Rapid uptake of N-ammonium and glycine-C, N by arbuscular and ericoid mycorrhizal plants native to a Northern California coastal pygmy forest

While it is well established that plants are able to acquire nitrogen in inorganic form, there is less information on their ability to ‘short circuit’ the N cycle, compete with microbes, and acquire nitrogen in organic form. Mycorrhizal fungi, known to enhance nutrient uptake by plants, may play a role in organic N uptake, particularly ericoid mycorrhizas. We asked the question—Can mycorrhizal fungi increase the ability of plants to take up organic N, compared to inorganic N? Here, we report on the abilities of three plant species, ericoid mycorrhizal Rhododendron macrophyllum and Vaccinium ovatum and arbuscular mycorrhizal Cupressus goveniana ssp. pigmaea, to acquire C and/or N from an organic and an inorganic N source. All three species are native to a California coastal pygmy forest growing in acidic, low-fertility, highly organic soils. In a pot study, glycine-α¹³C, ¹⁵N and ¹⁵N-ammonium were applied to pygmy forest soil for 17 or 44 h. Ericoid mycorrhizal species did not demonstrate a preference for either inorganic or organic sources of N while Cupressus acquired more NH₄-N than glycine-N. For all species, glycine-N uptake did not increase after 17 h suggesting glycine uptake and glycine immobilization occurred rapidly. Both glycine-N and glycine-C were recovered in shoots and in roots suggesting that all species acquired some N in organic form. Regression analyses of glycine-N and glycine-C recovery in root tissue indicate that much of the glycine was taken up intact and that the minimum proportion of glycine-N recovered in organic form was 85% (Cupressus) and 70% (Rhododendron). Regressions were non-significant for Vaccinium. For all species, glycine-N remained predominantly in roots while glycine-C was transferred to shoots. In contrast, NH₄-N remained in roots of ericoid plants but was transferred to shoots of arbuscular mycorrhizal Cupressus. Since net N mineralization rates in pygmy forest soils are low, our results suggest that organic N may be an important N source for plants in this temperate coniferous ecosystem regardless of mycorrhizal type. Acquisition of amino acid C by these species also may partially offset the carbon cost to plants of hosting mycorrhizal fungi.     

Transfer of N fixed by a legume tree to the associated grass in a tropical silvopastoral system

Below-ground transfer of nitrogen (N) fixed by legume trees to associated non-N₂-fixing crops has received little attention in agroforestry, although the importance of below-ground interactions is shown in other ecosystems. We used ¹⁵N natural abundance to estimate N transfer from the legume tree Gliricidia sepium (Jacq.) Kunth ex Walp. to C₄ grass Dichanthium aristatum (Poir.) C.E. Hubb. in a silvopastoral system, where N was recycled exclusively by below-ground processes and N₂ fixation by G. sepium was the sole N input to the system. Finding a suitable reference plant, a grass without contact with tree roots or litter, was problematic because tree roots invaded adjacent grass monocrop plots and soil isotopic signature in soil below distant grass monocrops differed significantly from the agroforestry plots. Thus, we used grass cultivated under greenhouse conditions in pots filled with agroforestry soil as the reference. A model of soil ¹⁵N fractionation during N mineralization was developed for testing the reliability of that estimate. Experimental and theoretical results indicated that 9 months after greenhouse transplanting, the percentage of fixed N in the grass decreased from 35% to <1%, due to N export in cut grass and dilution of fixed N with N taken up from the soil. The effect of soil ¹⁵N fractionation on the estimate of the reference value was negligible. This indicates that potted grass is a suitable reference N transfer studies using ¹⁵N natural abundance. About one third of N in field-grown grass was of atmospheric origin in agroforestry plots and in adjacent D. aristatum grassland invaded by G. sepium roots. The concentration of fixed N was correlated with fine root density of G. sepium but not with soil isotopic signature. This suggests a direct N transfer from trees to grass, e.g. via root exudates or common mycorrhizal networks.     

有机、无机肥料施用后土壤生物量C、N、P的变化及N素转化

研究结果表明,有机、无机肥施用后,土壤微生物量C、N、P开始增加很快,随着时间的推移,土壤微生物量C又有所降低,但生物量N和P则基本保持稳定。硫铵施入土壤后,微生物对肥料¹⁵N的生物固持10天后达到最高峰,以后被固持在体内的¹⁵N有一部分被逐渐释放出来,但一个月后仍有17%左右的¹⁵N被固持在微生物体内。硫铵与有机肥配合施用时,微生物对硫铵¹⁵N固持比例有所增加。有机肥中的¹⁵N被微生物固持的比例也较大,在肥料施入20天左右达到最大值,一个月后仍有19-25%存在于微生物体内。硫铵施用一个月后¹⁵N损失高达18%,有机肥中的N也有少量被损失。     

Tree girdling provides insight on the role of labile carbon in nitrogen partitioning between soil microorganisms and adult European beech

Nitrogen (N) cycling in terrestrial ecosystems is complex since it involves the closely interwoven processes of both N uptake by plants and microbial turnover of a variety of N metabolites. Major interactions between plants and microorganisms involve competition for the same N species, provision of plant nutrients by microorganisms and labile carbon (C) supply to microorganisms by plants via root exudation. Despite these close links between microbial N metabolism and plant N uptake, only a few studies have tried to overcome isolated views of plant N acquisition or microbial N fluxes. In this study we studied competitive patterns of N fluxes in a mountainous beech forest ecosystem between both plants and microorganisms by reducing rhizodeposition by tree girdling. Besides labile C and N pools in soil, we investigated total microbial biomass in soil, microbial N turnover (N mineralization, nitrification, denitrification, microbial immobilization) as well as microbial community structure using denitrifiers and mycorrhizal fungi as model organisms for important functional groups. Furthermore, plant uptake of organic and inorganic N and N metabolite profiles in roots were determined.Surprisingly plants preferred organic N over inorganic N and nitrate (NO₃⁻) over ammonium (NH₄⁺) in all treatments. Microbial N turnover and microbial biomass were in general negatively correlated to plant N acquisition and plant N pools, thus indicating strong competition for N between plants and free living microorganisms. The abundance of the dominant mycorrhizal fungi Cenococcum geophilum was negatively correlated to total soil microbial biomass but positively correlated to glutamine uptake by beech and amino acid concentration in fine roots indicating a significant role of this mycorrhizal fungus in the acquisition of organic N by beech. Tree girdling in general resulted in a decrease of dissolved organic carbon and total microbial biomass in soil while the abundance of C. geophilum remained unaffected, and N uptake by plants was increased. Overall, the girdling-induced decline of rhizodeposition altered the competitive balance of N partitioning in favour of beech and its most abundant mycorrhizal symbiont and at the expense of heterotrophic N turnover by free living microorganisms in soil. Similar to tree girdling, drought periods followed by intensive drying/rewetting events seemed to have favoured N acquisition by plants at the expense of free living microorganisms.     

Comparison of uptake of different N forms by soil microorganisms and two wet-grassland plants: A pot study

Two common plant species of temperate wet grasslands, Carex acuta and Glyceria maxima, were tested for their preferences in the uptake of different nitrogen (N) sources (amino acid, ammonium, nitrate) and their ability to compete for these sources with soil microorganisms. The experiment was a one-day incubation study with plants growing in soil obtained from the field, which was supplied with a solution containing the three N sources, one at a time labeled with ¹⁵N. The bulk of the N demand of both species was covered by nitrate-N, which was the dominant N form in the soil at the time of the experiment. Ammonium-N was taken up less strongly, and organic N formed only a negligible part of their nutrition. The assimilated inorganic N was preferentially transported to apical meristem of the youngest leaf, while organic N remained mostly in the roots. The fast-growing Glyceria took up more N and was a better competitor vis-à-vis soil microbes for rarer N forms than Carex. However, both plants were poor competitors for N vis-à-vis soil microbes, irrespective of the N form. Microbes took up nitrate ca. five times faster and organic N more than a hundred times faster than plants. Correspondingly, the calculated turnover time of microbial N was 17 days, compared to 40 days for N in plant roots. A significant amount of added ¹⁵N was found at non-exchangeable sites in the soil, which points to the importance of microbial N transformation and abiotic N fixation for N retention in soil. In summary, the preferential assimilation of inorganic N by the wetland plants studied here and their poor ability to compete for N with soil microbes over the short term agree with the results of studies carried out with other species from temperate grasslands.     

Effects of mycorrhizal roots and extraradical hyphae on N uptake from vineyard cover crop litter and the soil microbial community

The objectives of this study were to evaluate the contribution of arbuscular mycorrhizal (AM) fungal hyphae to ¹⁵N uptake from vineyard cover crop litter (Medicago polymorpha), and to examine the soil microbial community under the influence of mycorrhizal roots and extraradical hyphae. Mycorrhizal grapevines (Vitis vinifera) were grown in specially designed containers, within which a polyvinyl chloride (PVC) mesh core was inserted. Different sizes of mesh allowed mycorrhizal roots (mycorrhizosphere treatment) or extraradical hyphae (hyphosphere treatment) to access dual labeled ¹⁵N and ¹³C cover crop litter that was placed inside the cores after 4 months of grapevine growth. Mesh cores in the bulk soil treatment, which served as a negative control, had the same mesh size as the hyphosphere treatment, but frequent rotation prevented extraradical hyphae from accessing the litter. Grapevines and soils were harvested 0, 7, 14, and 28 days after addition of the cover crop litter and examined for the presence of ¹⁵N. Soil microbial biomass and the soil microbial community inside the mesh cores were examined using phospholipid fatty acid analysis. ¹⁵N concentrations in grapevines in the hyphosphere treatment were twice that of grapevines in the bulk soil treatment, suggesting that extraradical hyphae extending from mycorrhizal grapevine roots may have a role in nutrient utilization from decomposing vineyard cover crops in the field. Nonetheless, grapevines in the mycorrhizosphere treatment had the highest ¹⁵N concentrations, thus highlighting the importance of a healthy grapevine root system in nutrient uptake. We detected similar peaks in soil microbial biomass in the mycorrhizosphere and hyphosphere treatments after addition of the litter, despite significantly lower microbial biomass in the hyphosphere treatment initially. Our results suggest that although grapevine roots play a dominant role in the uptake of nutrients from a decomposing cover crop, AM hyphae may have a more important role in maintaining soil microbial communities associated with nutrient cycling.     

Effects of arbuscular mycorrhizal inoculation and phosphorus supply on the growth and nutrient uptake of Kandelia obovata (Sheue,Liu & Yong) seedlings in autoclaved soil

This study evaluated the interactive effect of arbuscular mycorrhizal fungi (AMF) inoculation and exogenous phosphorus supply on soil phosphotases, plant growth, and nutrient uptake of Kandelia obovata (Sheue, Liu & Yong). We aimed to explore the ecophysiological function of AMF in mangrove wetland ecosystems, and to clarify the possible survival mechanism of mangrove species against nutrient deficiency. K. obovata seedlings with or without AMF inoculation (mixed mangrove AMF), were cultivated for six months in autoclaved sediment medium which was supplemented with KH₂PO₄ (0, 15, 30, 60, 120 mg kg−¹). Then the plant growth, nitrogen and phosphorus content, root vitality, AMF colonization and soil phosphatase activity were analyzed. The inoculated AMF successfully infected K. obovata roots, developed intercellular hyphae, arbuscular (Arum-type), and vesicle structures. Arbuscular mycorrhizal fungi colonization ranged from 9.04 to 24.48%, with the highest value observed under 30 and 60 mg kg⁻¹ P treatments. Soil P supply, in the form of KH₂PO₄, significantly promoted the height and biomass of K. obovata, enhanced root vitality and P uptake, while partially inhibiting soil acid (ACP) and alkaline phosphotase (ALP) activities. Without enhancing plant height, the biomass, root vitality and P uptake were further increased when inoculated with AMF, and the reduction on ACP and ALP activities were alleviated. Phosphorus supply resulted in the decrease of leaf N–P ratio in K. obovata, and AMF inoculation strengthened the reduction, thus alleviating P limitation in plant growth. Arbuscular mycorrhizal fungi inoculation and adequate P supply (30 mg kg⁻¹ KH₂PO₄) enhanced root vitality, maintained soil ACP and ALP activities, increased plant N and P uptake, and resulted in greater biomass of K. obovata. Mutualistic symbiosis with AMF could explain the survival strategies of mangrove plants under a stressed environment (waterlogging and nutrient limitation) from a new perspective.     

Ecosystem partitioning of N-glycine after long-term climate and nutrient manipulations, plant clipping and addition of labile carbon in a subarctic heath tundra

Low temperatures and high soil moisture restrict cycling of organic matter in arctic soils, but also substrate quality, i.e. labile carbon (C) availability, exerts control on microbial activity. Plant exudation of labile C may facilitate microbial growth and enhance microbial immobilization of nitrogen (N). Here, we studied ¹⁵N label incorporation into microbes, plants and soil N pools after both long-term (12 years) climate manipulation and nutrient addition, plant clipping and a pulse-addition of labile C to the soil, in order to gain information on interactions among soil N and C pools, microorganisms and plants. There were few effects of long-term warming and fertilization on soil and plant pools. However, fertilization increased soil and plant N pools and increased pool dilution of the added ¹⁵N label. In all treatments, microbes immobilized a major part of the added ¹⁵N shortly after label addition. However, plants exerted control on the soil inorganic N concentrations and recovery of total dissolved ¹⁵N (TD¹⁵N), and likewise the microbes reduced these soil pools, but only when fed with labile C. Soil microbes in clipped plots were primarily C limited, and the findings of reduced N availability, both in the presence of plants and with the combined treatment of plant clipping and addition of sugar, suggest that the plant control of soil N pools was not solely due to plant uptake of soil N, but also partially caused by plants feeding labile C to the soil microbes, which enhanced their immobilization power. Hence, the cycling of N in subarctic heath tundra is strongly influenced by alternating release and immobilization by microorganisms, which on the other hand seems to be less affected by long-term warming than by addition or removal of sources of labile C.     

Relative importance of Ca,N, and P in enhancing mycorrhizal activity in leucaena leucocephala grown in an oxisol subjected to simulated erosion

Pot experiments were conducted in the greenhouse to determine the combined effects of lime, nitrogen and phosphorus and the relative importance of each of these nutrients in establishing nodulated and mycorrhizal Leucaena leucocephala (Lam.) de Wit Var. K8 in an oxisol subjected to simulated erosion. Leucaena was grown in the soil inoculated or not with the vesicular‐arbuscular mycorrhizal fungus Glomus aggregatum Schenck and Smith emend Koske, with or without a basal nutrient (basal) consisting of K, Mg, S, Zn, Cu, and B plus lime, N, and P (complete) or one of the latter three supplements.

The extent of mycorrhizal colonization of roots as well as mycorrhizal effectiveness, as measured by pinnule P content increased when the eroded soil was amended with combinations of all the nutrients and inoculated with G. aggregatum. Similar trends were observed when symbiotic effectiveness was measured in terms of shoot P, Cu, and Zn status and dry matter yield. Nodule dry matter was also responsive to amendment of the soil with the complete nutrients and to vesicular‐arbuscular mycorrhizal inoculation. Phosphorus was found to be the most important nutrient limiting mycorrhizal effectiveness in the eroded soil, followed by N and lime. It is concluded that lost nutrients, particularly P, need to be replaced before legumes can be established successfully on highly weathered eroded soils inoculated with vesicular‐arbuscular mycorrhizal fungi.     

Mechanisms behind soil N dynamics following cover restoration in degraded land in subtropical China

Purpose

Nitrogen (N) is an important nutrient for re-vegetation during ecosystem restoration, but the effects of cover restoration on soil N transformations are not fully understood. This study was conducted to investigate N transformations in soils with different cover restoration ages in Eastern China.

Materials and methods

Soil samples were collected from four degraded and subsequently restored lands with restoration ages of 7, 17, 23, and 35 years along with an adjacent control of degraded land. A ¹⁵N tracing technique was used to quantify gross N transformation rates.

Results and discussion

Compared with degraded land, soil organic carbon (SOC) and total N (TN) increased by 1.60–3.97 and 2.49–5.36 times in restoration land. Cover restoration increased ammonium and nitrate immobilization, and dissimilatory nitrate reduction to ammonium (DNRA) by 0.56–0.96, 0.34–2.10, and 0.79–3.45 times, respectively, indicating that restoration was beneficial for N retention. There were positive correlations between SOC content and ammonium and nitrate immobilization and DNRA, indicating that the increase in soil N retention capacity may be ascribed to increasing SOC concentrations. The stimulating effect of SOC on ammonium immobilization was greater than its effect on organic N mineralization, so while SOC and TN increased, inorganic N supply did not increase. Autotrophic and heterotrophic nitrification increased with increasing SOC and TN concentrations. Notably, heterotrophic nitrification was an important source of NO₃^??N production, accounting for 47–67% of NO₃^??N production among all restoration ages.

Conclusions

The capacity of N retention was improved by cover restoration, leading to an increase in soil organic carbon and total N over time, but inorganic N supply capacity did not change with cover restoration age.

     

The impact of trees, ectomycorrhiza and potassium availability on simple organic compounds and dissolved organic carbon in soil

The presence of tree roots and symbiotic mycorrhizal fungi is recognized to have a substantial impact on carbon dynamics in soils. In this study the effect of Pinus sylvestris seedlings and the ectomycorrhizal fungus Hebeloma crustuliniforme on a number of biogeochemical variables, mainly related to labile carbon pools was investigated. The impact of K limitation as a potential regulatory factor was also examined. Columns filled with E horizon ±plants and ±mycorrhizal fungi were incubated for 18.5 months. The results demonstrate that plants, as well as mycorrhizal fungi, significantly increased the concentrations of some simple organic acids, including oxalate, in soil solution. Observations for dissolved organic carbon were slightly contradictory but the cumulative amount found in drainage water was ∼20% higher in planted versus non-planted columns. Soil from planted treatments also showed more rapid mineralisation kinetics for oxalate. However carbon utilization (mineralisation vs. biomass) of oxalate and glucose by the soil microbial biomass was less influenced by plants. At harvest a component integration study of soil autotrophic and heterotrophic respiration was performed which revealed that both plant and mycorrhiza had a positive effect on the heterotrophic respiration. Potassium omission had little effect on the variables studied with the exception of the maximum mineralisation rate for oxalate, which increased when K was withdrawn. The results are discussed in the context of the dynamics of labile soil carbon pools and ecosystem C fluxes.     

Distribution of protozoa in scots pine mycorrhizospheres

The distribution of heterotrophic flagellates, naked amoebae, testate amoebae and ciliates was investigated in habitats created by Scots pine-Paxillus involutus and -Suillus bovinus ectomycorrhizospheres. The protozoa living on plant and fungal surfaces preferred the non-mycorrhizal pine roots over mycorrhizal roots or external mycelium. The testate amoebae were more abundant on external mycelium than on mycorrhizae regardless of the mycorrhizal fungal species. Numbers of protozoa were higher in the different habitats provided by S. bovinus mycorrhizospheres when compared with P. involutus mycorrhizospheres. Interestingly, the quality of the bacterial flora as food for the protozoa was affected by the mycorrhizal fungi even in the soils adjacent to non-mycorrhizal root tips of pine. These results demonstrate that mycorrhizal fungi create habitats differently suitable for protozoa living in boreal forest soil.     

Species-dependent partitioning of C and N stable isotopes between arbuscular mycorrhizal fungi and their C3 and C4 hosts

Natural ¹³C and ¹⁵N abundances of mycorrhizal fungi are increasingly used in ecology but reference data on arbuscular mycorrhizal fungi (AMF) are scarce. In experiments with nine phylogenetically dispersed AMF strains inoculated on leek (C3 plant) and sorghum (C4) in pot cultures, we measured the ¹³C/¹²C and ¹⁵N/¹⁴N ratios in shoots, roots, AMF spores as well as carbon isotope signature of the C16:1ω5 fatty acid (FA), which is diagnostic for AMF. Spore δ¹³C values varied among AMF strains on any given host. They were significantly lower than shoot δ¹³C for sorghum (−2.5‰ on average) while for leek, no clear C isotope partitioning between spores and host shoots was observed. The FA C16:1ω5 fatty acids were more ¹³C-depleted than spores, without correlation with spore δ¹³C values. For both, sorghum and leek, spore δ¹⁵N was higher (+1–2‰ on average) than for shoots. We found no evidence that isotopic partitioning between the partners drives ¹³C and ¹⁵N abundances in plant–AMF symbiosis. Mycorrhizal roots displayed relatively high δ¹³C typical for heterotrophic organs, and not a mix between AMF and plant signatures. Interestingly, inoculation slightly decreased shoot δ¹³C on leek but not on sorghum, as compared with non-mycorrhizal plants, suggesting that AMF improved the plant's water status, a parameter affecting the δ¹³C of C3 but not C4 plants. Phylogenetically closer AMF displayed more similar spore δ¹³C and induced similar ¹³C and ¹⁵N abundances on leek shoots, but this observation was not confirmed for sorghum. Plant and AMF isotopic abundances hardly correlated with other parameters related to plant development, mineral nutrition or root mycorrhizal colonisation, and these correlations were never consistent between sorghum and leek. Thus, isotopic abundances in plant–AMF symbiosis were rather constrained by each AMF/plant interaction. Nevertheless, our data provide a valuable reference for future investigations of AMF communities and AM symbiosis in situ.     

Competition for inorganic and organic N by blue oak (Quercus douglasii) seedlings, an annual grass, and soil microorganisms in a pot study

Sources of competition for limited soil resources, such as nitrogen (N), include competitive interactions among different plant species and between plants and soil microorganisms (microbes). To study these competitive interactions, blue oak seedlings (Quercus douglasii) were grown alone or grown together with an annual grass, wild oats (Avena barbata) in pots containing field soil. We injected ¹⁵N-labeled ammonium, nitrate or glycine into the soil of each pot and harvested plants 5 days later. Plant shoots and roots, soil microbial N and soil KCl-extractable N were analyzed for ¹⁵N content. When oak and grass were grown together, ¹⁵N recovery from the inorganic N treatments (NH₄⁺, or NO₃⁻) was 34, 9 and 4% for the grass, microbes and oak seedlings, respectively, and only 1% remained as KCl-extractable N. ¹⁵N recovery from the glycine treatment was 18, 22, 5% for the grass, microbes and oak seedlings, respectively, and 4% remained as KCl-extractable N. When oaks were grown alone, ¹⁵N recovery by soil microbes was 21, 48 and 40% in the NO₃⁻, NH₄⁺ and glycine treatments, respectively. N forms had no effects on ¹⁵N recovery in oak seedlings (7%) and in KCl-extractable N pool (13%). In general, total N recovery by the grass was much greater than by oaks. However, on a fine root surface area or length basis, oaks exhibited higher N uptake than the grass. Our results suggest that the high rooting density and rapid growth rate of the annual grasses such as Avena barbata made them superior competitors for available soil N when compared to blue oak seedlings and to microbes. Soil microbes were better competitors for organic than inorganic N when annual grasses were present, but preferred NH₄⁺ when competing only with oak seedlings.     

Soil tillage and herbicide applications in pea: arbuscular mycorrhizal fungi,plant growth and nutrient concentration respond differently

ABSTRACT

We conducted a field- and pot experiment with peas to investigate the impact of soil tillage and herbicide applications on arbuscular mycorrhizal fungi (AMF), plant growth, phosphorus concentrations, C:N ratio in plants and yield. The field study was carried out in a long-term soil tillage experiment where four tillage treatments have been compared. Field soil from the experimental plots were used for the pot experiment. AMF were not affected by herbicide (MCPB) application, neither in the field nor in the pot experiments. However, AMF root colonization was enhanced by reduced tillage, minimum tillage and no-tillage practices, compared to conventional tillage. In the pot experiment, plant growth and nodulation of pea roots was negatively affected by the high herbicide dosage. In the field experiment neither tillage nor herbicide treatment exert specific effects on root growth parameters, phosphorus concentrations, C:N ratio and plant dry matter. This work demonstrates that an appropriate herbicide usage coupled with conservation soil tillage techniques can favour AMF root colonization and benefit plant growth.

Abbreviations: AMF: arbuscular mycorrhizal fungi; CT: conventional tillage; RT: reduced tillage; MT: minimum tillage; NT: no tillage; P: Phosphorus; C:N ratio: carbon:nitrogen ratio     

Nitrogen retention and plant uptake on a highly weathered central Amazonian Ferralsol amended with compost and charcoal

Leaching losses of N are a major limitation of crop production on permeable soils and under heavy rainfalls as in the humid tropics. We established a field trial in the central Amazon (near Manaus, Brazil) in order to study the influence of charcoal and compost on the retention of N. Fifteen months after organic‐matter admixing (0–0.1 m soil depth), we added ¹⁵N‐labeled (NH₄)₂SO₄ (27.5 kg N ha^–1 at 10 atom% excess). The tracer was measured in top soil (0–0.1 m) and plant samples taken at two successive sorghum (Sorghum bicolor L. Moench) harvests. The N recovery in biomass was significantly higher when the soil contained compost (14.7% of applied N) in comparison to only mineral‐fertilized plots (5.7%) due to significantly higher crop production during the first growth period. After the second harvest, the retention in soil was significantly higher in the charcoal‐amended plots (15.6%) in comparison to only mineral‐fertilized plots (9.7%) due to higher retention in soil. The total N recovery in soil, crop residues, and grains was significantly (p < 0.05) higher on compost (16.5%), charcoal (18.1%), and charcoal‐plus‐compost treatments (17.4%) in comparison to only mineral‐fertilized plots (10.9%). Organic amendments increased the retention of applied fertilizer N. One process in this retention was found to be the recycling of N taken up by the crop. The relevance of immobilization, reduced N leaching, and gaseous losses as well as other potential processes for increasing N retention should be unraveled in future studies.     

Comparative effects of vesicular-arbuscular mycorrhizal inoculation and phosphorus fertilization on growth and phosphorus uptake of maize (Zea mays L.) and sorghum (Sorghum bicolor L.) plants under drought-stressed conditions

Maize (Zea mays L.) and sorghum (Sorghum bicolor L.) Moench (local variety called Masakwat) plants were grown in a sterilized low-P soil in the greenhouse for 12 weeks. Each plant species was either mycorrhizal with vesicular-arbuscular mycorrhizal (VAM) fungi, non-mycorrhizal but minimally fertilized with soluble P, or non-mycorrhizal but highly fertilized with soluble P. Drought stress was imposed after 4 weeks at weekly intervals. Under unstressed conditions, leaf area, shoot dry weights, xylem pressure, and soil water potentials were similar for VAM and the two non-mycorrhizal P-fertilized treatments but each of the VAM-infected species had a greater total root length. Total P uptake was similar for the maize treatments but higher for VAM than non-mycorrhizal P-fertilized sorghum treatments. Under drought-stressed conditions, the growth parameters and soil water potential were similar for all maize treatments but they were reduced by mycorrhizal inoculation in sorghum. Greater water extraction occurred in drought-stressed mycorrhizal sorghum. In both plant species, total P uptake and P uptake per unit root length (including unstressed species) were significantly enhanced in non-mycorrhizal P-fertilized treatments compared with the mycorrhizal treatment. Except for the root dry weight of sorghum plants, there were no differences in the growth parameters and P uptake between minimally and highly P-fertilized non-mycorrhizal treatments for either maize or sorghum. The increased total root length in drought-stressed mycorrhizal sorghum plants and the similar infected root lengths in unstressed and drought-stressed sorghum plants may have caused high C partitioning to drought-stressed mycorrhizal roots and therefore caused the reduced growth parameters in mycorrhizal plants compared to the non-mycorrhizal P-fertilized counterparts. The results indicate that P fertilization in addition to mycorrhizal inoculation may improve the drought tolerance of maize and sorghum plants.     

Evidence for recycling of N from plants to soil during the growing season

In spite of the known below-ground biomass production of plant roots that concurrently introduce significant amounts of carbon and nitrogen into the soil, the effects of these inputs on N cycling in the soil–plant system are seldom considered. Here, we report on two field experiments carried out between 1995 and 1997 at the FAM Research Station Scheyern: (1) a N-turnover experiment to determine the N fluxes derived from ¹⁵N-labeled clover residues incorporated into the plough layer of defined plots, and (2) a root production experiment to assess the above (shoot) and below ground (gross and net root) biomass production of winter wheat in different fields, but nearby the ¹⁵N plots. An initial 50% decrease in soil organic ¹⁵N at 0–20-cm soil depth was recorded between fall, 1996 (incorporation of clover straw) and spring, 1997 (138 days after incorporation), which was then followed by a period of stability in ¹⁵N levels in the soil organic N until the harvest of winter wheat (286 days after incorporation). This stability may be explained in two ways: (a) actual stability of clover-derived ¹⁵N remaining in the second phase, e.g., due to recalcitrant compounds or microbial immobilization; or (b) apparent stability, e.g., because the actual mineralization of clover-derived ¹⁵N in the soil was compensated by secondary inputs of organic ¹⁵N (recycling). Further results showed that the first explanation was unlikely, as (1) between 138 and 286 days after clover incorporation, the mean ¹⁵N signature in soil mineral N was 2.1 at.%, indicating a persistent mineralization of clover residues; and (2) a decrease in soil microbial biomass ¹⁵N occurred in the second phase, indicating a continued N turnover in the soil. The amount of clover-derived ¹⁵N accumulated below the plough layer at 20–110-cm soil depth (11.5%) between early spring and the harvest of wheat also corroborated the return of mineralized ¹⁵N into the soil being due to the root N inputs by winter wheat. Based on the depth distribution of winter wheat net root biomass (root production experiment) and on soil organic ¹⁵N depth distribution (¹⁵N-turnover experiment), the root N input into soil was estimated to be 282 kg ha⁻¹, equivalent to 54% of total net N assimilation of winter wheat. Thus, the results of this study give substantial evidence for a N loop between soil and growing plants, whereby a part of the net mineralized N taken up by plants is continuously returned into the soil by their roots. The implications of this N loop for the interpretation of ¹⁵N experiments and for plant nutrition are discussed.     

Medium-term transformations of organic N in a cultivated soil

We followed in situ the evolution of nitrogen recently incorporated into a soil under maize culture for 4 years. Each year, a different pair of plots treated by removal or return of maize crop residues received a single pulse of ¹⁵N-labelled fertilizer. Unlabelled fertilizer was otherwise supplied. In parallel, plots supplied with unlabelled fertilizer received a single pulse of ¹⁵N-labelled maize crop residues. Varying weather affected total and fertilizer-derived N in the crop and residual inorganic N in the topsoil, but it did not affect fertilizer-N immobilization and remineralization. There was no consistent effect of crop residue return on total soil N, immobilization of fertilizer N, or the decay kinetics of recently immobilized N. Recently incorporated organic N from crop residues and microbial immobilization of inorganic N displayed similar mid-term decay kinetics. Crop residue N and immobilized N enter a labile compartment with an average residence time of a few months. A proportion, estimated at 28%, enters a more stable compartment from which the mineralization was imperceptible in 4 years. Particle-size fractions >50 um, which receive most of the crop residue N, retained it for only a short time. The mid-term stabilization of N was mainly in soil fractions <50 um.     

Short-term competition between crop plants and soil microbes for inorganic N fertilizer

Agricultural systems that receive high amounts of inorganic nitrogen (N) fertilizer in the form of either ammonium (NH₄⁺), nitrate (NO₃⁻) or a combination thereof are expected to differ in soil N transformation rates and fates of NH₄⁺ and NO₃⁻. Using ¹⁵N tracer techniques this study examines how crop plants and soil microbes vary in their ability to take up and compete for fertilizer N on a short time scale (hours to days). Single plants of barley (Hordeum vulgare L. cv. Morex) were grown on two agricultural soils in microcosms which received either NH₄⁺, NO₃⁻ or NH₄NO₃. Within each fertilizer treatment traces of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ were added separately. During 8 days of fertilization the fate of fertilizer ¹⁵N into plants, microbial biomass and inorganic soil N pools as well as changes in gross N transformation rates were investigated. One week after fertilization 45-80% of initially applied ¹⁵N was recovered in crop plants compared to only 1-10% in soil microbes, proving that plants were the strongest competitors for fertilizer N. In terms of N uptake soil microbes out-competed plants only during the first 4 h of N application independent of soil and fertilizer N form. Within one day microbial N uptake declined substantially, probably due to carbon limitation. In both soils, plants and soil microbes took up more NO₃⁻ than NH₄⁺ independent of initially applied N form. Surprisingly, no inhibitory effect of NH₄⁺ on the uptake and assimilation of nitrate in both, plants and microbes, was observed, probably because fast nitrification rates led to a swift depletion of the ammonium pool. Compared to plant and microbial NH₄⁺ uptake rates, gross nitrification rates were 3-75-fold higher, indicating that nitrifiers were the strongest competitors for NH₄⁺ in both soils. The rapid conversion of NH₄⁺ to NO₃⁻ and preferential use of NO₃⁻ by soil microbes suggest that in agricultural systems with high inorganic N fertilizer inputs the soil microbial community could adapt to high concentrations of NO₃⁻ and shift towards enhanced reliance on NO₃⁻ for their N supply.