Soil carbon stocks in wetlands of New Zealand and impact of land conversion since European settlement

Freshwater wetlands provide a range of ecosystem services, one of which is climate regulation. They are known to contain large pools of carbon (C) that can be affected by land-use change. In New Zealand, only 10 % of the original freshwater wetlands remain due to conversion into agriculture. This study presents the first national estimation of C stocks in freshwater wetlands based on the compilation of soil carbon data from 126 sites across the country. We estimated C stocks for two soil sample types (mineral and organic) in different classes of wetlands (fen, bog, swamp, marsh, pakihi and ephemeral), and extrapolated C stocks to national level using GIS. Bogs had high C content and low bulk densities, while ephemeral wetlands were the reverse. A regression between bulk density and C content showed a high influence of the soil type. Average C densities (average ± standard error) were 1,348 ± 184 t C ha⁻¹ at full peat depth (average of 3.9 m) and 102 ± 5 t C ha⁻¹ (0.3 m depth) for organic soils, and 121 ± 24 t C ha⁻¹ (0.3 m depth) for mineral soils. At national level, C stocks were estimated at 11 ± 1 Mt (0.3 m depth) and 144 ± 17 Mt (full peat depth) in organic soils, and 23 ± 1 Mt (0.3 m depth) in mineral soils. Since European settlement, 146,000 ha of organic soils have been converted to agriculture, which could release between 0.5 and 2 Mt CO2 year⁻¹, equivalent to 1–6 % of New Zealand’s total agricultural greenhouse gas emissions.

Genotoxicity assessment of soils from wastewater irrigation areas and bioremediation sites using the Vicia faba root tip micronucleus assay

Genotoxicity potential of soils taken from wastewater irrigation areas and bioremediation sites was assessed using the Vicia faba root tip micronucleus assay. Twenty five soils were tested, of which 8 were uncontaminated soils and taken as the control to examine the influence of soil properties; 6 soils were obtained from paddy rice fields with a history of long-term wastewater irrigation; 6 soils were obtained from bioremediation sites to examine effects of bioremediation; and 5 PAH-contaminated soils were used to examine methodological effects between direct soil exposure and exposure to aqueous soil extracts on micronuclei (MN) frequency () in the V. faba root tips. Results indicate that soil properties had no significant influences on MN frequencies (p > 0.05) when soil pH varied between 3.4 to 7.6 and organic carbon between 0.4% and 18.6%. The MN frequency measured in these control soils ranged from 1.6‰ to 5.8‰. MN frequencies in soils from wastewater irrigation areas showed 2- to 48-fold increase as compared with the control. Soils from bioremediation sites showed a mixed picture: MN frequencies in some soils decreased after bioremediation, possibly due to detoxification; whereas in other cases remediated soils induced higher MN frequencies, suggesting that genotoxic substances might be produced during bioremediation. Exposure to aqueous soil extracts gave a higher MN frequency than direct exposure in 3 soils. However, the opposite was observed in the other two soils, suggesting that both exposure routes should be tested in case of negative results from one route. Data obtained from this study indicate that the MN assay is a sensitive assay suitable for evaluating genotoxicity of soils.

Extraction of compounds associated with water repellency in sandy soils of different origin

After an initial evaluation of several solvents, the efficiency of Soxhlet extractions with isopropanol/ammonia (s.g. 0.88) (70 : 30 v : v; 24 h) in extracting compounds associated with water repellency in sandy soils was examined using a range of repellent and wettable control soils (n = 15 and 4) from Australia, Greece, Portugal, The Netherlands, and the UK. Extraction efficiency and the role of the extracts in causing soil water repellency was examined by determining extract mass, sample organic carbon content and water repellency (after drying at 20°C and 105°C) pre- and post-extraction, and amounts of aliphatic C–H removed using DRIFT, and by assessing the ability of extracts to cause repellency in acid-washed sand (AWS).

Key findings are: (i) none of organic carbon content, amount of aliphatic C–H, or amount of material extracted give any significant correlation with repellency for this diverse range of soils; (ii) sample drying at 105°C is not necessarily useful before extraction, but may provide additional information on extraction effectiveness when used after extraction; (iii) the extraction removed repellency completely from 13 of the 15 repellent samples; (iv) extracts from all repellent and wettable control soils were capable of inducing repellency in AWS. The findings suggest that compounds responsible for repellency represent only a fraction of the extract composition and that their presence does not necessarily always cause repellency.

Investigation of compounds causing water repellency in the rhizosphere of sandy soils from a wide range of locations.

Although soils are generally considered to wet readily, some are actually water repellent at the surface and in the rhizosphere. This phenomenon occurs at low to moderate moisture contents and has been reported from soils under a range of vegetation types and from many regions around the globe. Water repellency in soils can have serious environmental implications including reduced seed germination and plant growth as well as irrigation efficiency, accelerated soil erosion, and enhanced leaching of agrochemicals through preferential flow. it has been proposed that water repellency is caused by the accumulation of hydrophobic organic compounds released as root exudates, microbial byproducts or from decomposing organic matter, which are deposited on mineral or aggregate surfaces, or are present as interstitial matter, Few studies to date have attempted to isolate and characterize these compounds and their structure is therefore only poorly understood, These studies have generally focussed on only a single soil or a small range of samples, have not included non-repellent soils as a control and have not always been able to demonstrate that the substances isolated are indeed responsible for repellency formation.

This study reports on the first part (extraction procedures) of a research programme addressing these gaps in current knowledge by investigating a wide range of severely repellent and wettable ‘control’ samples from different countries, and by including assessments of extraction efficiency and ability of extracts to cause repellency. Analytical methods include DRIFT (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) of soils and IR (Infrared) analysis of extracts.

Key findings are that (i) soil sample heating after extraction is valuable in assessing the effectiveness of the extraction procedure, (ii) Soxhlet extraction using isopropanol/ ammonia (70/30 v/v) was the most effective method in extracting hydrophobic compounds, while leaving the ability of extracted compounds to induce water repellency virtually unaffected, (iii) wettable control soils also contain hydrophobic substances capable of inducing water repellency, (iv) the amount of organic compounds extracted was poorly related to sample repellency, indicating that compounds responsible for repellency may only represent a small fraction of the extract, (v) differences in extraction efficiency between different samples indicate that the compounds responsible may differ generically and/or in terms of their bonding to minerals, and (vi) the combination of repellency assessments with DRIFT on soils and JR on extracts used with internal standards has considerable potential to allow quantification of CH bearing organic matter in the soil, the efficiency of extraction processes for its removal, and its significance in causing water repellency in soils.

Can mud (silt and clay) concentration be used to predict soil organic carbon content within seagrass ecosystems?

The emerging field of blue carbon science is seeking cost-effective ways to estimate the organic carbon content of soils that are bound by coastal vegetated ecosystems. Organic carbon (Corg) content in terrestrial soils and marine sediments has been correlated with mud content (i.e. silt and clay), however, empirical tests of this theory are lacking for coastal vegetated ecosystems. Here, we compiled data (n = 1345) on the relationship between Corg and mud (i.e. silt and clay, particle sizes <63 μm) contents in seagrass ecosystems (79 cores) and adjacent bare sediments (21 cores) to address whether mud can be used to predict soil Corg content. We also combined these data with the δ13C signatures of the soil Corg to understand the sources of Corg stores. The results showed that mud is positively correlated with soil Corg content only when the contribution of seagrass-derived Corg to the sedimentary Corg pool is relatively low, such as in small and fast growing meadows of the genera Zostera, Halodule and Halophila, and in bare sediments adjacent to seagrass ecosystems. In large and long-living seagrass meadows of the genera Posidonia and Amphibolis there was a lack of, or poor relationship between mud and soil Corg content, related to a higher contribution of seagrass-derived Corg to the sedimentary Corg pool in these meadows. The relative high soil Corg contents with relatively low mud contents (i.e. mud-Corg saturation) together with significant allochthonous inputs of terrestrial organic matter could overall disrupt the correlation expected between soil Corg and mud contents. This study shows that mud (i.e. silt and clay content) is not a universal proxy for blue carbon content in seagrass ecosystems, and therefore should not be applied generally across all seagrass habitats. Mud content can only be used as a proxy to estimate soil Corg content for scaling up purposes when opportunistic and/or low biomass seagrass species (i.e. Zostera, Halodule and Halophila) are present (explaining 34 to 91% of variability), and in bare sediments (explaining 78% of the variability).

Losses and recovery of organic carbon from a seagrass ecosystem following disturbance

Seagrasses are among the Earth's most efficient and long-term carbon sinks, but coastal development threatens this capacity. We report new evidence that disturbance to seagrass ecosystems causes release of ancient carbon. In a seagrass ecosystem that had been disturbed 50 years ago, we found that soil carbon stocks declined by 72%, which, according to radiocarbon dating, had taken hundreds to thousands of years to accumulate. Disturbed soils harboured different benthic bacterial communities (according to 16S rRNA sequence analysis), with higher proportions of aerobic heterotrophs compared with undisturbed. Fingerprinting of the carbon (via stable isotopes) suggested that the contribution of autochthonous carbon (carbon produced through plant primary production) to the soil carbon pool was less in disturbed areas compared with seagrass and recovered areas. Seagrass areas that had recovered from disturbance had slightly lower (35%) carbon levels than undisturbed, but more than twice as much as the disturbed areas, which is encouraging for restoration efforts. Slow rates of seagrass recovery imply the need to transplant seagrass, rather than waiting for recovery via natural processes. This study empirically demonstrates that disturbance to seagrass ecosystems can cause release of ancient carbon, with potentially major global warming consequences.

Potential for agricultural production on disturbed soils mined for apatite using legumes and beneficial microbe

Christmas Island has been mined for rock phosphate for over 100 years, and as mining will finish in the next few decades there is a need to develop alternative economies on the island, such as high value crop production. However, to conserve the unique flora and fauna on the island, only land previously mined will be considered for this purpose. As these soils have been severely perturbed by mining, strategies to improve soil quality parameters need to be undertaken before plant based industries can be considered. For instance, legumes and beneficial microbes have demonstrated a positive role in the remediation of degraded soils. Therefore, this study aimed to establish the scientific basis upon which agriculture can effectively be developed on s oils post phosphate mining. Six legume species (Glycine max (Soybean), Vigna radiata (Mungbean), V. unguiculata (Cowpea), Phaseolus vulgaris (Navybean), Cajanus cajan (Pigeon pea), and Lablab purpureus (Lablab)) were sown onto a two ha rehabilitated site t hat had previously been mined for rock phosphate. The soil had a pH of 7.0, and was high in P but low in Bo, Cu, K, Mg, N and S and had low organic C. The legumes were inoculated with their respective rhizobial inoculant or co-inoculated with the rhizobia and a plant growth promoting bacteria (PGPB) at three different fertilizer rates (nil, a low rate, and five times the low rate). With the exception of P. vulgaris, all the legume species survived. The application of fertilizer was essential for maximum biomass yields 18 weeks after sowing, however the lower fertilizer rate was sufficient to obtain maximum yields for some cultivars. The PGPB increased yields and nodulation of some of the legumes at different fertilizer levels. Although the legumes (except P. vulgaris) grew in the Christmas Island environment, selection of appropriate legume cultivars and inoculants plus optimization of the fertilizer regime is required for reliable agricultural productivity on the island.