Table 6 Description of the observations included in the meta-analysis
Variables | Categories | Subcategories | Unique papers (number of observations) |
|---|---|---|---|
Paper type | Peer-reviewed | Peer-reviewed | 83 (1,341) |
Gray literature | Dissertations | 5 (120) | |
Master’s theses | 3 (132) | ||
University repository | 1 (16) | ||
Experimental design | Field experiment | Field | 53 (893) |
Controlled experiments | Greenhouse | 19 (505) | |
Laboratory | 11 (211) | ||
Experimental continents | Africa | – | 16 (412) |
Asia | – | 35 (468) | |
Europe | – | 19 (254) | |
North America | – | 8 (85) | |
Oceania | – | 6 (149) | |
South America | – | 9 (231) | |
Feedstock type | Agricultural residue | – | 52 (844) |
Manure/digestate | – | 14 (180) | |
Wood-based | – | 42 (585) | |
Pyrolysis group (°C) |  ≤ 400 | Low | 30 (549) |
400 < ; > 600 | Medium | 52 (823) | |
 ≥ 600 | High | 14 (173) | |
Biochar application rate (t/ha) |  < 10 | – | 51 (664) |
10–30 | – | 49 (536) | |
30–50 | – | 17 (194) | |
50 <  | – | 19 (215) | |
Experiment duration (days) |  < 60 | – | 14 (246) |
60–160 | – | 28 (503) | |
160–402 | – | 23 (384) | |
402–730 | – | 20 (318) | |
730 <  | – | 9 (152) | |
Types of treatments | Biochar alone | – | 79 (1,020) |
Biochar with compost | – | 6 (33) | |
Biochar with manure | – | 11 (127) | |
Biochar with fertilizer | – | 30 (401) | |
Biochar with fertilizer & compost | – | 1 (6) | |
Biochar with fertilizer & manure | – | 2 (12) | |
Tillage application | Not tilled | – | 64 (1,047) |
Tilled | – | 29 (552) | |
Irrigation application | Not irrigated | – | 51 (1,053) |
Irrigated | – | 42 (556) | |
Base fertilizer application before and after biochar | Fertilizer applied | – | 42 (589) |
Fertilizer not applied | – | 51 (1,010) | |
SPPs | Soil total NPK | – | 61 (506) |
Soil mineral nitrogen | – | 35 (247) | |
Plant nutrient level | – | 15 (106) | |
N2O emission | – | 15 (53) | |
Soil NPK availability | – | 50 (468) | |
Potential CEC | – | 12 (37) | |
Effective CEC | – | 13 (49) | |
NUE | – | 5 (23) | |
Mineral N leaching | – | 5 (44) |