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Soil, land care and environmental research
RESEARCH ARTICLE

Effects of nitrogen addition on soil oxidisable organic carbon fractions in the rhizospheric and bulk soils of Chinese pines in north-western China

Hongfei Liu A * , Sha Xue A B * , Guoliang Wang A B and Guobin Liu C
+ Author Affiliations
- Author Affiliations

A College of Forestry, State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A&F University, Yangling 712100, Shaanxi, P. R. China.

B Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, Yangling 712100, Shaaxi, P. R. China.

C Corresponding author. Email: gbliu@ms.iswc.ac.cn

Soil Research - https://doi.org/10.1071/SR16358
Submitted: 21 December 2016  Accepted: 21 August 2017   Published online: 2 November 2017

Abstract

Increased atmospheric nitrogen (N) deposition caused by human activities has potentially important effects on ecosystem carbon (C) dynamics and different effects on C fractions with different stabilities and chemical compositions. A better understanding of the responses of different C fractions to N addition is vital for maintaining soil quality and protecting vegetation. In order to investigate the differential effects of N addition on total soil organic carbon (SOC) and four SOC fractions with increasing degrees of oxidisability in Pinus tabuliformis rhizospheric and bulk soils, a 6-year pot experiment was performed testing the effects of the addition of N at rates of 2.8, 5.6, 11.2, 22.4 and 44.8 g m–2 year–1 compared with a control (CK) group (no N addition). Addition of N addition had significant (P < 0.05) effects on SOC fractions of very labile C (C1) and recalcitrant C (C4), but negligible effects on total SOC (TOC) and SOC fractions of labile C (C2) and less labile C (C3). The C1 content and ratio of C1 to TOC in rhizospheres decreased following the addition of low levels (N2.8–N5.6) of N, but increased after the addition of high levels (N11.2–N44.8) of N, with minimum values obtained after the addition of 11.2 N g m–2 year–1. Low rates (N2.8–N5.6) of N addition considerably increased C4 and the ratio of C4 to TOC in the rhizosphere, whereas addition of high rates (N11.2–N44.8) of N decreased these parameters. The responses of C1 and C4 in the bulk soil to N addition were opposite. The SOC fraction was significantly higher in the rhizosphere than in the bulk soil, indicating large rhizospheric effects. However, increased N addition weakened these effects. These findings suggest that low rates (N2.8–N5.6) of N addition stabilise SOC against chemical and biological degradation, whereas increased rates of N addition increase the lability of SOC in the bulk soil. Thus, the rhizosphere plays a vital role in soil carbon stability and sequestration in response to N addition.

Additional keywords: Pinus tabuliformis, rhizosphere, nitrogen deposition, carbon stabilization.


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