Research Of Carbon And Nitrogen Regulation Technology For Soil Structure And Fertility Improvement In The Loess Upland

Improving soil physical structure and increasing the contents of soil organic carbon and nitrogen are an important guarantee to enhance soil quality and fertility level,which is closely related to crop production and food security.The Loess Plateau is an important grain producing area in China,with abundant light and heat resources and straw resources.As an important rain-fed agricultural area in China,the shortage of water resources,low soil quality,poor conservation capacity of water and fertilizer seriously restrict the high-quality development of agriculture in this area.Straw and organic fertilizer incorporation is an effective measure to improve soil quality.However,the effects of long-term straw incorporation on soil structure,carbon nitrogen distribution and retention need to be further studied,and the potential of nitrogen fertilizer reduction under different quality of exogenous carbon（C/N）incorporation is not clear in the Loess Plateau.Therefore,this study relied on two field positioning experiments,which were 5-year maize straw incorporation experiments,including no nitrogen fertilizer（CK）,single nitrogen fertilizer（N）,and nitrogen fertilizer combined with maize straw incorporation（NS）;and a two-year field experiment,including four different organic carbon sources（no exogenous carbon incorporation（CK）,soybean straw incorporation（S）,maize straw incorporation（Z）,and organic fertilizer incorporation（M））and three different nitrogen application rates(no nitrogen application（N₀）,reduced nitrogen application(N₁,180 kg·ha^-1),and conventional nitrogen application(N₂,225 kg·ha^-1)),were conducted to quantify the effects of maize straw incorporation for many years on soil physical structure,carbon and nitrogen contents,and the size distribution of water-stable aggregates,and the effects of different organic carbon sources incorporation combined with nitrogen fertilizer were further compared and analyzed to clarify the appropriate soil quality improvement technology in the arid area of the Loess Plateau.The main results are as follows:1.The application of nitrogen fertilizer for many years increased surface soil bulk density as a whole,while NS treatment significantly improved soil bulk density and porosity,increase soil aeration and water permeability.The particle size distribution of soil water-stable aggregates in 0-30 cm soil layer had no significant response to N treatment,but NS treatment increased the proportion of>2 mm and 0.25-2 mm aggregates in the 0-30 cm soil layer,and better promoted the synthesis and transformation of water-stable aggregates to larger particles,especially in the 0-10 cm and 10-20 cm soil layers.In the pre-sowing（PT）period of two years trial,compared with CK treatment,the percentage of>0.25 mm water-stable aggregates(WR_0.25)of NS treatment in 0-10 cm soil layer respectively increased by 4.21%and 8.63%;In the harvest period of two years trial,the percentage of WR_0.25 of NS treatment respectively increased by 5.65%and 11.07%.The average weight diameter（MWD）and geometric mean diameter（GMD）of water-stable aggregates of NS treatment increased in all soil layers,especially in the 0-10 cm soil layer during the PT period in 2020,compared with CK treatment and,the increase were 18.38%,34.77%,and compared with N treatment,the increase were17.53%,25.34%,respectively,which was beneficial to improve the stability of soil aggregates.2.The application of nitrogen fertilizer combined with maize straw incorporation for many years increased the contents of soil organic carbon（SOC）and total nitrogen（TN）,promoted the accumulation of carbon and nitrogen among all aggregate fractions,and facilitated the distribution of carbon and nitrogen in larger aggregates,while the distribution of carbon and nitrogen in each fraction of soil aggregates had no obvious response to long-term nitrogen application.For the 0-10 cm soil layer during PT period in two years trial,compared with CK,the SOC contents of NS treatment respectively increased by 46.72%and27.37%,and the TN contents respectively increased by 20.09%and 23.14%;For the 0-10 cm soil layer during R6 period in two years trial,compared with CK,the SOC contents of NS treatment respectively increased by 25.45%and 19.09%,and the TN contents respectively increased by 5.30%and 12.89%.In general,N treatment also increased the contents of SOC and TN in aggregates of 0-30 cm soil layer,but there was no significant difference between N treatment and CK treatment.The NS treatment increased the SOC and TN content of each particle size of aggregates in 0-30 cm soil layer,which was beneficial to the accumulation of carbon and nitrogen nutrients in large aggregates.3.Different organic carbon sources incorporation increased the proportion of large aggregates and improved the stability of aggregates,and the effect of organic fertilizer and soybean straw incorporation were better than that of maize straw incorporation,while the variation between different nitrogen fertilizer application rates did not reach a significant level.In terms of water-stable aggregates,soybean straw incorporation markedly increased the percentage of>2 mm and 0.25-2 mm fractions.The variation trend of organic fertilizer incorporation was similar to soybean straw,but the increase of>2 mm fraction was slightly lower than that of soybean straw,the increase of organic fertilizer incorporation under three nitrogen application rates was 25.71%,7.18%and 16.93%,respectively.Maize straw incorporation was more inclined to increase the percentage of 0.25-2 mm fraction.Three kinds of exogenous carbon incorporation all increased the MWD,GMD,>0.25 mm mechanically stable aggregates(DR_0.25)and WR_0.25,meanwhile reducing the soil aggregate destruction rate（PAD）and unstable group index(E_LT).The overall effect of organic fertilizer was the best,followed by soybean straw;moreover,different rates of nitrogen fertilizer application also had variety degrees of improvement on the aggregates stability indexes,but the differences between reduced nitrogen application and conventional nitrogen application were not significant.4.The three kinds of exogenous organic carbon incorporation increased the carbon and nitrogen contents of each particle size of aggregates to varying degrees,and the carbon and nitrogen contents of aggregates>2 mm increased the most.For the organic carbon contents of aggregates>2 mm,the effect of organic fertilizer incorporation was the best in the treatment without nitrogen fertilizer,which was 40.23%higher than that of CK treatment.In the treatment of nitrogen fertilizer reduction and conventional nitrogen application,soybean straw incorporation was the best,which increased by 18.54%and 13.55%respectively compared with CK treatment.For total nitrogen contents of aggregates>2 mm,the effect of soybean straw incorporation was the best in the treatment of no nitrogen fertilizer and conventional nitrogen application,which was 15.22%and 21.51%higher than that of CK treatment,respectively.In the treatment of nitrogen fertilizer reduction,organic fertilizer incorporation was the best,which was 13.83%higher than that of CK treatment.Three kinds of exogenous organic carbon incorporation increased the contribution rate of carbon and nitrogen for 0.25-2 mm particle size aggregates;In the treatment of nitrogen fertilizer reduction,soybean straw and organic fertilizer incorporation treatments also increased the contribution rate of carbon and nitrogen for>2 mm particle size aggregates.In the treatment of conventional nitrogen fertilizer application,three kinds of exogenous carbon simultaneously increased the contribution rate of carbon and nitrogen for>2 mm particle size aggregates.Therefore,the application of exogenous organic carbon incorporation combined with nitrogen fertilizer in the Loess Plateau can ameliorate the soil physical structure and improve the soil fertility level,what’s more,the effect of low C/N soybean straw or organic fertilizer incorporation combined with reduced nitrogen fertilizer is better,which can be used as an important technical measure to improve soil structure and soil quality in this area.