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Effects of nitrogen addition on soil oxidiszable organic- carbon fractions in the rhizospheric and bulk soils of Chinese pines in northwestern China
Increased atmospheric nitrogen (N) deposition caused by human activities has potentially important impacts on ecosystemic carbon (C) dynamics and different effects on C fractions with different stabilities and chemical compositions. A better understanding of the responses of different C fraction to N addition is vital for maintaining soil quality and vegetation protection. In order to investigate the differential effects of N addition on total soil organic carbon (SOC) and four SOC fractions with increasing degrees of oxidizability in Pinus tabuliformis rhizospheric and bulk soils, a six-year pot experiment with N addition was performed to test the following treatments: a control (CK) with no N addition and five N addition rate treatments with 2.8, 5.6, 11.2, 22.4, and 44.8 g m-2∙y-1. The results show that N addition had a significant effect (P<0.05) on SOC fractions of very labile C (C1) and recalcitrant C (C4), but a negligible effect on total SOC (TOC) and SOC fractions of labile C (C2) and less labile C (C3). The C1 and the percentages of C1 to TOC in the rhizospheres decreased at low levels of N addition but increased at high levels, and the minimum values were obtained from N11.2 (11.2 N g m-2∙y-1). Low N addition rates increased C4 and the percentages of C4 to TOC in the rhizosphere considerably, whereas high N addition rates decreased it. The C1 and C4 in the bulk soil responded oppositely to N addition. The SOC fraction contents were significantly higher in the rhizosphere than in the bulk soil, indicating large rhizospheric effects. However, increased N addition weakened these effects. These findings suggested that low N addition rates stabilize SOC against chemical and biological degradations, while increased N addition rates increase the lability of SOC in the bulk soil. Rhizosphere, thus, plays a vital for soil carbon stability and sequestration in response to N addition.
SR16358 Accepted 21 August 2017
© CSIRO 2017