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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

Hongfei Liu , Sha Xue , Guoliang Wang , Guobin Liu

Abstract

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