The Evolution Of The Contents,Carbon Fractions And Chemical Structureal Of Soil Organic Carbon During The Early Pedogenesis Of A Mollisol After Development From Parent Material

In northeast China, Mollisols are distributed between mountain and plain areas, and a large amount of slope croplands are formed along this terrain. Soil loss due to soil erosion and coal mining has occurred in northeast China, and has left parent materials closer to the surface or even exposed, and this affects the ability of soils to support food production. There is a need to develop strategies for restoration, sustainable use and protection of these soils. Knowledge of soil-forming processes from parent material in this region is poor, possibly because soil development takes at least several hundred years to produce an appreciable level of soil fertility under natural conditions. An experiment was established in 2004 at the State Key Experimental Station of Agroecology, Chinese Academy of Sciences, Hailun, Heilongjiang province, in order to address key components of soil formation under conditions representative of this region. Three ecological systems were imposed in this experiment:(1) natural fallow without weed control(Nat F);(2) alfalfa(Alfa);(3) crop rotations. The cropping system included four different mineral fertilizer and organic matter inputs: crop rotations without mineral fertilization(F0C0) and biomass removal; crop rotations with mineral fertilization and biomass removal(F1C0); crop rotations with mineral fertilizer and partial biomass incorporation(F1C1); crop rotations with mineral fertilizer and all biomass incorporated(F1C2). The main objective of this study was to investigate the changes in carbon sequestration, physical and chemical fractions of soil carbon, and chemical identity of soil C occurring during soil development from parent material under different management practices and land use. These findings have practical implications in describing how the combination of vegetation and management practices could be used to optimize soil development and fertility improvement. The main observations and conclusions were as follow:The parent material showed initial low fertility, with a soil organic carbon(SOC) content of 4.8 g kg-1, having been excavated out below 2 m depth from a comparatively closed environment. Over the 10 year period of development, SOC content in the parent material increased from 4.8 g kg-1 to 7.5-12.7 g kg-1 in the six treatments. The SOC in Nat F and Alfa treatments increased by 85% and 117%, respectively, while no fertilization and chemical fertilization of cropping systems increased SOC by 56% and 72%, respectively. The C sequestration efficiencies ranged from 0.27 to 0.85 g C kg-1 yr-1. The highest increase of SOC content took place in the treatments receiving organic inputs(37 Mg ha-1). In these treatments, the SOC content after 10 years of management was equivalent to 50% of that of a mature Mollisol(M1C0). No significant increase in SOC after long-term application of organic amendments was found in mature Mollisols, indicating that mature Mollisols are essentially at saturation level.The vertical distribution of SOC showed that the C content decreased with increasing soil depth in all treatments. Therefore, C accumulation occurred in the topsoil first, then gradually moved down..In the 300-400 mm layer, SOC content significantly increased only in the F1C2 treatment. The C input in subsoil as root litter and rhizodeposition considerably contributed to this SOC increase. Based on C sequestration efficiency and ability, we concluded that organic material applications and alfalfa can best improve soil fertility and increase C storage.Physical fractionation of SOC, based on separation by density, was used to evaluate the degree of protection of new C and the mechanism of its stabilization. The C content was lowest for material in the heavy fraction(HF) and highest in the occluded light fraction(o LF). However, most of the SOC, on a mass basis, was located in the HF, and therefore, the HF accounted for the largest portion of total SOC(78–90%). The amount of C in the o LF was significantly greater in all treatments compared to that of parent material(PM). The comparison between these organic fractions indicated that o LF is more sensitive than the free light fraction(f LF) to detect changes in soil due to land use and management practices at the beginning of soil improvement. Over the 10 year period of development, the C content in HF decreased in the order: F1C2>F1C1>Alfa>Nat F>F1C0>F0C0. The mount C in the HF was found to significantly correlate with cumulative C input.Chemical fractionation, based on solubility of SOC components in acid and alkali, was used as an indicator of the stabilization of organic inputs as they were transformed into more stable SOC. In the first 5 years, in parent material with low fertility, we found that HA that the amount of fulvic acids(FA) was greater than the amount of humic acids(HA). The C concentration of these fractions ranged from 15 to 19%, and 21 to 42%, respectively. The FA fraction is thought to be formed first, due to the simple structure and low molecular weight of these compounds, and later undergoes changes to HA. The OC in the FA fraction increased in the soybean/corn treatments. With the improvement of soil fertility, the C content of HA increased. Changes in the relative proportion of HA and FA in total SOC indicated that soil structure and C stability changed with time. The C content of humin ranged from 66 to 74%, and had a positive relationship with cumulative C input, showing that C inputs were incorporated into stable SOC in a relatively short period of time compared to the supposed timescale of soil formation.FTIR spectra were taken to provide the quality information of the types of compounds being accumulated as new SOC. The presence of significant amounts of aliphatic C was not observed in the original parent material. However, greater degree of aliphatic character was observed as soil fertility improved, as well as the intensity of remaining functional groups. As well as aromatic, protein and polysaccharide bands were found with as soil fertility improved. With the increase of cumulative C input, the degree of aliphatic character and aromatic gradually increased.Carbon-13 NMR spectra were taken to provide a general indication of the types of compounds being accumulated as new SOC. These spectra are generally divided into four main chemical-shift regions, representing alkyl C(0-45 ppm), O-alkyl C(45-110ppm), aromatic C(110-160 ppm), and carboxyl C(160-210 ppm). As soil fertility improved, the proportion of O-alkyl C, protein and polysaccharide were increased in all treatments. These founding indicated that soil C tended to be highly aliphatic, aromatic and stable.In general, the parent material at our experimental site was low in fertility and “starved” for C. For this reason, we observed a dramatic short-term effect of C inputs, by which C was stabilized in the mineral matrix. The dynamics of this accumulation was followed by using three distinct indices of SOC properties, providing insight into the mechanisms by which SOC accumulates, leading to improvements in soil fertility.