New and rapid techniques for estimating the stable fraction of organic carbon (C org ) in biochar are needed for carbon (C) sequestration accounting. In this study, 25 biochar samples produced from different feedstocks and pyrolysis temperatures were scanned using visible near infrared (NIR) spectroscopy and analysed using standard laboratory methods for reference data. Principal component analysis and linear discriminant analysis of preprocessed spectral data were used to extract relevant information and discriminate among biochars , respectively. Partial least squares regression was used to build calibration models between preprocessed spectral data and reference data, and the accuracy of calibration models in predicting biochar properties was then tested using a cross-validation procedure.Biochar indices related to C stability, such as aromatic C, fixed C, atomic H/C org ratio and the fraction of total C that is aromatic (fa) were successfully predicted (R 2 CV 0.92-0.94, RPD CV 3.26-4.22) using the NIR spectroscopy technique (NIR bands, 780-2500 nm). Aromatic C and fixed C could be predicted independently from fa. Other biochar properties, such as C org , H and O content, and atomic O/C org ratio, were predicted with an accuracy ranging from moderate to very high (R 2 CV 0.80-0.96, RPD CV 2.24-4.66). The study illustrates the potential of this rapid and low-cost technique for measuring biochar stability indices in routine analysis if accurate calibration models for each index are available.

The use of deep-rooting pasture species as a management practice can increase the allocation of plant carbon (C) below ground and enhance C storage. A 2-year lysimeter trial was set up to compare changes in C stocks of soils under either deep-or shallow-rooting pastures and investigate whether biochar addition below the top 10 cm could promote root growth at depth. For this i) soil ploughing at cultivation was simulated in a silt loam soil and in a sandy soil by inverting the 0 to 10 and 10-to 20-cm-depth soil layers, and a distinctive biochar (selected for each soil to overcome soil-specific plant growth limitations) was mixed at 10 Mg ha À1 in the buried layer, where appropriate and ii) three pasture types with contrasting root systems were grown. In the silt loam, soil inversion resulted in a general loss of C (2.0-8.1 Mg ha À1), particularly in the buried horizon, under shallowrooting pastures only. The addition of a C-rich biochar (equivalent to 7.6 Mg C ha À1 ) to this soil resulted in a net C gain (21-40% over the non-biochar treatment, P < 0.10) in the buried layer under all pastures; this overcame the loss of C in this horizon under shallow-rooting pastures. In the sandy soil, all pastures were able to maintain soil C stocks at 10-20 cm depth over time, with minor gains of C (1.6-5.1 Mg ha À1 ) for the profile. In this soil, the exposure of a skeletal-and nutrient-depleted soil layer at the surface may have fostered root growth at depth. The addition of a nutrient-rich biochar (equivalent to 3.6 Mg C ha À1 ) to this soil had no apparent effect on C stocks. More research is needed to understand the mechanisms through which soil C stocks at depth are preserved.

The use of biochar reduces nitrous oxide (N2O) emissions from soils under specific conditions yet the mechanisms through which interactions occur are not fully understood. The objectives of this glasshouse study were to investigate the effect of (i) biochar particle size, and (ii) the impact of soil inversion—through simulated mouldboard ploughing—on N2O emissions from soils to which cattle urine was applied. Pine biochar (550 °C) with two different particle sizes (<2 mm and >4 mm) was mixed either into the top soil layer at the original 0–10 cm depth in the soil column or at 10–20 cm depth by inverting the top soil to simulate ploughing. Nitrous oxide emissions were monitored for every two to three days, up to seven weeks during the summer trial and measurements were repeated during the autumn trial. We found that the use of large particle size biochar in the inverted soil had significant impact on increasing the cumulative N2O emissions in autumn trial, possibly through changes in the water hydraulic conductivity of the soil column and increased water retention at the boundary between soil layers. This study thus highlights the importance of the role of biochar particle size and the method of biochar placement on soil physical properties and the implications of these on N2O emissions.