| Huang-Huai-Hai Plain is an important grain production region in China,which is of great significance to the national food security.However,since small-size agricultural machinery plays a leading role in agricultural production,unreasonable plow layer structure affected by long term rotary tillage not only restricts the production potential of plowing layer,but also leads to low water and fertilizer use efficiency,exacerbating environmental pollution.Therefore,it is very important to construct a reasonable tillage structure to fully tap the potential of farming layer and ensure the sustainable development of agriculture in China.The plow pan is one of the most important problems to construct a reasonable tillage structure and many scholars have proposed different construction methods,but there is no unified conclusion for the distribution of plow pan and the degree of suitable removal.It is of great significance to explore a new way of ameliorating plow pan to sequestrate more carbon,mitigate the greenhouse gas emission and promote the crop production.Therefore,the characteristics and regional distribution of plow pan should be clarified,besides,the effect of breaking the plow pan on crop growth and soil environment and its related mechanism should also be explored.In order to solve the above problems,three main work were carried out which are as follows:（1）Regional investigation:In order to clarify the characteristics and regional distribution of the plow pan in the Huang-Huai-Hai Plain,two typical counties（Wuqiao County in Hebei province and Ling County in Shandong province）were selected at the jointing stage of winter wheat in 2014.（2）Field experiment:In order to explore the effect of breaking the plow pan by different degree on crop growth and soil environment,field experiment was conducted in Dezhou during 2014-2015.（DL15）.Eight treatments were conducted including the original plow pan（RT15）,breaking the plow pan by 1/3（DL20）,breakinb the plow pan by 2/3（DL25）,breaking the plow pan thoroughly（DL40）and the other four treatments with no N fertilizer including CK15,CK20,CK25 and CK40 in this research.（3）Soil column experiment labeled by 15N:The effect and its mechanism of plow pan with different thickness and bulk density on crop growth and soil environment were studied.The experiment was carried out under two soil texture conditions with four different thickness of plow pan（15 cm,10 cm,5 cm and 0 cm）also with there different bulk density of plow pan including 1.65 g·cm-3,1.75 g·cm-3and 1.85 g·cm-3.The main conclusions are as follows:（1）The characteristics and regional distribution of plow pan in Huang-Huai-Hai Plain:The plow pan was found in 74%-78%of the investigation points in Huang-Huai-Hai plain which indicated that the plow pan was ubiquitous.The average thickness of the plow layer is 14.7 cm,and the plow pan mainly distributed at 15-30 cm.The bulk density and penetrating resistance of the plow pan are significantly higher than that of the plow layer and subsoil layer（P<0.05）.The average bulk density of the plow bottom is about 1.54 g·cm-3 and the penetration resistance of plow pan at the jointing stage plow bottom of winter wheat varied from 1 371.0 to 4 256.0 kpa with the average penetration resistance of 2556.0 kpa.At present,the present situation of the plow pan in Huang-Huai-Hai plain is not conducive to the growth of crop roots and the full utilization of the soil nutrients.（2）The effect of plow pan on soil bulk density,penetration resistance and water dynamics:In this study,the soil bulk density and penetration resistance of 10-30 cm soil layer（P<0.05）were significantly reduced by completely or partially breaking the plow pan.The reduction rate of different treatments is DL40>DL25>DL20>RT15.Breaking the plow pan could increased the infiltration of water after precipitation or irrigation and reduce the ineffective loss of water.Breaking the plow pan could also increase the usage of water deep in the soil by crop during drought season and increase the soil water storage capacity during rainy season,the average water content of 20-70 cm under DL20,DL25 and DL40 treatments at seedling stage of winter wheat were significantly higher than that of RT15（P<0.05）,and the water consumption of winter wheat under DL20,DL25 and DL40 treatments increased by 4.9%-11.6%（P<0.05）compared with that of RT15 treatment.The average water content of 20-70 cm under DL20,DL25 and DL40treatments at seedling stage of summer maize were higher than that of RT15 treatment by 7.7%-15.8%（P<0.05）.However,the water consumption of summer maize decreased by 5.8%-10.5%（P<0.05）compared with that of RT15 treatment.There was a significant linear correlation between soil penetration resistance and soil moisture,bulk density（P<0.01）,the soil penetration resistance decreased with the increase of soil water content and increased with the increase of soil bulk density.The penetration resistance of plow pan was significantly affected by temperature and moisture.During the drought period,the soil penetrating resistance increased sharply,which further limited the uptake and utilization by crops and increased the water stress of crops.Besides,the penetrating resistance of the plow was affected by freezing and thawing.（3）The effect of different thickness of plow pan on photosynthetic characteristics,root growth and crop yield:Breaking plow pan could improve the net photosynthetic rate,stomatal conductance and transpiration of winter wheat and summer maize,and the leaf water use efficiency was also improved.However,the leaf net photosynthetic rate of summer maize decreased slightly after breaking the plow pan thoughtly.The different thickness of plow pan significantly affected the total dry weight of the root system and the spatial distribution of the roots in the soil.With the decrease of the thickness of plow pan,the total dry weight of winter wheat and summer maize increased gradually,and the total dry weight of winter wheat and summer maize decreased with the increase of bulk density.The crop roots mainly distributed in the soil layer of 0-15 cm under traditional plow pan,and the distribution ratio of roots deep in the soil increased after breaking the plow pan.At the plow pan bulk density of 1.65 g·cm-3,1.75g·cm-3 and 1.85 g·cm-3,the maximum depth the roots of winter wheat and summer maize could reach were 70 cm,50 cm,30 cm and 70 cm,50 cm,50 cm,respectively.Breaking the plow pan had different effects on crop growth with nitrogen application or non-nitrogen application.The dry weight of straw,dry weight of underground,grain yield and total biomass of winter wheat and summer maize straw increased by 6.8%-41.7%（P<0.05）and by14.1%-33.1%（P<0.05）compared with the traditional plow pan with non-nitrogen application.However,breaking the plow pan thoroughly is not conducive to crop grain yield with nitrogen application and the grain yield of DL25 treatment was the largest.The results showed that breaking the plow pan could increase the nitrogen uptake by crops,increase the efficiency of nitrogen fertilizer utilization,nitrogen fertilizer recovery and nitrogen productivity,but indexes mentioned above decreased slightly and even to a significant level（P<0.05）when breaking the plow pan thoughtly.（4）The impact of breaking the plow pan on the soil greenhouse gas（CO2,N2O and CH4）emissions and the comprehensive global warming potential of the ecosystem:Breaking the plow pan could significantly increase soil CO2 emissions and annual cumulative emissions（P<0.05）by increasing the soil disturbance.Howver,the net ecosystem productivity of wheat and maize ecosystem was increased by improving crop NPP which promoted the CO2 sequestration.There was no significant difference between DL25 and DL40 treatments（P>0.05）.Breaking the plow pan could significantly reduce the N2O annual cumulative emissions by shortening the duration of the emission peaks and weakening the intensity of emission peaks.And CH4 annual cumulative emissions under each treatment varied from-1.94 to-1.00 kg C·ha-1 and increased as the thickness of plow pan increased,which indicated the winter wheat and summer maize rotation system was CH4 sink.The main emission peaks under different treatments mainly concentrated in the summer maize season,which presented a seasonal variation.The difference of CO2 cummulative emission during winter wheat season between different treatments is the main reason for the difference of cumulative emissions during the whole year between different treatments.There was a significant correlation between soil respiration and soil temperature,however,no significant correlation was observed between soil respiration and soil moisture.The N2O emission under different treatments was mainly driven by events（irrigation,fertilization,precipitation,etc.）,and the intensity of N2O emission peak during maize season was significantly larger（P<0.05）than that of wheat season because of the more nitrogen application,concentrated precipitation and higher soil temperature.The N2O emission factor under different treatments varied from 0.43%to 0.73%which obvious smaller than that recommended by IPCC.The water content of soil surface（0-15 cm）and N2O emission flux showed significant correlation（P<0.01）.The comprehensive global warming potential of DL25 treatment was the smallest and the carbon fixation effect was the strongest by comprehensively considering the GWPSOILEXPORT and GWPNPP of each treatment.DL40 treatment showed the largest comprehensive global warming potential due to the larger GWPCO2 which reduced the carbon sequestration.（5）Breaking the plow pan counld affect the crop growth and soil environment by affecting the soil nutrient migration,distribution and balance.With the increase of soil depth,the nitrogen residue ratio decreased gradually.The nitrogen residue ratio in the deep soil increased as the thickness of the plow pan decreased.The proportion of residual nitrogen in the top soil layer of heavy loam was increased by 46.2%-82.4%than that in the light loam soil.Breaking the plow pan thoroughly could increase the uptake of nitrogen by crops and reduce the gaseous losses of nitrogen,but increased the residual nitrogen in the soil,which reduced the nitrogen availability.Breaking the plow pan partially（5cm）could not only promote the uptake of nitrogen by crops but also reduce the gaseous losses of nitrogen and the residual nitrogen in the soil.The nitrogen uptake by crops was 57.1%-60.6%in the heavy loam soil which was obviously higher that in the light loam soil,on the contrary,the gaseous losses of nitrogen in the heavy loam soil was lower than that in the light loam soil.The proportion of nitrogen uptaken by crops decreased with the increase of bulk density,while the proportion of residual nitrogen in the soil and gaseous loss of nitrogen increased with the increase of bulk density.In summary,the plow pan hindered the crop growth under the present situation of the cultivated layer structure in the Huang-Huai-Hai Plain,which restricted the utilization of the soil nutrient and water,limiting the potential of the cultivated layer.Comprehensively considering the economic benefits and environmental benefits,breaking the plow pan moderately is one of the best choices which can not only promote the crop yield,but also reduce the comprehensive global warming potential. |
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