Effect of land management on soil microbial N supply to crop N uptake in a dry tropical cropland in Tanzania

In Sub-Saharan Africa, conservation of available soil N during early crop growth, when N loss by leaching generally occurs, is important to improve crop productivity. In a dry tropical cropland in Tanzania, we assessed the potential role of soil microbes as a temporal N sink–source to conserve the available soil N until later crop growth, which generally requires substantial crop N uptake. We evaluated the effect of land management [i.e., no input, plant residue application before planting (P plot) with or without fertilizer application, fertilizer application alone, and non-cultivated plots] on the relationship between soil N pool [microbial biomass N (MBN) and inorganic N] and crop N uptake throughout the ∼120-d crop growth period in two consecutive years. In the P plot, MBN clearly increased (∼14.6–29.6 kg N ha[−1]) early in the crop growth period in both years because of immobilization of potentially leachable N, and it conserved a larger soil N pool (∼10.5–21.2 kg N ha[−1]) than in the control plot. Especially in one year in which N leaching was critical, increased MBN maintained a larger soil N pool in the P plot throughout the experimental period, and a delay of increased MB C:N ratio and a substantial decrease in MBN was observed, indicating better soil microbial N supply for crop N uptake during later crop growth. Therefore, plant residue application before planting should enhance the role of soil microbes as a temporal N sink–source, leading to the conservation of potentially leachable N until later phase of crop growth, especially in years in which N leaching is relatively severe. Although further studies are necessary, our results suggest that plant residue application before planting is a promising option to achieve better N synchronization

Effects of White Lupin and Groundnut on Fractionated Rhizosphere Soil P of Different P-Limited Soil Types in Japan

Phosphorus (P) is an essential nutrient for crop production, while most soil P is the less labile P associated with Aluminum (Al) and Iron (Fe) in acidic soils of Japan. The objectives of this study were to evaluate the effects of two contrasting P-efficient legumes (white lupin, WL (Lupinus albus L.); and groundnut, GN (Arachis hypogaea L.)) on rhizosphere soil P dynamics in different soil types of Japan, such as Al-rich volcanic-soil, Fe-rich red-yellow-soil, and sandy-soil, with or without historical fertilization managements (3 soil types × 2 managements = 6 soil samples). We conducted a 56-day pot experiment, and analyzed the plant P uptake and fractionated P of rhizosphere and bulk soils, based on the Hedley-fractionation method. We observed that GN P uptake was generally larger than that in WL in most soil types and managements. WL significantly decreased the labile P in most soils and also decreased the less labile inorganic P (Pi) and organic P (Po) in fertilized Red-yellow-soil, which has much crystalline Fe, though GN did not. In contrast, both WL and GN significantly decreased the less labile Pi in fertilized volcanic-soil, which has much non-crystalline Al. These results indicate that (1) characteristics of less labile P uptake by P efficient legumes were different between the soil types and managements, and (2) WL efficiently solubilized the less labile P than GN in fertilized red-yellow soil, while GN efficiently absorbed the larger amount of P than WL, especially in volcanic- and sandy-soil

No Tillage Increases SOM in Labile Fraction but Not Stable Fraction of Andosols from a Long-Term Experiment in Japan

No tillage (NT) fosters carbon (C) sequestration, increases soil organic matter (SOM) stock, and improves soil health. However, its effect on SOM accumulation in Andosol, which has high OM stabilization characteristics due to its specific mineral properties, remains unclear. In this study, we evaluated the effect of NT on SOM content and its distribution by the physical fractionation method and assessed the quality of accumulated SOM in each fraction. We collected soil samples at 0–2.5, 2.5–7.5, and 7.5–15 cm depths from NT and conventional tillage (CT) plots in a long-term (19 years) field experiment of Andosols in Ibaraki, Japan. The soil samples were separated into light fraction (LF), coarse-POM (cPOM: 0.25–2 mm), fine-POM (fPOM: 0.053–0.25 mm), and silt + clay (mOM: <0.053 mm). The C, nitrogen (N), and organic phosphorus (Po) contents of each fraction were analyzed. The C content of cPOM and fPOM in NT at 0–7.5 cm was higher than in CT, while there was no clear difference in the mOM fraction or deeper layer (7.5–15 cm). NT increased the C, N, and Po contents in the labile POM fractions at the surface layers but did not increase the stable fraction or change the quality

Impact of biochar and manure application on in situ carbon dioxide flux, microbial activity, and carbon budget in degraded cropland soil of southern India

International audienceBiochar application is attracting attention to be an effective soil organic carbon (SOC) management to prevent land degradation, though quantitative information of its effect on carbon dioxide (CO2) flux and associated microbial responses is still scarce, especially in degraded tropical agroecosystems. We conducted a 27-month field experiment with periodically measuring environmental factors, CO2 efflux rate, microbial biomass C (MBC), and SOC stock, and evaluated the impact of land management (control (C), biochar (B; 8.2 Mg C ha(-1)), farmyard manure (FYM) (M; 1.1 Mg C ha(-1) yr(-1)), and a mixture of both (BM) on CO2 flux, microbial responses (MBC and qCO(2) as microbial activity) and C budget, in tropical alkaline cropland of southern India. Based on the relationship between the CO2 efflux rate and environmental factors, cumulative CO2 flux was estimated at 2.4, 2.7, 4.0, and 3.7 Mg C ha(-1) in the C, B, M, and BM treatments, respectively. Biochar application increased soil moisture though did not affect CO2 flux, causing a positive C budget (6.7 Mg C ha(-1)), because of the limited response of microbes to increased soil moisture due to the small amount of SOC. Biochar and FYM combined application did not increase CO2 flux compared with FYM alone, contributing to the largest SOC increment (8.9 Mg C ha(-1)) with a positive C budget (9.1 Mg C ha(-1)), due to little difference of microbial responses between the two treatments. Hence, biochar application combined with FYM could be an effective SOC management in the degraded cropland of southern India

Root biomass and root traits varied spatially in a young Mediterranean agroforestry system: implications for soil C storage

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Effect of rhizobium inoculation on rhizosphere phosphorous dynamics and fertilised phosphorous use efficiency in a maize–pigeon pea intercropping system in weathered tropical soil

Abstract Introduction Low phosphorus (P) use efficiency (PUE) of fertiliser is a critical problem in sustainable crop production, especially in strongly weathered tropical soils with a high P‐fixation capacity. Both intercropping and rhizobium inoculation have shown to improve the P availability of rhizosphere soil, but the effect of a combined approach of using both intercropping and rhizobium inoculation is still unclear. In this study, we aimed to evaluate the effect of rhizobium inoculation on the soil–plant P dynamics and fertilised PUE under the intercropping system in strongly weathered tropical soil. Materials and Methods We conducted an 85‐day cultivation pot experiment with pigeon pea (PP) and maize using highly weathered tropical soil under eight treatments: monocropping (CS) or intercropping, with or without rhizobium (Bradyrhizobium elkanii USDA61) inoculation (−I, +I) and with or without P fertilisation (0P, 50P) (2 × 2 × 2 = 8 treatments). We evaluated the effects of intercropping and rhizobium inoculation on plant growth parameters, P dynamics of the rhizosphere and bulk soil using the Hedley P fractionation method, the amount of organic acid from plant roots as a plant P‐mobilising capacity, and fertilised PUE. Results Total plant P uptake per pot was significantly increased by intercropping but not by combining intercropping and rhizobium inoculation, resulting in better fertilised PUE only in intercropping. The available inorganic P (Pi) and less labile Pi of the soil were higher in the rhizosphere than those in the bulk by intercropping under 50P and were similar in PP + I under 50 P. The amount of organic acid per pot under 50P increased with each treatment, that is, intercropping and rhizobium inoculation, but not with their combination. Conclusion The intercropping system has a strong potential to improve PUE by stimulating the P‐mobilising capacity of intercropping plant roots, whereas rhizobium inoculation of the intercropping system did not improve PUE in this study

Horizontal and vertical variations of root distribution and traits, soil physical, chemical and microbial properties associated with CNP cycles in a young alley-cropping system under Mediterranean climate

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Forest understories controlled the soil organic carbon stock during the fallow period in African tropical forest: a 13C analysis

アフリカ熱帯林の焼畑-休閑サイクルにおいて休閑初期の草本植生の侵入が土壌肥沃度の回復を早めることを発見. 京都大学プレスリリース. 2019-07-31.Soil organic carbon (SOC) dynamics after slash-burn agriculture are poorly understood in African tropical forest, though recent studies have revealed C4 grass invasion as a forest understory influences SOC dynamics after deforestation. This study aimed to quantify the relative SOC contribution of C4 and C3 plants separately through the sequential fallow periods of forest (cropland, or 4–7, 20–30, or >50 years of fallow forest) in the tropical forest of eastern Cameroon. We evaluated the SOC stock and natural 13C abundance for each layer. The SOC stock was largest in 4–7 years fallow forest (136.6 ± 8.8 Mg C ha−1; 100 cm depth, and C4:C3 = 58:42), and decreased with increasing fallow period. SOC from C4 plants was larger in the 4–7 and 20–30 years fallow forests (57.2–60.4 ± 5.8 Mg C ha−1; 100 cm depth), while it clearly decreased in >50 years fallow forest (35.0 ± 4.1 Mg C ha−1; 100 cm depth), resulting in the smallest SOC in this mature forest (106.4 ± 12.9 Mg C ha−1; 100 cm depth). These findings indicate that C4 grass understories contributed to the SOC restoration during early fallow succession in the tropical forest of eastern Cameroon