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 To identify the role of soil properties and groundwater depth on the structure of a fire-protected neotropical savanna. We aimed to address: i) What is the contribution of soil properties and groundwater depth to the physiognomic variation? ii) Are these factors associated with patterns in woody structure? iii) Are open physiognomies maintained by shallow groundwater? Methods We measured soil properties, tree basal area, density, richness, and monitored groundwater depth in two types of grassland and three types of savanna during two years. We also investigated vegetation dynamics over three decades using remote sensing. Results There were no differences in soil properties between physiognomies, except for a greater soil organic matter content in flood-prone grasslands. Woody structure attributes were related to groundwater depth and clay content at coarse spatial scales (1 ha) and to groundwater depth and organic matter at fine spatial scales (100 m). Open savannas and grasslands remained unchanged and occurred where the groundwater depth was lower than 4 m. Conclusion Soil fertility did not drive the spread of closed physiognomies. Low clay content and shallow groundwater depth tended to decrease the woody cover, favoring the occurrence of open savannas and grasslands.

Description: Background and aims In temperate conifer forests, soil temperature is an important driver of fine root growth and winter root growth can occur during aerial dormancy. We hypothesize that in conifers, stocks of non-structural carbohydrates (NSC) in fine roots are high enough to provide energy for root growth and production throughout the year, even when photosynthesis is reduced. Methods We measured monthly root production (i.e. the number of roots undergoing elongation) and their elongation rate (RER) in mature Picea abies for one year, along a soil temperature gradient (three altitudes of 1400, 1700 and 2000 m). Every two months we harvested needles, branches, stem, large, medium and fine roots, and quantified starch and soluble sugars in each organ using analytical methods and near infra-red spectroscopy (NIRS). Soil water potential was monitored continuously. We analysed RER data with regard to climate variables and NSC levels of the current and preceding month. Results NIRS was a reliable method for measuring starch and soluble sugars. NSC was high in the crown and roots but very low in the trunk all year round. Soil temperature was positively correlated to RER (of the current month) between 0 and 8 °C, above which RER stabilised and was not explained by NSC levels or soil water potential. However, mean RER of fine roots in the month following the measurement of NSC was significantly and negatively correlated to soluble sugar and positively correlated with starch content. Very fine root starch content was also positively correlated with root production in the month following the starch measurement. Conclusion Soil temperature was a major driver of fine root elongation, but at low temperatures only. At soil temperatures 〉8 °C, no particular driver was dominant. NSC levels were negligible in the stem and root-bases, suggesting that wood production is a major sink that depletes carbohydrates. A large pool of NSC, principally in the form of starch, existed in fine roots of P. abies , and acted as an energy supply for root production throughout the year, even when photosynthesis was limited. Soluble sugars were depleted in fine roots during the growing season, but no relationship was found between fine root production and soluble sugars. The physiological mechanism by which NSC accumulation actively or passively occurs in fine roots is not known but could be due to the symplastic pathway of phloem unloading in conifer root tips, which suggests a passive mechanism.

Description: Background and aims Arbuscular mycorrhizal fungi (AMF) are important for plant nutrient and water acquisition. Much is known about how nutrient addition and environment affect AMF, but little is known about nutrient by environment interactions. We measured AMF colonization with nutrient additions and along an environmental gradient to assess these interactions. Methods We measured AMF colonization in roots of little bluestem ( Schizachyrium scoparium (Michx) Nash) with nutrient addition and across an environmental gradient. We assessed how AMF colonization changed across different fertilization treatments, and used ridge regression to determine nutrient, environment, and nutrient by environment interaction variables that predicted AMF colonization. Results The addition of nitrogen decreased AMF colonization, while mean annual temperature (MAT) and soil pH both positively predicted the percentage of AMF colonization in Schizachyrium scoparium . Additionally, we found an interaction term between MAT and phosphorus treatments that significantly affected percent AMF colonization. Conclusions Our results show the importance of understanding environmental conditions on AMF as well as nutrient by environment interactions when assessing how AMF respond to nutrient addition. Here we present a full-factorial nutrient addition study along an environmental gradient to assess how AMF root colonization is influenced by abiotic factors in addition to nutrients.

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: Aims In Ontario, Canada, acreage of red clover ( Trifolium pratense L.) intercropped with winter wheat ( Triticum aestivum L. em. Thell) has declined, despite well-documented soil and yield benefits. The decline has resulted from increasing prevalence of stand non-uniformity, which has been attributed in part to soil moisture deficits. We examined whether there are genotypic differences in drought response between red clover varieties. Methods A double-cut (Belle) and a single-cut variety (Altaswede) were grown under four different durations of drought (4, 8, 12 and 16 days below 15% relative soil water content, RSWC). Shoot dry weight, shoot relative water content (RWC), leaf area and crown water content were measured in control, drought and drought + recovery treatments. Results Belle used significantly more water during soil moisture deficit and had greater leaf area, shoot dry weight and RWC compared to Altaswede. In contrast, Altaswede had significantly higher survival rates than Belle, attributed to maintenance of meristematic tissue viability in the crown where re-growth, after shoot tissue desiccation, can occur. Conclusions By demonstrating genotypic variation in survival strategies of red clover, traits can be identified for the development of improved varieties. Varieties with higher survival rates during drought will result in more uniform stands and increased utilization of red clover for environmental and yield benefits.