Soil Nitrogen Conditions Approach Preinvasion Levels following Restoration of Nitrogen-Fixing Black Locust (Robinia pseudoacacia) Stands in a Pine–Oak Ecosystem

Restoring native plant communities on sites formerly occupied by invasive nitrogen‐fixing species poses unique problems due to elevated soil nitrogen availability. Mitigation practices that reduce available nitrogen may ameliorate this problem. We evaluated the effects of tree removal followed by soil preparation or mulching on native plant growth and soil nitrogen transformations in a pine–oak system formerly occupied by exotic nitrogen‐fixing Black locust (Robinia pseudoacacia) trees. Greenhouse growth experiments with native grasses, Andropogon gerardii and Sorghastrum nutans, showed elevated relative growth rates in soils from Black locust compared with pine–oak stands. Field soil nutrient concentrations and rates of net nitrification and total net N‐mineralization were compared 2 and 4 years since Black locust removal and in control sites. Although soil nitrogen concentrations and total net N‐mineralization rates in the restored sites were reduced to levels that were similar to paired pine–oak stands after only 2 years, net nitrification rates remained 3–34 times higher in the restored sites. Other nutrient ion concentrations (Ca, Mg) and organic matter content were reduced, whereas phosphorus levels remained elevated in restored sites. Thus, 2–4 years following Black locust tree removal and soil horizon mixing achieved through site preparation, the concentrations of many soil nutrients returned to preinvasion levels. However, net nitrification rates remained elevated; cover cropping or carbon addition during restoration of sites invaded by nitrogen fixers could increase nitrogen immobilization and/or reduce nitrate availability, making sites more amenable to native plant establishment.     

Restoration of a Native Grassland as Habitat for the Golden Sun Moth Synemon plana Walker (Lepidoptera; Castniidae) at Mount Piper, Australia

Synemon plana (golden sun moth) is a small diurnal moth from the family Castniidae, now listed as endangered in all regions of southeastern Australia in which it occurs. This study details an attempt to restore the native grassland habitat of S. plana at one site by increasing the cover of its probable food plant, Austrodanthonia eriantha (wallaby grass), to at least 40%, the average percentage cover determined from all currently inhabited sites. The percentage cover of A. eriantha increased when seedlings were planted, although there was no significant increase in cover by seeding caryopsides (dispersal units) or by weeding. However, combining weeding with planting seedlings, or seeding increased cover by 22% and 27% to 43% and 64%, respectively. Two hundred caryopsides per square meter (compared with only 49 seedlings/m²) were required to increase cover to at least 40% due to the low viability of seeds and low germination in the field. Therefore, to restore grasslands dominated by A. eriantha as suitable habitat for S. plana, seedlings should be planted and weeds removed.     

Applications of Citric Acid Industrial Wastewater and Phosphonates for Soil Remediation: Effects on Temporal Change of Cadmium Distribution

Citric acid industrial wastewater (CAIW) and phosphonates are potentially useful for enhancing remediation of metal-contaminated soil. This study aimed to investigate the effects of these enhancement agents on temporal change of Cd distribution in a clayey soil, which probably correlates with the metal lability and bioavailability in the environment. Sequential extractions were performed on the soil samples from batch desorption experiments between 5 and 4320 min. The CAIW primarily enhanced Cd extraction from weakly bound fractions (i.e., exchangeable and carbonate fractions), which was relatively fast and took place in the first 60 min. On the other hand, (nitrilotrimethylene)-triphosphonic acid (NTMP) and ethylene diaminetetra-methylenephosphonic acid (EDTMP) provided a more significant Cd extraction from exchangeable, carbonate, and oxide fractions, with the change of Cd distribution occurring over the first 300 min. The differences in the extent and temporal change of metal extraction reflected the weaker complexation strength of CAIW compared to that of NTMP and EDTMP. Moreover, an increase of pH from 4 to 10 shifted the remaining Cd from exchangeable fraction to carbonate and oxide fractions. These distribution changes indicated that both CAIW and phosphonates were efficient at removing weakly bound Cd fractions in a short period of time, but alkaline conditions could hinder their effectiveness.     

Are Sulfurous Soil Amendments (S0, Fe(II)SO4, Fe(III)SO4) an Effective Tool in the Restoration of Heathland and Acidic Grassland after Four Decades of Rock Phosphate Fertilization?

This paper deals with the complex issue of reversing long‐term improvements of fertility in soils derived from heathlands and acidic grasslands using sulfur‐based amendments. The experiment was conducted on a former heathland and acid grassland in the U.K. that was heavily fertilized and limed with rock phosphate, chalk, and marl. The experimental work had three aims. First, to determine whether sulfurous soil amendments are able to lower pH to a level suitable for heathland and acidic grassland re‐creation (approximately 3 pH units). Second, to determine what effect the soil amendments have on the available pool of some basic cations and some potentially toxic acidic cations that may affect the plant community. Third, to determine whether the addition of Fe to the soil system would sequester PO₄^? ions that might be liberated from rock phosphate by the experimental treatments. The application of S⁰ and Fe^(II)SO₄^? to the soil was able to reduce pH. However, only the highest S⁰ treatment (2,000 kg/ha S) lowered pH sufficiently for heathland restoration purposes but effectively so. Where pH was lowered, basic cations were lost from the exchangeable pool and replaced by acidic cations. Where Fe was added to the soil, there was no evidence of PO₄^? sequestration from soil test data (Olsen P), but sequestration was apparent because of lower foliar P in the grass sward. The ability of the forb Rumex acetosella to apparently detoxify Al³⁺, prevalent in acidified soils, appeared to give it a competitive advantage over other less tolerant species. We would anticipate further changes in plant community structure through time, driven by Al³⁺ toxicity, leading to the competitive exclusion of less tolerant species. This, we suggest, is a key abiotic driver in the restoration of biotic (acidic plant) communities.     

Phytotoxicity of Citric Acid and Tween® 80 for Potential Use as Soil Amendments in Enhanced Phytoremediation

Enhanced phytoremediation adding biodegradable amendments like low molecular weight organic acids and surfactants is an interesting area of current research to overcome the limitation that represents low bioavailability of pollutants in soils. However, prior to their use in assisted phytoremediation, it is necessary to test if amendments per se exert any toxic effect to plants and to optimize their application mode. In this context, the present study assessed the effects of citric acid and Tween® 80 (polyethylene glycol sorbitan monooleate) on the development of alfalfa (Medicago sativa) plants, as influenced by their concentration and frequency of application, in order to evaluate the feasibility for their future use in enhanced phytoremediation of multi-contaminated soils. The results showed that citric acid negatively affected plant germination, while it did not have any significant effect on biomass or chlorophyll content. In turn, Tween® 80 did not affect plant germination and showed a trend to increase biomass, as well as it did not have any significant effect on chlorophyll levels. M. sativa appeared to tolerate citric acid and Tween® 80 at the tested concentrations, applied weekly. Consequently, citric acid and Tween® 80 could potentially be utilized to assist phytoremediation of contaminated soils vegetated with M. sativa.