Soil fauna and organic amendment interactions affect soil carbon and crop performance in semi-arid West Africa

A field experiment was conducted at Kaibo in southern Burkina Faso on an Eutric Cambisol during the 2000 rainy season to assess the interaction of organic amendment quality and soil fauna, affecting soil organic carbon and sorghum (Sorghum bicolor L. Moench) performance. Plots were treated with the pesticides Dursban and Endosulfan to exclude soil fauna or left untreated. Sub-treatments consisted of surface-placed maize straw (C/N ratio = 58), Andropogon straw (C/N ratio = 153), cattle dung (C/N ratio = 40), sheep dung (C/N ratio = 17) or compost (C/N ratio = 10) and the control. Organic amendments were applied at a dose equivalent to the application of 40 kg N ha⁻¹. The presence of soil fauna increased soil total carbon by 32% and grain yield production by 50%. The interaction between high C/N ratio organic amendment, Andropogon straw (SA), and soil fauna reduced soil carbon build-up. We suggest that this is due to a priming effect of SA on soil organic matter in the presence of soil fauna. We also suggest that the interaction between soil fauna and easily decomposable organic amendment led to the smallest decrease in soil carbon build-up. It is concluded that in semi-arid West Africa, in the presence of soil fauna, soil carbon build-up is more affected by the quality of organic amendments than the quantity of carbon inputs. Sorghum grain yield production was significantly reduced in the absence of soil fauna. High C/N ratio organic amendment interacted negatively with soil fauna in its effects on crop performance. We propose that the effect of soil fauna on soil carbon build-up and crop performance can be optimised by using high quality organic matter or supplementing low-quality organic matter with inorganic nitrogen in semi-arid West Africa.     

Plant availability of phosphorus in superphosphate and a phosphate rock as influenced by earthworms

The effect of earthworms on the plant availability of phosphorus (P) in superphosphate and Chatham Rise phosphorite (CRP) was evaluated in a glasshouse experiment using perennial ryegrass over seven harvests. A mixed earthworm population of Lumbricus rubellus (Hoff.) and Allolobophora caliginosa (Savigny) was used. Increases in the yield of ryegrass in the presence of earthworms varied from 2 to 32%, whereas increases in P uptake by ryegrass ranged from 0 to 40% over seven harvests. With superphosphate, the initial increases in both ryegrass yield and P uptake by ryegrass in the presence of earthworms ranged from 20 to 40% at first harvest to less than 10% by the seventh. In marked contrast, earthworms increased the agronomic performance of pelletized CRP by 15 to 30% throughout the trial period. An increase in plant-available soil N concentrations due to earthworm activity probably explains the initial difference in the performance of superphosphate. The increased agronomic effectiveness of CRP appears to result from the incorporation and intimate mixing of the PR with the soil by earthworms. The implications of the results obtained in the present study to the interpretation of glasshouse and field trials evaluating P fertilizers are also discussed.     

Organic residues affect phosphorus availability and maize yields in a Nitisol of western Kenya

 The effects of organic residues and inorganic fertilizers on P availability and maize yield were compared in a Nitisol of western Kenya. Leaf biomass of Calliandra calothyrsus, Senna spectabilis, Croton megalocarpus, Lantana camara, Sesbania sesban, and Tithonia diversifolia were incorporated into the soil at 5 Mg ha^–1 for six consecutive seasons in 3 years and responses compared with those following the application of 120 kg N ha^–1, 0 kg P ha^–1 (0P); 120 kg N ha^–1, 10 kg P ha^–1; and 120 kg N ha^–1 25 kg P ha^–1 as urea and triple superphosphate (TSP); K was supplied in all treatments. Addition of Tithonia, Lantana and Croton increased soil resin-extractable P over that of fertilizer-amended soil throughout the first crop, but the amounts in the former treatments became similar to those for soils amended with inorganic fertilizers for subsequent crops. Addition of Sesbania, Calliandra and Senna had a similar effect on resin P as inorganic fertilizers. Total maize yields after six seasons were tripled by the application of Tithonia compared to 0P, and were higher than those of the Calliandra, Senna, Sesbania and Lantana treatments, and similar only to that of the Croton treatment. P recovered in the above-ground biomass and resin P, immediately after the implementation of the treatments, was higher in the Senna, Sesbania, Croton, Lantana and Tithonia (35–77%) treatments than in the inorganic fertilizer treatments (21–27%). The P content of organic residues, and the soluble C:total P ratio, were the main residue parameters predicting soil P availability and maize yield. All organic residues used in this study can replace inorganic fertilizers for the enhancement of P availability and maize production, while an additional benefit could be obtained from the use of Croton, Lantana and Tithonia. Received: 19 January 2000     

Rock phosphate-P enhances biomass and nitrogen accumulation by legumes in upland crop production systems in humid West Africa

Experiments were conducted during 1996–1998 in screen house and in the field in the humid forest zone of Côte d’Ivoire, to evaluate the effects of phosphorus (P) from phosphate rock (PR) on the performance of the root nodulating legume Crotalaria micans grown for 8 weeks. The experimental soils were acid Ultisols with <4 mg/kg extractable Bray-1 P. Tilemsi PR from Mali and triple superphosphate (TSP) were applied at 60 kg P ha^?1 (screen house) and 90 kg P ha^?1 (field) to the legume. Legume N-fixed (BNF) was estimated by the ¹⁵N-isotope dilution and δ ¹⁵N natural abundance methods, using Cassia obtusifolia L. as a non-fixing legume reference plant. Without P supply, and under the field conditions, C. micans produced less than 1 tonne of biomass and accumulated 29 kg N/ha. The application of PR-P enhanced legume N by about fourfold over the unfertilised control. There was no significant difference between the effects of TSP and PR. Phosphorus application mainly affected the total amount of N accumulated rather than the percentage derived from the atmosphere (%N dfa) per se. Furthermore, the cumulative effects of PR-P on the performance of C. micans greatly improved with time in the screen house. This study confirms that Tilemsi PR is an agronomically effective source of P for short-duration legume green manure (GM) even in the first year of its application to acid P-deficient soils in the West African humid zone.     

Collections of short-day onion germplasm in West Africa: a survey

Results are presented of a 1994 postal survey of holdings of local onion (Allium cepa L.) germplasm by institutions in West Africa. Data obtained from respondents included the number of accessions of onion or shallot held and the conditions in which onion seed was stored. Twenty-four replies were received and eighteen sites where collections were held were identified. The numbers of local A. cepa accessions in local collections varied in number from 38 down to nil, but many local collections were of less than five accessions. The largest number of national onion accessions was held at CERRA, Maradi, in Niger (16), and the largest regional collection was at the Station de Farako-bâ, Burkina Faso (38). Two sites held substantial shallot collections: CRA Bareng in Guinée and IDESSA, Bouaké, Côte d'Ivoire. Conditions under which onion seed was stored varied from low temperature, controlled storage in moisture-proof packs with desiccants, to storage in plastic envelopes under ambient laboratory conditions.     

我国水稻秸秆磷分布及其还田对土壤磷输入的贡献

我国水稻秸秆资源丰富,水稻秸秆还田是向土壤输入磷素的重要途径之一。对我国各省区不同季别水稻秸秆还田的土壤磷输入贡献进行测算,可有针对性地为水稻秸秆还田条件下土壤磷素优化管理及平衡调控提供科学参考和指导。本研究基于《中国农村统计年鉴》中水稻生产统计资料和文献调研参数,对2013—2018年我国主要稻区不同季别水稻秸秆磷养分资源时空分布特征以及单位播种面积水稻秸秆还田的土壤磷素输入量进行分析。结果表明,2018年我国主要稻区早稻、双季晚稻和中晚稻秸秆产量分别为2327万t、2783万t和13 527万t,长江中游和长江下游稻区的水稻秸秆资源量居于全国前列,分别占33.6%和21.8%。2013—2018年我国水稻秸秆磷(P_2O_5)养分产量呈缓慢增长的趋势,从2013年的59.7万t增加到2018年的62.8万t。2018年水稻秸秆磷养分资源主要分布在黑龙江(15.0%)、湖南(12.5%)、江苏(10.0%)、湖北(9.9%)和江西(9.6%)等省份。2013—2018年我国主要稻区早稻、双季晚稻及中晚稻秸秆还田的年均土壤磷养分输入量分别为13.9～15.1kg(P_2O_5)·hm~(-2)、16.0～20.9 kg(P_2O_5)·hm~(-2)和19.3～29.3 kg(P_2O_5)·hm~(-2)。从全国范围来看,早稻、双季晚稻和中晚稻秸秆还田下的土壤磷养分输入量平均分别为14.4 kg(P_2O_5)·hm~(-2)、18.2 kg(P_2O_5)·hm~(-2)和24.4 kg(P_2O_5)·hm~(-2)。基于上述测算结果,建议我国主要稻区各省份在水稻特别是中晚稻秸秆还田条件下,基于秸秆磷素携入量适当调整磷肥投入量,以实现土壤磷养分收支平衡,控制农田磷养分盈余及流失风险。     

Conservation tillage induced changes in organic carbon, total nitrogen and available phosphorus in a semi-arid alkaline subtropical soil

A multi-year experiment was conducted to compare the effects of conservation tillage (no-till and ridge-till) with conventional plow tillage on organic C, N, and resin-extractable P in an alkaline semi-arid subtropical soil (Hidalgo sandy clay loam, a fine-loamy, mixed, hyperthermic Typic Calciustoll) at Weslaco, TX (26°9′N 97°57′W). Tillage comparisons were established on irrigated plots in 1992 as a randomized block design with four replications. Soil samples were collected for analyses 1 month before cotton planting of the eighth year of annual cotton (planted in March) followed by corn (planted in August).

No-till resulted in significantly (p<0.01) greater soil organic C in the top 4 cm of soil, where the organic C concentration was 58% greater than in the top 4 cm of the plow-till treatment. In the 4–8 cm depth, organic C was 15% greater than the plow-till control. The differences were relatively modest, but consistent with organic C gains observed in hot climates where conservation tillage has been adopted. Higher concentrations of total soil N occurred in the same treatments, however a significant (p<0.01) reduction in N was detected below 12 cm in the ridge-till treatment. The relatively low amount of readily oxidizable C (ROC) in all tillage treatments suggests that much of the soil organic C gained is humic in nature which would be expected to improve C sequestration in this soil.

Against the background of improved soil organic C and N, bicarbonate extractable P was greater in the top 8 cm of soil. Some of the improvement, however, appeared to come from a redistribution or “mining” of P at lower soil depths. The results indicate that stratification and redistribution of nutrients were consistent with known effects of tillage modification and that slow improvements in soil fertility are being realized.     

Effects of tillage, organic resources and nitrogen fertiliser on soil carbon dynamics and crop nitrogen uptake in semi-arid West Africa

Tillage, organic resources and fertiliser effects on soil carbon (C) dynamics were investigated in 2000 and 2001 in Burkina Faso (West Africa). A split plot design with four replications was laid-out on a loamy-sand Ferric Lixisol with till and no-till as main treatments and fertiliser types as sub-treatments. Soil was fractionated physically into coarse (0.250–2 mm), medium (0.053–0.250 mm) and fine fractions (< 0.053 mm). Particulate organic carbon (POC) accounted for 47–53% of total soil organic carbon (SOC) concentration and particulate organic nitrogen (PON) for 30–37% of total soil nitrogen concentration. The POC decreased from 53% of total SOC in 2000 to 47% of total SOC in 2001. Tillage increased the contribution of POC to SOC. No-till led to the lowest loss in SOC in the fine fraction compared to tilled plots. Well-decomposed compost and single urea application in tilled as well as in no-till plots induced loss in POC. Crop N uptake was enhanced in tilled plots and may be up to 226 kg N ha⁻¹ against a maximum of 146 kg N ha⁻¹ in no-till plots. Combining crop residues and urea enhanced incorporation of new organic matter in the coarse fraction and the reduction of soil carbon mineralisation from the fine fraction. The PON and crop N uptake are strongly correlated in both till and no-till plots. Mineral-associated N is more correlated to N uptake by crop in tilled than in no-till plots. Combining recalcitrant organic resources and nitrogen fertiliser is the best option for sustaining crop production and reducing soil carbon decline in the more stabilised soil fraction in the semi-arid West Africa.     

Effectiveness of combined application of Kodjari phosphate rock,water soluble phosphorus fertilizer and manure in a Ferric Lixisol in the centre west of Burkina Faso

Increasing soil phosphorus and organic matter content for crop production while reducing the cost of production are required to facilitate the achievement of green revolution in Africa. Field and pot experiments were laid out during 2012 and 2013 to assess the effects of combined application of Kodjari phosphate rock (PR) and water soluble phosphorus on sorghum yields, P uptake and Lixisol characteristics in the centre west of Burkina Faso. Five P fertilizers treatments (zero P, 100% TSP (triple super phosphate), 100% PR, 50% PR + 50% TSP, 75% PR + 25% TSP) and two cow manure treatments (zero, 5 t ha^?1) were tested. In field experiment, 50% PR + 50% TSP was as effective as 100% TSP in increasing sorghum yield above the control by 30% in 2012 and 50% in 2013 and P uptake by 30% in both years. Manure had an additive effect on phosphorus fertilizers in increasing sorghum yields and P uptake. In pot experiment, increases of Ca uptake, soil pH and microbial P were observed with the application of 50% PR + 50% TSP. Our results suggest that formulation of fertilizer combining phosphate rock and mineral P would improve sorghum yields and income of smallholders.     

Problems and prospects in the utilization of animal traction in semi-arid west Africa: evidence from Niger

The use of animal draft power for soil tillage has long been a major theme of agricultural development programmes in semi-arid West Africa. The rationale for animal traction utilization ranges from increasing agricultural productivity and income to relieving the drudgery in farm work. Yet, its adoption remains low and localized. Previous research has identified the conditions which favour the adoption of animal traction, but evidence on the potential benefits realizable from its use is mixed. The issue of how to improve the adoption and profitable utilization of animal traction in semi-arid West Africa remains an important topic given that mechanization of agriculture is inevitable. This paper uses data collected from villages in two different agroclimatic zones of Niger to demonstrate the regional variability of potential gains from animal traction. Production functions were estimated to determine the impact of animal traction use on area cultivated, labour input and aggregate yield. Multi-year partial budget streams were also estimated to evaluate the profitability of animal traction at each location. The results show that the use of animal traction did not have any significant impact on area cultivated, but increased the yields of sorghum (Sorghum bicolor (L.) Moench) and maize (Zea mays L.) by 12–15%. It increased labour input by 15 man-hha⁻¹ (man hours per hectare) in the dry agroclimatic zone, but reduced it by 50 man-h ha⁻¹ in the wet zone. Rates of return on animal traction investment ranged from −4 to 18% in the dry zone and from 14 to 58% in the wet zone. These results suggest that the impact of animal traction on agricultural production varies by region and depends on a broad set of agroecological and economic conditions. The absence of these conditions in marginal areas strictly limits potential utilization and profitability. On efficiency grounds, it is recommended that animal traction should be promoted only in areas where the climate, soil and economic conditions permit its intensive and profitable utilization.     

Carbon sequestration in dryland ecosystems of West Asia and North Africa

The West Asia–North Africa (WANA) region has a land area of 1.7 billion ha, and a population of 600 million. Desertification and soil degradation are severe problems in the region. The problem of drought stress is exacerbated by low and erratic rainfall and soils of limited available water holding capacity and soil organic carbon (SOC) content of less than 0.5 per cent. The SOC pool of most soils has been depleted by soil degradation and widespread use of subsistence and exploitative farming systems. The historic loss of a SOC pool for the soils of the WANA region may be 6–12 Pg compared with the global loss of 66–90 Pg. Assuming that 60 per cent of the historic loss can be resequestered, the total soil‐C sink capacity of the WANA region may be 3–7 Pg. This potential may be realized through adoption of measures to control desertification, restore degraded soils and ecosystems, and improve soil and crop management techniques that can enhance the SOC pool and improve soil quality. The strategies of soil‐C sequestration include integrated nutrient management (INM) and recycling, controlled grazing, and growing improved fodder species on rangeland. Improved technologies for cropland include use of INM and biofertilizers, appropriate tillage methods and residue management techniques, crop rotations and cover crops, and water and nutrient recycling technologies. Through adoption of such measures, the potential of soil‐C sequestration in the WANA region is 0.2–0.4 Pg C yr⁻¹. Copyright © 2002 John Wiley & Sons, Ltd.     

Increased phosphorus availability to corn resulting from the simultaneous applications of phosphate rock, calcareous rock, and biochar to an acid sandy soil

Phosphorus (P) deficiency is one of the main constraints on crop production in Arenosols (acid sandy soil). The high cost of P fertilizers may represent an insurmountable obstacle in many poor countries, leaving the exploitation of their own calcareous and phosphate rocks as the only low-cost and long-term alternative. Biochar is suggested to have positive effects on soil properties; however, there is no published research on the synergistic effects of biochar and rocky materials in modifying soil properties. The aim of this study was to investigate the chemical and biochemical responses of an acid Arenosol treated with phosphate rock (PR), calcareous rock (CR), and biochar (BC), and the implications for corn yield. A soil from Marracuene District, Mozambique was used, where corn was grown for 90 d with the soil treated with:no addition (control), water-soluble zinc phosphite fertilizer (WSP), PR, WSP+CR, WSP+BC, WSP+CR+BC, PR+BC, and PR+CR+BC. Biochar was produced by pyrolysis of babycorn peels for 4 h at 450℃ and applied at 11 g kg^-1. The soil pH_H2O increased from about 4.54 in the control to 7.38 in the PR+CR+BC treatment. Easily oxidizable organic carbon, cation exchange capacity, and available P were higher in the treatments containing BC than in the control. The treatments containing CR and/or BC led to the highest activities of alkaline phosphomonoesterase, phosphodiesterase, and α-glucosidase, which increased P availability and gave the greatest biomass and yields. We suggest that biochar provides additional soluble P and supplies adsorption sites for phosphate, preventing its evolution to unavailable forms. Thus, PR applied together with BC contributed to an 840% yield increase compared to the control. The treatments containing WSP and BC facilitated phosphite oxidation to phosphate and increased crop yield.     

磷酸一钙施用模式对水稻有效性及土壤中有效磷分布的影响

利用温室盆栽试验研究了磷酸一钙不同施用模式对水稻的有效性及土壤中有效磷(P)分布的影响,并提出了水田中P肥的施用模式。结果表明:P肥撒施时,表层水中总P浓度最高,达4.8mg/L,而其他施肥模式下表层水中总P浓度可满足GB3838-2002Ⅲ类水水质要求;混施可为水稻提供立体的吸收空间,有效性高;沟施时,肥料P主要集中在施肥带上下约3cm的范围内;在水田中,施用P肥时应优先考虑混施,若土壤中P素有效性相对较高,应尽量在旱作时施用。     

Knowledge Levels and Perceived Effect of Climate Change on Extension Delivery in North West Province,South Africa

A simple random sampling technique was used to select 20% of the extension officers in North West Province, South Africa. Data on knowledge levels and perceived effect of climate change on extension delivery were collected and analyzed using frequency counts, percentages, and multiple regression analysis. The results show that a wide range of knowledge levels exists on climate change issues related to causes, effect, vulnerability, and mitigation and that extension officers will have to address new skill requirements for farmers, the need for specialized extension services, and changing and diversified livelihoods. The results have several implications for training and educating extension officers on climate change issues.     

Long-term microbial control of nutrient availability and plant biomass in a subarctic-alpine heath after addition of carbon, fertilizer and fungicide

A long-term field experiment lasting more than a decade was conducted on a subarctic fellfield to investigate effects of changes in nutrient availability on soil microbial C, N and P, soil nutrients, vascular plant biomass and plant-microbial interactions. Additions of NPK fertilizer, labile C (sugar) and fungicide (benomyl) were done in a fully factorial design, replicated in six blocks. The treatments were run for ten years and soil and vegetation samples were collected four years after initiating the experiment, and again after an additional 12 years, to evaluate the long-term effects. Labile C addition resulted in increased microbial biomass and nutrient immobilization after four years, and a long-term decrease in vascular plant biomass, thus suggesting the microorganisms to strongly control soil nutrient availability in periods of high microbial biomass. Fertilization increased the inorganic and total soil nutrient pools of N and P and the fine root biomass, but not the total aboveground vascular plant biomass. The vascular plant biomass increased due to benomyl addition thus indicating the plants to be strongly affected by the microbial community. Overall, the effects of benomyl resulted in more lasting changes in the soil compared to labile C and fertilizer addition. In relation to environmental changes, the indicated strong microbial control of the available nutrients in the fellfield ecosystem might limit ecosystem changes due to increased soil nutrient availability as otherwise expected in arctic soils.     

Ecological Engineering for Sustainable Food Production and the Restoration of Degraded Watersheds in Tropics of Low pH Soils: Focus on West Africa

Since upland rice under shifting cultivation is common, the mean paddy yield in sub-Sahara Africa (SSA), especially West Africa, has been stagnated at 1.3–1.7 t ha^?1 during the past 30 years of 1970–2000. There exist numerous small inland valley swamps (IVS), of which 10 million ha is capable of being turned into small scale irrigated rice fields, i.e., sawah* by simple and low cost ecological engineering technology with farmer's self-support efforts. The fertility of lowland soils in West Africa was the lowest, particularly available phosphorous status was extremely poor and low pH as well as low exchangeable bases among the three tropics of Asia, Africa and Latin America. However, because of multi-functional mechanisms of soil and water conservation and replenishment of nutrients, sustainable productivity of 1 ha of lowland sawah system can be equivalent more than 10 ha of upland fields. Geological fertilization, nitrogen fixation, neutralization of pH and increase phosphorous availability are the important functions of the sawah system. Sustainable development of sawah systems in IVSs can contribute to increase food production and to restore the degraded watersheds of SSA. *Sawah: The term sawah refers to leveled rice field surrounded by bund with inlet and outlet connecting irrigation and drainage. The term originates from Malayo-Indonesian. The English term, Paddy or Paddi, also originates from the Malayo-Indonesian term, Padi, which means rice plant. The term, Paddy, refers to rice grain with husk in West Africa of SSA. Most of the paddy fields in the Asian countries correspond to the definition of the term sawah. Paddy field is almost equivalent to sawah for Asian scientists. However, the term paddy fields refers to just a rice field including upland rice field in West Africa of SSA. Therefore in order to avoid confusion between the terms rice plant, paddy, and the improved man-made rice growth environment through ecological engineering, the authors propose to use the term sawah.     

Isotope techniques to study phosphorus cycling in agricultural and forest soils: a review

A sound understanding of nutrient dynamics in ecosystems is required in order to manage these systems on a sustainable basis. A valuable approach to studying phosphorus (P) dynamics in soil-plant systems has been the use of P isotope techniques. Isotope techniques used for studying P cycling in agricultural and forest soils are reviewed in this paper with particular reference to advances made in the part 15 years. A brief discussion of the properties of P isotopes and their measurements is included together with techniques for measuring exchangeable P in the soil, dissolution and decomposition rates of inorganic and organic P sources applied to the soil, rates of organic P immobilization and mineralization, rates of P release and retention in the soil, root activity and lifter decomposition rates in forest soils, and gene probing and hybridization. Basic principles, assumptions, procedures, limitations and merits of methods are discussed. These techniques have served as or have the potential to be valuable tools for advancing our understanding of P dynamics in soil-plant systems, and for studying the molecular characteristics of microbial communities in relation to the cycling of nutrients in the soil.     

A laboratory study of the effect of time and temperature on the decline in Olsen P following phosphate addition to calcareous soils

Abstract. The effects of time and temperature on the changes in Olsen P after phosphate application were studied in 13 calcareous soils from Pakistan, an Oxisol from Colombia and an Inceptisol from England. The phosphate sorption reactions were monitored in two stages. The short-term reaction (30 min shaking with added phosphate in the presence of the Olsen bicarbonate solution) showed that over this time the nature of the sorbing material and number of available sites for P adsorption were important but temperature was not. The extent of the short-term sorption was not related to the amount of calcium carbonate. In the long-term reaction (incubating the soils with phosphate at 10, 25 and 45 °C for one year) the amount of Olsen P decreased with time following a power relationship. Increased temperature increased the rate of reaction, following the Arrhenius principle i.e. Q₁₀⊃ 3 (activation energy 83 kJ mol^–1). The effects of time and temperature were well described by a modified power equation Y = a (1 + f_Tt ) ^-b , where Y is the amount of Olsen P extracted after time t , a is the Olsen P value after the short-term reaction (the initial value), f_T is the ratio of the rate constants at any two temperatures and b is a coefficient which represents the loss in extractability with time. On the basis of the initial Olsen P values and subsequent Olsen P values at different times and temperatures a unified decay curve Y/a = (1 + t )^–0.20 was developed where the initial Olsen P values are normalized to 1. The parameters of this equation allow, with limitations, the prediction of changes in Olsen P in these soils if the initial Olsen P value of the soil is known.