The Influence of Land Resources and Land Use Intensity on Soil Organic Carbon in Rice-Based Cropping Systems

Abstract

<p>The smallholder farmers of Bangladesh operate a variety of rice-based cropping systems (RBCS) with rotational crops in the wet and dry cropping seasons. RBCS continue to evolve over time to increase food security with the support of modern crop varieties, agrochemicals, irrigation and crop intensification. However, there is a concern for the productivity and sustainability of the traditional rice and rice cropping systems as well as those under increasing cropping intensity. Low soil organic carbon (SOC) content and soil fertility decline has been reported in many of the farming regions. This study in Dinajpur, northwestern Bangladesh sequentially examined the spatial and temporal trends of SOC and soil pH in the landscape, then synthesized smallholder’s cropping and management practices impact in land resource settings on SOC, total nitrogen (TN) and soil pH at field scale. Overall, it sought to comprehensively appraise the potential of SOC improvement under prevailing cropping and management practices in certain land resources. Finally, in order to preserve soil health, productivity and reduce the effects of climate change in Bangladesh, a novel effort was made to identify prospective RBCS that can sequester SOC in soils.</p> <p>Chapter 2 examined SOC, soil pH and the implications for long-term sustainability using soil legacy datasets of the 1990s and 2010s. SOC and pH data were categorized to soil physiographic types (Himalayan Piedmont plain, Tista Meander Floodplain and Barind Tract/Terrace) across land inundation types (highland-HL, medium highland-MHL, medium lowland-MLL) to determine which combination was more influential in spatial and temporal changes of these properties. SOC in the physiographic types were generally found to be low (8-12 g/kg), but significantly higher in MHL and MLL compared to HL. The change in SOC over 20 years was significant with a 14.5% increase of SOC from the 1990s values. There was a greater influence of land inundation type on SOC than the soil physiography. Furthermore, soil pH decreased by 0.5 units over 20 years with an approximately 50% increase in soils within a pH category of 4.6-5.5. The majority of soil pH values shift from slightly acidic to strongly acidic in the intervening 20 years period between soil sampling (0-15 cm). </p> <p>In Chapter 3, smallholder land resource endowment, cropping and management practice data, stratified by soil physiography, were collected through a farmer survey and the aim was to determine a farming system typology and to consider the likelihood of soil C improvement. A cluster analysis was performed that combined land resources (farm size, landholding, physiography, land type) and cropping practices (cropping system, cropping intensity, crop rotation) to determine a farming system typology. Results identified three farming system types, and these were then assessed as to the potential to improve soil C. The Medium SCIL (42.2%) was comprised of maize-rice, wheat-rice and veg.-rice cropping systems. These cropping systems were maintained with high to medium fertilization, low crop rotation, and medium to low crop residue retention in HL of the Piedmont plain, Floodplain and Terrace. The Low SCIL (30.1%) comprised of rice-rice cropping system. This cropping system was maintained with medium fertilization, no crop rotation and limited residue retention in MHL of the Piedmont plain, Floodplain and Terrace. The High SCIL (27.7%) comprised of crop diversified and mustard-rice cropping systems. These systems were maintained with high fertilization, crop rotation and crop residue retention in MHL of the Floodplain and Terrace. Thus, a farming system typology of the locality that featured six cropping systems and related management in land resource settings with varying likelihood of soil C improvement. </p> <p>Following the farming system typology, Chapter 4, presented SOC, TN and soil pH data at the field scale. The balance between land resources (physiography, land type, farm size, landholding) and cropping practices (cropping intensity, cropping system, fertilization, crop rotation, crop residue retention) was assessed and their explanatory power in explaining observed SOC were examined. Crop-diversified and mustard-rice systems that were divesified in rotational crops with high fertilization and crop residue retention recorded higher SOC (11.99 - 16.33 g/kg) than the other double and triple cropping systems. Nevertheless these RBCS were also located on inherently fertile Floodplain MHL compared to the less diverse cropping systems (maize-rice, wheat-rice, veg.-rice and rice-rice) in HL and MHL with limited crop residue retention. Wheat-rice system had the lowest SOC (5.46 g/kg), which was considered to be a consequence of low crop residue retention in HL of the Piedmont plain or Terrace. A strongly positive correlation between SOC and TN and a moderate negative trend between SOC and soil pH was found that aligned to SCIL farming system typology. </p> <p>In Chapter 5 the maximal capacity towards C saturation and sequestration potential of current cropping practices in land resource settings were quantified via saturation theory proposed by Six et al. (2002). The crop-diversified and mustard-rice systems under the stop-go triple cropping in MHL with 2:1 clay type in the Floodplain showed a higher C saturation capacity and lower deficit compared to the other triple and double cropping systems (maize-rice, wheatrice, veg.-rice and rice-rice). The triple and double cropping systems showed less pronounced differences in C saturation capacity amongst the systems, but high C saturation deficits.</p> <p>Theoretically these cropping systems have a high soil C sequestration need. Also sequestering C in the triple cropping systems that are practiced in HL that is inherently low in SOC would be dependent on increased C input. Majority of soils under a double cropped rice-rice system in MHL had a significant saturation deficit of C, these soils also have the greatest need to sequester more C. To generate a higher C input regime in the study region, the task is to diversify the long-established rice-rice cropping system with recommended fertilization, crop rotational diversity other than rice (maize, potato, mustard, vegetables), and higher crop residue retention after harvest.</p> <p>The spatial and temporal trajectory in SOC showed an increase in SOC content in 20 years but in general it remains at low to very low levels. A farming system typology of SCIL identified cropping and management practices in particular land resource settings. Cropping systems with higher land use intensification and high retention of crop residue, on the Floodplain and MHL inundation land type, provided the greatest opportunity for improved soil carbon management (SCM) compared to HL land types of the Piedmont plain, Floodplain and Terrace. The present trajectory of field scale SOC, TN and soil pH showed variability among the cropping systems. Crop-diversified and mustard-rice systems in MHL land type under the stop-go triple cropping intensity with high crop rotational diversity, fertilization and crop residue retention recorded the highest SOC content of the RBCS studied. Thus, under present conditions SOC improvement was observed with these cropping systems compared with the less diverse cropping systems (maize-rice, wheat-rice, veg.-rice and rice-rice) with low crop residue retention in HL and MHL land types. The latter RBCS are at risk of further soil C decline without concerted efforts to increase C inputs. The triple cropping systems in HL land type and the double cropping rice-rice system in MHL land type require sustainable land use intensification. Rotational crops such as maize, jute, potato, tomato, legumes and mustard, vegatebles, and short duration crop varieties of these crops, by cultivating with minimum tillage can help to develop a sustainable land use intensitifaction in the region. This might ensure more C input in soil in an improved management scenario. Further the double and triple cropping systems are in greater need of C sequestration as shown by high C saturation deficits. The better-performing cropping systems that are on a trajectory towards a theoretical maximum saturation capacity could be targeted for sequestering more C in soil to mitigate climate change in Bangladesh. Most RBCS need to consider sustainable land use intensification in certain land resources setting so that the goal of C sequestration can be realistically achieved in all settings. Therefore the balance between LRs/land resources (physiography, land type, farm size, landholding) and CPs/cropping practices (cropping intensity, cropping system, fertilization, crop rotation, crop residue retention) are important considerations for farming and management priority-setting in soil C improvement pathways, soil health and productivity of RBCS, and climate change mitigation in Bangladesh.</p>