Effects of ammonium sulphate application on the rhizosphere,fine-root and needle chemistry in aPicea abies (L.) Karst. stand

Rhizosphere, fine-root and needle chemistry were investigated in a 28 year old Norway spruce stand in SW Sweden. The uptake and allocation pattern of plant nutrients and aluminium in control plots (C) and plots repeatedly treated with ammonium sulphate (NS) were compared. Treatments started in 1988. Current year needles, one-year-old needles and cylindrical core samples of the LFH-layer and the mineral soil layers were sampled in 1988, 1989 and 1990. Compared to the control plots, pH decreased significantly in the rhizosphere soil in the NS plots in 1989 and 1990 while the SO₄-S concentration increased significantly. Aluminium concentration in the rhizosphere soil was generally higher in the NS plots in all soil layers, except at 0–10 cm depths, both in 1989 and 1990. Calcium, Mg and K concentrations also increased after treatment with ammonium sulphate. Ammonium ions may have replaced these elements in the soil organic matter. The NS treatment significantly reduced Mg concentrations in fine roots in all layers in 1990. A similar trend was found in the needles. Ca concentrations in fine roots were significantly lower in the NS plots in the LFH layer in 1990 and the same pattern was found in the current needles. The N and S concentrations of both fine roots and needles were significantly higher in the NS plots. It was suggested that NS treatment resulted in displacement of Mg, Ca and K from exchange sites in the LFH layer leading to leaching of these cations to the mineral soil. Further application of ammonium sulphate may damage the fine roots and consequently adversely affect the water and nutrient uptake of root systems.     

Measured and modelled differences in nutrient concentrations between rhizosphere and bulk soil in a Norway spruce stand

The gradient in soil characteristics from the bulk soil to the root surface is important to roots and to the organisms that live in the rhizosphere. Our ability to measure ion concentrations at the root surface is extremely limited, and models are largely untested. We used data from a well studied Norway spruce stand in SW Sweden to compare the measured difference in nutrient concentrations between rhizosphere and bulk soil with the difference predicted by a steady-state simulation model based on ecosystem budgets of nutrient uptake. The simulation model predicted depletion of NH₄, Ca, Mg, K in the rhizosphere, which shows that budgeted uptake rates were greater than the mass flow of bulk solution towards the root. In plots treated with ammonium sulphate, the model predicted an accumulation of S in the rhizosphere. In contrast, the observed rhizosphere concentrations were generally enriched in nutrients, relative to bulk soil. Collecting rhizosphere soil adhering to root surfaces may not be an appropriate method for describing the concentration gradient around the root. In addition, the simulation model omits some processes affecting conditions in the rhizosphere that are important to explaining nutrient uptake.     

Changes in the extractability of cations (Ca,Mg and K) in the rhizosphere soil of Norway spruce (Picea abies) roots

Nutrient concentrations in the rhizosphere soil can be higher, lower or remain unchanged compared to the bulk soil, but relatively little is known about such changes for basic cations in the rhizosphere of tree roots. A modified root container technique of studying rhizosphere processes was employed. Plexiglas cylinders were horizontally split by a membrane with 30 M mesh size into an upper compartment for root growth and a root-free lower compartment, each with an inner diameter of 5 cm and a height of 10 cm. One 2-year-old Norway spruce (Picea abies) seedling was transplanted from a nursery into each cylinder. Plants were not specifically inoculated, but roots were colonised by a mix of ectomycorrhizal fungi originating from the nursery. The nutrient poor mineral soil used in the experiment was taken from a forest site in Bayerischer Wald, southern Germany. The soil was either supplied with a mix of Ca, Mg and K, or not supplied with these cations. Plants were harvested 30 weeks after transplanting. The nylon membrane between the root compartments restricted root growth to the upper compartment, so that by the end of the experiment a root mat was formed at the top side of the membrane. In the lower compartment, soil nearest to the root mat was regarded as rhizosphere soil while soil in a distance from the root mat was regarded as bulk soil. In the upper compartment, rhizosphere soil was obtained at the end of the experiment by gently shaking the roots. The soils were analysed for Ca, Mg and K contents following two different soil extraction methods. In the fertilised treatment, H₂O-extractable Ca and Mg were accumulated in the rhizosphere. In contrast, K (NH₄Cl-extraction) was depleted in the rhizosphere. In the bottom tube, the depletion of K (NH₄Cl-extraction) was restricted to 1 cm distance from the root mat. In unfertilised soil, Ca, Mg and K concentrations did not differ clearly between rhizosphere and bulk soils. The results indicated that the occurrence of cation gradients in the rhizosphere depended on the level of soil nutrient supply. Distinct rhizosphere effects were measured by conventional soil extraction methods only when the soil was freshly fertilised with mineral elements prior to the experiment. In this case, K depletion in the rhizosphere reflected higher K uptake by the fertilised Norway spruce plants. For low-nutrient soils, novel techniques are required to follow subtle changes in the rhizosphere.     

Root and needle litter decomposition responses to enhanced supplies of N and S in a Norway spruce forest in southwest Sweden

The effects of additions of ammonium sulfate (NS) on the decomposition of litter derived from Norway spruce roots (< 2 and 2 - 5 mm in diameter) in the humus and mineral soil layers (0 - 15 cm) of a Norway spruce stand in southern Sweden were investigated over a 6-year period. To this purpose, litterbags were incubated in the humus layer and in the mineral soil in June 1996, with roots collected from NS and control (C) plots incubated in the NS and C plots, respectively. The N concentrations in fine roots (< 2 mm) in the NS- plots were higher than those in 2 - 5 mm roots in both humus and mineral soil layers. In the humus layer, N concentrations in the fine roots in the C- and NS- plots were 12.8 and 15.7 mg g ^{? 1}, respectively. By the end of the fifth year the < 2 mm roots in humus layer had lost 48.5 and 50% of their mass in the C and NS plots, respectively, while the corresponding values for the 2 - 5 mm diameter class were 44 and 54%. The fresh root litter may be a sensitive indicator to responses to enhanced N and S deposition, although decomposition rates of both litter types are affected.     

Root distribution in a Norway spruce (Picea abies (L.) Karst.) stand subjected to drought and ammonium-sulphate application

Results of the spatial distribution of fine roots are reported from a Norway spruce (Picea abies (L.) Karst.) in SW Sweden stand subjected to drought (D) and ammonium-sulphate application (NS). The sampling was carried out by excavating monoliths in segments of 0.5 × 0.5 × 0.1 m to a depth of one meter. Root data also included in the study were obtained by excavating whole trees and soil coring.The data suggest a fairly deep distribution pattern of fine roots (< 1 mm in diameter) in the study area compared with other forest sites in SW Sweden. The weight fraction of living fine roots in the LFH-horizon amounted to 53, 36 and 55% of the total fine-root biomass and 12, 30 and 32% of the total fine-root necromass (dead fine roots) in the control, D and NS-treatment areas respectively. Drought seemed to result in a redistribution of fine roots to deeper mineral soil horizons. Ammonium sulphate application led to the reverse, viz, a concentration of fine roots to the LFH-horizon. A significantly smaller fine-root necromass was indicated in the LFH-horizon of the control areas compared with both the D and NS-treatment areas, suggesting a high mortality of fine roots in these areas. A heavy dry matter fraction accumulates in roots > 1 mm in diameter and in stumps. These root fraction, were frequently found between the trees, although the stump constitutes an important fraction in terms of dry weight.     

Water-soluble carbon in roots of rape and barley: impacts on labile soil organic carbon, arylsulphatase activity and sulphur mineralization

Investigating the impact of plant species on sulphur (S) availability in the rhizosphere soil is agronomically important to optimize S fertilization. Bulk, rhizosphere soils and the roots of field-grown rape and barley were sampled 7 times (every fortnight), from March to June, at plant maturity. Root carbon (C) and nitrogen (N) in water extract, along with soil SO₄²⁻-S, labile soil organic-C (HWC) and -N (HWN) in hot water extract, as well as soil arylsulphatase activity were then monitored. The average concentrations of both HWC and HWN were observed in the following decreasing order: rape rhizosphere soil >barley rhizosphere soil >bulk soil. In parallel, the average contents of water extractable-C and -N in rape roots were higher than those in barley roots. These results suggest that soil C and N contents in hot water extract (including rhizodeposition) were correlated with C and N released by roots. Great ARS activities found in rape rhizosphere soil were accompanied by great SO₄²⁻-S mineralization over time. Finally, bulk and rhizosphere soils of rape and barley were pooled from the seven samplings and incubated with the corresponding pooled root water-soluble C of both plant species and glucose-C. After 1 and 9 weeks, a greater net S mineralization (gross mineralization - immobilization) was observed with rape root water-soluble C than with barley root water-soluble C and glucose-C. Conjointly, we found a higher average value of ARS activity in rape rhizosphere than in barley rhizosphere soil. Our findings suggest that plant species, via their rhizodeposition, determine the dynamic of S in soil.     

Effects of liming on rhizosphere chemistry and growth of fine roots and of shoots of sessile oak (Quercus petraea)

Soil acidification can be detrimental to root growth and nutrient uptake, and liming may alleviate such acidification. In the following study, seedlings of sessile oak (Quercus petraea Liebl. M.) were grown in rhizotrons and subjected to liming (L) or gypsum (G) treatments and compared with the control (C). In order to study and interpret the impact of these calcium rich treatments on fine root development and tree growth, the following parameters were assessed: fine root biomass, fine root length, seedling development (height, diameter, leaves), seedling biomass, nutrient content of roots and seedlings, bulk soil and soil solution chemistry and rhizosphere soil chemistry. The results show that liming increased bulk soil pH, exchangeable Mg, Ca and the Ca/Al molar ratio, and decreased exchangeable Al, mainly in the A-horizon. Gypsum had a similar but smaller impact on exchangeable Al, Ca, H⁺ and the Ca/Al molar ratio in the A-horizon, but reacted with depth, so that exchangeable Mn, Mg and Ca were increased in the B-horizon. In the rhizosphere, the general pattern was determined by the treatment effects of the bulk soil. Most elements were more concentrated in the rhizosphere than in bulk soil, except for Ca which was less concentrated after liming or gypsum application. In the B-horizon rhizosphere pH was increased by the treatments (L > G,C) close to the root tips. Furthermore, the length of the zone with a positive root-induced pH increase was greater for the limed roots as compared with both the other treatments. Fine root growth was stimulated by liming (L > G,C) both in terms of biomass and length, whereas specific root length was not obviously affected apart from the indication of some stimulation after liming at the beginning. The live:dead ratio of fine roots was significantly higher in the limed rhizotrons as compared to the control (G not assessed), indicating lower mortality (higher longevity). Shoot growth showed greater lime-induced stimulation (L > G,C) as compared to root growth. As a result the shoot:root ratio was higher in the limed rhizotrons than in the control (L > G,C). Liming induced a higher allocation of P, S, Mg, Ca and K to the leaves, stem and twigs. Gypsum showed similar effects, but was only significant for S. Liming increased the foliar Ca/Al ratio by both increasing foliar Ca and decreasing foliar Al, whereas gypsum did not clearly improve foliar nutrition. This study suggests that a moderate application of lime can be successful in stimulating seedling growth, but that gypsum had no effect on seedling growth. It can be concluded that this lime-induced growth stimulation is directly related to the improved soil fertility status, and the alleviation of Al toxicity and acid stress, resulting in better foliar nutrition. The impact of liming on fine roots, as a consequence, was not limited to a stimulation of the total amount of fine roots, but also improved the root uptake performance. This revised version was published online in June 2006 with corrections to the Cover Date.     

不同连栽代数杉木人工林细根生长、分布与营养物质分泌特征

用土钻法研究了不同连栽代数杉木细根生长与分布,结果表明,杉木连栽1代后,细根生物量明显减少,第1代杉木活细根生物量的范围为646．4－799．7g.m-2,而第2代则为284．4－536．9g.m-2,在一定距离范围内,离树体的距离越远,细根分布越少,杉木连栽后,主要减少了表层土壤（0－10cm)细根的生长,根系分泌物的分析表明,杉木连栽1代后,根系阳离子NH4＋,Na ,K ,Ca2 ,Mg2 的分泌量没有变化,而阴离子NO3-,Cl-,SO4 2-,HPO4^2-,的量都减少,但只有Cl-和HPO4^2-差异显著（P＜0．05）,第2代杉木根系分泌物中的HPO4^2-量仅为第1代的1／65,根－土界面磷交换过程的阐明有利于说明第2代杉木根系HPO4^2-分泌量减少的原因,杉木连栽后,表层土壤细根生长的减少和根系分泌行为的改变可能是生产力下降的重要原因。     

西南喀斯特石漠化环境适生植物构树细根、根际土壤化学计量特征

为了解西南喀斯特石漠化适生植物构树(Broussonetia papyrifera)对贫瘠土壤养分环境的适应策略, 及其细根、根际土壤的化学计量特征对石漠化等级的响应, 该研究以西南喀斯特石漠化环境适生植物构树为研究对象, 运用生态化学计量学方法, 开展不同等级石漠化环境构树细根、根际土壤有机碳(C)、全氮(N)、全磷(P)、全钾(K)、全钙(Ca)及全镁(Mg)养分含量特征及C、N、P化学计量特征研究。结果表明, 除Ca含量外, 喀斯特石漠化环境适生植物构树细根、根际土壤的养分含量均处于较低水平; 细根N:P为12.59, 表明构树生长受N和P共同限制; 随着石漠化等级的增加, 细根C、N含量和C:N、C:P呈先降后升的变化趋势, K、P含量是则表现为先升后降, Ca、Mg含量和N:P无明显变化规律; 不同等级石漠化环境中的构树根际土壤N、P、K、Ca含量呈不同的变化趋势, 而C、Mg含量及C、N、P化学计量特征的变化较不显著; 细根与根际土壤的化学计量特征之间存在显著的相关性, 二者的C、P、Ca、Mg含量、C:N、C:P分别对应呈显著正相关关系, 而N含量呈极显著负相关关系; 细根的K含量则较为稳定, 几乎不受根际土壤养分的影响。     

Effects of wood ash and nitrogen fertilization on fine root biomass and soil and foliage nutrients in a Norway spruce stand in Finland

We determined the effects of wood ash fertilization, given together with nitrogen (WAN), and nitrogen given together with P, B and Cu (SSF), on soil and foliage nutrients and fine root biomass in a 45-year-old Norway spruce stand in southern Finland. Fine roots were sampled 9 years, and the soil 10 years after ash (3 t/ha) and nitrogen (150 kg/ha) application. Fine root biomass tended to be lower, the necromass higher, and the fine root distribution relatively deeper on the WAN than on the control and SSF plots. The response of fine root biomass to WAN was probably related to changes in soil acidity. pH, base saturation, total and extractable concentrations of Ca, K, Mg and P, and total B, Cd, Mn, Ni and Zn concentrations in the organic layer were significantly higher on the WAN plots than on the SSF and the control plots with no ash and nutrient addition. On the WAN plots, the pH was 1.2 pH-units higher, the exchangeable Ca concentrations fourfold and those of Mg over twofold compared to the control plots. WAN increased the concentrations of K but decreased those of Mn and Ni in the needles compared to the control and SSF treatment. Even though ash and nitrogen fertilisation tended to decrease the fine root biomass, this decrease was not likely to affect tree growth during a 10-year period.     

Soil solution chemistry in the rhizosphere of roots of sessile oak (Quercus petraea) as influenced by lime

This study describes the soil solution chemistry in the rhizosphere of fine roots of sessile oak ( Quercus petraea (M.) Liebl.) grown in rhizotrons. A control was compared with soils treated with an equivalent CaCO₃ of 1.4 t ha^-1 CaO. Solution samples were extracted from the B-horizon using micro suction cups with a suction of _∼40 kPa. Two series of experiments were carried out: one irrigated with rain water (age of seedling 2 to 4 months) and one irrigated with demineralized water (age of seedlings 1.5 to 2 months). Half of the sampling points were choosen close to the roots and half in the bulk soil. In both experiments there was generally no rhizospheric gradient after liming. In contrast, in the control, depletion in the rhizosphere occurred for most of the ions studied (Mg, Ca, Al, K, NO₃ ^-, NH₄ ⁺, Cl^-) in the demineralized water experiment, but this was different when rainwater was used. The latter effect is probably due to the higher solution concentrations in the rainwater experiment but could also be a result of root damage due to low Ca/Al ratios in the rhizosphere solution. It was concluded, that liming improved the chemical composition in the rhizosphere soil solution by increasing overall solute concentration to levels enabling sufficient and easier nutrient uptake by roots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Impact of common European tree species and Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) on the physicochemical properties of the rhizosphere

Trees play a crucial role in nutrient cycling and ecosystem fertility, notably through rhizosphere processes. The aim of this study was to compare soil physicochemical properties between bulk soil and rhizosphere of several tree species, and to compare rhizosphere properties between fertilized and non-fertilized conditions. The soil sampling was performed in Breuil-Chenue forest (North-East of France) in seven stands: native forest (old beech (Fagus sylvatica L.) and oak (Quercus sessiliflora Smith) coppice with standards; CwS), beech, oak (Quercus petraea [Matt.] Liebl.), Douglas-fir and fertilised Douglas-fir, Norway spruce (Picea abies Karst.) and fertilised Norway spruce. Systematic soil sampling was performed at 0–3, 3–10, and 10–23 cm in 20 calibrated pits. The rhizosphere of the different species was generally enriched in C, N, Ca, Mg, and K. Interestingly, the same positive effect was observed in the fertilised plots. The rhizosphere effect varied between tree species for C, “base” cations, pH_water and cation exchange capacity. This study reveals that interactions between roots, microorganisms and soil can enrich the pool of nutrients in the rhizosphere compared to bulk soil whatever the soil fertility conditions, and that the magnitude of the rhizosphere effect depends on tree species.     

Fine-root morphology and uptake of 32P and 35S in a Norway spruce (Picea abies (L.) Karst.) stand subjected to various nutrient and water supplies

An investigation of fine (< 1 mm in diameter) and small (1–2 mm in diameter) roots in the organic soil layer was carried out in a Norway spruce forest stand with different treatments of water and nutrients, including control (C); ammonium sulphate application (NS); nitrogen-free fertilization (V); irrigation with liquid fertilization (a complete nutrient solution) (IF); NS followed by artificial drought (ND); V followed by artificial drought (VD). In order to evaluate the vitality and function of the fine roots, the following approaches were used: i) classification of fine roots, based on morphological characteristics; ii) nutrient uptake bioassay, using ³²P-phosphate and ³⁵S-sulphate; iii) nutrient concentration in fine roots and its relation to nutrient uptake. The NS treatment showed effects on the fine and small roots, with a decrease in amount of living roots, and a decrease in the total amount of fine and small roots. The VD treatment resulted in increased amounts of living small roots, while the ND treatment showed the opposite, as compared with the V and NS treatments, respectively. The uptake of P was negatively related to the P supply, with a higher P uptake for C and NS fine roots than for IF and V fine roots. The specific root length (SRL, m g^-1 DW) decreased for NS fine roots and increased for IF fine roots, indicating a further increase in uptake for NS roots and a decreased uptake for IF roots if calculated on a root length basis. So far, the NS and IF treatments maintain a considerable increase in above-ground biomass with a significantly reduced root biomass and standing crop.     

Biomass, morphology and nutrient contents of fine roots in four Norway spruce stands

Fine root systems may respond to soil chemical conditions, but contrasting results have been obtained from field studies in non-manipulated forests with distinct soil chemical properties. We investigated biomass, necromass, live/dead ratios, morphology and nutrient concentrations of fine roots (<2 mm) in four mature Norway spruce (Picea abies [L.] Karst.) stands of south-east Germany, encompassing variations in soil chemical properties and climate. All stands were established on acidic soils (pH (CaCl₂) range 2.8–3.8 in the humus layer), two of the four stands had molar ratios in soil solution below 1 and one of the four stands had received a liming treatment 22 years before the study. Soil cores down to 40 cm mineral soil depth were taken in autumn and separated into four fractions: humus layer, 0–10 cm, 10–20 cm and 20–40 cm. We found no indications of negative effects of N availability on fine root properties despite large variations in inorganic N seepage fluxes (4–34 kg N ha⁻¹ yr⁻¹), suggesting that the variation in N deposition between 17 and 26 kg N ha⁻¹ yr⁻¹ does not affect the fine root system of Norway spruce. Fine root biomass was largest in the humus layer and increased with the amount of organic matter stored in the humus layer, indicating that the vertical pattern of fine roots is largely affected by the thickness of this horizon. Only two stands showed significant differences in fine root biomass of the mineral soil which can be explained by differences in soil chemical conditions. The stand with the lowest total biomass had the lowest Ca/Al ratio of 0.1 in seepage, however, Al, Ca, Mg and K concentrations of fine roots were not different among the stands. The Ca/Al ratio in seepage might be a less reliable stress parameter because another stand also had Ca/Al ratios in seepage far below the critical value of 1.0 without any signs of fine root damages. Large differences in the live/dead ratio were positively correlated with the Mn concentration of live fine roots from the mineral soil. This relationship was attributed to faster decay of dead fine roots because Mn is known as an essential element of lignin degrading enzymes. It is questionable if the live/dead ratio can be used as a vitality parameter of fine roots since both longevity of fine roots and decay of root litter may affect this parameter. Morphological properties were different in the humus layer of one stand that was limed in 1983, indicating that a single lime dose of 3–4 Mg ha⁻¹ has a long-lasting effect on fine root architecture of Norway spruce. Almost no differences were found in morphological properties in the mineral soil among the stands, but vertical patterns were apparently different. Two stands with high base saturation in the subsoil showed a vertical decrease in specific root length and specific root tip density whereas the other two stands showed an opposite pattern or no effect. Our results suggest that proliferation of fine roots increased with decreasing base saturation in the subsoil of Norway spruce stands.     

Root elongation of seedling peas through layered soil of different penetration resistances

Field soils contain localized zones of larger penetration resistance within peds and compacted layers, while cracks and biopores offer low resistance pathways to roots. Root responses to such localized conditions have not been investigated in detail. This study examined what happens to the root elongation rate when roots grew through a layer of hard soil into a layer of looser soil for a 4 day period. The experiment was performed twice; firstly with the shoot in continuous darkness, and secondly with it exposed to a day-night cycle to prevent etiolation of the shoot. Pea seedlings were grown in columns of a sandy loam soil which was packed to bulk densities of 0.85, 1.1, 1.3 or 1.4 Mg/m³ in the top layer and 0.85 Mg/m³ in the bottom layer. The root elongation rate in the top layer of 1.4 Mg/m³ soil (penetrometer resistance=1.8 MPa) was only 55% of the elongation rate in the top layer of 0.85 Mg/m³ soil (penetrometer resistance=0.06 MPa). The elongation rate of roots that had grown through the top layer of 1.4 Mg/m³ soil into the bottom layer of loose soil was reduced by some residual effect of the mechanical impedance. The root elongation rate in the bottom layer of loose soil decreased as the penetrometer resistance of the top layer of soil increased. The daily elongation rate of the roots in the bottom layer that had grown through the 1.4 Mg/m³ soil averaged only about 65% of the elongation rate of the roots that had grown through the 0.85 Mg/m³ soil. This residual effect of mechanical impedance on root elongation persisted for at least 2 days and was more severe in the day-night cycle experiment than in the dark experiment. These results have important implications for modelling root elongation in any soil in which the soil strength changes with distance or with time.     

Fertilization effects on fineroot biomass, rhizosphere microbes and respiratory fluxes in hardwood forest soils

Fertilizer-induced reductions in CO(2) flux from soil ((F)CO(2)) in forests have previously been attributed to decreased carbon allocation to roots, and decreased decomposition as a result of nitrogen suppression of fungal activity. Here, we present evidence that decreased microbial respiration in the rhizosphere may also contribute to (F)CO(2) reductions in fertilized forest soils. Fertilization reduced (F)CO(2) by 16-19% in 65-yr-old plantations of northern red oak (Quercus rubra) and sugar maple (Acer saccharum), and in a natural 85-yr-old yellow birch (Betula allegheniensis) stand. In oak plots, fertilization had no effects on fine root biomass but reduced mycorrhizal colonization by 18% and microbial respiration by 43%. In maple plots, fertilization reduced root biomass, mycorrhizal colonization and microbial respiration by 22, 16 and 46%, respectively. In birch plots, fertilization reduced microbial respiration by 36%, but had variable effects on root biomass and mycorrhizal colonization. In plots of all three species, fertilization effects on microbial respiration were greater in rhizosphere than in bulk soil, possibly as a result of decreased rhizosphere carbon flux from these species in fertile soils. Because rhizosphere processes may influence nutrient availability and carbon storage in forest ecosystems, future research is needed to better quantify rhizo-microbial contributions to (F)CO(2).     

Soil strength and water content influences on corn root distribution in a sandy soil

Initial field observations revealed a shallow corn (Zea mays L.) root system on a Zimmerman fine sand in a corn/soybean (Glycine max L.) rotation. Since root distribution influences crop water and nutrient absorption, it is essential to identify factors limiting root growth. The objective of this study was to determine the factor(s) limiting corn rooting depth on an irrigated fine sand soil. Bulk density, saturated hydraulic conductivity, and soil water retention were measured on undisturbed soil cores. Corn root distribution assessed at tasseling over a 3-yr period showed an average of 94% of total root length within the upper 0.60 m of soil with 85% in the upper 0.30 m of soil. Mechanical impedance was estimated with a cone penetrometer on two dates with differing water contents. Cone penetrometer measurements greater than 3 MPa indicated mechanical impedance in soil layers extending from 0.15 to 0.35 m deep. Penetration resistance decreased as soil water content increased. However, soil water contents greater than field capacity were required to decrease penetration resistance below the 3 MPa threshold. Such water saturated conditions only occurred for short periods immediately after precipitation or irrigation events, thus roots usually encountered restrictive soil strengths. The soil layer from 0.15 to 0.60 m had high bulk density, 1.57 Mg m^-3. This compacted soil layer, with slower saturated hydraulic conductivities (121 to 138 mm hr-1), held more water than the soil above or below it and reduced water movement through the soil profile. Crop water use occurred to a depth of approximately 0.75 m. In conclusion, a compacted soil layer confined roots almost entirely to the top 0.60 m of soil because it had high soil strength and bulk density. The compacted layer, in turn, retained more water for crop use.     

Depletion of macro-nutrients from rhizosphere soil solution by juvenile corn, cottonwood, and switchgrass plants

In situ sampling of rhizosphere solution chemistry is an important step in improving our understanding of soil solution nutrient dynamics. Improved understanding will enhance our ability to model nutrient dynamics and on a broader scale, to develop effective buffers to minimize nutrient movement to surface waters. However, only limited attention has been focused on the spatial heterogeneity and temporal dynamics of rhizosphere solution, and still less is known about how rhizosphere solution chemistry varies among plant species. Nutrients in rhizosphere soil solution and changes in root morphology of juvenile corn (Zea mays L. cv. Stine 2250), cottonwood (Populus deltoids L.), and switchgrass (Panicum virgatum L.) were monitored using mini-rhizotron technology. Plants were grown for 10 days in a fine-silty, mixed, superactive, mesic Cumulic Hapludoll (Kennebec series). Micro-samples (100–200 μL) of rhizosphere and bulk soil solution were collected at 24-h intervals at a tension of ?100 kPa and analyzed for P, K, Ca, and Mg concentration using Capillary Electrophoresis techniques. Plants were harvested at the end of the 10-day period, and tissue digests analyzed for nutrient content by Inductively Coupled Plasma Spectroscopy. Corn plants produced roots that were 1.3 times longer than those of cottonwood, and 11.7 times longer than those of switchgrass. Similar trends were observed in number of root tips and root surface area. At the end of 10 days, rhizosphere solution P and K concentrations in the immediate vicinity of the roots (<1 mm) decreased by approximating 24 and 8% for corn, and 15 and 5% for cottonwood. A rhizosphere effect was not found for switchgrass. After correction for initial plant nutrient content, corn shoot P, K, and Mg were respectively 385, 132, and 163% higher than cottonwood and 66, 37, and 10% higher than switchgrass. Cottonwood shoot Ca concentration, however, was 68 to 133% higher than that of corn or switchgrass. There was no difference in root P concentration among the three species. Nutrient accumulation efficiency (μg nutrient mm^?1 root length) of cottonwood was 26 to 242% higher for P, 25 to 325% higher for Ca, and 41 to 253% higher for Mg than those of corn and switchgrass. However, K accumulation efficiency of corn was four to five times higher than that of the cottonwood and switchgrass. Nutrient utilization efficiency (mg of dry weight produced per mg nutrient uptake) of P, K, and Mg was higher in cottonwood than in corn and switchgrass. These differences are element-specific and depend on root production and morphology as well as plant nutrient status. From a practical perspective, the results of this study indicate that potentially significant differences in rhizosphere solution chemistry can develop quickly. Results also indicate that cottonwood would be an effective species to slow the loss of nutrients in buffer settings.     

Soil fertility and fine root dynamics in response to 4 years of nutrient (N,P, K) fertilization in a lowland tropical moist forest,Panama

The question of how tropical trees cope with infertile soils has been challenging to address, in part, because fine root dynamics must be studied in situ. We used annual fertilization with nitrogen (N as urea, 12.5 g N m^?2 year^?1), phosphorus (P as superphosphate, 5 g P m^?2 year^?1) and potassium (K as KCl, 5 g K m^?2 year^?1) within 38 ha of old‐growth lowland tropical moist forest in Panama and examined fine root dynamics with minirhizotron images. We expected that added P, above all, would (i) decrease fine root biomass but, (ii) have no impact on fine root turnover. Soil in the study area was moderately acidic (pH = 5.28), had moderate concentrations of exchangeable base cations (13.4 cmol kg^?1), low concentrations of Bray‐extractable phosphate (PO₄ = 2.2 mg kg^?1), and modest concentrations of KCl‐extractable nitrate (NO₃ = 5.0 mg kg^?1) and KCl‐extractable ammonium (NH₄ = 15.5 mg kg^?1). Added N increased concentrations of KCl‐extractable NO₃ and acidified the soil by one pH unit. Added P increased concentrations of Bray‐extractable PO₄ and P in the labile fraction. Concentrations of exchangeable K were elevated in K addition plots but reduced by N additions. Fine root dynamics responded to added K rather than added P. After 2 years, added K decreased fine root biomass from 330 to 275 g m^?2. The turnover coefficient of fine roots <1 mm diameter ranged from 2.6 to 4.4 per year, and the largest values occurred in plots with added K. This study supported the view that biomass and dynamics of fine roots respond to soil nutrient availability in species‐rich, lowland tropical moist forest. However, K rather than P elicited root responses. Fine roots smaller than 1 mm have a short lifetime (<140 days), and control of fine root production by nutrient availability in tropical forests deserves more study.     

What controls variation in carbon use efficiency among Amazonian tropical forests?

Why do some forests produce biomass more efficiently than others? Variations in Carbon Use Efficiency (CUE: total Net Primary Production (NPP)/ Gross Primary Production (GPP)) may be due to changes in wood residence time (Biomass/NPP_wood), temperature, or soil nutrient status. We tested these hypotheses in 14, one ha plots across Amazonian and Andean forests where we measured most key components of net primary production (NPP: wood, fine roots, and leaves) and autotrophic respiration (R_a; wood, rhizosphere, and leaf respiration). We found that lower fertility sites were less efficient at producing biomass and had higher rhizosphere respiration, indicating increased carbon allocation to belowground components. We then compared wood respiration to wood growth and rhizosphere respiration to fine root growth and found that forests with residence times <40 yrs had significantly lower maintenance respiration for both wood and fine roots than forests with residence times >40 yrs. A comparison of rhizosphere respiration to fine root growth showed that rhizosphere growth respiration was significantly greater at low fertility sites. Overall, we found that Amazonian forests produce biomass less efficiently in stands with residence times >40 yrs and in stands with lower fertility, but changes to long‐term mean annual temperatures do not impact CUE.