Yield gaps, nutrient use efficiencies and response to fertilisers by maize across heterogeneous smallholder farms of western Kenya

The need to promote fertiliser use by African smallholder farmers to counteract the current decline in per capita food production is widely recognised. But soil heterogeneity results in variable responses of crops to fertilisers within single farms. We used existing databases on maize production under farmer (F-M) and researcher management (R-M) to analyse the effect of soil heterogeneity on the different components of nutrient use efficiency by maize growing on smallholder farms in western Kenya: nutrient availability, capture and conversion efficiencies and crop biomass partitioning. Subsequently, we used the simple model QUEFTS to calculate nutrient recovery efficiencies from the R-M plots and to calculate attainable yields with and without fertilisers based on measured soil properties across heterogeneous farms. The yield gap of maize between F-M and R-M varied from 0.5 to 3 t grain ha-1 season-1 across field types and localities. Poor fields under R-M yielded better than F-M, even without fertilisers. Such differences, of up to 1.1 t ha-1 greater yields under R-M conditions are attributable to improved agronomic management and germplasm. The relative response of maize to N-P-K fertilisers tended to decrease with increasing soil quality (soil C and extractable P), from a maximum of 4.4-fold to -0.5- fold relative to the control. Soil heterogeneity affected resource use efficiencies mainly through effects on the efficiency of resource capture. Apparent recovery efficiencies varied between 0 and 70% for N, 0 and 15% for P, and 0 to 52% for K. Resource conversion efficiencies were less variable across fields and localities, with average values of 97 kg DM kg-1 N, 558 kg DM kg-1 P and 111 kg DM kg-1 K taken up. Using measured soil chemical properties QUEFTS over-estimated observed yields under F-M, indicating that variable crop performance within and across farms cannot be ascribed solely to soil nutrient availability. For the R-M plots QUEFTS predicted positive crop responses to application of 30 kg P ha-1 and 30 kg P ha-1 + 90 kg N ha-1 for a wide range of soil qualities, indicating that there is room to improve current crop productivity through fertiliser use. To ensure their efficient use in sub-Saharan Africa mineral fertilisers should be: (1) targeted to specific niches of soil fertility within heterogeneous farms; and (2) go hand-in-hand with the implementation of agronomic measures to improve their capture and utilisation

Effects of Total Resources, Resource Ratios, and Species Richness on Algal Productivity and Evenness at Both Metacommunity and Local Scales

The study of the interrelationship between productivity and biodiversity is a major research field in ecology. Theory predicts that if essential resources are heterogeneously distributed across a metacommunity, single species may dominate productivity in individual metacommunity patches, but a mixture of species will maximize productivity across the whole metacommunity. It also predicts that a balanced supply of resources within local patches should favor species coexistence, whereas resource imbalance would favor the dominance of one species. We performed an experiment with five freshwater algal species to study the effects of total supply of resources, their ratios, and species richness on biovolume production and evenness at the scale of both local patches and metacommunities. Generally, algal biovolume increased, whereas algal resource use efficiency (RUE) and evenness decreased with increasing total supply of resources in mixed communities containing all five species. In contrast to predictions for biovolume production, the species mixtures did not outperform all monocultures at the scale of metacommunities. In other words, we observed no general transgressive overyielding. However, RUE was always higher in mixtures than predicted from monocultures, and analyses indicate that resource partitioning or facilitation in mixtures resulted in higher-than-expected productivity at high resource supply. Contrasting our predictions for the local scale, balanced supply of resources did not generally favor higher local evenness, however lowest evenness was confined to patches with the most imbalanced supply. Thus, our study provides mixed support for recent theoretical advancements to understand biodiversity-productivity relationships

Processes determining intercrop productivity and yields of component crops

This review examines how intercrop productivity is determined, by analysing several key physiological processes which affect yields of component crops. Availability of environmental resources to each of the component crops is important in determining combined intercrop productivity, and hence the analysis is based on capture of environmental resources and efficiency of conversion of captured resources into growth of harvested organs of the component crops. It is emphasized that the competitive abilities of component crops, which determine their biomass production and often yields, vary greatly according to growth environment, and hence cultural manipulation can adjust the balance of their yields. Intercrops are most productive when their component crops differ greatly in growth duration so that their maximum requirements for growth resources occur at different times. For high intercrop productivity, plants of the early-maturing component should grow with little interference from the late-maturing crop. The latter may be affected somewhat by the associated crop, but a long time period for further growth after the harvest of the first crop should ensure good recovery and full use of available resources. Compared with a sole crop, the reduced size of non-harvested organs of the late-maturing crop can result in improved assimilate partitioning to the harvested organ during the later part of the growth period and consequently a higher harvest index. Because of the differences in growth rhythm between component crops, there tends to be little interaction between relative performance of component crops and growth environment and hence productivity of this type of intercrop is often insensitive to management interventions. In contrast, when growth durations of component crops are similar the crops compete more intensely for available resources. Their relative performances can then be greatly affected by small changes in growth environment. 'Additive' intercrops of this type may nevertheless be productive, particularly where growth resources are more completely captured than in corresponding sole crops. However, if non-replenished growth resources are utilized too rapidly, the less-competitive component may suffer greatly. 'Replacement' intercrops of this type are not so productive in high-yielding environments. When the growth environment is not favourable however, their total lower plant populations compared to additive intercrops may allow yields of replacement intercrops to be less depressed. Where similar-duration crops are grown in variable environments, replacement intercrops may therefore be preferred due to their greater yield stability. Where a dominant crop uses available resources excessively and inefficiently, agronomic manipulation in favour of the usually suppressed component seems most likely to improve the productivity of the whole intercrop. Intercrop productivity depends on the genetic constitution of component crops, growth environment (atmospheric and soil) and agronomic manipulations of microenvironment. The interaction of these factors should be optimized so that the limiting resource is utilized most effectively in the intercrop. An understanding of the sharing of resources among component crops will help identify more appropriate agronomic manipulations and cultivars for intercrops