Heat,Water,Salt And Carbon Fluxes Under Plastic Mulched Drip Irrigation In An Oasis

The arid oasis in the Northwest of China is an important grain and cotton production area,which has been challenged by extreme water shortage,severe soil salinization,and fragile eco-environment,therefore it demands the efficient water saving method such as plastic mulched drip irrigation?PMDI?.Over the past 20 years,the widely applied PMDI and declining of groundwater level has altered the underlying surface characteristics and consequently affected the heat-water-salt-carbon fluxes in both the field and regional scales.However,due to the lack of observation and further studies about these fluxes,it has largely limited the scientific management of water and land resources in irrigation districts and cannot be well depicted in the land surface model.This research was conducted at the ecological-hydrological experimental station of Tsinghua University in Korla Oasis to obtain comprehensive observations with eddy covariation,chamber,soil water and salinity and crop sampling methods in a representative PMDI field from 2012 to 2016.To achieve aims of the research,the temporal and spatial variation and controlling factors of heat-water-salt-carbon fluxes was analyzed,carbon fluxes both above the ground and canopy were separated to get the carbon budget and the effect of plastic mulched drip irrigation on carbon sequestration potential and the coupling of water and carbon fluxes was discussed.Plastic mulch separates the whole field into two parts,i.e.with mulch and without mulch.It increases the spatial heterogeneity of fluxes and provids a natural contrast experiment for research.Plastic mulch increases the albedo,reduces the soil heat flux,and causes the non-energy closure.Plastic mulch decreases evaporation,which enables nearly 70%of irrigation and precipitation be used for evapotranspiration,as a result the efficiency of irrigation water use has largely increased.The deep water exchange flux is calculated according soil water balance equation.With the decline of groundwater level,the deep water exchange flux at seedling and harvest stages is weakened,and the return of salt from groundwater is reduced.For each relationship between groundwater depth and soil salinity obtained,when the depth of groundwater table reaches 3.5 m,PMDI combined with winter or spring flood irrigation can both suppress the increase of salinity and not cause harm to the ecological water demand of natural vegetation.Plastic mulched and non-mulched drip irrigation contrast experiments show that furrows,plant holes and plastic mulch are important pathways for CO₂ emissions from the soil;frequent irrigation and precipitation make soil moisture as a major controlling factor for soil respiration.Plastic mulch promots soil respiration,but the difference in soil respiration between plastic mulched and non-mulched filed has decreased with the increase of precipitation in the past three years.The impact of plastic mulch on soil respiration is related to the amount of precipitation during the growing season.Carbon budget is obtained with the separation of the net carbon flux above soil and canopy.The PMDI field is a weak carbon source with Net Biome Productivity?NBP?of 67.12 g C m^-2 when crop harvesting is considered.Plastic mulched drip irrigation promotes the heterotrophic respiration so as to soil respiration and ecosystem respiration(R_eco)and the ratio of R_eco to Gross Primary Productivity?GPP?is 0.93,which causes the reduction of carbon sequestration ability and the decline of soil organic carbon.The coupling characteristics of water and carbon at leaf,plant and canopy scale indicate that water use efficiency in PMDI field is smaller than non-PMDI field both at leaf and canopy scale as of the increase of ET.Based on the results,it is concluded that PMDI promotes soil temperature and moisture,prohibites soil evaporation and the return of soil salt from the groundwater;it also increases soil respiration,decreases the carbon sequestration ability;and it increases ET so as to decrease crop water use efficiency.This study addresses the challenges existing in the observation and study of the heat-water-salt-carbon fluxes in the PMDI field in arid oasis.With the study,we analyze the temporal and spatial variation characteristics and controlling factors of these fluxes.And the carbon sequestration potential and the coupling characteristics of water and carbon fluxes are also detected.The above research results are of great significance for parameterizing in land surface models and the management of water and carbon resources in PMDI field under the condition of climate change.