Description: Background and aims Soil nitrogen (N) immobilization from cover crop residues may help suppress weeds. We established a gradient of cereal rye shoot biomass to determine the extent that soil N can be immobilized and its effect on redroot pigweed ( Amaranthus retroflexus L.). Methods A microplot study was conducted in no-till cereal rye ( Secale cereale L.)—soybean ( Glycine max L. (Merr.)) systems at two sites in eastern USA. Microplots received 0, 2000, 5000, 8000, 12,000 or 15,000 kg ha −1 of cereal rye shoot biomass, and were injected with two mg 15 N kg −1 soil 5 cm below the soil surface. Pigweeds were sown and allowed to germinate. Results Maximum rates of cereal rye shoot decomposition were observed at ≥5000 kg ha −1 . Although cereal rye shoot N declined, shoots became enriched with 15 N, indicating fungal transfer of soil N to shoots. Soil inorganic N declined by an average of 5 kg N ha −1 . Pigweed tissue N and biomass were reduced in the presence of cereal rye. The magnitude of pigweed N reduction was similar across all shoot application rates. Conclusions We found weak evidence for a cereal rye shoot-based N immobilization mechanism of weed suppression. Our results indicate N immobilization may be primarily due to root residues.

Description: Background and aims Fungi play vital roles in organic matter decomposition, and mineralisation of phosphorus and nitrogen, are significant plant and animal pathogens, and major mutualistic symbionts with the roots of higher plants. Despite their importance, relatively little is known about the effects of livestock grazing on different functional groups of fungi. Methods We used structural equation modelling to examine how grazing by domestic livestock and native herbivores, and aridity, plant cover and soil carbon influenced four functional groups of soil fungi (ectomycorrhizal fungi, arbuscular mycorrhizal fungi, dung saprobes, plant pathogens) from three microsites (tree, shrub, grass) at 54 woodland sites across 0.4 million km 2 of dryland in eastern Australia. Results Structural equation modelling showed that aridity influenced fungi indirectly by affecting different herbivores and by changing plant cover, which had different effects on different fungal groups. Rabbit grazing had a direct negative effect on ectomycorrhizal and arbuscular mycorrhizal fungi, most likely by disrupting hyphal networks through soil disturbance. Increased cattle grazing was directly positively associated with fungal dung saprobe abundance, and indirectly, negatively associated with dung saprobes by suppressing the positive effects of soil carbon. Sheep had direct and indirect negative effects on the abundance of plant pathogens. Conclusions Grazing was always an important predictor of the relative abundance of all fungal groups, either directly or indirectly. Thus, overgrazing is likely to have substantial effects on a range of important soil processes controlled by these microorganisms. Overall, our work indicates that increasing grazing, linked to on-going land use intensification to support a growing global population, will have major impacts on fungal functional groups.

Description: Background and aims Crucifers grown as cover crops are known to reduce sulphate leaching (S catch-crop service) and release large amounts of mineral sulphate for the subsequent cash crop once incorporated into the soil (S green-manure service). Crucifer-legume cover crop mixtures are effective to obtain high nitrogen related services, but few data exist on their performances for S-related services. Our study aimed to assess performances of a wide variety of bispecific crucifer-legume mixtures designed to provide soil S catch-crop and S green-manure services. Methods A two-year field experiment was conducted at two sites near Toulouse, France (silt clay loam soil) and Orléans, France (sandy loam soil) in which cultivars from eight crucifer species and nine legume species were tested as sole and bispecific cover crops. Results Crucifer-legume mixtures and crucifer sole cover crops provided the same level of S catch-crop service (12 kg S ha −1 ), significantly higher than that of legume sole cover crops (4 kg S ha −1 ). Similarly, crucifer-legume mixtures provided almost the same level of S green-manure service (5.5 kg S ha −1 ) as crucifer sole cover crops (6.5 kg S ha −1 ). Conclusion Our results demonstrate the compatibility and complementarity of certain crucifer and legume species when grown together to provide S and N catch-crop and green-manure services. For a same cover crop species no strong cultivar effect has been highlighted in our growing conditions.

Description: Aims Frost is a major risk factor for grain production in Australian farming systems and appears to be increasing in severity and occurrence due to changing climate. In this study we assessed the role of potassium (K) and micronutrients in alleviating floret sterility (FS) and yield loss in wheat crops subject to frost. Methods Field experiments with K application in 2015 and 2016 were conducted in frost-prone, low soil K fields in the grain belt of Western Australia. Following frost events the heads reaching anther dehiscence were tagged and FS was measured 5–6 weeks later. We also measured leaf K concentration, photosynthesis and antioxidant activity, and grain yield. Results In 2015 K supply decreased FS from 32% at nil K to 24% at 80 kg K ha −1 . In 2016 the FS values varied from 30 to 〉95%. Although there was no effect of K on FS at extreme frosts (FS 〉95%), applying 20–80 kg K ha −1 reduced FS by 10–20% and increased yield by 0.2–0.4 t ha −1 at less severe frosts. The decrease in FS was associated with increasing leaf K concentrations in the range 1.5–2.6%, higher photosynthesis and less oxidative stress at anthesis, but K supply did not provide extra protection from frost damage at leaf K 〉 2.6%. Foliar micronutrients at booting and heading did not affect FS in either year due to adequate micronutrient levels in the topsoil. Conclusions Improved plant K status can increase grain set and yields in wheat under frost, likely by maintaining physiological functions such as cell osmoregulation, plant photosynthesis and anti-oxidant systems. Plant K requirement in frost prone parts of the landscape is greater than in the areas with low risk of frost damage.

Description: Background and aims Belowground (BG) residues may contribute significant amounts of N and P via nutrient cycling to following crops, particularly in low fertiliser input systems or where all the above-ground residues are removed. Reports of simultaneous measures of nitrogen (N) and phosphorus (P) release from mature crop residues, especially from intact root systems in situ , are rare. A single stem-feed of 15 N followed by 33 P was used to (i) estimate total amounts of N and P accumulated in situ BG by mature canola and lupin plants including the stoichiometry of N and P in recovered and non-recovered components of these BG residues, and ii) simultaneously trace and quantify the relative release of that N and P. Methods One set of pots were destructively sampled at lupin and canola maturity to quantify total accumulation of BG P and BG N, including estimated N and P in unrecovered roots plus root-derived materials (RD N and RD P). Shoots were removed from a second set of pots into which wheat was sown after a 3 weeks fallow. Release of P and N from the decomposing in situ 33 P/ 15 N labelled lupin and canola BG residues was assessed as uptake in 5 weeks old wheat. Results Canola root had higher P and lower N concentrations than lupin. Canola total BG P was greater than lupin with a higher proportion as estimated RD P. Estimated RD N was similar in both species but lupin had more N in roots and so higher total BG N. C:P ratio of lupin roots was 708:1 and 188:1 for canola. Root C:N ratio was 39:1 for canola and 24:1 for lupin. N:P ratio for lupin roots was wider (29:1) than canola (5:1), but N:P ratio of RD fractions was similar (6:1 canola; 7:1 lupin). Proportion of BG P released and taken up by wheat was 21% after canola and 19% after lupin, and since total amount of BG P was much greater for canola the quantity of P released was double that after lupin. Proportion of lupin BG N (37%) released was similar to that for canola BG N (33%) although a larger amount of N was released from lupin given the larger BG N pool. Conclusion The proportion of P and N released from in situ BG residues of mature canola and lupin and taken up by wheat in this short term study was broadly inversely related to C:P, C:N and N:P ratio of recovered roots but results suggest a likely influence also of N:P ratio of the unrecovered BG residues. Quantities of N and P released were a function of the estimated total amount of plant N and P accumulated in situ BG.

Description: Aims Belowground legume nitrogen (N) composed of roots and rhizodeposition is an important N input to soils, but published data of belowground N vary broadly, probably due to extrapolation from short-term experiments and dissimilar growing conditions. We quantified belowground N inputs of red clover ( Trifolium pratense L.) during two consecutive years in a clover-grass sward along a soil nutrient availability gradient. Methods We established a red clover-perennial ryegrass ( Lolium perenne L.) model sward in microplots located in field plots of the DOK experiment, which has a 33-year history of organic and conventional cropping, resulting in a soil nutrient availability gradient. Four treatments were examined: the zero fertilisation control, bio-organic with half and full dose manure application, and the conventional system with mineral fertilisation at full dose. We studied the development of clover aboveground and belowground N using multiple pulse 15 N urea leaf labelling. Results Belowground clover N increased over time and with rising nutrient availability and was proportional to aboveground clover N at all times. Belowground clover N amounted to 40% of aboveground clover N during two consecutive years, irrespective of the nutrient availability status. Belowground clover N development was initially dominated by fast root growth, followed by enhanced root turnover during the second year. Potassium availability limited clover growth and total N accumulation in treatments with low nutrient availability. Conclusions Belowground red clover N inputs could be estimated from aboveground N by a constant factor of 0.4, regardless of the nutrient availability and cultivation time. Root turnover led to a distinct absolute increase of N rhizodeposition over time. Hence, N rhizodeposition, with an 80% share of belowground N, was the predominant N pool at the end of the second year.

Description: Background and aims Rooting plasticity is critical for plants exploiting patchy soil-water resources, but empirical evidence remains controversial due to complex root/soil interactions in natural and agricultural environments. We compared cultivated and wild Chenopodium populations from distinct agroecological background to assess their rooting plasticity when exposed to contrasting wet-dry soil profiles in a controlled environment. Methods Four treatments of increasing dryness were applied during 6 weeks in plants of Chenopodium hircinum , Chenopodium pallidicaule and two ecotypes (wet- and dry-habitat) of Chenopodium quinoa grown in rhizotrons. Root system architecture and growth were sequentially mapped. At the end of the experiment, plant and root morphological traits and dry biomass were measured. Results Contrary to the other two species, C. quinoa showed accelerated taproot growth in dry soil conditions. The dry-habitat C. quinoa ecotype showed consistently higher plant traits related to longer, coarser, and more numerous root segments which give it a faster taproot growth and sustained root branching at depth in dry soil. Conclusions High rooting plasticity confers the advantage of fast root elongation and deep soil exploration under soil water deficit. Variation in intrinsic root traits and plastic responses among Chenopodium populations controls their root foraging capacity facing patchy soil-water resources.

Description: Background and aims Carpobrotus edulis invades coastal areas throughout the world, decreasing plant diversity and hampering restoration efforts by changing soil properties. Some of its effects on soils are known but there is a knowledge gap about the effects in rocky areas and micronutrients that we aimed to fill for dunes and rocky habitats with temperate-humid climate. Methods We compared invaded vs non-invaded paired plots in two dune and two rocky areas by measuring 18 variables in litter and 24 in soils (0–5 and 5–10 cm layers). Results Invasion effects increased with the accumulated alien necromass, decreased with soil depth and are substrate-dependent: soil pH, Al, Fe and P increased in dunes, while these variables and Mg, Cu and Zn decreased in rocky sites. Carpobrotus necromass is richer in Mg and Ca and poorer in Al, Co, Cu, Fe, Ni and Zn than native necromass. Conclusions Invader effects on soils are largely mediated by its necromass, which has contrasting characteristics with the autochthonous necromass. Carpobrotus edulis ability to discriminate against Al uptake, while favouring Mg and Ca uptake, and its lower requirement (or higher resorption) of key micronutrients (Co, Cu, Fe, Ni, Zn) than native vegetation could partly explain its invasiveness.

Description: Background and aim Humic substances (HS) are known to influence plant physiological processes, enhancing crop yield, plant growth and nutrient uptake. The present study sought to gain a better understanding of the specific effects of HS application on the abundance of metabolites in plant tissues, using mass spectrometry analyses. Methods Arabidopsis thaliana plants, grown in hydroponic conditions, were treated for 8 h with indole-3-acetic acid (IAA), HS from International Humic Substances Society (IHSS) and HS from earthworm faeces (EF), respectively. Humic substances structural characteristics were analyzed by 1 H NMR an FT-IR spectroscopies. Root and leaf free amino acids, sugar alcohols and carbohydrate contents, and leaf amino acids from protein hydrolysis were identified and quantified by gas chromatography-mass spectrometry (GC/MS), and liquid chromatography-mass spectrometry (LC/MS). Canonical discriminant analysis (CDA) was used to evaluate the influence of the treatments on the studied parameters. Results EF treatment had the highest influence on metabolite profiles compared to the control, IAA and IHSS. CDA analysis highlighted a clear distinction between EF and IHSS plant physiological responses, depending on the different chemical and structural properties of the HS. IAA-treated plants showed no significant difference from the control. Conclusions A better understanding of the specific effects of different HS, also related to their chemical characteristics, might serve as a basis for the identification of marker compounds for HS bioactivity.

Description: Aims The detailed Se distribution in plants has been poorly described. This study was performed to determine the optimal dose of selenite for enhanced Medicago sativa growth and comprehensively explore the distribution characteristics of Se in this plant. Methods Alfalfa pot experiments were conducted in a growth chamber. The plants were grown in sterilized quartz sand and treated with different selenite levels for 21 days to determine the effect of Se on growth. Plants exposed to 1 and 10 μM selenite were used to clarify the Se distribution in alfalfa. Results Alfalfa growth was significantly stimulated ( P  〈 0.05) under 〈5 μM Se. Three linear correlations were found between the applied Se doses and Se concentrations in alfalfa roots, stems, and leaves. The following patterns of Se concentrations were observed: root〉leaf〉stem in the organs; rhizodermis and cortex〉stele in the tissues; and younger leaf 〉mature leaf〉older leaf and marginal leaf〉midrib〉internal leaf in the leaves. In addition, Se concentration in the cytoplasm fraction was significantly higher than that in cell wall fraction at 1 μM selenite, and the opposite result was found at 10 μM selenite. Conclusions Appropriate selenite addition (1 μM) benefited alfalfa. Se binding in the rhizodermis and cortex of the root caused relatively low transport efficiency of Se from the root to the shoot. Se redistributions may be a possible important factor affecting the transportation of Se in shoot and Se was transported along with the transpiration stream within a single leaf. Cell wall immobilization might be a major strategy to protect plant organs from potential Se toxicity.