Solid farm manure and compost

Summary
Animal houses: Aerobic microbial activity in the surface layers of the deep litter caused a temperature increase to a maximum of 40-50oC at 10 cm depth. Urea is the source of ammonium in the litter. A high proportion of the ammonium is found more than 10 cm from the surface, and most of the ammonium is absorbed by the straw and thereafter transformed by microorganisms to non-volatile organic N. Due to absorption and immobilisation ammonia volatilization is less than 10% of the N, that is excreted and collected in the straw litter. In the deep litter no nitrification and denitrification were measured, and in consequence no or very low concentration of nitrous oxide was determined. The nitrification and denitrification processes were inhibited by low oxygen partial pressure, high temperatures and a high ammonia concentration. In the layers more than 10 cm from the surface the oxygen concentration was low and CH4 was produced. No oxidation of CH4 occurred in the oxic surface layers of the deep litte and the daily CH4 emission constituted ca. 10% of the total emission of CH4 and CO2 from the deep litter.

Storage: In a laboratory experiment 53% and 46%, of the initial C and N content were lost from composted manure, and losses from anaerobic stored manure were 24% C and 18% N. Urine N accounted for the highest proportion of total N lost from both the composted and anaerobic stored manure. The C to N ratio of the manure composted in the laboratory was 14, which was considerably lower than the C:N ratio of the deep litter composted at full scale, therefore, the emission of N during composting in the laboratory was higher than from full scale composting of deep litter. It was tested if an increase in immobilisation through cutting and mixing would reduce losses of N. The loss of N during cutting and mixing was high and this treatment did, therefore, not reduce the total loss of N during pre-treatment and storage. The convection of air through the deep litter heaps was reduced by a cover of porous tarpaulin or compaction with a front loader, this treatment reduced ammonia volatilization due to both the lower air exchange and a reduced heating of the heap. Covering the compost with porous tarpaulin or compacting the compost reduced emission losses to 12-18% of total N compared to a loss of 28% during composting of untreated deep litter. Most of the N loss was due to ammonia volatilization, leaching accounted for about one fifth of the N losses and little N was lost due to denitrification in the heaps with unmixed litter. Leaching losses of K were 11-15% of the amount pre-sent at initiation of the experiment, compaction and a cover reduced the volume of liquid leaching from the heaps and K loss. Less than 0.3% of the total N was emitted as nitrous oxide and CH4 emission was between 0.01 and 0.03% of the C in the stored deep litter.

Field: More CO2 evolved from soil incubated for 9 months with anaerobic stored manure when compared with incubated composted manure. The cumulated C loss during storage and the following incubation with soil was 59% of C in composted manure and 52% of C in the anaerobic stored manure. Thus, quantitatively a similar amount of C was retained in soil irrespective of the treatment during storage. The crop recovery of urine 15N from anaerobic stored manure in wheat was higher than the recoveries of N from straw of faeces in manure, because less N had been lost from the anaerobic stored manure. The yield respond of barley after amendment of compost and of fresh deep litter, showed that composting reduced the fertilizer efficiency of the deep litter.

Holistic approach: The findings of the study were used for the development of a holistic farm model. The model calculations showed, that composting reduced N and C availability in the field, considerably. Thus N would be less available on a farm with deep litter and composting than on a farm with slurry, and C availability at the same level. In the field, the organic N content will increase on organic farms, but only on farms with deep litter applied directly in the field (no storage) the organic C will increase.