Effects Of Exogenous Nitrogen And Crop Straw Addition On Soil Co₂ Emissions

In this study, the effects of exogenous nitrogen and crop straw addition on soil CO2 emissions were investigated by a series of indoor incubation experiments between March 2013 to July 2014. In order to investigate the effects of exogenous nitrogen addition on CO2 emissions and enzyme activities of a forest soil, a incubation experiment was performed. The soil used for incubation experiment was collected from a broad-leave forest soil. There was 60 g dry soil in each jar used for soil incubation. There were 17 exogenous nitrogen addition treatments, which were 0.0000,0.0010,0.0013,0.0016,0.0019,0.0022,0.0025,0.0028,0.0036,0.0045, 0.0055,0.0067,0.0078,0.0091,0.0200,0.0501,0.1000 g. Soil CO2 emission rates were measured 1,2,4,6,8,15,18, and 22 days after the exogenous nitrogen addition by using an infra-red gas analyzer. In order to investigate the priming effects of crop straw amendments on the broad-leave forest soil, a incubation experiment was performed. Five types of crop straw (winter wheat, rice, maize, soybean, and peanut) were used in the experiment and the soil without straw added was control (CK). There were three levels (0.6,1.2, and 2.4 g) for each straw type. Soil CO2 emission rates were measured 1,2,4,6,8,11,15,18, and 22 days after the crop straw amendments by using the infra-red gas analyzer. In order to investigate the effects of winter wheat straws with different genotypes on decomposition coefficient of crop straw, a incubation experiment was performed. The soil samples used for the experiment were taken from a cropland. Six genotypes of wheat straws (HM19, HM20, HM24, JM22, WM6 and YL9) were used in the experiment and the soil without straw added was control (CK). There were four levels (0.6,1.2,1.8, and 2.4 g) for each straw type. Soil CO2 emission rates were measured 1,2,4,6,8,11,17, 24, and 31 days after the crop straw amendments by using a infra-red gas analyzer. Activities of urease, invertase and catalase, dissolved organic carbon (DOC), and pH were also measured after incubation.Results showed that the accumulative forest soil CO2 emissions for all treatments increased significantly with the increase of incubation days. Soil CO2 emissions for most nitrogen addition treatments were lower than that for CK treatment, which indicated that the addition of exogenous nitrogen inhibited soil CO2 emissions. The addition of nitrogen can increase the soil urease activity; a linear regression function can be used for fitting the relationship between soil urease activity and the amount of exogenous nitrogen added. Within low nitrogen addition range, soil invertase activity increased significantly with the increase of exogenous nitrogen. There was no significant linear relationship between soil catalase activity and exogenous nitrogen addition. Further investigation indicated that, within low nitrogen addition range, the relationship between DOC content and the amount of exogenous nitrogen can be fitted with a linear function. Nitrogen addition induced the increase of soil pH.Further investigation indicated that, the basal soil CO2 emission, i.e. soil organic carbon (SOC) mineralization in CK, was significantly (P<0.001) lower than the CO2 emission from straw-amended soils. Given a specific straw type, forest soil CO2 emission was significantly and positively correlated with the amount of straw inputs. The decomposition coefficients, i.e. the slopes of the linear function, of winter wheat, rice, maize, soybean, and peanut straws, were 0.275,0.593,0.895,0.890, and 1.344 mg·g-1 per gram of straw, respectively. Winter wheat straw amendment induced an about 2-fold increase of basal soil CO2 emission, showing a positive net priming effect (PE). Regression analysis indicated that soil CO2 emission increased with the increase of urease, invertase, and catalase activities. Relationships between soil CO2 emission and the activity of these three enzymes implied that soil enzyme activity might play an important role in soil CO2 emissions and relevant PEs. Crop straw amendments also significantly (P< 0.05) increased the DOC content. Soil CO2 emission increased with the increase of the DOC content. Regression analysis showed that soil CO2 emission increased linearly with the increase of pH. Further analysis indicated that the decomposition coefficient of different crop straws increased linearly with the increase of pH.The CO2 emission from straw-amended soils was significantly higher than the CO2 emission from CK, which indicated that crop straw amendments significantly increased cropland CO2 emission. Different genotypes might exert great difference in decomposition coefficient of wheat straw. It induced an about 2-fold difference between the highest and lowest decomposition coefficient of wheat straw. Winter wheat straw amendment increased the DOC content. Given a specific genotypes of wheat straw, soil CO2 emission significantly increased with the increase of the amount of straw inputs. Regression analysis indicated that soil urease, invertase and catalase activities were significantly and positively correlated with the amount of winter wheat straw inputs. Soil CO2 emission increased with the increase of urease, invertase, and catalase activities. Further analysis indicated that there was a significant linear relationship between the decomposition coefficient of wheat straw and soil pH. Soil CO2 emission decreased linearly with the increase of pH.