Effects Of Elevated CO₂ On Soil Carbon And Nitrogen Fractions And Roots Of Maize In A Semiarid Region

Under the background of elevated atmospheric CO₂ concentration,understanding the responses of crop,soil organic carbon and nitrogen in fa rmland to elevated CO₂ concentration play extremely significant roles in assessing soil fertility,soil organic carbon and nitrogen cycles in farmland,and agriculture sustainability.A field experiment was conducted to study the contents of organic carbon,total nitrogen,dissolved organic carbon,dissolved total nitrogen,readily oxidized organic carbon and roots in spring maize field under three different environmental conditions: natural atmospheric CO₂ concentration（as the control,marked as CK）,open-top chamber（OTC）system with CO₂ concentration of 700 μmol·mol-1（marked as OTC+CO₂）and OTC system with current atmospheric CO₂ concentration（marked as OTC）.And our study provided theoretical basics and basic datas to optimize farmland nutrient manage ment and soil sustainability.The main results were showed as follows:（1）Two years’ spring maize field experiment results showed that soil organic carbon and its fractions had different responses to the elevated CO₂ concentration,the interaction between OTC and elevated CO₂ concentration.In general,compared with the OTC,the elevated CO₂（OTC+CO₂）had no significant effects on the contents of soil organic carbon and readily oxidizable organic carbon in cropland under the condition of RN225 and MN225 at most growth periods of maize in 2017 and 2018.But the effect of elevated CO₂ on the contents of dissolved organic carbon varied with film mulching and growth period,and the elevated CO₂ had no effect on it under the condition of no film mulching（RN225）but increased it at individual growth stages under the condition of film mulching（MN225）in both experimental years.Compared with the natural atmospheric control（CK）,the interaction between OTC and elevated CO₂ concentration（OTC+CO₂）treatment markedly decreased the soil organic carbon and readily oxidizable organic carbon in cropland at most growth periods,and the effect of 2017 was more significant than that of 2018.The OTC+CO₂ had no effect on dissolved organic carbon under the condition of no film mulching（RN225）in 2018,but the effect of OTC+CO₂ on it varied with growth seasons in 2017,and increased at the early growth stages,decreased at the later growth stages under the condition of film mulching（MN225）.（2）Total nitrogen and dissolved total nitrogen were affected by elevated CO₂ concentration and the interaction between OTC and elevated CO₂ concentration during spring maize growing stages in 2017 and 2018,and the effect varied with years,growth period and mulching.Generally speaking,under the single condition of elevated CO₂ concentration（compared with the OTC）,OTC+CO₂ significantly increased soil total nitrogen at R6 stage under the condition of RN225 and MN225 in 2018,but had no effect on it at other stages under the condition o f RN225 and MN225 in both years.The dissolved total nitrogen had markedly seasonal variations,which decreased significantly at the vegetative growth stages and increased significantly at the reproductive growth stages.However,under the compound condition of OTC and elevated CO₂ concentration（compared with the CK）,soil total nitrogen was decreased effectively at most growth stages,by 3.77%～12.48%.The OTC+CO₂ decreased the dissolved total nitrogen at vegetative growth stages but had no effect on it at reproductive growth stages under the condition of no film mulching（RN225）,increased the dissolved total nitrogen at most growth stages under the condition of film mulching（MN225）,by 8.96% ～ 288.70%.The results also showed that film mulching could generally increase the dissolved total nitrogen under the interaction between OTC and elevated CO₂ concentration.（3）The elevated CO₂ concentration and OTC+CO₂ treatment had significant effects on maize root system under the two conditions of RN225 and MN225 during the growth process of maize in 2016,2017 and 2018.Compared with the OTC,the elevated CO₂ had a promoting effect on root biomass under the conditions of film mulching or no film mulching,and the range was 0.07%～348.43%.Similar to the root biomass,the root carbon content was only increased in 0-20 cm soil layers but not significantly.And the effect of elevated CO₂ on root nitrogen and root C/N ratio varied with the film mulching.The OTC+CO₂ reduced the root nitrogen under the condition of no film mulching,by 23.5%～40.10%,but increased the root nitrogen under the condition of film mulching and increased the C/N ratio under the condition of no film mulching,by 0.91%～19.99% and 7.78%～49.29%,respectively.Compared with the CK,the interaction between OTC and elevated CO₂ concentration（OTC+CO₂）had same effect on root biomass,root carbon content and root nitrogen content（under the condition of film mulching）as that of elevated CO₂ concentration.However,the OTC+CO₂ effectively increased the root nitrogen content under the condition of RN225,by 0.40%～27.07%.In addition,the experimental datas from 2016 to 2018 also showed that most of the maize roots were distributed in the 0-40 cm soil layer,and the root biomass decreased gradually with the deepening of the soil layer.And the order of nitrogen content of maize root system was 0-10 cm<10-20 cm<20-30 cm in different soil layers.In conclusion,under the current experimental conditions,the effect of elevated CO₂ on spring maize roots has a positive effect,but the impact on soil carbon and nitrogen in cropland varies with years,growth period and mulching.The specific indicators of carbon and nitrogen have different responses to the elevated CO₂ concentration and the interaction between OTC and elevated CO₂ concentration.The results of this study can provide a theoretical basis for soil carbon and nitrogen cycling and maize growth and development under climate change conditions.