Biochar effects on salt-affected soil properties and plant productivity: A global meta-analysis.

Biochar has been recognized as a promising practice for ameliorating degraded soils, yet the consensus on its effects remains largely unknown due to the variability among biochar, soil and plant. This study therefore presents a meta-analysis synthesizing 92 publications containing 987 paired data to scrutinize biochar effects on salt-affected soil properties and plant productivity. Additionally, a random meta-forest approach was employed to identify the key factors of biochar on salt-affected soil and plant productivity. Results showed that biochar led to significant reductions in electrical conductivity (EC), bulk density (BD) and pH by 7.4%, 4.7% and 1.2% compared to the unamended soil, respectively. Soil organic carbon (by 55.1%) and total nitrogen (by 31.3%) increased significantly with biochar addition. Moreover, biochar overall enhanced plant productivity by 31.5%, and more pronounced increases in forage/medicinal with higher salt tolerance than others. The results also identified that the soil salinity and biochar application rate were the most important co-regulators for EC and PP changes. The structural equation model further showed that soil salinity (P < 0.001), biochar pH (P < 0.001) and biochar specific surface area (P < 0.01) had a significant negative effect on soil EC, but it was positively impacted by biochar pyrolysis temperature (P < 0.05). Furthermore, plant productivity was positively affected by biochar pH (P < 0.001) and biochar feedstock (P < 0.01), while negatively influenced by biochar pyrolysis temperature (P < 0.01). This study highlights that woody biochar with 7.6 < pH < 9.0 and pyrolyzed at 400-600 °C under 30-70 t ha-1 application rate in moderately saline coarse soils is a recommendable pattern to enhance forage/medicinal productivity while reducing soil salinity. In conclusion, biochar offers promising avenues for ameliorating degradable soils, but it is imperative to explore largescale applications and field performance across different biochar, soil, and plant types.

Biochar addition and reduced irrigation modulates leaf morpho-physiology and biological nitrogen fixation in faba bean-ryegrass intercropping.

Intercropping legume with grass has potential to increase biomass and protein yield via biological N2-fixation (BNF) benefits, whereas the joint effects of biochar (BC) coupled with deficit irrigation on intercropping systems remain elusive. A 15N isotope-labelled experiment was implemented to investigate morpho-physiological responses of faba bean-ryegrass intercrops on low- (550 °C, LTBC) or high-temperature BC (800 °C, HTBC) amended sandy-loam soil under full (FI), deficit (DI) and partial root-zone drying irrigation (PRD). LTBC and HTBC significantly reduced intrinsic water-use efficiency (WUE) by 12 and 14 %, and instantaneous WUE by 8 and 16 %, respectively, in faba bean leaves, despite improved photosynthetic (An) and transpiration rate (Tr), and stomatal conductance (gs). Compared to FI, DI and PRD lowered faba bean An, gs and Tr, but enhanced leaf-scale and time-integrated WUE as proxied by the diminished shoots Δ13C. PRD enhanced WUE as lower gs, Tr and guard cell length than DI-plants. Despite higher carbon ([C]) and N concentration ([N]) in faba bean shoots amended by BC, the aboveground C- and N-pool of faba bean were reduced, while these pools increased for ryegrass. The N-use efficiency (NUE) in faba bean shoots was reduced by 9 and 14 % for LTBC and HTBC, respectively, but not for ryegrass. Interestingly, ryegrass shoots had 52 % higher NUE than faba bean shoots. The N derived from atmosphere (% Ndfa) was increased by 2 and 9 % under LTBC and HTBC, respectively, while it decreased slightly by reduced irrigation. Quantity of BNF in faba bean aboveground biomass decreased with HTBC coupled with reduced irrigation, mainly towards decreased biomass and soil N uptake by faba bean. Therefore, HTBC might not be a feasible option to improve WUE and BNF in faba bean-ryegrass intercropping, but PRD is permissible as the clear trade-off between BC and PRD.