Meta-analysis on how manure application changes soil organic carbon storage

Manure application to agricultural soils is widely considered as a source of nutrients and a method of maintaining levels of soil organic carbon (SOC) to mitigate climate change. At present, it is still unclear which factors are responsible for the SOC stock dynamics. Therefore, we analyzed the relationship between SOC stock changes and site characteristics, soil properties, experiment characteristics and manure characteristics. Overall, we included 101 studies with a total of 592 treatments. On average, the application of manure on agricultural soils increased SOC stocks by 35.4%, corresponding to 10.7 Mg ha(-1). Manure applications in conventional tillage systems led to higher SOC stocks (+2.2 Mg ha(-1)) than applications under reduced tillage. Soil organic carbon increase upon manure application was higher in soils under non-tropical climate conditions (+2.7 Mg ha(-1)) compared to soils under sub-tropical climate. Larger SOC increases after manure application were achieved in intermediate and shallow topsoils (in 0-15 cm by 9.5 Mg ha(-1) and in 16-20 cm by 13.6 Mg ha(-1)), but SOC stocks were also increased in deeper soils (>20 cm 4.6 Mg ha(-1)), regardless of the tillage intensity. The highest relative SOC increase (+48%) was achieved if the initial SOC was below 1% but the absolute SOC increased with increasing initial SOC. Clay soils showed higher SOC increase rates compared to sandy soils (+3.1 Mg ha(-1)). Acidic soils showed comparable relative effects but a higher stock difference than neutral (+5.1 Mg ha(-1)) and alkaline soils (+5.1 Mg ha(-1)). The application of farmyard-, cattle- and pig manure showed the highest SOC increases (50%, 32% and 41%, respectively), while green manure and straw showed only minor effects. If manure applications were combined with additional mineral fertilizer, the SOC increases were higher (+1.7 Mg ha(-1)) compared to manure alone. Higher applied amounts generally led to higher SOC stocks. However the annually applied amount is only important under conventional tillage, non-tropical climate conditions, and pH-neutral as well as SOC-rich or SOC-depleted soils and if no additional mineral fertilization is applied. Further studies should focus on the SOC dynamics under tropical climate conditions and factors influencing a potential carbon saturation. In both cases, the number of data was too small. For this reason, additional field studies should be conducted primarily in the tropics. On the other hand, long-term field trials should be re-assessed or newly established to specifically investigate potential saturation effects and long-term (>20 years) fertilizer effects and carbon sequestration.