Characteristics Of Soil Organic Carbon Fraction And Chemical Composition Under Long-term Fertilization In Upland Soil Of China

Soil organic carbon (SOC) is an essential component with key multifunctional roles in soil fertility as well as in global carbon cycling and global climate change. Fertilizer amendment to agricultural soil has long been widely used as a common management practice to maintain or increase soil fertility and SOC level. However, from the point of carbon sequestration, identify the key fractions and the best fertilizer management practices for SOC accumulations as well as explore the changes of SOC chemical composition under different fertilizer managements across different cropping systems in China are still a challenge that is yet to be fully resolved.Based on four cropland long-term fertilizer experiments in China, changes in SOC and total nitrogen (TN) in bulk soil, microbial biomass carbon (MBC) and nitrogen (MBN) and physical fractions as well as the changes in SOC chemical composition were examined using physical fractionation, chloroform fumigation extraction method and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Four soil physical fractions are coarse free and fine free particulate organic carbon (cfPOC and ffPOC), intra-microaggregate POC (iPOC) and mineral associated organic carbon (MOC). The SOC chemical composition were presented as alkyl C, O-alkyl C, aromatic C and carbonyl C. The fertilization treatments included:unfertilized control, chemical nitrogen (N), phosphorus (P) and potassium (K) (NPK), NPK plus straw (NPK.S, hereafter straw return), and NPK plus manure (NPKM and 1.5NPKM, hereafter manure). The major results and conclusions were as follows:(1) SOC and TN in bulk soil and fractions varied among the different fertilizations treatments and the effects were significant higher under manure treatment. Compared with control, manure significantly increased all tested parameters at the three sites while straw return had no effects on all indexes at Gongzhuling site. Chemical fertilization had no effects on SOC, TN, MBN and MOC, but significantly decreased MBC in Gongzhuling and Qiyang sites.(2) Free particulate fraction was the sensitive indicator of SOC and TN changes to fertilizations, while SOC and TN were most sequestreated in mineral associated fraction. SOC and TN in fractions distributed as MOC> iPOC> cfPOC> ffPOC with the highest increase in cfPOC (329.3%) and cfPTN (431.1%), and the lowest in MOC (40.8%) and MTN (45.4%) under manure. SOC significantly positively correlated with MBC, cfPOC, ffPOC, iPOC and MOC (R2=0.51-0.84, P<0.01), while TN with cfPTN, ffPTN, iPTN and MTN (R2=0.45-0.79, P<0.01). Principal component analyses explained 86.9-91.2% variance of SOC, TN, MBC, MBN, SOC and TN in each fraction.(3) SOC chemical composition determined showed profound changes under four fertilization treatments, compared with the initial soil sample.22 years different fertilization decreased the proportions of aromatics C on average by 3.4-5.6% while increased O-alkyl C on average by 3.9-7.3%. However, no appreciable differences of the NMR spectra were observed among different fertilizer types in 2012.(4) O-alkyl C contributed most to the increased SOC stock and soil properties attribute to the SOC conversion rate of NMR-determined functional groups. The soil carbon concentrations of each NMR-determined functional group were positively linear correlated with cumulative C input, and the conversion rate (the slope of linear regression between SOC sequestered and cumulative C input) was highest in O-alkyl C (3.3-8.4%). Meanwhile, the conversion rate of functional group was mainly controlled by soil properties (C:N ratio, pH, clay content,26.79%) rather than climate factors (18.60%).In conclusion, increased stock with concurrent chemical composition changes of soil organic carbon under long-term continuous fertilization in three wheat-maize cropping systems of China were observed. Manure addition was the best fertilization for improving soil fertility while straw return should be taken into account climate factors in Chinese croplands. Additionally, there is a need to optimize soil conditions to achieve increased conversion rate of function group and finally sequestration of more C in soil.