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_dry ha^-1 , resulting in the addition of 26 (Solano), 36 (Merced), and 125 (Sacramento) Mg biosolids-C ha^-1 . 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^-1 yr^-1 . 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^-1 yr^-1 , 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.

The effects of organic matter amendments (OMA) on soil fertility in permanent cropping systems like orchards is under-studied compared to annual cropping systems. We evaluated experimentally the impact of OMAs on soil fertility in almond (Prunus dulcis) orchards over a two-year period with annual applications. Two OMAs, derived from composted green waste (GWC) or composted manure wood chips (MWC), were applied as surface mulch and compared to a control at two sites with different soil textures (sandy loam and loamy sand). OMAs increased soil moisture content (0–0.1 m depth) at both sites by 27–37%. Both amendments increased soil inorganic N at the sandy loam (GWC: 194%; MWC: 114%) and loamy sand (GWC: 277%; MWC: 114%) sites the month following application, but soil inorganic N concentrations quickly decreased to values similar to those of control plots. After two-years, the GWC and the MWC amendments increased the soil cation exchange capacity (CEC) by 112% and 29%, respectively, in the sandy loam site, but no change was observed in the loamy sand site. The greatest increase in soil extractable K occurred in the GWC-amended plots at the sandy loam site even though the initial K concentration of MWC was higher. Both OMAs increased soil organic carbon (SOC) after two years, but the SOC increase in the GWC-amended plots was greater. Our results suggest that OMAs can significantly improve soil fertility after one or two annual applications, and that fertility gains appear to be dependent on soil texture than the nutrient concentrations of the OMA.