Influence of recycled organic waste amendments on carbon pools, greenhouse gas emissions, and nematode indicators of soil health

To optimize outcomes for plant and soil health, greater understanding is needed about how management practices affect interrelationships between soil organic carbon and soil food webs, which has important implications for greenhouse gas emissions. This study examined the effects of applying three different organic amendments, from sources previously considered waste. The amendments included: 1) compost composed of municipal food waste and yard trimmings 2) biochar produced from woody debris and 3) liquid food hydrolysate derived from grocery store organics. These amendments were compared to either fertilized (N+) or unfertilized (N-) controls. In an almond orchard field trial, both biochar and compost increased pools of total carbon and large macroaggregates compared to N+ and N- controls, but only compost increased the carbon pool, permanganate oxidizable carbon (POXC). Average daily nitrous oxide (N 2 O) emissions during the first year were significantly higher in N+ and food hydrolysate than in biochar, compost and N- treatments, likely due to their lower C:N ratio. Biochar decreased the total number of nematodes and root herbivores compared to N- controls, while compost, food hydrolysate and N+ treatments increased the abundance of root-herbivores. Food hydrolysate additionally increased the ratio of key fungal-feeding/bacterial-feeding nematodes compared to N+ treatments. Fertilizer treatments increased tree growth while both food hydrolysate and fertilizer increased leaf nitrogen and potassium. The incubation study showed that food hydrolysate released more net mineralized inorganic N than other amendments or N+ treatments and that compost consistently increased microbial biomass carbon. These results may be used to optimize the selection of organic amendments and indicate that there are benefits and tradeoffs in terms of their effects on carbon storage, soil biology and greenhouse gas emissions. • In the field, biochar and compost increased total carbon and large macroaggregates. • Compost increased biologically active carbon and microbial biomass carbon. • Fertilizer and food hydrolysate increased cumulative nitrous oxide emissions. • Biochar decreased the total number of nematodes and root herbivore nematodes. • Compost, food hydrolysate and fertilizer stimulated root herbivore nematodes.