Effects Of Afforestation On Soil Organic Carbon Content And Composition Characteristics In Northeastern China

Soil organic carbon (SOC) is important in terrestrial ecosystem carbon pool and have significant influence in soil fertility, water retention, soil microbial activity and other soil physical and chemical properties. In recent years, afforestation is an important project in China for controlling soil degradation, biodiversity loss and increasing C sequestration. To investigate SOC sequestration mechanism, it is nessary to evaluate the the influence of afforestation on SOC content and composition. This study was based on the comparasion between arable lands under continuous cultivation and adjacent afforested lands converted from croplands for6,12and25years, in northeastern China. Changes in SOC content were analysised and impact of afforestation on SOC distribution in soil profile was evaluated with Daubechies discrete wavelet transform. The pathway of forest litter derived C incorporated into SOC was investigated on sequential density fraction with δ13C analysis. SOC composition was analyzed by pyrolysis gas-chromatography-mass-spectrometry. Main conclusions were following:(1) There was an initial decrease in SOC after afforestation followed by a gradual increase. The initial decline was observed to last for at least6years. Subsequent accumulation of C was at25years forest. N content decreased in5and25years forest compared with dajacent arable soil, respectively. The influence of afforestation on SOC and N was significant in0-60cm depth and is not significant in60-160cm depth. After planting trees of25years, C/N increased in0-60cm forest sites compared with adjacent arable sites, indicating the SOC was less decomposed by microbial organism in forest sites than that in arable sites. Effect of afforestation on SOC was related with the annual input of C from forest and the C decompostion of former arable SOC. The SOC and N distribution was influenced by the Fluvents soil forming process and coarse texture.(2) In6years forest, afforestation effect in SOC content distribution is not significant, and forest and arable SOC content changed significantly in about80cm depth in10cm scale. In12years forest, afforestation significantly influenced SOC content distribution in10and40cm scale, and in25years forest, afforestaion significantly influenced SOC content distribution in20and40cm scale. In6years forest, afforestion effect was not significant in N content distribution, and forest and arable N content changed significantly in about60cm depth in10cm scale. In12years forest, N content changed significantly in deeper than100cm in10,20and40cm scale. In25years forest, afforestation significantly influenced N content distribution in10and20cm scale.(3) Afforestation decreased SOC content in<2.5g cm-3fractions of6years forest sites, but increased SOC content in<2.0g cm-3fractions of25years forest sites. No significant incluence in SOC concentration was observed in all density fractions of12years forest. Afforestation did not affect the soil mass, SOC and N proportional weight distribution. Most forest litter-derived C entered the low density fractions of soil firstly, then passed through low density aggregates to higher density organic-mineral associations. A small proportion of the forest litter-derived C could rapidly diffuse and absorb on single mineral particles. Former arable derived C which was associated with denser fractions were depleted slowly than which was associated with lighter fractions. The pattern of forest litter incorporation and former litter derived C depletion suggested that microaggregate protection and association between organic material and mineral controlled the C sequestration within the afforestation soil system.(4) Afforestion influenced the SOC chemical composition. There was no significant correlation between litter composition and SOM composition. Afforestion increased lignin abundance in12years forest topsoil but decreased lignin contribution in6years forest topsoil, likely due to the increasing litter input to the topsoil and inhibited lignin decomposition rates. Twelve years and twenty-five years forest soil contained abundant less-degraded material. While the arable soil (5-10cm), adjacent of25years forest, contained substantial strongly decomposed plant material and microbially altered material and showed the effect of vegetation burning.