Accumulation of physically protected organic carbon promoted biological activity in macro-aggregates of rice soils under long term rice cultivation
Authors: Liu Yalong , Wang Ping , Li Lianqing , Cheng Kun , Zheng Jufeng , Filley Timothy , Zhang Xuhui , Zheng Jinwei , Pan Genxing 
Published: April 2016

Abstract. While carbon stabilization had been increasingly concerned as ecosystem properties, the link between carbon stabilization and soil biological activity had been yet poorly assessed in soil dynamics of carbon and aggregation. In this study, topsoil samples were collected from rice soils derived from salt marsh under different lengths of rice cultivation up to 700 years from a coastal area of China. Particle size fractions (PSF) of soil aggregates were separated using a low energy dispersion protocol. Carbon fractions in the PSFs were analyzed with either FTIR spectros copy or chemical fractionation. Soil microbial community of bacterial, fungal and archaeal were analyzed with molecular fingerprinting using specific gene primers. Soil respiration and carbon gain from maize straw amendment as well as enzyme activities were respectively measured, using lab incubation protocols. While the PSFs were dominated by fine sand (200–20 μm) and silt (20–2 μm) fractions, the mass proportion both of sand (2000–200 μm) and clay (< 2 μm) fraction increased with prolonged rice cultivation. Soil organic carbon was enriched mostly in coarse sand fraction (40–60 g/kg), followed by the clay fraction (20–25 g/kg), but depleted in the silt fraction (~ 10 g/kg). Contents of recalcitrant C pool were higher (33–40 % of total SOC) in both coarse sand and clay fractions than in fine sand and silt fractions (20–29 % of total SOC). However, the ratio of LOC/SOC showed a weak decreasing trend with decreasing size of the PSFs. Total soil DNA content in the size fractions followed a similar trend to that of SOC. Bacterial and archaeal gene abundance were concentrated in both sand and clay fractions but that of fungi in sand fraction only, but increased with prolonged rice cultivation in both sand and clay fractions. Change in community diversity with sizes of the PSFs was found of fungi and weakly of bacterial but not of archaeal. Soil respiration quotient (Respired CO2-C to SOC) was highest in silt fraction, follo wed by the fine sand fraction but lowest in sand and clay fractions in the rice soils cultivated over 100 years. Whereas, scaled by total DNA concentration, respiration was higher in silt fraction than in other fractions for these rice soils. For the size fractions other th an clay fraction, soil DNA concentration, archaeal gene abundance, normalized enzyme activity and carbon sequestration was seen increased but SOC- and DNA-content scaled soil respiration decreased, more or less with prolonged rice cultivation. Carbon chemical stability and respiration were in a similar between sand and clay fractions but correlations of total DNA contents and bacterial gene abundance as well as normalized enzyme activity to SOC and labile OC content were only observed in sand fraction only. Our findings suggested that carbon accumulation and stabilization was prevalent in both sand and clay fraction, only the coarse sand fraction was found responsible for bioactivity dynamics in the rice soils.