K. Y. Chan

A pot trial was carried out to investigate the effect of biochar produced from greenwaste by pyrolysis on the yield of radish (Raphanus sativus var. Long Scarlet) and the soil quality of an Alfisol. Three rates of biochar (10, 50 and 100 t/ha) with and without additional nitrogen application (100 kg N/ha) were investigated. The soil used in the pot trial was a hardsetting Alfisol (Chromosol) (0–0.1 m) with a long history of cropping. In the absence of N fertiliser, application of biochar to the soil did not increase radish yield even at the highest rate of 100 t/ha. However, a significant biochar × nitrogen fertiliser interaction was observed, in that higher yield increases were observed with increasing rates of biochar application in the presence of N fertiliser, highlighting the role of biochar in improving N fertiliser use efficiency of the plant. For example, additional increase in DM of radish in the presence of N fertiliser varied from 95% in the nil biochar control to 266% in the 100 t/ha biochar-amended soils. A slight but significant reduction in dry matter production of radish was observed when biochar was applied at 10 t/ha but the cause is unclear and requires further investigation. Significant changes in soil quality including increases in pH, organic carbon, and exchangeable cations as well as reduction in tensile strength were observed at higher rates of biochar application (>50 t/ha). Particularly interesting are the improvements in soil physical properties of this hardsetting soil in terms of reduction in tensile strength and increases in field capacity.

Despite the recent interest in biochars as soil amendments for improving soil quality and increasing soil carbon sequestration, there is inadequate knowledge on the soil amendment properties of these materials produced from different feed stocks and under different pyrolysis conditions. This is particularly true for biochars produced from animal origins. Two biochars produced from poultry litter under different conditions were tested in a pot trial by assessing the yield of radish (Raphanus sativus var. Long Scarlet) as well as the soil quality of a hardsetting Chromosol (Alfisol). Four rates of biochar (0, 10, 25, and 50 t/ha), with and without nitrogen application (100 kg N/ha) were investigated. Both biochars, without N fertiliser, produced similar increases in dry matter yield of radish, which were detectable at the lowest application rate, 10 t/ha. The yield increase (%), compared with the unamended control rose from 42% at 10 t/ha to 96% at 50 t/ha of biochar application. The yield increases can be attributed largely to the ability of these biochars to increase N availability. Significant additional yield increases, in excess of that due to N fertiliser alone, were observed when N fertiliser was applied together with the biochars, highlighting the other beneficial effects of these biochars. In this regard, the non activated poultry litter biochar produced at lower temperature (450°C) was more effective than the activated biochar produced at higher temperature (550°C), probably due to higher available P content. Biochar addition to the hardsetting soil resulted in significant but different changes in soil chemical and physical properties, including increases in C, N, pH, and available P, but reduction in soil strength. These different effects of the 2 different biochars can be related to their different characteristics. Significantly different changes in soil biology in terms of microbial biomass and earthworm preference properties were also observed between the 2 biochars, but the underlying mechanisms require further research. Our research highlights the importance of feedstock and process conditions during pyrolysis on the properties and, hence, soil amendment values of biochars.

To compare the effectiveness of different pasture species in restoring soil quality, changes in concentration and quality of soil organic carbon (C) were measured in the surface 10 cm of an Oxic Paleustalf (red earth) in the semiarid area of New South Wales, Australia, at the end of 4 years under lucerne (Medicago sativa cv. Trifecta), Consol lovegrass (Eragrostis curvula), and barrel medic (Medicago truncutulata cv sephi). Before the investigation, the soil had been degraded by 50 years of cropping. Soil samples were analyzed for water stable aggregation, mineralizable N, and C by three procedures: Total carbon (C) by dry combustion, oxidizible C by potassium permanganate, and oxidizible C by potassium dichromate/sulphuric acid with varying concentrations of acid. Higher dry matter production caused lucerne to be was more effective than barrel medic in increasing soil organic carbon concentration. Compared with fallow plots, total soil organic carbon concentration increased by 16, 26, and 11%, respectively, in the Consol lovegrass, lucerne, and barrel medic treatments. Nevertheless, even in the case of lucerne, the 26% increase in organic carbon in the 0-10-cm layer at the end of 4 years (7.87 vs. 9.88 g/kg) represented only 15% of the total loss in organic carbon after 50 years of cropping. Most (78-92%) of the organic carbon increases under the various pastures were of the more labile forms, as indicated by their removal under much milder oxidizing conditions than those recommended in the standard methods for organic carbon determination. Significant improvements in structural stability and nitrogen availability were detected in the perennial pasture soils. Our results suggested that the amount of organic carbon oxidizible by a modified Walkley-Black method, which involves using only half the amount of sulphuric acid, is a more sensitive indicator of the improvement in soil quality parameters under investigation, namely increases in mineralizable nitrogen and water stable aggregation. Further research is needed to verify these findings over a range of soil types and agroecosystems.