Soil biodiversity, carbon cycling and crop plant biomass responses to experimental biochar amendment of agricultural soil (Dundee, UK)

72 plant-soil mesocosms were used; they were 38l in volume (380 x 380 x 300 mm).

Soil fauna samples were taken for nematodes on 21–22 June 2011, 28–29 August 2012 and 20 August 2013, and for microarthropods (collembola, mites) on 20 August 2013. On each occasion, each mesocosm was sampled in three random locations with a 3.5 cmØ corer to 10 cm depth. Each soil core was split vertically into two halves, one half designated for nematode extraction and the other for microarthropod extraction. The three replicate halves were pooled into a single sample for each pot, with fresh weight recorded prior to invertebrate extraction. For nematode extraction, soil samples were placed in a Baermann funnel system for 24 h wet extraction. Microarthropods were collected into alcohol-filled vials using Tullgren funnels for 24 h. Following extraction of invertebrates, the soil was oven-dried and weighed to determine soil dry weight. Nematodes and microarthropods (mites and collembola) were counted under a light microscope, and abundance values were converted to standardised densities by calculating individuals per g of dry soil.

To quantify annual aboveground primary production, barley and ryegrass biomass was collected by cutting the vegetation biomass to 1 cm above the soil surface using handheld shears in September of each year, 2011– 2013. Root biomass was determined in August 2013 by taking one soil core from each mesocosm (3.5 cm Ø, 10 cm depth). Only the top 10 cm were analysed so that root data would correspond to the same soil stratum as sampled for invertebrates. Separation of roots from soil was accomplished using washing, sieving (1mm mesh) and handpicking. Once separated, the plant material was oven-dried prior to weighing.

Phospholipid fatty acid (PLFA) analysis was used in order to quantify the dry weight-based mass of markers for microbial biomass and fungal-to-bacterial ratio in the soil in different treatments. One soil sample per mesocosm (3.5 cm Ø core to 10 cm depth) was taken in August 2013 and stored at −20 °C prior to freeze-drying at −20 °C. A subsample (1 g) of the freeze-dried soil was subsequently taken for phospholipid fatty acid (PLFA) analysis. Three measures of microbial community structure were derived. Total PLFA provided a measure of overall microbial biomass; the 16:1ω5 fatty acid marker was used as a proxy measurement for arbuscular mycorrhizal biomass and the fungal-to-bacterial PLFA ratio was calculated by dividing the fungal PLFA marker (18:2ω6,9) by the summed bacterial PLFA markers (i15:0, a15:0, 15:0, i16:0, 16:1ω7, a17:0, i17:0, cy17:0, cis18:1ω7, cy19:0).

Ecosystem respiration and net ecosystem exchange (NEE) of CO2 fluxes from each mesocosm were quantified monthly. An IRGA EGM-4 connected to a gas sampling chamber (45,693 cm3) was used. The chamber was inlaid with Propafilm C on all five sides to allow light transmission so NEE of CO2 could be measured. Ecosystem respiration was measured by using an aluminium cover to exclude light from the chamber. Prior to the onset of the experiment, chamber air-tightness was confirmed by injecting a known concentration of SF6 into a chamber connected to a trial pot, then using a gas chromatograph to monitor SF6 levels over the course of one hour. Net CO2 efflux data were expressed as positive values whereas net CO2 uptake data were expressed as negative values.