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. 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 conducted to assess the effects of
long-term management measures on plant biomass, ecosystem organic carbon, and nitrogen stocks in a karst alpine grassland of the Yunnan-Guizhou
Plateau. Our results showed that grazing significantly reduced the
aboveground biomass (CG, MG, and RG) and root biomass (MG and RG)
compared to GE, but there were no differences between the results
obtained using the various grazing methods. The root/shoot ratio
increased by 55.98% in CG and decreased by 52.96% and 37.14% in MG
and RG, respectively, compared to that achieved with GE. Soil organic
carbon (SOC) of GE was higher than that of CG, MG, and RG in each soil
layer, while it was significantly higher at 0–10 cm and 20–30 cm.
Grazing promoted the mean total N content with a significant increase in
CG and RG, and significantly increased total P content in each soil
layer compared to GE. Grazing significantly decreased the C/N and C/P
ratios at each soil depth. GE significantly increased the ecosystem
organic carbon stocks (EOCs) and decreased the ecosystem total nitrogen
stocks (ETNs). Although there were no significant changes among the
grazing methods, the EOCs increased by 22.29% (MG) and 16.31% (RG) and
ETNs increased by 7.76% (RG) when compared to those obtained with CG.
EOSs were positively correlated with SOC, stoichiometry (C: N: P), and
aboveground biomass, while being negatively correlated with TP; ETNs
were positively correlated with N and P, while being negatively
correlated with C/N, C/P, aboveground biomass, and root biomass. Our
results indicate that GE can provide significant improvements in plant
recovery and ecosystem organic carbon storage, whereas RG is beneficial
for promoting both EOCs and ETNs under the condition of pasturing
utilisation in karst grasslands.