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Declines in soil fertility and limited access to inorganic nitrogen (N) fertilizer constrain crop production in Sub-Saharan Africa. The incorporation of organic residues could increase nutrient mineralization and replenish soil carbon (C), however, the effect that long-term residue management (10+ years) has on maize (Zea mays L.) yields and soil nutrient pools (C and N) is largely unknown. In four identical long-term trials in Kenya that differ by soil type and climate, we compared maize yield and soil C (0–0.15 m) across four treatments comparing organic inputs of contrasting C:N ratios: tithonia [Tithonia diversifolia (Hemsl.) A. Gray] + N (120 kg N ha−1); tithonia − N (0 kg N ha−1); maize stover + N (120 kg N ha−1); maize stover − N (0 kg N ha−1). On average, maize yields were 92% greater under tithonia compared to maize stover at the sandy sites and in general followed this trend: tithonia + N > tithonia − N = stover + N > stover − N. The continuous application of tithonia also increased soil C and N pools; for instance, mineralizable C was up to 57% greater than that of maize stover at the sandy sites. Increases in yield and nutrient pools under tithonia were less apparent at the clay sites. When exploring the relationship between soil C and maize yield, we found that both mineralizable and processed pools were related to agronomic performance. Our findings demonstrate that low C:N residues could significantly benefit crop production and enhance soil organic matter at sandy sites across Kenya.

Dynamics of ammonia-oxidizing bacterial (AOB) and archaeal (AOA) abundance was assayed in a tropical Humic Nitisol during two cropping seasons of a long-term field experiment situated in the central highlands of Kenya. Since 2002, soils were treated yearly with biochemically contrasting organic inputs (4 Mg C ha−1 year−1) of Tithonia diversifolia (TD; C/N ratio: 13; lignin: 8.9 %; polyphenols: 1.7 %), Calliandra calothyrsus (CC; 13; 13; 9.4) and Zea mays (ZM; 59; 5.4; 1.2) combined with and without 120 kg CaNH4NO3 ha−1 season−1. In 2012 and 2013, soils (0–15 cm) were sampled at young growth (EC30) and flowering (EC60) stages of maize and subjected to DNA-based amoA gene quantification. ZM and TD increased AOB abundance by 9 and 19 %, respectively, compared to CC, while AOA remained unaffected. This was ascribed to high organic N in TD and lower lignin and polyphenol contents in ZM than in CC. In CC, formation of polyphenol-protein complexes limited microbial access to N. Sole use of mineral N or its combination with organic inputs decreased AOA abundance by 35 % but not AOB as a consequence of pH reduction by mineral N. Overall, AOB was more responsive than AOA to input quality that became most pronounced under optimal soil moisture conditions found in 2013 and at EC60 in 2012. We recommend prolonged study periods, considering also rRNA-based analyses to explore the dynamics of active nitrifying communities as a consequence of interrelations of contrasting organic inputs, crop growth stages and seasonality in agricultural soils.

Maize production in western Kenya is commonly limited by P deficiencies and aluminum phytotoxicity. Due to high costs of imported fertilizers and lime, focus is now shifting to solutions that utilize local resources. We tested the effect of three inorganic P sources i.e., triple superphosphate (TSP), Minjingu phosphate rock (MPR) and Busumbu phosphate rock (BPR), each applied in combination with two organic materials (OMs) i.e., farmyard manure (FYM) and Tithonia diversifolia green manure (tithonia), or with urea on soil chemical properties related to soil acidity, P availability and maize yields for three consecutive seasons in western Kenya. The OMs and inorganic P sources were applied to provide 20 and 40 kg P ha−1 respectively in their combination. Where urea was used, the inorganic P sources were applied at 60 kg P ha−1. Maize did not respond to application of TSP, MPR or BPR with urea in the first two seasons. However, after three seasons, maize significantly responded to application of MPR with urea. FYM was more effective than tithonia in increasing the labile inorganic P pools but it gave lower maize yields than tithonia which was more effective in reducing the exchangeable Al. It appears that the ability of an OM to lower the exchangeable Al is more important in increasing maize yields than its ability to increase P availability. The effectiveness of the inorganic P sources in increasing maize yields followed the order of their effectiveness in increasing available P, i.e., TSP > MPR > BPR, once Al phytotoxicity was reduced by application of tithonia but the difference between TSP and MPR was not significant. The extra maize yield obtained by the additional 40 kg P ha−1 from the inorganic P sources was, however, in most cases not substantial enough to justify their use. Economic considerations may therefore favour the use of tithonia or FYM when applied alone at 20 kg P ha−1 than when combined with any of the inorganic P sources used in this study at a total P rate of 60 kg ha−1.

Phosphorus release from decomposing leaf biomass of Calliandracalothyrsus Meissner, Tithonia diversifolia Hensley A.Gray and Tephrosia vogelii Hook.f. agroforestry species applied alone or combined with triple super phosphate (TSP) was studied at World Agroforestry Centre (ICRAF) laboratory for 56 days using an incubation method. The effects of above treatments on maize yield were evaluated in the field at Rubona, southern province of Rwanda between the years 2001 and 2004. The net cumulative phosphorus (P) mineralised ranged from 16.2 to 212.2 mg P kg−1. The net P mineralisation rates from green manure, TSP applied alone or combined with green manure decreased in the order green manure > green manure + TSP > TSP > lime > control. The best plant residues quality for predicting P mineralisation is total P, C, and C:P & C:N ratios. Relative to the control, leaf biomass combined with TSP resulted in six times higher maize grain yield at the end of the experiment i.e., from 0.9 to 7.1 t ha−1. In the fourth season, application of Tithonia diversifolia Hensley A.Gray green manure combined with TSP at 50 kg P ha−1 resulted in higher maize yield (25% increase) than TSP and Tithonia diversifolia Hensley A.Gray (9% increase) applied alone at the similar rate. Therefore, application of plant residues and TSP alone might not be sufficient to meet maize plant P requirements and to achieve the yield potential of maize in the Rubona soils unless supplemented with mineral fertilisers.

Soil fertility replenishment is a critical factor that many farmers in the tropical American hillsides have to cope with to increase food crop production. The effect of three planted fallow systems (Calliandra houstoniana-CAL, Indigofera zollingeriana-IND, Tithonia diversifolia-TTH) and a crop rotation (maize/beans-ROT) on soil nitrogen mineralization, organic matter and phosphorus fractions was compared to the usual practice of allowing natural regeneration of native vegetation or natural fallow management (NAT). Studies were conducted on severely degraded Colombian volcanic-ash soils, 28 months after fallow establishment, at two on-farm experimental sites (BM1 and BM2) in the Cauca Department. Tithonia diversifolia had a significantly higher contribution to exchangeable Ca, K and Mg as well as B and Zn; the order of soil nutrient contribution was TTH > CAL > IND > NAT > ROT. On the other hand, lND had significantly higher soil NO 3 − –N at both experimental farms as compared to all the other fallow system treatments. For the readily available P fraction, CAL and ROT had significantly higher H2O–Po and resin-Pi, respectively, in the 0–5 cm soil layer; whereas TTH showed significantly higher values for both H2O–Po and resin-Pi in the 5–10 cm soil layer. Significant effects were observed on the weights of the soil organic matter fractions which decreased in the order LL (Ludox light) > LM (Ludox intermediate) > LH (Ludox heavy). Indigofera zollingeriana showed greater C, N and P in the soil organic matter fractions than all the other fallow treatments, with NAT having the lowest values. It is concluded that planted fallows can restore soil fertility more rapidly than natural fallows.

The application of nitrogen in a soil under agricultural production is subject to several pathways including de-nitrification, leaching and recovery by an annual crop. This is as well greatly influenced by the management practices, nitrogen source and soil conditions. The main objective of this study was to investigate the loss of nitrogen (N) through nitrous oxide (N2O) emissions and mineral N leaching and uptake by annual crop as influenced by the N source. The study was carried out at Kabete in Central Kenya. Measurements were taken during the second season after two seasons of repeated application of N as urea and Tithonia diversifolia (tithonia) leaves. Results obtained indicated that nitrous oxide (N2O) emissions at 4 weeks after planting were as high as 12.3 -g N m -2 h-1 for tithonia treatment and 2.9 -g N m-2 h-1 for urea treatment. Tithonia green biomass treatment was found to emit N2O at relatively higher rate compared to urea treatment. This was only evident during the fourth week after treatment application.Soil mineral N content at the end of the season increased down the profile. This was evident in the three treatments (urea, tithonia and control) investigated in the study. Urea treatment exhibited significantly higher mineral N content down the soil profile (9% of the applied N) compared to tithonia (0.6% of the applied N). This was attributed to the washing down of the nitrate-N from the topsoil accumulating in the lower layers of the soil profile. However, there was no significant difference in N content down the soil profile between tithonia treatment and the control. It could be concluded that there was no nitrate leaching in the tithonia treatment. Nitrogen recovery by the maize crop was higher in the urea treatment (76% of the applied N) as compared to tithonia treatment (55.5% of the applied N). This was also true for the residual mineral N in the soil at the end of the season which was about 7.8% of the applied N in the urea treatment and 5.2% in the tithonia treatment.From this study, it was therefore evident that although there is relatively lower N recovery by maize supplied with tithonia green biomass compared to maize supplied with urea, more nitrogen is being lost (through leaching) from the soil¿plant system in the urea applied plots than in tithonia applied plots. However, a greater percentage (37.8%) of the tithonia-applied N could not be accounted for and might have been entrapped in the soil organic matter unlike urea-applied N whose greater percentage (92%) could be accounted for.

A 2-year field experiment was conducted to evaluate the effects of Tithonia diversifolia green manure combined with either Minjingu phosphate rock (MPR) or triple super phosphate (TSP) on soil chemical properties that influence P availability, P pools and maize grain yields, on a Chromic Acrisol in Morogoro, Tanzania. Leafy biomass of tithonia was applied before maize planting for two consecutive growing seasons. Treatments compared were the control, MPR and TSP each at 80 kg P ha−1; tithonia alone at 2.5, 5.0, and 7.5 Mg ha−1 dry matter and tithonia combined with MPR or TSP at 40 kg P ha−1. Tithonia led to significant increases in soil pH, exchangeable Ca, labile (resin and NaHCO3-Pi), and moderately labile inorganic P (NaOH-Pi). It reduced exchangeable Al and P sorption. Application of MPR alone had liming effects and resulted in increase in labile P. Combining tithonia with MPR had similar but more intense effects. Triple superphosphate alone led to acidification and this was reversed when TSP was co-applied with tithonia. Increasing the application rates of tithonia either alone or in combination with TSP or MPR led to more pronounced liming effects but the differences between 2.5 and 5.0 Mg tithonia ha−1 were not significant due to moisture stress that was experienced during the season. The P and Ca concentrations of the maize plants at tasselling increased with the application of tithonia alone or combined with MPR or TSP, and were significantly correlated with maize grain yields (r = 0.75 and 0.64 for MPR and TSP, respectively). Tithonia added consecutively for 2 years increased total maize grain yields by 70% compared to that in the control. The relative agronomic effectiveness (RAE) of MPR increased from 46% in the first year of application to > 142% in the second year, indicating that the initially slow dissolution of MPR improved by combined application of tithonia and MPR, attributed to reduction of P sorption. It is concluded that tithonia can enhance P availability from the Chromic Acrisol through modification of soil properties associated with P transformation and availability. In cases where tithonia is found within the farmers’ fields its combined application with MPR can increase maize yields at a much-reduced cost associated with tithonia procurement.

Decline in crop yields is a major problem facing smallholder farmers in Kenya and the entire Sub-Saharan region. This is attributed mainly to the mining of major nutrients due to continuous cropping without addition of adequate external nutrients. In most cases inorganic fertilizers are expensive, hence unaffordable to most smallholder farmers. Although organic nutrient sources are available, information about their potential use is scanty. A field experiment was set up in the sub-humid highlands of Kenya to establish the chemical fertilizer equivalency values of different organic materials based on their quality. The experiment consisted of maize plots to which freshly collected leaves of Tithonia diversifolia (tithonia), Senna spectabilis (senna) and Calliandra calothyrsus (calliandra) (all with %N>3) obtained from hedgerows grown ex situ (biomass transfer) and urea (inorganic nitrogen source) were applied. Results obtained for the cumulative above ground biomass yield for three seasons indicated that a combination of both organic and inorganic nutrient source gave higher maize biomass yield than when each was applied separately. Above ground biomass yield production in maize (t ha−1) from organic and inorganic fertilization was in the order of senna+urea (31.2), tithonia+urea (29.4), calliandra+urea (29.3), tithonia (28.6), senna (27.9), urea (27.4), calliandra (25.9), and control (22.5) for three cumulative seasons. On average, the three organic materials (calliandra, senna and tithonia) gave fertilizer equivalency values for the nitrogen contained in them of 50, 87 and 118%, respectively. It is therefore recommended that tithonia biomass be used in place of mineral fertilizer as a source of nitrogen. The high equivalency values can be attributed to the synergetic effects of nutrient supply, and improved moisture and soil physical conditions of the mulch. However, for sustainable agricultural production, combination with mineral fertilizer would be the best option.