Abstract
Afforestation is a prevalent practice carried out for soil recovery and carbon sequestration. Improved understanding of the effects of afforestation on soil organic carbon (SOC) content and dynamics is necessary to identify the particular processes of soil organic matter (SOM) formation and/or decomposition that result from afforestation. To elucidate these mechanisms, we have used a sequential density fractionation technique to identify the transfer mechanisms of forest derived C to soil fractions and investigate the impact of afforestation on SOC sequestration. Surface soil samples from continuous maize crop land (C4) and forest land (C3), which had been established 5, 12 and 25 yr, respectively, on the Northeast China Plain were separated into five density fractions. SOC, nitrogen (N) concentration and δ13C data from the three forests and adjacent cropland were compared. Afforestation decreased SOC concentration in the < 2.5 g cm-3 fractions from 5 yr forest sites, but increased SOC content in the < 2.0 g cm-3 fractions from 25 yr forest sites. Afforestation did not affect soil mass distribution, SOC and N proportional weight distributions across the density fractions. The < 1.8 g cm-3 fractions from 12 and 25 yr forests showed higher C/N and lower δ13C as compared to other fractions. Incorporation of forest litter-derived C occurred from low density (< 1.8 g cm-3) fractions to aggregates of higher density (1.8-2.5 g cm-3) through aggregate recombination and C transport in the pore system of the aggregates. Some forest litter-derived C could transfer from the light fractions or directly diffuse and adsorb onto mineral particles. Results from this study indicate that microaggregate protection and association between organic material and minerals provide major contribution to the SOC sequestration in the afforested soil system.
Highlights
Carbon emission from land use conversion during the last two centuries is approximately 100~150 Pg C [1], and around one half of which is attributed to loss of soil organic matter (SOM) [2]
High C/N in the < 1.8 g cm-3 fractions indicated that the coarse plant debris in this fraction had been partly decompsed because the C/N of this fraction was lower than fresh litters [21, 37]. This fraction from 12 and 25 yr forest showed higher C/N and lower δ13C compared to other fractions, and held the highest forest C propotion and the highest decay rates of old C. These results indicated that forest litter derived C mainly entered into these fractions and the soil organic carbon (SOC) turnover rates was higher in these factions than others
Afforestation resulted in significant decrease in SOC concentration for the 5 year old forest and increase in SOC concentration in the 12 and 25 year old forest
Summary
Carbon emission from land use conversion during the last two centuries is approximately 100~150 Pg C [1], and around one half of which is attributed to loss of soil organic matter (SOM) [2]. Conversion of forest to agricultural land can result in the depletion of SOM, while. Previous land management [7], climate characteristics, and the quantity and quality of C input can influence soil C turnover rates following forest establishment [8, 9]. Improved understanding of the effects of afforestation on SOC content and dynamics is necessary to accurately characterize the particular processes of SOM formation and decomposition that result from changes in land use
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