| Due to the increasing vegetable fields,the use of chemical fertilizers has also increased.Plastic film mulching and nitrogen application are two important agricultural measures.There were a lot of researches on the effects of crop growth,yield and soil physical and chemical properties.However the effects of plastic film mulching and nitrogen application on soil aggregates in farmland ecosystems still rare to see,especially on the vegetable field ecosystem.Taking pepper-radish rotation as the research object,the field observation experiment was conducted for three years in common vegetable fields in Southwest China.Four nitrogen concentration gradients were set under mulching and no mulching management measures(CN0,CN1,CN2,CN3,FN0,FN1,FN2,FN3).In this experiment,the soil was taken from the vegetable field in April 2017,and the stratified sampling method was used to stratify 0-10 cm,10-20 cm,20-30 cm,30-40 cm,40-50 cm at each sampling point.Soil samples are layered and mixed according to the level,and then enough samples are taken by the quadruple method to remove the gravel and root system.After air dried and passed through 0.25 mm and 2 mm sieves respectively,the soil physical and chemical properties were determined.The impact of plastic film mulching and nitrogen application on soil aggregate stability,content and storage of organic carbon and total nitrogen in aggregates,active organic carbon and nitrogen fractions(labile oxidized carbon,dissolved organic carbon,dissolved total nitrogen,microbial biomass carbon and microbial biomass nitrogen)and physical fractions of organic carbon were analyzed.For this thesis we explore the effects of plastic film mulching and nitrogen application on the carbon sequestration capacity and nutrient cycling of soil aggregates,and provide reference for farmland management measures in China.The main results are as follows:1 Effects of plastic film mulching and nitrogen application on soil stability of vegetable fieldsIn the 0-50 cm soil depth,the different nitrogen treatments with and without mulching were mainly macroaggregate(>0.25 mm).Among them,the FN0 treatment's macroaggregate(>0.25 mm)were the highest,accounting for 80.29 % of the total fraction,which was significantly higher than the CN0 treatment.However,under low nitrogen and medium nitrogen conditions,there was no significant difference between the treatment with or without mulching.CN3 and FN3 treatments have the lowest content of macroaggregate(>0.25 mm),accounting for 47.8% and 50.01% of the total fraction respectively.It is indicated that under non-nitrogen conditions,treatment with mulching can improve the stability of soil aggregates,while nitrogen application has no significant effect on the stability of soil aggregates.When nitrogen fertilizer exceeds a certain application rate,it will reduce the stability of macroaggregate in soil,make it convert to smaller fractions.The MWD and GMD values of FN2 treatment were 0.87 mm and 0.88 mm respectively,which were significantly higher than other treatments,indicating that the soil agglomerates under FN2 treatment had the best stability.2 Effects of plastic film mulching and nitrogen application on soil organic carbon and total nitrogen in soil aggregates of vegetable fields(1)Under the condition with and without mulching,the content of organic carbon in the aggregates of different fractions decreased with soil depth increasing,while the total nitrogen content fluctuated,and there was no obvious rule.The content of organic carbon in different treatments is higher in agglomerates with larger fractions.Under the condition with and without mulching,the content of organic carbon in the agglomerates increased with the increase of nitrogen application rate,and the increase rate under mulching conditions was higher.It indicated that both mulching and nitrogen treatment could improve the organic carbon content of soil aggregates.Under the conditions without mulching,there was no significant difference in total nitrogen content of aggregates of different fractions under different nitrogen treatments.Under the conditions with mulching,the total nitrogen content in the low-nitrogen and medium-nitrogen treatments decreased significantly compared with the non-nitrogen treatment,with a decrease of 40.11 % and 39.84 %.(2)Under the condition with and without mulching,except for CN1 treatment,other treatment with nitrogen application had higher organic carbon storage in macroaggregate(>0.25 mm)than treatment without nitrogen application.Among them,organic carbon storage of FN1,FN and FN3 treatments in macroaggregate(>0.25 mm)was significantly higher than FN0 treatment,and the growth rates were 33.45%,70.61% and 43.84%,respectively.Under conventional conditions,there was no significant difference in total nitrogen storage under different nitrogen treatments for different fraction aggregates.Under the conditions with mulching,the in total nitrogen storage of low-nitrogen and medium-nitrogen treatments were significantly lower than the non-nitrogen treatment,with a decrease of 42.86 % and 41.19 %,respectively.Under the conditions of low nitrogen and medium nitrogen,the aggregate total nitrogen storage of mulching treatments were significantly lower than conventional treatment.3 Effects of plastic film mulching and nitrogen application on active organic carbon and nitrogen fractions(1)Under the condition with and without mulching,different nitrogen treatment significantly increased the content of LOC in different aggregates fraction in soil(P<0.05)with the increase of nitrogen application rate.Under the same nitrogen application conditions,the film treatment can increase the content of LOC in the agglomerates with larger fraction,and the LOC content in the agglomerates with smaller fraction decreases.LOC is mainly concentrated in the intermediate agglomerates.Under conditions without mulching,there was no significant difference in the distribution ratio of LOC content of agglomerates(>0.25 mm)in different treatments(P>0.05).Under the conditions with mulching,the distribution ratio of LOC decreased with the increase of nitrogen application rate,and the decrease was low-nitrogen(20.88 %),medium nitrogen(11.28 %),high nitrogen(23.52 %)respectively.Under non-nitrogen and high-nitrogen conditions,the distribution ratio of LOC content in the agglomerate(>0.25 mm)significantly increased by 37.33 % and 26.61 %(P<0.05).It is indicated that both the film and the nitrogen treatment can increase the LOC content.The film treatment promotes the distribution of LOC to the agglomerates with larger fraction,and the LOC content shifts to the agglomerates with smaller fraction with the nitrogen application increasing.(2)Under the conditions without mulching,CN2 and CN3 treatment can significantly increase the DOC content in different fractions of aggregates except large macroaggreate,and CN1 treatment can increase the DOC content of silt and clay macroaggreate.Under conditions with mulching,different nitrogen treatments can significantly improve the DOC content of other fraction aggregates in addition to large agglomeration.Under the same nitrogen application conditions,the DOC content of the agglomerates with different fractions had no significant differences in the treatment without mulching(P>0.05).The DOC is mainly concentrated in the intermediate agglomerates.With the increase of nitrogen application rate,the DOC distribution of aggregates(>0.25 mm)decreasing,and the decrease rate of CN3,FN1,FN2 and FN3 were 30.24 %,21.49 %,18.66 % and 31.75%,respectively(P<0.05).It shows that nitrogen application promotes the transfer to agglomerates with smaller fraction while the DOC content increasing.Under the same nitrogen application conditions,the film treatment can increase the DOC distribution of aggregates with larger fraction.(3)Under the conditions with and without mulching,different nitrogen treatments can increase the DON content of aggregates with different fractions significantly(P<0.05).Under the same nitrogen application conditions,the DON content of different aggregates in treatment with mulching was higher than that without mulching.DON is mainly concentrated in the intermediate agglomerates.There was no significant difference in DON distribution ratio of large aggregate(>0.25 mm)between the CN1,CN2,CN3 treatments and the CN0 treatment(P>0.05).The FN1,FN2 and FN3 treatments significantly reduced DON distribution in agglomerates(>0.25 mm),the decrease rate were 28.88 %,14.21% and 26.29 % respectively(P<0.05).It is indicated that under the conditions with mulching,nitrogen application will result in a shift of DON from large aggregates to smaller aggregates.Under no-nitrogen conditions,the film treatment significantly increased the DON distribution ratio of macroaggregate and microaggregates,with an increase of 44.78 % and 46.57 %.Under the conditions of low nitrogen and medium nitrogen,the DON distribution in microaggregates increased significantly under film treatment,with an increase of 61.66 % and 71.17 %(P<0.05).It is indicated that under certain nitrogen application conditions,the film treatment can improve the distribution of DON to agglomerates in larger particle diameters.(4)Different nitrogen treatments under conditions with and without mulching can significantly increase the MBC content of agglomerates in different fractions.Under the same nitrogen application conditions,MBC content of large agglomerates and intermediate agglomerates in treatment with mulching was significantly higher than that without mulching.MBC is mainly concentrated in intermediate agglomerates.The MBC ratio of large agglomerate(>0.25 mm)in CN3 treatment was significantly reduced,with a decrease of 19.73 %(P<0.05).The FN1,FN2 and FN3 treatments significantly reduced the MBC distribution ratio of macroaggregate(>0.25 mm),the decrease was 20.52 %,16.42 % and 30.85 % respectively(P<0.05).It indicates that nitrogen application will promote the transfer of MBC to agglomerates with smaller fractions.Under the conditions of non-nitrogen,low-nitrogen and high-nitrogen,the film treatment increased the proportion of MBC distribution of large agglomerates,which increased by 75.25 %,25.07 % and 39.04 %.It is indicated that the film treatment can improve the distribution of MBC to agglomerates with a large particle diameters.(5)Under conditions without mulching,different nitrogen treatments can significantly increase the MBN content of agglomerates(>0.25 mm),and the CN3 treatment increased most.Under the conditions with mulching,different nitrogen treatments also increased the MBN content of agglomerates in different fractions.Under the same nitrogen application conditions,the film treatment had little effect on MBN content in different fraction agglomerates.MBN is mainly concentrated in intermediate agglomerates.There was no significant difference between the CN1,CN2,CN3 and the CN0 treatment in MBN distribution ratio of large agglomerate(>0.25 mm)(P>0.05).The FN1,FN2 and FN3 treatments reduced the distribution ratio of MBN in large aggregate(>0.25 mm)compared with non-nitrogen treatment,the decrease rate were 15.84 %,12.24 % and 19.89 %,respectively.Under the conditions of no nitrogen and high nitrogen,the treatment with mulching improved the distribution of MBN to agglomerates with larger fraction.Under low nitrogen and medium nitrogen conditions,the treatment with mulching distributes MBN into the microaggregates.It is indicated that under the condition with and without mulching,nitrogen application will result in a shift of MBN into smaller aggregates.Under the same nitrogen application condition,the treatment with mulching will result in a shift of MBN into larger aggregates.4 Effect of plastic film and nitrogen application on physical components of soil organic carbon(1)Under normal conditions,medium nitrogen treatment significantly increased the content of cPOC by 33.27 %;high nitrogen treatment significantly reduced the content of fPOC by 42.80 %;different nitrogen treatment reduced the organic component of s+c_m carbon content.Under the conditions of film mulching,the treatment of medium nitrogen and high nitrogen reduced the content of cPOC by 39.76 % and 62.69 %,and increased the content of fPOC by 76.70 % and 21.05 %.Under conventional and film-mulching conditions,except for nitrogen treatment in the film,other nitrogen treatments significantly increased the content of iPOC(P<0.05).The results showed that the mean content of organic carbon in the fractions of SOC,cPOC,fPOC,iPOC,s+c_m and s+c fraction in the 0-50 cm soil depth was no significant difference(P>0.05).Under the same nitrogen application conditions,the content of organic carbon in s+c and s+c_m fraction in the treatment with mulching was higher,while the mean content of organic carbon of cPOC,fPOC and iPOC was higher in the treatment without mulching.It indicates that the film treatment will convert the organic carbon in the soil into chemical and biochemically protected organic carbon.Under conditions with and without mulching,the content of organic carbon in cPOC,fPOC and iPOC in the treatment with low nitrogen and medium nitrogen was significantly higher than that without nitrogen treatment,while the treatment with high nitrogen was lower than that without nitrogen.It indicated that proper nitrogen application would increase the content of organic carbon in soil with higher activity.The sum of the organic carbon contents of cPOC,fPOC and iPOC in CN2 treatment is up to 7.99 g/kg.(2)The iPOC has the lowest distribution ratio of organic carbon of the SOC fractions,and it's about 8 % under different treatments(P<0.05).The cPOC and fPOC being considered to be active in non-protected organic carbon pool,accounted the highest proportion in CN2,CN3,FN1 and FN2 treatment,all above 50 %.The proportion of non-protected organic carbon pool in FN0 treatment was low,while it's high in the s+c friction in the pool of organic carbon which has chemical protection,which was 51.70 %.It showed that CN2,CN3,FN1 and FN2 treatments had higher activity of soil organic carbon and higher fertility.(3)cPOC is the most sensitive indicators of SOC and its fractions,so it can be a good indicator of the impact on plastic film mulching and nitrogen application on soil organic carbon pool. |
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