Effect of amorphous Al hydroxide addition on soil organic matter decomposition with bamboo plantation

Soil organic matter (SOM) accumulated rapidly in soils with Lei bamboo (Phyllostachys praecox f. Preveynalis) plantation in Southeast China. Accompanied by dramatic acidification, the Al oxides in soil greatly influence SOM transformation and stabilization. However, the effect of Al oxides in SOM decomposition in acidified soils is not still well known. The aim of this study was to investigate the effect of amorphous Al hydroxide (AAH) addition on SOM decomposition in Lei bamboo plantation with severe acidification for a better understanding of the influence of acidification on SOM preservation. Soil samples (derived from Quaternary sand shale) in a cultivation time sequence were collected from bamboo stands. SOM composition was analyzed by C-13 cross-polarization magic-angle spinning (CPMAS) NMR, and soil Al oxides were fractionated. A batch of incubation experiment with various AAH additions, i.e., 0, 2, 10, 20, and 40 g kg(-1), respectively, was conducted in lab. Fifteen soil samples of 50 g in 1000 ml plastic containers with three replicates were incubated under a temperature of 28 A degrees C and moisture of 80 % at field water capacity. The total microbial activity represented by fluorescein diacetate (FDA) hydrolysis was determined as well as CO2 release. SOM in the Lei bamboo field increased rapidly from 31.0 to 79.2 g kg(-1) (P < 0.05) 15 years after transplanting; however, soil pH decreased dramatically from 5.47 to 3.32 (P < 0.05). Simultaneously, soil exchangeable Al increased from 3.85 to 198 mg kg(-1), and the ratio of soil free Al oxide to total Al (Al-d/Al-T) increased from 2.60 to 3.67 %. The 2-month incubation results indicated that the addition of AAH suppressed SOM decomposition on the whole as a maximum of 60.7 %. The inhibitory effect increased with the increase in AAH addition rate. However, the inhibitory rate in SOM decomposition decreased with the increase in SOM content under the same AAH addition. The total microbial activity represented by FDA was also suppressed significantly by the addition of AAH up to 52.8 %. The effect of AAH on SOM decomposition is mainly responsible for physical protection and enzyme activity suppression. With a high ratio of SOM/AAH, AAH could serve as a core of SOM-AAH-SOM complexation to increase the labile part of SOM for microbial decomposition. Such a mechanism might explain the stimulation of SOM decomposition under the high SOM/AAH ratio.