Ammonia and greenhouse gases losses from mechanically turned cattle manure windrows: A regional composting network

An on-farm composting network operates in the Basque Country (northern Spain), in which solid manure produced in livestock farms (mostly dairy and beef cattle) is composted through windrow turning. This network aims to produce a valuable resource (compost) for the farmers whereas the volume of the solid manure was reduced at farm level The objective of the study was to assess the gaseous losses (NH₃ and GHG) from 6 on-farm composting windrows (either deep litter systems or solid fraction after slurry separation) after turning operations. Monitored turning events occurred 1 to 4 months after establishing the heaps on the field. Ammonia and greenhouse gas (GHG) losses were estimated by the open and close chamber techniques, respectively. Results showed overall low emission rates related to the long degradation period of the windrows. Maximum NH₃ release was at 2.0 mg m^-2 d^-1 after the second/third turning events. Baseline N₂O losses were below 50 mg m^-2 d^-1, with maximum rates close to 500 mg m^-2 d^-1 some days after turning works. Methane emissions were mostly below 100 mg m^-2 d^-1, while CO₂ losses were lower than 25 g m^-2 d^-1. Carbon dioxide peaks (≈250 g m^-2 d^-1) were reached after the second/third turnings. Overall, gaseous N and C losses accounted for 0.1 and 1% of the initial N and C content of the windrows, respectively. The present study concluded that two/three turning operations in aged solid manure-derived compost windrows do not have significant effects on NH₃ and GHG losses. The magnitude of the gaseous losses from on-farm composting systems is dependent on the manure management practices at farm level (e.g. moment of windrow stacking).