Ronald F. Kühne’s research while affiliated with University of Göttingen and other places

The new techniques proposed for agriculture in the Amazon region include rotational fallow systems enriched with leguminous trees and the replacement of biomass burning by mulching. Decomposition and nutriente release from mulch were studied using fine-mesh litterbags with five different leguminous species and the natural fallow vegetation as control. Samples from each treatment were analyzed for total C, N, P, K, Ca, Mg, lignin, cellulose content and soluble polyphenol at different sampling times over the course of one year. The decomposition rate constant varied with species and time. Weight loss from the decomposed litter bag material after 96 days was 30.1 % for Acacia angustissima, 32.7 % for Sclerolobium paniculatum, 33.9 % for Iinga edulis and the Fallow vegetation, 45.2 % for Acacia mangium and 63.6 % for Clitoria racemosa. Immobilization of N and P was observed in all studied treatments. Nitrogen mineralization was negatively correlated with phenol, C-to-N ratio, lignin + phenol/N ratio, and phenol/phosphorus ratios and with N content in the litterbag material. After 362 days of field incubation, na average (of all treatments), 3.3 % K, 32.2 % Ca and 22.4 % Mg remained in the mulch. Results confirm that low quality and high amount of organic C as mulch application are limiting for the quantity of energy available for microorganisms and increase the nutrient immobilization for biomass decomposition, which results in competition for nutrients with the crop plants.

Application of legume green manure (GM) is suggested to be effective in increasing the availability of native soil phosphorus (P) and the dissolution and utilization of phosphate rock (PR)-P by food crops. Experiments were conducted to study the dynamics of extractable P (P extracted by Bray-1-extracting solution) of an Ultisol amended with or without GM residues of contrasting P concentrations in the absence of growing plants. In two separate experiments, GM residues of Aschynomene afraspera (a flood-tolerant legume) and of Crotalaria micans (upland) with varying P concentrations were added to an acidic soil amended with PR-P or triple superphosphate (TSP) in plastic bottles. Soil moisture was brought to field capacity of the soil in the upland experiment and saturated with distilled water in the lowland setup. This was done to simulate aerobic upland and anaerobic lowland soil conditions in the relevant plastic bottles. Only P concentration of the residues added varied, while lignin and C : N ratios were similar. A temperature of 25°C was maintained throughout the experiment. Changes in soil extractable Bray-1-P were measured at the end of the incubation period (60 or 80 d). In the aerobic soils, extractable P in the combined PR+GM or TSP+GM treatments was significantly lower than in the PR- or TSP- treated soils. The amendment with GM residues alone significantly increased Bray-1-P over the unamended control in the case of the inorganic P-fertilized GM residues. The trend in extractable P was similar in the soils incubated under anaerobic conditions. However, in the case of PR, concentrations of P extracted by Bray-1 solution did not significantly change in the presence or absence of GM. The results suggest that the incorporation of GM residues with low P concentration does not lead to a net P release in upland or lowland soils. These results have implications for nutrient cycling in farming systems in W Africa as most of the soils are poor and very low in available P.

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 8weeks. The experimental soils were acid Ultisols with <4mg/kg extractable Bray-1 P. Tilemsi PR from Mali and triple superphosphate (TSP) were applied at 60kg P ha−1 (screen house) and 90kg 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 1tonne of biomass and accumulated 29kg 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.

Quantification of biological nitrogen fixation (BNF) is fundamental to identifying potential green manure legumes for their enhanced nitrogen contribution to sustained crop production. A screenhouse experiment compared BNF by Aeschynomene afraspera L. as affected by phosphorus, using the 15N-isotope dilution and the 15N methods. Rice (Oryza sativa L.) was grown as a non-fixing plant. Phosphorus significantly increased the percent N derived from the atmosphere (% Ndfa), whereby the 15N method tended to overestimate BNF contribution on average by 20% compared to the 15N dilution. Data confirmed rice as a non-fixing plant for estimation of BNF by flood-tolerant legumes.

We studied the impact of plant leaf residue decompo sition and nutrient release of nitrogen and phosphorus of two weed species - Imperata cylindrica and Chromolaena odorata - and one native forest species - Phyllanthus discoideus - on soil enzyme activities in a pot experiment in the humid tropics of central Cameroon. We tested th e impact of plant leaf residue types on decomposition rate, nutrient release and enzyme act ivities in soil. We measured mass loss, nutrient release of nitrogen (N) and phosphorus (P) from decomposing residues, and soil enzymes of the carbon cycle (ß-glucosidase), N cycle (prote ase) and P cycle (acid and alkaline phosphatase) over 120 days. Mass loss from both chromolaena and phyllanthus residues started immediately and was rapid, whereas mass loss from decomposing imperata leaves was only 24% after 90 days. Nitrogen and P release was greater from decomposing chromolaena and phyllanthus leaves than from imperata residues. After 120 days, chromolaena and phyllanth us plant leaf residues had released nearly three times as much of its initial pools of N and P than had imperata plant leaf residue. Beta-glucosidase activity was strongly affected by plant leaf residue types and mass loss. Knowledge on resource-use efficiency of invading weeds might help understand the processes affecting nutrient availability in soils through th eir return of plant leaf residues differing in quality.

The use of plant materials for sustaining soil nutrient in slash-and mulch agricultural system has been reported causing nutrient immobilization by soil microbes, which in turn leads to inhibition of crop growth. Nutrient immobilization occurs mainly at the beginning of plant materials amendment. Immobilization period is very crucial to recognize in order to set a strategy of mulching management in sustainable agricultural system. A simulation on soil microbes-maize competition was carried out in green house experiment to answer the question whether amendment with plant materials could improve P uptake by maize or whether it would cause reduction of available P due to microbial consumption (immobilization) during growing period. Three percent (w/w of soil) of the fallow vegetation materials (Ficus, Albizia, Chromolaena, Macaranga and Trycospermum) were mixed with the top part of pre-incubated soil in the pots (1-3 cm). This set of treatment was contrasted with control (without amendment). Seeds of maize were sown at the time of plant materials amendment. Maize growth, soil P availability, P uptake and soil microbial P were investigated during growing period. Amendment with plant materials into the soil significantly declined the maize biomass and P uptake particularly at 4 th and 6 th week after sowing. Extension of growing period up to 10 week enables the recovery of P uptake by maize grown on amended soil. The reduction of P uptake was due to declining of P concentration in the plant and inorganic P in the soil solution. At the period, in which, the biomass and uptake of maize grown on the amended soil were lower than those of control, the microbial-biomass P in the amended soil was significantly higher than in the control. The presence of plant on the amended soil resulted in a reduction of microbial biomass P.

Rice (Oryza sativa L.) demand in West Africa is unmet because of insufficient production. Legume fixed N [biological N fixation (BNF)] may sustainably increase rice productivity in low-input systems. However, P deficiency limits BNF on the acid soils encountered in the region, despite the prevalence of phosphate rock (PR). Pot and field experiments were conducted in Côte d'Ivoire in 1996-1998 to study the impact of combined legume and PR on rice performance. Triple superphosphate and PR were applied at rates of 60 (pot) and at 90 (field) kg P ha-1 to rice and the legume Aeschynomene afraspera grown for 8 wk and then incorporated before rice transplanting. Legume fixed N was determined by 15N isotope dilution. Under field conditions, addition of PR doubled the biomass of A. afraspera. Irrespective of P source, P application increased the amount of BNF-N (three- to eightfold) to 36 mg N plant-1 in pots and to 84 kg N ha -1 in the field. Nitrogen derived from the air was correlated with legume P uptake (r = 0.97***, where *** = significant at the 0.001 level) and nodulation (r = 0.91**, where ** = significant at the 0.01 level). The synergy of PR and BNF on N and P cycling improved P nutrition and total biomass of subsequent lowland rice under pot conditions. Combining legume green manure (GM) with PR enhanced soil extractable Bray-1 P and may thus play an important role in improving the availability of PR. Under field conditions, due to asynchrony in GM nutrient release and demand, the impact of the combined GM-PR treatment on rice yield was minimal.

Soil degradation and depletion of soil nutrients are among the major factors threatening sustainable cereal production in the Ethiopian highlands. Fertilizer use as a means of mitigating nutrient depletion has become widely accepted. The fate of fertilizer nitrogen (N) applied to a semidwarf bread wheat system was determined in microplots receiving 41 kg N ha−1 in the form of urea labelled with 5.617 % atom excess 15N, without and with the application of phosphorus (P) at 20 kg P ha−1. The study was conducted in three different agro-ecological zones at 2200, 2400 and 2800 m above sea level (masl) (Kulumsa, Asasa and Bekoji, respectively) in the south-eastern Ethiopian highlands. Yield and yield response to N diminished with elevation, while the response to P increased with elevation, with a significant increase in wheat grain yield observed at Bekoji (2800 masl). N harvest index and N use efficiency improved in response to P application at Asasa (2400 masl). Crop removal accounted for 25–64 and 36–64 % of the applied N without and with P, respectively. The corresponding fractions of 15N recovered in the soil ranged from 2.9 to 3.5 % and from 3.2 to 3.7 %, respectively. Recoveries of applied 15N were 67 and 52 % at Kulumsa and Asasa, but only 25 % at the highest elevation site, Bekoji. Fertilizer P significantly enhanced N recovery at Asasa (60 %) and Bekoji (36 %), but not at Kulumsa, where the soil had a high level of residual P from previous applications. The fraction of 15N recovered in the soil–plant system was rather low at Bekoji. However, the rate of 15N recovery was significantly improved (by 37 %) when fertilizer P was added to the soil. Across all locations, the unaccounted 15N may have been lost primarily through denitrification and volatilization.

We evaluated the temporal variation of microbial biomass C, &#35-glucosidase, acid phosphomonoesterase (acP), alkaline phosphomonoesterase (alP), and protease activity over 18 consecutive months. The likely causes for the seasonal variability at a non-degraded and a degraded site in south-western Nigeria were identified. Microbial biomass, alP, and &#35-glucosidase activity were sensitive indicators of soil quality changes over time. Microbial biomass C correlated significantly with soil moisture conditions and soil organic matter-related parameters. AlP and &#35-glucosidase activities were not controlled by climatic conditions over the course of two rainy seasons and one dry season but were temporally related to microbial biomass C and total C and N. Due to the steadiness of the alP activity over time the enzyme is considered a suitable indicator with which to monitor long-term changes of soil quality. Single sampling during the course of a year is adequate. Both microbial biomass and &#35-glucosidase activity fluctuated highly. They were sensitive indicators to monitor short-term variations of soil quality with. Sampling for microbial biomass ought to be limited to the rainy seasons, whereas the measurement of &#35-glucosidase activity need not be restricted seasonally. Due to the short-term variability found, sampling should be repeated. AcP and protease activity fluctuated highly during the course of a year and exhibited pronounced inter-seasonal differences. The marked seasonal changes could not be ascribed to soil moisture conditions and only poorly to major ecological soil processes. This was more pronounced for acP than for protease. Hence, neither parameter was considered a sensitive and meaningful indicator of soil quality changes over time.

This book contains 23 peer-reviewed papers presented during the 'International Symposium on Balanced Nutrient Management Systems' which was held between 9 and 12 October 2000 in Cotonou, Republic of Benin. This book is presented in seven sections (i) general introduction; (ii) variability on physical and socioeconomic factors and its consequences for selection of representative areas for integrated nutrient management (INM) research; (iii) soil processes determining nutrient dynamics, particularly N and P; (iv) interactions between organic and mineral nutrient sources; (v) improved utilization of rock phosphate; (vi) decision support systems to improve resource use at farm level: on-farm testing of improved technologies; and (vii) recommendations. The currently accepted INM approach advocates the use of organic resources and mineral fertilizer inputs to redress nutrient depletion and sustain crop production. It also ensures that development of nutrient management strategies is problem-driven and involves farmers that are the end-users of such technologies.