Modifications of the carbon and nitrogen allocations in the plant (Triticum aestivum L.) soil system in response to increased atmospheric CO2 concentration

The aim of this work was to examine the response of wheat plants to a doubling of the atmospheric CO₂ concentration on: (1) carbon and nitrogen partitioning in the plant; (2) carbon release by the roots; and (3) the subsequent N uptake by the plants. The experiment was performed in controlled laboratory conditions by exposing fast-growing spring wheat plants, during 28 days, to a ¹⁴CO₂ concentration of 350 or 700 L L^–1 at two levels of soil nitrogen fertilization. Doubling CO₂ availability increased total plant production by 34% for both N treatment. In the N-fertilized soil, the CO₂ enrichment resulted in an increase in dry mass production of 41% in the shoots and 23% in the roots; without N fertilization this figure was 33% and 37%, respectively. In the N-fertilized soil, the CO₂ increase enhanced the total N uptake by 14% and lowered the N concentration in the shoots by 23%. The N concentration in the roots was unchanged. In the N-fertilized soil, doubling CO₂ availability increased N uptake by 32% but did not change the N concentrations, in either shoots or roots. The CO₂ enrichment increased total root-derived carbon by 12% with N fertilization, and by 24% without N fertilization. Between 85 and 90% of the total root derived-¹⁴C came from respiration, leaving only 10 to 15% in the soil as organic ¹⁴C. However, when total root-derived ¹⁴C was expressed as a function of root dry weight, these differences were only slightly significant. Thus, it appears that the enhanced carbon release from the living roots in response to increased atmospheric CO₂, is not due to a modification of the activity of the roots, but is a result of the increased size of the root system. The increase of root dry mass also resulted in a stimulation of the soil N mineralization related to the doubling atmospheric CO₂ concentration. The discussion is focused on the interactions between the carbon and nitrogen allocation, especially to the root system, and the implications for the acquisition of nutrients by plants in response to CO₂ increase.Abbreviations N soil fertilization without nitrogen - N soil fertilization with nitrogen     

A comparison of mineral uptake and translocation by above-ground and below-ground root systems ofSalix syringiana

Nutrient uptake and translocation by above-ground adventitious roots and below-ground roots of woodySalix syringiana saplings were studied with gamma spectrometry. Each of four radionuclides (⁷⁵Se,¹³⁸Cs,⁵⁴Mn, and⁶⁵Zn) administered to adventitious and belowground roots were detected in stems and leaves within one month. Nuclides tended to be immobilized in the leaves and branches closest to the adventitious roots that absorbed them, while nuclides absorbed from below-ground sources were distributed more evenly throughout the plant. The capacity of adventitious roots to acquire nutrients from above-ground sources suggests they function as a potential auxiliary pathway of nutrient uptake and might enhance plant nutrient status where below-ground root uptake it hindered by adverse soil conditions.     

The effect of aluminium and media on the growth of mycorrhizal and nonmycorrhizal highbush blueberry plantlets

A factorial experiment was conducted to determine the effect of aluminium (0 and 600M) and media (sand, and 1:1 sand:soil) on mycorrhizal (M) and non-mycorrhizal (NM) highbush blueberry plantlets. There were no differences in nutrient uptake and total plant dry weight between M and NM plantlets. However, more root growth, as determined by dry weight, was observed in M than NM plantlets. The plantlets growing in sand had more dry weight than did those in the soil medium. Although the root growth and shoot growth were reduced by the 600M Al treatment, the direct effect of Al on plantlet growth was not clear due to Al and P interactions. Plant nutrient uptake was reduced by high concentrations of Al, suggesting that high Al concentration limited the ability of roots to acquire most of the nutrients. Mycorrhizal cortical cell infection levels of 15–20% wene maintained in the roots in soil medium but decreased to about 5% over the 6 weeks of the experiment in the sand medium. Although M plantlets accumulated more Al in their roots, Al was readily transported to the leaf tissues of M and NM plantlets.     

Chlorsulfuron reduces rates of zinc uptake by wheat seedlings from solution culture

Wheat plants differing in zinc efficiency (Excalibur; Zn-efficient, Gatcher and Durati; Zn-inefficient) were grown in HEDTA chelate-buffered nutrient solution in controlled conditions and supplied with 0 or 40 g chlorsulfuron L^-1 . Zinc uptake rates of 12-d-old plants were measured over 80 or 90 minutes using⁶⁵ Zn added to nutrient solutions. Increasing the zinc concentration of the solution increased the rate of zinc uptake, while the percentage of zinc transported to shoots was decreased. Addition of chlorsulfuron to uptake solutions for 90 minutes did not influence rate of zinc uptake or transport of zinc to shoots. Pretreating plants with chlorsulfuron for 5 days decreased zinc uptake rates, but transport to shoots was proportionally increased. Three-day pretreatment with chlorsulfuron was the minimum required for significant differences in uptake and transport of zinc to occur. Plants exposed to chlorsulfuron for 3 days required a further 5 days of growth in chlorsulfuron-free solutions before uptake rates recovered to control plant rates. It is concluded that chlorsulfuron deleteriously but reversibly affects uptake of zinc across the plasma membrane after prolonged exposure.     

EGTA对Cd胁迫下蓖麻Cd积累和营养元素吸收的影响

以‘淄蓖麻5号’蓖麻品种为材料,通过盆栽试验研究了重度Cd土壤污染（100 mg·kg^-1）条件下,不同浓度（0、0.5、1.0、2.0 mmol·kg^-1）外源螯合剂——乙二醇双(2-氨基乙基醚)四乙酸(EGTA)对蓖麻植株生长、Cd积累和营养元素吸收的影响,探讨外源螯合剂调控Cd污染土壤上植物生长和修复效应。结果显示：（1）在Cd胁迫下,土壤中外源添加0.5~2.0 mmol·kg^-1EGTA使蓖麻根系鲜、干重比不添加EGTA对照不同程度降低,但植株总干重没有受到显著影响。（2）外源EGTA能有效促进Cd从蓖麻根部向地上部的转移,2.0 mmol·kg^-1的EGTA处理使蓖麻叶片Cd 含量显著增加了41.34倍;与不添加EGTA对照相比,外源EGTA处理蓖麻叶片中Cd积累量随添加EGTA的浓度增加而显著大幅度增加14.0~45.6倍,占相应植株总积累量的36.89%~58.63%,而茎中Cd积累量增加幅度较小,根中Cd积累量则显著降低。（3）Cd胁迫条件下,外源EGTA对蓖麻各器官矿质元素含量的影响不一,EGTA促进K向蓖麻地上部的转运,同时抑制Mg向植株地上部转运;随土壤添加的EGTA浓度提高,蓖麻植株对Ca吸收表现为低促高抑,叶片Zn含量和植株Cu含量逐渐增加,叶片和根系Fe含量及植株各器官Mn含量显著增加。与无Cd胁迫对照相比,EGTA在提高植株Cd积累的同时,降低了根系对K的吸收。研究表明,Cd胁迫显著抑制了蓖麻植株的生长,适宜浓度的外源EGTA对Cd的这种抑制有显著的缓解作用;外源EGTA改变了Cd在蓖麻根、茎、叶中的积累分布情况,提高了Cd从根系向地上部,尤其是向叶片的转移能力,从而强化了蓖麻对Cd污染土壤的修复效率;在采用EGTA强化植物修复Cd污染土壤时,应适量增施K肥以保证植株的正常生理代谢。     

Do oaks have different strategies for uptake of N, K and P depending on soil depth?

The uptake of nutrients from deep soil layers has been shown to be important for the long-term nutrient sustainability of forest soils. When modelling nutrient uptake in forest ecosystems, the nutrient uptake capacity of trees is usually defined by the root distribution. However, this leads to the assumption that roots at different soil depths have the same capacity to take up nutrients. To investigate if roots located at different soil depths differ in their nutrient uptake capacity, here defined as the nutrient uptake rate under standardized conditions, a bioassay was performed on excised roots (<1 mm) of eight oak trees (Quercus robur L.). The results showed that the root uptake rate of ⁸⁶Rb⁺ (used as an analogue for K⁺) declined with increasing soil depth, and the same trend was found for . The root uptake rate of , on the other hand, did not decrease with soil depth. These different physiological responses in relation to soil depth indicate differences in the oak roots, and suggest that fine roots in shallow soil layers may be specialized in taking up nutrients such as K⁺ and which have a high availability in these layers, while oak roots in deep soil layers are specialized in taking up other resources, such as P, which may have a high availability in deep soil layers. Regardless of the cause of the difference in uptake trends for the various nutrients, these differences have consequences for the modelling of the soil nutrient pool beneath oak trees and raise the question of whether roots can be treated uniformly, as has previously been done in forest ecosystem models. Responsible Editor: Herbert Johannes Kronzucker.     

Elevated CO2 evokes quantitative and qualitative changes in carbon dynamics in a plant/soil system: mechanisms and implications

It is hypothesized that carbon storage in soil will increase under an elevated atmospheric CO₂ concentration due to a combination of an increased net CO₂ uptake, a shift in carbon allocation pattern in the plant/soil system and a decreased decomposition rate of plant residues. An overview of several studies, performed in our laboratory, on the effects of elevated CO₂ on net carbon uptake, allocation to the soil and decomposition of roots is given to test this hypothesis. The studies included wheat, ryegrass and Douglas-fir and comprised both short-term and long-term studies.Total dry weight of the plants increased up to 62%, but depended on nutrient availability. These results were supported by the data on net ¹⁴CO₂ uptake. A shift in ¹⁴C-carbon distribution from shoots to roots was found in perennial species, although this depended on nutrient availability.The decomposition experiments showed that roots cultivated at 700 L L^–1 CO₂ were decomposed more slowly than those cultivated at 350 L L^–1 CO₂. Even after two growing seasons differences up to 13% were observed, although this was found to be dependent on the nitrogen level at which the roots were grown.Both an increased carbon allocation to the soil due to an increased carbon uptake, whether or not combined with a shift in distribution pattern, and a decreased decomposition of root residues will enhance the possibilities of carbon sequestration in soil, thus supporting our hypothesis. However, nutrient availability and the response of the soil microbial biomass (size and activity) play a major role in the processes involved and require attention to clarify plant/soil responses in the long term with regard to sustained stimulation of carbon input into soils and the decomposability of roots and rhizodeposition. Soil texture will also have a strong effect on decomposition rates as a result of differences in the protecting capacity for organic matter. More detailed information on these changes is needed for a proper use of models simulating soil carbon dynamics in the long term.     

Nutrient concentration in wheat and soil under allelopathy treatments

Allelopathy is related to soil nutrient availability and allelochemicals can change the soil and therefore the plant nutrient status. Wheat is one of the most important crops for the production of human food in the world. Alhagi maurorum and Cardaria draba are the most important weeds in wheat fields. We performed experiments to assess the allelopathic effect of A. maurorum and C. draba shoots on mineral nutrient concentrations in pot-grown wheat plants and soil. The presence of dry powder of A. maurorum and C. draba shoots reduced concentrations of macronutrients (NO₃ ^?, K⁺, Ca²⁺ and P) and micronutrients (Fe²⁺ and Cu²⁺) in roots and shoots of wheat plants, whereas it did not affect concentrations of Mg²⁺, Mn²⁺ and Zn²⁺. Allelopathic effect of A. maurorum was significantly greater than that of C. draba. There was a significantly positive correlation between wheat growth and ion concentration. There was a significantly negative correlation between the soil nutrient concentration and plant nutrient concentration across the treatments. These results suggest that allelopathy increases the nutrient availability in the soil because of the decrease in absorption by plants.     

Lead uptake from solution by perennial ryegrass and its transport from roots to shoots

Summary The uptake of lead by roots and its transport to the shoots was examined with perennial ryegrass in solution cultures. Root uptake as measured by the decrease in concentration of lead in an aqueous solution containing 1 mg Pb/l as Pb (NO₃)₂ was rapid, almost complete, and unaffected by removing the shoots or killing the roots. Lead bound in the roots was not released by exchange with Ca or Ba ions. The distribution of lead within the plant was examined at intervals after a single, 3-day exposure to various levels of lead added to a nutrient solution. The total uptake, or lead burden, increased with increasing rates of addition and ranged from 281 to 9969 g/Pb per 3 plants. The proportion of the lead reaching the shoots at the first harvest (7 days after adding lead) was 3.5 to 22.7 per cent of total uptake, the lower value being for plants with the greatest burden. Transport to the shoots continued throughout the experimental periods of 21 and 28 days but did not exceed 28.9 per cent of total uptake. The concentration of lead in shoots at the first harvest ranged from 0.2 to 58.4 ppm and that in the corresponding roots from 5.5 to 5310 ppm. At later harvests, and after cutting, the concentration in the shoots decreased; an exception was in plants with the greatest lead burden. It is concluded that roots of actively growing ryegrass provide a barrier which restricts the movement of lead to the above-ground parts of plants, and so to animals or man.     

Acquisition of phosphorus and copper by VA-mycorrhizal hyphae and root-to-shoot transport in white clover

White clover (Trifolium repens L.) plants were grown in a calcareous soil in pots with three compartments, a central one for root growth and two outer ones for growth of vesicular-arbuscular (VA) mycorrhizal (Glomus mosseae [Nicol. & Gerd.] Gerdemann & Trappe) hyphae (hyphal compartments). Phosphorus (P) was applied at three levels (0, 20 and 50 mg kg⁻¹ soil) in the outer compartments in mycorrhizal treatments. Root and shoot dry weight were increased in mycorrhizal plants with hyphal access to outer compartments. Growth of the mycorrhizal hyphae in the outer compartments was not significantly affected by variation in P level in these compartments. However, both concentration and amount of P in roots and shoots sharply increased with increasing P supply in the outer (hyphal) compartments. With increasing P levels the calculated delivery of P by the hyphae from the outer compartments increased from 34% to 90% of total P uptake. Hyphal access to the outer compartments also significantly increased both concentration and quantity of Cu in the plants. The calculated delivery of Cu by the hyphae from the outer compartments ranged from 53% to 62% of total Cu uptake, irrespective of the P levels and the amounts of P taken up and transported by the hyphae. However, the distribution of Cu over roots and shoots was largely dependent on P levels. With increase in P level in the outer compartments the calculated hyphal contribution to the total amount of Cu in the shoots increased from 12% to 58%, but decreased in the roots from 75% to 46%. In conclusion, uptake and transport by VA-mycorrhizal hyphae may contribute substantially not only to P nutrition, but also to Cu nutrition of the host.     

Reduction and transport of Fe from siderophores

Soils contain siderophores produced by bacteria and fungi; however, the role of siderophores in Fe nutrition of plants is uncertain. The Strategy I plant cucumber (Cucumis sativus L.) was used in an investigation of ferric chelate reduction activity and uptake and transport of Fe from ferric hydroxyethylethylenetriacetic acid (FeHEDTA) and ferric N,N–di–(2–hydroxybenzoyl)–ethylenediamine– N,N-diacetic acid (FeHBED) and the hydroxamate siderophores, ferric rhodotorulic acid (FeRA) and ferric ferrioxime B (FeFOB). Cucumber seedlings were grown in a hydroponic medium without Fe or supplied with 10 M FeHEDTA. Iron-deficient cucumber roots readily reduced FeHEDTA, while Fe-sufficient roots had low levels of ferric chelate reduction activity. The siderophore FeRA was reduced by Fe-deficient roots at 8% of the rate of FeHEDTA, while FeFOB was not reduced. The highly stable synthetic chelate FeHBED was reduced at 16% the rate of FeHEDTA. Fe transport to shoots by Fe-deficient seedlings from the slowly reducible complexes ⁵⁹FeRA and ⁵⁹FeHBED was, respectively, 74% and 73% of that transported from ⁵⁹FeHEDTA. The ferrous complexing agent, bathophenanthrolinedisulfonic acid (BPDS), had a strong inhibitory effect on uptake and transport of Fe from ⁵⁹FeHEDTA or ⁵⁹FeRA into shoots. An average of 11% as much Fe was transported to shoots of Fe-deficient seedlings from ⁵⁹FeFOB as from ⁵⁹FeHEDTA. Neither the Fe nutritional status of the seedlings nor the presence of BPDS influenced the uptake and transport of Fe from ⁵⁹FeFOB. It is concluded that cucumber roots may take up substantial amounts of Fe from FeRA and FeHBED following reduction, while small amounts of Fe may be taken up from FeFOB by a mechanism not involving reduction of the ferric siderophore at the root surface.     

Influence of external zinc and phosphorus supply on Cd uptake by rice (Oryza sativa L.) seedlings with root surface iron plaque

Rice seedlings were grown in hydroponic culture to determine the effects of external Zn and P supply on plant uptake of Cd in the presence or absence of iron plaque on the root surfaces. Iron plaque was induced by supplying 50 mg l⁻¹ Fe²⁺ in the nutrient solution for 2 day. Then 43-day-old seedlings were exposed to 10 μmol l⁻¹ Cd together with 10 μmol l⁻¹ Zn or without Zn (Zn–Cd experiment), or to 10 μmol l⁻¹ Cd with 1.0 mmol l⁻¹ P or without P (P–Cd experiment) for another 2 day. The seedlings were then harvested and the concentrations of Fe, Zn, P and Cd in dithionite–citrate–bicarbonate (DCB) extracts and in roots and shoots were determined. The dry weights of roots and shoots of seedlings treated with 50 mg l⁻¹ Fe were significantly lower than when no Fe was supplied. Adsorption of Cd, Zn and P on the iron plaque increased when Fe was supplied but Cd concentrations in DCB extracts were unaffected by external Zn or P supply levels. Cd concentrations in shoots and roots were lower when Fe was supplied. Zn additions decreased Cd concentrations in roots but increased Cd concentrations in shoots, whereas P additions significantly increased shoot and root Cd concentrations and this effect diminished when Fe was supplied. The percentage of Cd in DCB extracts was significantly lower than in roots or shoots, accounting for up to 1.8–3.8% of the plant total Cd, while root and shoot Cd were within the ranges 57–76% and 21–40% respectively in the two experiments. Thus, the main barrier to Cd uptake seemed to be the root tissue and the contribution of iron plaque on root surfaces to plant Cd uptake was minor. The changes in plant Cd uptake were not due to Zn or P additions altering Cd adsorption on iron plaque, but more likely because Zn or P interfered with Cd uptake by the roots and translocation to the shoots.     

Soil boron and selenium removal by three plant species

High concentrations of boron (B) and selenium (Se) naturally found in the environment are detrimental to sustainable agriculture in the western USA. Greenhouse pot experiments were conducted to study B and Se uptake in three different plant species; Brassica juncea (L.) Czern (wild brown mustard), Festuca arundinacea Schreb. L. (tall fescue), and Brassica napus (canola) were grown in soil containing naturally occurring concentrations of 3.00 mg extractable B kg^–1 and 1.17 mg total Se kg^–1 soil. During the growing season, four intermediate harvests were performed on wild mustard and tall fescue. Final harvest I consisted of harvesting wild mustard, canola, and clipping tall fescue. Final harvest II consisted of harvesting wild mustard, which had been planted in soil in which wild mustard was previously grown, and harvesting previously clipped tall fescue. The greatest total amount of above ground biomass and below surface biomass was produced by tall fescue. Plants were separated into shoots and roots, weighted, and plant tissues were analyzed for total B and Se. The highest concentrations of tissue B were recovered in shoots of wild mustard and canola at final harvest I, while roots from tall fescue contained the highest concentrations of B irrespective of the harvest. Tissue Se concentrations were similar in all plants species. Soils were analyzed for residual B and Se. Extractable soil B concentrations at harvest times were lowered no less than 32% and total Se no less than 24% for all three species. The planting of wild mustard, canola, or tall fescue can reduce water-extractable B and total Se in the soil.     

Root Development and Nutrient Uptake

Root system formation proceeds in close coordination with shoot growth. Accordingly, root growth and its functions are regulated tightly by the shoot through materials cycling between roots and shoots. A plant root system consists of different kinds of roots that differ in morphology and functions. The spatial configuration and distribution of these roots determine root system architecture in the soil, which in turn primarily regulates the acquisition of soil resources like nutrients and water. Morphological and physiological properties of each root and the concomitant tissues further affect nutrient uptake and transport, while the root traits that are related to such acquisition also depend on the kinds of nutrients and their mobility in the soil. In addition, mechanisms involved in the uptake and transport of mineral nutrients recently have been elucidated at the molecular level. A number of genes for acquisition and transport of various mineral nutrients have been identified in model plant systems such as Arabidopsis thaliana, and rice, and in other plant species. An integration of studies on nutrient behavior in soils and the morphological and physiological functions of root systems will further elucidate the mechanism of plant nutrient uptake and transport by roots, and offer a real possibility of genetically improving crop productivity in problem soils.

     

Comparison of plant uptake and plant toxicity of various ions in wheat

The effects of varying solution concentrations of manganese (Mn), zinc (Zn), copper (Cu), boron (B), iron (Fe), gallium (Ga) and lanthanum (La) on plant chemical concentrations, plant uptake and plant toxicity were determined in wheat (Triticum aestivum L.) grown in a low ionic strength (2.7×10^–3 M solution culture). Increasing the solution concentration of Mn, Zn, Cu, B, Fe, Ga and La increased plant concentrations of that ion. Asymptotic maximum plant concentrations were reached for Zn (10 mg kg DM^–1 in the roots), Ga (2 mg kg DM^–1 in the tops and 18 mg kg DM^–1 in the roots) and La (0.4 mg kg DM^–1 in the tops and 4 mg kg DM^–1 in the roots). Plant ion concentrations were, on average, 3 times higher in the roots than the tops for Mn and Zn, 7 times for Cu, 9 times for Fe, 12 times for Ga and 15 times for La. In contrast, B concentrations were higher in the tops than the roots by, on average, 2 times. The estimated toxicity threshold (plant concentration at which a rapid decrease in yield occurred) in the tops was 0.4 mg g DM^–1 for B, 2 for Zn, 0.075 for Cu and 0.09 for La and in the roots 0.2 mg g DM^–1 for B, 5 for Zn, 0.3 for Cu and 3 for La. Plant uptake rates of the ions (as estimated by the slope of the relationship between solution ion concentrations and plant ion concentrations) was in the order B 250 mg kg DM^–1 M ^–1). Plant toxicity was estimated as the reciprocal of the plant concentration that reduced yield by 50% (change in relative yield per mg ion kg DM^–1). The plant toxicity of the ions tested was in the order Mn相似文献   

Phosphorus transport to the xylem and its regulation by water flow

Summary The effects of water flow on phosphorus uptake by roots and on its subsequent translocation to shoots were separated by giving short-term pulses of ³²P-labelled nutrient to intact tomato plants. At the end of a 5 min pulse, all the ³²P taken up by the plants was confined to the roots. Only about half of this ³²P was later translocated to shoots; there was very little translocation after 4 hours.Experiments after long-term labelling showed that only a small part of the total P in the root is readily translocated to shoots. This P appears to be in part of the symplast and contributes about 75% of the P transported to the xylem sap. The rest is presumably derived by leakage from vacuoles.A slow rate of water flow reduced both uptake into the symplast and the translocation to the shoots of P which had already been absorbed by the roots. This was conclusively demonstrated by giving a ³²P pulse before reducing the rate of water flow; ³²P not translocated to shoots was partly retained by the roots and partly lost to the external solution. Water flow also accelerates transport to the xylem of previously-absorbed P in excised roots.It is concluded that the major effect of water flow on phosphorus transport to shoots occurs after phosphorus uptake by the roots, probably during radial transport to the xylem.     

Influence of hydraulics on the uptake of ammonium by two freshwater plants

相似文献   

A H+-ATPase That Energizes Nutrient Uptake during Mycorrhizal Symbioses in Rice and Medicago truncatula

Most plant species form symbioses with arbuscular mycorrhizal (AM) fungi, which facilitate the uptake of mineral nutrients such as phosphate from the soil. Several transporters, particularly proton-coupled phosphate transporters, have been identified on both the plant and fungal membranes and contribute to delivering phosphate from fungi to plants. The mechanism of nutrient exchange has been studied in plants during mycorrhizal colonization, but the source of the electrochemical proton gradient that drives nutrient exchange is not known. Here, we show that plasma membrane H⁺-ATPases that are specifically induced in arbuscule-containing cells are required for enhanced proton pumping activity in membrane vesicles from AM-colonized roots of rice (Oryza sativa) and Medicago truncatula. Mutation of the H⁺-ATPases reduced arbuscule size and impaired nutrient uptake by the host plant through the mycorrhizal symbiosis. Overexpression of the H⁺-ATPase Os-HA1 increased both phosphate uptake and the plasma membrane potential, suggesting that this H⁺-ATPase plays a key role in energizing the periarbuscular membrane, thereby facilitating nutrient exchange in arbusculated plant cells.     

Photoassimilated carbon allocation in a wheat plant-soil system as affected by soil fertility and land-use history

Background and aims

Carbon (C) cycling in terrestrial ecosystems is influenced by the distribution of photo-assimilated C in the plant-soil system. Photo-assimilated C allocation in a wheat cropping system was examined to identify the links between soil fertility, C partitioning and soil C sequestration.

Methods

A pulse labelling experiment was conducted where ¹⁴CO₂ was introduced to wheat plants grown in two groups of soils of varying fertility: arable soils spiked with nutrients, and soils with differing land-use histories. Wheat shoot, root and soil samples were taken 1, 14 and 28 days after pulse labelling to examine the fluxes of ¹⁴C in the plant-root-soil system.

Results

The partitioning of ¹⁴C in wheat plant-root-soil system was found to vary with time, nutrient spiked soil fertility and land-use history. At the end of the experiment using spiked soils, a positive correlation was observed between the allocation of ¹⁴C in the shoots and soil fertility, whereas in the roots, this relationship was negative. The overall allocation of ¹⁴C in the plant-root system differed significantly between the land-use histories; while in the spiked arable soils ¹⁴C allocation in the shoots and roots systematically followed their fertility status.

Conclusions

There was a weak relationship between C allocation and soil fertility in the soils of different land-use history compared to the strong relationship in the spiked arable soils. This suggests that other factors in the soils under different land uses were more important than nutrient status alone in driving photo-assimilated C allocation. This study demonstrated that soil fertility and land-use history have a crucial role in the allocation of photo-assimilated C in the plant-soil system and are important factors by which C sequestration in soil may be impacted.     

The effect of soil pH on rhizosphere carbon flow of Lolium perenne

Perennial rye-grass plants were grown at 15°C in microcosms containing soil sampled from field plots that had been maintained at constant pH for the last 30 years. Six soil pH values were tested in the experiment, with pH ranging from 4.3–6.5. After 3 weeks growth in the microcosms, plant shoots were exposed to a pulse of ¹⁴C-CO₂. The fate of this label was determined by monitoring ¹⁴C-CO₂ respired by the plant roots/soil and by the shoots. The ¹⁴C remaining in plant roots and shoots was determined when the plants were harvested 7 days after receiving the pulse label. The amount of ¹⁴C (expressed as a percentage of the total ¹⁴C fixed by the plant) lost from the plant roots increased from 12.3 to 30.6% with increasing soil pH from 4.3 to 6. Although a greater percentage of the fixed ¹⁴C was respired by the root/soil as soil pH increased, plant biomass was greater with increasing soil pH. Possible reasons for observed changes in the pattern of ¹⁴C distribution are discussed and, it is suggested that changes in the soil microbial biomass and in plant nitrogen nutrition may, in particular be key factors which led to increased loss of carbon from plant roots with increasing soil pH.