| Soil organic carbon(SOC)is an important indicator of soil quality and the guarantee of improving and maintaining crop yield.Unreasonable agricultural management measures could result in the decline of SOC and it is not conductive to the increase of crop yield.The present study using CCRI 50 as the experimental material and was conducted at Dafeng(33°20'N,120°46'E),Jiangsu,China from 2015 to 2017 and the Pailou Experimental Station(118°50' E,32°02' N)from 2016 to 2017.The both experiments included four amounts of nitrogen fertilizer(0,75,150 and 300 kg N ha-1).The field experiment included two ways of straw returned(no straw returned and straw returned),while the pot experiment included three ways of straw returned(no straw returned,straw returned and biochar returned).The filed experiment was aimed to study the effect of different rates of N application and straw amendment on the growth of cotton,the distribution and stability of aggregate,labile SOC fractions and the stability of SOC under the prior excessively N inputs.The pot study in Pailou experimental station was aimed to study the difference of growth of cotton and SOC fractions response to the straw and biochar addition.The main results of this study are as follows:1.Effect of different rates of nitrogen fertilizer application and straw amendment onthe growth of cotton and seed cotton yieldIncreasing the amount of nitrogen fettilizer was more conducive to the increase of seed cotton biomass per boll and boll number under the straw or biochar retured treatment.Increasing the rates of nitrogen fertilizer application significantly increased the biomass of seed cotton,the biomass of the cotton,number of fruiting branches and positions and the net photosynthetic rate of functional leaves in the main stem after the squaring stage.Whereas,the straw amendment had little effect on the growth of cotton in the field experiment.The seed cotton yield was significantly affeected by nitrogen fertilizer in 2015-2016,whereas,it was markedly affected by both N fertilizer and straw amendment in 2017.At the rate of 75,150 and 300 kg N ha-1 N application,compared with the no straw returned treatment,the seed cotton yield of the straw returned treatment was increased by 36.8%,7.2%and 2.9%,respectively.It was indicated that there would be a certain time for the straw returned to promote the yield.At a certain nitrogen level,straw and biochar application significantly increased the cotton yield.But there was no marked significant difference in seed cotton yield,total biomass,morphological traits and photosynthetic performance between straw and biochar treatment.The biochar addition significantly increased the enlongation of the cotton root.2.Effect of different rates of nitrogen fertilizer application and straw amendment on the soil organic carbon pool and aggregate stabilityCompared with N fertilizer applied alone,combined application of nitrogen fertilizer and straw was more beneficial to the increase the mass percentage of 8-0.25 mm mechanically stable aggregate and water-stable aggregate,while the mass percentage of macroaggregate(2-0.25 mm),microaggregate(0.25-0.053 mm)and silt&clay(<0.053 mm)was decreased.It was indicated that the application of nitrogen fertilizer and straw was beneficial to the formation of larger macroaggregate.In addition,the mean weight diameter and geometric mean diameter were increased with the amount of nitrogen fertilizer and it was higher in the straw returned treatment.It seems that the ability of the protection for aggregate-associated organic carbon was enhanced by the increment of aggregate stability.The soil structure was more loosen due to that the fractal dimension D was decreased caused by the application of nitrogen and straw.Furthermore,the mechanically stable and water-stable aggregate was more stable in 20-40 cm than in the 0-20 cm depth.3.Effect of different rates of nitrogen fertilizer application and straw amendment on the stability of soil organic carbonCompared with compined application of nitrogen fertilizer and straw,N application alone had less effect on soil aggregate-associated organic carbon(AOC)content at 0-20 cm depth and the cumulative amount of AOC at 0-40 cm depth.In addition,the straw application had a greater promotion effect on AOC at 20-40 cm depth.The promotion effect of straw returning to soil on the AOC of macroaggregate increased with the amount of N application,whereas,the promotion for the AOC of microaggregate and silt&clay had the largest effect under the rate of 150 kg N ha-1 application.Compared with N application alone,compined application of nitrogen fertilizer and straw increased the cumulative mineralization of AOC by 27.7%in the 0-20-cm layer and by 80.9%in the 20-40-cm layer for different rates of N and aggregate sizes.It mens that the the biochemical stability of SOC was decreased caused by straw addition.Macroaggregates showed better protection of AOC in 0-20 cm depth,but the opposite result was observed at 20-40 cm depth.Straw returned treatment had a significantly higher value of the proportion of labile carbon pool compared with nitrogen fertilizer applied alone in a certain amount of nitrogen fertilizer application,while the straw amendment had no marked effect on lability index.4.Effect of different rates of nitrogen fertilizer application and straw amendment onsoil pH and soil organic carbon fraction.The combined application of nitrogen with straw or nitrogen with biochar were beneficial to the accumulation of total soil organic carbon and the content of microbial biomass carbon(MBC),particulate organic carbon(POC)and dissolved organic carbon(DOC).The straw application had no significantly effect on MBC,POC and DOC at the 20-40 cm soil layer.The straw returned was most favorable for SOC accumulation at the rate of 150 kg N ha-1,while the promotion effect of biochar returned on SOC increased with the incrase of nitrogen application rate.Both the rates of nitrogen and straw application had no significant effect on soil pH,while,the soil pH was decreased with the application of nitrogen fertilizer in the pot experiment.The straw returned could slow down the decline of the soil pH caused by the nitrogen fertilizer.The application of biochar significantly increased the soil pH and total soil organic carbon,whereas,there was no significant difference of labile organic carbon fractions between straw and biochar treatment.In conclusion,the combined application of straw or biochar can enhance the mass percentage of larger aggregate,soil aggregate stability,the content of SOC,POC,MBC,DOC and the physical stability and chemical stability of SOC.But the combined application of straw reduced the biochemical stability of SOC.Biochar significantly increased the elongation of root though there was no significant difference in cotton growth between the application of biochar and straw.The effect of rates of nitrogen fertilizer and straw application on soil pH was not consistent in the field and pot experiment,which was mainly caused by the different soil conditions.The combined application of 150 kg N ha-1 and straw was most conducive to the simultaneous improvement of cotton yield and soil quality. |
PDF Full Text Request