Effects Of Combined Application Of Biochar With Nitrogen Fertilizer On Soil Aggregate Composition,Microbial Biomass Carbon And Nitrogen And Crop Yield In Upland Red Soil

Positive effects of biochar application on soil and crop yield have recently been demonstrated in many studies,which mainly focused on the short-term incubation experiments.Investigaitons on the influences of biochar on agricultural soil in the longer term are urgently needed,and the comprehensive impacts of biochar on soil and crop for longer periods need further evaluation.Therefore,this study determined the effects of biochar combined with nitrogen fertilizer on soil physicochemical properties,soil aggregate composition and stability,aggregate-associated C and N,microbial biomass carbon and nitrogen and crop yield in typical upland red soil in Jiangxi province following three consecutive crop rotation.The field experiment was conducted with a 7×4 factorial design at biochar rates of 0,2.5,5,10,20,30 and 40 t/hm2(designated as C0,C1,C2,C3,C4,C5 and C6 correspondingly)and nitrogen fertilizer rates of 0,60,90,120 kg N/hm2(designated as NO,Nl,N2 and N3 correspondingly).The results were as follows:(1)Biochar significantly improved the physicochemical properties of upland red soil in the three consecutive years,whereas nitrogen fertilizer had no effect on soil properties.Soil pH,soil organic carbon,cation exchange capacity,soil porosity,saturated hydraulic conductivity increased and soil bulk density decreased with increasing biochar amount in all years.In the first rapeseed harvest season after biochar incorporation,the ?10 t/hm2 biochar treatments increased soil pH remarkably,of which C6N2 treatment enhanced soil pH by 0.60 with a 12.68%increase.Soil pH in the biochar-amended treatments(?10 t/hm2)tended to decline continuously with the cultivation years.In the third sweet potato harvest season,biochar application at the rate of 40 t/hm2(C6N2 treatment)increased soil pH by 0.35 with a 7.25%increase when compared with C0N2 treatment.Biochar with the application rate of>20 t/hm2 remarkablly enhanced soil organic carbon and cation exchange capacity with these treatments seeing a>35%increase in soil organic carbon and>30%increase in cation exchange capacity for the three consecutive years.Biochar application resulted in less variation in soil bulk density and soil porosity than in saturated hydraulic conductivity.In the third sweet potato harvest season,C6N2 treatment increased the soil porosity and saturated hydraulic conductivity by 11.71%and 126.57%respectivley,and reduced the soil bulk density by 0.17 g/cm3 with the decrease rate being 13.59%.(2)Biochar significantly affected the composition and stability of soil aggregate.In the second rapeseed harvest season after its application,biochar significantly enhanced the content of>2 mm dry aggregates in the 0-15 cm soil layer,the content of>1 mm dry aggregates in the 15-30 cm soil layer and the content of>0.5 mm water-stable aggregates in both soil layers whilst the content of<0.25 mm water-stable aggregates decreased remarkably with the increment of biochar applicaiton.Applying biochar significantly reduced the percentage of aggregate destruction(PAD0.25)and soil fractal dimension,and increased the mean weight diameter(MWD)and geometric mean diameter(GMD).The decline of fractal dimension of dry aggregates in biochar-amended treatments was 2.26-9.18 times as much as that of water-stable aggregates in the 0-15 soil layer,and 1.68-3.86 times in the 15-30 cm soil layer.These results suggest that biochar can improve the resistance of soil aggregate to physical stresses,and its improvement for mechanical tensile strength was more obvious than that for water-stability.Biochar exerted a significant influence on the dry aggregate-associated organic carbon and total nitrogen in the 0-15 soil layer while little difference was observed in the 15-30 cm soil layer.After combined application of biochar and nitrogen fertilizer for three years,the soil organic carbon and total nitrogen in all fractions increased with increasing biochar amount and decreasing particle size in the 0-15 soil layer.Both soil organic carbon and total nitrogen were the highest in the<0.25 mm fraction under C6N2 treatment with a 39.61%increase in soil organic carbon(equivalent to 3.77 g/kg)and a 14.69%increase in total nitrogen(equivalent to 0.18 g/kg)respectively.Biochar had a greater impact on the distribution of nitrate nitrogen than that of ammonium nitrogen.In the 0-15 cm soil layer,C4N2 and C6N2 treatment both significantly enhanced the content of nitrate nitrogen in all fractions,while the ammonium nitrogen was significantly higher than control only in the>0.5 mm fraction under C6N2 treatment.(3)Biochar significantly enhanced the soil microbial biomass carbon(MBC)and nitrogen(MBN)in the third tuber bulking season and sweet potato harvest season.When biochar was applied at comparatively low amount(i.e.,less than 5 t/hm2),biochar application with or without nitrogen fertilizer had no effect on MBC and MBN.When biochar rate was increased to 20 t/hm2,both MBC and MBN significantly increased with the increment of biochar or nitrogen fertilizer use.The largest increase of MBC or MBN occurred in C4N3 treatment in the third tuber bulking stage while the C4N3 and C6N3 treatment showed the highest MBC and MBN respectively in the sweet potato harvest season.(4)The yields of rapeseed and sweet potato both increased with the increment of biochar or nitrogen fertilizer application and maintained a high increase rate even when the absolute yields decreased in the subsequent two years.The highest rapeseed yields were observed in C6N2,C5N3 and C6N2 treatments correspondingly in the three harvest seasons with the increases over the C0N2 treatment being 35.71%,50.88%and 69.16%respectively.The C6N3 treatment had the highest sweet potato yileds for the three years after biochar application,and increased the sweet potato yileds by 66.17%,101.82%and 103.42%respectively.Compared with rapeseed,sweet potato was much more sensitive to the improvement of soil properties and crop yield caused by biochar applicaiton.The pearson correlation coefficients between crop yields and soil physicochemical properties for sweet potato were all greater than those for rapeseed.Sweet potato yields increased remarkably even at the low biochar rate(10 t/hm2),and the increase amplitude far exceeded rapeseed for the three consecutive years.