| Aboveground net primary production(ANPP) affects all ecosystem processes and is an effective integrator of the effects of climatic variables on a grassland ecosystem.The various practices required for grazing management will continue to increase as necessitated by the reported rate of reduction in productivity coupled with degradation of Inner Mongolian steppe ecosystems.The current study was conducted to(i)examine the responses of ANPP to different grazing intensities and its relationship with soil factors(ii)study the effects of grazing intensity on herbage growth and dry matter intake and(iii)study the effect of grazing intensity on C3 and C4 species and soil nutrient contents,in addition to the effects of grazing intensity,year,soil layer on belowground biomass(BGB)in Hulunber grasslands,Northeastern China.The grazing experiment was designed with six grazing intensities(0.00,0.23,0.34,0.46,0.69,and 0.92 Animal Units ha-1,where 1 Animal Unit[AU]=500 kg of adult cows).Three replicates were used for each stocking rate,with each replicated area covering a 5-ha paddock.The 18 plots were randomly distributed over a total homogeneous area of 90ha.The grazing intensities were achieved by using 0,2,3,4,6,and 8 young cows with body weight ranging from 250 to 300 kg per plot.Constant grazing was held for 120 days,from June to September,during the 2017 and 2018 growing seasons.Our study focused on the processes involved in the degradation induced by grazing over a spacious range of grazing intensities to identify the sensitive indicators of short term system.We found that the relationship between ANPP and grazing intensity supported the moderate disturbance hypothesis.The ANPP reached the maximum under light and moderate grazing intensity(G0.23?G0.34),especially in dry years,it was significantly higher than that of the control and heavy grazing.Precipitation was an important factor affecting the response of plants to different grazing intensities.Annual precipitation was positively correlated with ANPP and the decrease of ANPP caused by grazing intensities.Compared with dry years,there were higher grazing tolerance and higher forage growth rate in wet years.Yields were mainly affected by the sampling time factors.Considerable effects of grazing intensity on yield were recorded.The greatest decrease in aboveground biomass(AGB)was a decrease of 64.1%and59.3%,in 2017 and 2018 respectively,on the G0.92 treatment compared with the G0.00 treatment.There was a positive correlation between yearly precipitation and ANPP as well as grazing intensity which reduced the ANPP.The BGB significantly decreased(P<0.001)with an increase of grazing intensity over the years.The BGB decrease under the grazing condition was caused by the decrease in the volume of the source of C assimilation organ and the enhancement of the re-transport of root carbohydrate to stem meristem.The proportion of the C3 species recorded was higher than the C4species in 2017 and 2018(86.6 and 84.1%,respectively).Overall,the soil organic carbon(SOC)was significantly higher in G0.00 and G0.23(P<0.05),while total nitrogen(TN),and total phosphorus(TP)were not significantly different among grazing intensities.Compared with the G0.00 plots,grazing intensity did not significantly alter the SOC.At low and moderate grazing intensities(G0.23 to G0.46),the content of TN was significantly affected and TP were not significantly affected,but at the high grazing intensities of G0.69 and G0.92,the TN and TP content increased significantly(P<0.05).Plant's tolerance to grazing occurred for only a short term since high grazing intensities with stable relative growth rates were considered as an indication of grazing effects on relative growth rate.Precipitation was also a determinant of the responses of plants to increasing grazing levels.Higher grazing tolerance of plants and higher herbage growth rate was found in the wet year compared with a dry year.Our results highlight the negative effects of heavy grazing intensity on plant composition and soil nutrient loss.Also,further studies are needed to comprehensively understand the effect of grazing intensity on grassland ecosystems in order to provide information for sustainable management practices.Knowing the effect of grazing intensities on ecological systems will provide useful information for assessing the current scenarios of grazing management and conducting timely adaptive practices to maintain the ability of grasslands in long term systems.Our findings demonstrate that short-term grazing basically altered the standing crop of aboveground biomass,and canopy structural traits,which has a long-term evolutionary grazing history and high resources availabilities.We found that grazing intensity and annual grazing intensity had significant effects on the plant height(p<0.05).Significant differences(p<0.05)were observed among the treatments for plant height in both 2017 and 2018.Cattle grazing at each grazing intensity resulted in apparent reduction in the total plant height mainly in the both 2017 and 2018 under intermediate and heavy grazing,whereas no significant effect was found at light grazing rates in the both 2017 and 2018season,much like the case for grazing rate-induced changes in plant height,with increasing grazing intensity and duration;canopy height and coverage decreased substantially with increasing stocking rates,whereas significant changes in plant density occurred only at heavy grazing;and significant negative linear relations were found between the standing crop of biomass and grazing intensity in each individual year.Decreasing trends in plant coverage with increasing grazing intensity were found in each of the 2 years.The species abundance was not different among grazing intensities except heavy grazing treatment(G0.69)which recorded the highest value among all treatments.Our findings imply that monitoring changes in species composition,canopy traits,and standing crop of biomass in grassland communities can provide important references for assessing current grazing management scenarios and conducting timely adaptive practices to maintain the long-term ability of grassland systems to perform their ecological functions. |
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