Spatial and temporal dynamics of agricultural residue resources in the last 30 years in China.

The availability and distribution of biomass resources are important for the development of the bioenergy industry in a region. Biomass resources are abundant in China; however, the raw material is severely deficient, which makes the Chinese bioenergy industry an embarrassment and a contradiction. Unclear reserves and distribution and changing trends of biomass resources are the reason for this situation. A collection coefficient model of Chinese agricultural residue resources was established and the spatial and temporal pattern dynamics of agricultural residue resources in the last 30 years were analyzed. The results show that agricultural residue resources increased in stages from 1978 to 2011, including a rapid increase from 1978 to 1999, a significant fall from 2000 to 2004, and a slow increase from 2004 to 2011. Crops straw and livestock manure are the main ingredients of agricultural residue resources with proportions of 53-59% and 31-38%, respectively. However, the former has gradually decreased, while the latter is increasing. This mainly resulted from the strategic reorganization of the Chinese agriculture structure and the rapid development of large-scale livestock breeding and agricultural mechanization. Large regional differences existed in Chinese agricultural residue resources, and three distribution types formed, including resource-rich areas in North China, Northeast and Inner Mongolia, resource-limited areas in Central and Southwest China, and resource-poor areas along Northwest and Southeast coasts. This pattern is a reverse of the distributions of climatic conditions, water resources, economic development, human resources, and technological levels. Finally, it can be predicted that livestock manure and biomass conversion technology at low temperature will play increasingly significant roles in bioenergy industry development.

[Temporal and Spatial Distribution, Utilization Status, and Carbon Emission Reduction Potential of Straw Resources in China].

Based on the crop yield data of China and each region from 1981 to 2020 (excluding data from Hong Kong, Macao, and Taiwan), by using the grain-straw ratio method, this study estimated the total amount of crop straw and collectable amount of crops, including corn, rice, wheat, other cereals, cotton, rapeseeds, peanuts, beans, tubers, sesame, fiber crops, sugarcane, and beetroots, and the spatial and temporal distribution characteristics of resource density and per capita resources of crop straw were analyzed. This study analyzed the current utilization mode, development, and change of crop straw in China. Finally, we used the life circle assessment (LCA) method to estimate the carbon emission reduction potential of biochar prepared from crop straw. The main findings were:from 1981 to 2020, the temporal distribution trend of theoretical crop straw resources and collectable straw resources in China generally showed a steady growth trend, and the two increased from 3.33×108 t and 3.04×108 t in 1981 to the highest values of 7.70×108 t and 6.63×108 t in 2020, with a net increase of 4.37×108 t and 3.59×108 t, respectively. The net increase in rice, wheat, and corn straw resources was 3.69×108t, accounting for between 77% and 85% of the total crop straw and always occupying the main position of straw resources in China. The proportion of wheat straw in the total amount of straw was maintained at approximately 20%, rice straw resources decreased from 44% to 28.4%, and corn straw increased from 19.9% to 34.2% from 1981 to 2020. In 2020, the total theoretical resources of crop straw in China were 7.72×108 t, and the source structures were:rice 28.4%, wheat 21.45%, corn 31.45%, other cereals 1.4%, beans 3.4%, tubers 0.82%, cotton 2.28%, peanuts 2.97%, rapeseeds 3.4%, sesame 0.12%, fiber crops 0.06%, beetroots 0.67%, and sugarcane 0.84%. As to the spatial distribution of crop straw resources in China in 2020, the locations with straw resources ≥ 60 million tons included Heilongjiang, Henan, and Shandong, of which Henan had as much as 88.56 million tons; those with between 40 million and 60 million tons included Hebei, Inner Mongolia, Jiangsu, and Anhui; those with between 20 million and 40 million tons included Liaoning, Jilin, Jiangxi, Hubei, Hunan, Sichuan, Yunnan, and Xinjiang; and the straw resources in the rest of the region were below 20 million tons. Rice straw was mostly distributed in the middle and lower reaches of the Yangtze River and the Northeast region, of which the amount of Heilongjiang rice straw was the largest, with 31.86 million tons; wheat straw was mainly distributed in North China, with Henan having the most abundant resources (48.04 million tons). Corn straw was mainly distributed in Northeast China and North China, of which Heilongjiang and Inner Mongolia corn straw resources were relatively rich, with 33.18 million tons and 29.90 million tons, respectively. Crop straw resource density and per capita resources were shared in 2020 in China. The average density of crop straw resources in China was 4.61 t·hm-2, and the average densities of crop straw resources in various agricultural areas were 5.39 t·hm-2 in Northeast China, 5.42 t·hm-2 in North China, 4.45 t·hm-2 in the Mengxin Region, 4.44 t·hm-2 in the middle and lower reaches of the Yangtze River, 3.92 t·hm-2in Tibet, 3.40 t·hm-2 in the Loess Plateau, 3.08 t·hm-2 in South China, and 2.91 t·hm-2 in Southwest China. The average per capita share of straw resources was 0.55 t. The average values of per capita straw resources in each region were:1.46 t in the Northeast area, 1.20 t in the Mengxin Region, 0.47 t in North China, 0.44 t in the middle and lower reaches of the Yangtze River, 0.40 t in the Loess Plateau, 0.37 t in the Southwest area, 0.33 t in the Qinghai-Tibet area, and 0.20 t in the South China area. The utilization of crop straw in China was diversified. Fertilizer and feed were the main utilizations, accounting for 62.1% and 15.4%, respectively. In 2020, collectable crop straw resources for the preparation of biochar totaled 2.04×108 t in China. Renewable energy replaced fossil fuels in the process of preparing biochar, which could reduce CO2e(CO2e:CO2 equivalent) emissions by 1.45×108 t. Biochar could sequester approximately 4.63×108 t of CO2e; biochar application was able to reduce chemical fertilizer application to achieve a CO2 emission reduction of 8.58×105 t; and biochar application could promote crop yield in order to reduce CO2e emissions by approximately 7.77×106 t. The inhibition of N2, respectively. In the process of biochar preparation and application, the total greenhouse gas emission was 3.32×107 t, and the net greenhouse effect emission reduction reached 5.86×108 t, i.e., it could sequester 0.88 t CO2e per ton of raw materials. The net greenhouse gas emission reduction of unused straw was 6.73×107 t in 2020. With the continuous harvest of grain crops in China, the potential of biochar preparation and carbon sequestration will increase yearly. Using crop straw to prepare biochar has great potential and will be one of the most effective ways to achieve carbon emission reduction in agriculture. It is suggested that government departments should pay attention to the preparation of biochar, support the field experiments of biochar application effects after applying soil on policy and funds, and then introduce relevant biochar standards to ensure the scientific application of biochar prepared by crop straw according to local conditions, so as to achieve the dual benefits of carbon emission reduction and soil remediation and yield increase.

Effects of straw decayed products of four crops on the amelioration of soil acidity and maize growth in two acidic Ultisols.

Variable charge soils have low agricultural productivity associated with low pH, low cation exchange capacity (CEC), and low pH buffering capacity (pHBC). As a result of rapid acidification rates, these soils are prone to infertility resulting from Al phytotoxicity and deficiency of P, Ca, Mg, and K, and thus require amendments that can ameliorate soil acidity and enhance soil CEC and pHBC. A 30-day pot experiment was carried out using a clay Ultisol and a sandy Ultisol amended with straw decayed products (SDPs) of peanut, pea, canola, and rice. The results showed that applying SDPs increased the soil CEC, organic matter content, and exchangeable base cations in the two Ultisols. The ameliorative effects of the SDPs were superior for the sandy Ultisol than for the clay Ultisol. The addition of SDPs significantly increased soil pH and pHBC of the two Ultisols, and simultaneously decreased soil exchangeable Al3+. Among them, the greatest effect was found in the treatment with pea straw decayed products (PeaSD). The soil pHs of clay Ultisol and sandy Ultisol treated with PeaSD were respectively 5.70 and 7.37 and were 1.26 and 2.63 pH units higher than those of control. Also, applying SDPs increased maize seedling biomass in both soils and the most significant effect was found in the treatment with PeaSD, which were 0.97 (clay Ultisol) and 2.5 (sandy Ultisol) times higher than in the respective controls. The results of this study demonstrated that carefully selected straws for SDP production can effectively improve soil chemical properties, enhanced soil pHBC, and thus promote agricultural sustainability.