Quantitative Evaluation Of Carbon Footprint And Fertilizer Nitrogen Fate In Agricultural Production

Global greenhouse gas（GHG）emissions due to human activities have grown rapidly since pre-industrialization.The increase in CO₂ emissions can be primarily attributed to fossil fuel combustion and land use change,while CH₄ and N₂O emissions have come mainly from agriculture.In China,agriculture made a significant contribution of 11%to the national anthropogenic GHG emissions,which accounted for 52%and 84%of the national anthropogenic methane（CH₄）and nitrous oxide（N₂O）emissions,respectively.Significant technical mitigation potential has been suggested for global agriculture mainly by improved crop production management.Quantifying the carbon footprint（CF）of crop production can help identify key options to mitigate greenhouse gas emissions from agriculture.The objectives of the present study were,（1）to quantify the CFs of the major crops production including grain crops,vegetables and fruits in China using farm survey data based on the life cycle assessment（LCA）method,and to characterize the contribution of individual inputs involved in farming practices to the overall CFs;（2）to compare the CFs of crop production among climate regions,farm scales and management patterns;（3）to assess how much N taken up by crops was fertilizer-derived through synthesizing published data that traced the fate of 15N-labeled fertilizer.It aimed also to provide information for policy-makers to identify key options for climate change mitigation in China’s agriculture.The main results obtained were as follows:1.Quantifying the carbon footprint（CF）of China’s crop production can help identify key options to mitigate greenhouse gas emissions from agriculture.Using farm survey data from eastern China,the carbon footprints of three major grain crops（rice,wheat and maize）were assessed by quantifying the greenhouse gas emissions from individual inputs and farming operations with the LCA methodology.The farm carbon footprint（FCF）in terms of farm area was estimated to be 6.0±0.1,3.0±0.2,and 2.3±0.1 t CO₂-eq ha^-1,and the product carbon footprint（PCF）in terms of grain produced was 0.80±0.02,0.66±0.03,and 0.33±0.02 t CO₂-eq t^-1 grain for rice,wheat and maize, respectively.Use of synthetic nitrogen fertilizers contributed 44%-79%and mechanical operations 8%^-15%,of the total carbon footprints.Irrigation and direct methane emission made a significant contribution by 19%and by 25%,on average respectively for rice production.However,irrigation was only responsible for 2%-3%of the total carbon footprints in wheat and maize.The carbon footprints of wheat and maize production varied among climate regions,and this was explained largely by the differences in inputs of nitrogen fertilizers and mechanical operations to support crop management.Moreover,a significant decrease（22%-28%）in the product carbon footprint both of wheat and maize was found in large sized farms,compared to smaller ones.This study demonstrated that carbon footprint of crop production could be affected by farm size and climate condition as well as crop management practices.Improving crop management practices by reducing N fertilizer use and developing large scaled farms with intensive farming could be strategic options to mitigate climate change in Chinese agriculture.2.This study quantify the CFs of vegetables production in Nanjing using farm survey data based on the LCA method,and also characterized the relation of CF with nutrition value of vegetables and economic benefit gained by farmers.In this survey,farm CFs（CF per unit of area）of greenhouse vegetable production in term of land use were within the range of 0.7^-10.4 t CO₂-eq ha^-1.There no difference of farm CF between the kinds of vegetables.But the product CF（CF per unit of yield）of Chinese cabbage and amaranth were 0.34 kg CO₂-eq kg^-1 and 0.38 kg CO₂-eq kg^-1 respectively,significantly higher than others(0.07-0.17 kg CO₂-eq kg^-1).Furthermore,the income-scaled CF nutrition-scaled CF of Chinese cabbage and amaranth(1.95 kg CO₂-eq USD^-1 and 1.82 kg CO₂-eq USD^-1)was significantly higher than others.However,the nutrition-scaled CF of water spinach and amaranth(0.12 kg CO₂-eq ANV^-1 and 0.36 kg CO₂-eq ANV^-1 on average respectively)turned lower than Chinese cabbage,tomatoes,cucumbers and potatoes.The biggest contributor to the total emisions was chemical fertilizers（55%-82%）,then followed by organic fertilizers（2%-21%）,irrigation（1%-26%）.Farm machinery,films and pesticides accounted for a small proportion（5%,5%and 2%,respectively）.Reducing synthetic N fertilizer application and increasing organic fertilizer use could be one of the most important measures to realize climate change mitigation in Chinese vegetables production.However,how to balance nutrition requirement and farmers’incomes is still a great challenge.3.The CF in terms of land use,nutrition value and income of fruit production were respectively quantified using farm survey data based on the life cycle assessment method up to the farm gate.The fruit crops included apple,peach,pear,orange and banana,the five major fruit types of China.The farm CF ranged from 2.9 t CO₂-eq ha^-1 to 12.8 t CO₂-eq ha^-1 across the surveyed orchards.Whereas,the product CF ranged from 0.07 kg CO₂-eq kg^-1 to 0.7 kg CO₂-eq kg^-1 fruit.While the product CF on average of orange and pear(0.14 and 0.18 kg CO₂-eq kg^-1 fruit,respectively)was significantly lower than apple,banana and peach(0.24,0.27and 0.37 kg CO₂-eq kg^-1 fruit,respectively).However,the nutrition-scaled CF of orange(0.5 kg CO₂-eq g^-1 Vc on average)was significantly lower than others(3.0-5.9 kg CO₂-eq g^-1 Vc).Furthermore,the income-scaled CF of orange and pear(1.20 and 1.01 kg CO₂-eq USD^-1 on average respectively)turned higher than apple,banana and peach(0.87-0.39 kg CO₂-eq USD^-1).Among the inputs,synthetic nitrogen fertilizer use made a major contribution（by 47%-75%）to the total GHG emissions,varying with the types of fruits.Moreover,higher fruit yields but lower product CFs were found under high efficiency management compared to low efficiency management.Thus,increasing organic fertilizer use and improving nitrogen fertilizer use efficiency could be options to cut CF of these fruit productions.This study suggests some tradeoffs in product CFs between fruit nutrition value and fruit growers’ income.Low carbon production and consumption policy and marketing mechanism should be developed to cut down carbon emissions from fruit production sector and given the nutrition,income and climate change mitigation to a balance.4.This study quantified the influence of the land management mechanism and different farmland management pattern on the CF of crop production,and suggested developing scale operation and intensive management can improve the efficiency of agricultural production and the great potential of GHG mitigation.Both household and aggregated farm scales were surveyed to obtain the data of rice production and farming management practices in a typical rice cultivation area of Northern Jiangxi,China.The CFs of the different rice systems including early rice,late rice and single rice under household and aggregated farm scale were calculated.In general,early rice had the lower CF in term of land use and grain production being 4.54±0.44 t CO₂-eq ha^-1 and 0.62±0.1 kg CO₂-eq kg^-1 grain than single rice(6.84±0.79 t CO₂-eq ha^-1 and 0.80±0.13 kg CO₂-eq kg^-1)and late rice(8.72±0.54 t CO₂-eq ha^-1 and 1.1±0.17 kg CO₂-eq kg^-1 grain).The emissions from nitrogen fertilizer use accounted for 33%of the total CF on average,and the direct CH₄ emissions for 57%.The results indicated that the CF of double rice cropping under aggregated farm being 0.86±0.11 kg CO₂-eq kg^-1 grain was lower by 25%than that being 1.14±0.25 kg CO₂-eq kg^-1 grain under household farm,mainly due to high nitrogen use efficiency and low methane emissions.Therefore,developing the aggregated farm scale with efficient use of agro-chemicals and farming operation for greater profitability could offer a strategy for reducing GHG emissions in China’s agriculture.Here rice carbon footprint in different farm types including small household,intensive farm and organic farm from jiangsu region were evaluated.The results showed that rice carbon footprint under small household management was the highest(0.80 kg CO₂-eq kg^-1), then followed by organic farm(0.56 kg CO₂-eq kg^-1),and that under intensive farm was the lowest(0.50 kg CO₂-eq kg^-1).Compared to small household management,the emissions from fertilizer,pesticides,irrigation and methane emissions in intensive and organic farm significantly reduced.The farm CF in term of land use in organic farm was low because of organic fertilizer instead of chemical fertilizer,and biological pesticide instead of chemical pesticides.Considering the yield and economic benefits,the product CF of rice production in intensive farm was lower than in organic farm.Therefore,developing intensive management not only can obtain high yield but also realize low carbon emissions in agriculture.5.Based on statistics and measurement of global data from literatures,it revealed the important way to increase nitrogen fertilizer use efficiency is to improve the residue Nin soil absorbed by crops,and also indicated the important direction on reasonable nitrogen fertilizer application and mitigation.Understanding how to manage these N inputs to identify the most efficient practices has been an organizing principle of agronomic research.Yet soil N stocks-even in heavily fertilized systems-are substantial,and mineralization of soil N may be an important source of nutrients.We synthesized published data that traced the fate of 15N-labeled fertilizer in order to assess how much N taken up by crops was fertilizer-derived.We found that less than half of the N taken up by crops was derived from N fertilizer-the majority of N in crops was from other sources.This was true for maize（only 41%from current-year N fertilizer）,rice（32%）and small grains（37%）,including barley,wheat,millet,oats,and sorghum.Recovery of N in organic fertilizers（manure,green manure,compost,etc.）was low（29%）,but N recovery in subsequent years（10%）was greater than that for mineral fertilizers.We suspect that the majority of non-fertilizer N found in crops comes from turnover of soil and crop residue N,though the observed amount of N recovered from previous years’ mineral fertilizer inputs were small（5%in subsequent two years for mineral fertilizers）.Our work suggests that focusing on efficient recovery of N fertilizer inputs may neglect the most important sources of crop N and that efforts to enhance agricultural sustainability should focus on effective management of N supplied through soil processes.This thesis quantified carbon footprints of China’s crop production（including grain crops,greenhouse vegetables and fruit）,and compared the CFs between the different climate regions,farm scales and farm types,in order to identify important mitigation options in crop production,and also to provide basic information for policy makers.At the same time,through the collection of data from field experiment with 15N labeled nitrogen fertilizer,the thesis revealed the fate of fertilizer nitrogen in cropland ecosystem,and points out that soil N is the most important source of crop N,should pay attention to the management of the soil N.This study suggests some tradeoffs in CFs between product nutrition value and farmers’ income.Low carbon production and consumption policy and marketing mechanism should be developed to cut down carbon emissions from agricultural production sector.