Soil Greenhouse Gas Emission From Sown Pastures In Hexi Oasis

Sown pasture produces 35.4%of the world's livestock products and support 13%of the population;livestock products of grass-feeding account for 85.4%of global livestock products and emit 68%of agricultural GHGs.Increasing population and improving living standards have rigidly increased the demand for cultivated grassland-livestock production systems.Increasing the productivity of sown pasture and reducing environmental footprints such as GHG emissions are important of the"carbon neutrality and emission peak"strategy.To this end,the effects of forage types,nitrogen application and utilization patterns on soil GHG emissions from sown pasture were studied in Hexi Oasis.The main findings are as follows:(1)Characteristics and factor analyses of soil GHG emissions for eight kinds of forage.In 2014,eight kinds of forage with two life forms of perennial and annual,two nitrogen-fixing kinds of legume and grass,and two photosynthetic kinds of C3 and C4.For the grass yield:sorghum sudan hybrid(Sorghum bicolor×S.sudanenese,S)>corngrass(Zea mays×Zea Mexicana.,C)and alfalfa(Medicago sativa,A)>maize(Zea mays,M)>common vetch(Vicia sative,V),barley(Hordeum vulgare,B),wheat(Triticum aestivum,W)=rye(Secale cereale,R),for the crude protein(CP)content:A,V>C,B,W,R>M,S and for CP yield:A>S,C>V,M>B,W,R.Cumulative soil CH₄flux during the growth period A,S,C,W>B,W,R>V.Cumulative N₂O emissions:A,S,C,M>B,W,R>V.Soil GHG emissions per unit of hay yield were 0.96–1.62 kg/kg hay,respectively were:M,W?B,R?A,V?C?S.Soil GHG emissions per unit of CP yield were6.89-14.58 kg/kg CP,respectively were:M?R,W?B>C,S>A,V.Soil GHG emissions per unit of metabolic energy(ME)yield were 0.100-0.200 kg/MJ ME,respectively were:B,W,R>M>A,V,C,S.Soil CO₂ emission flux and CH₄ uptake flux showed a"hump-shaped"curve with the changes of hay and CP yield,soil cumulative CO₂ emission was significantly positively correlated with cumulative CH₄uptake and cumulative N₂O emission during the growth period.Soil CO₂ emission,CH₄ absorption and N₂O emission had positive correlations with soil temperature(ST),available nitrogen(AN),p H,soil organic carbon(SOC)and total nitrogen(TN),but had negative correlations with soil water content(SWC).The Q₁₀ value of soil respiration to temperature change was higher in grass pasture than in legumes pasture.The contribution of ST to soil respiration was 72.79%,the contributions of ST and AN to soil respiration respectively were75.54%and 8.53%in legume pasture.The contributions of ST and p H to soil CH₄ absorption respectively were 70.39%and 2.65%in grass pasture and 66.35%and 7.22%in legume pasture.The contributions of ST and SWC to soil N₂O emission respectively were 71.95%and 6.24%in grass pasture,the contributions of ST and AN to soil N₂O emission respectively were51.18%and 18.96%in legume pasture.(2)Effects of N application on soil GHG emissions of four kinds of forageFrom 2016 to 2017,two C3 forages(barley and rye)and two C4 forages(corngrass and sorghum sudan hybrid)were chosen to established plot,and four gradients of N application respectively were 0,150,200 and 250 kg/ha.The soil GHG dynamics and properties,and forage yield were determined.In two C3 and two C4 grass pastures,with the range of N application rate of 0-250 kg/ha,the cumulative CO₂ emissions and CH₄ uptake in the growing season were positively correlated with the N application rate,the cumulative N₂O emissions in the growing season increased exponentially with the increase rates of N applied.The N₂O emission factor(EF)was positively correlated with the N application rate,and the linear slope was:C3>C4.Under the N application,the cumulative CH₄ uptake in the growing season was positively correlated with the hay yield;under the N rate of 250 kg/ha,the cumulative N₂O emission was negatively correlated with the hay yield;under the N rate of 200 and 250 kg/ha,the cumulative N₂O emissions were negatively correlated with the CP production.Soil CO₂ and N₂O emissions per unit of hay and per unit of CP yield were higher in C3 pasture than in C4pasture.Soil CO₂ and CH₄ fluxes per unit of hay and per unit of CP yield were negatively correlated with the N application rate.The N₂O emissions per unit of hay and per unit of CP yield increased with the increase of N application in a"saddle-shaped".The Q₁₀ value of soil respiration to ST change was higher in C4 pasture than in C3pasture.The contributions of ST and p H to soil respiration respective were 76.12%and4.81%in C3 pasture,the contribution of ST to soil respiration was 72.21%in C4 pasture.The contributions of ST and p H to soil CH₄ absorption respectively were 77.50%and1.67%in C3 pasture,the contribution of ST to CH₄ absorption was 63.00%in C4pasture.The contributions of AN,SWC and ST to soil N₂O emission respectively were74.00%,12.21%and 2.02%in C3 pasture,the contributions of AN and ST to soil N₂O emission respectively were 73.15%and 9.40%in C4 pasture.(3)Effects of grazing on soil GHG emissions of two kinds of sown pastureFrom 2014 to 2017,annual pasture(barley)and perennial pasture(mixed with tall fescue and alfalfa)under grazing and mowing were chosen for measuring forage yield,soil properties and GHG emissions.For both annual and perennial sown pastures,grazing increased hay and CP yields by 44.3%and 81.2%in annual pasture and by 18.2%and 14.9%in perennial pasture.Grazing reduced soil CO₂ emissions by 5.04%and12.84%,reduced CH₄ absorption by 11.8%and 13.8,and reduced N₂O emissions by9.7%and 7.6%,respectively in annual and perennial pastures.Grazing reduced CO₂ emissions per hay and CP yield by 40.4%and 55.8%,23.0%and 20.5%,reduced CH₄ absorption per hay and CP yield by 40.8%and 56.1%,23.5%and 21.2%,reduced N₂O emissions per hay and CP yield by 40.3%and 55.5%,19.8%and 17.3%,respectively in annual and perennial pastures.The Q₁₀ values of soil respiration to ST change in annul pasture were higher for mowing than grazing.The contributions of ST,p H and SOC to soil respiration respective were 77.0%,1.4%and 0.8%in annual mowing pasture;the contribution of ST to soil respiration was 77.0%in annual grazing pasture.The Q₁₀ values of soil respiration to ST change in perennial pasture were higher for grazing than mowing.The contributions of ST and SOC to soil respiration respective were 7.2.5%and 4.7%in perennial mowing pasture,the contributions of ST,SOC and p H to soil respiration were76.2%,3.1%and 1.1%in perennial grazing pasture.