2018 ASHS Annual Conference

Biodegradable and organic mulches are potentially sustainable alternatives to polyethylene film in vegetable production, but annual soil incorporation of biobased mulch may interfere with subsequent crop growth. However, it may be possible to speed degradation (and minimize nitrogen immobilization) by inoculating residues with compost extracts. Little is known about the microbial composition of compost extracts from different feedstocks and how these extracts influence plant growth following mulch incorporation. We completed a series of biological tests to characterize 10 compost extracts, including nematode extraction, protozoa most probable number assay, fatty acid methyl ester (FAME) profiles and microscopic counts for total bacterial and fungal indices, a colorimetric assay, and a lettuce seed phytotoxicity bioassay. Based on test results, three non-phytotoxic compost extracts (from vermicompost, yardwaste, and chicken manure feedstocks) were selected to explore effects on lettuce growth in residue-rich soils in a greenhouse. Extracts contained a diverse range of bacterial (6-12 nmol FAME/mL), fungal (0.67-2.15 nmol FAME/mL), protozoa (4,660-466,780/mL), and nematode (0-22/mL) concentrations. Soil-incorporated mulch residues included alfalfa (equivalent to 11.2 Mg/ha), straw (4.5 Mg/ha), and wood particle-loaded polylactic acid (PLA) mulch (3.8 Mg/ha), and all residues were inoculated with extracts at a nitrogen-adjusted rate of 3.4 kg N/ha (2,000-4,800 L/ha). The interacting effects of residue and extract influenced lettuce growth. High carbon residues, including PLA mulch, reduced growth and extracts had no effect in these treatments; however, inoculation of alfalfa residue with yardwaste and chicken manure compost extracts increased lettuce growth 60% to 72%, respectively. The growth-promoting extracts contained less microbial biomass and microfauna than the vermicompost extract, which performed comparably to the control. Results suggest that certain compost extracts can enhance lettuce seedling growth when applied to high nitrogen residues before soil incorporation.