Effects Of Long-term Manure And Fertilizers On Soil Active Organic Carbon Fractions In Typical Cropland Of China

Soil organic carbon (SOC) plays a key role in several physical, chemical and biological soil processes that contribute productivity of agricultural soils. More researches are mainly focused on the dynamics of total SOC, its relationship to the total carbon, and particle fractions of SOC in the upper surface (0-20cm) layer of soil. A few studies were conducted to examine the active and passive fractions of SOC in different soils, much less along further soil profile or under long-term application of manure and fertilizers. Thus, in the present study, an attempt has been made to evaluate the effects of long-term manure and fertilizers on active organic carbon fractions in0-20,20-40and40-60cm soil layers. I analyzed the changes of SOC, the amount and relative proportions of active SOC fractions in different depths, and relationships between active fractions and the total SOC to explore how the long-term application of manure with fertilizers affect (ⅰ) the increment of active fractions of SOC (ⅱ) and the distribution of active fractions in the soil depth.This study was carried out in five long-term fertility experiments (Gongzhuling, Zhengzhou, Chongqing, Jinxian and Qiyang) varying in soil type, cropping patterns and agro-climatic conditions in the arable cropping regions of China. The treatments examined were:(1) No-fertilization (control);(2) Fallow, where no crop was grown since the initiation of the experiment;(3) NPK;(4) NPK combined with crop straw (NPKS);(5) NPK with livestock manure (NPKM); and (6) higher application rate of NPKM (1.5NPKM). Soil samples were collected at0-20,20-40and40-60cm depths and analyzed for total C, total SOC, and four fractions of SOC:very labile (Cfrac1=12N H2SO4), labile (Cfrac=18N-12N H2SO4), less labile (Cftac3=24N-18N H2SO4) and non-labile (Cfrac4=Total SOC-24N H2SO4). The main results are summarized as follows:1. The application of manure and fertilizers increased the SOC when compared with the initial C content of1990. At Gongzhuling site, the NPK, NPKS, NPKM and1.5NPKM treatments increased the SOC content by14,17,54and69%in0-20cm and15,28,52and80%, respectively in20-40cm soil layers. The NPK, M, NPKM and1.5NPKM treatments increased the SOC content by28,90,82and121%, respectively in0-20cm soil layer at Qiyang. The highest increase of SOC was179%observed in1.5NPKM treatment at Zhengzhou. The1.5NPKM and M had the highest SOC in all five sites.2. The fertilization increased the different fractions of SOC compared with the initial C content of1990. At Gongzhuling, the NPK, NPKS, NPKM and1.5NPKM treatments increased the Cfrac, and Cfrac2content by6,14,39and48%, and12,27,49and50%, respectively in0-20cm soil layer. The Cfrac4was increased by120and102%in1.5NPKM treatment followed by83and61%in NPKM at0-20cm and20-40cm soil layers. At Qiyang, the highest increase of126,113and158%was recorded in M, NPKM and1.5NPKM treatments in Cfrac4at0-20cm soil layer. While, at Zhengzhou, the highest increase of63,71,141and179%was observed in Cfrac4in NPK, NPKS, NPKM and1.5NPKM treatments. In all5sites, the inorganic and manure fertilization treatments increased the both labile and recalcitrant C pools compared to control treatment. Significant correlations were found among different fractions of SOC. This suggests that these C fractions are in dynamic equilibrium with each other. The increase or decrease in one fraction may shift this equilibrium and also affect the size of the other fractions.3. Manure and fertilizer application increased the recalcitrant C fraction (Cfrac3and Cfrac4) of total SOC compared with control and fertilizer only treatments. At Gongzhuling and Chongqing, the significantly highest recalcitrant C pool of66.1and56.6Mg ha-1was observed in1.5NPKM treatment. While, at Jinxian the significantly highest recalcitrant C pool of39.6Mg ha-1was found in NPK+M treatment. The long-term manure and fertilizers application decreased the labile:recalcitrant ratio of SOC compared with the no-fertilization control. The SOC retention was higher in the NPK and manure treated plots compared to control plots. The active C pool (Cfrac1and Cfrac2) constituted52-56%and recalcitrant C pool (Cfrac3and Cfrac4) constituted44-48%of total SOC at all five sites.4. Manure and fertilizers showed the different roles in increasing the fraction contents with the soil depth. Manure increases the recalcitrant fraction (Cfrac3and Cfrac4) in deep layer. For Qiyang, the recalcitrant C fraction was55%of total SOC at20-40cm in M treatment compared with the surface layer (51%). Fertilizers showed to increase the active fractions (Cfrac1and Cfrac2) in the surface layer much. For Gongzhuling, the labile fraction was55%of total SOC in0-20cm soil layer compared with50%at20-40cm in NPK treatment. The labile:recalcitrant ratio of SOC was lower in20-40cm soil layer compared with the surface layer in all the treatments. A larger proportion of more oxidizable C was recorded in upper layer. With the increase in depth the proportion of resistant C fractions was increased.5. Serial H2SO4concentrations were used to divide SOC into different fractions. But, the very labile fraction measured by12N H2SO4was higher than the active carbon measured by traditional KMnO4method. So, I modified this method and decreased the concentration of H2SO4to6N, The results revealed that labile fractions determined by the KMnO4and6N H2SO4methods showed a significantly positive correlation in four sites. Therefore, it is suggested that6N H2SO4method may be used for determination of labile fraction of SOC.I concluded that the combined application of manure and fertilizers has the potential to significantly increase the SOC and its fractions in agricultural soils of China. The inorganic plus manure fertilizer treatments contained a larger proportion of total SOC in recalcitrant fraction compared with NPK and no-fertilizer treatments indicating that manure application helped in promoting the formation of SOC in the recalcitrant fraction and thus protecting SOC from decomposition losses. Fertilizer treatment had the higher labile:recalcitrant ratio in surface layer than the sub-surface layer implied that fertilizer helped to increase the surface SOC activity. The higher labile:recalcitrant SOC ratio in surface layer than deeper soil layer indicating that improving the depth distribution may be practical suggestion to achieve C retention.