Soil carbon response to long-term biosolids application.

Astudy was conducted in three agroecosystems in California (Sacramento, Solano, and Merced counties) that received biosolids applications for 20 yr. Management varied in application rates and frequencies, resulting in average cumulative amount of biosolids applied of 74 (Solano), 105 (Merced), and 359 (Sacramento) Mg biosolids ha , resulting in the addition of 26 (Solano), 36 (Merced), and 125 (Sacramento) Mg biosolids-C ha . Measurements included soil organic carbon (SOC) and total nitrogen (N) concentrations from 0 to 100 cm and microbial biomass C (MBC) and microbial biomass N (MBN) from 0 to 30 cm in biosolids-amended and control sites. Biosolids treatments had greater amounts of SOC and total N at all sites, and MBC and MBN were greatest at Sacramento and Solano. The largest increases in SOC were at the site that received the lowest cumulative loading rate of biosolids (Solano), where SOC content to 100 cm was 50% greater in amended soils (p < .001). Net changes in soil C stocks to 30 cm were 0.4 ± 0.1 (Solano), -0.04 ± 0.1 (Merced), and 0.3 ± 0.2 (Sacramento) Mg C ha yr . These values change when considering deeper soil depths (0-100 cm) to 0.5 ± 0.1 (Solano), 0.2 ± 0.2 (Merced), and 0.216 ± 0.2 (Sacramento) Mg C ha yr , reflecting differences in C stocks changes in surface and subsurface soils across sites. Rates of C storage per dry Mg of biosolids per year applied were 1 ± 0.2 (Solano), 0.5 ± 0.4 (Merced), and 0.04 ± 0.1 (Sacramento). Our results suggest that local controls on soil C stabilization are more important than amendment application amount at predicting climate benefits and that accounting for soil C changes below 30 cm can provide insight for sequestering C in agroecosystems.