Soil organic carbon dynamics in the agricultural soils of Bangladesh following more than 20 years of land use intensification

Soil organic carbon (SOC) is a key soil quality indicator, as it is a source and storage of plant nutrients and plays a vital role in soil fertility and productivity maintenance. Intensification of agriculture is known to cause SOC decline; however, much of the evidence stems from field-scale experimental trials. The primary aim of this study is to investigate how more than 20 years of agricultural land use intensification in Bangladesh has influenced SOC levels at landscape levels. This was achieved by revisiting in 2012 four sub-sites from the Brahmaputra and Ganges alluviums which were previously sampled (1989-92) by the Soil Resource Development Institute and collecting 190 new samples. These were located at different elevations and subjected to differing amounts of inundation. The SOC was determined using the same method, potassium dichromate wet oxidation, used in the 1989-92 campaign. A comparison of the SOC in the 2012 samples with their historic levels (1989-92) revealed that overall SOC declined significantly across both alluviums as well at their four sub-sites. Further analysis, however, showed that SOC has declined more at higher sites. The higher sites are inundated to a limited level, which makes them suitable for growing multiple crops. Among the land types considered here, the low land sites (because of their topographical position) remain inundated for a greater part of the year, allowing a maximum of only one crop of submerged rice. As a result of reduced biomass decomposition due to anaerobic conditions when inundated, and lower land use/cropping intensity, SOC accretion has occurred in the lower land sites. The SOC levels in South Asian countries are inherently low and agricultural land use intensification fuelled by growing food production demand is causing further SOC loss, which has the potential to jeopardise food security and increase the environmental impact of agriculture.

Research on cropping intensity mapping of the Huai River Basin (China) based on multi-source remote sensing data fusion

As a key input variable to many global climates, land surfaces and crop models, cropping intensity (CI) accurately assesses and predicts crops' output, in view of the global decline in food production in recent years due to declining natural resources, urban expansion and declining quality of arable land. Hence, research on CI mapping can have a contribution to solve this problem. Unfortunately, existing remote sensing data for CI mapping research, including Moderate Resolution Imaging Spectroradiometer (MODIS) and Landsat images, are not adequate for obtaining CI information at higher spatial and temporal resolution. In this regard, we develop an algorithm to extract CI based on per-pixel physiognomy. To be specific, the algorithm is based on the Google Earth Engine (GEE) platform and constructs a high temporal (10 days) spatial (30 m) resolution dataset with the fusion of Landsat 7/8 and Sentinel-2 A/B image data and extracts CI information using a time series of peak discovery method, threshold method and phenological period feature extraction to obtain the 2018 Chinese Huai River Basin (HRB) CI map. Our results suggest that the overall accuracy (OA) of CI extraction in the HRB is 92.72%, with a kappa coefficient of 0.864. The single-season crop, double-season crop and three-season crop account for 41.6%, 57.7% and 0.7% of the total farmland area, respectively. Compared to existing CI identification and extraction methods, this approach achieves higher accuracy in the identification and extraction of CI information over a larger area.

Residue retention promotes soil carbon accumulation in minimum tillage systems: Implications for conservation agriculture

Crop residue retention and minimum tillage (including no-tillage, NT, and reduced tillage, RT) are common conservation tillage practices that have been extensively applied for improving soil health and reducing the negative environmental impact caused by intensive farming. However, the effects of minimum tillage, coupled with crop residue retention (including no-tillage plus residue retention, NTR, and reduced tillage plus residue retention, RTR), on soil organic carbon (SOC) stock have not been systematically analyzed. Using a dataset consisting of 1928 pairs of data points from 243 studies, we conducted a global meta-analysis to evaluate the effects of crop residue retention and minimum tillage on SOC stock in the 0-30 cm soil and how these effects varied with soil (soil sampling depth and texture), environmental (climate) and crop management conditions (cropping intensity), as well as treatment duration. We found that regardless of the climatic condition, crop management, or residue retention, minimum tillage alone increased the overall mean SOC stock. Specifically, NT and RT increased SOC stock by 11 and 6%, respectively, in comparison to conventional tillage (CT). Compared with CT, NTR and RTR increased SOC stock by 13 and 12%, respectively. The above effects were greater in the topsoil (62% of data points from the 0-15 cm depth) than in the subsoil (38% of data points from the 15-30 cm depth). Moreover, residue retention enhanced the resistance of SOC turnover to agricultural and environmental factors; mean annual temperature (coefficient = 0.15), soil pH (0.14), and experimental duration (0.08) were critical for increasing SOC stock with minimum tillage alone, while the response ratio of SOC stock under coupled residue retention and minimum tillage was insensitive to changes in those factors. Additionally, double cropping generally increased SOC stock cross all conservation tillage practices compared to multiple cropping. Therefore, we conclude that minimum tillage coupled with residue retention in a double-cropping system is the most promising management system for increasing SOC stocks in the 0-30 cm soil in croplands Our finding can inform sustainable soil management practices aimed at increasing resistance of SOC in croplands to climate change and soils degradation induced by intensive agriculture.

Multiple cropping systems of the world and the potential for increasing cropping intensity

Multiple cropping, defined as harvesting more than once a year, is a widespread land management strategy in tropical and subtropical agriculture. It is a way of intensifying agricultural production and diversifying the crop mix for economic and environmental benefits. Here we present the first global gridded data set of multiple cropping systems and quantify the physical area of more than 200 systems, the global multiple cropping area and the potential for increasing cropping intensity. We use national and sub-national data on monthly crop-specific growing areas around the year 2000 (1998-2002) for 26 crop groups, global cropland extent and crop harvested areas to identify sequential cropping systems of two or three crops with non-overlapping growing seasons. We find multiple cropping systems on 135 million hectares (12% of global cropland) with 85 million hectares in irrigated agriculture. 34%, 13% and 10% of the rice, wheat and maize area, respectively are under multiple cropping, demonstrating the importance of such cropping systems for cereal production. Harvesting currently single cropped areas a second time could increase global harvested areas by 87-395 million hectares, which is about 45% lower than previous estimates. Some scenarios of intensification indicate that it could be enough land to avoid expanding physical cropland into other land uses but attainable intensification will depend on the local context and the crop yields attainable in the second cycle and its related environmental costs.