When the manure hits the land: Carbon sequestration potential of organic matter amendments to California annual grasslands

Background/Question/Methods

Grasslands are well known for their potential to accumulate and store C, decreasing carbon dioxide concentrations in the atmosphere. Organic matter amendments such as livestock manure have been proposed as a means to increase C sequestration in grasslands while enhancing net primary productivity. However, recent studies suggest that manure amendments may be a source of greenhouse gas emissions and may not increase soil C. To determine the impact of manure additions on soil C pools, we sampled soils from 2 or 3 fields at each of 10 sites in grazed annual grasslands in California. At each site, the fields received different manure additions (none or annual addition of solid or liquid dairy cow manure). Soils were sampled at intervals to 50 cm-depth and analyzed for chemical and physical properties. We used the DAYCENT biogeochemical model to explore potential mechanisms for the patterns observed. 

Results/Conclusions

Soil C content ranged widely, from 60 to 220 Mg C/ha (0-50 cm). Variations were greater across sites than within sites. Soil C concentrations decreased with depth (4.1±0.2 %C in 0-5 cm and 1.5±0.1%C in 30-50 cm) but >50% of the total C content (Mg C/ha) was located below 10 cm. Surprisingly, fields with manure addition had similar soil C content as controls (127±12 and 127±15 Mg C/ha, respectively). This suggests manure applied to the surface may decompose before it is incorporated into the subsurface.  Soil C content was inversely correlated with soil bulk density but was not correlated with soil texture. Greater bulk densities may limit the transport of C through the soil, superseding clay’s ability to complex with organic matter. The model predicted a large increase in soil C content (15-20 Mg C/ha) with manure additions, counter to field measurements. The model may be underestimating the lability of the material under field conditions. The model also found that soil texture explained a small portion (<2 Mg C/ha) of the variability in soil C pools.  Our results suggest manure additions to annual grasslands do not consistently increase C sequestration due to their rapid decomposition in surface soil environments.