5.3.2.1. Comparison between the different methodologies to calculate soil C (and N) stocks
The soil TC (and TN) stocks were first computed by using the more common calculation at a fixed depth (FD). In order to avoid any inaccuracy resulting from differences in soil BD caused by the presence of biochar and plant roots, calculations on soil TC (and TN) then were made using an alternative methods of a fixed mass (FM) and an equivalent soil mass approach (ESM). The soil mass and the respective soil TC and TN stocks are presented in the Table A4.01 and Table A4.02 in Appendix 4. Both the FM and the ESM methods showed a strong positive correlation (r = 0.967 and r = 0.958, respectively) with the net difference in soil TC stocks (i.e., TCControl – TC biochar‐amended) calculated for each nominal depth using the FD method. The strong correlation value indicates that both the FM and ESM methods have a similar trend in calculating the differences in soil TC stocks between biochar treatments. The application of ESM calculations is often used when there are no soil BD data and only the TC concentration and the soil mass data are needed. On the other hand, the FM method amends the soil C stocks calculated at a fixed depth by subtracting the additional C due to differences in BD at each nominal depth. As data on soil BD (Appendix 3 and 4; Calvelo Pereira et al., 2015) were available for all the soil samples, it was considered convenient here to use the FM approach for a detailed assessment of TC changes with depth in both pastures with different root systems and biochar as soil amendment.
5.3.2.2. Change in soil C and N stocks in experiment 1
Soil TC and TN stocks for the TK soil down to a nominal depth of 30 cm were calculated at T0 and T2 using the FM method; and values are shown in Table A4.01 and Table A4.02 in Appendix 4. At T0, the soil TC stock was 64.1 Mg C ha–1, with 29, 48, and 23% of it distributed at 0–10, 10–20, and 20–30 cm soil layers, respectively. At the end of the experiment and in the absence of biochar, the TK soil under ryegrass and chicory always had a lower soil TC stocks (53.2 and 58.0 Mg C ha–1, respectively) compared with the corresponding TC stock at T0 (64.1 Mg TC ha–1). The mixture of cocksfoot and red clover at T2, however, showed a higher soil TC stock (73 Mg TC ha–1) compared with the TC stock at T0. When comparing between pasture species, the mixture of
cocksfoot and red clover always showed the highest soil TC (P < 0.05), regardless the addition of PI‐350 biochar (Table A4.01, in Appendix 4). At T2, the application of PI‐350 biochar to the TK soil significantly increased (P < 0.05) the soil TC of all pasture compared with the control treatment (Table A4.01 in Appendix 4). The highest increase of soil TC was recorded in the ryegrass with PI‐350 biochar (an increase of 9.9 Mg C ha–1) compared with the other pasture types (mixture pasture: 4.3 Mg C ha–1; chicory: 5.3 Mg C ha–1).
The soil TN stocks showed a similar pattern to the soil TC stocks described above (Table A4.02 in Appendix 4). At T2, the soil TN stocks showed lower values than those at T0, with N losses ranging from 1.2 to 2.1 Mg N ha–1. The application of PI‐350 biochar did not affect TN stocks.
5.3.2.3. Changes in soil C and N stocks in experiment 2
Soil TC and TN stocks for the Motuiti soil were calculated at T0 and T2 using the FM method down to a nominal depth of 30 cm, and are shown in Table A4.01 and Table A4.02 in Appendix 4. In the absence of biochar, the soil TC stock at T0 was 50.6 Mg C ha–1, with 30, 61, and 9% of it being distributed in the 0–10, 10–20 and 20–30 cm depth. After 2 years (T2) of pasture establishment and the absence of biochar addition, all pasture was recorded as having have soil TC stocks similar to those at T0 (T0 = 50.6 Mg C ha–1; T2 = 52.5 – 54.0 Mg C ha–1). The addition of the ash‐rich BG‐550 biochar to this sandy MS soil did not influence the soil TC stocks under any pasture treatment (Table A4.01 in Appendix 4). Interestingly, the Motuiti soil amended with BG‐550 biochar under ryegrass showed a slightly lower TC stock (P > 0.05) compared with the corresponding control treatment (47.4, and 52.5 Mg C ha–1, respectively). Similar trends to those described for TC were also observed in the TN stocks (Table A4.02 in the appendix 4), with losses of N being also observed in the ryegrass, especially at the 20‐30 cm soil depth.
5.3.3. Effect of biochar on net changes in TC and TN stocks compared to non‐biochar treatments at T2
The net effect of biochar addition to soil TC and TN stocks was tested by assessing the net difference in soil TC (and TN) stocks [i.e., TC (or TN)Control – TC (or TN)biochar‐amended]
at T2 for each nominal depth considered (Table A4.01 and Table A4.02 in the appendix 4). In experiment 1 on the TK soil, the application of PI‐350 biochar (equivalent to 7.6 Mg C ha–1) resulted in a net increase (ranging from 6 to 16%) in the TC stocks for the soil profile down to nominal depth of 30 cm (Fig. 5.5). Changes in the soil TC stocks were always more apparent for the 10–20 cm depth these being always significant (Fig. 5.6). The treatment of TK soil amended with PI‐350 biochar under ryegrass showed a higher net C gain compared with the other pasture species (Fig. 5.5). The addition of PI‐350 biochar (equivalent to 0.03 Mg TN ha–1) did not influence the net difference in TN stock (Fig. 5.5), as expected.
In the experiment 2 on the Motuiti soil, the addition of BG‐550 biochar (equivalent to 3.6 Mg C ha–1 and 0.14 Mg N ha–1) did not changed the net soil TC and TN stock difference at T2 for the entire soil profile to a nominal depth of 30 cm (Fig. 5.6).
Figure 5.5 Net gain and losses of TC and TN stocks in the biochar‐amended
treatments compared to the non‐amended ones [Δ TC and Δ TN: (stockscontrol
– stocksbiochar‐amended), in percentage] after 2 years of pasture establishment in
the TK soil: a) ryegrass, b) cocksfoot and red clover mixture, c) chicory. Value followed by * showed a significant difference at P < 0.05
Figure 5.6 Net gain and losses of TC and TN stocks in the biochar‐amended
treatments compared to the unamended ones [Δ TC and Δ TN: (stockscontrol –
stocksbiochar‐amended), in percentage] after 2 years of pasture establishment in
the MS soil: a) ryegrass, b) cocksfoot and red clover mixture, c) lucerne . Value followed by * showed a significant difference at P < 0.05