Nitrification-Denitrification Of Fertilizer Nitrogen In Winter Wheat/Summer Maize Rotation System

Nitrification-denitrification is one of most important pathway resulting in nitrogen loss from soil, it also cause N₂O emission simultaneously. Either from the point of agricultural view or from the point of environmental view nitrification-denitrification is an important soil biological process. Unfortunately information on arable soil nitrification-denitrification was absent in Northern China. Therefore in this paper a continual measurement of denitrification with acetylene inhibition method in a soil core incubation system, and nitrification was carried out with different nitrogen rate, irrigation and straw treatments in two field in winter wheat/summer maize rotation system. The main results are as follows:Two year's study in situ showed that summer maize growth season is the key period for soil denitrification in the rotation system, meanwhile nitrogen loss by nitrification-denitrification mainly happened within 1-2 weeks after fertilizer nitrogen application while soil Nmin and water content were relatively higher.Nitrogen loss by denitrification with the optimum fertilizer nitrogen application was comparable to the CK treatment, while the conventional fertilization method increased dentrification-nitrogen loss by 2-3 times than the optimum fertilization. Nitrogen fertilization method based on soil Nmin test can greatly reduce nitrogen loss by denitrification. The effect of straw on nitrification was not significant while denitrification increased with straw incorporated into the soil.Factor affecting soil denitrification was soil water status in early maize growth stage while in late stage that was soil water content and low temperature together.Soil nitrification completed within 1-2 weeks after fertilization in winter wheat growth season. Relative low soil water content and soil temperature inhibited denitrification, which was quite lower in wheat season than in maize season. Fertilizer nitrogen application had not showed any effect on denitrification when soil water content was very low.High spatial and temporal variability of denitrification comes from unevenly distribution of soil Nmin, water status in space and time. Spatial gradient of soil Nmin due to fertilizer nitrogen banded application at maize third leaf stage resulted in denitrification variability till to about 20 days later when tenth leaf stage fertilizer nitrogen would be applied.Denitrification rate decreased greatly from soil surface to subsoil. The soil dentrification rate in 15-30 cm was relatively high amounting to 10.7%-33.5% of that in 0-15cm soil layer, and the total nitrogen loss by denitrification in the soil (15-60 cm) was equal to 14%-51% of that in the surface soil (0-15cm). Available C addition significantly enhanced denitrification either for surface soil or for subsoil, when the increased effect was even higher for subsoil. The study confirmed surface soil being the main soil layer for denitrification. On the other hand, subsoildenitrification should not be underestimated when nitrogen budget is calculated in summer maize season.Similar trend was found between soil denitrification dynamic and N2O production/ emission dynamic in the rotation system. Summer maize growth season was also the key period for N2O production, especially within 1-2 weeks after fertilizer nitrogen was applied. Soil N2O production was mainly attributed to nitrification-denitrification, meanwhile under some particular condition soil nitrification was the main source of N2O production/emission such as in the period just after nitrogen topdressing in summer maize season.Nitrogen loss induced by Nitrification-denitrification was 4.5%(for fertilizer nitrogen rate at 120 kg/hm2)-18.3% (for fertilizer nitrogen rate at 360 kg/hm2) in winter wheat growth season and 58.2%(for fertilizer nitrogen rate at 120 kg/hm2)-63.4%(for fertilizer nitrogen rate at 360 kg/hm2) in summer maize growth season based on indirect estimation, while that was trace in winter wheat growth season and trace-13.39% in summer growth season based on direct estimation. Reasons for big difference in nitrogen losses estimated by two methods need further investigation.