Soil active organic carbon is the most important carbon pool and a good indicator in ecosystem management due to its great significance in soil carbon cycling and soil quality.In order to investigate the effect of biochar (BC) addition on soil organic matter fractions,apple tree twigs were used to produce BC at 300,400,500 and 600℃,respectively.Elemental analysis and Fourier transform infrared (FTIR) spectroscopy were used to determine the characteristics of BC.Four kinds of BC were added into soils at five application rates (0,0.5%,1%,2% and 3%) and incubated at 25℃ in lab for over 360 days.Soil organic carbon (SOC),microbial biomass carbon (MBC),water soluble organic carbon (WSOC) and readily oxidized organic carbon (ROC) were measured during the incubation.The mass fraction of carbon (C) in the generated BC ranged from 62.20%-80.01%,while hydrogen (H) ranged from 2.72%-5.18% and Oxygen (O) ranged from 15.98%-30.92%.The increasing temperature increased the mass fraction of C,while decreased the O and H mass content,as well as the ratio of H/C and O/C.The addition of BC significantly increased SOC,and the treatments amended with BC500 had the highest increments.Compared with the control treatment (CK),the addition of BC produced at temperatures below 400℃ increased the contents of MBC,WSOC and ROC during the incubation,at the end of the incubation,BC300 treatments significantly increased the contents by 38.25%,82.09% and 63.53%(P<0.05),respectively;BC400 treatments significantly increased the contents by 26.07%,65.61% and 48.09%(P<0.05),respectively;while lower contents of MBC,WSOC and ROC were found in the treatments amended with BC produced at temperatures above 400℃ after 40-60 d incubation.After 360 d of incubation,the contents of MBC,WSOC and ROC were significantly decreased by 0.27%,13.48% and 14.67% in BC500 treatments and 7.80%,14.66% and 15.79% in BC600 treatments (except for the MBC in BC500 treatment)(P<0.05).The relative contents of ROC ranged from 3.39% to 15.65%,BC application decreased the relative content of ROC,suggesting that the increase was in proportion to the stability of organic carbon in the soil.Considering the content and quality of SOC,when the BC products were applied to the Loutu soil,500℃ was the optimal temperature for preparing apple-derived BC due to its significant increase of the soil organic carbon and a slight decrease of the relative content of soil active organic carbon.
Keywords: active organic carbon; biochar; microbial biomass carbon; organic carbon; readily oxidized organic carbon; water soluble organic carbon.