Response Of Soil Aggregates And Their Associated Carbon And Nitrogen To Biotic And Abiotic Measures Under Drying-wetting Cycles

In recent years,extreme weather events,including heavy rainfall and extreme drought,have increased significantly.Such events as well as agricultural irrigation led to cycles of rewetting of dry soils in most terrestrial ecosystem.The drying and wetting cycles（DW cycles）of soils significantly affects soil microstructure and aeration,aggregate turnover,microorganism and enzyme activities and carbon（C）and nitrogen（N）mineralization.Soil aggregates,play an important role in the protection of soil organic matter,as they function as the carriers of C and N.Explicitly expounding how soil aggregates and their associated C and N and the characteristics of soil carbon and nitrogen mineralization under DW cycles and establishing the relationship between soil aggregates and C and N mineralization are crucial for better understanding the biogeochemical cycle of terrestrial ecosystems under the global climate change conditions.Previous studies differed greatly in experimental design of DW cycles,and most of these studies have focused on how DW cycles affects aggregates and the redistribution of C and N within aggregates in bare soil,while aggregate turnover and their associated C and N regulated by plants,soil fauna and conservation tillage were less examined.Herein,this study systematically investigated the aggregate distribution and their associated OC and N under DW cycle.Four soils with different C and clay contents were used in this study:an Entisol（from Hechuan,Chongqing）,an Ultisol（from De’an,Jiujiang,Jiangxi Province）,an Anthrosol（from Yangling,Xianyang,Shaanxi Province）and a Mollisol（from Nenjiang,Heihe,Heilongjiang Province）.A laboratory microcosm incubation experiment was designed to assess the influence of DW sequences and frequencies on soil aggregate distribution and their associated organic C（OC）,potassium permanganate oxidized C（POXC）,total and mineral N（TN,MIN）,and on extracellular enzyme activity associated with C-（Eny-C）and N-acquisition（Eny-N）and the characteristics of C and N mineralization.Moreover,ryegrass（Lolium perenne L.）,earthworm（Metaphire tschiliensis）,super absorbent polymers（polyacrylamide,PAM）and gravel mulch were introduced into soils underdoing DW cycles to explore the regulation effects of biological（ryegrass and earthworm）and abiotic（PAM and gravel mulch）measures on the aggregates and their associated C and N.The main results are as follows:1.DW sequences（drying-wetting-drying-wetting,2DW;wetting-drying-wetting-drying,2WD）significantly affects soil aggregates and their associated C and N,and such effects varied with soil types.Compared to constant wetting（CW）control,the proportion of large aggregates（5-8 and 2-5 mm）in Entisol,Ultisol and Anthrosol under2DW treatment were higher,and the proportion of large aggregates in Entisol,Ultisol and Mollisol under 2WD treatment were lower.The OC in each aggregate of Entisol,Anthrosol and Mollisol were higher but the OC associated with aggregates in Ultisol were lower than CW control.The 2DW and 2WD treatments minimally affected POXC,TN and Min-N in aggregates as well as N mineralization,but significantly increased cumulative CO₂ emission of four soils.2.The proportion of large aggregates of Entisol,Ultisol and Anthrosol were higher under DW cycles with different frequencies（2DW,4DW,8DW and 16DW）than CW control,but the proportion of large aggregates of Mollisol were lower than CW control.Compared to CW control,the OC in each aggregate of Entisol and Mollisol were higher,while the OC in each aggregate of Ultisol were lower under four DW treatments.Four DW treatments increased the POXC in each aggregate of Entisol but decreased them of Anthrosol.The 2DW,4DW and 8DW treatments decreased but 16DW treatment increased the POXC in each aggregate of Ultisol.Four DW treatments increased the aggregate associated TN of Entisol but decreased them of Ultisol.For the MIN of Ultisol,the MIN was highest under 2DW treatment and higher than CW control,but the MIN decreased with the increase of DW frequency,which were all lower than CW control.Four DW treatments had minimally effects on the aggregate associated TN of Anthrosol and Mollisol and on the aggregate associated MIN of Entisol,Anthrosol and Mollisol.The cumulative CO₂ emission of four soils under 2DW,4DW,8DW and16DW treatments were significantly higher than CW control,but did not affect the N mineralization.3.Earthworm increased the soil nutrient availability and promoted ryegrass growth.The effects of earthworm and ryegrass on aggregates distribution were dependent on soil moisture condition and soil type.For CW condition,the large aggregates in Anthrosol and Mollisol soils with ryegrass（V treatment）and with ryegrass and earthworm（VE treatment）were lower than bare soils（CK treatment）.For 4WD condition,the large aggregates in Anthrosol under V treatment were higher,but the large aggregates in Anthrosol and Mollisol under VE treatment were lower than CK treatment.However,the effects of ryegrass and earthworm on aggregate associated C and N were soil type depended.For CW and 4WD conditions,the OC in 1-2,0.25-1 and<0.25 mm aggregates of Anthrosol and Mollisol were higher under V and VE than CK treatment.The POXC in each aggregate of Anthrosol were higher under V and VE treatments,but were lower under E than CK treatment.However,the POXC in each aggregate of Mollisol were higher under E and VE treatment,but were lower under V than CK treatment.The TN in each aggregate of Anthrosol were higher under V and VE than CK treatment,while the TN of Mollisol showed a opposite trend.The MIN in each aggregate of Anthrosol and Mollisol were higher under E treatment,and the MIN in each aggregate of Mollisol were lower under V and VE treatment,compared with CK treatment.Furthermore,the V,E and VE treatment improved the Eny-C and Eny-N in Anthrosol for CW and 4WD condition,and these treatments increased the Eny-C and Eny-N in Mollisol under CW condition,but decreased them under 4WD condition.4.The regulation effects of ryegrass and earthworm on soil aggregate and their associated C and N under DW cycles were related to soil type.For Anthrosol,the proportion of large aggregates in CK and VE treatment were lower under 4WD than CW condition,and such difference was greater in VE treatment,but the proportion of large aggregates in V treatment were higher under 4WD than CW condition.For Mollisol,the proportion of large aggregates in CK,P,E and PE treatment were lower under 4WD than CW condition,and such difference was greater in VE treatment.The OC in 0.25-1 mm aggregates of VE treatment were lower in Anthrosol but higher in Mollisol under 4WD condition,compared to corresponding CW control.The 4WD cycle decreased the POXC in each aggregate of Anthrosol,but increased them of Mollisol.The DW cycles had no significant effects on aggregate associated TN and MIN were not significant,but significantly affected Eny-C and Eny-N,and such effects varied with soil treatments and soil type.For Anthrosol,the 4WD treatment had no significant effects on Eny-C and Eny-N in CK and VE treatment,but the Eny-C and Eny-N were higher in VE than CK treatment.The 4WD treatment increased Eny-C in V and E treatment but decreased Eny-N in V treatment.For Mollisol,the 4WD cycle increased Eny-C in CK treatment,while decreased it in VE treatment.The 4WD cycle did not affect Eny-N in CK treatment,while decreased it in V and VE treatment.5.DW cycles（4DW vs.CW）increased the proportion of large aggregates in Entisol,Ultisol and Anthrosol,but decreased them in Mollisol.The 4DW cycles had no significant effects on aggregate associated C and N for four soils,and the regulation effects of PAM and gravel mulch were not significant.The addition of PAM（PAM,P treatment;PAM+gravel mulch,PG treatment）can promote the formation of large aggregates,and increased the aggregate associated OC and TN of Entisol,Ultisol,Anthrosol and Mollisol and the POXC in 0.25-1 and<0.25 mm aggregates of Entisol under CW and 4DW conditions.The effects of PAM and gravel mulch on aggregate associated MIN were dependent on soil type.The MIN in each aggregate of Entisol,Ultisol and Anthrosol were higher in P and PG treatment,but were lower in soils with gravel much（G treatment）than CK treatment.The MIN in each aggregate of Mollisol were not affected by PAM and gravel mulch.Above all,for soils with low C and N（i.e.,Entisol,Ultisol and Anthrosol）,PAM could promote while gravel mulch inhi bit N mineralization;for soils with high C and N（Mollisol）,PAM and gravel mulch had little effects on its N mineralization.P and PG treatment decreased Eny-C and Eny-N of four soils,while G treatment had little effects on Eny-C and Eny-N.In conclusion,this dissertation clarified that the response of aggregates and their C and N to the sequence and frequency of DW cycles were significantly related to soil types and DW cycles with different sequences and frequencies promoted the C mineralization in varied soil types,but had no significant effect on N mineralization.And this study confirmed the key role of plant and soil fauna in soil aggregates and their associated C and N and their crucial regulation effects under DW cycles.This study also revealed that conservation tillage measures significantly affected soil aggregates and their associated C and N and N mineralization,however,had no significant regulation effects on aggregates and their associated C and N under DW cycles.These results were precious for a deep understanding of the response of soil C and N cycles to DW cycles in complex terrestrial ecosystem,and also provided more theoretical basis for soil management under the scenario of future climate changes.