Long-term CH3Br and CH3Cl flux measurements in temperate salt marshes

Fluxes of CH3Br and CH3Cl and their relationship with potential drivers such as sunlight, temperature and soil moisture, were monitored at fortnightly to monthly intervals for more than two years at two contrasting temperate salt marsh sites in Scotland. Manipulation experiments were conducted to further investigate possible links between drivers and fluxes. Fluxes followed both seasonal and diurnal trends with highest fluxes during summer days and lowest (negative) fluxes during winter nights. Mean (± 1 sd) annually and diurnally-weighted net emissions from the two sites were found to be 300 ± 44 ng m−2 h−1 for CH3Br and 662 ± 266 ng m−2 h−1 for CH3Cl. The fluxes from this work are similar to findings from this and other research groups for salt marshes in cooler, higher latitude climates, but lower than values from salt marshes in the Mediterranean climate of southern California. Statistical analysis generally did not demonstrate a strong link between temperature or sunlight levels and methyl halide fluxes, although it is likely that temperatures have a weak direct influence on emissions, and both certainly have indirect influence via the annual and daily cycles of the vegetation. CH3Cl flux magnitudes from different measurement locations depended on the plant species enclosed whereas such dependency was not discernible for CH3Br fluxes. In 14 out of 18 collars with vegetation CH3Br and CH3Cl net fluxes were significantly positively correlated. The CH3Cl/CH3Br net-emission mass ratio was 2.2, a magnitude lower than mass ratios of global methyl halide budgets (~22) or emissions from tropical rainforests (~60). This is likely due to preference for CH3Br production by the relatively high bromine content in the salt marsh plant material. Extrapolation based solely on data from this study yields salt marsh contributions of 0.5–3.2% and 0.05–0.33%, respectively, of currently-estimated total global production of CH3Br and CH3Cl, but actual global contributions likely lie between these values and those derived from southern California

The routes and kinetics of trichloroacetic acid uptake and elimination in Sitka spruce (Picea sitchensis) saplings via atmospheric deposition pathways

A major flux of trichloroacetic acid (TCA) to forests is via wet deposition, but the transfer of TCA into tree foliage may occur by an above- or below-ground pathway. To investigate the routes and kinetics of TCA uptake, two groups of 10 Sitka spruce saplings (with an equivalent number of controls) were exposed to a single application of 200 μg TCA in solution, either to the soil only, or sprayed as a mist to the foliage only. The needle foliage was subsequently analysed regularly for TCA for 3 months during the growing season. Significant uptake into current year (C) needles was observed from both routes just a few days after application, providing direct evidence of an above-ground uptake route. Uptake of TCA was also observed in the previous year needle class (C+1). Kinetic modelling of the data indicated that the half-life for within-needle elimination (during the growing season) was 50±30 days. Most of the applied TCA appeared to be degraded before uptake, either in the soil, or externally on the sapling foliage

Rhizosphere microbial roles in phosphorus cycling during successive plantings of Chinese fir plantations

The rhizosphere, a critical interface involving soil, plant roots, and microorganisms, plays a vital role in the feedback processes between plants and soil, especially under phosphorus (P) limiting conditions typical of subtropical forests. This study used a chronosequence design to investigate first-fourth successive planting rotations of Chinese fir (Cunninghamia lanceolata) plantations in Fujian Province, China, with stands around 17 years old. We employed a modified P fractionation assay and metagenomic sequencing to explore P cycling in the rhizosphere across different plantation rotations. Rhizosphere labile P concentrations increased significantly in the fourth rotation, alongside a consistent upward trend in moderately labile P throughout successive plantings. Conversely, stable P and residual P concentrations declined during successive plantings, signaling a shift toward more accessible P forms. From metagenomic analyses, the proportion of P transport processes (transportation of phosphonate, phosphate, and inorganic phosphate) gradually increased. Notably, abundances were significantly higher in the rhizosphere soil of the fourth Chinese fir planting rotation of the K01126 gene (involved in phosphate ester mineralization), the phnD gene (associated with phosphonate transport), functional genes related to the solubilization of inorganic phosphate, such as pqqB, pqqC, pqqE, and ppa, and the phoB gene (linked to P-starvation response regulation). The results indicate that functional microbes of the rhizosphere, dominated by Proteobacteria and Acidobacteria, are instrumental in changing P cycling processes during successive plantings. The successive planting rotations of Chinese fir plantations significantly and positively impacted on the gene abundance related to the activation and uptake of P in the rhizosphere. Based on these insights, specific strategies, such as regular monitoring and application of phosphate fertilizer and adjusting rotation timing based on the soil rhizosphere P status, and incorporating native broad-leaved tree species are suggested to promote efficient P cycling, thus supporting sustainable forest management practices

Fluxes of trichloroacetic acid through a conifer forest canopy

Controlled-dosing experiments with conifer seedlings have demonstrated an aboveground route of uptake for trichloroacetic acid (TCA) from aqueous solution into the canopy, in addition to uptake from the soil. The aim of this work was to investigate the loss of TCA to the canopy in a mature conifer forest exposed only to environmental concentrations of TCA by analysing above- and below-canopy fluxes of TCA and within-canopy instantaneous reservoir of TCA. Concentrations and fluxes of TCA were quantified for one year in dry deposition, rainwater, cloudwater, throughfall, stemflow and litterfall in a 37-year-old Sitka spruce and larch plantation in SW Scotland. Above-canopy TCA deposition was dominated by rainfall (86%), compared with cloudwater (13%) and dry deposition (1%). On average only 66% of the TCA deposition passed through the canopy in throughfall and stemflow (95% and 5%, respectively), compared with 47% of the wet precipitation depth. Consequently, throughfall concentration of TCA was, on average, 1.4 × rainwater concentration. There was no significant difference in below-canopy fluxes between Sitka spruce and larch, or at a forest-edge site. Annual TCA deposited from the canopy in litterfall was only 1–2% of above-canopy deposition. On average, 800 μg m−2 of deposited TCA was lost to the canopy per year, compared with estimates of above-ground TCA storage of 400 and 300 μg m−2 for Sitka spruce and larch, respectively. Taking into account likely uncertainties in these values (±50%), these data yield an estimate for the half-life of within-canopy elimination of TCA in the range 50–200 days, assuming steady-state conditions and that all TCA lost to the canopy is transferred into the canopy material, rather than degraded externally. The observations provide strong indication that an above-ground route is important for uptake of TCA specifically of atmospheric origin into mature forest canopies, as has been shown for seedlings (in addition to uptake from soil via transpiration), and that annualized within-canopy elimination is similar to that in controlled-dosing experiments

Land cover influence on catchment scale subsurface water storage investigated by multiple methods:Implications for UK Natural Flood Management

Study region: United Kingdom (UK). Study Focus: ‘Natural flood management’ (NFM) schemes manipulating land use and other catchment features to control runoff are increasingly promoted across the UK. Catchment water storage and mixing processes influence runoff, but our understanding of the effects of land cover change on these processes is still limited. This study combined hydrometric, isotopic and geochemical measurements to investigate land cover versus potential topographic, soil and geological controls. It compared storage-discharge dynamics in nine nested catchments within a 67 km2 managed upland catchment in southern Scotland. Storage and mixing dynamics were characterised from hydrometric data using recession analysis and from isotopic data using mean transit time and young water fraction estimates. To give information on water sources, groundwater fraction was estimated from end member mixing analysis based on acid neutralising capacity.New hydrological insights: The analysis showed low but variable sub-catchment scale dynamic storage (16–200 mm), mean transit times (134–370 days) and groundwater fractions (0.20–0.52 of annual stream runoff). Soil hydraulic conductivity was most significantly positively correlated with storage and mixing measures, whilst percentage forest cover was inversely correlated. Any effects of forest cover on increasing catchment infiltration and storage are masked by soil hydraulic properties even in the most responsive catchments. This highlights the importance of understanding dominant controls on catchment storage when using tree planting as a flood management strategy