Theoretical evidence that root penetration ability interacts with soil compaction regimes to affect nitrate capture

Background and AimsAlthough root penetration of strong soils has been intensively studied at the scale of individual root axes, interactions between soil physical properties and soil foraging by whole plants are less clear. Here we investigate how variation in the penetration ability of distinct root classes and bulk density profiles common to real-world soils interact to affect soil foraging strategies.MethodsWe utilize the functional–structural plant model ‘OpenSimRoot’ to simulate the growth of maize (Zea mays) root systems with variable penetration ability of axial and lateral roots in soils with (1) uniform bulk density, (2) plow pans and (3) increasing bulk density with depth. We also modify the availability and leaching of nitrate to uncover reciprocal interactions between these factors and the capture of mobile resources.Key ResultsSoils with plow pans and bulk density gradients affected overall size, distribution and carbon costs of the root system. Soils with high bulk density at depth impeded rooting depth and reduced leaching of nitrate, thereby improving the coincidence of nitrogen and root length. While increasing penetration ability of either axial or lateral root classes produced root systems of comparable net length, improved penetration of axial roots increased allocation of root length in deeper soil, thereby amplifying N acquisition and shoot biomass. Although enhanced penetration ability of both root classes was associated with greater root system carbon costs, the benefit to plant fitness from improved soil exploration and resource capture offset these.ConclusionsWhile lateral roots comprise the bulk of root length, axial roots function as a scaffold determining the distribution of these laterals. In soils with high soil strength and leaching, root systems with enhanced penetration ability of axial roots have greater distribution of root length at depth, thereby improving capture of mobile resources.     

Spring ephemeral herbs and nitrogen cycling in a northern hardwood forest: an experimental test of the vernal dam hypothesis

In the late 1970s R.N. Muller and F.H. Bormann posited their ”vernal dam” hypothesis, stating that spring-ephemeral herbs in deciduous forests serve as a temporary sink for N when overstory trees are dormant, and then release this N later, in the summer, when the trees are active. This hypothesis has gained wide acceptance, yet two of its critical assumptions have never been experimentally tested: (1) that N taken up by spring ephemerals would otherwise be lost from the ecosystem, and (2) that N from senesced ephemeral tissues contributes to increased rates of summertime N mineralization. To test these assumptions, I quantified patterns of N cycling and loss from a set of paired plots, half of which served as controls and from half of which all spring-ephemeral plants were removed. There were no significant differences in NO₃ ^– leaching between plots with and without spring ephemeral vegetation. These results are consistent with the relatively low rates of N uptake by the dominant spring ephemeral, Allium tricoccum, and its apparent preference for NH₄ ⁺, which is far less mobile in soil than NO₃ ^–. In addition, based on sequential sampling, I found that soil microorganisms took up 8 times as much N during the spring than did spring-ephemeral herbs (microbial uptake=3.19 vs. plant uptake=0.41 g N m^–2), suggesting that microbial immobilization of N is the dominant sink for N during this season. Removal of spring ephemeral vegetation also had no effect on summertime rates of net N mineralization. Furthermore, the addition of spring ephemeral litter to soil+forest floor microcosms did not significantly increase rates of N mineralization in a laboratory incubation. Instead, this experiment demonstrated the overwhelming influence of forest floor litter in controlling the release of mineral N from these soils. Overall, neither assumption of the vernal dam hypothesis holds true in this ecosystem, where patterns of N cycling and loss appear to be dominated by microbial decomposition of forest floor material and soil organic matter. Received: 24 August 1999 / Accepted: 23 March 2000     

Buffering capacity through cation leaching of Pinus radiata D. Don canopy

The change in the composition of atmospheric deposition as it passes through the forest canopy on two 10-year-old Pinus radiata D. Don forests (Manzanal and Posadero) was studied in the Basque Country, analysing the concentration of different constituents in bulk precipitation and throughfall. The precipitation at the study sites was bimodal with two maxima in the year; one in late spring and another one in late autumn. Posadero had a mean annual precipitation of 1223.6 mm with an interception loss of 27.5% by the forest canopy and Manzanal had a mean annual precipitation of 978.6 mm with the interception loss being 22.2% of it. Constituent concentrations followed a similar seasonal variation, increasing during the summer when precipitation decreases. At both study sites the chemical species analysed in bulk precipitation and throughfall were characterised by the fact that they came from three distinct sources: acidic pollution, marine and terrestrial origin. Concentration of constituents in bulk precipitation in Manzanal was higher than in Posadero, most probably due to the smaller amount of precipitation that falls in this study site. The precipitation at Manzanal had a significantly lower pH than at Posadero. The amounts of sulphate, nitrate-nitrogen, organic nitrogen and protons that fell in the bulk precipitation at Manzanal (the polluted site) were higher than those that fell at Posadero (less polluted site). The concentration of organic nitrogen in the bulk precipitation of the polluted site was significantly related to the hydrocarbon concentration measured in the atmosphere in the nearby town of Muskiz. Throughfall in Manzanal had higher amounts of sulphate, nitrate-nitrogen, calcium, magnesium, sodium, potassium and chloride than in Posadero. This fact suggests that both dry deposition and canopy leaching were an important source of throughfall constituents in Manzanal. The amount of manganese measured in Posadero throughfall was higher than that found in Manzanal throughfall. The pH in the throughfall did not show any significant difference between sites and was significantly higher than in bulk precipitation. Thus, canopies in the study sites seem to be able to neutralise very efficiently the acidic load of bulk deposition. Despite this buffering capacity of the canopies, the soil at Manzanal appeared to be more acidic than at Posadero, probably due to the liberation of protons in the rhizosphere when the neutralising pacity of the canopy is `recharged'. This soil acidification may be leading to a greater solubilization of aluminium in the polluted site which could suffer from cation nutrient deficiencies in the future.     

溶液pH值及模拟酸雨对两种磺酰脲类除草剂在土壤中行为的影响

采用平衡振荡法和土柱淋洗法,研究了溶液pH及模拟酸雨对土壤中苄嘧磺隆和甲磺隆行为的影响.结果表明,Freundlich方程能较好地描述苄嘧磺隆和甲磺隆的吸附等温线,水-土壤系统pH升高能明显地降低这两种除草剂在土壤中的吸附,促进其在土壤中的迁移,且吸附常数(K_f)与土壤有机质含量、粘土含量呈正相关,而与土壤pH呈负相关.pH值高的模拟酸雨对除草剂在土壤中淋溶贡献较大,且淋溶量随雨量的增大而增大.除草剂在土壤中的淋溶与土壤性质密切相关,有机质含量和粘粒含量较高的土壤对除草剂的持留能力较强.     

Nitrate leaching from three afforestation chronosequences on former arable land in Denmark

In regions dominated by agricultural activities, nitrogen (N) is recognized as a major pollutant in aquatic environments. In north‐western Europe, afforestation of agricultural land is part of a strategy to improve water quality. In Denmark, former arable land has been afforested during the past 40–50 years. This study evaluated the effect of afforestation of former arable land on nitrate leaching, based on three afforestation chronosequences. Precipitation, canopy throughfall and soil water were collected and soil moisture was monitored at two Danish locations, Vestskoven (nutrient‐rich, medium deposition) and Gejlvang (nutrient‐poor, high deposition). Afforestation was performed using Norway spruce [Picea abies (Karst.) L.] and common oak (Quercus robur L.) at Vestskoven and Norway spruce at Gejlvang. The results suggest that afforestation of former arable land initially leads to lower nitrate leaching than that occurring under the former agricultural land use, and largely below the standard of 50 mg NO⁻₃ L⁻¹ for groundwater to be utilized as drinking water. Nitrate concentrations became almost negligible in forest stands of 5–20 years of age. However, after canopy closure (>20 years) nitrate concentrations below the root zone and nitrate leaching tended to increase. This was attributed to increased N deposition with increasing canopy development and decreased N demand once the most N‐rich biomass compartments had been built up. Nitrate leaching started to increase at a throughfall deposition level of about 10 kg N ha⁻¹ yr⁻¹. Compared with nutrient‐poor sandy soils, nutrient‐rich clayey soils appeared more vulnerable to disturbance of the N cycle and to increased N deposition, leading to N saturation and enhanced nitrate leaching. In approximately the first 35 years after afforestation, nitrate leaching below the root zone was generally higher below oak than below Norway spruce.     

Nitrification and mineralization potentials in a limed Ultisol in the humid tropics

Summary We studied the effect of liming on the rates of mineralization and nitrification in a coarse-textured kaolinitic Ultisol. Soil samples were taken from field plots which received lime rates from 0 to 4mt/ha three years prior to the study. The pH of the soil samples varied from 4.2 to 6.1. Ammonification of soil organic N and added urea source proceeded readily and was not affected by lime rate. Nitrification occurred in both limed and unlimed soils but the rate of nitrification depended upon the rate of lime application. Soil pH, exchangeable Ca and exchangeable A1 were significantly correlated with the amount of NO₃-N accumulated at the end of the 65 days incubation period. Nitrification of NH₄-N from ammonium sulfate was absent in soils receiving lower rates of lime which gave pH values ranging from 4.2 to 4.8. Added ammonium source was nitrified readily after a 3-week delay period in the soil (pH 6.1) which received a higher rate of lime (4 mt/ha).     

Loading of assimilates in wheat leaves. II. The path from chloroplast to vein

Abstract. An improved method is described for micro-autoradiography of water-soluble substances alter freeze-substitution of plant tissue in which water is rigorously excluded. Resin sections are cut and flattened dry, and dry photographic emulsion is mounted on them. When the location of ¹⁴C in wheat leaves after assimilation of ¹⁴CO₂ was studied with this method, it was found that ¹⁴C entered the intermediate veins before the laterals and entered both types of veins along the flanks of the veins adjacent to the phloem. High concentrations of ¹⁴C were found in small spaces in the cells of mesophyll and vein parenchyma; these spaces coincide with the nuclei. The concentration of ¹⁴C in these nucleus-associated spaces was as high as that reached at later times in the sieve tubes. Water washed the ¹⁴C out of these spaces of the sections and the label in the washings was predominantly in sucrose. The high ¹⁴C concentrations of the nucleus-associated spaces were particularly easily leached. It is concluded that the raising of the sugar concentration to the high levels found in sieve tubes can take place in these leaves in a special space in each mesophyll cell not, or not only, at the boundary of the sieve tubes.     

Fluxes of elements in rain passing through forest canopies in south-eastern Australia

The elemental content of rainfall (bulk deposition), throughfall and stemflow was measured inPinus radiata D. Don andEucalyptus forests in Gippsland, Victoria. Accessions in rainfall (mg m^–2 year^–1) averaged: organic-C 551, NO₃ ^–-N 96, NH₄ ⁺-N 62, total-N 303, K⁺ 382, Na⁺ 2250, Ca²⁺ 1170, and Mg²⁺ 678. The mean pH of rainfall was 5.9. Concentrations of all elements were greater in throughfall than in rainfall, and generally greater in stemflow than in throughfall. However, pH of pine throughfall was higher than that of rainfall, and pH of eucalypt throughfall was lower than that of rainfall. There was a net efflux of inorganic-N from pine crowns to rainfall, whilst in eucalypts there was generally net sorption of inorganic-N from rainfall. In both species organic-N was leached from the crowns and the net efflux of total-N from eucalypt crowns (50 mg m^–2 year^–1) averaged one-quarter of that in pines. Increases in the organic-C content of throughfall relative to rainfall in eucalypts were two to four times those in pines. Increases in the content of other elements in throughfall were comparable in pines and eucalypts and within the ranges K⁺ 615–1360, Na⁺ 480–-1840, Ca²⁺ 123–780 and Mg²⁺ 253–993 mg m^–2 year^–1. However, enrichment of Ca²⁺ may have been due to dust trapped in the canopies. Stemflow contributed significantly to the total amounts of elements reaching the forest floor in water.     

Stemflow nutrient inputs to soil in a successional hardwood forest

Stemflow and throughfall from a regenerating (8-year-old) southern Appalachian hardwood forest were collected to examine the relative importance of tree bole nutrient leaching in response to acid deposition. Samples from nine (2 m²) stemflow collection plots were analyzed for four dormant season and 11 growing season rainstorm events. Results showed that, relative to throughfall fluxes, stemflow accounted, on average, for approximately 8.5% of total water reaching the forest floor during both dormant and growing season storms. Relative to foliar leaching, K-, SO₄-, and PO₄ ions appear to be the most easily leached ions from young tree stems. Proportional nitrate and base cation stemflow fluxes increased significantly (p<0.05) with growing-season storm-event duration, suggesting that the stemsurface nutrient pool is depleted by precipitation more slowly than the foliar pool. On average, proportional stemflow fluxes of SO₄ (12%) and K (14%) were consistently higher than reported maximum values for more mature forest stands, which indicates that small-scale stemflow inputs of ions such as these to the forest floor may be important in early successional ecosystems.