How Does Recycling of Livestock Manure in Agroecosystems Affect Crop Productivity, Reactive Nitrogen Losses, and Soil Carbon Balance?

Recycling of livestock manure in
agroecosystems to partially substitute
synthetic fertilizer nitrogen (N) input is recommended to alleviate
the environmental degradation associated with synthetic N fertilization,
which may also affect food security and soil greenhouse gas (GHG)
emissions. However, how substituting livestock manure for synthetic
N fertilizer affects crop productivity (crop yield; crop N uptake;
N use efficiency), reactive N (Nr) losses (ammonia (NH3) emission, N leaching and runoff), GHG (methane, CH4;
and nitrous oxide, N2O; carbon dioxide) emissions and soil
organic carbon (SOC) sequestration in agroecosystems is not well understood.
We conducted a global meta-analysis of 141 studies and found that
substituting livestock manure for synthetic N fertilizer (with equivalent
N rate) significantly increased crop yield by 4.4% and significantly
decreased Nr losses via NH3 emission by 26.8%, N leaching
by 28.9% and N runoff by 26.2%. Moreover, annual SOC sequestration
was significantly increased by 699.6 and 401.4 kg C ha–1 yr–1 in upland and paddy fields, respectively;
CH4 emission from paddy field was significantly increased
by 41.2%, but no significant change of that was observed from upland
field; N2O emission was not significantly affected by manure
substitution in upland or paddy fields. In terms of net soil carbon
balance, substituting manure for fertilizer increased carbon sink
in upland field, but increased carbon source in paddy field. These
results suggest that recycling of livestock manure in agroecosystems
improves crop productivity, reduces Nr pollution and increases SOC
storage. To attenuate the enhanced carbon source in paddy field, appropriate
livestock manure management practices should be adopted.