Summary

Minimising nutrient losses through promoting effective recycling of nitrogen (and other nutrients) is a key issue in maintaining the sustainability of many smallholder, mixed farming systems. However, it is only recently that the dynamic role that may be played by livestock in mediating N transfers has come to be recognised. This includes the potentially high sensitivity of the N dynamics of the farming system as a whole to changes in livestock feeding management. The varying quality (e.g. N, lignin and polyphenol content) of livestock diets influences feed digestibility, partitioning of nutrients amongst different ruminant tissues and partitioning of excreted nutrients between faeces and urine. The consequences of variation in these partitions for the value of excreta in providing nutrients to the soil and hence supporting crop production, have been poorly understood and under-researched.

Documents

	Final Technical Report
	Sustainable Nutrient Cycling in Crop/Livestock Systems

This LPP project has used both experimental and simulation modelling techniques to examine the implications of changes in N dynamics in animals for the subsequent behaviour of N in soils to which their excreta has been added and for plant growth on those soils. A significant, novel feature of the approach taken in R6283 has been to integrate animal metabolism experiments with soil and plant growth studies conducted under controlled conditions by using characterised manure from the animal experiments. An on-farm study was then established to verify the implications of the results of the experimental studies under field conditions. Despite a number of difficulties during the establishment phase, delivery of project outputs has now been completed to schedule.

Animal metabolism studies indicated the high degree of variation that may be induced in manure quality by dietary manipulation. Changes in both level and form of N supplementation radically altered the total amounts of N excreted and its partitioning between faeces and urine. The results of the studies indicated that the presence of dietary polyphenols might also affect the extent to which faecal N was bound to neutral detergent fibre with considerable implications for the dynamics of N release on the incorporation of manure into the soil.

The dynamics of N mineralisation (measured in leaching tubes) was affected considerably by the provenance of the manure (i.e. the diet that had produced it). Manures produced from diets supplemented with Calliandra calothyrsus and Macrotyloma axillare had similar N mineralisation patterns, with net cumulative N release occurring at around week 16 following incorporation. However, N mineralisation from these manures was much faster than mineralisation from manure derived from diets supplemented with poultry manure. Nonetheless, there was evidence that N was immobilised for at least 12 weeks following applications of all manure types suggesting that application at planting (a common local practice) may not always be most effective in promoting crop growth.

The potential significance of these observations was confirmed by seedling growth studies conducted in pots in which the highest dry matter assimilation was observed after 12 weeks in treatments where no manure had been added. Reductions in DM yields associated with the addition of manure produced from the different treatment diets ranged from 6% to 27% in comparison with the un-manured control - a finding that was consistent with the pattern of immobilisation of soil N observed in the leaching tube experiment. Such a lag period between application of manure to the soil and a net release of N has considerable significance for planning organic matter applications in practice. Furthermore, these results would suggest that dietary factors might need to be taken into account in doing this. Early manure applications before planting could provide better synchrony of crop N demand with N release from added manure. Alternatively, the beneficial effects of manure application may only be realised in the growth of a later season's crop.

An on-farm study conducted in the Tea, Coffee and Semi-arid agro-ecological zones to the east of Nairobi, and again using characterised manure, examined whether these experimental findings are borne out in the farmer's field. Initial findings (from one season's crop) were somewhat ambiguous. Lower rates of crop dry matter assimilation in manured and littered plots were consistent with the observations of N immobilisation under controlled conditions. However, the differences amongst litter and manure types observed in the mineralisation and pot growth experiments were not repeated under field conditions. These observations might longer-term studies for confirmation as data were not available on mineralisation beyond 24 weeks (i.e. beyond the first crop). However, it is also likely that appropriate manure and litter handling techniques for conservation of N may be a priority area for research if the potential benefits for soil fertility of the interactions of organic resources with animals are to be realised.