To understand the effects of agroforestry on soil biological processes we
assessed the conditions in Pinus radiata plantations of
50, 100, 200, and 400 stems/ha after 25 years of growth, and in a
grassland. Agroforestry resulted in a 15–25% decline in soil
organic C and N compared with grassland, and had a significant negative
influence on soil microbial biomass. There was less microbial C and N in soils
under 50–400 stems/ha of P. radiata than in
soils under grassland (0 stems/ha). Soil carbon decomposition and
microbial activity were measured by trapping the carbon dioxide produced by
incubating soils over a 60-week period. The results showed that soil C
decomposition rates were ~1.5 times as much (c. 15 mg
CO2-C/kg soil) in soil from grassland as in that
from plots with 50 or100 stems/ha (c. 10 mg
CO2-C/kg soil), and were further reduced to one half
(c. 5.5 mg CO2-C/kg soil) in
the plots with 200 or 400 stems/ha. The soils under
P. radiata gave off less carbon dioxide per unit of
biomass (the metabolic quotient) than soils under grassland. These shifts in
microbial biomass and its metabolic quotients appear to be associated with
differences in the quantity and ‘quality’ of inputs and soil
organic matter decomposition rates, and to reflect the land use change from
grassland to forest. Given the general ability of soil microbial biomass to
recolonise depopulated areas after tree harvest, we see no problem in
restoring populations of these soil organisms vital in controlling nutrient
cycling after tree felling, provided adequate adjustments to soil pH are made.
Metode-Metode Penetapan Biomassa Mikroorganisme Tanah secara Langsung dan Tidak Langsung: Kelemaahn dan Keunggulannya(Direct and Indirect Methods of Soil Microbial-Biomass Determination: Weakness and Strength)