Seasonal Patterns Of Soil Respiration And Soil Biochemical Properties Under Nitrogen Addition

Soil respiration is an important part of terrestrial ecosystem carbon cycling, it has a great impact on atmospheric CO2. The North China Plain is one of the major grain production areas, excessive nitrogen application is widespread in this area, it is not beneficial for sustainable development of agricultural production. This study was conducted for wheat-maize rotation system in the North China Plain and N fertilizer was applied at four different levels: 0, 120, 180 and 240 kg N ha-1(denoted as N0, N12, N18 and N24, respectively). We explored the effect of N addition on soil respiration, biochemistry properties and yield, and illuminated the mechanism of how N addition affects soil respiration, and then suggested rational nitrogen fertilizer application amount. Our study provided theoretical basis for optimizing nitrogen fertilizer application management in the North China Plain agroecosystems, increasing soil carbon stabilization and improving soil properties. The main results are summarized as follows:1. N addition significantly increased soil total respiration and the promotion effect became more and more significant with the continuous N fertilizer applications. Compared with N0, N12, N18 and N24 significantly increased soil total respiration by 19%, 23% and 29%, respectively.2. Soil heterotrophic respiration(Rh)/soil total respiration(Rs) was higher than soil autotrophic respiration(Ra)/Rs. Ra /Rs of N0, N12, N18 and N24 was 39%, 40%, 44% and 45%. It indicated that N addition increased Ra /Rs by improving root biomass.3. Soil temperature and moisture were the dominant environmental factors of soil respiration. The exponential relationship between soil respiration and soil temperature under all treatments was found, soil temperature explained about 66% of soil respiration and Q10 ranged from 1.49 to 2.68. No significant correlation between soil respiration and moisture was found when soil moisture was optimum, but soil respiration exhibited significant and negative correlation with soil moisture when soil moisture was above the optimum value. The multiple regression model including both soil temperature and moisture could explain 67% of the seasonal variations in soil respiration.4. N addition altered soil respiration by influencing soil enzyme activity. During 2013-2014 winter wheat and summer maize growing season, N24 significantly decreased soil cellobiohydrolase activity(CBH), the significant and negative correlation between soil respiration and CBH was found. During 2014 summer maize growing season, N addition significantly increased soil β-glucosidase(BG) and β-xylosidase activities(BXYL), soil respiration exhibited significant and negative correlation with BG and BXYL.5. N addition significantly increased soil respiration by improving easily extractable glomalin(EEG). During 2013-2014 winter wheat growing season, compared with N0, N12, N18 and N24 significantly increased EEG by 9%, 9% and 20%, respectively. During 2014 summer maize growing season, EEG of N0, N12, N18 and N24 was 0.33 mg?g-1、0.36 mg?g-1、0.40 mg?g-1 and 0.46 mg?g-1, respectively. The significant and positive correlation between soil respiration and EEG was found during 2013-2014 winter wheat and summer maize growing season.6. N addition promoted crop growth by improving soil NH4+-N and NO3--N contents and then significantly increased soil respiration. During 2013-2014 winter wheat growing season, compared with N0, N12, N18 and N24 significantly increased soil NH4+-N content by 17%, 24% and 38%, respectively. Soil NO3--N content of N24>N18>N12>N0. During 2014 summer maize growing season, N addition could not significantly influence soil NH4+-N content but significantly increased soil NO3--N content. Compared with N0, N12, N18 and N24 significantly increased soil NO3--N content by 153%, 161% and 232%, respectively. Soil NH4+-N and NO3--N contents showed a significant and positive correlation with soil respiration.7. N addition significantly increased soil organic carbon(SOC) and total nitrogen content(TN), but TN significantly decreased when N addition amount was above 180 kg?ha-1. During 2013-2014 winter wheat growing season, compared with N0, N12, N18 and N24 significantly increased SOC by 9.64%, 9.64% and 9.99%, respectively. During 2014 summer maize growing season, compared with N0, N12, N18 and N24 significantly increased SOC by 5.03%, 6.26% and 7.38%, respectively. During 2013-2014 winter wheat growing season, TN of N18 and N24 was significantly higher than N0 and N12. During 2014 summer maize growing season, TN of N18 was also significantly higher than N0 and N12, but TN of N24 significantly decreased compared with N18.8. N addition significantly increased yield, nitrogen agronomic efficiency(NAE) was high when N addition amount was 180 kg N ha-1. During 2013-2014, compared with N0, N12, N18 and N24 significantly increased yield by 20%, 24% and 29%, respectively. During 2014-2015, compared with N0, N12, N18 and N24 significantly increased yield by 45%, 53% and 66%, respectively. NAE of N18 was high during most growing seasons. By the analysis of yield and NAE, we concluded that the optimum N addition amount of summer maize and winter wheat growing season at our study site was 180 kg N ha-1.