| The Qinghai Tibetan alpine meadow stores vast quantities of soil organic carbon(SOC),which has an integrally functional role in sustaining global carbon cycle.In nutrient deficient alpine ecosystem,increasing population and global climate change increase soil bearing capacity and result in the aggravation of soil degradation.Despite the popular practice of chemical fertilization could increase plant productivity,there remains unknown in terms of the effects of fertilization on the accumulation and stability of SOC.Based on a field experiment in the Qinghai Tibet Plateau initiated in 1999,four amendment levels were chosen including amending ammonium phosphate((NH4)2HPO4)at 0(NPO),30(NP30),90(NP90)or 120(NP120)g m-2 yr-1.We aimed to determine SOC dynamics and its fractions in terms of physical protection and chemical composition,as well as their relationship with microbial community and soil enzyme activities,and to reveal the mechanisms of SOC stabilization in such susceptible alpine ecosystem.The main results are as followed:1)Compared to the NPO of no extra chemical fertilizer,nitrogen(N)and phosphorus(P)inputs had no significant influences on total SOC content(P>0.05),but increased soil available P by an average of 109.2%,and decreased soil pH(P<0.05).On average,the NO 3--N content increased by 68.7%at intermediate and high levels of N and P inputs(NP90 and NP120),compared to the other nutrient inputs.2)N and P inputs significantly increased total phospholipid fatty acid(PLFA)contents by an average of 29.6%.On average,N and P inputs significantly increased the bacterial PLFA by 28.3%,gram negative bacteria by 23.2%and gram positive bacteria by 40.2%,but decreased fungal PLFA by 13.9%and arbuscular mycorrhizal fungi(AMF)by 28.8%,respectively,resulting in an 23.6%decrease in fungal to bacterial ratio.N and P inputs also reduced activities of the measured C-cycling associated enzyme activities such as the hydrolases(i.e.,?-glucosidase,?-glucosidase,cellobiohydrolase and ?-xylosidase)and oxidase(i.e.,phenol oxidases and peroxidases).3)Under intermediate and high N and P inputs,when compared to the NPO and low nutrient inputs,proportion of macroaggregates decreased.Compared with the NPO,low and high N and P inputs significantly reduced C content of free light fraction by an average of 28.37%,but high N and P inputs increased mineral-associated fraction by an average of 8.6%,while it had no effects on the particulate organic carbon occluded within macroaggregate and microaggregate.4)The integration of the major regions of 13C-NMR revealed that,on average,N and P inputs significantly decreased the relative intensity of alkyl C by 10.1%but increased aromatic C by 12.0%and carbonyl C by 4.0%,resulting in 9.8%decrease in aliphaticity,2.8%decreased in hydrophobicity,and 13.2%increase in aromaticity.5)Linear regression analysis and structural equation modelling(SEM)indicated that the pathway of P availability directly modified SOC physical protection,chemical composition,microbial community and enzyme activity.Moreover,the reduced soil aggregate stability and increased mineral-associated C were mainly attributed to the decreased proportion of fungi and the increased bacteria.Together,long-term fertilization induced acidity exacerbation,aggregation degeneration,microbial community towards bacteria as well as the loss of oxidative enzyme activities contributed to the increase of mineral associated C and aromaticity.Notably,soil available P rather than N could play a central role in re-shaping the pattern of physical and chemical stabilization of SOC.Under N and P intensive inputs,the shift from moderately physical protection to highly chemical stability of SOC implicated the pivotal roles of P management in C cycling of alpine ecosystem.Regarding to the long-term effects of chemical fertilization on SOC stock and its stability,future studies need take account of the advanced analytical techniques and large tempo-spatial scales to provide in-depth insights of SOC fate in alpine ecosystem. |
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