May Balabane

Relationship of soil organic matter dynamics to physical protection and tillage

Tillage has been reported to reduce organic matter concentrations and increase organic matter turnover rates to a variable extent. The change of soil climate and the incorporation of aboveground C inputs within the soil lead to no unique effect on biodegradation rates, because of their strong interaction with the regional climate and the soil physical properties. The periodical perturbation of soil structure by tools and the subsequent drying‐rewetting cycles may be the major factor increasing organic matter decomposition rates by exposing the organic matter that is physically protected in microaggregates to biodegradation. This paper reviews the assessed effects of tillage on organic matter, the scale, extent and mechanisms of physical protection of organic matter in soils. # 2000 Elsevier Science B.V. All rights reserved.

Strategies of heavy metal uptake by three plant species growing near a metal smelter

Some higher plant species have developed heavy metal tolerance strategies which enable them to survive and reproduce in highly metal-contaminated soils. We have investigated such heavy metal uptake and accumulation strategies of two absolute metallophyte species (Armeria maritima ssp. halleri and Cardaminopsis halleri) and one pseudometallophyte (Agrostis tenuis) growing near a former metal smelter. Samples of plant parts and soil were analysed for Zn, Cd, Pb, and Cu. In soil, there were two dominant types of metal concentration gradients with depth. Under the absolute metallophytes, extremely high metal contents were measured in the surficial Ah horizon, followed by a strong decrease in the underlying soil horizons (L(11) and L(12)). Under the pseudometallophyte, metal concentrations in the Ah horizon were much lower and fewer differences were observed in metal concentrations among the Ah, L(11), and L(12) horizons. The concentrations of Zn, Cd, Pb, and Cu in Agrostis tenuis roots were greater than concentrations in leaves, indicating significant metal immobilisation by the roots. For C. halleri, Zn and Cd concentrations in leaves were >20,000 and >100 mg kg(-1), respectively, indicating hyperaccumulation of these elements. Armeria maritima ssp. halleri exhibited root concentrations of Pb and Cu that were 20 and 88 times greater, respectively, than those in green leaves, suggesting an exclusion strategy by metal immobilisation in roots. However, Zn, Cd, Pb, and Cu concentrations in brown leaves of Armeria maritima ssp. halleri were 3-8 times greater than in green leaves, suggesting a second strategy, i.e. detoxification mechanism by leaf fall.

Maize root-derived soil organic carbon estimated by natural 15c abundance

The natural enrichment of 13C in maize plant material, compared to that of indigenous soil organic carbon, was used in the field to estimate the quantity of underground maize-derived C at harvest in October. Maize-derived C in the soil fractions coarser than 200 μm accounted for 12% of the C of the aboveground plant parts. Half of it was situated in the 20 cm wide × 20 cm deep soil portion situated beneath the plant. The soil fraction size-class 200–2000 μm contained 25% of the total and was the dominant size-class in the deeper layers. Because of the low resolution of the 13C tracer, only a 95% confidence interval could be obtained for the quantity of solid material finer than 200 μm: between 1 and 9% of the aerial parts of maize. Contribution of maize-derived C to soil organic matter in early August was not significantly different from that at the harvest date.

Mutual effects of soil organic matter dynamics and heavy metals fate in a metallophyte grassland

Abstract Metallophytes, plant species that grow only on soils rich in metals, are used for bioremediation of polluted soils. However, little is known about the functioning of such soil–plant systems. We have investigated soil organic matter (SOM) dynamics and its effects on the fate of heavy metals under a metallophyte grassland highly polluted by industrial dust fallout. Both litter and soil horizons were sampled. Particle-size and density fractionations were carried out to separate particulate organic matter (POM), i.e. the light fraction >50 μm. All samples were analysed for C, N, Zn, Pb, and Cd. Bulk densities were determined for all horizons and stocks of elements per unit area were calculated. Compared to broad uncontaminated temperate grasslands, SOM displayed similar quantities but differed significantly as to its quality and dynamics. The main differences were a lack of incorporation of plant returns in the soil profile and an imbalance of SOM composition towards more POM and less fine humified material. Zn, Pb, and, to a lesser extent, Cd were located mainly in the organic-rich, superficial soil layer. Heavy metal concentrations of POM of different sizes were similar within each horizon. Heavy metal concentrations of total POM increased strongly according to a depth–time scale. Our results suggest a selective decomposition of portions of metallophyte-derived debris with initially low heavy metal concentrations and resistance to biodegradation of those portions with initially high heavy metal concentrations. Such a mechanism may constitute a process of mutual protection, in this soil–plant system, of plant debris towards biodegradation and of heavy metals towards mobility.

Soil structure and characteristics of organic matter in two orchards differing in earthworm activity

By consuming plant remains and soil, earthworms incorporate organic matter (OM) into the soil and form biogenic soil structures, which can affect OM dynamics. We carried out a (micro)morphological study of soil structure development and OM distribution in two orchards (45 year) in a Dutch calcareous marine loam: RI − without, and KR + with high earthworm activity, the result of different levels of heavy metal contamination from fungicides. In both soils, sedimentary–stratification was absent to 60 cm depth and equal amounts of biogenic calcite spheroids were counted, suggesting similar earthworm activity in the past. In RI − the current vol.% of worm-worked groundmass in thin sections was 6% in the Ah and 7% in the Bw horizon compared with 51% in the Ah and 16% in the Bw horizon of KR + . Disappearance of earthworms with time in RI − gave rise to a compacted physicogenic soil structure with angular and prismatic aggregates and the absence of earthworm biopores. Due to restricted fragmentation and incorporation of OM fragments in casts, a litter layer formed at the soil surface. OM coatings were present in fissures and root pores of the Ah horizon, indicating the absence of mixing of organic and mineral soil materials. OM fragments were relatively coarse (>50m) and heterogeneously distributed through the Ah groundmass. Stronger decalcification in RI − than in KR + could be ascribed to higher production of organic acids in the litter layer of RI − and the absence of soil homogenisation by earthworms. In KR + earthworm activity was high, which has resulted in a biogenic structure with granular and subangular blocky aggregates and many worm casts and biopores. Particulate OM was relatively fine ( <50m) and encapsulated with clayey material in casts and micro-aggregates. The organic C content was not significantly ( P< 0.05) higher in the Ah horizon of KR + , than in the Ah horizon of RI − (15.7 and 13.7 g kg −1 , respectively). The lower C mineralization rates in KR + below 6 cm depth, however, might be an indication of higher microbial substrate-use efficiency or physical protection of OM against decomposition. The latter explanation would accord with the observed encapsulation of OM in micro-aggregates, and with studies on other management systems that favour biogenic aggregate formation. The quality of the soil macro- and microstructures, degree of soil compaction and decalcification and soil OM dynamics were strongly determined by the occurrence of earthworms in soils.

Input of fertilizer-derived labelled n to soil organic matter during a growing season of maize in the field

Abstract Fertilizer N was applied as 15N-labelled ammonium nitrate to a maize crop grown under field conditions in north-western France. After labelled-N was supplied (in May), plant and soil samples (to 80 cm depth) were collected at the 10-leaf stage (in June), flowering (in August) and harvest (in October). At harvest, applied N was recovered quantitatively in the plant and soil system (100 ± 6%): 71 ± 4% in above-ground plant parts, 26 ± 3% in the soil organic phase and 3 ± 3% as residual fertilizer in the soil. From mid-May to late-June, microbial immobilization accounted to a large extent for fertilizer N input to soil organic matter (20kgN ha−1). Recently-immobilized N in the topsoil (0–35 cm) was associated mainly with the clay particle-size fraction: 47, 32, 17 and 4% with fine clay, coarse clay, silt and fine sand fractions, respectively. From late-June to mid-August (when maize displays its maximum root growth rate) another 20kg ha−1 of fertilizer N were incorporated as soil organic N. From mid-August to the end of the growing season in October, no significant variation in the amount of fertilizer-derived organic N in the soil was recorded. Pathways of in situ input of fertilizer N to soil organic matter were approached by comparing soil organic N labelling with 13C natural labelling of the same soil samples by maize C. Particle-size fractions > 200 μm incorporated labelled-N mainly through maize underground biomass production. At harvest, 20% of the fertilizer-derived organic N present in the soil was located in these fractions as root material. Microbial immobilization of fertilizer N, associated with native C, contributed largely to the N-labelling of the 200μm were more enriched in recently-immobilized N than in total N, reflecting active associated soil organic matter.

Metal extraction by Arabidopsis halleri grown on an unpolluted soil amended with various metal-bearing solids: a pot experiment.

Most studies dealing with phytoremediation have considered metal extraction efficiency in relation to metal concentration of bulk soil samples or metal concentration of the soil solution. However, little is known about the effect of various metal-bearing solids on plant growth and metal extraction of hyperaccumulators. In this study, we investigated the ability of Arabidopsis halleri to grow and extract metals from different substrates consisting in an unpolluted soil amended with various metal-bearing solids collected in soils around a Zn smelter complex. The metal-bearing solids used as amendments were: fresh and decomposing organic residues in the soil, a soil clay fraction and two waste slags. Pure mono-metallic salt (ZnSO4) was also used. Two series of substrates were produced, one moderately polluted, and the other highly polluted. An additional substrate was formed by the unamended soil, and used as an unpolluted control. Zn, Cd, Cu, and Pb were measured in the substrates, and in the roots and shoots of A. halleri. The dry matter yield of A. halleri was shown not to depend on the nature of the metal-bearing solid used, except when Cu-toxicity was suspected. On highly-polluted substrates, Zn extraction by A. halleri depended on the nature of metal-bearing solids used, showing the following trend: pure mono-metallic salt > waste slags and soil clay fraction > fresh and decomposing organic matter. We explained these differences by the high solubility of Zn in the mono-metallic salt, whereas in the mineral metal-bearing solids and in both fresh and decomposing organic matter, Zn release required mineral weathering or organic matter mineralization, respectively. This work clearly showed that phytoremediation studies have to consider the nature of metal-bearing solids in contaminated soils to better predict the efficiency of plant extraction.