Hot-water extractable carbon studies as an indicator on status of forests and their regeneration

Abstract

Soil is an important substratum that affects plant regeneration. In this study, the organic matter of the soils of the Bukit Timah Nature Reserve Primary and Secondary Forests was investigated as an indicator of soil fertility. The Hot-Water Extraction technique was used to extract organic matter present in soil samples of Primary and Secondary Forests of the Bukit Timah Nature Reserve (BTNR). Total organic carbon content found in the Hot-Water Extractable Carbon (HWEC) extracts across different points of a 20 m x 20 m plot from both BTNR Primary and Secondary Forests were analysed. Total above-ground biomass (AGB) of plants of each forest plot was estimated using the individual diameter at breast height (dbh) of the plant. BTNR Primary Forest plot had a higher AGB of 29713 kg per plot than that of the Secondary Forest plot which had an AGB of 7380 kg per plot.<br><br>Hot-water Extractable Carbon (HWEC) levels present in 0-10 cm depth soil samples from Primary Forest which ranged from 1.93 to 3.93 mg g^-1 of soil were higher than that of Secondary Forest which ranged from 1.64 to 2.10 mg g^-1 of soil. HWEC levels present in 10-20 cm depth soil samples from Primary Forest which ranged from 0.897 to 1.47 mg g^-1 of soil were also higher than that of Secondary Forest which ranged from 0.536 to 1.08 mg g^-1 of soil. Mass of chye sim (Brassica chinensis) shoots grown on BTNR Primary Forest and Secondary Forest soils was used to assess the effect of HWEC on their growth. HWEC levels in soil samples affected the mass of chye sim (Brassica chinensis) shoots grown. Mean dry mass of chye sim (Brassica chinensis) shoots cultivated on 0-10 cm depth soil samples from BTNR Primary Forest ranged from 6.22 to 9.6 mg and were higher than those cultivated on 10-20 cm depth soil samples which ranged from 5.17 to 8.24 mg. Mean dry mass of chye sim (Brassica chinensis) shoots cultivated on 0-10 cm depth soil samples from Secondary Forest ranged from 6.66 to 8.21 mg while the mean dry mass of shoots cultivated on 10-20 cm depth soil samples ranged from 6.97 to 12.8 mg.<br><br>Carbohydrate precursors –oxirane, amides and amines were present in the HWEC of soil samples from BTNR Primary Forest and Secondary Forest. Acetyl semicarbazone was present in very large quantities in HWEC of soil samples from BTNR Primary Forest.<br><br>Alkaline-hydrolysed leaves of a Primary Forest predominant tree species (Shorea curtisii) and a Secondary Forest predominant tree species (Campnosperma auriculata) HWECs of concentrations from 0 to 600 mg/L were respectively administered to chye sim (Brassica chinensis) seedlings. Chye sim (Brassica chinensis) shoots supplied with 600 mg/L Shorea curtisii HWEC had the highest mean dry mass of 52.2 mg while that supplied with 200 mg/L Campnosperma auriculata HWEC had the highest mean dry mass of 40 mg. Amines were present in both types of alkaline-hydrolysed leaf HWECs.<br><br>There was high correlation between HWEC levels with at least one of the four fertility-related parameters– Water Soluble Carbon (WSC) levels, nitrite and nitrate levels in WSC and HWEC extracts, field capacity and microbial DNA concentration at p < 0.05 significance level.<br><br>Greater biomass of chye sim (Brassica chinensis) cultivated on forest soils with higher HWEC suggested that HWEC may be an important determinant of forest regeneration. Other than the total organic carbon content of HWEC affecting plant growth, different nitrogen composition in the carbon compounds may play an important role as well.