Cropland Changes Enhance Carbon Sequestration in Northwest China From 1995 to 2015

18 Background: The Northwest China has experienced dramatic changes in agricultural 19 land area in recent years; the effects of these changes on carbon storage are unknown 20 and cannot guide further land development policies related to carbon emissions. In 21 this study, we evaluated the effects of cropland changes (reclamation and transfer) 22 during 1995 - 2015 on carbon storage in Northwest China by using land use data, 23 carbon density data, and statistical yearbooks with the Intergovernmental Panel on 24 Climate Change (IPCC) method. 25 Results: The results indicated that the area of cropland increased by 1.48 × 106 ha 26 from 1995 to 2005, resulting in a total carbon sequestration of 12.46 Tg, in which 27 conversion of cropland to forest (11.16 Tg) and other land to cropland (8.92 Tg) were 28 the main sources of the increase in carbon storage. Specifically, regional carbon 29 sequestration due to cropland changes exhibited an increasing trend during 1995 - 2002 30 (dominated by cropland transfer), a gradually decreasing trend during 2002 - 2009 31 (dominated by cropland reclamation), and stabilization since then (during 2009 - 2015). 32 Conclusions: These results suggest that the development of high carbon density lands 33 or the conversion of low carbon density lands are critical to increasing future carbon 34 sequestration due to cropland change. We used a novel approach of combining land use data, carbon density data, and statistical yearbooks to assess the impact of 36 cropland change on carbon storage; this method is promising in applications which guide agricultural land - use management. IPCC method to investigate carbon storage caused by cropland change (reclamation and transfer) between 1995 and 2015 in Northwest China. The method used in our study showed a clear temporal evolution of cropland change and provided promising applications in guiding agricultural land - use. Our results showed that cropland changes acted as a carbon sink in Northwest China, with a total carbon sequestration of 12.46 Tg, in which the conversion of cropland to forest (11.16 Tg) and other land reclamation to cropland (8.92 Tg) affected by ecological engineering programs and characteristics of ecological environment were the main sources of an increase in carbon storage. Spatial variability in carbon storage in different provinces was mainly related to natural geographical features in these regions, which limited the types and areas of cropland conversion. Cropland reclamation and transfer dominated different periods, resulting in fluctuations in the temporal evolution of carbon sequestration. These results indicate that to increase carbon sequestration in cropland change, it is essential to promote development of land cover types with high carbon density, or the conversion of low carbon density lands to cropland. Meanwhile, we propose that a balance should be sought between the transfer and reclamation of cropland to allow for a steady increase in carbon storage over time. Our results also indicate that will likely continue the future area. water arid resource

degrees of success [29]. Generally, chamber measurements are particularly well suited 82 for in situ and laboratory-based studies [29]. At regional or global spatial scales, 6 empirical statistical models (e.g., bookkeeping), remote sensing models (e.g., CASA) 84 or process-based ecosystem models (e.g., TEM; LPJ) are usually used for evaluating 85 the effects of LUCC on carbon budgets [28,30,35,52]. However, accuracy of 86 representation of both temporal evolution and spatial heterogeneity of carbon storage 87 with modeling approaches is limited by the availability of land use data, and the 88 model itself. Hence, more precise annual information of LUCC is needed to analyze 89 temporal evolution and spatial heterogeneity of carbon budgets. Further, most of the based on objective assessment of regional carbon storage, which is conducive to 110 stable and sustainable development in arid and semi-arid regions.

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The overall approach used in this study consisted of two parts. First part included 113 land use data and provincial statistical yearbooks for calculating annual area of 114 cropland change, and mapping spatial distribution. Second part involved vegetation 115 and soil carbon density data combined with the area of annual change in cropland for 116 calculating carbon storage. The structure of this study is shown in Fig. 1.   Shanxi provinces, accounting for 32.2% of the total land area of China, and an area of 123 approximately 3.10 million km 2 (Fig.2). Most of the area exhibits a typical continental 124 climate, with very low mean annual precipitation (below 250 mm), mean annual and basins [4]. The natural landscape changes from east to west from forest and 131 typical grassland, to a grassland-desert and desert; vegetation cover shows a gradually 132 decreasing trend.   The annual cropland reclamation and transfer area in the study period was 159 calculated using the following equations: 160 ) ..., , , Where IC P (%) is the coefficient of annual increase in cropland area. Calculation 172 method of DC P (%) is similar to IC P , except that the formula (2) is changed to the 173 following: Where DC S  is the annual decrease of cropland area,  The annual area of cropland reclamation or transfer (ha) was obtained by  188 We obtained vegetation carbon density information for each land use type (   (Table 3) Table 2 Here Table 3 Here  The spatial distribution of cropland change was similar for both periods (Fig. 5). 245 The most dramatic change took place in Xinjiang province, especially in its northern 246 part. Cropland change in Qinghai province, which is mainly covered by grassland, 247 occurred mainly within that cover type. Additionally, cropland changes were notable    Northwest China (Fig.7a). The increase in carbon storage was mainly from grassland, Northwest China (Fig.7b). The increase in carbon storage was mainly from cropland  The trends in area and carbon storage caused by cropland changes were relatively 305 consistent in spatial characteristics for both periods of study (Fig.8)   to forest and grassland (Fig.7). On the one hand, in our study, 2.03×10 6 ha of other in fluctuations in the temporal evolution of carbon sequestration (Fig. 6, 7). At the  Strengths and limitations of this study, and future work 433 We also simulated future changes in cropland using CA-Markov models based on 434 the historical transition rules (Supplement material, Fig.S2, Table.

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Availability of data and materials 516 Land-use dataset is derived from LC-ESA through https://www.esa-landcover-cci.org/.

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The annual area of cropland is derived from available a series of provincial statistical 518 yearbooks (http://data.cnki.net/; Table S3). Vegetation and soil carbon density data 519 are shown in Table 1 and Table S4.

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Competing interests 521 The authors declare that they have no competing interests.      Location of Northwest China and its administrative divisions Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 6
Changes in soil, vegetation and total carbon storage over time Changes in carbon storage due to different cropland conversion types

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