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

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.