Impacts Of Management Regimes On Soil N₂O Emission From Mollisols In Agroecosystem

In recent decades, the greenhouse effect and other environmental problems have been paidmore and more attention. When people concern about the carbon dioxide （CO₂）, methane （CH₄）,nitrous oxide （N₂O） and other greenhouse gas emissions, the targets are usually factory emissions,automobile exhaust and Freon emissions.Whereas soil N₂O （the largest source in the biosphere） isusually ignored, although N₂O has a high global warming potential. Based on the Mollisols ofagricultural ecosystem in Northeast China, we measured soil N₂O emissions and their influencingfactors under different agricultural management regimes （fertilization, farming systems,continuous cropping, and land use patterns）, explored the relationships between soil N₂Oemissions and the environmental factors, and aimed to provide a theoretical basis for nitrogencycle in agroecosystem and the issue of N₂O source or sink.There were six treatments in the fertilization experiment: no fertilizer （CK）, chemicalfertilizer （NPK）, fertilizer plus straw （NPK+MS1）, fertilizer plus double the straw （NPK+MS2）,fertilizer plus pig manure （NPK+OM1） and fertilizer plus triple the pig manure （NPK+OM2）.Compared with the control, under the treatment NPK+MS2(106±17.9mg N·m^-2), NPK+OM1(119±7.7mg N·m^-2) and NPK+OM2(117±11.9mg N·m^-2), the cumuative N₂O emissions weresignificantly improved by66%,86%and83%（P<0.05）. However, NPK fertilization alone (64±0.7mg N·m^-2) and NPK+MS1(89±15.7mg N·m^-2) had no remarkable influences on soil N₂Oemissions （P>0.05）. During the growing season, soil N₂O emissions significantly increased globalwarming potential under the treatment NPK+MS2, NPK+OM1and NPK+OM2compared with thecontrol. The index model analysis showed that soil temperature can explain31-96%of theseasonal variations in N₂O emissions except for the control treatment. Compared with soiltemperature, fertilization practice was the dominant factor on affecting the N₂O emissions. Ourresults suggest that the influences of organic amendmens on soil N₂O emissions from Mollisolsprimarily vary with the type of the applied organic amendmens.Soil mineral nitrogen content under the conventional tillage is64%higher than that under theno-tillage treatment （P<0.05）. The correlation of soil organic carbon and soil C/N was notsignificant （P>0.05）. The average soil temperature in the depth of5cm （T5） and10cm （T10） wasboth conventional tillage> no-tillage, and the average water content of soil porosity （WFPS） wasno-till （45.9%）> conventional tillage （35.4%）. Compared with the no-tillage, the soil N₂O flux and cumulative emissions of conventional tillage were significantly higher. The cumulative soil N₂Oemissions under the no-till and conventional tillage were100and201mg N·m^-2, respectively, andsoil N₂O fluxes were33and59μg N·m^-2·h^-1, respectively. Soil mineral nitrogen content explained65%of the cumulative N₂O emissions. Under the conventional tillage, the correlation of soiltemperature and soil N₂O flux is significant （P<0.05）, whereas the correlation is not significantunder the no-till treatment （P>0.05）. The correlation of soil WFPS and soil N₂O flux is notsignificant both in no-tillage and conventional tillage treatments （P>0.05）.Soil mineral nitrogen under continuous cropping was wheat continuous cropping> maizecontinuous cropping> soybean continuous cropping, and soil mineral nitrogen explained15%ofthe N₂O emissions. Due to the early maturity of wheat and the removal of aboveground parts afterharvest, soil received adequate light and thus the soil temperature （T5） of wheat continuouscropping was higher than other treatments after harvest until September. In the three continuouscropping systems, the average size of the soil WFPS was maize continuous cropping （44.8%）>wheat continuous cropping （41.1%）> soybean continuous cropping （40.5%）. Compared withsoybean continuous cropping, the soil N₂O fluxes under maize and wheat cropping were higher by78%and43%, respectively. In the soybean, maize, and wheat cropping systems, the cumulativesoil N₂O emissions were61,110, and84mg N·m^-2, respectively. Throughout the crop growingseason, the correlation of soil temperature and soil N₂O flux was significant （P<0.05） only in themaize continuous cropping. In the maize and soybean continuous cropping systems, soil WFPS canexplain the seasonal variation in soil N₂O flux by23%and45%, respectively （P<0.05）.Soil mineral nitrogen and land use patterns were closely related. The size of mineral nitrogencontent was grassland> cropland> bare land. Soil mineral nitrogen content could explain62%ofsoil cumulative N₂O emissions. With the increase of soil mineral nitrogen content, soil cumulativeN₂O emissions increased. Under different land use patterns, soil temperature in the depth of5cm（T5） ranged from6°C to37°C, with the highest average value in farmland （22.0°C）, the second inbare land （21.6°C）, and the lowest in grassland （18.9°C）. The mean soil WFPS in the threedifferent land use patterns approciately maintained at40%, with40.8%in grassland,46.0%in bareland, and38.3%in farmland. The average N₂O flux was farmland> bareland> grassland, whichwere21.44,20.36, and16.56μg N·m^-2·h^-1, respectively. The correlation of N₂O flux and T5, T10,WFPS were not significant in both grassland and bare land, and only in the farmland system, thecorrelation of soil N₂O flux with T5, T10, and WFPS reached a significant level （P<0.01）. Theresults suggest that the inflencing factors are different under different land use patterns. In thefarmland ecosystem, soil temperature and moisture were the key factors affecting soil N₂Oemissions. In conclusion, the influencing mechanisms and degrees of different management regimes onsoil N₂O emissions were different. Soil N₂O emissions can be inflenced to different degree by thetype of the applied organic amendmens, no-tillage, crop type, and land-use change.