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Phosphorus (P) acquisition is key for global food production. Arbuscular mycorrhizal fungi (AMF) help plants acquire P and are considered key for the design of sustainable agroecosystems. However, how the functioning of AMF varies across agricultural soils and responds to management practices is still unknown. Here, we collected soils from 150 cereal fields and 60 non-cropped grassland sites across Europe, and in a greenhouse experiment, we tested the ability of AMF in these soils to forage for radioisotope-labelled 33P from a hyphal compartment. Hyphal-mediated P uptake was 64% higher in non-cropped grassland compared to cropland soils. Soil pH and organic carbon best explained the hyphal-mediated P uptake in the grasslands, while the use of fungicide in croplands reduced P uptake in the croplands by 43%. Our results suggest that land-use intensity and fungicide use are major deterrents to the natural capacity of AMF to contribute to sustainable crop production.

Forests dominated by beech (Fagus sylvatica L.) cover large parts of Europe where they occupy a broad ecological niche in terms of soil fertility. This indicates a large potential to adapt to different soil conditions over long time periods. Recent changes in tree mineral nutrition across Europe raise the question to what degree beech can acclimate to changing soil conditions in the short term. In this study, we aimed at assessing the plasticity of root traits and rhizosphere properties of young beech trees from populations that are adapted to either high or low nutrient supply, when growing in soils differing in their fertility. We sampled beech saplings from two forest sites of contrasting nutrient supply, most distinctly in terms of phosphorus. We grew them for 2 years in rhizoboxes in mineral soil either from their own site or from the other site. We assessed the influence of the factors “plant origin” and “current soil” on root traits and rhizosphere properties. Fine root traits related to growth (biomass, length), architecture (branching), and morphology (diameter) responded strongly to the factor “current soil.” Provenance (factor “plant origin”) modified the response. The modifying effect was consistent with an influence of the plant status in those nutrients, which were not in sufficient supply in the soil. An additional genotypic difference in the sensitivity of the beech saplings to different soil nutrient supply could not be excluded. Fine root parameters normalized for length, mass, or volume (root tip density and frequency, specific root length and area, and root tissue density) did not differ among the treatments. Differences in percentage of mycorrhizal root tips and rhizosphere parameters related to phosphorus mobilization potential (pH, abundance of organic acid anions, and phosphatase activity) were small and mainly determined by the “current soil.” Provenance had only a minor modifying effect, possibly due to differences in the ability of the plants to transfer carbon compounds from the shoot to the root and the fungal partner. Our results indicate a high plasticity of young beech trees to adapt their root system to different soil nutrient supply, thereby also taking into account internal nutrient reserves.

Soil microbial community coalescence, whereby entire microbial communities mix and compete in a new environmental setting, is a widespread phenomenon whose applicability for targeted root microbiome assembly has not been studied. Using a legume shrub adapted to nutrient poor soil, we tested for the first time how the assembly of communities of rhizobial root nodule symbionts is affected by the interaction of coalescence and fertilization. Seedlings of the rooibos [Aspalathus linearis (Burm.f.) Dahlg.], were raised in pairwise mixtures of soil from cultivated and uncultivated land of five farms, as well as the individual mixture components. A fragment of the symbiosis maker gene, nodA, was sequenced to characterize the taxonomic turnover of the rhizobia associated with all root nodules at the age of eight month. Soil mixing promoted taxonomic turnover in the rhizobial communities, while fertilization amplified such turnover by increasing the number of rhizobia that became more abundant after soil mixing. Soil mixing and fertilization had a synergistic effect on the abundance of a particular taxon, which was rare in the component soils but became highly abundant in fertilized plants raised in soil mixtures. These findings provide the first evidence that fertilizer addition can interact with soil microbial community coalescence, probably through increasing the chances for rare strains to prioritize root nodule colonization. The combination of soil mixing and fertilizer addition may be a still unexplored measure to (re)introduce root microbial mutualists in arable land.

Background and aims: Legumes integrated in crop rotations are intended to improve crop nitrogen (N) supply and yield. In conservation agriculture (CA) systems under low input conditions on highly weathered tropical soils, experimental evidence for these benefits is lacking. To understand the mechanisms and evaluate the impact of the legume N on the subsequent crop, an in-depth study on N dynamics in the soil-plant system was conducted. Methods : In Madagascar, a CA based crop rotation with the perennial forage legume Stylosanthes guianensis (stylo) and upland rice (rice/stylo – stylo - rice/stylo) was established under three fertilization regimes. In addition, rice was grown in a non-CA bare fallow rotation without fertilizer. Over the three years N2 fixed in stylo shoots, the incorporation of stylo shoot (mulch) N into soil N pools and mulch N uptake by rice was quantified using 15N techniques and mulch and stylo root residue decomposition was investigated in a litterbag study. Results: N2 fixed in stylo shoots ranged from 96 to 122 kg N ha−1. Between 50 to 70% of stylo mulch and root residues decomposed during the third cropping season. Without fertilizer, grain yield of rice after the fallow with stylo was about 70% greater than after bare fallow, corresponding to 11 kg N ha−1 greater N uptake. Recoveries of stylo mulch N after rice harvest were on average 64% in soil, with about 3% in each of the microbial and mineral N pools, with 39% on the soil surface, and 6% in the rice crop. The N input via stylo seed, leaf litter and belowground N totalled about three times the amount of N contained in stylo mulch, which usually is considered as major rice N source. Conclusions: Legumes, like stylo, can improve crop N supply and yield in low input CA cropping systems on highly weathered tropical soils. To explain the impact and mechanisms involved requires a consideration of all legume-N components beyond the mulch N present at the onset of the rice-cropping season.

Effective use of soil phosphorus (P) for crop production requires an understanding of how P pools are stabilized and cycled within soil aggregates, rather than assuming that P dynamics, particularly organic P, closely follow those of C. The main goal of this study was to compare C and N cycling with P dynamics in soil aggregate fractions under two distinct crop species, maize (Zea mays) and pigeon pea (Cajanus cajan) in a highly-weathered Lixisol. We found that while C and N follow an open cycle, whereby C and N are mineralized from microaggregates during macroaggregate turnover and partially exit the soil system as gas and leachate, P has a relatively closed cycle, where most of the mineralized and solubilized P from microaggregates is lost from the plant-available pool via sorption to the unaggregated silt and clay-sized particles (

Soil phosphorus (P) tests are designed to indicate plant-available inorganic orthophosphate (Pi), but fail to account for Pi that may become available through organic phosphorus (Po) mineralization. This P source may be especially important in soils with low concentrations of solution and labile Pi. We assessed gross Po mineralization and immobilization using labeling with 33P in four calcareous Alfisols with varying concentrations of Olsen soil test P that were collected from forage fields of dairy farms in Ontario, Canada. Rapid microbial 33P uptake during incubation was found for the soils with the lowest available Pi as indicated by both Olsen soil test P and water-extractable Pi. The tracer incorporation into microbial P after 8 days ranged from 7 to 44% of applied 33P and was negatively related to water-extractable Pi following a power-type relationship. As concentrations of microbial P were similar in all soils, this suggests faster turnover of P in the microbial biomass at water-extractable Pi below 0.1 mg P kg−1 soil. Daily gross Po mineralization rates ranged from 0.2 to 2.8 mg P kg−1 soil d−1 and contributed 7–56% of the isotopically-exchangeable P in 8 days. Based on these findings, microbial processes have the potential to make a significant contribution to forage P nutrition.

Inoculation with phosphorus (P) solubilizing bacteria is being proposed to increase P availability for plants by mineralization and solubilization of non-available soil and fertilizer P. Solubilization of inorganic P compounds by bacterial strains has repeatedly been shown on agar plates and in liquid media. However, the effects of inoculation on P availability to plants growing in soils, either in pot or field studies, are inconsistent and do not allow to separate between direct effects on P availability and indirect effects such as improved plant health. This differentiation could be achieved using 33P isotopic labeling. We applied the 33P isotopic dilution method in a pot and in an incubation experiment to study gross P fluxes in a calcareous soil inoculated with the P solubilizing bacteria Pseudomonas protegens CHA0. We hypothesized that the inoculant dilutes the specific activity (33P/31P) in the soil solution or in the plant shoots because of P solubilization beyond the P mobilization by the endogenous microbial biomass. To this end, we conducted a plant growth experiment with Lolium multiflorum var. Gemini and an incubation experiment. In both experiments, the soil was amended or not with a calcium P rich sewage sludge ash, and both treatments were conducted with and without inoculation. The inoculant was able to solubilize P from sewage sludge ash under controlled conditions in liquid media. However, it did not enhance P release from soil or from sewage sludge ash in the incubated soil. Inoculation of the soil reduced organic P mineralization by the soil microbial biomass, which was supported by a simultaneous decrease in soil respiration. Thus, any inorganic P solubilized by the inoculant might have been offset by less basal organic P mineralization. Increased P uptake of inoculated Lolium multiflorum at first harvest was attributed to an indirect effect, since the specific activity in shoots of inoculated Lolium multiflorum was not decreased. Although sewage sludge ash contained very little water-soluble P, an increase in P availability following sewage sludge ash addition could be shown using 33P isotopic dilution, while biological processes remained unchanged. While in this study, the inoculant did not increase P availability, the approach presented here can give insight into the mechanisms underlying beneficial effects of inoculants.

Background and aims In Malawi, strategies are being sought to boost maize production through improvements in soil fertility. This study assessed the impact of intercropping maize (Zea mays) with pigeon pea (Cajanus cajan) in Lixisols of Malawi on yield, biological N fixation, soil aggregation, and P forms within soil aggregates. Methods Maize and pigeon pea were grown intercropped in pots, with varying degrees of root interaction in order to understand the relative importance of biochemical versus physical rhizospheric interactions. Following harvest, soils were separated into aggregate fractions using wet-sieving, and the nutrient content of all fractions was assessed. Results The proportion of macroaggregates and microaggregates increased by 52 and 111%, respectively, in the intercropping treatment compared to sole maize, which significantly increased organic P storage in the microaggregates of intercropped compared to sole maize (84 versus 29 mg P kg−1, respectively). Biologically fixed N increased from 89% in the sole pigeon pea to 96% in the intercropped system. Conclusions Intercropping maize with pigeon pea can have a significant and positive impact on soil structure as well as nutrient storage in these high P-sorbing soils. This is caused primarily by physical root contact and to a lesser degree by biochemical activities.

Decreasing phosphorus (P) concentrations in leaves of beech (Fagus sylvatica L.) across Europe raise the question about the implications for forest health. Considering the distribution of beech forests on soils encompassing a broad range of nutrient availability, we hypothesized that this tree species exhibits high phenotypic plasticity allowing it to alter mass, and nutrient allocation in response to local nutrient availability. To test this, we grew two groups of 12–15 year old beech saplings originating from sites with high and low soil P availability for 2 years in mineral soil from their own site and in soil from the other site. After two growing seasons, P concentrations in leaves and stem, as well as mass allocation to leaves and fine roots were affected by both soil and plant origin. By contrast, relative P allocation to leaves and fine roots, as well as P concentrations in fine roots, were determined almost entirely by the experimental soil. Independent of the P nutritional status defined as average concentration of P in the whole plant, which still clearly reflected the soil conditions at the site of plant origin, relative P allocation to leaves was a particularly good indicator of P availability in the experimental soil. Furthermore, a high plasticity of this plant trait was indicated by a large difference between plants growing in the two experimental soils. This suggests a strong ability of beech to alter resource allocation in response to specific soil conditions., Frontiers in Plant Science, 10, ISSN:1664-462X

In many parts of the world, soils deficient and/or toxic in micronutrients reduce potential soybean (Glycine max) yields. The objective of our study was to grow plants in low to high concentrations of boron (B) and zinc (Zn) to determine how B and Zn deficiencies and toxicities affect soybean growth and interact with other essential nutrients in roots, leaves, and seeds. We found that B significantly affected levels of all essential nutrients except manganese and iron, while Zn significantly affected all essential nutrients in at least one plant tissue tested. Some of the physiological responses and nutrient interactions were cultivar-dependent. This study showed how deficiencies and toxicities of B and Zn affect plant growth and how B and Zn fertility interacts with many of the other essential nutrients.

Background and Aims Soil microbial communities contribute to organic phosphorus cycling in a variety of ways, including secretion of the PhoD alkaline phosphatase. We sampled a long-term grassland fertilization trial in Switzerland characterized by a natural pH gradient. We examined the effects of phosphate depletion and pH on total and active microbial community structures and on the structure and composition of the total and active phoD-harboring community. Methods Archaeal, bacterial and fungal communities were investigated using T-RFLP and phoD-harboring members of these communities were identified by 454-sequencing. Results Phosphate depletion decreased total, resin-extractable and organic phosphorus and changed the structure of all active microbial communities, and of the total archaeal and phoD-harboring communities. Organic carbon, nitrogen and phosphorus increased with pH, and the structures of all total and active microbial communities except the total fungal community differed between the two pH levels. phoD-harboring members were affiliated to Actinomycetales, Bacilliales, Gloeobacterales, Planctomycetales and Rhizobiales. Conclusions Our results suggest that pH and associated soil factors are important determinants of microbial and phoD-harboring community structures. These associated factors include organic carbon and total nitrogen, and to a lesser degree phosphorus status, and active communities are more responsive than total communities. Key players in organic P mineralization are affiliated to phyla that are known to be important in organic matter decomposition.

1) The extent to which plants can reduce nutrient concentrations in soil and thereby compete with others may increase with nutrient mobility. Hyphae of arbuscular mycorrhizal fungi (AMF) can extend the soil volume from which plants acquire phosphorus (P), thus increasing competition for these resources with neighbours. In this study we tested whether the suppression of hyphal interconnections between neighbour plants mitigates their competitive interactions and consequently affects plant community structure. 2) We used custom-built microcosms that used a wire system to suppress the development of a common mycorrhizal network (CMN) between plant neighbours. We applied this CMN treatment to plants without neighbours (competition-free controls), with conspecific neighbours (monocultures), or with heterospecific neighbours (two and four species communities), all assembled from two pools of four separate temperate grassland species each. We analyzed changes in species and community-level productivity and P acquisition. 3) The CMN treatment affected species differently. Most species had reduced shoot biomass while root biomass increased with CMN disconnection. Productivity and nutrient acquisition of Plantago lanceolata in four-species mixtures was negatively affected, leading to a less even distribution of P among species, but community-level P acquisition was not affected. On average, two-species and four-species mixtures produced similar community biomass and had the same P content as monocultures. 4) Synthesis: CMN disconnection did affect competitive interactions among species only little. One explanation may be that the absence of pronounced competitive hierarchy among the species investigated led to relatively symmetric interactions among species that were stable with respects to additional CMN-effects. Another explanation is that CMN effects are less important in natural soils with natural AMF communities than experiments with few AMF strains and often sterilized soils suggest.

Plant and Soil, 290 (1-2), ISSN:0032-079X, ISSN:1573-5036

Nutrient Cycling in Agroecosystems, 88 (3), ISSN:1385-1314, ISSN:1573-0867

Plant and Soil, 284 (1-2), ISSN:0032-079X, ISSN:1573-5036

Plant and Soil, 370 (1-2), ISSN:0032-079X, ISSN:1573-5036

Exploiting the natural variability of Brachiaria forage germplasm to identify forage grasses adapted to infertile acid soils that contain very low available phosphorus (P) is an important research objective for improving livestock production in the tropics. The objective of this study was to determine the differences in the release of root biochemical markers, i.e. carboxylates and acid phosphatases (APases), during the development of P deficiency in signalgrass and ruzigrass. We used the hydroxyapatite pouch system in hydroponics to simulate conditions of low P supply in acid soils to test the response of well-adapted signalgrass (Brachiaria decumbens cv. Basilisk, CIAT 606) and less-adapted ruzigrass (B. ruziziensis cv. Kennedy, CIAT 654). We monitored shoot and root growth and other physiological and biochemical components that are important for root functionality at weekly intervals for 3 weeks. We found that monocarboxylate exudation was not associated with the plant’s physiological P status, while exudation of oxalate and secreted-APases increased with declining plant P concentrations in both grasses. Ruzigrass showed higher exudation rates and grew faster than signalgrass, but could not maintain its initial fast growth rate when P concentrations in plant tissue declined to 1.0 mg P/g dry matter. Oxalate was the dominant exuded carboxylate for signalgrass after 21 days of growth and this response might confer some eco-physiological advantages in signalgrass when grown in low-P acid soils., Tropical Grasslands - Forrajes Tropicales, 5 (3), ISSN:2346-3775

Organic fertilizers are important in agroecosystems, but the fate of specific forms of added phosphorus (P) in the long-term is not well understood. We studied the effects of 62 years of application of three organic fertilizers (dairy manure, compost and dried sewage sludge) on organic P forms in the topsoil as characterized by solution 31 P NMR spectroscopy and enzyme additions in a field experiment on a slightly acidic Luvisol in Switzerland. Changes in inorganic P extracted with NaOH–EDTA were related to the P-input–output balances of the treatments, which ranged between − 13 kg P ha − 1 yr − 1 in the non-fertilized control and + 93 kg P ha − 1 yr − 1 with sewage sludge application. The majority of extractable P in the applied organic fertilizers (68–91%) was in the form of orthophosphate. The fertilizers differed with respect to specific organic P compounds identified by 31 P NMR, with myo -inositol hexakisphosphate ( myo -IP 6 ) found in compost and dried sewage sludge, but absent from dairy manure. A small amount of scyllo -inositol hexakisphosphate ( scyllo -IP 6 ) was additionally found in compost. However, the specific forms of organic P in topsoil extracts remained unaffected, with similar concentrations of myo -IP 6 , scyllo -IP 6 , pyrophosphate and degradation products of phospholipids and nucleic acids present in all treatments. Using enzyme additions, a significant accumulation of non-hydrolyzed P in soil amended with dried sewage sludge was observed, but it accounted for only 9% of the total accumulation of P in this treatment compared to the treatment with manure addition which had an even input–output P balance. The similar composition of organic P in all treatments suggests that specific organic P compounds added with the three organic fertilizers were completely transformed in the topsoil or lost from it.

Degradation of tropical pastures on highly weathered soils is linked to soil structural decline and associated losses of organic matter, which could both also affect soil phosphorus (P) dynamics and availability. Our aim was to elucidate these linkages by examining the effect of pasture degradation on the contents and forms of P contained within aggregate size classes and macroaggregate fractions (collectively referred to as soil structural components). We conducted a study on nine farms in a deforested part of the Colombian Amazonia, each farm with degraded and productive Brachiaria spp. pastures. Topsoil (0–10 cm) samples were separated by wet-sieving into aggregate size classes, and macroaggregates were further separated into occluded fractions. Soils and structural components were analyzed for concentrations of total P, available P (extracted using anion exchange resins), NaOH–EDTA extractable organic and inorganic P, and enzyme-hydrolyzable organic P (simple monoester-like, DNA-like, myo -inositol hexakisphosphate-like) in the NaOH–EDTA extracts. Degraded pasture soils had significantly fewer large macroaggregates and more microaggregates, both with significantly lower organic P concentrations (in mg P kg − 1 of structural component) than those in productive pasture soils. At the same time, total and extractable inorganic P concentrations in bulk soil and structural components did not differ between pasture types, suggesting a shift from organic to non-extractable P upon degradation. Soils under productive pastures contained 37% more organic P than degraded pasture soils (in mg P kg − 1 soil), mainly in large macroaggregates. The organic P concentrations were strongly correlated with C concentrations across all soil structural components, suggesting similar stabilization mechanisms for organic P and C. In bulk soil and most structural components, around 60% of organic P was enzyme-hydrolyzable. The lower contents of all enzyme-hydrolyzable as well as enzyme-stable organic P in degraded vs. productive pasture soils indicate a reduction of all organic P classes during pasture degradation. The large macroaggregates and, in particular, microaggregates occluded within this fraction were identified as an important site for organic P storage. Our results highlight a linkage between soil structure and organic P, both of which can play an important role in maintaining the productivity of pastures established on highly weathered soils.

Microbial nutrient limitation was investigated in a 53-year-old field experiment in the Central-West of Burkina Faso under sorghum–cowpea rotation, comparing three fertilization practices: mineral fertilizer (MIN), mineral fertilizer and farmyard manure (MINFYM), and a non-fertilized control (CON). We assessed microbial N and P limitation after removal of C limitation by (i) determining microbial N and P, (ii) assessing respiration kinetics in incubated soil samples amended with easily available C (glucose) alone or in combination with N and/or P, or not amended, and (iii) evaluating changes in microbial biomass and community composition at the peak of microbial respiration by microbial P and phospholipid fatty acid (PLFA) analyses. Microbial N and P were very low in all fertilization practices, but greater in MINFYM than in CON. Easily available C was the first factor limiting microorganisms in all fertilization practices. After removal of C limitation, most indicators suggested N and P co-limitation in CON. In contrast, respiration kinetics in MINFYM and MIN were only N-limited, while biomass formation in MINFYM was also P-limited. PLFA analyses indicated preferential fungal growth on the added C, and P limitation of changes in microbial community composition in MIN. Long-term application of fertilizers mostly alleviated secondary microbial nutrient limitation by P but not by N, and C always remained the primary limiting factor for microbial growth.

Cover crops provide benefits in agricultural systems with high P availability (i.e., optimal or excessive soil P for plant growth) by reducing losses of soil phosphorus (P) via erosion and leaching, and potentially by increasing soil P availability when P is released during residue decomposition. We quantified P transfer from cover crop residues to soil pools and a subsequent wheat crop in a greenhouse experiment. Soils from two field experiments in California (Davis and Salinas) were labeled with carrier-free 33P and amended at a rate of 15 mg P kg−1 soil with cover crop residues (rye, oat, fava bean, vetch, mustard, rye-legumes mixture) or water-soluble mineral fertilizer. We analyzed plants and several soil pools – resin, microbial, and organic – for P and 33P. In both soils, residues and water-soluble mineral fertilizer had a similar effect on soil pools and wheat P uptake, except for higher microbial and organic P with residues in the Davis soil. Residues contributed 35–40 % (Davis) or 20–25 % (Salinas) of the P taken up by wheat, and 13–22 % (Davis) or 8–14 % (Salinas) of residue P was recovered in wheat. Our results demonstrate that P taken up by cover crops can cycle rapidly in agricultural systems with high soil P availability, with direct benefits for soil P availability and few differences among these cover crop species.

Soil chemistry and biota heavily influence crop plant growth and mineral nutrition. The stress-severity and optimal resource allocation hypotheses predict mutualistic symbiotic benefits to increase with the degree of metabolic imbalance and environmental stress. Using two cross-factorial pot experiments with the same biologically active calcareous soil, one time highly saline and nutrient-deficient, and the other time partially desalinated and amended with mineral soil fertilizer, we explored whether these general predictions hold true for zinc (Zn) nutrition of bread wheat in mycorrhizal symbiosis. Increased arbuscular mycorrhizal (AM) fungal root colonization positively correlated with plant Zn nutrition, but only when plants were impaired in growth due to salinity and nutrient-deficiency; this was particularly so in a cultivar-responsive to application of mineral Zn fertilizer. Evidence for direct involvement of AM fungi were positive correlations between Zn uptake from soil and frequency of fungal symbiotic nutrient exchange organelles, as well as the quantitative abundance of AM fungi of the genera Funneliformis and Rhizophagus, but not Claroideoglomus. Combined partial soil desalination and fertilization swapped the dominance ranking from Claroideoglomus spp. to Funneliformis spp. Positive growth, nitrogen, and Zn uptake responses to mycorrhization were contingent on moderate soil fertilization with ZnSO4. In agreement with the predictions of the stress-severity and optimal resource allocation hypotheses, plants limited in growth due to chemically adverse soil conditions invested relatively more into AM fungi, as evident from heavier root colonization, and took up relatively more Zn and nitrogen in response to mycorrhization, than better growing and less mycorrhized plants. It thus appears that crop plant cultivar-dependent mycorrhization and Zn fertilizer-responsiveness may reinforce each other, provided that there is bioavailable Zn in soil and plant growth is impaired by suboptimal chemical soil conditions.

Crop production in subsistence agriculture in tropical Africa is still sustained mainly by short-to medium-term fallows to recuperate natural fertility of the soils. Microbes play a pivotal role both in the process of soil fertility restoration and in nutrient acquisition by the crops. Here we ask the question how the duration of fallow affects the composition of indigenous arbuscular mycorrhizal fungal (AMF) communities and their contribution to maize nutrition and growth, in acidic, low P soils of southern Cameroon. This question has been addressed in a bioassay where soils collected from continuously cropped fields, short-term fallows dominated by Chromolaena odorata and long-term fallows (secondary forests) have been sterilized and back- and cross inoculated with living soils from the different land-use systems. Particular microbes larger than the pore size of the filter paper (mainly the fungi including the AMF) contained in the cropped and short-fallowed soils caused greater growth and P uptake stimulations to the maize as compared to the forest soil. By using molecular profiling, we demonstrated a shift in the composition of AMF communities along a gradient of fallow duration, changing from dominance by Rhizophagus in the forest fallow soil, to dominance by Claroideoglomus under cropland. Our results contradict the hypothesis that deterioration of quality of root symbiotic communities would be responsible for a rapid yield decline following deforestation, and indicate a positive feedback of cropping on mycorrhizal functioning under conditions of shifting agriculture in tropical Africa.

In many tropical regions, slash-and-burn agriculture is considered as a driver of deforestation; the forest is converted into agricultural land by cutting and burning the trees. However, the fields are abandoned after few years because of yield decrease and weed invasion. Consequently, new surfaces are regularly cleared from the primary forest. We propose a reclamation strategy for abandoned fields allowing and sustaining re-cultivation. In the dry region of south-western Madagascar, we tested, according to a split-plot design, an alternative selective slash-and-burn cultivation technique coupled with compost amendment on 30–year-old abandoned fields. Corn plants (Zea mays L.) were grown on four different types of soil amendments: no amendment (control), compost, ashes (as in traditional slash-and-burn cultivation), and compost + ashes additions. Furthermore, two tree cover treatments were applied: 0% tree cover (as in traditional slash-and-burn cultivation) and 50% tree cover (selective slash-and-burn). Both corn growth and soil fertility parameters were monitored during the growing season 2015 up to final harvest. The amendment compost + ashes strongly increased corn yield, which was multiplied by 4–5 in comparison with ashes or compost alone, reaching 1.5 t/ha compared to 0.25 and 0.35 t/ha for ashes and compost, respectively. On control plots, yield was negligible as expected on these degraded soils. Structural equation modeling evidenced that compost and ashes were complementary fertilizing pathways promoting soil fertility through positive effects on soil moisture, pH, organic matter, and microbial activity. Concerning the tree cover treatment, yield was reduced on shaded plots (50% tree cover) compared to sunny plots (0% tree cover) for all soil amendments, except ashes. To conclude, our results provide empirical evidence on the potential of recultivating tropical degraded soils with compost and ashes. This would help mitigating deforestation of the primary forest by increasing lifespan of agricultural lands. ISSN:2045-7758

Zinc (Zn) deficiency in human populations depending on cereals as a main source of Zn is a global malnutrition problem. In this field study, we investigated the potential of green manure application to increase soil Zn availability and wheat grain Zn concentrations (biofortification) on a Luvisol with different long-term fertilizer management. We also studied cadmium (Cd), as wheat is a major contributor of this undesired non-essential element to human diets. Clover (Trifolium alexandrinum L.), mustard (Sinapis alba L.) or no green manure was grown on field plots which had been managed with farmyard manure or mineral fertilizers for 65 years in Switzerland. After green manure incorporation into the soil, spring wheat (Triticum aestivum L.) was grown on all plots. The “diffusive gradients in thin films” (DGT) method and DTPA extraction were used to compare soil Zn and Cd availability among the treatments. In contrast to mustard, clover increased soil mineral nitrogen concentrations and wheat biomass; however, neither increased grain Zn concentrations. DGT-available Zn and Cd increased temporarily after both farmyard manure and mineral nitrogen fertilizer application. Higher DTPA-extractable soil Zn and Cd, lower wheat grain yields, but higher grain Zn concentrations were obtained with farmyard manure compared to mineral fertilizers, independent of the green manure treatment. Farmyard manure added Zn, Cd and organic matter that increased the soil binding capacity for Zn and Cd. The decomposition of clover residues caused higher wheat grain yields, but only marginally lower grain Zn concentrations. The absence of a stronger dilution of grain Zn was probably due to organic acid and nitrogen release from decomposing clover, which facilitated Zn uptake by wheat. The study revealed that both long- and short-term field management with organic matter alters soil Zn and Cd concentrations but that the long-term effects dominate their uptake by wheat, in Zn sufficient soil.

Phosphorus availability may shape plant–microorganism–soil interactions in forest ecosystems. Our aim was to quantify the interactions between soil P availability and P nutrition strategies of European beech (Fagus sylvatica) forests. We assumed that plants and microorganisms of P-rich forests carry over mineral-bound P into the biogeochemical P cycle (acquiring strategy). In contrast, P-poor ecosystems establish tight P cycles to sustain their P demand (recycling strategy). We tested if this conceptual model on supply-controlled P nutrition strategies was consistent with data from five European beech forest ecosystems with different parent materials (geosequence), covering a wide range of total soil P stocks (160–900 g P m−2

The abundance, distribution and functions of soil fungi in alpine ecosystems remain poorly understood. We aimed at linking the fungal community structure with soil enzymatic activities in the rhizospheres of several plants associating with mycorrhizal fungi (arbuscular, ecto- and ericoid) and growing along a soil developmental gradient on the forefield of an alpine glacier. Fungal communities in roots and in rhizosphere soils were assessed using a site-tailored set of quantitative PCR assays with fluorescent hydrolysis probes. Enzymatic activities of mycorrhizal roots and rhizosphere soils were assessed using fluorogenic substrates. In this study we addressed: i) whether and how the structure of fungal communities and enzymatic activities in rhizosphere soils change along the soil developmental gradient, ii) whether the type of mycorrhiza shows a clear relationship to the pattern of enzymatic activities in the rhizosphere, and iii) how the structure of fungal communities and enzymatic activities in rhizosphere soils is related to plant species abundances along the soil chronosequence. The results suggest that plant identity affected the structure of both ecto- and arbuscular mycorrhizal fungal communities in rhizosphere soil and roots, whereas the community of non-mycorrhizal fungi was rather dictated by the soil developmental stage. Both plant identity and associated mycorrhizal fungi affected the enzymatic activity in the rhizosphere soil. Species-specific elevations of rhizosphere enzyme activities were detected for Salix helvetica (chitinase and α-glucosidase), Rhododendron ferrugineum (α-glucosidase and sulfatase), and Agrostis gigantea (phosphatase and xylosidase). These results indicate different functional roles played by different types of mycorrhizal symbiosis in a young alpine ecosystem.

Producing a P fertilizer from sewage sludge ash (SSA) is a strategy to efficiently recycle P from a secondary raw material. The P speciation in four SSAs was characterized before and after the removal of heavy metals by a thermo-chemical treatment that involved CaCl addition. We chose complementary techniques to determine the direct P speciation, including X-ray powder diffraction, solid-state P direct-polarization magic-angle spinning nuclear magnetic resonance, and X-ray absorption near edge structure. Results from these techniques were compared with operational and functional speciation information obtained from a sequential P extraction and a plant biotest with Italian ryegrass grown on a soil-sand mixture with little available P. The speciation of P in untreated and thermo-chemically treated SSAs depended on their elemental composition. At a molar ratio of Ca:P ≤ 2, SSAs contained combinations of polymorphs of AlPO, β-tricalcium phosphate, and apatite-like P species. In SSAs with a molar ratio of Ca:P > 2, an apatite-like molecular environment was predominant. The thermo-chemical process induced an increase in crystalline phases and enhanced the crystallinity of the P species. The structural order of the bulk sample was the most decisive parameter in controlling the P availability of the studied SSAs to plants. We conclude that, to produce a high-quality fertilizer and despite of the successful heavy metal removal, the thermo-chemical process requires further development toward enhanced P bioavailability.

Phosphorus (P) is the second essential nutrient for plant growth but can become an ecological and economical concern in case of over-fertilization. Soil P dynamic is influenced by many parameters like soil physical-chemical properties and farming practices. A better understanding of the factors controlling its distribution is required to achieve best P crops management. In Switzerland, the FRIBO network was launched in 1987 and comprises of 250 sites covering a wide diversity of soils and three different land uses (croplands, grasslands and mountain pastures) across the Fribourg canton. A spatial investigation of the different P forms for the FRIBO network led to the following main conclusions: i) The P status in agricultural soils was significantly different among the three land uses encountered, with the highest mean values of available P found in croplands (from 2.12 to 81.3 mg.kg-1 according to the indicator used), whereas total P was more abundant in permanent grasslands (1186.2 mg.kg-1), followed by mountain pastures (1039.0 mg.kg-1) and croplands (935.0 mg.kg-1). A full characterization of the soil P status provides necessary data on P distribution related to soil properties and land use, and should help to develop more accurate estimation procedures and fertilization strategies in a near future; ii) Environmental variables derived from digital elevation model (DEM) only explained a small part of the spatial variation of the different P forms (20 to 25%). Thus, the geostatistic analyses revealed that land use play a major role in soil P distribution. However, this pattern was less visible for total P than for available P. Future studies should include more data points as well as additional variables such as parent material and soil type to accurately estimate the role of soil parameters on the distribution of P-related forms.

Maintaining the productivity of tropical pastures is a major challenge for the sustainable management of tropical landscapes around the globe. To address this issue, we examined linkages between soil organic matter (SOM), aggregation, and phosphorus (P) dynamics by comparing productive vs. degraded pastures in the deforested Amazon Basin of Colombia. Paired plots of productive (dominated by planted Brachiaria spp.) vs. degraded pasture were identified on nine farms in the Department of Caqueta and sampled during the rainy season of 2011. Aboveground pasture biomass production and nutrient content were measured. Surface soils (0–10 cm) were also fractionated by wet sieving, and C, 13C, N and P contents were analyzed for the bulk soil and various aggregate size classes. Productive pastures yielded more than double the aboveground biomass compared to degraded pastures (during a 35 day regrowth period following cutting), with over 60% higher N and P contents in this material. Similar trends were observed for the standing litter biomass and nutrient contents. Soil aggregate stability was found to differ between pasture types, with a mean weight diameter of 3590 vs. 3230 μm in productive vs. degraded pastures, respectively. Productive pastures were found to have 20% higher total soil C and N contents than degraded pastures. While there was no difference in total P content between pasture types, organic P was found to be nearly 40% higher in soils of productive vs. degraded pastures. Differences in total SOM between pasture types were largely explained by a higher C content in the large macroaggregate fraction (>2000 μm), and more specifically in the microaggregates (53–250 μm) occluded within this fraction. These findings confirm the role of microaggregates within macroaggregates as a preferential site for the physical stabilization of SOM, and furthermore, suggest that it may serve as a useful diagnostic fraction for evaluating management impacts on SOM in tropical pasture systems. Similar to trends observed for C and N, total P content was 25% higher in the microaggregates within large macroaggregates of productive vs. degraded pasture soils. This correspondence between C and total P contents in large macroaggregate fractions, along with elevated levels of organic P in productive pastures, suggests that this P is likely in an organic form and that there is a close link between soil structure, SOM dynamics and the maintenance of organic P in these soils. Given the potential relevance of organic P for efficient P cycling in these soils, our findings offer critical new insight for the management of SOM and aggregate-associated P pools in tropical pasture systems.

A precise assessment of the phosphorus (P) nutrition status of plants is necessary for an efficient P management in agricultural grasslands. Plant mineral analysis is a tool to identify the nutrition status of grasslands and several P indicators derived thereof are available. However, the interpretation of plant P indicators in grassland samples is complex due to variation in botanical composition, changing nutrient concentrations during growth and interactions between nutrients. The aim of this study was to compare indicators on the P nutrition status of plants and eventually to improve their application in agricultural grasslands. We studied three agricultural grassland types in Switzerland that were managed at different intensity either for high biodiversity or high forage production. Each grassland type was for 5–25 years subjected to treatments of low, recommended or high rates of P, nitrogen (N) and potassium (K) fertilizer. The recommended P rates ranged from 15 to 48 kg P ha −1 year −1 . We measured plant P, N and K concentrations in the aboveground biomass of grasses, legumes and forbs. The evaluated P indicators were: P concentration, N:P and K:P ratios and the P nutrition index (PNI). The PNI is calculated as P concentration divided by the linear relationship describing optimal sward P concentration as a function of N concentration (PNI = 100P/(0.06 × 5N + 1.5)). We observed significant yield reduction compared to the expected yield only when one or more nutrients were omitted from fertilization. Fertilizer P input higher than recommended did not significantly increase yields above the yield expected for the respective management and altitude. Under P limiting conditions, forbs and legumes had significantly higher P concentrations than grasses. Additionally, the proportion of legumes affected the P indicators integrating N. Therefore, we used the P indicators in the grass fraction, which was always the main botanical fraction. In grasses all P indicators differentiated between P fertilized and non-P-fertilized treatments. Concentrations of P from 2.1 to 3.0 mg g −1 indicated sufficient P supply, while yield was reduced at lower and not increased at higher concentrations, suggesting luxurious P consumption. Our results for N:P and K:P suggested optimal ranges of 5.5–9.0 and 6.0–10.5, respectively. The PNI showed a clear differentiation between deficient, sufficient or surplus fertilizer inputs. For the precise and correct interpretation of the plant P nutrition status in agricultural grasslands under different management, we propose to use the PNI in the grass fraction. Finally, the interpretation of the indicators was valid for the agricultural grasslands managed at different intensities in spite that the grass fraction was composed by different species.

This chapter addresses the following questions: (i) is phosphorus (P) that passes through the “organic pathway”, i.e. that is found at some point in time in an organic form or within soil organisms or plant residues completely used by plants (100 % use by plants) and (ii) does P that passes through the “organic pathway” fully remain in the soil/plant system (zero losses to the environment). Positive answers to these questions would lead to the conclusion that P management could be optimized by increasing soil organic P content and soil biological activity. The chapter begins with a short presentation of the actors of the “organic pathway”, and how their dynamics influence soil P transformations. Then we show how the “organic pathway” controls soil P availability in natural systems and how soils under native vegetation can lose significant amounts of P during their pedogenesis. Afterwards we discuss the role of the “organic pathway” in determining P availability and limiting P losses in agricultural systems. The analysis of exemplary cropping systems studied in the Swiss DOK field experiment (bio-Dynamic, bio-Organic and conventional (K) cropping systems) shows that it is not possible to reach 100 % apparent P recovery and that the recovery is negatively correlated with soil available inorganic P (Pi). A high apparent P fertilizer recovery is observed in the system in which the soil shows the highest biological activity. This high recovery can be explained by the low P inputs in this treatment and by the transfer of P through the soil biota . However, this high recovery can also be interpreted as an artifact in the calculation of the apparent recovery rate, concealing a higher use of P derived from the soil in this treatment. The higher use of soil P by crops can be explained by the higher root colonization by mycorrhizal fungi and by the good soil structure observed in this treatment. In permanent grasslands, the very high microbial P content observed in the upper soil horizon suggests that the “organic pathway” plays a major role in taking up and releasing P to the plants. Finally, the importance of this pathway for limiting P losses from agricultural systems is discussed. Whereas it might have a role for conserving P in the soil profile, it is probably not the most important point to address for limiting P losses from agricultural systems.

The impact of soil chemical parameters (pH and base saturation) and soil depth on fine root dynamics in six acidic European beech (Fagus sylvatica L.) stands with low soil base saturation was examined. The results were compared with published data from other European forests. Fine roots were sampled by sequential coring down to 1 m depth. Production, mortality, turnover and estimated age were calculated, and the ages of fine roots from two stands were measured by radiocarbon. In all stands, fine root biomass, necromass, production and mortality decreased significantly with soil depth. No relationships were observed between any of the soil chemical parameters and the fine root biomass, necromass, production and mortality. A significant negative correlation was found between soil pH and fine root turnover when our data were combined with the published data. The ages of fine roots estimated with sequential coring ranged from 0.9 to 2.9 years, and with radiocarbon from 3 to 26 years. These divergent ...

The productivity of white yam (Dioscorea rotundata Poir) must increase to sustainably meet the demand of the increasing populations in the developing world where this is a staple crop. Although thi...

In this study we wanted to know whether diversity in fertilization practices between and within urban vegetable sites exists and how does this diversity affect soil and crop quality. We sampled soils (at 0–15 and 15–30 cm depth) and lettuce leaves (Lactiva sativa) at four vegetable growing sites in Ouagadougou called Boulmiougou, Hopital, Tanghin and Wayalguin. We determined the pH, total carbon (C), nitrogen (N), phosphate (P), and inorganic P contents, and available P and heavy metals (Cd, Cu, Pb, Cr, Zn, and Ni) contents in these soils. We evaluated also the lettuce leaves yields and we analyzed them for their N, P, K, Ca, Mg, Na and Cd, Cu, and Zn contents. We also interviewed farmers about their fertilization practices. We observed a large diversity of fertilization practices between and within sites. We found that site identity explained a very large fraction of the variability of fertilization practices, soil chemical properties and lettuce leaves production and element contents. The effect of the site was explained by the proximity to nutrient sources, the location of the site in the landscape, the presence of farmers’ association, and farmers’ skills. Solid waste and waste water were major sources of nutrients at the sites located inside the city (Hopital, Tanghin, and Wayalguin) while at Boulmiougou which is located in the periphery, manure and mineral fertilizers were the preferred sources of nutrients. The proximity of the site Tanghin to a major road led to the highest soil Pb content which was attributed to atmospheric pollution. The higher lettuce leaves yields in site Boulmiougou was attributed to good technical skill of farmers. The use of solid waste and waste water led to N, P and metals accumulation in soil. Zn and Cu content in the lettuce leaves were negatively correlated to soil pH, while Cd content in lettuce leaves was correlated to the use of water soluble P fertilizers.

Lower P-input levels in organic than conventional farming can decrease soil total and available P, which can potentially be resupplied from soil organic P. We studied the effect of 30 y of conventional and organic farming on soil P forms, focussing especially on organic P. Soil samples (0–20 cm) were taken in a field experiment with a nonfertilized control, two organic systems receiving P inputs as animal manure, and two conventional systems receiving only mineral P or mineral P and manure. Soils were analyzed for total, inorganic, organic, and microbial P, by sequential P fractionation and by enzyme additions to alkaline soil extracts. Samples taken prior to starting the experiment were also analyzed. Average annual P balances ranged from –20 to +5 kg ha–1. For systems with a negative balance, labile and moderately labile inorganic P fractions decreased, while organic and stable inorganic P fractions were hardly affected. Similar quantities and proportions of organic P extracted with NaOH-EDTA were hydrolyzed in all soils after addition of an acid phosphatase, a nuclease, and a phytase, and enzyme-stable organic P was also similar among soils. Thus, neither sequential fractionation nor enzyme addition to alkaline soil extracts showed an effect of the type of applied P (manure vs. mineral) on organic P, suggesting that organic P from manure has largely been mineralized. Thus far, we have no indication that the greater microbial activity of the organic systems resulted in a use of stable P forms.

Non-sorted solid urban wastes (SUW) are used by periurban cereal farmers in Africa. There is however limited information on how these SUW affect soil quality and cereal production and quality. In order to answer this question we identified around Ouagadougou (Burkina Faso) sites cultivated with sorghum (Sorghum bicolor (L.) Moench) that had received SUW for less than 5 years, for more than 5 years and less than 10 years and for more than 10 years. We sampled soils at 0–15 and 15–30 cm depth and we analysed pH, total carbon (C), nitrogen (N) and inorganic phosphate (P) content and P and heavy metals availability. We also measured at some of these sites sorghum production and nutrient and metal contents in sorghum grain and straw. Our results show that the 0–15 cm horizon of the soils that had received SUW for less than 5 years had lower pH, available P and heavy metals contents and produced lower yields than those that had been amended with SUW for more than 10 years. Maximum grain yield was obser...

Yams (Dioscorea spp.) are known to be very demanding in terms of soil fertility. The influence of deleterious soil fertility on the growth of yams, however, has not yet been studied. A field study was carried out in Cote d'Ivoire, in a fertile forest site and in a low fertility savanna site, submitted to identical climatic conditions, to determine how soil fertility affects Leaf Area Index, Leaf Area Ratio, Crop Growth Rate, Tuber Bulking Rate, Radiation Interception, Radiation Use Efficiency and final tuber yields in D. alata and D. rotundata, grown in the presence of adequate fertilizers. While germination and establishment of both species were not affected by the site, growth parameters of D. alata were superior to those of D. rotundata. This resulted in higher yields of D. alata, when compared to D. rotundata. Only the radiation use efficiency was higher in D. rotundata than in D. alata. Both species reached higher Crop growth rate and Tuber bulking rate and produced higher fresh tuber yields...

Water yam (Dioscorea alata) is a staple crop for many of the poorest in the tropics. Yam tuber response to fertiliser application has been reported to be erratic. We hypothesised that this variable response could be due to the development of a suboptimal root system that would not allow the plant to capture the nutrients added with the fertiliser. To verify this hypothesis we studied during two seasons (2006 and 2007) in Central Cote d'Ivoire, the growth of yam root system in relation to plant growth as affected by fertiliser input, using sequential root coring. In addition, we assessed the horizontal and vertical distribution of yam roots using the wall profile method in the same site. Three root types (seminal, adventitious and tubercular roots) were identified. Roots grew within the mound until 100 days after planting (DAP) and afterwards, extended radially in the soil outside of the mound remaining in the first soil horizon (15 cm depth). Maximum root extension was observed at 160 DAP. Root length density and root mass density were higher at higher soil temperature whereas the average root diameter decreased at higher soil temperature. Specific root length did not change with soil temperature. Root growth parameters were not affected by fertiliser application. Roots were mostly distributed in clumps. As tuber yield formation was independent from root growth we conclude that the root system of this cultivar did not limit tuber productivity. Other factors have to be considered to explain the variable response of yam to mineral fertiliser additions.

Concerted use of legumes and of functional diversity in grassland forage systems can provide major contributions to the challenges of agricultural systems being productive yet environmental friendly. Acquisition and transformation of nitrogen (N) resources by legumes and grasses were studied in a temperate grassland experiment near Zurich (Switzerland) to investigate mechanisms driving effects of functional diversity in mixed swards and to optimise mixtures for efficient resource use. Grass–legume interactions and N availability were varied by manipulating legume percentage of the sward (0–100%) and N fertiliser application (50, 150 or 450 kg of N ha−1 year−1). 15N technology quantified N derived from symbiotic (Nsym) and non-symbiotic (Nnonsym) sources. Generally, acquisition of Nsym by the entire mixture was stimulated by grasses. As a result, strong overyielding of Nsym occurred (e.g. 75 and 114% for year 1 and 2 at N150) and mixtures with only 60% and 37% legumes (year 1 and 2) already attained the same Nsym yield as pure legume stands. Legumes stimulated Nnonsym acquisition by the entire mixture, largely via increased uptake by the grass component. Thus, overyielding of Nnonsym of 31% occurred in year 1 (N150). Mutual grass–legume interactions stimulated acquisition of Nsym, acquisition of Nnonsym and efficient transformation of N into biomass compared to either monocultures. These effects of functional diversity can substantially contribute to productive and resource efficient agricultural grassland systems and were maximised in mixtures with 40–60% legumes.

Plant and Soil, 341 (1-2), ISSN:0032-079X, ISSN:1573-5036

The biomass production and the nutritive value of three tropical shrub legumes with condensed tannins (CT) (Calliandra calothyrsus, Flemingia macrophylla, Leucaena leucocephala) and two without CT (Cratylia argentea, Desmodium velutinum) as nitrogen-rich feed supplements for ruminants were assessed in two field experiments in Colombia. In one experiment, conducted on two different typical tropical soils (mollisol vs. oxisol; pH, 7·8 vs. 5·0; P content, 43 vs. 2 mg kg DM⁻¹), the effects of low and high levels of fertilizer application with P, K, Ca, Mg and S were tested in both the dry and rainy seasons. In a second experiment on the oxisol, the effect of a lower level of application of either P or S fertilizer was assessed. On the oxisol, C. calothyrsus and F. macrophylla had the highest biomass production (93 and 100 g DM plant⁻¹ in 9 weeks respectively) but the dry season caused extremely low DM yields in all species tested. Leucaena leucocephala did not grow on the oxisol but had the highest biomass production on the mollisol (454 g DM plant⁻¹ in 9 weeks). On the oxisol, the mineral concentrations of the forage were below the requirements of ruminant livestock. Fertilizer treatment had no clear effects on the nutrient composition of the forage. The non-CT shrub C. argentea had the highest concentrations of most minerals in its forage.

Brachiaria forage grasses are widely used for livestock production in the tropics. Signalgrass (Brachiaria decumbens cv. Basilisk, CIAT 606) is better adapted to low phosphorus (P) soils than ruzigrass (B. ruziziensis cv. Kennedy, CIAT 654), but the physiological basis of differences in low-P adaptation is unknown. We characterized morphological and physiological responses of signalgrass and ruzigrass to low P supply by growing both grasses for 30 days in nutrient solution with two levels of P supply using the hydroxyapatite pouch system. Ruzigrass produced more biomass at both levels of P supply whilst signalgrass appears to be a slower-growing grass. Both grasses increased biomass allocation to roots and had higher root acid phosphatase and phytase activities at low P supply. At low P supply, ruzigrass showed greater morphological plasticity as its leaf mass density and lateral root fraction increased. For signalgrass, morphological traits that are not responsive to variation in P supply might confer long-term ecological advantages contributing to its superior field persistence: greater shoot tissue mass density (dry matter content) might lower nutrient requirements while maintenance of lateral root growth might be important for nutrient acquisition in patchy soils. Physiological plasticity in nutrient partitioning between root classes was also evident for signalgrass as main roots had higher nutrient concentrations at high P supply. Our results highlight the importance of analyzing morphological and physiological trait profiles and determining the role of phenotypic plasticity to characterize differences in low-P adaptation between Brachiaria genotypes.

Summary 1 Increasing plant species richness often increases biomass production in nutrient-poor semi-natural grasslands. If such positive diversity–productivity effects also apply to nutrient-rich agricultural grasslands, mixtures could improve resource-use efficiency in the vast area used for forage production. We therefore quantified the diversity–productivity effects in nutrient-rich agricultural grasslands using four-species grass–legume mixtures. 2 The sown overall density and species proportions of Lolium perenne, Dactylis glomerata, Trifolium pratense and Trifolium repens were varied in a 3-year field experiment to investigate the effects of species richness (1, 2, 4 species) and species proportion (0, 3, 10, 25, 40, 50, 70, 90, 100% sowing proportion) on productivity under a nitrogen fertilization of 50, 150 or 450 kg N ha−1 year−1. 3 The four-species mixtures reached up to twice the yield of the average of the four species’ monocultures (overyielding up to 106%), predominantly due to combining grass and legume species. Mixtures were up to 57% more productive than the most productive monoculture (transgressive overyielding). Both these diversity–productivity effects appeared across a broad range of species proportions and persisted at the two lower levels of N fertilization for 3 years. 4 Mixtures fertilized with 50 kg N ha−1 year−1 produced yields comparable to grass monocultures fertilized with 450 kg N ha−1 year−1, if the legume proportion was about 50 to 70%. Diversity–productivity effects were reduced at the highest level of N fertilization, where they virtually disappeared in the third year. Increased N fertilization also accelerated the observed general trend towards D. glomerata dominated and legume-poor swards. 5 Synthesis and applications. Diversity–productivity effects led to consistent transgressive overyielding in intensively managed grasslands, suggesting a highly increased resource-use efficiency in mixtures. Performance better than monocultures can be achieved with grass–legume mixtures that have a low number of species, across a wide range of species proportions and in nutrient-rich conditions. Processes such as niche complementarity and positive interspecific interactions leading to diversity effects proved to be highly relevant and widely applicable for intensive forage production. Such diversity–productivity effects could allow reduced inputs of N fertilizer without loss of productivity in different grassland production systems.

Annals of Botany, 103 (2), ISSN:1095-8290, ISSN:0305-7364

Nutrient Cycling in Agroecosystems, 83 (3), ISSN:1385-1314, ISSN:1573-0867

Conservation tillage is widely practiced in semi-arid climates, mostly in small grain crop rotations. It is implemented to a much lesser extent in cool and humid climates of Europe, mainly due to a lack of knowledge about agronomic and ecological impacts. This study was conducted in light- to medium-textured soils in the Swiss midlands from 1995 to 2000. The aim was to determine whether tillage intensity impacted wheat yield and measure the effects of the preceding crop and the level of N fertilisation. Conventional tillage (CT) with ploughing, minimum tillage (MT) with a chisel, and no-tillage (NT) were studied in the following crop rotation: winter wheat (Triticum aestivum L.)–oilseed rape (Brassica napus L.)–winter wheat–maize (Zea mays L.). Winter wheat was grown at either 0 kg N ha−1 (N0) or at the recommended N supply (N1). Harvest residues of all the crops were left on the field. In the early stages wheat development under NT was slightly slower than under CT and MT, but at maturity the shoot biomass was 2% higher under NT than under the other tillages. The grain yield decreased by 3% under NT compared to CT and MT, mainly due to fewer ears per area and a significantly lower thousand-kernel weight. Wheat planted after oilseed rape had significantly higher shoot biomass and grain yield than wheat planted after maize. At the beginning of the vegetation season there were small but significant differences in the soil mineral N content among the tillage treatments. Accordingly, the level of N fertiliser applied was adjusted to ensure similar N availability in all tillage systems. The relative reduction in grain yield under NT compared to CT and MT was similar with and without N fertilisation. Thus, N availability was not a limiting factor for the yield of NT wheat in this study.

Summary The fate of carbon (C) in grassland soils is of particular interest since the vast majority in grassland ecosystems is stored below ground and respiratory C-release from soils is a major component of the global C balance. The use of 13 C-depleted CO2 in a 10-year free-air carbon dioxide enrichment (FACE) experiment, gave a unique opportunity to study the turnover of the C sequestered during this experiment. Soil organic matter (SOM), soil air and plant material were analysed for d 13 C and C contents in the last year of the FACE experiment (2002) and in the two following growing seasons. After 10 years of exposure to CO2 enrichment at 600 ppmv, no significant differences in SOM C content could be detected between fumigated and non-fumigated plots. A 13 C depletion of 3.4& was found in SOM (0–12 cm) of the fumigated soils in comparison with the control soils and a rapid decrease of this difference was observed after the end of fumigation. Within 2 years, 49% of the C in this SOM (0–12 cm) was exchanged with fresh C, with the limitation that this exchange cannot be further dissected into respiratory decay of old C and freshly sequestered new C. By analysing the mechanistic effects of a drought on the plant-soil system it was shown that rhizosphere respiration is the dominant factor in soil respiration. Consideration of ecophysiological factors that drive plant activity is therefore important when soil respiration is to be investigated or modelled.

Phosphorus (P) losses from many agricultural soils are driven by fast transport processes, such as surface runoff, shallow interflow and macropore flow. Close interaction of rain with topsoil layers (0–5 cm), rich in available P, results in high P concentrations in these rapid runoff pathways. This is particularly pronounced in permanent grassland soils typically characterized by a P enrichment of the topsoil. One option to mitigate P losses from grassland is therefore to reduce soil P content, particularly available P, in this critical surface layer. Omitting application of P fertilizer will reduce soil P in the long term but more drastic methods may be necessary to achieve P loss reductions in shorter timescales. In this study, three management options to reduce P in runoff from grassland soils were evaluated in a field experiment lasting 2 years on two grassland sites: (i) no P inputs, (ii) no P inputs plus soil tillage and (iii) no P inputs plus tillage combined with application of a P sorbing soil amendment (iron oxide). Withholding P inputs did not decrease either the available P in the soil or the P concentrations in runoff from sprinkler experiments over a 2-year period. Both the treatments which included tillage caused immediate reductions in both soil available P and runoff P, with Fe ⁄ OH amendment being more effective. However, in the second year the effects of both tillage treatments were less apparent and only the Fe ⁄ OH treatment significantly decreased runoff P. The results indicate that the long-term enrichment of soils due to P application in excess of plant needs is unlikely to be remedied by short-term treatments. However, further work is required to assess the applicability of our conclusion to other sites.