Net changes of soil C stocks in two grassland soils 26 months after simulated pasture renovation including biochar addition

Abstract

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-1 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-1), particularly in the buried horizon, under shallow-rooting pastures only. The addition of a C-rich biochar (equivalent to 7.6 Mg C ha-1) 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-1) 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-1) 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.