts for PCBs. This suggested that the most important pathway for uptake of PCBs by grass was at the aerial part, and the difference of PCBs concentrations between leaves and roots was about four orders of magnitude. Removing and risk transfer of PCBs or other organic pollutants by grass need to be investigated and assessed further.

Phosphorus Recycling from Wastes

The plant phosphorus (P) uptake by Italian ryegrass using organic wastes such as sewage sludge, biochar, composted sewage sludge and spent wetland filters as fertilizer was investigated in pot experiments after manipulating the wastes pH by mixing with other wastes giving acidification by acetic acid and compost leachate, and liming by concrete waste and lime. Pots with no fertilizer and with mineral fertilizer served as control. Available P was measured both with passive diffuse gradient thin film (DGT) samplers and by P uptake in the grass. The pH in the treated waste was about 4 in the acid treatment, and more than 11 in the high pH treatment. The pH in the pot during the grass production was adjusted to normal levels. The P uptake increased up to 56% after pH treatment of the waste. The P uptake responded both to the high and low pH treatments, probably due to the prevalence of different P species. The more extreme pH treatments gave the highest uptake. The DGT uptake gave the same broad picture in the pots fertilized with biochar but not in pots with sludge, and, also, without the separation between high and low treatments. The passive samplers correlated relatively well with the measured grass uptake. A principal component (PCA) analysis showed that the P uptake as measured by the DGT correlated with P, potassium (K) and silicon (Si) concentrations, at to a lower degrade with iron (Fe) and lead (Pb), and was uncorrelated with nickel (Ni) and cupper (Cu). In summary we can say that the pH treatment of the different organic wastes increased the plant P availability. The smallest increase was in the wetland filter that also had the lowest P total uptake. Also the lupin treatment increased the plant P uptake.

Analysis of Clodinafop-propargyl Herbicide Transport in Soil Profile under Vetiver Cultivation using HYDRUS-1D and Modified PRZM-3 Models

Chemicals such as herbicides used in agriculture, can be a source of soil and ground water pollution. Computer simulation models provide an efficient and cost-effective alternative that can be easily adapted to many field situations with varying soil types and different land uses. In the present study HYDRUS-1D and modified PRZM-3 (Pesticide Root Zoone Model) models were used to simulate clodinafop-propargyl (Topik) herbicide in soil profile and plant uptake under vetiver cultivation The measured data were obtained from treatments with two concentrations of the herbicide applied to the vetiver grass (C1V and C2V) and without vetiver grass (C1S and C2S). The vetiver grass uptake of clodinafop-propargyl was 1198.8 and 1107.4 mg.ha-1. The observed data showed that in surface layer, 0-10 cm, clodinafop-propargyl concentration was increased because of volatilization of herbicide from the deeper soil layer to soil surface, upward flow of water in soil and cracks in soil surface. The measured data for C2V treatment was used for calibration and the measured data from the other treatments were used for validation of above models. The statistical parameters showed that modified PRZM-3 model was more accurate than HYDRUS-1D for predicted of herbicide concentration in soil and vetiver uptake. The error of HYDRUS-1D model was about 2.5 times higher than the modified PRZM-3 model. The predicted cumulative uptake of clodinafoppropargyl by vetiver grass through modified PRZM-3 model was about 1.5 times higher than that predicted by HYDRUS-1D model. In general, plant uptake of herbicide estimated by HYDRUS-1D model was more accurate than modified PRZM-3 model.

Changes of soil organic carbon stocks and CO2 emissions at the early stages of urban turf grasses’ development

Urbanization coincides with remarkable expansion of turf grasses and their increasing role in environmental processes and functions, including carbon (C) sequestration. Soil organic carbon (SOC) stocks in turf grass soils are substantial, however, an intensive soil respiration is also likely. Therefore C sequestration in turf grasses remains uncertain, especially at the early stages after development, when C uptake and CO2 emissions are unbalanced. We analyzed changes in SOC stocks and CO2 emissions at the experimental turf grasses in Moscow megapolis during the three years period after establishment. An influence of the three contrast depths of organic layers (5, 10 and 20 cm) on soil and biomass C and on the ornamental functions of turf grasses was studied. Total CO2 emission from the turf grasses during the observation period exceeded C uptake in grass and root biomass by two to three times. Therefore the turf grasses at the early stages of development are important source of biogenic C. Although the C losses were substantial, CO2 emission decreased and C uptake in biomass increased by the end of the observation period. The highest ratio of sequestered and emitted C was obtained for the thick turf grass soil constructions with a 20 cm organic layer. The highest ornamental value, indicated by the projective cover and sprout density, was also obtained for the thick turf grasses, which is essential to consider for developing the best management practices and sustainable turf grass soil constructions.

Tree–grass competition varies across select savanna tree species: a potential role for rooting depth

Tree–grass competition is recognized as an important ecological process in savannas and woodlands, yet we only have a limited understanding of how it varies across tree species, particularly in relation to water use and root morphology. We grew seedlings of three African savanna tree species either with or without grasses in a greenhouse experiment over 8 months. We measured soil moisture content at three depths throughout the course of the experiment and measured seedling height, root depth, and root allocation as a function of depth at the end of the experiment. The tree species differed markedly in their response to grass competition, ranging between a reduction >50 % in relative growth rate to no effect. The species also differed in terms of relative root mass allocation with depth, despite a lack of maximum rooting depth differences. Against expectations, the species with the deepest rooting allocation was highly susceptible to competition from grasses. Grass presence led to a reduction of soil moisture in relation to controls and the reduction increased as a function of depth. Analysis of soil moisture reductions during dry periods indicated that grass uptake was dominant in the upper soil layers, suggesting that lower water availability at depth associated with the presence of grass was the result of reduced flow to deeper layers. We hypothesize that shallow-rooted tree species may be able to successfully engage in exploitative competition with grasses, but deep-rooted species may be exposed to interference competition when water is limiting.

Soil tests for predicting nitrogen supply for grassland under controlled environmental conditions

SUMMARYMineralized soil nitrogen (N) is an important source of N for grassland production. Some soils can supply large quantities of plant-available N through mineralization of soil organic matter. Grass grown on such soils require less fertilizer N applications per unit yield. A reliable, accurate and user-friendly method to account for soil N supply potential across a large diversity of soils and growing conditions is needed to improve N management and N recommendations over time. In the current study, the effectiveness of chemical N tests and soil properties to predict soil N supply for grass uptake across 30 Irish soil types varying in N supply potential was investigated under controlled environmental conditions. The Illinois soil N test (ISNT) combined with soil C : N ratio provided a good estimate of soil N supply in soils with low residual mineral N. Total oxidized N (TON) had the largest impact on grass dry matter (DM) yield and N uptake across the 30 soil types, declining in its influence in later growth periods. This reflected the high initial mineral N levels in these soils, which declined over time. In the current study, a model with ISNT-N, C : N and TON (log TON) best explained variability in grass DM yield and N uptake. All three rapid chemical soil tests could be performed routinely on field samples to provide an estimate of soil N supply prior to making N fertilizer application decisions. It can be concluded that these soil tests, through their assessment of soil N supply potential, can be effective tools for N management on grassland; however, field studies are needed to evaluate this under more diverse growing conditions.

Phosphate amendment of metalliferous tailings, Cannington Ag–Pb–Zn mine, Australia: implications for the capping of tailings storage facilities

This study appraised the use of phosphate fertilisers in immobilising metals in mine tailings to prevent their uptake into Curly Mitchell grass (Astrebla lappacea), when grown on capped, phosphate-amended tailings. Leaching experiments showed that Pb mobility was reduced by both bone meal and superphosphate amendment. Bone meal amendment also reduced Cd mobility. By contrast, Cd, Mn and Zn mobility increased in superphosphate-amended tailings due to increased acid production and, Cu was mobilised in bone meal-amended tailings possibly through the formation of soluble metal-complexing organic compounds. Arsenic and Sb were mobilised in both treatments due to phosphate ligand exchange. Greenhouse trials used Curly Mitchell grass grown on 1-m-high columns stacked with waste materials and different amendments overlain by clean topsoil. Curly Mitchell grass showed substantial uptake of Cd, Mn, Pb and Zn from unamended tailings and waste rock, where these were penetrated by the plant’s root system. Addition of phosphate fertilisers to the surface of tailings did not result in reduced metal uptake by Curly Mitchell grass. In tailings capped with limestone, the limestone layer formed an effective physical barrier preventing root penetration into the tailings and led to substantially reduced metal uptake in grass. The study demonstrates that thorough mixing of waste materials and fertilisers as well as irrigation may be required for successful phosphate immobilisation of metals in base metal tailings. Alternatively, the placement of a thin layer of crushed limestone on top of the tailings pile prior to capping may lead to the formation of a chemical and physical barrier and prevent the transfer of environmentally significant elements into the above-ground biomass of Mitchell grasses.

Mercury emission and plant uptake of trace elements during early stage of soil amendment using flue gas desulfurization materials

A pilot-scale field study was carried out to investigate the distribution of Hg and other selected elements (i.e., As, B, and Se), i.e., emission to ambient air, uptake by surface vegetation, and/or rainfall infiltration, after flue gas desulfurization (FGD) material is applied to soil. Three FGD materials collected from two power plants were used. Our results show Hg released into the air and uptake in grass from all FGD material-treated soils were all higher (P < 0.1) than the amounts observed from untreated soil. Hg in the soil amended with the FGD material collected from a natural oxidation wet scrubber (i.e., SNO) was more readily released to air compared to the other two FGD materials collected from the synthetic gypsum dewatering vacuum belt (i.e., AFO-gypsum) and the waste water treatment plant (i.e., AFO-CPS) of a forced oxidation FGD system. No Hg was detected in the leachates collected during the only 3-hour, 1-inch rainfall event that occurred throughout the 4-week testing period. For every kilogram of FGD material applied to soil, AFO-CPS released the highest amount of Hg, B, and Se, followed by SNO, and AFO gypsum. Based on the same energy production rate, the land application of SNO FGD material from Plant S released higher amounts of Hg and B into ambient air and/or grass than the amounts released when AFO-gypsum from Plant A was used. Using FGD material with lower concentration levels of Hg and other elements of concern does not necessary post a lower environmental risk. In addition, this study demonstrates that considering only the amounts of trace elements uptake in surface vegetation may under estimate the overall release of the trace elements from FGD material-amended soils. It also shows, under the same soil amendment conditions, the mobility of trace elements varies when FGD materials produced from different processes are used. Implications This study evaluated environmental impact associated with land application of FGD material by considering the overall mass balance of an element in a life cycle starting from coal combustion process to the stage of land application. The approach allows the relative significance of trace element release potential among different FGD materials, as well as the distribution of trace elements in different release pathways (i.e., air emission, uptake in grass, and soil leachate), to be compared based on different environmental management schemes.

A field evaluation of coated urea with biodegradable materials and selected urease inhibitors

Urease inhibitor and biodegradable polymer coatings are two most suitable startegies to increase urea fertilizer efficiency. Coating of urea with selected inhibitors can increase the crop production by slowing down the hydrolysis process of urea in the soil. For this purpose, a field experiment was conducted to evaluate the effects of newly coated urea with biodegradable materials, CuSO 4 and ZnSO 4 . Guinea grass was used as a test crop. Six treatments (urea alone, palm stearin + Cu coated urea, agar + Cu coated urea, gelatin + Cu coated urea, Cu coated urea, and Zn + Cu coated urea) were applied to field plots as nitrogen sources at the recommended dose of urea (217 kg ha -1 ), with triple super phosphate (100 kg ha -1 ) and muriate of potash (217 kg ha -1 ). The soil and plant analyses were conducted by standard method for mineral N, soil pH and nutrient content of plant. The outcomes of the study showed 50% increase in dry matter yield and N uptake of grass and 20% reduction in ammonium accumulation on the soil surface in the coated urea plots. A concomitant decrease in soil pH to 4.2 or 4.3 was measured after 15 days of fertilizer application, which was due to the acidic effects of CuSO 4 and ZnSO 4 . All coated urea treatments facilitated significantly for better pasture production and nutrient uptake. Key words: Coated urea, micronutrients, pasture, urease inhibitor.

Effects of tomato cannery water forage irrigation on nitrogen and carbon in a clay loam soil

Land application is a common method for treatment and disposal/reuse of process water and solids from food processing in Central Valley California, USA. Application rates are commonly based on N as contamination of ground and surface waters are of environmental concern. Contemporary disposal practices employ a crop to consume the N applied. Tomato processing wastewaters and semi-solid wastes were evaluated using homogeneous soil columns, and long-term monitoring of the cannery land application site. The 105-day outdoor column study included the smectitic field soil of clay loam texture, Bermudagrass, and three cannery water (CW) loading rates and a control (urea) applied weekly. NO 3 − and BOD leaching were low during the simulated irrigation season. Based on N grass uptake, leaching losses, and soil N, gaseous N losses were 50, 64, and 61% of the N applied from 313, 471, and 627 kg N ha −1 CW applications. However, gaseous N losses exceeded N applied by all treatments and included significant native soil N loss ranging from 1500 to 2600 kg ha −1. N losses indicated a “priming effect” due to mineralizable organics, and a significant decrease in soil C was seen in all treatments. The study results point toward denitrification as the major cause of gaseous N losses where ammonia volatilization was found to be a lessor cause of N loss. Long-term field monitoring results showed greater N impacts from land incorporation of semi-solid wastes than from wastewater irrigation, consistent with the soil column findings. The study demonstrated that CW having high amounts of labile organics applied to a soil of low permeability can result in heightened gaseous N loss rates, especially by denitrification. Gaseous N losses in addition to crop N uptake lessened the possibility of contamination of ground and surface waters.

Application of Humic Substances Results in Consistent Increases in Crop Yield and Nutrient Uptake

ABSTRACT The effect of humic substances on yield and nutrient uptake of grass, maize, potato and spinach was investigated in six field experiments and two pot experiments in a high input cropping system. The humic substances originating from leonardite formations in Canada were applied as liquid solution to the soil (Humifirst liquid) or as a solid incorporated in mineral fertilizers (Humifirst incorporated). Formal meta-analysis of the results of all executed experiments showed that the application of humic substances had an overall positive effect on dry matter yield of the crops and this effect was statistically significant for Humifirst incorporated. In the case of permanent grassland, humic substances promoted mainly the production of the first grass cut, which has the highest grass quality among all cuts during the growing season. Tuber production on the potato field trial showed a high response on the application of humic substances. Total potato yield increased with 13 and 17% for Humifirst liquid and Humifirst incorporated, respectively. The effect of humic substances on maize yield was limited, probably due to the rather high nutrient status of both soils. Finally, the formal meta-analysis showed a consequent increase in nitrogen and phosphorus uptake of all studied crops as well. The effect on potassium and magnesium uptake was also mainly positive, while sodium and calcium uptake were not affected in most of the experiments.

Perchlorate in an urban lawn environment

Environmental context. The extent of perchlorate contamination in our environment is inadequately known. We examined perchlorate content in precipitation, soil water, soil cores and grass clippings in an urban lawn environment. Our results show that perchlorate is present in the lawn environment at concentrations that may adversely affect human health. Abstract. Perchlorate contamination in groundwater is a concern owing to the likelihood that low concentrations of perchlorate may disrupt normal thyroid function. Lawns fertilized with perchlorate-containing fertilizers can adversely affect groundwater. We examined perchlorate concentrations from June 2006 to January 2007 in precipitation, grass clippings, soil cores and soil water at eight lawn environments in Suffolk County, NY, a county where lawns comprise more than 25% of the land use. Measured concentrations in soil waters were as high as 255 μg L–1, with average concentrations of 69 μg L–1 beneath sites treated with organic fertilizer, 1.2 μg L–1 beneath sites treated with chemical fertilizer and 0.34 μg L–1 beneath sites not fertilized. Although concentrations were dependent on the type of fertilizer applied, the patterns of leaching and grass uptake were similar at all sites.

Impact of hemiparasitic Rhinanthus angustifolius and R. minor on nitrogen availability in grasslands

Root hemiparasites like Rhinanthus angustifolius C.C. Gmel and R. minor L. have a potential to accelerate the restoration of semi-natural grasslands because they may decrease above-ground biomass of the vegetation. This, in turn, may be beneficial for species diversity. It is known that hemiparasites often accumulate high nutrient concentrations in their above-ground parts, resulting in high quality litter. Because of the short life cycle of many parasitic plants, litter is released early in the season and the main part is not removed from the grassland by hay-making. This has been shown to yield an increased nutrient availability locally. We performed an introduction experiment with R. angustifolius and R. minor in three semi-natural grasslands in Flanders (Belgium). In the second year after sowing, the above-ground nitrogen (N) content of the grasses and of the potential host vegetation (excluding the hemiparasite), was increased in the parasitized plots. The reduction of grass (and legume) above-ground biomass in parasitized plots resulted in a decrease in the total above-ground N uptake of grasses, host and total vegetation (ex- and including the parasite, respectively) of the parasitized plots compared to the control. Furthermore, with a tracer experiment (15N), we demonstrated that the N from the added tracer was relatively less available in parasitized plots, suggesting larger soil N pools in these treatments. This is probably the consequence of increased mineralization, resulting from the high-quality, parasitic litter. Further experiments should be conducted to investigate the impact of hemiparasitic Rhinanthus spp., e.g. on the availability of other nutrients such as phosphorus.

Legume defoliation affects rhizosphere decomposers, but not the uptake of organic matter N by a neighbouring grass

Legume–grass interactions have a great influence on grassland primary production and it was recently shown how defoliation of a legume can increase the transfer of fixed N to a neighbouring grass. It has also been shown that defoliation of a plant can increase soil microbial activity and lead to better soil N availability in the rhizosphere of the defoliated plant. We combined these two perspectives and tested whether defoliation of a legume (Lotus corniculatus) can enhance N nutrition of the neighbouring grass (Holcus lanatus) by increasing growth of soil decomposer biota and the availability of soil organic matter N for grass uptake. We grew mixtures of L. corniculatus and H. lanatus in grassland soil that included 15N-labelled L. corniculatus litter. In half of the systems, we subjected L. corniculatus to a defoliation treatment mimicking insect larvae feeding. At destructive harvests 1, 3, 9 and 30 days after the last defoliation event, we determined how L. corniculatus defoliation affected decomposer microbes, protozoa and nematodes and whether these changes among decomposers created a feedback on the growth and 15N uptake of the neighbouring H. lanatus. Defoliation reduced the growth and litter-N uptake, but increased shoot N concentration of L. corniculatus. Of the soil variables measured, defoliation doubled the number of bacterial-feeding protozoa, but did not affect the abundance of decomposer microbes and bacterial- and fungal-feeding nematodes. Defoliation did not have statistically significant effects on H. lanatus shoot growth, shoot N concentration or litter-N uptake. Our results demonstrate how defoliation-induced changes in legume ecophysiology can affect the growth of decomposers in soil. However, these effects did not appear to lead to a significant change in the availability of soil organic N to the neighbouring grass. It seems that when positive effects of legume defoliation on grass N nutrition are found in grassland ecosystems, these are more likely to be explained by direct transfer of fixed N rather than changes in the availability of soil organic matter N.

Precipitation thresholds for CO2 uptake in grass and shrub plant communities on Walnut Gulch Experimental Watershed

In semiarid ecosystems, precipitation is the major driving force for carbon uptake and subsequent plant growth. The hypothesis for this study was that the timing, frequency, and precipitation amount would produce different precipitation thresholds for uptake of carbon dioxide in grass and shrub plant communities. Eight years of precipitation data were used to determine the amount needed for carbon dioxide uptake thresholds in spring and summer seasons. Bowen ratio energy balance systems were used to measure carbon dioxide and moisture fluxes. In spring at the shrub site, close to or above long‐term average spring precipitation of 59 mm was required to produce an uptake response. At the grass site a minimum of 23 mm was needed to produce an uptake response, which was much less than the long‐term precipitation average of 68 mm. At both sites, spring or multiple summer responses reduced the threshold values for a summer response. Summer threshold ranges for the shrub site were 57–94 mm with a spring response and 123–140 mm without. Grass site summer thresholds were 51–95 mm with a spring response and 80–148 mm without. Summer precipitation threshold values were higher than spring values relating to the high summer evapotranspiration demand. The influence and variability of precipitation timing and frequency on carbon dioxide uptake threshold values resulted in no definitive conclusions as to differences between the grass and shrub plant communities, except that the grass site had slightly lower thresholds. Precipitation timing and frequency influence on total carbon uptake in some situations were more important than total precipitation. The lower grass site threshold values, along with a shift in climate toward more frequent and smaller precipitation events, may give grass ecosystems a competitive growth advantage.

Spatial distribution and transport of heavy metals in soil, ponded-surface water and grass in a pb-contaminated watershed as related to land-use practices

The aim of this study was to investigate the spatial distribution of heavy metal in soil and evaluate the dissolution of metal from soil to ponded-surface water, leaching through soil profiles and metal uptake in grass as related to different land-use practices. The data provided a scientific basis for best-management practices for land use in Khli Ti watershed. The watershed has a Pb-contamination problem from the previous operation of a Pb-ore concentrator and abandoned Zn-Pb mine. Sampling sites were selected from a land-use map, with land-use types falling into the following four categories: forest, agricultural land, residential area and road. Soil, ponded-surface water, grass samples and soil profiles were collected. The study related soil characteristics from different land-use practices and locations with observed metal concentrations in ponded-surface water and soil. High enrichment factors of Pb and As in soil were found. Partitioning coefficient, K(d) values were in the order: Cr > Pb > Ni > Cu > Cd > Zn. Soil disturbance from land-use activities including tillage and traffic increased leaching of trace metal from soils. Pb in soil was significantly taken up by grass even though the Transfer Factor, TF values were rather low. Agricultural activities in the watershed must be limited. Moreover, land encroachments in the upper and middle part of the watershed which have high potential of Pb must be strictly controlled in order to reduce the Pb contamination from non-point sources.