Long-Term Grazing Exclusion Increases Ecosystem Carbon Stock but Decreases Nitrogen Stock in the Karst Alpine Grassland of China

Abstract

Grazing management practices are a major factor regulating nutrient cycling and plant growth in grasslands. However, the response of long-term grazing regimes to ecosystem carbon and nitrogen accumulation and plant productivity remains uncertain in karst landscapes. The purpose of this research is to assess the effects of long-term management measures on plant biomass and ecosystem organic carbon (EOCS), total nitrogen (ETNS), and phosphorus (ETPS) stocks in a karst alpine grassland. A 17 year-long field experiment, constituting the methods of grazing exclusion (GE), continuous grazing (CG), mowing and grazing (MG), and rotational grazing (RG) was established in 2001. Plant and soil samples were collected in 2017. Grazing treatments significantly reduced the aboveground and root biomass (MG and RG) compared to GE. SOC of GE was higher than that of CG, MG, and RG in each soil layer. Grazing promoted the mean TN content with a significant increase in CG and RG. GE significantly increased the EOCS and decreased the ETNS and ETPS. The EOCS increased by 22.29% (MG) and 16.31% (RG) and ETNS increased by 7.76% (RG), and MG significantly decreased ETNS compared to CG. EOCS were positively related to SOC, stoichiometry, and aboveground biomass, while being negatively correlated with TP; ETNS were positively related to TN and TP, while being negatively correlated with C/N, C/P, aboveground biomass, and root biomass; ETPS were positively correlated with TN and TP and negatively correlated with NH4+-N and stoichiometry. GE can provide significant improvements in plant recovery and ecosystem carbon stock, whereas RG is beneficial for promoting ecosystem nutrient stock under the condition of pasturing utilisation in the karst grasslands.