Conversion effects of farmland to Zanthoxylum bungeanum plantations on soil organic carbon mineralization in the arid valley of the upper reaches of Yangtze River, China.

Farmland conversion to forest is considered to be one of the effective measures to mitigate climate change. However, the impact of farmland conversion to forest land or grassland on soil CO₂ emission in arid areas is unclear due to the lack of comparative information on soil organic carbon (SOC) mineralization of different conversion types. The SOC mineralization in 0–100 cm soil layer in farmland (FL), abandoned land (AL) and different ages (including 8, 15, 20 and 28 years) of Zanthoxylum bungeanum plantations were measured by laboratory incubation. The size and decomposition rate of fast pool (C_f) and slow pool (C_s) in different land-use types and soil layers were estimated by double exponential model. The results showed that: 1) Farmland conversion increased the cumulative CO₂-C release (C_min) and SOC mineralization efficiency, and those indexes in AL were higher than that in Z. bungeanum plantations. The C_min and SOC mineralization efficiency of 0–100 cm soil increased with the ages of Z. bungeanum plantation. Both C_min and SOC mineralization efficiency decreased with the increase of soil depth; 2) Both soil C_f and C_s increased after farmland converted to Z. bungeanum plantations and AL. The C_s in the same soil layer increased with the ages of Z. bungeanum plantation, and the C_f showed a "V" type with the increased ages of Z. bungeanum plantation. The C_f and C_s decreased with the increase of soil depth in all land-use types; 3) Farmland conversion increased the decomposition rate of C_f (k₁) in all soil layer by 0.008–0.143 d⁻¹ and 0.082–0.148 d⁻¹ in Z. bungeanum plantations and AL, respectively. The k₁ was obviously higher in the 0−20 cm soil layer than that in other soil layers, while the decomposition rate of C_s (k₂) was not affected by FL conversion and soil depth; and 4) The initial soil chemical properties and enzyme activity affected SOC mineralization, especially the concentrations of total organic nitrogen (TON), SOC, easily oxidizable organic carbon (EOC) and microbial biomass carbon (MBC). It indicated that the conversion of farmland to Z. bungeanum plantations and AL increases SOC mineralization, especially in deeper soils, and it increased with the ages. The conversion of farmland to Z. bungeanum plantation is the optimal measure when the potential C sequestration of plant-soil system were taken in consideration. [ABSTRACT FROM AUTHOR]