| On the eastern Qinghai-Tibetan Plateau,there is an interesting mountain landscape where plant community on hills can be divided sharply into southward meadows and northward shrublands by ridges.It's very important to evaluate soil organic carbon and nitrogen cycling under global warming on the mountain area of Qinghai-Tibetan Plateau by studying the aspect-community complex effects on soil organic carbon and nitrogen mineralization,but so far little is known about biochemical characteristics,the quality of soil organic carbon(OC)and its relation to decomposability of soil OC and nitrogen under these aspect-community complex types.In order to evaluate the aspects-community complex effects on the decomposablility of soil OC and nitrogen,two hills were randomly selected in the year of 2011 on the eastern Qinghai-Tibetan Plateau(Tianzhu Zang Autonomous County,Gansu Province).Soil hydrothermal condition,p H value,texture,total organic carbon(OC)content and its composition,total nitrogen(N)content,microbial biomass C and N,soil enzyme activities,soil carbon and nitrogen mineralization were detected,respectively.Soil carbon mineralization was measured for 49 days at 18 °C and 25 °C under laboratory condition and soil N mineralization was analyzed in situ(undisturbed soil)incubation method.The results were shown as followed:1.Over the growing season(from June to September),the daily soil temperature at 0–15 cm soil layer was 2–9 °C higher under meadows than under shrublands,contrasted with 2–5 times greater soil water contents under shrublands than under meadows in the upper 15 or 30 cm soil layer.Besides,soil textures were both silt loam under these two types of plant community;the soil under shrublands had a significant lower p H value and inorganic C concentration across depths than that under meadows.2.More than half of soil OC was present as particulate fraction across all samples.Averaging over depths,shrublands contained 87.7-114.1 g OC and 7.7-9.3 g N per kg soil,which were63-78% and 26-31% higher than those in meadows,respectively.Meanwhile the C/N ratio of soil organic matter was 11.4-12.3 under shrublands,being 29-40% higher than that under meadows.In contrast with the greater soil organic carbon and total N contents in shrublands,soil microbial biomass and ?-glucosidase and urease activities were consistently smaller compared with meadows across sites.3.Non-cellulosic carbohydrates content in soil under shrublands was significantly smaller than under meadow(130 mg g–1 OC vs.153 mg g–1 OC);while the lignin contents were similar(20 mg g–1 OC on average)at 0–15 cm soil layer in southward meadows and northward shrublands,but were higher at 15–30 cm soil layer under shrublands than that under meadows(17 mg g–1 OC vs.12 mg g–1 OC).Compared to the soils under meadows,the soils under shrublands contained more abundant microbial-derived sugars and microbial-transformed ligneous substances in OC.4.By the end of 49 days' incubation,total CO2–C evolution from soils under meadows was significantly higher than that under shrublands(15.0–16.2 mg g-1 OC vs.9.7–12.8 mg g-1 OC).Across all soil samples over two sites,total CO2–C evolved per g OC was closely correlated to enrichments of non-cellulosic carbohydrates and microbial biomass.This indicates that the greater soil OC decomposability under meadows was associated with its larger abundances of readily mineralizable fractions compared with shrublands.However,temperature increase effect on soil OC mineralization changed with soil depth but not with plant community types.This indicates that increase soil temperature had a larger proportion of soil OC mineralized in the upper soil layer than in the lower layer,but had a similar proportion of soil OC mineralized in the soil between meadows and shrublands.5.Over the growing season,gross and net soil N mineralization rates were both greater in soil under meadows than that under shrublands at both sites.And the total cumulative net N mineralization was 104–121% greater under meadows(7.1–8.7 g m–2)than under shrublands(3.5–3.9 g m–2)in the upper 15 cm soil layer or 134–178% greater under meadows(22.1–22.3 g m–2)than under shrublands(7.9–9.5 g m–2)in the upper 30 cm soil layer across sites.Despite the smaller soil N mineralization in shrublands than in meadows,a greater soil N availability was measured in shrublands compared with meadows,linked to a potentially smaller plant N uptake due to slower growth of shrubs than herbaceous grasses.This suggests that the mineralization of soil N and plant uptake are less tightly coupled in the north-facing shrubland than in the south-facing meadow,highlighting that aspect-community complex had a profound effect on the cycling of soil N in the mountainous areas of Tibetan Plateau.In conclusion,our findings suggest that the aspect-community complex induced notable heterogeneity of soil physicochemical characteristics,soil OC and nitrogen pool size,the composition of soil OC,decomposability of soil OC and nitrogen on the eastern Qinghai-Tibetan Plateau,highlighting the complexity in predicting soil C and N dynamics in future climatic and land management scenarios. |
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