Durum wheat culture requires a high level of N fertilization to achieve ideal protein concentration for semolina and pasta quality, contributing to N losses. Optimizing plant N use efficiency could improve agro-environmental balance. In the current paper, we studied the impact of the marine (DPI4913) and fungal (AF086) extracts (biostimulants) applied on leaves on growth, N absorption and N fluxes in durum wheat in field and greenhouse experiments. In the field, 15NO3− and 15NH4+ were injected into the soil; in the greenhouse, N of the flag-leaf was labelled with 15NH4+. Flag-leaf senescence was studied by estimating leaf chlorophyll concentration. In greenhouse, biostimulants increased grain yield, total N in plant and the proportion of plant N in ears. When water was limited in greenhouse experiment, neither biostimulants had any effect. In the field, DPI4913 increased soil fertilizer-derived 15N accumulated in grains. In the greenhouse, biostimulants increased the proportion of 15N applied to the flag-leaf recovered in grains and accelerated leaf senescence. For plants treated with biostimulants, flag-leaf N resorption increased. Biostimulants had a larger positive impact on mineral N root uptake than on N remobilization. In conclusion, our study has shown that DPI4913 and AF086 can promote plant growth and grain yield, N uptake and remobilization. Thus, these biostimulants could be used to optimize durum wheat N fertilization and contribute to reduced N losses.

A 2-year fertilization experiment was conducted to study the effect of different ratios of organic (pig) manure on wheat yield and nitrogen use efficiency (NUE). The four treatments were no nitrogen (N) (CK); 100% chemical fertilizer N (urea; T1); 70% chemical fertilizer N + 30% organic manure N (T2) and 50% chemical fertilizer N + 50% organic manure N (T3), with the same amount of applied nitrogen (120 kg/ha). The results showed the maximum grain yield (3049 kg/ha), crop nitrogen uptake (216 kg/ha), NUE (65.4%) and accumulated nitrate nitrogen (NO3−-N in 0–200 cm, 142 kg/ha) were observed in the T1 among all treatments in the first year. However, the largest grain yield (5074 kg/ha), crop nitrogen uptake (244 kg/ha) and NUE (82.5%) were under T2 treatment in the second year. Furthermore, T2 had the maximum NO3−-N content in 0–100 cm layer (116 kg/ha), especially 0–40 cm layer, and the lowest NO3−-N content in 100–200 cm (58.8 kg/ha). However, 50% organic manure N in T3 increased apparent nitrogen loss by 39.0% compared to that in T2. Therefore, 30% organic manure N application was more conducive for enhancing wheat yield and NUE and promoting environmental safety after 1-year fertilization time.

Deficit irrigation (DI) improves water use efficiency (WUE), but the reduced water input often limits plant growth and nutrient uptake. The current study examined whether arbuscular mycorrhizal fungi (AMF) could alleviate abiotic stress caused by low phosphorus (P) fertilization and DI.

A greenhouse experiment was conducted with potato grown with (P1) or without (P0) P fertilization, with AMF (M1+: Rhizophagus irregularis or M2+: Glomus proliferum) or AMF-free control (M−) and subjected to full irrigation (FI), DI or partial root-zone drying (PRD).

Inoculation of M1+ and M2+ maintained or improved plant growth and P/nitrogen (N) uptake when subjected to DI/PRD and P0. However, the positive responses to AMF varied with P level and irrigation regime. Functional differences were found in ability of AMF species alleviating plant stress. The largest positive plant biomass response to M1+ and M2+ was found under FI, both at P1 and P0 (25% increase), while plant biomass response to M1+ and M2+ under DI/PRD (14% increase) was significantly smaller. The large growth response to AMF inoculation, particularly under FI, may relate to greater photosynthetic capacity and leaf area, probably caused by stimulation of plant P/N uptake and carbon partitioning toward roots and tubers. However, plant growth response to AMF was not related to the percentage of AMF root colonization. Arbuscular mycorrhizal fungi can maintain and improve P/N uptake, WUE and growth of plants both at high/low P levels and under FI/DI. If this is also the case under field conditions, it should be implemented for sustainable potato production.

The contribution of nitrogen (N) fertilizer placement to management of annual weeds in spring wheat grown under two tillage systems was examined at sites located in three ecoregions of the northern Great Plains from 1994 to 1996. The effect of three different fertilizer placements on narrowleaf and broadleaf weed emergence, growth, and N uptake were determined in a no-till and a conventional tillage system. Fertilizer was broadcast, subsurface-banded in the spring prior to seeding, or side-banded at the time of seeding. Tillage system generally did not influence weed and crop response to fertilizer placement. Broadcast-applied fertilizer was more effective than banded fertilizer in promoting wild oat and broadleaf weed emergence and growth over the growing season. Weed densities, biomass, and N uptake measured early in the growing season averaged 20 to 40% less, and grain yield of wheat at maturity averaged 12% higher where fertilizer was side-banded compared to broadcast. In contrast, the addition of fertilizer, regardless of placement, was detrimental to green foxtail because of enhanced crop competitiveness. Banding fertilizer at recommended rates can be an effective cultural practice for managing weeds in no-till and conventional tillage wheat-cropping systems in semiarid to subhumid regions of the northern Great Plains, but it is not reliable when used as the sole method of weed management.

Information on nitrogen fertilizer effects on crop–weed competitive interactions might aid in developing improved weed management programs. A controlled environment study was conducted to examine the effect of three N rates on the competitive ability of four weed species grown with wheat. The four weed species were chosen to represent species that varied in their growth responsiveness to nitrogen (N): Persian darnel (low), Russian thistle (low), redroot pigweed (high), and wild oat (high). Wheat and each weed species were grown in a replacement series design at N rates of 60, 120, and 240 mg N kg−1 soil. The competitive ability of the low N-responsive species, Persian darnel and Russian thistle, was not influenced by N rate, supporting our hypothesis that N rate would have little effect on the competitiveness of species responding minimally to N. Conversely, the competitiveness of the high N-responsive species redroot pigweed progressively improved as N rate increased. However, wild oat competitiveness was unaffected by N fertilizer rate. There is some evidence from this study to suggest that fertilizer management strategies that favor crops over weeds deserve greater attention when weed infestations consist of species known to be highly responsive to higher soil N levels. Information gained in this study will be used to advise farmers of the importance of strategic fertilizer management in terms of both weed management and crop yield.

Weeds cause crop loss indirectly by reducing the quantity of resources available for growth. Quantifying the effects of weed interference on nitrogen (N) supply, crop growth, and N nutrition may assist in making both N and weed management decisions. Experiments were conducted to quantify the effect of N addition and weed interference on soil nitrate-N (NO3-N) over time and the dependence of corn growth on NO3-N availability, determine the corn N nutrition index (NNI) at anthesis, and evaluate if relative chlorophyll content can be utilized as a reliable predictor of NNI. Urea was applied at 0, 60, and 120 kg N/ha to establish N treatments. Season-long weedy, weed-free, and five weed interference treatments were established by delaying weed control from time of crop planting to the V3, V6, V9, V15, or R1 stages of corn development. Soil NO3-N ranged from 20 kg N/ha without N addition to 98 kg N/ha with 120 kg N/ha added early in the season, but crop and weed growth reduced soil NO3-N to 10 kg N/ha by corn anthesis. Weed presence reduced soil NO3-N by up to 50%. Average available NO3-N explained 29 to 40% of the variation in corn shoot mass at maturity. Weed interference reduced corn biomass and NNI by 24 to 69%. Lack of N also reduced corn NNI by 13 to 46%, but reduced corn biomass by only 11 to 23%. Nondestructive measures of relative chlorophyll content predicted corn NNI with 65 to 85% accuracy. Although weed competition for factors other than N may be the major contributor to corn biomass reduction, the chlorophyll meter was a useful diagnostic tool for assessing the overall negative effects of weeds on corn productivity. Further research could develop management practices to guide supplemental N applications in response to weed competition.

Field experiments were conducted to study the competition effect of winter wheat planted in a square arrangement and Italian ryegrass planted randomly on biomass yields of both species, ryegrass seed yield, N use efficiency, and progeny seed germination. Increases in wheat density up to 800 plants/m2 reduced ryegrass seed yield by 87% but increased its harvest index up to 42% compared to its monoculture yield. Species densities and their interactions accounted for 66 to 73% of the total variation in per-unit area biomass of species, and their association was more favorable to ryegrass biomass than wheat. Seeds of each species had three times greater nitrogen percentage than did shoots. Intra- and interspecific competition increased nitrogen percentage in wheat seeds. In Italian ryegrass, only interspecific competition increased N percentage in seeds. Although total nitrogen uptake by winter wheat was three times greater than in Italian ryegrass, Italian ryegrass was two times more efficient than wheat at producing biomass per unit of N taken up and specific leaf area at heading stage in mixture. Germination percentages of progeny seeds of both species in mixtures were greater in presence of high densities of the companion species than in their monocultures. Nitrogen was not the main limiting factor for competition between winter wheat and Italian ryegrass in this study.

Managing fertilizer in cropping systems may be an important component of integrated weed management programs. A field study was conducted to determine the effect of various application methods of the 15N-enriched nitrogen (N) fertilizer on N uptake in green foxtail, wild mustard, and spring wheat and on crop–weed competition. N application methods consisted of ammonium nitrate in solution applied broadcast on the soil surface, applied in pools on the soil surface at 20-cm intervals between every second wheat row, and point injected 10 cm deep at intervals similar to those of the surface pools. An unfertilized control treatment was also included. N uptake by green foxtail throughout the growing season was often greater from surface broadcast than from surface pools or point-injected N and was sometimes greater from surface pools than from point-injected N. In contrast, N uptake by wild mustard was rarely affected by the fertilizer placement method. In the presence of weeds, the ranking of N uptake by wheat usually was point injected > surface pools > surface broadcast. Weed biomass was often greater with surface broadcast than with either surface pools or point-injected N. In the absence of weeds, wheat yields were similar across the three N application methods. However, in the presence of green foxtail, wheat yields were greater with point-injected N than with surface broadcast N in two of the 3 yr and with surface pools of N in one of the 3 yr. In the presence of wild mustard, wheat yields were greater with surface pools and point-injected N compared with the unfertilized control in two of the 3 yr, whereas yields with broadcast N were never greater than the unfertilized control. Study findings suggest that point-injected N results in suppressed weed growth, not by reduced N uptake by weeds but instead by greater N uptake by wheat that increases its competitiveness with weeds. Information gained in this study will be utilized to develop a more integrated program for weed management in spring wheat.

Not much is known about the nitrogen (N) uptake capacity and N-form preference of tropical trees. In a replicated labelling experiment with 15N-ammonium, 15N-nitrate and dual-labelled glycine applied to saplings of six tree species from southern Ecuadorian montane forests, we tested the hypotheses that (1) the saplings of tropical trees are capable of using organic N even though they are forming arbuscular mycorrhizas, and (2) with increasing altitude, tree saplings increasingly prefer ammonium and glycine over nitrate due to reduced nitrification and growing humus accumulation. Three- to 5-y-old saplings of two species each from 1000, 2000 and 3000 m asl were grown in pots inside the forest at their origin and labelled with non-fertilizing amounts of the three N forms; 15N enrichment was detected 5 days after labelling in fine roots, coarse roots, shoots and leaves. The six species differed with respect to their N-form preference, but neither the abundance of ammonium and nitrate in the soil nor altitude (1000–3000 m asl) seemed to influence the preference. Two species (those with highest growth rate) preferred NH4+ over NO3−, while the other four species took up NO3− and NH4+ at similar rates when both N forms were equally available. After 13C-glycine addition, 13C was significantly accumulated in the biomass of three species (all species with exclusively AM symbionts) but a convincing proof of the uptake of intact glycine molecules by these tropical montane forest trees was not obtained.

Organic cropping systems that utilize winter grown cereal–legume cover crop mixtures can increase plant available nitrogen (N) to a subsequent cash crop, but the rate of N release is uncertain due to variations in residue composition and environmental conditions. A study was conducted to evaluate N availability from rye (Secale cereale L.)–hairy vetch (Vicia villosa Roth) cover crop mixtures and to measure the response of organically grown sweet corn (Zea mays L.) to N provided by cover crop mixtures. Nitrogen availability from pure rye, pure hairy vetch, and rye–vetch mixtures was estimated using laboratory incubation with controlled temperature and soil moisture. Sweet corn N response was determined in a 2-year field experiment in western Washington with three cover crop treatments as main plots (50:50 rye–vetch seed mixture planted mid September, planted early October, and none) and four feather meal N rates as subplots (0, 56, 112 and 168 kg available N ha−1). Pure hairy vetch and a 75% rye–25% hairy vetch biomass mixture (R75V25) released similar amounts of N over 70 days in the laboratory incubation. But, the initial release of N from the (R75V25) treatment was nearly 70% lower, which may result in N release that is better timed with crop uptake. Cover crops in the field were dominated by rye and contained 34–76 kg ha−1 total N with C:N ranging from 18 to 27. Although time of planting and management of cover crop quality improved N uptake in sweet corn, cover crops provided only supplemental plant available N in this system.

The aim of this investigation was to determine the NH4Cl concentration threshold, above which negative physiological effects would occur in the nitrophytic lichen Xanthoria parietina. Over a 10 month period, X. parietina thalli growing on roof tiles were exposed weekly to NH4Cl concentrations of 0·04, 0·17, 0·34 or 0·69 M. Nitrogen (N) uptake from ammonium and the concentrations of total thallus N and biont markers (chlorophyll a, ergosterol and chitin) were measured on four occasions, over the experimental period. Xanthoria parietina was able to assimilate a significant quantity of the applied ammonium. However, lichens exposed to the two higher concentrations suffered damage to both the photobiont and the mycobiont, as evidenced by reduced chlorophyll a and ergosterol concentrations, while lichens exposed to the two lower concentrations showed no significant changes in either chlorophyll a or ergosterol that could be related to the ammonium inputs. Xanthoria parietina tolerated a weekly irrigation of at least 0·17 M NH4Cl, corresponding to an N deposition of c. 1000 kg ha−1 yr−1, suggesting that this species has a very high tolerance to N pollution.

We studied the transport of 15N from a soil compartment separated from a plant root compartment by a hydrophobic polytetrafluoroethylene (PTFE) membrane to plants in the presence and absence of arbuscular mycorrhizal fungi (AMF). We have previously shown that this type of membrane efficiently inhibits mass flow and diffusion of mobile ions in the soil solution in an abiotic system, but can be penetrated by the hyphae of mycorrhizal fungi. Mycorrhizal tomatoes (Lycopersicon esculentum) colonized by Glomus mosseae were grown at two N fertilizer concentrations in a root compartment. A PTFE membrane was placed between the root compartment and an adjoining soil compartment that was inaccessible to the roots but accessible to the AMF hyphae (hyphal compartment). Additional N was applied to the hyphal compartment using uniformly 15N-labelled NH4NO3. There was a flux of 15N from the hyphal compartment to the plants even in the absence of mycorrhizal fungi. However, this flux was much higher in mycorrhizal plants, which had much higher N concentrations in their shoots and roots than did the non-mycorrhizal control plants. This was particularly apparent when the root compartment had a low N fertilizer concentration. Of the total N content of mycorrhizal plants, c. 42 and 24% at the low and high N fertilizer concentrations, respectively, were estimated to originate from the hyphal compartment by transport through AMF hyphae. In the presence of mycorrhizal fungi, the flux of 15N was about three times higher than in their absence. The results show that AMF can access a soil compartment separated by a PTFE membrane, and can contribute substantially to N uptake by plants.

Root growth and physiological responses to elevated CO2 were investigated for three important Mojave Desert grasses: the C3 perennial Achnatherum hymenoides, the C4 perennial Pleuraphis rigida and the C3 annual Bromus madritensis ssp. rubens. Seeds of each species were grown at ambient (360 μl l−1) or elevated (1000 μl l−1) CO2 in a glasshouse and harvested at three phenological stages: vegetative, anthesis and seed fill. Because P. rigida did not flower during the course of this study, harvests for this species represent three vegetative stages. Primary productivity was increased in both C3 grasses in response to elevated CO2 (40 and 19% for A. hymenoides and B. rubens, respectively), but root biomass increased only in the C3 perennial grass. Neither above-ground nor below-ground biomass of the C4 perennial grass was significantly affected by the CO2 treatment. Elevated CO2 did not significantly affect root surface area for any species. Total plant nitrogen was also not statistically different between CO2 treatments for any species, indicating no enhanced uptake of N under elevated CO2. Physiological uptake capacities for NO3 and NH4 were not affected by the CO2 treatment during the second harvest; measurements were not made for the first harvest. However, at the third harvest uptake capacity was significantly decreased in response to elevated CO2 for at least one N form in each species. NO3 uptake rates were lower in A. hymenoides and P. rigida, and NH4 uptake rates were lower in B. rubens at elevated CO2. Nitrogen uptake on a whole root-system basis (NO3+NH4 uptake capacity × root biomass) was influenced positively by elevated CO2 only for A. hymenoides after anthesis. These results suggest that elevated CO2 may result in a competitive advantage for A. hymenoides relative to species that do not increase root-system N uptake capacity. Root respiration measurements normalized to 20 °C were not significantly affected by the CO2 treatment. However, specific root respiration was significantly correlated with either root C[ratio ]N ratio or root water content when all data per species were included within a simple regression model. The results of this study provide little evidence for up-regulation of root physiology in response to elevated CO2 and indicate that root biomass responses to CO2 are species-specific.

Studies on the rhizomatous clonal sedge Carex bigelowii, at a fellfield site in subarctic Swedish Lapland examined the ecological potential of rhizomes as nitrogen uptake systems. Direct application of a solution of 15NH415NO3 to the rhizomes of C. bigelowii significantly enriched the 15N content of the plant tissue. The pattern of enrichment indicated movement of labelled nitrogen into rhizomes and adjoining tissue (including roots and shoots), showing both uptake and translocation via the rhizome system. There was a gradient of decreasing 15N enrichment with increasing distance from the point of labelling, and estimation of the total mass of nitrogen taken up via labelled rhizomes showed low levels and rates of uptake. Quantification of the size of rhizome and root systems of C. bigelowii at the study site indicates that 80% of the living biomass may be below ground, and that the surface area of the rhizome system of C. bigelowii is approx. two-thirds that of the roots. The rhizome system of C. bigelowii can therefore act as a route for nitrogen uptake, with the potential to exploit almost as great a volume of soil as the root system. This mechanism of nitrogen uptake may play an important role in the Arctic, where many species have a clonal, rhizomatous growth form. In addition, plant growth in many arctic ecosystems is limited by low soil nutrient availability, a result of low temperatures (leading to slow soil decomposition rates) and patchy resource distribution. Nitrogen uptake via rhizomes may provide plants with the capacity to take advantage of transient nutrient supplies, and may partly compensate for the cost of developing and maintaining persistent rhizome networks in ecosystems where nutrient resources are in short supply.