Plant and Soil

Mohamed, S.; Smouni, A.; Neyra, M.; Kharchaf, D.; Filali-Maltouf, A.

doi: 10.1023/A:1004838218642pmid: N/A

Thirty isolates of root-nodulating bacteria obtained from Acacia cyanophylla, A. karroo, A. cyclops, A. tortilis (subsp. raddiana ), Faidherbia albida and Acacia sp., grown in different regions of Libya, were studied by performing numerical analysis of 104 characteristics. Three fast- and one slow-growing reference strains from herbaceous and woody legumes were included. Five distinct clusters were formed. The fast-growing reference strains were separated from the isolates whereas the slow-growing was included in cluster 4. With the exception of one cluster, the majority of clusters were formed regardless of the host plant or site of origin. Based on plant tests, generation times, acid production and carbon utilization the isolates were diverse (fast and slow-growing isolates). Like slow-growing isolates, most of the fast-growing isolates appeared to be non-specific, nodulated many species from the same genus notably F. albida , known to nodulate only with slow-growing strains. Most clusters grew at temperatures 35 °C and 37 °C; some grew at temperatures above 40 °C. The majority of isolates grew at acid and alkaline pH and only one isolate grew below pH 4. Most isolates were able to utilize many amino acids as nitrogen sources and to reduce nitrate. Urea was hydrolysed by all clusters. Monosaccharides and polyols were used by slow and fast-growing isolates as the only carbon sources whereas assimilation of disaccharides varied: Some isolates, like slow-growing isolates, failed to utilize these carbon sources. Most isolates were unable to utilize polysaccharides. Regarding tolerance to NaCl on agar medium, the majority of isolates were unable to grow at a concentration of 2% NaCl, but some were highly resistant and there was one isolate which grew at 8% NaCl. Most isolates were resistant to heavy metals and to antibiotics.

Carrera, A.L.; Sain, C.L.; Bertiller, M.B.

doi: 10.1023/A:1004841917272pmid: N/A

We analysed the main plant strategies to conserve nitrogen in the Patagonian Monte. We hypothesized that the two main plant functional groups (xerophytic evergreen shrubs and mesophytic perennial grasses) display different mechanisms of nitrogen conservation related to their structural and functional characteristics. Evergreen shrubs are deep-rooted species, which develop vegetative and reproductive growth from spring to late summer coupled with high temperatures, independently from water inputs. In contrast, perennial grasses are shallow-rooted species with high leaf turnover, which display vegetative growth from autumn to spring and reproductive activity from mid-spring to early-summer, coupled with precipitation inputs. We selected three evergreen shrubs ( Larrea divaricata Cav., Atriplex lampa Gill. ex Moq. and Junellia seriphioides (Gilles and Hook.) Moldenke) and three perennial grasses ( Stipa tenuis Phil., S. speciosa Trin. and Rupr. and Poa ligularis Nees ex Steud.), characteristic of undisturbed and disturbed areas of the Patagonian Monte. N concentration in expanded green and senesced leaves was estimated in December 1997 (late spring) and June 1998 (late autumn). Deep-rooted evergreen shrubs displayed small differences in N concentration between green and senesced leaves (low N-resorption efficiency), having high N concentration in senesced leaves (low N-resorption proficiency). Shallow-rooted perennial grasses, conversely, showed high N-resorption efficiency and high N-resorption proficiency (large differences in N concentration between green and senesced leaves and very low N concentration in senesced leaves, respectively). The lack of a strong mechanism of N resorption in evergreen shrubs apparently does not agree with their ability to colonize N-poor soils. These results, however, may be explained by lower N requirements in evergreen shrubs resulting from lower growth rates, lower N concentrations in green leaves, and lower leaf turnover as compared with perennial grasses. Long-lasting N-poor green tissues may, therefore, be considered an efficient mechanism to conserve N in evergreen shrubs in contrast with the mechanism of strong N resorption from transient N-rich tissues displayed by perennial grasses. Evergreen shrubs with low N-resorption efficiency provide a more N-rich substrate, with probably higher capability of N mineralization than that of perennial grasses, which may eventually enhance N fertility and N availability in N-poor soils.

Schack-Kirchner, Helmer; Wilpert, Klaus; Hildebrand, Ernst

doi: 10.1023/A:1004806122105pmid: N/A

The external mycelium forms the major part of the absorbing surface of mycorrhized tree roots. Because the macro pore space of acid forest soils is selectively depleted of mobile nutrient cations, it is ecologically important, whether soil hyphae grow into the soil aggregates or not. Seedlings of Norway Spruce ( Picea abies (L.) Karst.) with defined mycorrhiza were grown in unsterilized soil cores taken from the A and B-horizon of a limed and an unlimed cambisol on triassic sandstone in the Northern Black Forest, Germany. Water-tension treatments were 10, 30, 160 and 900 hPa. On ground and polished vertical cuts stained with acridine orange, we identified and measured the location of hyphae and characterized their micropedological environment using an image analyzing system. Mean length density varied between 17 m/cm 3 and 100 m/cm 3 and was independent of aeration parameters. The percentage of hyphae completely embedded in the soil matrix varied between 30% and 8% and decreased significantly with increasing CO 2 concentration in the soil air. Of the hyphae in the soil matrix, 70% were located in a 50 μm shell around the macro pores. Pair correlation functions show, that the majority of soil hyphae occur in clusters with diameters below 100 μm. Between 60% and 80% of randomly chosen circles with 250 μm diameters were completely devoid of hyphae. The inefficient opening up of the intra-aggregate space by soil hyphae is explained by the very slow oxygen diffusion between air-filled macro pores and the intra-aggregate space and mechanical restrictions for hyphae growth.

Brand, J.D.; Tang, C.T.; Graham, R.D.

doi: 10.1023/A:1004830506175pmid: N/A

Two glasshouse experiments were conducted to examine the effects of nutrient supply and rhizobial inoculation on the performance of Lupinus pilosus genotypes differing in tolerance to calcareous soils. In experiment 1, plants were grown for 84 days in a calcareous soil (50% CaCO 3 ; soil water content 90% of field capacity) at four nutrient treatments (no-added nutrients, added nutrients without Fe, added nutrients with soil applied FeEDDHA, added nutrients with foliar applied FeSO 4 ). In experiment 2, plants were grown for 28 days with supply of NH 4 NO 3 without inoculation or inoculated with Bradyrhizobium sp. ( Lupinus ). Chlorosis in the youngest leaves was a good indicator of the relative tolerance of the genotypes to the calcareous soil in both experiments, except the treatment with FeEDDHA at 5 mg kg −1 soil which was toxic to all genotypes. Chlorosis scores correlated with chlorophyll meter readings and chlorophyll concentrations. The foliar application of FeSO 4 did not fully alleviate chlorotic symptoms despite concentrations of active or total Fe in the youngest leaves being increased. Adding nutrients and chemical nitrogen did not change the severity of chlorosis or improve the growth of the plant. The nutrient supply did not alter the ranking of tolerance of genotypes to the calcareous soil. The results suggest that nutrient deficiency or poor nodulation was not a major cause of poor plant growth on calcareous soils and that bicarbonate may exert a direct effect on chlorophyll synthesis. The mechanism for tolerance is likely to be related to an ability to exclude bicarbonate or prevent its transport to the leaves.

Spehn, Eva; Joshi, Jasmin; Schmid, Bernhard; Alphei, Jörn; Körner, Christian

doi: 10.1023/A:1004891807664pmid: N/A

The loss of plant species from terrestrial ecosystems may cause changes in soil decomposer communities and in decomposition of organic material with potential further consequences for other ecosystem processes. This was tested in experimental communities of 1, 2, 4, 8, 32 plant species and of 1, 2 or 3 functional groups (grasses, legumes and non-leguminous forbs). As plant species richness was reduced from the highest species richness to monocultures, mean aboveground plant biomass decreased by 150%, but microbial biomass (measured by substrate induced respiration) decreased by only 15% ( P = 0.05). Irrespective of plant species richness, the absence of legumes (across diversity levels) caused microbial biomass to decrease by 15% ( P = 0.02). No effect of plant species richness or composition was detected on the microbial metabolic quotient (qCO 2 ) and no plant species richness effect was found on feeding activity of the mesofauna (assessed with a bait-lamina-test). Decomposition of cellulose and birchwood sticks was also not affected by plant species richness, but when legumes were absent, cellulose samples were decomposed more slowly (16% in 1996, 27% in 1997, P = 0.006). A significant decrease in earthworm population density of 63% and in total earthworm biomass by 84% was the single most prominent response to the reduction of plant species richness, largely due to a 50% reduction in biomass of the dominant `anecic' earthworms. Voles ( Arvicola terrestris L.) also had a clear preference for high-diversity plots. Soil moisture during the growing season was unaffected by plant species richness or the number of functional groups present. In contrast, soil temperature was 2 K higher in monocultures compared with the most diverse mixtures on a bright day at peak season. We conclude that the lower abundance and activity of decomposers with reduced plant species richness was related to altered substrate quantity, a signal which is not reflected in rates of decomposition of standard test material. The presence of nitrogen fixers seemed to be the most important component of the plant diversity manipulation for soil heterotrophs. The reduction in plant biomass due to the simulated loss of plant species had more pronounced effects on voles and earthworms than on microbes, suggesting that higher trophic levels are more strongly affected than lower trophic levels.

Pakrou, Naser; Dillon, Peter

doi: 10.1023/A:1004838323594pmid: N/A

The paper presents integrated measurements of N fixation, net mineralisation, pasture yield and change in soil mineral N over a 12 month period for dairy pastures on a sandy loam soil in the South East of South Australia. The two adjacent pastures studied were an irrigated perennial white clover-ryegrass and an annual non-irrigated subterranean clover with mixed annual grasses. This produced the most comprehensive mineral N balance reported for grazed pastures, to the authors' knowledge, allowing calculation of gaseous and leaching losses of N (210 kg ha −1 in the irrigated and paddock and 81 kg ha −1 in the non irrigated paddock) primarily from urine patches. In both paddocks these losses were about three times the N yield in milk (61 and 28 kg N ha −1 respectively) and were replenished by biological N fixation (294 and 100 kg N ha −1 ). However, mineralisation of soil organic N, excretal N and pasture residues (687 and 438 kg N ha −1 ) was the major source of mineral N for cycling and losses. The results demonstrate the enormous impact of pasture management on N fluxes and reinforce the importance of livestock urine on the magnitude of N fluxes including gaseous and leaching losses.

Bañuelos, G.S.; Zambrzuski, S.; Mackey, B.

doi: 10.1023/A:1004881803469pmid: N/A

This two-part study compared the efficacy of different plant species to extract Se from soils irrigated with Se-laden effluent. The species used were: Brassica napus L. (canola), Brassica juncea Czern L. and Coss (Indian mustard), and Hordeum vulgare L. (barley). In Study 1 we irrigated the plants with a saline effluent containing 0.150 mg Se L −1 , while in Study 2 , the same species were planted in a saline soil selenized with 2 mg Se L −1 . Plants were simultaneously harvested 120 days after planting. In Study 1 , there were only slight effects of treatment on dry matter (DM) yield. Plant Se concentrations averaged 21 μg Se g −1 DM for the Brassica species, and 4.0 μg Se g −1 DM for barley. Total Se added to soils via effluent decreased by 40% for Brassica species and by 20% for barley. In Study 2 , total DM decreased for all species grown in saline soils containing Se. Plant Se concentrations averaged 75 μg g −1 DM for Brassica species and 12 μg Se g −1 DM for barley. Total Se added to soils prior to planting decreased by 40% for Brassica species and up to 12% for barley. In both studies, plant accumulation of Se accounted for at least 50% of the Se removed in soils planted to Brassica and up to 20% in soils planted to barley. Results show that although the tested Brassica species led to a significant reduction in Se added to soil via use of Se-laden effluent, additional plantings are necessary to further decrease Se content in the soil.

Plénet, D.; Mollier, A.; Pellerin, S.

doi: 10.1023/A:1004835621371pmid: N/A

Biomass accumulation by crops depends on both light interception by leaves and on the efficiency with which the intercepted light is used to produce dry matter. Our aim was to identify which of these processes were affected for maize ( Zea mays L., cv Volga) field crops grown under phosphorus (P) deficiency. In the preceding paper (Plénet et al., 2000), it was shown that P deficiency severely reduced leaf growth. In this paper, the effect of P deficiency on the radiation-use efficiency (RUE) was investigated. The experimental work was carried out in 1995, 1996 and 1997 on a long-term P fertilisation trial located on a sandy soil in the south-west of France. Three P fertilisation regimes have been applied since 1972: no- P (P0 treatment) and different rates of P fertiliser (P1.5: 1.5 times the grain P export and P3: 3 times the grain P export). These fertilisation regimes have led to contrasted levels of soil P supply. Only slight differences were observed between the P1.5 and P3 treatment for above-ground biomass accumulation and grain yield. Conversely the grain yield was significantly reduced in P0 (−11%). Above-ground biomass production was severely reduced, with the maximum difference between treatment (−60% in P0) occurring between 400 and 600 °C days after sowing. The lower biomass production in P0 was accounted for by the reduced amount of photosynthetically active radiation (PAR) absorbed by the canopy, which was itself the consequence of the reduced leaf area index (see Plénet et al., 2000). The calculated RUE were found to depend on the plant stage, especially during the pre-flowering period, and on the average air temperature. No effect of P deficiency was observed on the calculated RUE, even during the period when above-ground biomass accumulation was the most severely reduced. These results obtained in field crop conditions strengthen the idea that P deficiency affects plant growth, especially leaf growth, earlier and to a greater extent than photosynthesis per unit leaf area.

Hagedorn, Frank; Bucher, Jürg; Tarjan, David; Rusert, Peter; Bucher-Wallin, Inga

doi: 10.1023/A:1004831401190pmid: N/A

The objectives of this study were to estimate how soil type, elevated N deposition (0.7 vs. 7 g N m −2 y −1 ) and tree species influence the potential effects of elevated CO 2 (370 vs. 570 μmol CO 2 mol −1 ) on N pools and fluxes in forest soils. Model spruce-beech forest ecosystems were established on a nutrient-rich calcareous sand and on a nutrient-poor acidic loam in large open-top chambers. In the fourth year of treatment, we measured N concentrations in the soil solution at different depths, estimated N accumulation by ion exchange resin (IER) bags, and quantified N export in drainage water, denitrification, and net N uptake by trees. Under elevated CO 2 , concentrations of N in the soil solution were significantly reduced. In the nutrient-rich calcareous sand, CO 2 enrichment decreased N concentrations in the soil solution at all depths (−45 to −100%). In the nutrient-poor acidic loam, the negative CO 2 effect was restricted to the uppermost 5 cm of the soil. Increasing the N deposition stimulated the negative impact of CO 2 enrichment on soil solution N in the acidic loam at 5 cm depth from −20% at low N inputs to −70% at high N inputs. In the nutrient-rich calcareous sand, N additions did not influence the CO 2 effect on soil solution N. Accumulation of N by IER bags, which were installed under individual trees, was decreased at high CO 2 levels under spruce in both soil types. Under beech, this decrease occurred only in the calcareous sand. N accumulation by IER bags was negatively correlated with current-years foliage biomass, suggesting that the reduction of soil N availability indices was related to a CO 2 -induced growth enhancement. However, the net N uptake by trees was not significantly increased by elevated CO 2 . Thus, we suppose that the reduced N concentrations in the soil solution at elevated CO 2 concentrations were rather caused by an increased N immobilisation in the soil. Denitrification was not influenced by atmospheric CO 2 concentrations. CO 2 enrichment decreased nitrate leaching in drainage by 65%, which suggests that rising atmospheric CO 2 potentially increases the N retention capacity of forest ecosystems.

Tian, G.; Kolawole, G.O.; Kang, B.T.; Kirchhof, G.

doi: 10.1023/A:1004873206350pmid: N/A

Legume cover crops are a potential means for overcoming N depletion in the derived savanna of West Africa. A 3-year trial was, therefore, conducted near Ibadan, southwestern Nigeria to measure the N contribution of 13 legume cover crops as compared to urea –N, using a N fertilizer replacement index for a maize test crop. Two series of trials involved the following legume cover crop species: Aeschynomene histrix, Centrosema brasilianum, Centrosema pascuorum, Chamaecrista rotundifolia, Cajanus cajan, Crotalaria verrucosa, Crotalaria ochroleuca, Lablab purpureus, Mucuna pruriens, Psophocarpus palustris, Pseudovigna argentea, Pueraria phaseoloides and Stylosanthes hamata . Trials were undertaken using a complete block design. Cover crops were planted in 1994 (Series 1) and 1995 (Series 2) in separate sites and each series was subsequently slashed and planted for one season with maize ( Zea mays ) in 1995 and 1996. At the 50% flowering stage, N concentration of above-ground vegetation of cover crops ranged from 21 to 38 g N kg −1 . Nitrogen accumulated by 4.5-month old cover crops ranged from 14 to 240 kg N ha −1 , depending on species and year. Cover crops increased grain yield of the subsequent maize crop by 25–136% over the control without N application. Nitrogen uptake by the maize crop was higher following cover crops than after maize or natural grass. The N fertilizer replacement index of cover crops for maize ranged from 11 ( A. histrix ) to 96 kg N ha −1 ( C. cajan ) in Series 2. Perennial ( C. brasilianum, S. hamata, C. cajan, P. phaseoloides and C. verrucosa ) and annual ( C. rotundifolia , M. pruriens, C. ochroleuca and L. purpureus ) species could potentially save 50 to 100 kg N ha −1 for maize crops. The cover crops accumulated more N in the wetter than in the drier year. However, the N fertilizer replacement index was higher for subsequent maize grown in the drier year. The cover crop-N recovery in maize was also higher than the urea-N uptake in the drier year. The N fertilizer replacement indexes can be predicted using the above-ground biomass amount of cover crops at 20 weeks after planting (drier year) or the N concentration at that stage (wetter year).