The Response Of Dryland Soil Profiles And Surface Emissions Of Greenhouse Gas To Crops,Nitrogen Fertilizer And Plastic Mulching

As important greenhouse gas,carbon dioxide?CO₂?,nitrous oxide?N₂O?and methane?CH₄?have a strong impact on global climate change.Dryland farmland accounts for approximately 70%of our national land,and it plays an essential role in ensuring food security.Dryland region also acts as an important source and sink of greenhouse gas.Therefore,it is a necessity to study the processes of the production,transportation and consumption of greenhouse gas and their driving factors before fully understanding the sink and source capacity of soil and taking measures to reduce greenhouse emissions.In order to understand the role of maize plants,N fertilization and film mulching in regulating the production,consumption and net emissions of soil greenhouse gas,we performed an experiment in a spring maize field at the Changwu Agricultural and Ecological Experimental Station.We measured subsoil concentrations at depths of 0-50 cm and surface emissions of CO₂,N₂O and CH₄ and calculated diffusive fluxes using the gradient method to test our hypotheses:?1?The presence of maize plants could affect soil profiles of dynamics and surface emissions of greenhouse gas directly by root respiration and indirectly by altering soil temperature,moisture and nutrients;?2?N fertilization and plastic mulching may alter soil profiles of dynamics and surface emissions of greenhouse gas by regulating plant growth,and the effect might vary with plant phenology.Our main findings were presented as follows:?1?Compared with the unplanted treatments?unplanted and unfertilized,C0;unplanted and N fertilized,CN?,the presence of maize plants?planted and unfertilized,P0;planted and N fertilized,PN?significantly decreased soil temperatures and WFPS at depths of 10-50 cm and soil mineral N in the upper 0-20cm layers during the maize growing season.The relative diffusion coefficients within soil profiles were also decreased by the presence of maize plants.N fertilization significantly enhanced the aboveground dry matter,N uptake and grain yield.Compared with the P0 treatment,PN treatment significantly decreased soil water-filled pore space?WFPS?during the maize growing season at depths of 10-20cm in 2014 and 10-50 cm in 2015.The relative diffusion coefficients in the 0-10 and10-20 cm layer were also significantly increased by the presence of maize plants.?2?Soil CO₂ concentrations and fluxes in the planted treatments corresponded with maize growth.CO₂ concentrations and fluxes in the planted soil were significantly higher compared to those in the unplanted soil.N inputs had no significant effect on CO₂ fluxes within soil profiles in the unplanted soil.However,N fertilization significantly decreased?P<0.05?the CO₂ concentrations in the planted soil at depths of 10,20,and 30 cm in 2015 and increased total CO₂ fluxes of the 0-50cm soil layers during the maize growing season by 4%?P=0.29?in 2014 and by 14%?P<0.05?in 2015,with the cumulative plant-derived CO₂ fluxes enhanced by 19%and 20%,respectively.The contributions of the plant-derived CO₂ effluxes to the total CO₂ effluxes of the 0-50 cm soil layers were affected by maize growth,with two year mean values of 0.46 and 0.51 for the P0 and PN treatment,respectively.?3?Peaks of soil profile concentrations mainly occurred following heavy rainfall;Surface N₂O emissions usually peaked after N fertilization following heavy rainfall.Soil moisture and temperature were critical factors affecting the N₂O concentrations and surface emissions.Nitrogen fertilization significantly increased the soil N₂O concentrations and surface emissions.The consumption of soil water by maize plants resulted in a soil condition less suitable for N₂O production.Therefore,the cumulative surface emissions and effluxes at a depth of 10 cm during the maize growing season?PN treatment?were decreased by 9%and 18%in 2014 and by 15%and 18%in 2015 compared to values of the CN treatment.Compared to the unplanted soil,the presence of maize plants significantly decreased the N₂O concentrations at depths of 10-40 cm during the maize growing season.The modeled N₂O fluxes at a depth of 10 cm presented a similar pattern to the chamber measurements.However,there was a discrepancy between the concentration gradient and chamber methods when the fluxes were high,mainly because the gradient method could not detect N₂O production and consumption process above the uppermost gas sampler?0-10 cm?.?4?Seasonal variations of CH₄ fluxes varied with plant phenology.The CH₄fluxes in the 0-20 cm layers accounted for 49%-58%of the total fluxes in the 0-50cm layers.CH₄ uptake rates were significantly and positively correlated with soil temperatures and negatively correlated with soil WFPS.The presence of maize plants significantly increased the cumulative CH₄ fluxes at depths of 0-30 cm and surface CH₄ uptake during the maize growing season.This was most likely that maize plants decreased soil moisture and thus increased gas diffusivity and CH₄uptake.N fertilization had no significant effect on soil CH₄ concentration,fluxes and surface uptake in the unplanted soil.In the planted soil,N fertilization significantly increased the cumulative CH₄ uptake during the maize growing season by 9%and15%,and the cumulative CH₄ fluxes at depths of 10 and 10-40 cm in 2014 and 2015,respectively.This was mainly because N fertilization affected soil uptake of CH₄mainly by regulating soil water.?5?Compared with the unplanted and unmulched treatment?CN?,the unplanted and mulched treatment?CNM?generally increased soil temperatures during the maize growing season.Compared with the planted and unmulched treatment?PN?;the planted and mulched treatment?PNM?mainly increased soil temperatures from sowing time to late June.Plastic mulching markedly increased soil WFPS regardless whether or not maize plants were planted.Plastic mulching stimulated maize emergence and development.Mulching practice significantly increased the mean soil CO₂ emissions and concentrations within soil profiles.The CO₂ concentrations within soil profiles and emissions were increased by plastic mulching mainly from planting time to late August and during the whole growing season,respectively in the planted and unplanted treatments.Compared to the unmulched treatments,plastic mulching significantly increased soil profiles of N₂O concentrations and the two-year mean cumulative N₂O emissions during the maize growing season by12%-38%.Under unplanted treatment,mulching significantly decreased the cumulative CH₄ uptake during the maize growing season by 19%in 2014 and 26%in 2015 and decreased CH₄ concentrations at depths of 10-30 cm in both years.Under planted treatment,the plastic mulching significantly decreased the cumulative CH₄ uptake by 18%in 2014 and 27%in 2015.Plastic mulching significantly decreased CH₄ concentrations at depths of 10-20 cm in 2014 and 10-50 cm in 2015,respectively.?6?Our results were generally in agreement with our hypotheses through two years of field experiment.The presence of maize plants significantly increased CO₂concentrations and diffusive fluxes at depths of 10-50 cm and net CO₂ emissions.N fertilization increased the total CO₂ fluxes at a depth of 0-50 cm mainly by improving plant-derived CO₂ fluxes during the late growing season.In the unplanted treatments,the mulching practice generally increased the subsoil CO₂ concentrations and surface emissions during the maize growing season.However,the mulching practice primarily increased subsoil CO₂ concentrations and surface emissions from sowing time to middle August in the planted soil.The presence of maize plants decreased N₂O concentrations at a depth of 10 cm and surface emissions mainly by decreasing soil WFPS.Mulching practice and N fertilization increased subsoil N₂O concentrations and surface emissions,respectively.The presence of maize plants increased the CH₄ diffusive fluxes at depths of 10-30 cm and surface uptake by decreasing soil WFPS,which mainly occurred after jointing.N fertilization decreased soil moisture due to the increased maize growth,which in turn further increased CH₄ uptake.Plastic mulching decreased subsoil CH₄ concentrations and surface uptake by increasing soil moisture and physical obstruction.