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Temporal Variations of Soil Organic Carbon and Ph at Landscape Scale and the Implications for Cropping Intensity in Rice-based Cropping Systems

Md. Noor E. Alam Siddique
Soil Resource Development Institute, Ministry of Agriculture, Rangpur 5402, Bangladesh

Lisa Lobry de Bruyn
School of Environmental and Rural Science, University of New England, Armidale, NSW 2350, Australia

Chris N. Guppy
School of Environmental and Rural Science, University of New England, Armidale, NSW 2350, Australia; cguppy@une.edu.au (C.N.G.); yosanai@une.edu.au (Y.O.)

Yui Osanai
School of Environmental and Rural Science, University of New England, Armidale, NSW 2350, Australia; cguppy@une.edu.au (C.N.G.); yosanai@une.edu.au (Y.O.)

Abstract/ Executive Summary:

Landscape scale assessment of temporal variations in soil organic carbon (SOC) contents and soil pH and the implications for long-term agricultural sustainability was determined using legacy datasets collected over two periods separated by 20 years: the 1990s and 2010s. Soil data on SOC and pH were categorized according to the prevailing land types (based on inundation as highland (HL), medium highland (MHL), and medium lowland (MLL)), and physiographic types (i.e., Himalayan Piedmont plain, Tista Floodplain and Barind tract/Terrace) to determine which variable or combination of variables was more influential in spatial and temporal changes of these properties. SOC contents in the physiographic types were generally found to be low, varying between 8 to 12 g/kg. While, SOC contents were significantly higher in MHL and MLL compared with HL that experienced less inundation. The change in SOC contents over 20 years was significant with a 14.5% increase of SOC. There was a greater influence of land type compared with physiography on SOC contents over time. Inundation land types and associated cropping intensity were considered likely to influence SOC of soils under rice-based cropping systems. Furthermore, the levels of 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 results shift from slightly acidic to strongly acidic in the intervening 20-year period between samplings. Soil acidification is potentially a combination of inefficient and excess use of ammonium-based fertilizers with higher application rates and low input from residues. We conclude that acidification may continue with more intensive land use. However, trends in SOC contents over time under certain combinations of physiography and land type either increased slightly or showed a significant loss and in the latter, specifically, the role of land management is not clear. The legacy datasets would be useful for monitoring spatial and temporal soil quality trends at a regional scale, but has limited capacity to capture field level variations in soil properties as data on smallholder cropping practice and management were not collected. Therefore, future research examining the role of management in SOC and pH dynamics at the field-scale would guide the use of fertilizers, crop residue management, and amelioration of acidic soil, to improve the sustainability of rice-based cropping systems in Bangladesh.

Keywords: Soil carbon; Soil pH; Paddy soils; Land inundation; Crop intensification; Fertilizers

Location: Dinajpur district

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Non-commodity: Carbon sequestration

Objectives:

The primary objective was to identify the spatial patterns in SOC content and soil pH due to the effects of the physiographic types, land types, and then both factors combined. The secondary objective was to identify the net change in SOC and soil pH by year (i.e., time periods, 1990s and 2010s), and then the interactions with physiography and land types were assessed to understand their contribution, and which areas were more or less vulnerable to SOC and soil pH over 20 years.

Methodology:

2.1. Study Area Description Dinajpur district lies between 26⁰040 north latitude and 89⁰180 east longitude in the Northwestern region of Bangladesh. The total area of Dinajpur is 3438 km2. The cropped area was 2707 km2 in 2014–2015. Dinajpur is among one of the most intensively used agricultural areas in Bangladesh. The region has a humid, wet and hot subtropical climate with distinct summer, monsoon and winter seasons. The majority of soils are located in three physiographic types, i.e., Piedmont plain, Tista Floodplain and Barind Tract/Terrace. The soils of Piedmont plain and Tista Floodplain are noncalcareous grey soils (i.e., Gleysols) and Terrace is shallow grey soils (i.e., Planosols). The lands of Dinajpur possess three land types, based on flooding during the monsoon and/or flood season, which are HL (i.e., land above the normal flooding level) and MHL (i.e., land flooded up to 90 cm for at least two weeks) and the remainder is MLL (i.e., land flooded up to 90–180 cm for more than two weeks). The surface soil (i.e., 0–15 cm generally) texture is mainly loam and silt loam, but varies from silt loam to sandy loam and silty clay loam to clay loam [33]. Farmers throughout the year depend on monsoon rain and/or irrigation, and commonly practice double crops (i.e., Aman rice in monsoon and Boro rice in winter/dry season rice) or triple crops (i.e., Aman and Boro rice with rotation crop) in MHL and HL or single crop (i.e., Aman rice) in MLL areas. Land management is characterized by conventional farming, agrochemicals, irrigation in dry season and high yielding crop varieties. Puddling (i.e., wet-tillage) is a traditional soil management practice for land preparation before transplanting of seedlings in paddy fields, which involves plowing and harrowing of surface soil in water-saturated conditions. After paddy crop harvest, crop residues and stubbles are often removed for animal fodder and homestead purposes in Dinajpur. 2.2. Soil Legacy Data, Cropping Intensity and Fertilizer SOC and pH datasets from two separate periods (i.e., 1990s and 2010s) were extracted from legacy soil data that was collected for a national semi-detailed soil survey program, conducted by the Soil Resource Development Institute in Bangladesh. The first soil survey initiative was published in the Land and Soil Resource Utilization Guide (LSRUG) (recognized as Upazila Nirdeshika) for each upazila’s (sub-district) in Bangladesh. The second phase of the survey (i.e., follow-up soil survey) was published as reports with a soil polygon map (1:50,000). Dinajpur district consists of thirteen upazila, soil survey datasets were accessed from the primary survey (first phase) and follow-up survey (second phase). These soil datasets are referred to as “legacy data of 1990s and 2010s periods”. The potential contribution of increasing cropping intensity and fertilization on the trends of SOC and soil pH was explored from 1990 to 2019 in Dinajpur. The cropping intensity (%), which is [(gross cropped area (total cropping area sown once and/or more in a year)/Net-cropped area (total cropping area) × 100)]; and fertilizers (Nitrogen (N), Phosphorus (P), Potassium (K), Sulphur (S)) usage (kg/ha) data from 1990 to 2019 were collected from the District Agriculture Office and Bangladesh Bureau of Statistics (BBS). 2.3. Statistical Analysis For data analysis and visualization, the statistical software R for Windows version 1.2.5019 and Microsoft Excel were used. Statistical significance was taken at the p < 0.05 level. Data were tabulated and visualized for physiography, land types and the interactions between the two factors. The contents of SOC and soil pH in physiographic types, land types were visualized in line and bar graphs. To establish the relationships statistically among the variables, SOC and soil pH were considered as dependent variables, and soil sampling years (i.e., time periods 1990s and 2010s), physiography and land type were considered as independent variables. Three-way partial (unbalanced) factorial linear regression models were carried out for the analysis of variance. The partial factorial model was used because the land type category MLL is only present in the Floodplain and is absent in the Piedmont plain and Terrace physiography, so that the model was not constrained by the number of categories within variables. For comparing the means of SOC and soil pH between the 1990s and 2010s for each physiography across land types the Fisher’s Least Significance Difference (LSD) post hoc multiple comparison tests was applied. Prior to analysis, the box-cox transformation was applied to the dependent variables (i.e., SOC and soil pH), ANOVA assumptions were checked and the normality distributions were assessed through Q-Q plots.

Source of Information: Agronomy 2021, 11, 59. https://www.mdpi.com/journal/agronomy

Article Link (Online Journal): https://doi.org/10.3390/agronomy11010059

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

The spatial and temporal trend in the contents of SOC derived from legacy datasets of the 1990s and 2010s identified that in general SOC contents was influenced by both physiography and land inundation type, and that the magnitude and direction of the temporal changes in SOC contents also depended on physiology and land type. The effect of land type is greater on SOC than physiography. Despite the overall increase of 14.5% in SOC contents in 20 years, we identified one area, MLL under the Floodplain physiography, to be the most vulnerable area for SOC losses, as it had a sharp decline (40.9%) in SOC contents over the 20 years. The decline of SOC in MLL could be related to lower soil carbon inputs from single cropping, greater periods of fallow and less crop residue retained but such detailed land management information was not available from legacy data to allow such conclusions to be drawn. Nevertheless, MLL constituted a small area (5%) of the landscape. However, in the 1990s, the high SOC contents (20.3 g/kg) of MLL in the Floodplain physiography indicates that this area has a better prospect for greater storage of SOC through management practices matched to specified land type. Furthermore, there was a small but significant increase in SOC contents in HL and MHL of the Floodplain and HL of the Terrace over the 20 years. With the legacy datasets, it is imprecise whether field-scale variation of SOC contents is more responsive due to variations in cropping systems and management as information on these factors were not collected in the legacy data. However, identifying what factors have contributed to this increase in SOC contents will ensure that this increasing trend will continue and reach the optimal SOC contents that can maintain agricultural productivity in these areas. The dominant trend observed in soil pH datasets was a 0.5 unit decline in soil pH that was consistent across the physiography and land type. The number of soil samples falling within the strongly acidic pH range (4.5–5.5) during 2010s was found to be higher (by 50%) than the number of slightly acidic (5.5–6.5) soil samples during 1990s. This indicates that soil acidification is increasing over time, which is likely to impact highland crops of the Piedmont plain and Terrace more so than wetland paddy in the Floodplain soils. The decrease in soil pH could be linked to a higher rate of fertilizer application, due to more crops being planted per unit of area. It is therefore essential to optimize fertilizer application depending annual cropping practice and soil testing. The inclusion of organic manure and soil amendment is also essential to balance soil pH. This would help farmers to reduce and control soil acidification by optimizing fertilizer rates, so that land degradation could be avoided and the productivity of RBCS would be maintained in future. This study provides valuable information on the trends in SOC and pH levels at the regional level in Bangladesh. Soil legacy data enables the assessment of soil quality change over time, but this baseline legacy information requires careful interpretation to initiate and facilitate decision-making in soil fertility management at field-scale. Therefore, further investigation is warranted to understand SOC dynamics and soil pH at a field-scale, focusing on different cropping systems and cropping intensity (i.e., single/double/triple cropping), land management, crop residue management and crop choice by the smallholder farmers. Field-scale knowledge will be crucial in developing an effective management strategy to avoid land degradation and further soil.

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