Browsing by Subject "Electrical resistivity tomography"

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Crop yield and fate of nitrogen fertilizer in maize-based soil conservation systems in Western Thailand
(2021) Wongleecharoen, Chalermchart; Cadisch, Georg
The increase in food demand and land scarcity in high-potential lowland areas have forced cropping intensification with a transformation of land use from subsistence to permanent agriculture in remote hillside in Southeast Asia. This change and inappropriate land use are the prime cause of soil degradation by erosion, which have negatively affected the agricultural systems productivity and sustainability in Thailand. Therefore, vulnerable land in sloping terrain is classified as unsuitable for continuous production of arable crops unless conservation measures are introduced to stabilize the landscape. Even though conservation practices can stabilize sloping land, farmers have not been widely adopted the measures due to various constraints, such as crop area loss and crop-tree competition. To improve land use management, a two-year study (2010-2011) was conducted at the Queen Sirikit research station (13°28’N, 99°16’E), Ratchaburi Province, Thailand, on a hillside with a slope of around 20%. The treatments consisted of (T1) maize (Zea mays L.) mono-crop under tillage and fertilization, (T2) maize intercropped with chili (Capsicum annuum L.) under tillage and fertilization, (T3) maize intercropped with chili, application of minimum tillage plus Jack bean (Canavalia ensiformis (L.) DC) relay cropping and fertilizer application, (T4) maize intercropped with chili, application of minimum tillage with Jack bean relay cropping and fertilizer application plus perennial hedges of Leucaena leucocephala (Lam.) de Wit, (T5) as T3 but without fertilization, and (T6) as T4 but without fertilization. There was an additional plot of chili sole cropping to calculate the land equivalent ratio (LER). The first part of the study evaluated yield performance and nitrogen use efficiency (NUE) of crops using the 15N isotope technique under diverse fertilized cropping systems during the first year. Maize grain yields were lower in T2 (3.1 Mg ha-1), T3 (2.6 Mg ha-1) and T4 (3.3 Mg ha-1) than in the control (T1) (6.7 Mg ha-1). The total returns from maize and chili yields were 1,914, 5,129, 3,829, 3,900, 3,494, and 2,976 USD ha-1, for T1, T2, T3, T4, T5 and T6, respectively. Higher economic returns in mixed crop systems, by selling both maize and chilies, compensated for the maize area loss by intercropping. Maize 15NUE was highest in T2 (53.5%), being significantly higher than in T1 (47.0%), T3 (45.5%), and T4 (45.7%). Overall system’s NUE in T2 (56.8%) was comparable to T1 (53.8%) and T4 (54.5%) but significantly lower in T3 (48.6%). Minimum tillage and hedgerows (despite their positive filter effect) did not increase NUE but adversely affected maize growth during the establishment phase. The second part of the study examined nitrogen fertilizers fate and quantified partial nitrogen budgets at plot level over two cropping seasons for various maize-based cropping systems with or without fertilizer application. Overall plant uptake of fertilizer 15N applied to maize was 48.6-56.8% over the first season, while residual fertilizer 15N recovery of plants was only 2.3-4.9% over the subsequent season. The quantity of applied labelled N remaining in the soil at the end of season 1 and season 2 was 6.2-28.1% and 7.7-28.6%, respectively. Thus, 60.0-76.0% in season 1 and 12.7-31.3% in season 2 of the applied fertilizer 15N were accounted for within the plant-soil system. Consequently, 24.0-40.0% and 12.9-16.1% of labelled fertilizer N were not accounted for at the end of season 1 and season 2, respectively. The derived N balance over two years revealed severe soil N depletion under T1 (-202 kg N ha-1), T5 (-86 kg N ha-1) and T6 (-48 kg N ha-1), and a slightly negative N budget under T2 (-5 kg N ha-1). In contrast, T3 (87 kg N ha-1) and T4 (62 kg N ha-1) had positive N balances. The increase of N input via additional N fertilizer applied to chili and symbiotic N2 fixation of legumes, and the reduction of N losses by soil erosion and unaccounted fertilizer N (probably lost via leaching, volatilization and denitrification) were the main factors of the positive N balances under maize-chili intercropping systems with conservation measures and fertilization (T3 and T4). Maize yield decline under T1, T2, T5 and T6 in season 2 was related to negative N balances, while maize yield increase under T3 and T4 was related to positive N balances. However, maize-chili intercropping with fertilization had some advantage (LER > 1.0) relative to sole species cropping. Moreover, total returns from crop yields in season 2 of all maize-chili intercroppings (1,378-1,818 USD ha-1) were higher than chili sole cropping (1,321 USD ha-1), which pointed to its crucial role in decreasing production risk by reducing yield loss by pests and diseases observed in chili plants. The third part of the study used combined data of stable isotope discrimination and electrical resistivity tomography (ERT) to improve understanding of competition at the crop-soil-hedge interface. Hedges significantly reduced maize grain yield and aboveground biomass in rows close to hedgerows. ERT revealed water depletion was stronger in T1 than in T4 and T6, confirming time domain reflectometry (TDR) and leaf area data. In T4, water depletion was higher in maize rows close to the hedge than rows distant to hedges and maize grain δ13C was significantly less negative in rows close to the hedge ( 10.33‰) compared to distant ones ( 10.64‰). Lack of N increased grain δ13C in T6 ( 9.32‰, p ≤ 0.001). Both methods were negatively correlated with each other (r= 0.66, p ≤ 0.001). Combining ERT with grain δ13C and %N allowed identifying that maize growth close to hedges was limited by N and not by water supply. In conclusion, the results suggested a significant positive interaction between mineral N fertilizer, intercropping systems and soil conservation measures in maintaining or improving crop yields and N balances in Thailand’s hillside agriculture. Simultaneously, combining ERT imaging and 13C isotopic discrimination approaches improved the understanding of spatial-temporal competition patterns at the hedge-soil-crop interface and pointed out that competition in maize-based hedgerow systems was driven by nitrogen rather than water limitation. Therefore, sustainable agriculture might be achieved if farmers in Thailand combine soil conservation measures with appropriate and targeted N fertilizer use.
Evaluation of geo-physics methods to study the effects of land use on salinity in rice production systems in the Vietnam Mekong Delta
(2023) Nguyen, Van Hong; Asch, Folkard
In the Vietnam Mekong Delta (VMD), salinity is a major concern for rice production, which is highly susceptible to saltwater intrusion due to its proximity to the sea and tidal influences. Climate change induced sea level rise, reduced upstream freshwater flows and land subsidence are exacerbating the problem. As a result, saltwater intrudes into the rivers, canals and aquifers of the VMD, reducing the availability of freshwater for irrigation and agricultural use. As the worlds largest rice exporter, the impacts of salinity on rice production in the VMD is significant and poses a serious threat to food security. Addressing the impact of salinity on rice production in the VMD requires a comprehensive approach to assess salinity from the topsoil to the subsoil layers. Therefore, this study was conducted to evaluate salinity issues in rice production systems and figure out the link between rice production systems in the VMD and salinity by applying geophysical methods. Geophysical methods were used in this study including Electromagnetic Induction (EMI) and Electrical Resistivity Tomography (ERT). EMI measures electrical conductivity, while ERT measures electrical resistivity, which is the inverse of conductivity and is closely related to soil salinity. ERT was employed to assess salinity of the subsurface to a depth of 40 m, while EMI was used to detect topsoil salinity up to 1.5 m depth. The case study, Tra Vinh province, in the VMD was chosen for the soil salinity investigation. Soil salinity measurement was conducted during dry season in different land-use types related to rice production systems in the VMD. The field measurements were carried out in two consecutive dry seasons, the dry season of 2019-2020 and dry season of 2020-2021. The first measurement was carried out at five case study sites with different cropping patterns to validate the ERT data and to compare the two methods to determine the best method for investigating soil salinity. Five boreholes were drilled to a depth of 40 m to validate the ERT measurement for subsurface salinity. In the following dry season, ERT and EMI were measured in the extensive survey, with the measurement sites selected along four typical geological transects in the Tra Vinh province. With a desire to use ERT alone to assess soil salinity, ERT data was then used to predict topsoil salinity along with EMI measurement. However, the results from ERT measurement seems to underestimated topsoil salinity due to the lack of measurement on the fields compared to the EMI. The conductivity data collected from different land use types, showed that double rice crop fields are the most prone to salinity than other cropping patterns such as, triple rice, triple/double rice, and single rice. In general, however, topsoil salinity is not a critical issue in the study area compared to potential salinity from the near surface water table, which varies from a relatively shallow depths, from 2 m to 5 m depth, identified by ERT using a resistivity of less than 3 Ωm as the threshold for saline water. The saline water table is the Tra Vinh province increase with the proximity to the sea. As analyzing from resistivity maps, saltwater intrudes the subsurface groundwater from two directions: rivers along the province, and from the sea. Therefore, the double rice, single rice fields and small area of triple rice fields distributed along the two main rivers in the Tra Vinh province are highly affected by saline subsurface water. From the results we see that salinity affect land use in rice production system, and not another way round. Furthermore, we would like to prove for the first time the capability of ERT and EMI in evaluating soil salinity in the rice cultivation fields in the VMD. In addition, we suggested the powerful methods to capture and monitor saltwater intrusion into the rice fields from top to subsurface, which is necessary to improve and protect rice production.