Study On The Accumulation Of Soil PhytOC And Its Factors In Grasslands Of Northern China

Phytoliths can occlude a portion of organic carbon(approximately from 0.1% to 6%)during their formation in plant cell tissues,which is considered as phytolith-occluded carbon(PhytOC).Phytolith carbon sequestration is one of the stable biogeochemical carbon sequestration mechanisms at a long-term scale,and thus may play an important role in decreasing atmospheric CO2 and global carbon sink.Grassland is one of the most important terrestrial ecosystems due to its large distribution area,high aboveground net primary productivity(ANPP)and rich silicon(Si)concentration of plants.However,grassland is also one of the most human-disturbed ecosystems,and grassland degradation is the most representative phenomenon.In the current study,grasslands of different types,degraded grasslands of different degrees and forests converted from grasslands were selected as the objects in North China.Soil phytoliths were isolated by a wet oxidation method followed by a heavy liquid suspension method.Soil PhytOC was determined by the potassium dichromate method.The aim of this study was to investigate the effect of grassland type,grassland degradation,and land use change on distribution and accumulation of soil phytoliths and PhytOC in grasslands.Moreover,factors for the stability of soil phytoliths were clarified,and scientific and reasonable managements were suggested to increase phytolith carbon sinks in grassland ecosystems.These managements could provide some scientific evidences for biogeochemical carbon sink and its regulation mechanism in grassland ecosystems.We obtained the following three conclusions according to the above researches.In three grasslands of different types(typical steppe,meadow steppe and meadow),this study showed that the soil phytolith storage was the highest in meadow(33.44 ± 0.91 t ha–1),followed by meadow steppe(26.8 ± 0.98 t ha–1)and typical steppe(21.19 ± 4.91 t ha–1),which was consistent with the trend of phytolith returning fluxes in aboveground litterfall.The PhytOC storages in typical grassland,meadow steppe and meadow soils(0–60 cm)were significantly different,showed the average of 0.23 ± 0.02,0.29 ± 0.02 and 0.39 ± 0.01 t ha–1,respectively.The turnover time of soil phytoliths in typical steppe,meadow steppe and meadow soil was 665,529 and 607 years,respectively.The results showed that the flux of litterfall phytolith returning played an important role in soil phytolith and PhytOC storages in grasslands.The relative stability of soil phytoliths decreased as follows: typical meadow > meadow > meadow steppe.The stability of soil phytoliths in grasslands may be affected by different grassland types.Climate and soil environments could affect the accumulation of PhytOC in the soils by influencing the stability of soil phytoliths.Based on the study of degraded grasslands of four different degrees,we found that the storages of soil phytolith and PhytOC significantly decreased with the following order: lightly degraded grassland > non-degraded grassland ≈ moderately degraded grassland > seriously degraded grassland.The soil phytolith and PhytOC storages in non-degraded grassland,lightly degraded grassland,moderately degraded grassland and seriously degraded grassland were 12.97 ± 2.15,15.90 ± 0.65,14.35 ± 0.79,13.22 ± 1.07 t ha–1 and 0.11 ± 0.02,0.16 ± 0.02,0.12 ± 0.01,0.07 ± 0.01 t ha–1,respectively.Grassland degradation reduced the plant PhytOC production by 35%,50% and 56% in the lightly degraded grassland,moderately degraded grassland and seriously degraded grassland,respectively.Human disturbances,especially grazing and harvesting,are major factors for the storages of soil phytoliths and PhytOC in degraded grasslands in China.The restoration of degraded grassland could not only curb the deterioration of grassland ecological environment,but also greatly increase the potential of phytolith carbon sequestrations,enhancing the role of degraded grasslands in mitigating global climate change.A comparative study of phytoliths and PhytOC in grassland and forest soils showed that the soil phytolith storage(0–60 cm)was 48.55 ± 3.97 t ha–1 in the grasslands and significantly higher than that in the forest soil(21.20 ± 8.82 t ha–1)after grassland afforestation.Similarly,the storage of soil PhytOC in grassland(0.23 ± 0.03 t ha–1)was significantly higher than that in forest(0.16 ± 0.01 t ha–1).The change of land use,caused by grassland afforestation,will lead to the reduction of soil phytolith carbon sequestration,and has effects on the silicon cycle of grassland ecosystem.