Articles | Volume 14, issue 10
Biogeosciences, 14, 2685–2696, 2017
Biogeosciences, 14, 2685–2696, 2017

Research article 31 May 2017

Research article | 31 May 2017

Effects of long-term mowing on the fractions and chemical composition of soil organic matter in a semiarid grassland

Jiangye Li1, Qichun Zhang1, Yong Li1, Yimeng Liu2, Jianming Xu1, and Hongjie Di1 Jiangye Li et al.
  • 1Zhejiang Provincial Key Laboratory of Agricultural Resources and Environment, Key Laboratory of Environment Remediation and Ecological Health, Ministry of Education, Zhejiang University, Hangzhou 310058, China
  • 2School of Economics and Resource Management, Beijing Normal University, Beijing 100875, China

Abstract. The grassland ecosystem is a significantly important terrestrial carbon pool. Intensive mowing is common to meet the need of increased livestock. However, little information on the quality and quantity of soil organic matter (SOM) under different mowing managements has been documented. In this work, in order to evaluate the impacts of different mowing managements on the quality and quantity of SOM, the fractions and chemical composition of SOM under different mowing managements were determined using traditional fractionation methods and spectroscopy technologies, including advanced nuclear magnetic resonance (NMR) (e.g. cross-polarization magic angle spinning 13C-NMR, CPMAS 13C-NMR) and Fourier-transform infrared (FTIR) based on a 13-year field mowing trial with four treatments: unmown (M0), mowing once every second year (M1/2), mowing once a year (M1) and mowing twice a year (M2). The results showed that compared with M0, M1/2 and M1 significantly enhanced the SOM accumulation and increased the stability of SOM by enhancing humification, while M2 limited SOM accumulation and microbial biomass. Substituted alkyl carbon (C) was the major organic C type in the grassland ecosystem, and it made up over 40 % of the total C. M1/2 and M1 significantly increased stable C functional groups (alkyl C and aromatic C) by degrading labile C functional groups (O-alkyl and carbonyl C) and forming recalcitrant humus, while M2 had opposite effects. The consistent increase in the values of NMR indices reflecting the degradation degree, hydrophobicity and aromaticity of SOM in M1 reflected the fact that M1 had the largest contribution to increasing the stability of SOM, while these values in M2 were similar to those in M0. Significant correlations between different SOM fractions and nitrogen (N) mineralization, and between the contents of different C functional groups and net soil organic nitrogen mineralization or microbial biomass C, indicated that the shifts in SOM fractions and chemical composition were closely related to soil microbial biomass and activity. Therefore, in view of the quality and quantity of SOM and the sustainable development of grassland ecosystems, M1 was the optimal mowing management, while M2 should be avoided in the semiarid grassland.

Short summary
(1) Solid state 13C-NMR is a powerful tool to study the nature of soil organic carbon. (2) Moderate mowing improved the stability of the grassland soil organic carbon pool. (3) Heavy mowing decreased the stability of the grassland soil organic carbon pool. (4) Mowing once a year is the most recommended practice in the semiarid grassland. (5) Mowing twice a year should be avoided for the long term in the semiarid grassland.
Final-revised paper