UC Davis

Composts are increasingly used as soil amendments to enhance soil health and agricultural sustainability. However, their effects on nitrogen (N) availability, and therefore their fertilizer potential, greenhouse gas (GHG) emissions, carbon (C) sequestration, and crop water use efficiency (WUE) are unclear. We conducted a three-year field experiment in sub-surface drip-irrigated tomatoes to investigate the effects of co-compost (FW; food waste and green waste) and green waste compost (GW) on yield and fertilizer N use, as co-composts are gaining research interest as a means of diverting organic waste from landfills and increasing the nutrient value of GW compost. Three FW and GW compost rates (0, 9 t ha-1, or 18 t ha-1) and four fertilizer N levels (0%, 70%, 85% and 100% of recommended rate) were examined, and treatment combinations were chosen to replace a certain fertilizer N input with compost N. 15N-labeled fertilizer was used to determine fertilizer N crop use efficiency (true NUE), while apparent NUE was determined by comparing crop N uptake between treatments and no compost controls. In years two and three, FW and GW sustained and/or increased yield compared to no compost when low fertilizer rates were applied (0% N and 70% N), with lower apparent and true NUE observed in the compost treatments compared to no compost when 70% N and 85% N was supplied. These results suggest that compost served as an N source or primed the mineralization of soil N, potentially replacing fertilizer N crop uptake. Fertilizer N remaining in topsoil post-harvest was greatest for FW compared to GW and controls, while no difference in nitrate leaching potential was found among treatments, except in year two FW had the lowest nitrate leaching potential. These findings did not consistently produce statistically significant effects but show the potential role of compost in immobilizing fertilizer N and priming soil N mineralization. Within the same field experiment, we also evaluated the effects of FW and GW compost on GHG fluxes and cumulative emissions, soil C content, and nematode populations and diversity as a potential biological mechanism for C sequestration. FW and GW composts had no significant effects on the fluxes or annual cumulative emissions of N2O or CH4, likely because emissions under sub-surface drip irrigation were already low and the considerable variation in low fluxes obscured the detection of significant changes. The application of compost significantly increased soil respiration (carbon dioxide (CO2) emissions). The level of fertilizer N played a significant role in regulating N2O and CH4 emissions, as these fluxes decreased with lower levels of fertilizer N. Soil C content increased with the addition of FW and GW composts compared to the no-compost controls in the 0-15 cm soil layer. This increase was not observed in deeper soil layers of 15-30, 30-60, or 60-90 cm. FW and GW compost treatments reduced δ13C values in the topsoil, indicating that the newly added soil C was derived from the compost. Furthermore, compost application increased the presence of certain individual genera of bacterial- and fungal-feeding nematodes compared to the no-compost controls, offering insights into potential biological mechanisms for C decomposition and sequestration. However, no significant treatment effects were observed on individual genera of nematodes. These findings indicate that although compost may not directly lead to reductions of GHG, it holds potential for mitigating climate change by sequestering C in agricultural soils. We also conducted a one-year field experiment in two, surface drip-irrigated, super-high density olive orchards (in Woodland and Stockton, California, hereafter named the MR and ST sites, respectively) to investigate the effects of compost and fertilizer N management on yield and intrinsic WUE (iWUE; the ratio between net CO2 fixation and stomatal conductance). The olive industry in California is rapidly growing, and there is a need for irrigation and N management guidelines to be updated, taking climate change impacts on the state’s diverse microclimates and increased tree density into consideration. At each field site in our trial, two GW compost rates (0 or 9 t ha-1) and three fertilizer N rates (84, 112, or 140 kg N ha-1 at the MR site, and 28, 42, or 56 kg N ha-1 at the ST site) representing a low, medium, and high fertilizer rate were applied. During the growing season, monthly leaf sampling and concurrent soil sampling were conducted for analyzing δ13C, iWUE, and δ18O, which are proxies for plant water status, and soil gravimetric water content (GWC), NH4+, and NO3-. Compost increased yield compared to no-compost controls at the lowest fertilizer N level at the ST site, while no treatment effects on yield were observed at the MR site. Compared to no-compost controls, compost tended to increase olive δ13C and iWUE at lower fertilizer N levels, indicating increased stomatal closure and plant water stress. In contrast, compost decreased δ13C and iWUE at the highest fertilizer N level, indicating that additional N decreased compost-induced plant water stress. These treatment effects on δ13C and iWUE were more significant at the MR site than the ST site, and the overall iWUE was lower at the ST site than the MR site, indicating less plant water stress in the ST site, likely due to higher precipitation, older tree age, and thus larger root zones for water uptake masking treatment effects. The result of compost increasing iWUE, suggesting that the compost increased water stress for olive trees, was unexpected because compost typically improves soil water-holding capacity for crop uptake. However, in our study, soil GWC results contradicted these typical compost effects on soil water, as there was generally a lack of significant treatment effects on GWC at both field sites. There were also no consistent compost or fertilizer N effects on olive δ18O, an indicator for transpiration, and little to no treatment effects on soil NH4+ or NO3−. Soil NH4+ and NO3− did not show significant correlation with plant iWUE or δ18O, whereas soil GWC was positively correlated with iWUE at the MR site. Our results suggest olive iWUE was regulated more by soil water content than plant-available N content.