Experimental site and set up
The experiment was conducted in the backyard of the tree nursery of the Kenya Forestry Research Institute (KEFRI) at Maseno, western Kenya (0° 1′ 0″ S, 34° 36′ 0″ E; 1503 m.a.s.l.). This location allowed easy access to the experiment to farmers from the surroundings, controlled management and safe handling of sampling and measurements. The storage methods were designed to resemble local practices of manure storage and were conducted during the same period as that in which farmers normally store manure. Western Kenya is characterised by a bimodal rainfall regime that allows two cropping seasons per year (Fig. 1). At the beginning of each rainy season farmers plough their fields and apply the manure that has been stored during the previous season. Crop residues are added to the manure heap/pit as the crop harvest proceeds, and is rarely done all at once. Excreta is also added to the stored manure throughout the storage period. For this experiment, however, it was necessary to premix excreta and crop residues in a representative proportion observed locally (Castellanos-Navarrete 2007). This was done at the beginning of the experiment, and no other material was added during storage. It was assumed that it takes farmers about 2 months of manure collection to accumulate an amount (fresh mass) that represents a heap/pit of typical dimensions (±300 kg). For this reason, the experiment was set up in October 2006 (about 2 months after planting time) and terminated in April 2007 (cf. Fig. 1). The number of treatments and replications in the experiment were limited by the amount of manure of relatively homogeneous quality (from the same origin) that could be gathered.
Between 11 and 25 October 2006, cattle excreta were collected at the experimental farm of Maseno University, from a herd of 20 dairy (crossbred Friesian and Ayrshire) cattle in lactation, with an average milk production of 8 L cow−1 day−1, and an average body weight of 300 kg. During this period, cows were fed a daily diet consisting of: 70 kg fresh weight cow−1 of Napier grass, 3.5 kg cow−1 of dairy meal (dry cows got only 1 kg a day) and c. 1 kg FW cow−1 of banana stalks. Molasses were added to the ration at a rate of 7 L in the daily bulk of dairy meal (of 70 kg). Water consumption was 80 L cow−1 day−1. Cows were kept on a paddock dominated by unpalatable grass species from 7.30 to 13.00 h and from 16.00 to 18.00 h daily. Fresh excreta were collected mostly from hard floored stalls, containing small fractions of urine, and partly from the yard around the zero grazing units, which may have been contaminated slightly with soil. Only fresh faeces excreted the same day were collected; those exposed to weather for more than 24 h (rained on or sun dried) were not collected. All excreta collected were stored under a roof (with open walls) in an uncovered pile resting on a plastic sheet. The excreta were almost completely free of plant material such as bedding or feed refusals. The total quantity of excreta at the end of the collection period of 14 days was ca. 2.7 t fresh weight (c. 20% DM content). Maize stover was collected from a single field at Maseno University farm. It had been removed from the field after harvest on the last week of August 2006 and piled in the open air for c. 60 days. The total amount of maize stover collected was c. 200 kg air dry weight.
Layout of the experiment
Three treatments were laid out: (i) compost heaps in open air (HOA), (ii) compost pits in open air (POA) and (iii) compost heaps under roof (HUR), with three experimental units per treatment (Fig. 2). While the dimensions and environmental conditions of the HOA and POA treatments reproduced the conditions under which farmers store manure, the HUR was tested as an affordable improvement of storage conditions. The HUR were located under a 2.3 m-high roof made of semi-transparent glass fibre sheets inclined for drainage, and open walls. The HOA and POA treatments were located in an adjacent field (about 15 m away from the HUR site) that was fenced. The experimental units were placed contiguously without randomisation, due to the impossibility of randomising the treatment under roof. All treatments, both under roof and open air were shaded by nearby trees and buildings during early morning and late afternoon. To avoid run-on of rain water towards the experimental units, 30 cm-deep ditches were dug across the slope, upslope along the width of the experiment. To avoid contamination between experimental units due to drainage and/or run-off, heap and pit replicates were placed 1 m away from each other and 20 cm-high ridges were built in between them. A drainage ditch was also dug between the rows of heaps and pits in the open air site. The soil on which the heaps were placed was levelled (terraced) prior to the experiment.
The fresh excreta collected from the dairy farm was mixed thoroughly to homogenize differences in quality that could have been caused by different residence times in the collection pile (ranging from 1 day to 15 days). The excreta were then mixed with maize stover (previously chopped to 20 cm) in a volume ratio 2:3 using wheelbarrow load counts (6:9). The weights (± standard error) of wheelbarrow loads of fresh excreta, dry maize stover and of the manure mixture were determined: fresh excreta, 51.7 ± 0.58 kg; dry maize stover (chopped), 2.3 ± 0.12 kg; manure mix 2:3 v/v excreta:stover: 28.3 ± 2.66 kg. Heaps of approximately conical shape of 1.5 m basal diameter and 70 cm height were built with the manure mix, averaging 332 ± 15 FW kg per heap. Pits of 1 × 1 m and 0.6 m deep were dug and filled with the same amount of the mix of excreta and stover used to build the heaps. Two additional heaps were built as controls, without replications, and placed next to the rest of the heaps: one heap of pure excreta and one heap of pure maize stover. They had the same shape and dimensions as the other heaps and were sampled for laboratory analysis as the rest of the treatments. The results of these analyses were used to cross-check those from the main treatments. The quality and nutrient composition of the materials used in the experiment is given in Table 1.
Management and monitoring
The experiment started on October 28, 2006 and the heaps and pits were turned three times during storage (as practiced locally). On December 7, January 9 and February 8 (40 days, 73 days and 103 days of storage, respectively) all the material was removed from the heaps/pits and weighed, and heaps/pits were re-built by placing the former surface material at the bottom and vice versa to simulate the operation of turning the heap/pit. On April 26 all the material from the heaps/pits was removed and weighed. A rain gauge and a max/min thermometer were installed next to the experiment to record daily rainfall and air temperatures. Temperature was measured every morning at 09.00 h and every afternoon at 13.00 h at the centre of the heap/pits throughout the experiment. pH was monitored in the field on 1:2.5 suspensions of samples taken from the heaps twice a week using pH strips.
Sampling and laboratory analysis
Six samples of excreta of approximately 0.5 kg each were taken from the initial collection pile at the end of the collection period, two close to the upper surface of the pile, two from the centre and two from the bottom. These samples were quickly but thoroughly mixed on a plastic film and three sub-samples of 0.3 kg were packed in polythene bags, sealed and stored at 4°C in a coolbox prior to analysis of mineral N, total N, P and K, water and ash content and pH. Three samples of approximately 0.3 kg each were collected from the centre of the bulk of maize stover (chopped to 20 cm pieces and mixed throughout) and analysed for total N, P, K and water content. After mixing excreta and stover in the selected proportion (2:3 v/v) and prior to the construction of manure pits and heaps, three samples were taken, mixed, and a composite sample of 0.3 kg was sent to the laboratory for analysis of mineral N, total N, P and K, water and ash content and pH. This procedure was repeated three times during the process of heap/pit building (i.e. three composite samples were sent for analysis).
Samples were taken from the heaps/pits every 30 days throughout the experiment, from three points around the centre (about 20 cm from it) on three imaginary axes separated by an angle of 120 degrees, and about 20 cm deep into the heap/pit (one auger-head deep). These samples were mixed to generate a composite sample of each experimental unit (cf. Fig. 2), totalling nine composite samples per sampling date. Samples were taken at 40 days, 73 days, 102 days and 182 days of storage (the original plan was to sample at 30 days, 60 days, 90 days and 180 days of storage, but heavy rains delayed the first sampling by 10 days). Samples for mineral N were taken before the compost was removed for weighing. Samples were sealed in plastic bags, stored at 4°C in a coolbox and sent to the laboratory for analysis of mineral N and water contents and pH the following day. Composite samples (including the surface, centre and bottom positions) were taken from the manure mix at every turning and at the end of the experiment and analysed for mineral N, total N, P and K, ash and water contents. After sub-sampling the remaining material was returned to the heaps/pits.
Soil sampling for mineral N
To assess N losses through leaching from stored manure, soil samples were taken from underneath the heaps/pits and from control soil on the same experimental field. Samples were taken on December 7 (after 40 days storage) at 0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm and 80–100 cm depth from the soil beneath the open air heaps (HOA) and at 60–80 cm and 80–100 cm from beneath the pits (POA) and at 0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm and 80–100 cm depth from the soil beneath the pure manure and maize stover control heaps. The soil beneath the pits was sampled while the material was being removed for weighing and turning. The soil beneath the heaps under roof (HUR) was only sampled for the upper 0–20 cm layer. Samples were also taken at five different points within the experimental field were the experiment was placed. These samples were bulked per depth, corresponding to 0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm and 80–100 cm. Samples were stored at 4ºC and analysed for mineral N (NH4-N and NO3-N) the following day.
Samples were oven dried at 55°C and ground to pass through a 1-mm sieve. Organic matter was analysed by loss on ignition (Okalebo et al. 2002). A 10 g sample was taken and ignited at 550°C for 8 h and the ash weighed on a fine balance. To convert the percent organic matter content to total C the loss on ignition was multiplied by the coefficient 0.526 determined by Kirchmann and Witter (1992). While we acknowledge that C contents in organic matter will vary according to its composition and change during composting, within the approximate range of 40 – 60%, this value was seen as more conservative than the 58% often assumed in simple calculations for soil organic matter (Stevenson 1986). N, P and K were analysed after complete oxidation of the materials by a modified Kjeldahl digestion using sulphuric acid (Okalebo et al. 2002). Samples were pre-treated with sodium salicylate to convert NO3 to NH4, and hydrogen peroxide was added as oxidising agent. N was determined from 5 mL aliquot of the digestion mixture using an auto-analyser (Sklalar Analytical BV, The Netherlands), K was determined by flame photometry and P colorimetrically using the molybdate-blue method. Mineral N was determined in potassium chloride extracts through a cadmium-reduction method (Dorich and Nelson 1984). The pH was determined on water extracts (1:2.5 manure/water), as described by Anderson and Ingram (1993).
A simple laboratory test was conducted to measure the potential for P leaching from manure during storage. PVC tubes of 10 cm diameter and 20 cm long were filled with 100 g (DW) manure. An iron mesh, filter paper and sterile sand were placed at the bottom of the manure columns. Water was applied daily during 6 days, totalling of 600 mL (equivalent to ca. 75 mm rain in a week). The leachate was collected on days 1, 2, 3 and 6 and analysed for P concentration colorimetrically.
The dry weight of the heaps/pits (kg) at each weighing date was calculated from their fresh weights and dry matter fractions (%). The concentrations of C, N, P and K were used to calculate the total content of these elements per heap/pit, and expressed in kg SU−1, where SU stands for storage unit and corresponds to a pit or a heap containing ca. 100 kg of manure dry matter. The three experimental units of each treatment were considered replicates in ANOVA’s performed to test the effects of storage practice (pit open air, heap open air, heap under roof), days of storage (40, 73, 102, 182) and their interaction. Single exponential models of the form: Y
) × exp
-rt were used to describe statistically the changes in manure dry weight and in its total content of C, N, P and K (kg) during the period of measurement. The parameters Y
(in Y units) represent the initial and final levels of Y
, respectively, and r is the relative rate of change of the state variable over time (t). The analyses were done using GenStat, 10th release.
The measurement of changes in carbon during manure storage in heaps, in the open air and under roof, were used to fit the C mineralization model of Yang and Janssen (2000), in which the organic matter is treated as a single component. Pits were discarded because the stored manure was contaminated with run-on soil particles, so that changes in C could not be attributed reliably only to decomposition. The model of Yang and Janssen is based on the principle that the logarithm of the average relative mineralization rate (K) of a substrate considered as a whole is linearly related to the logarithm of decomposition time (t, years). The equation is: K = R t
-S, or: logK = logR − S logt, where R (dimension t
S - 1) represents K at t = 1, and S (dimensionless, 1 ≥ S ≥ 0) is a measure of the rate at which K decreases over time, also called the speed of `ageing’ of the substrate. The quantity of the remaining substrate, Y
, is calculated by Y
1 - S), where Y
is the initial quantity of the substrate. The actual relative mineralization rate (k) at time t is proportional to K, according to k = (1-S)K. This model was tested against an assembly of 136 sets of data collected from trials conducted in 14 countries all over the world, covering periods of months to tens of years and materials of widely different quality (Yang and Janssen 2000).
Both the statistical model fitted against the measurements of organic matter in manure over time and the model of Yang and Janssen (2000) were used to generate graphs to assist in estimating CO2 emissions from manure stored under different conditions over variable periods of time.