Technology Evaluation For Greenhouse Gas Mitigation In Cropland Ecosystem And Case Studies

In spite of the increased studies on greenhouse gas(GHG) emissions mitigation technologies, there is still a lack of systematic indices and method for reliable evaluation of their overall impacts in cropland ecosystem. In this study, we conducted analysis on each index relating to greenhouse gas emissions, and the time scale to show the effect of mitigation practices on grain yield, soil carbon sequestration and N2O(CH4) emissions, and made comparison among the methods to estimate grain yield, soil carbon sequestration and N2O(CH4) emissions in terms of advantages and disadvantages. Finally, an evaluation method was proposed to evaluate various GHG mitigation technologies in cropland. In addition, we conducted case studies on GHG mitigation technology evaluation in winter wheat-summer maize rotation system of North China and spring maize mono-cropping system of Northeast China(NEC), which is of theoretical and practical significance to popularize mitigation technology and achieve GHG mitigation in agriculture. The main results were decribed as follows:(1) After analyzing each index following the principles of representativeness, objectivity, completeness, dominance and operability, we proposed evaluation indices for mitigation technologies based on the current situation of China. Crop yield per unit area was proposed as a constrained index; greenhouse gas emissions intensity(GHGi), defined as GHG emissions per unit of produced yield, was proposed as the comprehensive index, and the calculation of GHGi involved yield, change of soil organic carbon, direct and indirect N2 O emissions, paddy CH4 emissions and direct and indirect emissions from croplands inputs(mainly from combustion, producton, transportation); and the economic benefit was proposed as feasible index, including variable costs, income from selling grain, government subsidy and canbon trade.(2) On the basis of the proposed comprehensive evaluation indices, we further analysed the time scale to show the effect of mitigation practices on grain yield, soil carbon sequestration and N2O(CH4) emissions, and determined it as 20 years; and we made comparison among the three methods, including field experiment, IPCC Tier1 and Tier2, and process-based model, to estimate grain yield, soil carbon sequestration and N2O(CH4) emissions in terms of advantages and disadvantages, and proposed the estimation method: field experiment and process-based model were combined to estimate grain yield, soil carbon sequestration and N2O(CH4) emissions; combining field survey with literature retrieve to estimating GHG emissions mainly from production and transportation of cropland input; the calculation of economic benefit need the combination of field survey, literature retrieve and model simulation. The evaluation process was proposed as follows: firstly, the grain yield of surveyed conventional practices and optimized practice were compared after 20-year simulation, then when the grain yield was not significantly decreased compared with surveyed conventional practices, GHG intensity was compared to the value of surveyed conventional practices, and when the GHG intensity was not reduced, the analysis of economic feasibility was conduced.(3) After the directly calibration of parameters relating to the change soil organic carbon based on the previous research, we conducted validity test on APSIM(Agricultural Production Systems s IMulator) using the experimental data at Huantai of Shandong Province in North China and the experimental data in Northeast China, and the results showed that the calibrated model can well simulated the crop phenological stage, yield and N2 O emissions with R2 of >0.5. Although there existed potential problems in term of daily N2 O emissions simulation by APSIM, it can well reflect the effect of different N(N) application on the total emissions in a growing season and a year.(4) The aggregated GHG emissions(GHGA) and GHG intensity(GHGi) of surveyed farmers’ conventional practices in North China was 9.46 t CO2-eq ha-1 and 0.60 kg CO2-eq kg-1 yield, recpectively. After analying on GHG mitigation of different techonloies using the proposed evaluagion method, the results were as follows: the N input in the surveyed region was highly overused; reducing N rate from 581 kg N ha-1(surveyed conventional N rate) to 390 kg N ha-1 had higher mitigation porential compared to other management(i.e. different irrigation amount and different combined organic N rate), and can reduce the GHGA and GHGi by 2.68 t CO2-eq ha-1 and 0.16 kg CO2-eq kg-1 yield recpectively; while replacing half of basal N(chemical fertilizer) with organic N in conventional practices increased soil carbon sequestration, but it also brought more GHG emissions from the production and transportation of organic fertilizer and diesel fuel combustion, and finally reduced GHGi only by 0.02 kg CO2-eq kg-1 yield.(5) The aggregated GHG emissions(GHGA) and GHG intensity(GHGi) of surveyed farmers’ conventional practices in Northeast China was 4.17 t CO2-eq ha-1 and 0.45 kg CO2-eq kg-1 yield, recpectively. After analying on GHG mitigation of different techonloies using the proposed evaluagion method, the results were as follows: the N input in the surveyed region was not highly overused; although the emissions from N input(mainly including producton and transportation) accounted for the highest proportion(38%), but it had no big mitigation portential; straw returning to field for conventional practices can greatly reducing GHG emissions and GHG intensity by 1.18 t CO2-eq ha-1 and 0.13 kg CO2-eq kg-1 yield; reducing the N rate to 150 kg N ha-1 with all residue retention for conventional practices can additionally reduce the GHGA and GHGi by 0.44 t CO2-eq ha-1å'Œ 0.04 kg CO2-eq kg-1 yield without significantly affecting crop production(P=0.81).(6) The results on economic feasibility of different mitigation techonloies were as follows: at current situation of wheat-maize cropping system in North China, only reducing N rate was feasible in the economy compared to reducing irrigation times and different proportion of organic N in basal N. Although there was no obvious increase in economic benefit, reducing N rate can play an important role in reducing GHG emissions and N leaching. At current situation of spring maize mono-cropping system in Northeast China, the mitigation technologies, including all residue retention and replacing basal N(chemical fertilizer) with organic N, were all not feasible in the economy. But aiming to reducing soil degradation and GHG emissions caused by the reduction of soil organic carbon, government subsidies should be added at least 800 Yuan ha-1 to promote straw returning in the survey region when the added cost from machine operation, pesticides and labour for straw returning was considered. In addition, the current carbon trade price in China was not important to promote GHG mitigation in the typical cropping systems of these two regions.